Mar 26, 2002 - Family, Gustavo & Family, André, Kika, Isabella, Adri & Family, ...... during the summer, but an eventual strong evaporation in the tidal flat areas ...
Long- to Short-term Morphodynamic Evolution of the Tidal Channels and Flats of the Dithmarschen Bight, German North Sea.
Dissertation zur Erlangung des Doktorgrades der Mathematisch-Naturwissenschaftlichen Fakultät der Christian-Albrechts-Universität zu Kiel
vorgelegt von
Nils Edvin Asp Neto Kiel 2003
Referent: Prof. Dr. Roberto Mayerle Korreferent: Prof. Dr. Rolf Köster Tag der mündlichen Prüfungen: .........................................06.02.04 und 09.02.04 Zum Druck genehmigt: ....................................................................Kiel, 17.05.04
Gedruckt mit Unterstützung des Deutschen Akademischen Austauschdienstes (DAAD)
Table of Contents: 1. Introduction ………………………..……………………………………..………….… 1.1. Objectives ……...………………………………………...…………………………....
1 4
2. The study area (“state of the art”) ..…………………………………………………. 2.1. Location and general aspects ..……………………………………………………….. 2.2. Geological characteristics, configuration and age of the tidal basin …….…………... 2.2.1. Holocenic sea-level rise, sediment supply and distribution …………..……………. 2.2.1.1. Superficial sediments …………………………………………………………….. 2.3. Climate and hydrodynamic conditions ……...……………………………………….. 2.3.1. Hydrology ………………………………………………………………………….. 2.4. Morphology and morphodynamics …………………..……….……………………....
5 5 6 7 10 10 12 14
3. Materials and Methods ………………..………………………………..…………….. 3.1. Data basis ………………………………………………………….……..…………... 3.1.1. Bathymetric data of the Piep channel system (BSH) ..……………..…………….… 3.1.2. Water levels (ALR – Husum/Gewässerkunde Büsum) ….……………...…...….…. 3.1.3. Sediment cores ……………...…..……………………………………..………….... 3.1.4. Current measurements at the Piep channel system (Promorph data bank) ...………. 3.1.5. Wind data (FTZ data bank) ..........................……………..……………………….... 3.2. Field measurements …………………….……………………………………………. 3.2.1. Periodic bathymetric measurements ……………..….………………………..……. 3.2.2. Side-scan sonar and reflection seismic profiling ……………………..……………. 3.2.3. Sediment cores ………………………………………..…………...……………….. 3.3. Modelling …………………………...………………………………………..………. 3.3.1. Digital elevation models (DEM) ...……………………………………………….... 3.3.2. Hypsometric analysis …………....……………………………………..…………... 3.3.3. Numerical modelling of tidal flow .…………………………………..……………. 3.4. Systematic, organization and presentation of results …………………………………
17 17 17 18 18 18 19 19 19 21 23 25 25 35 26 28
4. Long-term morphodynamics (Millennia to Centuries): Holocene evolution of the inner German Bight (natural and anthropogenic processes) …………………….…... 4.1. End of the ice age and sea-level rise …………………………………………………. 4.2. Succession of geomorphologic configurations during Holocene ……………………. 4.3. Beginning of settlement and anthropogenic influence in the area ………………….... 4.3.1. History of land reclamation in Dithmarschen since Middle Ages ………………….
31 31 32 36 36
5. Medium-term morphodynamics (Decades): Land reclamation and natural processes ………………………………………………………………………….…….... 5.1. Natural processes in medium-term morphodynamics ……………………………….. 5.1.1. Meandering ……………………………………………………………………….... 5.1.2. Migration of outer sandbanks …………………………………………………….... 5.1.3. Depth-limitation by consolidated layers ………….................................................... 5.2. Recent land reclamation in the Meldorf Bight ...……………………………………..
41 41 41 44 47 59
6. Short-term morphodynamics (few years to months) ...……………………………... 6.1. Morphological changes at evaluated areas …………………………………………... 6.1.1. Cross-section “A” - Norderpiep ……………………………………………………. 6.1.2. Cross-section “B” - Süderpiep ……………………………………………………... 6.1.3. Cross-section “C” - Piep …………………………………………………………....
61 61 61 63 67 i
6.1.4. Channel slope across from Büsum ……………………………………………….... 6.1.5. Cross-section Sommerkoog-Steertloch ……………………………………………. 6.2. Hydrodynamics of the Piep channel system .…………………….....………………... 6.2.1. General aspects of water levels and tidal currents …………………………………. 6.2.2. Lateral tidal asymmetry ………....….………………………………..…………….. 6.2.3. General aspects of the wave action ……………………………………………….... 6.3. Seasonality of the morphological changes …………………………………………...
70 72 75 75 87 95 97
7. Reconstruction of paleo-morphologies and tidal conditions during Holocene: An innovative approach …………………………………………………………………….. 103 7.1. Reconstruction of paleo-morphologies ………………………………………………. 103 7.2. Reconstruction of early tidal conditions ……………………………………………... 106 8. Summary and Conclusions …………………………………………………………... 8.1. Summary and correlation of the morphological processes in the different evaluated temporal-spatial scales ……………………………………………………………………. 8.2. Conclusions …………………………………………………………………………... 8.3. Some recommendations for further work …………………………………………….
113 113 117 121
9. References …………………………………………………………………………….. 123 10. Appendices …………………………………………………………………………... 131
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List of Figures: Figure 2.1: Location of the study area. ……………………………………………...…...…….… Figure 2.2: The sea-level curve for the region in the last 9,000 years (after MENKE, 1976 and STREIF & KÖSTER, 1978). …………………………..…………………..…………...……….….. Figure 2.3: The schematic representation of the Holocene sediment package (after DITTMER, 1938). …………………………………………………………………………………..…….…... Figure 2.4: Annual air and water temperature (monthly averages) variation on List, from measurements from 1969 to 1991 (after RIECKE, 1998 & BECKER, 1998). ………………….….. Figure 2.5: Monthly average velocities (A) and prevailing wind directions (B), measured at the “Hallig Hooge”, between 1975 and 1982 (after MENGELKAMP ET AL., 1998). ……………..…..... Figure 2.6: Frequency of storm surges (3m above NN) from 1868 to 1991 at the gauge station Büsum (after KESPER, 1992). ………………………………………..……………..………….… Figure 2.7: Morphology of the study area. ……………………………………………..…….…..
5 8 9 11 11 13 14
Figure 2.8: Schematic representation of the shoals/flats height control by waves and currents (after GÖHREN, 1968). ………………………………………………..………………………...… 16 Figure 3.1: Location of the PROMORPH hydrodynamic data (after MAYERLE ET AL., 2003). … 17 Figure 3.2: Location of field measurements in the scope of geology/morphology analysis. …….
20
Figure 3.3: Schematic representation of the bathymetric measurements and data processing. ….
21
Figure 3.4: Principle of operation of towed dual-channel side-scan sonars (modified from D’OLIER, 1979 (a) and BLONDEL & MURTON, 1997 (b)). ………………………………………. 22 Figure 3.5: Coverage of side-scan sonar and reflection seismic profiling. ………………………. 23 Figure 3.6: Principle of operation of continuous sub-bottom seismic profiling system (after FIGGE, 1980). …………………………………………………………………………………….. 23 Figure 3.7: Vibracore operation using A – “Bobo”, for coring in the channels; and B – using a tripod for drilling in the tidal flats during low water. ……………………………………………. 24 Figure 3.8: Northwest European Continental Shelf Model Nesting. ……………………………. 27 Figure 3.9: A reconstructed paleo-bathymetry and adapted GBM. ………………………………
28
Figure 4.1: Sea-level rise (a) and Holocene sediment deposits (b). ………………………………
31
Figure 4.2: General aspect of the area in several periods (WIELAND, 1990). …………………….
35
Figure 4.3: Schematic reconstruction of the Holocene evolution of the area, compiled from different authors, as referred above, and complemented with results from this study. .................. 36 Figure 4.4: Settlement situation in the Dithmarschen area in the 15th century (after MEIER, 2001). 37 Figure 4.5: Dike construction in the Dithmarschen area (after PRANGE, 1986). ............................
38
Figure 4.6: Part of the chart from C.C. ZAHRTMANN (1846) extracted from LANG (1975). ……..
39
Figure 4.7: General aspect of the area today (bathymetry from 1996). …………………………..
39
Figure 5.1: Schematic representation of estuarine meanders and their development (after AHNERT, 1960). ............................................................................................................................... 42 Figure 5.2: Overview of the middle-term morphological changes on the study area. .................... 43 Figure 5.3: Location of the mobile outer sandbanks. …………………………………………….. Figure 5.4: The morphological development of the northern and central part of Tertius sandbank between 1977 and 1999. .................................................................................................................. Figure 5.5: The morphological development of the southern part of Tertius sandbank between 1977 and 1999. ................................................................................................................................ Figure 5.6: The morphological development of the sandbank D-Steert between 1977 and 1999 (the plotted line corresponds to the mean low water - 1.6 m). ........................................................ Figure 5.7: The Dithmarscher Klei. ................................................................................................
45 46 46 47 49
Figure 5.8: Medium-term development at several channel cross-sections with respect to the depth of the Dithmarscher Klei. ...................................................................................................... 50 iii
Figure 5.9: Formation of steps and slump balls during the erosional process in the northern part of the cross-section A – Norderpiep. ............................................................................................... 51 Figure 5.10: Interpretation of the side-scan sonar measurements carried out during July 2000. .... 52 Figure 5.11: Interpretation of the side-scan sonar measurements carried out during July 2001. .... Figure 5.12: Interpretation of the side-scan sonar measurements carried out during March 2002 (vicinity of cross-section B – Süderpiep). ....................................................................................... Figure 5.13: Sediments distribution in the cross-section B – Süderpiep based in side-scan sonar measurements in comparison with a bathymetric measurement and the depth of the consolidated silt-clay layers. ................................................................................................................................. Figure 5.14: A section of a seismic profile over a structure interpreted as the scour referred by LÜNEBURG (1969). .......................................................................................................................... Figure 5.15: A - General distribution of salt domes in the German Bight (after WALTER, 1992) and B - in detail at the study area with the position of the seismic profiles. The number 1 indicates the localization of the section of seismic profile shown in the figure 5.14. ..................... Figure 5.16: Aerial photographs of the area in 1966 (A - from KÖNIG, 1972) and in 1994 (B Gewässerkunde Büsum /ALR – Husum). ....................................................................................... Figure 6.1: Local morphology and position of the cross-section A – Norderpiep. .........................
53 54 54 57 58 60 61
Figure 6.2: Bathymetric changes in the cross-section A – Norderpiep. ..........................................
62
Figure 6.3: Comparison of the bathymetric measurements at cross-section A – Norderpiep. ........
63
Figure 6.4: Local morphology and position of the cross-section B – Süderpiep. ...........................
64
Figure 6.5: Modifications of the profile in the cross-section B – Süderpiep. .................................
65
Figure 6.6: Comparison of the different measurements realized in the cross-section B – Süderpiep. ........................................................................................................................................ 65 Figure 6.7: Side-scan sonar imageries from the measurements during July 2000. ......................... 66 Figure 6.8: Local morphology and position of the cross-section C – Piep. ....................................
68
Figure 6.9: Modifications of the profile in the cross-section C - Piep. ...........................................
68
Figure 6.10: Comparison of the bathymetric measurements at cross-section C – Piep. .................
69
Figure 6.11: Seasonal and annual variations at cross-section C – Piep. .........................................
69
Figure 6.12: Local morphology and position of the field area across from Büsum. .......................
70
Figure 6.13: Comparison of the bathymetric measurements at the corner across from Büsum. .....
71
Figure 6.14: Local morphology and position of the cross-section Sommerkoog-Steertloch. .........
72
Figure 6.15: Morphological changes of the evaluated cross-section in the SommerkoogSteertloch. ........................................................................................................................................ Figure 6.16: Isobaths of –2 m (NN) showing the supposed direction of bedload sediment transport (bypassing = arrows). ....................................................................................................... Figure 6.17: Definition of terms (German Industry Norm) involving tidal course and water levels (PN = Gauge Zero level, 5 m below NN). ....................................................................................... Figure 6.18: Comparison of ebb and flood phase’s duration during June, September and December 2000, for the cross-sections Norderpiep, Süderpiep and Piep. ...................................... Figure 6.19: Comparison of maximum ebb and flood cross-sectional averaged current velocities. Figure 6.20: Correlation of the difference between the maximum flood and ebb currents with the difference in the corresponding tidal range (a) and with the difference in the duration of the flood and ebb phase (b), separate by neap-spring cycle. .......................................................................... Figure 6.21: Correlation of the difference between the maximum flood and ebb currents with the difference in the corresponding tidal range (a) and with the difference in the duration of the flood and ebb phase (b), separate by cross-section. .................................................................................. Figure 6.22: Correlation of the difference between the flood and ebb tidal range and the difference in the duration of the flood and ebb phase, separate by cross-section. .......................... Figure 6.23: Ebb and flood phase duration, compared with mean water level at gauge Büsum and wind measurements (30-days averaged) at station Büsum. ............................................................. iv
73 74 75 76 78
80 81 82 85
Figure 6.24: Schematic representation of the location of cells and ensembles of cells used in the evaluation of lateral asymmetry for each cross-section. .................................................................. Figure 6.25: Ebb and flood current pattern in the cross-section A – Norderpiep (spring cycle, 22nd March 2000). ............................................................................................................................ Figure 6.26: Ebb and flood current pattern in the cross-section B – Süderpiep (spring cycle, 21st March 2000). ................................................................................................................................... Figure 6.27: Ebb and flood current pattern in the middle ensemble of the cross-section B – Süderpiep (spring cycle, 21st March 2000). ..................................................................................... Figure 6.28: Ebb and flood current pattern in the cross-section C – Piep (spring cycle, 23rd March 2000). .................................................................................................................... .......................... Figure 6.29: Ebb and flood current pattern in the cross-section A – Norderpiep (neap cycle, 5th December 2000). ............................................................................................................................. Figure 6.30: Ebb and flood current pattern in the cross-section B – Süderpiep (neap cycle, 5th December 2000). ............................................................................................................................. Figure 6.31: Ebb and flood current pattern in the middle ensemble of the cross-section B – Süderpiep (neap cycle, 5th December 2000). ................................................................................... Figure 6.32: Ebb and flood current pattern in the cross-section C – Piep (neap cycle, 6th December 2000). ............................................................................................................................. Figure 6.33: Simulated wave heights based on an input of waves from west, with 2 m height and 6s period (after WILKENS, 2003). .................................................................................................... Figure 6.34: Description of cores taken in the study area (after RAMLI, 2002). .............................
96 100
Figure 7.1: Location of considered previous studies (a), cores and measurements (b).
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Figure 7.2: Reconstructed morphologies corresponding to the periods around 7,000 y. BP (A), 5,000 y. BP (B) and 4,000 y. BP (C) and sea-levels around – 10 m (A), –5 m (B) and –3 m (C), compared to the present morphology (D). ……………………………………………………….. Figure 7.3: Calculated tidal ranges at selected points using the CSM over the current morphology with respect to the sea-level and the corresponding age. ………………………………………… Figure 7.4: The tidal ranges using the modified GBM (nested in the CSM) with the reconstructed morphologies. …………………………………………………………………………………….. Figure 7.5: Tidal range and maximum tidal currents for the area of Spiekeroog using the same morphology with different sea-levels (a) and using the developed ancient morphologies (b). …... Figure 7.6: Tidal ranges and corresponding maximum current velocities for the location Büsum using the developed ancient morphologies. ………………………………………………………. Figure 7.7: Tidal course for the location Büsum using the reconstructed ancient morphologies. ..
87 89 90 90 91 92 93 93 94
105 107 108 109 109 110
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List of Tables: Table 3.1: Detailed periodic measurements in the different areas of interest. ...…………………
20
Table 3.2: Location, length and altitude of the collected cores. …………..……………………..
25
Table 6.1: Mean maximum ebb - flood cross-sectional averaged current velocities ……………..
78
List of Formulas: Formula 3.1: Hypsometry calculation ………………………………………...…………………
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Preface The study presented here was submitted to the Mathematical and Natural Sciences Faculty of the University of Kiel, in Germany, attaining the Ph.D. degree. This study was carried out in the framework of the research project PROMORPH, funded by the German Ministry of Research and Education. The PROMORPH project, with the aim to predict medium-scale morphodynamic changes, combines a variety of field measurements with numerical model simulations. This study was also counting on the financial and technical support from the Research and Technology Center Westcoast in Büsum of the Kiel University, where this study was carried out, and the German Academic Exchanges Service (DAAD), due to the scholarship to support my Ph.D. studies. I would like to sincerely thank the support from DAAD in cooperation with the CAPES (Council for Research Support of Brazil) that propitiated me the possibility to study and live in Germany in the last 4 years, representing a very important and interesting experience for my personal and professional life. I would like to thank also Ms. Helga Wahre and Ms. Maria Salgado Martinez from DAAD for their very competent support and kind attention to the scholarship holders from Brazil. This study was focusing the morphodynamics of a tidal flat area in the west coast of Germany and one of the main tasks was correlate morphodynamic processes in different time scales. This was really a challenge, but I hope at the end I could contribute a little bit to the comprehension of the morphodynamics from tide-dominated coasts by studying the tidal flats of Dithmarschen. The cooperation of several colleagues from FTZ and from CORELAB in the project PROMORPH generated a lot of data and knowledge that improve substantially this study. Without this cooperation it would have been impossible to carry out almost the totality of measurements and acquire the data used here. In this context I would like to thank sincerely my supervisors Prof. Dr. Roberto Mayerle and Dr. Klaus Ricklefs for introduce me to the “world” of tide-dominated coasts and for contribute so much for my understanding of geology, morphodynamics and modelling. I would like to thank sincerely also the colleagues MSc. Jort Wilkens, Dr. Poerbandono, Dr. Kalle Runte, Dr. Christian Winter, Dr. Fernando Toro, Dr. Christian Reimers, Dr. Friedrich Abegg and Mr. Burkhard Meier for the priceless contribution and support in the last 4 years. I also would like to thank Dr. Klaus Schwarzer (Institute for Geosciences of Kiel University) for his friendly co-operation in performing seismic and sonar measurements in the vii
area, as well as for the support in the interpretation of the results. I thank also Prof. Dr. Rolf Köster (Institute for Geosciences of Kiel University) for his precious suggestions and contribution as co-referent of my thesis. Thanks to Prof. Dr. Wolfgang Rabbel, Prof. Dr. Peter Janle, Prof. Dr. Ulrich Sommer and Prof. Dr. Helmut Hillebrand for the support and carry out the exams. Thanks also to Dr. Ingrid Austen for her very helpful corrections and kind and suggestions for my thesis. I really appreciate the collaboration of the German authorities for dispose several data sets for the development of this work, especially bathymetric data (Federal Maritime and Hydrographic Agency - Hamburg, Germany), water level measurements (Susanne Timm and Kai Saßmannshausen: Gewässerkunde Büsum /ALR – Husum) and cores (Dr. Reiner Schmidt – LANU/Schleswig-Holstein). Thanks also to Dr. Guilherme Lessa (Geosciences Institute – Federal University of Bahia - Brazil) for his very helpful suggestions about the study of morphodynamic evolution of tide-dominated coastal environments. Thanks a lot to my colleague and friend Dr. Eduardo Siegle for his precious support in all fields involved in our research and in the quotidian life in the last 12 years. Thanks Eduardo! I am also very grateful to Germany and to the Germans that welcomed me with patience and interest. During the last years I met a lot of very nice people and I would like to thank everyone. Thanks also to all my friends and colleagues from FTZ for being so nice and helpful to me and making possible the survival of a shy Brazilian guy in the very small and quite isolated Büsum. Special thanks for Wolfgang, Burkhard, Heiner, Gero, Klaus Ricklefs and Klaus Vanselow, Britta, Martina, Fiete, Kerstin, Jens, Sabine, Kai, Ilka, Dirk Meier, Karl, Thomas, Sebastian, Ralf, Dani und Daniela, Franciscus, Marion, Margrit, Jürgen, Erika, Susanne, Burger, Uwe Becker and Uwe Hansen, Ario, Thomas Größe… I thank all people from the Institute! Thanks also for Octopus and its dream team Peti and Heike for very good drinks and meals, and especially for very nice company and atmosphere in the long winter nights! Thank you my friends from Brazil, that even 12,000 km faraway, were always standing by me. Thanks Rafael Laranja, Daniel Dias, and all colleagues from Rio Grande (FURG) and Porto Alegre (UFRGS). Furthermore, I would like to thank my parents and all my relatives, for believe on me and for invest so much time and energy in me: thank you very much mom, dad, Dália, Chico viii
& Family, Gustavo & Family, André, Kika, Isabella, Adri & Family, Luciano & Family, Duca & Family, Délio & Family, Dario, Ribarcki … Thanks a lot my daughter Lulu, for being so nice. Simone my girlfriend, thanks a lot for your love, support and patience, and thank God that I got all of you! My friends, especially Denilson, Fernando and Maurício & Family, thanks a lot for your support and happiness, for the excellent weekends and for have been made Germany much more cozy for me, as well as for the attention and comprehension in the difficult moments. Without the support and attention of all of you, I had never gone far, I had never achieved something.
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Abstract The present study describes the results of an investigation aimed at the long- to shortterm morphodynamics of a tidal flat area. The investigation area is the coastal region of the Dithmarschen District on the German North Sea coast, characterized by a mean tidal range of 3.2 m, being widely tide-dominated. In the scope of short- to middle-term time-scales (weeks to months / years to decades) morphological changes were evaluated by periodic bathymetric measurements. Furthermore, the hydrodynamics was evaluated by current velocity and water level measurements, including also information about the wave climate in the region, to explain the observed morphological changes and characterize the hydrodynamic regime of the study area. The periodic bathymetric measurements carried out during 2000 to 2003 at preselected channel cross-sections reveal important morphological changes, which can be summarized in two main trends, including a gradual lateral migration of the channels and a seasonal cycle of erosion (autumn/winter) - deposition (spring/summer). Current measurements over entire tidal cycles show important lateral asymmetric pattern for ebb and flood currents in the main channels that result in the so-called estuarine meandering and, combined with seasonal variations in the hydrodynamic regime, explains partially the observed morphological changes. Besides, the hydrodynamic analyzes have showed that the channels in the outer parts are weak flood-dominated (shorter flood phases with higher current velocities), while the channels in the inner parts are weak ebb-dominated. Particularities of the morphological
changes
at
each
evaluated
cross-section
are
related
to
especial
geomorphological characteristics of each area, which were also evaluated by cores, shallow reflection seismic and side-scan sonar measurements. These particularities include mainly landward migration of outer sandbanks and channel incision hindrance by consolidated clayey layers. The results show that some outer sandbanks migrate landwards in rates of up to 130 m/year and this results in channel constriction and incision, which is hindered in several places by the so-called Dithmarscher Klei, a consolidated clayey layer with top found usually between 15 to 20 m depth. The superficial mapping of this layer was also developed in the scope of the present work. Furthermore, regarding long-term time scales (for instance centuries to millennia) the morphodynamic Holocene evolution of the study area was evaluated. On the basis of the mentioned geological surveys and previous studies, several geomorphological evolutionary stages of the inner German Bight during the Holocene (7,000, 5,000 and 4,000 y. BP) were reconstructed and by using numerical modelling on morphodynamics, the corresponding paleo tidal conditions were simulated. It was found out x
that in the early Holocene, with a lower sea-level, tidal range and tidal currents were substantially slighter than today resulting in a relative increased wave energy significance. The results indicate that this relative increase of significance of wave energy resulted in the formation of a beach ridge system around 4,000 y. BP, which is well-known from geological findings at the study area, contrasting with the wide tidal flats found today. Besides, the results show an overall flood-dominance in the hydrodynamic regime for the early stage (7,000 y. BP). With the further increase of sea-level and gradual sediment fill of the basin, tidal range and tidal currents also have increased and ebb-currents became gradually more significant, while the area became tidal-dominated. With archeological historical and geological information from previous and from the present study, the Holocene evolution and the current morphodynamics of the area were connected, including the effects of land reclamation in the area, which is ongoing in the area since the Middle Ages and is influencing the natural morphodynamic evolution of the area substantially.
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Zusammenfassung Diese Arbeit beschreibt die Ergebnisse von Untersuchungen der lang- und kurzfristigen morphodynamischen Evolution eines Wattgebietes. Das Untersuchungsgebiet ist die Küstenregion von Dithmarschen an der deutschen Nordsee Küste, die mit einem mittleren Tidenhub von 3.2 m charakterisiert und weitgehend tide-dominiert ist. Die morphologischen Änderungen in kurz- bis mittelfristigen Zeitskalen (Wochen bis Monaten / Jahre bis Jahrzehnten) wurden durch wiederholten bathymetrischen Messungen untersucht. Weiterhin wurde
die
Hydrodynamik
des
Gebietes
mittels
Strömungsgeschwindigkeits-
und
Wasserstandmessungen evaluiert. Informationen über das Wellenklima wurden auch berücksichtigt. So wurden die Hauptursachen von den beobachteten morphologischen Änderungen auch identifiziert und die Hydrologie des Gebietes charakterisiert. Die periodischen bathymetrischen Messungen, die während 2000 bis 2003 in vorausgewählten Rinnenquerschnitten ausgeführt wurden, zeigen wichtige morphologische Änderungen. Dies können unter zwei Haupttrends zusammengefasst werden, einschließlich der seitlichen Verschiebung von Kanäle und einem saisonbedingten Zyklus von Erosion (Herbst/Winter)Ablagerung (Frühling/Sommer). Die durchgeführten Strömungsgeschwindigkeitsmessungen zeigen wichtiges seitliches asymmetrisches Muster für Ebb- und Flutströmungen in den Haupt Kanälen, die im sogenannten Ästuarinmäandrierung resultieren, und, kombiniert mit saisonbedingten Variationen in der Hydrologie, die beobachteten morphologischen Änderungen zum großen Teil erklärt. Die Messungen im Rahmen der Hydrodynamik zeigen noch, dass die Außenteile der Hauptkanäle Flut-dominiert sind (kürzere Flutphase mit höher Strömungsgeschwindigkeiten), während der Innenbereich der Kanäle Ebb-dominiert ist. Besondere Aspekte der morphologischen Änderungen für jeden Untersuchungsfeld werden von eigen geomorphologischen Charakteristiken erklärt, die auch durch Sedimentkerne, reflexionsseismischen
und
Seitensicht-Sonar
Messungen
untersucht
wurden.
Diese
Besonderheiten der geomorphologischen Entwicklung des Gebietes schließen landwärts Wanderung von außen Sandbänken und Erosionslimitierung in verschiedenen Stellen durch kohäsiv-konsolidierten Tonschichten (z.B. Dithmarscher Klei) ein. Die Ergebnisse zeigen, dass einige außen Sandbänke in Raten von bis zu 130 Metern/Jahr landwärts wandern, und dies in Verengerung und Vertiefung von Rinnenquerschnitt resultiert. Anderseits wird dies in mehreren Stellen vom sogenannten Dithmarscher Klei gehindert. Die Lage dieser Sedimentschicht ist maist zwischen 15 und 20 Metern festgestellt. Die Kartierung dieser Schicht wurde auch im Rahmen dieser Studie durchgeführt. Weiterhin, die langfristige xii
Morphodynamik (Jahrhunderten bis Jahrtausend) wurde im Rahmen der holozän Evolution des Gebietes untersucht. Auf Basis der erwähnten geologischen Messungen und vorherigen Studien wurden drei verschiedenen geomorphologischen Zwischenstufen der inneren Deutschen Bucht während des Holozäns (7000, 5000 und 4000 J. BP) rekonstruiert. Mit numerischen Modelle für Morphodynamik wurden denn die entsprechenden PaläoTideverhältnisse simuliert. Es wurde herausgefunden, dass mit einem niedrigeren Meerspiegel im frühen Holozän der Tidenhub und die Tideströmungen wesentlich geringer als im Gegenwart waren, was wiederum in relativ wichtigeren Welleneinfluss resultiert hat. In der Tat, vorherigen Untersuchungen und Ergebnisse dieser Studie zeigen, dass vor 4000 Jahre die Küsteregion von Dithmarschen durch Strandwälle, Barriere und Nehrungen charakterisiert war. Dies wurde auf eine Wellen-dominierte Küste hindeuten, im Kontrast mit den breiten Wattenfläche (tide-dominiert), die das Gebiet heute ausprägen. Die Ergebnisse zeigen auch das für die Stufe von 7000 J. BP die Tideprozesse deutlich Flut-dominiert waren. Mit der weiteren Zunahme des Meerspiegels und die allmähliche Ausfüllung der Tidebecken mit Sedimente, hat den Tidenhub und die Tideströmungen allgemein deutlich zugenommen, aber vor allem haben die Ebbströmungen zugenommen. So das um 4000 J. BP Flut- und Ebbströmungen nahezu gleich waren. Die gesamte Region wurde allmählich zu Tidedominiert und Watten haben sich gebildet. Mit der Hilfe von archäologischen, historischen und geologischen Informationen von vorläufigen und die gegenwärtige Studie, wurde die langfristige Evolution des Gebietes mit der gegenwärtigen Morphodynamik (kurzfristige Entwicklung) eingegliedert. Dies fasst die Geschichte der Landgewinnung im Dithmarschen um, die seit Mittelalter die natürliche morphologische Entwicklung des Gebietes wesentlich beeinflusst hat.
xiii
Resumo Este trabalho apresenta os resultados de uma investigação da evolução morfodinâmica a curto, médio e longo prazo de uma região de planícies de maré. A área de investigação é a região litoral do Distrito de Dithmarschen na costa leste alemã (Mar do Norte), caracterizada por uma amplitude de maré de 3,2 m, resultando no domínio da maré no regime hidrodinâmico da área. Em escalas temporais de curto e medio prazo (semanas a meses / anos a décadas), mudanças morfológicas foram avaliadas através de medições batimétricas periódicas. Além disso, o regime hidrodinâmico foi avaliado em termos de variações do nível d’água e velocidades de corrente, também incluindo informação do clima de ondas da área, com os objetivos de explicar as mudanças morfológicas observadas e caracterizar a morfodinâmica da área de estudo. As medições batimétricas efetuadas durante 2000 a 2003 em seções transversais pre-selecionadas dos principais canais de maré da área revelam mudanças morfológicas importantes que podem ser resumidas em duas tendências principais, incluindo a gradual migração lateral dos canais e um ciclo sazonal de erosão (outono/inverno) – sedimentação (primavera/verão). Medições de velocidades de corrente e níveis d’água durante ciclos de maré completos revelam uma importante assimetria lateral entre correntes de vazante e enchente, que resulta no processo de formação de meandros estuarinos. Isto, em associação com variações sazonais no regime hidrodinâmico, explica grande parte das mudanças morfológicas observadas. Resultados também demonstram que a fase de enchente nas partes externas dos canais de maré principais é usualmente mais curta do que a fase de vazante, apresentando também velocidades de corrente mais intensas. Por outro lado, as partes mais internas dos canais de maré apresentam fases de vazante usualmente mais curtas, com velocidades correspondentemente mais elevadas do que durante as fases de enchente. Aspectos geomorfológicos especiais de cada área avaliada foram também estudados por meio de testemunhos, sísmica rasa de reflexção e sonar de varredura lateral. Estas particularidades incluem migração em direção a costa de bancos arenosos na parte externa das planícies de maré, e também a redução e obstrução de erosão nos canais através de camadas consolidadas de sedimentos finos. Os bancos arenosos das partes externas apresentam taxas de migração de até 130 m/ano em direção a costa, o que resulta em alguns casos no estreitamento de canais, levando a uma tendencia de erosão (aprofundamento), o que por sua vez é, em alguns casos, impedido pela presença da denominada Dithmarscher Klei, uma camada lamosa consolidada, com seu topo geralmente encontrado em profundidades de 15 a 20 m. O mapeamento desta camada também foi efetuado durante este estudo. Com relação a escala de longo prazo, a xiv
evolução holocênica da área de estudo foi investigada. Com base nas pesquisas geológicas mencionadas anteriormente e estudos prévios, várias estágios evolutivos da geomorfologia da área foram reconstruídos (7000, 5000 e 4000 a. BP) e através de modelos numéricos em morfodinâmica, as condições pretéritas de maré foram calculadas. Os resultados revelam que para os três estágios simulados a amplitude e as correntes de maré eram substancialmente reduzidas, em comparação com as condições observadas na atualidade. Os resultados demostram que com a redução em importância das correntes de maré, a energia de ondas era relativamente mais importante. De fato, trabalhos anteriores demonstram que por volta de 4000 a. BP a regia avaliada era caracterizada por cordões litorâneos e barreiras, em contraste com as extensas planícies de maré que ocupam a área no presente. Com elevação do nível do mar e o preenchimento da bacia com sedimentos, a amplitude de maré também sofreu um aumento gradual e as correntes de maré vazante sofreram um aumento relativo, tornando-se aproximadamente equivalente as correntes de enchente por volta de 4000 a. BP, enquanto que para o estágio por volta de 7000 a. BP o regime hidrodinâmico era amplamente dominado por correntes de enchente. Com informações arqueológicas, históricas e geológicas de estudos anteriores e do presente estudo, a evolução holocênica e a morfodinâmica atual da área foram integradas, incluindo a gradativa incorporação de áreas de planície de marés ao continente pela construção de diques, que vem sendo efetuada na região desde a Idade Média, influenciando substancialmente a evolução morfodinâmica natural da área de estudo.
xv
xvi
Introduction
1. Introduction This study was carried out in the tidal flats of the Dithmarschen Wadden Sea area, at the German North Sea coast, between the river Elbe and the river Eider, comprising the socalled Dithmarschen Bight. The hydrodynamic of the region is characterized by a mesotidal regime, according to the classification of HAYES (1975), with a mean tidal range of 3.2 m, which leads to open and wide tidal flats. EHLERS (1988) stressed that according to its characteristics and landforms the area could be classified as a low macrotidal environment. Shallow sandbanks in the outer part prevent the inner tidal flats from wave energy of the open sea, acting similarly to barrier islands. This investigation aims the study of the morphodynamics (morphological changes and associated hydrodynamic) in the Dithmarschen Bight, in short to long-term time scales ensuing previous works and trying to enhance the knowledge of understanding the morphological evolution of this area. The considered time-scales vary from weeks to months (short-term), years to centuries (medium-term) and centuries to millennia (long-term – Holocene). With regard to the Holocene morphodynamic evolution of the area, the most relevant factor is the post-glacial sea-level rise. Besides, depending on the rates of sea-level rise or the time scale considered, several other factors are relevant. In the early Holocene the sea-level rise, with rates around 2 m/100 years, resulted in the rapid accommodation space generation, which controlled the evolution of the coastal areas in the German Bight (BEETS & VAN DER SPECK, 2000). In the inner German Bight the sealevel rise decelerated to rates around 0.2 m/100 years between 7,000 and 6,000 y. BP (STREIF, 1986). After this period the sedimentation rates surpass the creation of accommodation space and several other factors became substantially important for the evolution of the inner German Bight (BEETS & VAN DER SPECK, 2000). In an overview the factors controlling the evolution of any coastal depositional sedimentary environment (basin) can be referred as follows: The age of the basin, availability of sediments, its geological characteristics, especially the sea level history and the antecedent topography, dynamic conditions (also climate) and the often underestimate morphodynamic. According to the first three factors (age, sediment avail. and geological charact.), barrier coasts like found in the German Bight may range from openwater lagoons to sediment filled salt-marshes or tidal flats (LESSA & MASSELINK, 1995). Two hydrodynamic factors, i.e. the wave height and the tidal range, with their interactions are rather important in determining the geomorphological processes in coastal 1
Introduction
areas in medium-term scales. In this context, coasts may vary in a wide range of configurations between tidal and wave domination (HAYES, 1975, 1979). Despite of the wave conditions, the tidal range varies clearly in the North Sea coast. It increases from west to east and from north to south in the inner German Bight according to the position of amphidromic points, obviously influencing the landforms along the coast in the German Bight (STREIF, 1986). HAYES (1979) developed a model to classify the coastal morphological variations according to the tidal regime. The German Bight (Wadden Sea) corresponds quite well with the HAYES’ scheme (DIJKEMA, 1980; STREIF, 1986), but EHLERS (1988) pointed out that the boundaries between micro, meso and macrotidal environments proposed by DAVIES (1964) and used by HAYES, seem to be drawn rather arbitrarily. According to the geological features found there, EHLERS (1988) proposed a modification of the HAYES’ scheme for the German Bight, which classifies the study area (Dithmarschen) as low macrotidal. There, the tidal flow is unrestricted and the fresh-water inflow is insignificant. So the area is clearly tidedominated. The tidal basin of Dithmarschen can be referred as a mature or sediment filled basin. The huge amount of sediments deposited there during the Holocene can be only explained by a residual inward sediment transport acting in long-term scales, in contrast with a dynamic equilibrium situation found today in the area (ASP
ET AL.,
2003). This residual inward
sediment transport is mainly attributed to the tidal currents (POSTMA, 1962). VAN STRAATEN & KUENEN (1957) elaborated a clear theory to explain the residual inward transport in tidal inlets based on the simplifying assumptions that: A) Current velocities at each separate point vary with time as a sinusfunction; and B) That the current velocities at each stage of the tide decrease from point to point in direct and linear proportion from the distance of the inlet to the coast; As pointed out by VAN STRAATEN & KUENEN, the velocities of tidal currents during the propagation of the tidal wave are depth-dependent. The decrease of average depth from the outer tidal inlets to the coast explains the decrease of average velocities. Besides this, two facts corroborate to a residual transport inwards: the velocity required to bring a particle from the bottom into suspension is higher than the velocity to keep it in suspension resulting in a scour lag, and it also still taking some time before the particle reaches the bottom, after the velocity dropped below the critical velocity, resulting in a so-called settling lag. Parallel to the scour and settling lag effects, a tidal asymmetry, i.e. stronger maximum tidal currents during flood than during ebb, is very important for the net sand import into a 2
Introduction
tidal basin, since the sand transport is proportional to the third (or higher) power of the effective flow velocity (DRONKERS, 1986; VOS & VAN KESTEREN, 2000). According to BOON & BYRNE (1981) a flood-dominant sediment transport can be a priori expected for every tidal basin at the early stages (not filled) of its formation and evolution. With the gradual infilling of a tidal basin, the relative intertidal storage also increases gradually. According to AUBREY & SPEER (1985) a relative large intertidal storage favours ebb-dominance. In this context, the evolution of the tidal basins of the Dithmarschen area would be characterised by a clear flood-dominant sediment transport pattern in an early phase of the Holocene evolution, that with the gradual infilling of the basin, especially after the deceleration of the sea-level rise, was gradually reduced. In the last 2,000 years the sea-level changes were characterised only by small variations (STREIF & KÖSTER, 1978). With a relatively stable sea-level and an advanced infilling state of the basin, the further evolution of the area was mainly controlled by the local dynamic. The interaction of strong tidal currents in the channels and wave action at the outer sandbanks, awards the area a changing morphology. These morphological changes are welldocumented in the historic and scientific literature, that date back to the 16th century (LANG, 1975). These changes have been also distinctly influenced by human activities, especially through land reclamation, i.e. dike construction, that began in this region in the early 11th century (PRANGE, 1986). Regarding to the morphological evolution, more detailed investigations have been carried out since the beginning of the 20th century. Most of those studies have a descriptive character and might not have considered the effects of dike construction properly, especially concerning long-term changes. Even when the infilling of the tidal basins of Dithmarschen is mainly related to tides, the wave action certainly has corroborated for the infilling of the basin. The tidal range in the North Sea is supposed to had increased during the Holocene in association with the sea-level rise, especially in an initial transgressional phase, with rapid sea-level rise (POST, 1976; FLEMMING & DAVIS, 1994). It leads to the conclusion that during the early Holocene the tidal range was much more reduced than today. Consequently, the wave-dominated sediment transport was also much more important than today, as showed in the stratigraphic register of the area (HUMMEL & CORDES, 1969; FLEMMING & DAVIS, 1994; VAN
DER
MOLEN & VAN
DIJCK, 2000 and ASP ET AL., 2003).
3
Introduction
1.1. Objectives In this context, the following general objective was formulated for the present work:
It was aimed to analyze the morphodynamic evolution of the Dithmarschen tidal flats (German North Sea Coast), especially in medium to short-term scales using several bathymetric, hydrodynamic and stratigraphic (shallow seismic, cores and side-scan sonar) measurements. These were combined with numerical model simulations, to extend the findings to the study of evolution and long-term changes of the Dithmarschen tidal flats. To reach this general objective the following specific aims were pursued:
1) To identify and explain the main morphological changes in the area, especially in pre-selected channel cross-sections, in short to mediumterm time scales based on geological, morphological and hydrodynamic measurements. 2) To characterize the domain according to its hydrodynamic conditions, by measuring currents and water levels, with especial interest in tidalasymmetries in the pre-selected cross-sections and along the main channel system. 3) To use the findings of the morphodynamic processes at the present to recognize and understand the morphological changes in medium-term time scale, mainly focused on the effects of land reclamation in the seventies in the study area, also supported by bathymetric measurements of the last 30 years. 4) To find links between medium and long-term evolution of the study area using geological and historical data. 5) To
use
the
acquired
knowledge
of
short
to
medium-term
morphodynamics to improve a conceptual evolutionary model of the long-term evolution of the study area, mainly based on geological findings
and
supported
by
numerical
model
simulations
on
hydrodynamics. To achieve these goals several measurements on geology, morphology and hydrodynamics were carried out. Furthermore, extensive data-banks and numerical modelling in morphodynamics were used. 4
The Study Area
2. The study area 2.1. Geographic location and general aspects The study area comprises the tidal flats between the rivers Eider and Elbe, the socalled Dithmarschen Bight in the Wadden Sea at the German North Sea coast (fig. 2.1). The core study area comprises the Piep channel system and its associated tidal flats (about 600 km2), with the south and north limits at the latitudes 54°01’N and 54°12’N, and the west and east boundaries respectively at about the 08°30’ and 09°00’ longitudes (see polygon in the fig. 2.1). So, the east and west limits are represented respectively by the coastal or dike line and by the intertidal outer sand shoals (D-Steert and Tertius).
Figure 2.1: Location of the study area.
5
The Study Area
According to the different time scales and approaches used in the present work, the study area was extended or reduced. For analyses on the long-term evolution of the area, regional aspects are important and consequently the entire Dithmarschen Bight, or even the whole German Bight was considered. For detailed and frequent measurements aiming the analyses of short-term morphological changes and hydrodynamics, small areas were selected (see figure 3.2 for location). Another important aspect is the redefinition of the east boundary of the study area according to the post-transgression history of the area, which is represented by the progradation of the coastline, also sensibly incremented from the Middle Ages up to now, due to land reclamation. So the east boundary of the study area was redefined according to the approximate coastal form and position of each period evaluated. 2.2. Geological characteristics, configuration and age of the tidal basin The North Sea area is a part of the North Germany depression, an old sedimentary basin. It has a Pre-Cambrian basis covered by a sediment package usually as thick as 4 km. Sediments with an age of 415 x 106 years, were reached in a depth of about 3,800 m, according to a bore in the Southeast North Sea. Another bore near the island Sylt shows the age of 419 x 106 years for the oldest deposits (WALTER, 1992). The North Sea is almost totally covered by soft sediments. The bottom morphology was formed essentially during the Quaternary, especially in the Pleistocene. The climatic variations between glacial and interglacial Ages during that period generated important sealevel changes. The glacial periods and the subsequent sea-level rises were the most important factors that molded the morphology of the North Sea and its coast. The most relevant cycles in this context are the Elsterian glacial and the Saalian glacial, with the corresponding subsequent marine transgressions of the Holstein and the Eem interglacial periods, with ages of about 195,000 to 230,000 and 115,000 to 125,000 y. BP, respectively (STREIF, 1990). During the last glacial age (Weichselian) the whole North Sea was dry. The formation of the Wadden Sea started during the Flandrian Transgression (from 10,000 y. BP), with the rise of the sea level and intrusion of brackish-marine water in the North Sea. The region of Dithmarschen started to be influenced by the sea-level about 8,000 y. BP. Around 7,000 y. BP the rise of the sea-level decreased and the tidal flat sedimentation became more widespread. In the areas of West and North Frisian Islands this deceleration in the sea-level rise favored the formation of barrier islands by increasing the transport of sandy sediments towards the coast. 6
The Study Area
Differences of the subsurface, sediment disposability and supply, tidal range and wave regime led to the formation of different kinds of coastal environments, ranging from wave dominated coasts (or beaches) to tidal flats. The totality of these different coastal environments, including the coasts of the Netherlands, Germany and Denmark, compose the so-called Wadden Sea. There barrier islands and their shallow tidal basins occupy most parts of the coast. In the innermost part of the German Bight barrier islands are absent and open tidal flats, like the Dithmarschen tidal flats, can be found. This absence of genuine barrier islands in the inner German Bight is mainly attributed to the tidal range. According to DIJKEMA (1980), the size of barrier islands decreases towards the inner parts of the German Bight in relation to the increase in tidal range. In places with mean tidal range exceeding 2.9 m, barrier islands are usually absent. Several other geological factors are supposed to have contributed to the absence of barrier islands. Due to the presence of a depression in the Pleistocene surface, corresponding to the so-called “Elbe-Urstromtal”, an ancient ice-marginal valley, the Pleistocene surface is quite deep near the coast in the inner German Bight, compared to the adjacent areas, where barrier islands occur. Besides, the depth of the Pleistocene underground, the course of the Elbe-Urstromtal, the ancient Weser and Eider rivers result in the deposition of an extensive mud deposit in the inner shelf of the inner German Bight (FIGGE, 1980). This might have represent a hindrance in the eventual sediment supply, hence the North Sea sands and the abrasion of the Pleistocene underground would be an important source for barrier islands formation or maintenance in the area (EHLERS, 1988; ZEILER ET AL., 2000). Barrier islands, and their corresponding shallow tidal basins and inlets, are quite vulnerable systems with a very sensitive morphology to changes in the physical system (DE VRIEND, 1996). Open tidal basins, like the tidal flat of Dithmarschen, would be even more sensitive. They would respond quite quickly and distinctively to changes in the tidal regime (amplitude, asymmetry), wave climate (wave height and direction, storms, chronology of events) or an accelerated sea-level rise. 2.2.1. Holocenic sea-level rise, sediment supply and distribution In a general approach, the history of the holocenic sea-level rise in the Dithmarschen area can be divided in two phases. The first phase began at the end of the last glaciation and is marked by a rapid sea-level rise, in the order of 2 m/100 years (STREIF, 1986). This lasted till approximately 7,000 to 6,000 y. BP, when the sea-level rise decelerated substantially. The second phase began in that period and extended until today and is characterized by a slow sea7
The Study Area
level rise with rates around 0.2 m/100 years (fig. 2.2), superimposed by secondary variations comprising stabilization or slight drop phases (MENKE, 1976; STREIF & KÖSTER, 1978). BEETS & VAN DER SPEK (2000), studying the Holocene evolution of the coastal zone of Belgium and the Netherlands, could also distinguish these 2 phases. According to them, during the first phase of the Holocene sea-level rise the rates of accommodation space generation surpass the sediment supply and in a general aspect it was characterized by the intrusion of the rising sea-level in the lower reaches of river valleys of the more elevated Pleistocene sand body (FLEMMING & DAVIS, 1994). During the second phase, the sediment supply would have surpassed the accommodation space generation and the coasts stabilized or progradated (BEETS & VAN DER SPEK, 2000). According to HUMMEL & CORDES (1969) the main sedimentation phase in the northern Dithmarschen started around 5,000 y. BP, in agreement with the other studies.
Figure 2.2: Sea-level curve for the region in the last 9,000 years (after MENKE, 1976 and STREIF & KÖSTER, 1978).
In Dithmarschen the general history of this post-Pleistocene deposition starts with a peat layer above Pleistocene glacio-fluvial sands. Due to a rapid sea-level rise at the beginning of the post-Pleistocene transgression, the peat was partly eroded and subsequently silty clay was deposited. The in-fauna of this widely spread clayey deposit indicates that the sedimentation happened under permanent submarine conditions. This layer, the so-called Dithmarscher Klei, reaches a thickness of up to 10 m. The Dithmarscher Klei corresponds today to a kind of natural basement that hinders erosional processes to scour the streambed in the tidal channels, due to its cohesive and consolidated character. However, the younger sandy sediments are much more mobile and their redeposition leads to the pronounced morphological changes in the area. 8
The Study Area
In the following period the early Holocene silty clay was partly discordant, partly concordant overlaid by a sequence of sandy sediments with some interlayered cohesive, muddy deposits. The facies-change from clay sediments to increasingly sandy deposits, indicates a change in the environment from deeper water to intertidal shallow water conditions. The composition of the deposits corresponds strongly to the recent tidal flat sediments. The thickness of these recent intertidal deposits is about 20 m. Figure 2.3 shows a representation of the different sediment layers deposited during the Holocene in the study area. "GEEST"
20 m 10 m 0m - 10 m - 20 m - 30 m
High to the Mean Sea-level (NN)
OUTER SAND BANKS
DYKES (COASTAL LINE)
- 40 m FINE SAND
CONSOLIDATED MUD (KLEI)
PEAT
MUDDY SAND
COARSE SAND AND GRAVEL
PLEISTOCENE
Figure 2.3: Schematic representation of the Holocene sediment package (after DITTMER, 1938).
As explained above, the amount of Holocene sediments deposited in the study area is also associated with the Holocene sea-level rise. In the last 2,000 years the sea-level has experienced only weak oscillations (STREIF, 1986). A general trend of rising sea-level can be recognized for this period, but this rise would be as big as 0.1 m/100 years. Besides, several authors refer to indications of a rising sea-level in the area in the last 2 centuries, as well as in almost all coastal areas around the world. According to FÜHRBÖTER & JENSEN (1985), from analysis of several German tidal gauges, the average rate of sea-level rise in the region was 0.325 m/100 years between 1934 and 1983. For the period between 1959 and 1983 it was 0.637 m/100 years. For the next decades sea-level rise of similar rates are expected for the region (SPIEGEL, 1997). As a result, the embankments (dikes) will hinder the normal evolution of the tidal flats, reclaiming new intertidal areas and thus generating a high sediment deficit. Therefore it is expected that the area will experience important morphological and hydrodynamic changes also as a response to the sea-level rise. 9
The Study Area
2.2.1.1. Superficial sediments The composition of the surface sediments is mainly fine to medium sands with varying proportions of silt and clay. On the tidal flats there is a general tendency of a reduction of the grain size from outer areas towards the coast, what reflects the already mentioned decrease in the hydrodynamic energy towards the shore. Tidal flats on the Wadden Sea are very often classified according to the percentage of mud (silt and clay), like explained by FIGGE,
ET AL.
(1980): Sandy tidal flats (Sandwatt):
mainly composed of fine sand, with less than 5% mud, and usually 0 to 10% middle sand (1); Mixed tidal flats (Mischwatt): Mud content can vary between 5 and 50% (2); and Muddy tidal flats (Schlickwatt): Comprise the tidal flats, where the mud content more than 50% is (3). In the Dithmarschen tidal flats these three types can be found. According to measurements carried out by REIMERS (2000), the superficial grain size distribution in the area is generally characterized by sandy sediments in the central and outer parts, with usually less than 5% mud. Locally, sand banks provide protected areas, where the mud content reaches also values of about 20%. Towards the coast the mud content increases rapidly, reaching easily more than 50%. The mean grain size on the tidal flats is usually fine sand, but very fine sand fraction uses to reach values of about 40 to 60%. However, in the supratidal sandbanks medium sand is also abundant. RAMLI (2002) collected several sediment cores in the shallow parts of the study area in collaboration with the present study. The results of analyses of the cores support the conclusion about the tendency of increasing mud content towards the coast, not only for the surface, but also for the first 3-4 meters of the sediment package. However, it was showed that local factors like channel migration are often much more important than general tendencies. Combined side-scan sonar measurements and bottom sampling carried out by VELADIEZ (2001) indicate that the bed sediments of the channels in the area are mainly composed of fine to medium sand, with local areas of consolidated mud and shell banks. 2.3. Climate and hydrodynamic conditions The climate in the area is of a sub-oceanic type of the tempered latitudes. The mean max. air temperature in summer is around 16 °C and in winter around 1 °C. This oscillation in the air temperature is followed by the variation in the water temperature. Figure 2.4 shows the monthly averages of water temperature at the north part of the island Sylt, located about 80 km north of the Dithmarschen tidal flats. 10
The Study Area
20
Air Water
18 16 Temperature °C
14 12 10 8 6 4 2 0 -2 Jan.
Feb.
March
April
May
June
July
Aug.
Sept.
Oct.
Nov.
Dec.
Figure 2.4: Annual air and water temperature (monthly averages) variation at List, from measurements between 1969 and 1991 (after RIECKE, 1998 and BECKER, 1998).
The precipitation is in the order of 800 mm/y and the main rain period is between August and December. The salinity oscillates normally between 20‰ in the winter and 28‰ during the summer, but an eventual strong evaporation in the tidal flat areas during the summer or eventual strong rainfalls during the winter may cause salinity values up to 33‰ and 15‰, respectively (BECKER, 1998). Westerly winds (SW-W) are dominating in frequency and magnitude in the investigation area. Figures 2.5a and 2.5b show the monthly average velocities and prevailing wind directions, measured at the “Hallig Hooge”, an island in the Wadden Sea just 40 km north of the study area, for a period between 1975 and 1982. Comparisons between data collected in the study area in the last years and the Hallig Hooge data show no important differences, making it possible to use those data for a general description of the climate of the area. 9
Wind Velocity in m/s
Calm: 0.1%
Annual Average
N 12
%
10 8
8
6 4 2
W
7
O
0
6
A
SW Jan.
SW Feb.
SW E March April
NW May
NW June
W July
W Aug.
SW Sept.
SW Oct.
Month / Predominant Direction
SW Nov.
SW Dec.
SW Jan.
B
S
Figure 2.5: Monthly average velocities (A) and prevailing wind directions (B), measured at the “Hallig Hooge”, between 1975 and 1982 (after MENGELKAMP ET AL., 1998).
11
The Study Area
2.3.1. Hydrology Tides The hydrology of the area is dominated by a semidiurnal tidal regime. From the mouth of the Elbe Estuary in the south to the Eiderstedt peninsula in the north the mean tidal range is about 3.2 m varying locally between 3.1 to 3.4 m. Referred to the German water level datum "NN", the mean high water at Büsum is about +1.6 m NN and the mean low water is about – 1.6 m NN. The difference between neap and spring tidal range is approximately 0.9 m. According to SPIEGEL (1997) east of a line connecting the supra tidal sands of "Blauortsand" and "Trischen" 73% of the tidal basin is covered by intertidal areas. There, the tidal prism is in the order of 414 x 106 m3. The daily motion of this water mass generates high current velocities in the channels. The magnitude of these alternating tidal flow in the main tidal channels can reach maximum values of about 2 m/s with mean maximum velocities of about 1.2 to 1.5 m/s. On the intertidal flats the maximum tidal induced currents are mainly in the range of 0.3 m/s and seldom exceed 0.5 m/s. However here the superposition of tidal and wind driven currents can lead to magnitudes of more than 1 m/s under storm conditions. More information about tides and currents is given in SIEFERT ET AL. (1980, 1983) and WIELAND ET AL. (1984). Wave climate The wave climate in the investigation area differs very much depending on the exposition and the water depth. In the inlets close to the open sea, significant wave heights of up to 4 m can occur. Measurements near Sylt carried out between 1986 and 1993 show that the characteristic open sea waves come from the directions northwest, west and southwest, with a significant wave height of 2.5m and a period around 7.3 seconds (OPTIMIERUNG DES KÜSTENSCHUTZES AUF SYLT - Phase 2, 1994). Due to the bathymetric conditions these waves do not penetrate very far into the Wadden Sea area. In general the wave height decreases towards the shore or shallower areas. Measurements performed on open tidal flats seldom show values of more than 0.5 m signif. wave height. In the tidal flats that are located behind the breaker zone of the seaward boundary (outer sandbanks), the waves are usually more associated with local wind fields. Storms Associated with the normal wave climate and tides, frequent storms influence the coastal zone on the German Bight. Their action can be separated in the storm surge effect and the storm waves. Storm surges in the area can result in an upset of the water level of more than +5 m NN (EHLERS, 1988). 12
The Study Area
During the Middle Ages, the North Sea experienced periods of high frequency of storms, that caused important morphological changes and motivated dike construction in the area (PETERSEN & ROHDE, 1991). Figure 2.6 shows that in the last years the region has experienced a new increment in the frequency of storms (storm surge 3 m above NN). It is expected that their effect in the morphological changes in the area has also been incremented. An increased water level due to storm surges can enable higher waves to penetrate in the inner parts of the tidal flats. It is expected that under this conditions morphological changes take place. These changes would be more intense in the tidal flats, since the main channels are usually deeper than the depths reached by wave base under stormy conditions. Especially after a storm, the water volume that flows during a tidal cycle can be much amplified. The additional water volume accumulated in the tidal basin will flow out when the wind weakens. This might result in increases in the current velocities and also changes in the current patterns, especially because of the increased water level that allows the water to flow more intense over the tidal flats than in a normal situation, when the flow is only really intense in the channels. Further information about the effect of storms on the morphology of the Wadden Sea is still missing, especially because of their unpredictability, resulting in a lack of information and/or measurements at the time before a storm and the limitations to collect data during or after a storm. 16 14
Number of Storms
12 10 8 6 4 2
1988
1984
1980
1976
1972
1968
1964
1960
1956
1952
1948
1944
1940
1936
1932
1928
1924
1920
1916
1912
1908
1904
1900
1896
1892
1888
1884
1880
1876
1872
1868
0
Years
Figure 2.6: Frequency of storm surges from 1868 to 1991 at the gauge station Büsum (after KESPER, 1992).
13
The Study Area
2.4. Morphology and morphodynamic Regarding to the morphology, the Dithmarschen tidal flats are especially characterized as wide-open sandy tidal flats. The system is mainly composed of the Piep tidal channel and its adjacent tidal flats, dominating the morphology of the inner or central investigation area. The channel has the shape of a lying “Y”, in which the northern and southern inlets ("Norderpiep" and "Süderpiep", respectively) form the connection to the open North Sea. From the point of intersection of the two sub channels, the actual "Piep" stretches in a more or less straight W-E line towards the city of Büsum. Here and in the two inlets the mean water depth in the central part is about 10m with maximum values of 25 m. East of Büsum, the "Piep" splits up into three second order channels (so-called “Wöhrdener Loch”, “Kronenloch” and “Sommerkoog-Steertloch”) and finally into several tidal creeks which scatter in the tidal flat area of the Meldorf Bight (fig. 2.7).
Figure 2.7: Morphology of the central study area.
The intertidal areas form a belt of sandbanks and shoals of roughly 15 km width along the coast. The seaward banks exhibited relatively complex outlines with finger-shaped interand supratidal extensions stretching some kilometers westwards. At the transition zone from these sands to more sheltered areas, some lune-shaped supratidal banks like "Blauort" or the almost fully evolved barrier island of "Trischen" can be found. Compared to the seaward banks the tidal flats in the inner parts have a more compact shape, but they are much more internally structured by gullies and tidal creeks. With the exception of the areas that were strongly affected by land reclamation during the last decades (e.g. inner part of the Meldorf Bight), the transition between the mainland and the tidal flats is formed by a belt of salt marshes with a typical width of about some ten to hundred meters. The combination of strong tidal currents in the channels and wave action at the outer sandbanks and shallow parts awards the area a changing morphology. These morphological 14
The Study Area
changes are documented in the historic and scientific literature, that date back to the 16th century (LANG, 1975). These changes have been also influenced by the human activity, especially through land reclamation, i.e. dike construction, that began in the region early in the 11th century (PRANGE, 1986). The most dynamic morphological units are found at the western boundary of the tidal flats, where wave action interferes with strong tidal and wind driven currents, within waves. The banks and shoals in this region exhibit the strongest migration rates. A good example is the D-Steert bank in the outermost west of the investigation area. This sandbank shifted its position during the period from 1973 to 1997 some hundred meters in southeasterly direction and changed its outline dramatically (RAUSCH, 2000). Although the inner parts seem to be more stable on the first view, intensive morphodynamic transformations also take place there. These are mainly caused by channel migration processes. Since they are the main subject of the present study, more information about the morphology and morphological changes of the area is given in the chapters about results and discussion. Another marked characteristic of the study area is that the mean height of the flats is distinctly below the high-water level and supratidal areas are relative sparse. This would be related to the high currents and wave energy in the area, especially during storms, when wave attack is supposed to “cut” the top of sandbanks (fig. 2.8). It led to the weak coverage of vegetation, specially the expected salt marsh vegetation. It can be found only direct in front of the dikes and in protected areas behind of the supratidal sandbanks Blauort and Trischen (see also fig. 2.7). The weak siliciclastic sediment supply does not enable the further growth of supratidal banks and the subsequent dune formation, that would propitiate protection during storms, favoring vegetation colonization and having also a synergetic effect in the sedimentation. These characteristics as a whole determine that the area is represented by open tidal flats, instead barrier islands and or salt marshes. Parallel to the description of the morphology of the area and its changes, it makes sense to discuss the hydrodynamic and its interactions with the morphology, because of their strong and direct correlation. In a simplified point of view, the “normal” (expected) morphological evolution of a tidal flat is determined by the direction and magnitude of the sediment transport in an area. Modifications in the morphology or in its evolutionary trends are usually caused by asymmetries in the flux (for instance tidal currents), also resulting in asymmetries in the sediment transport.
15
The Study Area
Figure 2.8: Schematic representation of the shoals/flats height controlled by waves and currents (after GÖHREN, 1968).
As sediment transport responds nonlinearly to the current velocity, the net transport through a channel is quite sensitive to asymmetries in the tidal current velocities and stage curves (DE VRIEND & RIBBERINK, 1996). Important types of asymmetry are the hypsometry effect, the coriolis effect and the topography-induced rectification, that are also occurring in the Dithmarschen tidal flats. A general explanation about the hydrology of the area is given in the topic 2.3.1, and more detailed explanations about these asymmetries (hydrodynamic) and its interactions with the morphology are presented in the results and discussion.
16
Materials and Methods
3. Materials and Methods 3.1. Data basis As pointed out above, the present study was developed in collaboration with the PROMORPH project, counting on its extensive data basis, as well as data from several different German institutions. The measurements and data are more detailed described in the annual internal reports of the PROMORPH project from 2000 and 2001. The data basis comprises mainly tidal current and water level records, but includes also wave, wind, cores descriptions, sediment transport data and sediment characteristics. The figure 3.1 shows an overview of the available data. Each component of the data basis used is described below.
Figure 3.1: Location of the PROMORPH data (after MAYERLE ET AL., 2003).
3.1.1. Bathymetric data of the Piep channel system (BSH) Bathymetric data covering the whole Piep channel system and associated tidal flats (about 600 km2) from the BSH (German Federal Agency for Navigation and Hydrography), realized over more than 27 years (1974 to 2001), were used. These data sets are separated according to the year or month of measurement, composing annual or monthly data series. The data is available in digital format including position (in the German Gauß-Krüger coordinate system) and depth. These data sets cover especially the main channels. Besides, several annual measuring campaigns, like the data sets of 1977, 1979, 1983, 1987, 1990, 1993 and 1996, present also a good coverage of the tidal flats and second order channels. Analysis was mainly focused in those data sets. Each annual data set comprises usually a density information of one value every 100 m at the main channels and 200 m at the tidal flats. 17
Materials and Methods
3.1.2. Water levels (ALR – Husum/Gewässerkunde Büsum) Concerning water level records, the data basis used in the present work stem of 9 different water level gauges installed in the Dithmarschen tidal flat area (fig. 3.1). These comprise eight so-called summer gauges usually in operation between May and October, and one permanent gauge (at Büsum). However, the main analysis is based on the permanent gauge in the main channel at Büsum, because it enables analysis over several years including seasonal variations. According to their locations, the gauges can be divided in two groups covering the inner and outer parts of the Piep channel system. The permanent gauge, at Büsum, can be included in the group of gauges of the inner part, but its location in the main channel avoids strong distortions in the tidal flow, that can be found in small tidal creeks. The location of the tidal gauges can be seen in the figure 3.1. Water levels were mainly used for calculations of tidal ranges, effects of winds (storms), duration of tidal phases, as well as for the correction of the bathymetric measurements. Further, the analysis of water levels was used in the interpretation of the identified morphological changes. 3.1.3. Sediment cores For a reconstruction of the Pleistocene surface all available data from previous studies were used. Due to the kind cooperation of Dr. Reiner Schmidt (LANU – Schleswig-Holstein), the descriptions of a set of more than 30 cores collected in the area between 1936 and 1987 reaching the Pleistocene were made available for this study. The information from the cores was essential for the validation and complementation of the seismic data. The localization of the cores is showed in the figure 3.2, in association with the field measurements in the scope of geology/morphological analysis.
3.1.4. Current measurements at the Piep channel system (Promorph data bank) Numerous current measurements were carried out using ADCPs (Acoustic Doppler Current Profilers) between 2000 and 2002 in the Piep channel system, in the scope of the PROMORPH project. The data bank includes results of stationary ADCPs and vesselmounted ADCPs for profiling. The data from the stationary ADCPs represent continuous detailed current measurements covering periods of about 2 months for a point of the channel. Profiling however, enables to cover entire cross-sections and, due to the sequential acquire of transects, entire tidal cycles (12.5 hrs.). The interesting data for the present work corresponds to the 18
Materials and Methods
vessel-mounted profiling. Each data set covers a period of up to 14 hours, comprising 20 to 75 profiles, according to the length of each cross-section (MAYERLE
ET AL.,
2003). It was
obtained using a 1,200 kHz vessel-mounted ADCP configured with 12 seconds averaging ensemble and 0.5 m depth cell length, mainly at three cross-sections: A – Norderpiep, B – Süderpiep and C – Piep, which were also evaluated in terms of morphological changes (fig. 3.2). Although, data from several other profiles are also disposable (fig. 3.1). 3.1.5. Wind data (FTZ data bank) The used wind data was obtained at the meteorological station of the Research and Technology Center Westcoast (FTZ), at Büsum (fig. 3.1). There, the city and the presence of the dikes impose reductions of the wind velocity in the order of 10 to 20%, but comparisons with other station at the harbor entrance (almost without influence of the dikes and the mainland) indicated no important alterations in the wind direction at the used station. These factors enabled the use of this data in the present study. The wind measurements were used only indirectly as an indicator of stormy periods, regarding to the velocities and also considering the wind directions. High velocities combined with west or east directions can result in extreme high or low water levels, respectively. 3.2. Field measurements To evaluate the medium- to short-term morphological changes in the Piep channel system, several measurements were also carried out in the scope of the present study. These comprise mainly bathymetric (morphological) and geological measurements. The last one includes side-scan sonar and reflection seismic sub-bottom profiling, as well as sediment cores. Figure 3.2 shows the location of the measurements. 3.2.1. Periodic bathymetric measurements For a detailed investigation of the morphological changes on the Piep system several channel cross-sections were chosen (fig. 3.2). Measurements were carried out periodically with a vessel-mounted echo-sounder and a D-GPS positioning system. The used echo-sounder has a frequency of 200 kHz and an accuracy of 0.1 m.
19
Materials and Methods
Figure 3.2: Location of field measurements in the scope of geology/morphology analysis.
As the sound speed in the water varies according to its density, the obtained data was corrected according to the calculated density (i.e. sound speed) of the water during each measurement, influenced by local water temperature and salinity. These were measured simultaneously with the bathymetric measurements due to a CTD (ConductivityTemperature-Depth). Furthermore, the effect of tidal variation in the measured depths was corrected due to water levels, obtained continuously at the tidal gauge of Büsum. The bathymetric measurements covered usually a 500 m wide section of the channels, comprising several transversal and longitudinal profiles, with regular intervals of about 100 m. The evaluated cross-sections present a length between 800 m (Kronenloch) and 3,500 m (Süderpiep). Each data set was further interpolated in a digital elevation model (DEM). Figure 3.3 summarizes the procedure involving the bathymetric measurements and data processing. Detailed information about the generation of DEMs is summarized in the section 3.3.2. Although the mean interval between two consecutive measurements for each cross-section is about three to four months, it can vary between one month to one year (table 3.1). Table 3.1: Detailed periodic measurements in the different areas of interest (fig. 3.2).
2000 2001 2002 2003 06 09 12 05 07 12 03 06 08 09 10 11 12 03 04 05 08 Norderpiep (A) X X X X X X X Fieldwork areas
Süderpiep (B) Piep (C) Büsum SommerkoogSteertloch 20
X
X X
X X
X X
X X
X
X X
X X X
X
X X
X X
X X
X X
X
X
X
Materials and Methods
Figure 3.3: Schematic representation of the bathymetric measurements and data processing.
Assessing the quality of the echo-sounder measurements and data processing in the scope of the present study, four main error sources were identified: The precision of the used equipments (echo-sounder and positioning system); the water level correction (tides); the variation of sound speed in the water, due to variations of salinity and temperature; and the data interpolation during elaboration of DEM’s. The maximum combined error of each measurement was estimated in the order of 0.3 m. In this context, variations smaller than 0.5 m were not considered in the analysis of morphological changes. 3.2.2. Side-scan sonar and reflection seismic profiling As the stratigraphy of the Holocene sediments influence substantially the morphological evolution of the study area, and the history of the morphological evolution is, in turn, written in the stratigraphic register, this was detailed studied in the scope of the present research due to side-scan sonar and reflection seismic profiling, as well as cores. The principle of operation of a side-scan sonar is essentially similar to an echosounder. A pulse of electrical power is applied via a switching unit to the transducer causing it to vibrate, thereby creating pressure waves, which are projected into the water. On reflection from the seabed, the pressure waves are picked up by the receiving transducer or hydrophone, converted into electrical energy and usually recorded on a continuously running paper recorder or as digital images (D’OLIER, 1979). 21
Materials and Methods
Periodic side-scan sonar measurements were carried out using the side-scan sonar KLEIN digital sonar model 595, 100/500 kHz (simultaneous dual frequency). A schematic representation of the operation principle of a side-scan sonar can be seen in the figure 3.4. Campaigns in the scope of side-scan sonar measurements were carried out during July 2000, September 2000, July 2001 and March 2002. The coverage of each measuring campaign can be seen in figure 3.5.
a)
b)
Figure 3.4: Principle of operation of towed dual-channel side-scan sonars (modified from D’OLIER, 1979 (a) and BLONDEL & MURTON, 1997 (b)).
The basis of operation of a sub-bottom profiler is, like the side-scan sonar and the echo-sounder, the emission of a continuous series of sound pulses from a towed source and, after reflection from the seabed, the receiving and recording of the returning signals (fig. 3.6). Additionally in this case, the reflection from discontinuities within the underlying sediments and rocks are also received and recorded, being of main interest (D’OLIER, 1979). The continuous seismic reflection sub-bottom profiling was carried out mainly with the so-called “Boomer” system, using an energy range of signal from 100 to 300 joules and a frequency range of signal from 0.5 to 15 kHz during the measuring campaign of September 2000 (see fig. 3.5). Sub-bottom profiling was also obtained with a 3.5 kHz Sub-bottom Profiler during the measuring campaign of July 2000 (see also fig. 3.5).
22
Materials and Methods
Figure 3.5: Coverage of side-scan sonar and reflection seismic profiling.
Figure 3.6: Principle of operation of continuous sub-bottom seismic profiling system (after FIGGE, 1980).
3.2.3. Sediment cores To study short- to medium-term morphological changes, several cores were taken in the Dithmarschen tidal flat between September and November 2001. They reached lengths of 1.8 to 5.5 m and were taken usually in intertidal areas. The technique used to take those cores 23
Materials and Methods
was the so-called Vibracoring. The used system is mainly based on a portable version, especially developed for unconsolidated sediments on land and underwater environments. More detailed descriptions are given by LANESKY ET AL. (1979). Cores in several channels were also obtained using a special boat developed at the Research and Technology Center Westcoast (FTZ-Westküste). The so-called Bobo (Bohrboot – coring boat) is a small but robust and stable aluminum boat, with an opening in the underside and a kind of tripod, constituting a “drilling platform”. Figure 3.7 shows both kinds of vibracoring methods used.
A Figure 3.7: Vibracore operation using A – “Bobo”, for coring in the channels; and B - a tripod for drilling in the tidal flats during low water.
B
The choice of the sampling points was mainly based on the preliminary results of morphological changes in the study area due to comparison of bathymetric measurements of the last decades. Locations with significant sedimentation in the last years were selected, trying to keep a homogeneous coverage. 20 cores in 10 locations were taken (1 retort for each station) covering the inner, central and outer part of the area. The coordinates of the cores are given in table 3.2. Figure 3.2 shows the location of the cores. Positioning of each station was done using a portable GPS, enabling positioning directly at the stations during the coring. As in the bathymetric measurements the positions were later converted in Gauß-Krüger coordinates using the software N1Koor to facilitate the graphic representation.
24
Materials and Methods
Table 3.2: Location, length and altitude of the collected cores.
Core 1 2 3 4A 5B 6A 7B 8 9 10
Geographic Coordinates Latitude Longitude 54°09.838' 54°09.837' 54°05.448' 54°03.324' 54°03.881' 54°06.264' 54°07.634' 54°06.510' 54°06.954' 54°06.519'
8°39.313' 8°39.314' 8°37.372' 8°57.458' 8°54.791' 8°51.683' 8°54.841' 8°44.377' 8°49.974' 8°47.251'
Gauß-Krüger Coordinates East North 3477549 3476300 3475393 3497296 3494386 3491003 3494449 3483040 3486173 3489143
Altitude Core Length (m to Mean low water) (m)
6003780 5998500 5995647 5991642 5992678 5997104 5999640 5997583 5997589 5998388
3.6 1.8 3.5 3.7 4.6 3.1 4. 6 5.5 3.5 4.8
0.5 0.5 -0.1 -0.2 -0.5 -0.3 -0.5 1 -0.1 0.6
3.3. Modelling 3.3.1. Digital elevation models (DEM) The bathymetric data obtained in the present study and the data from the BSH were used to generate digital elevation models (DEM), enabling mathematical comparisons between the different bathymetries. For the data sets from the BSH, grid spacings between 50 and 200 m were used, according to the data density and coverage area. For the carried out bathymetric measurements, grid spacings between 10 and 15 m were used. According to the information density, such grid spacings seem to be most appropriated. The mathematical grids corresponding to each data series were compared and the differences between two different grids were calculated. These differences are the digital representation of the morphological changes during the period that separate the two grids. This procedure was done using a software for contouring and 3D surface mapping: Surfer 7.02, from the Golden Software, Inc. The gridding method “Triangulation with Linear Interpolation” was used. 3.3.2. Hypsometric analysis Hypsometric analysis is the study of the distribution of ground surface area, of a landmass with respect to elevation (STRAHLER, 1952). It can be referred as an empirical model to reproduce numerically a specific morphology. This method was firstly used to study terrestrial erosional basins. Subsequently its application was extended to coastal (depositionary) basins, especially tidal basins including inlets, tidal flats and lagoons (BOON, 1975). Hypsometric analysis was used in the present study to assist the reconstruction of paleo-morphologies and the analysis of morphological changes in medium-term time scales, 25
Materials and Methods
especially to quantify the effects of land reclamation. The term Basin Hypsometry was first suggested by BOON & BYRNE (1981). It comprises the study of distribution of basin surface area with height. In the abscissa the area (dry area) is represented and in the ordinate the water level. The minimum (dry) area (AMIN) is reached during mean high water (maximum flooded area), which also corresponds to the maximum water level. The maximum (dry) area (AMAX) corresponds to the total area of the basin. This situation is the minimum water level, called here water level zero. In the graphic representation the water level (or depth) is called “d”, and the dry area “a”. The water levels and dry areas can be also converted in relative values, varying from 0 to 1, by dividing each value by the maximum ones (d/DMAX and a/AMAX). In the present study the calculated hypsometric curves were converted into relative hypsometric curves, which enable more easily comparisons of curves, even if they correspond to quite different basin areas. The mean high water (MHW) corresponds to d = 1 (d = DMAX). According to the presence of supratidal sandbanks the dry area “a” would never be zero (AMIN), because those areas are not flooded during MHW (DMAX). But for a better graphic representation the dry area during MHW is considered zero. At the hypothetical dry situation (d = 0) the dry area corresponds to a = 1 (a = AMAX). STRAHLER (1952) developed an equation to model Hypsometric function that was further improved by BOON (1975). The modified formula gives the dimensionless area as:
where A = AMAX , r = AMIN / AMAX. The parameter r controls the slope of the curve at the inflection point, representing the amount of basin curvature. The parameters r and γ are determined empirically. The morphologies (DEM’s) were further used to calculate volumes and surface areas. Using any reference level (d) varying from the maximum depth (d = 0) to the MHW (d = 1) the hypsometric calculations were carried out. For each reference level the corresponding area (a) was calculated and the hypsometric curves for the different morphologies generated. 3.3.3. Numerical modelling of tidal flow The data basis and measurements referred above have been used to calibrate and validate several morphodynamic models that have been set-up in the scope of the 26
Materials and Methods
PROMORPH project. A process-based model comprising modules for simulation of flow, waves, sediment transport and bottom evolution was set-up, calibrated and validated. Comparisons of simulations and measurements demonstrate the reliability of the developed modules (WILKENS ET AL., 2001; WILKENS & MAYERLE, 2002; MAYERLE & PALACIO, 2002; WINTER & MAYERLE, 2003; MAYERLE
ET AL.,
2003). The Delft3D modelling system,
developed by Delft Hydraulics in the Netherlands, has been employed (ROELVINK & VAN BANNING, 1994). The general set-ups of the existing models were used in this study. In this study the model system was adapted to investigate the tidal flow conditions of several reconstructed paleo-morphologies. Because of the relevancy of the tidal flow in the morphodynamics of the area and the supposed variations in the tidal regime during the Holocene evolution of the area, the carried out simulations were based on the tidal flow module. To simulate the hydrodynamic conditions during early periods of the Holocene sealevel rise and for the present situation, a large-scale system of nested models covering the entire North Sea was used. The model nesting has been used successfully in the prediction of water levels and to provide boundary conditions for coastal area model (MAYERLE & PALACIO, 2002). Figure 3.8 shows the grids of the model nesting used in this study. It comprises two models. The larger one covers the north-west European Continental Shelf area (CSM) with grid spacing of about 9 km (VERBOOM
ET AL.,
1992). A more refined model
covering the German Bight area (GBM) with grid spacing ranging from 0.5 km to 1.9 km, is nested with the CSM (MAYERLE & PALACIO, 2002). DK
N S
North Sea
IRL
DK
UK
D NL B
F
Continental Shelf Model (CSM)
D
100 km
NL
100 km
German Bight Model (GBM)
Figure 3.8: Northwest European Continental Shelf Model Nesting.
For handling the morphologies of the early stages of the Holocene evolution the GBM model was modified, especially near the coastline and river courses (fig. 3.9). Along the open boundaries of the CSM with the ocean, 10 harmonic constituents (M2, S2, N2, K2, O1, K1, 27
Materials and Methods
Q1, P1, NU2 and L2) are prescribed. It is assumed that these constituents can also be considered for early Holocene situations, since the tidal conditions in the North Atlantic are supposed to have been relative stable during the Holocene (POST, 1976).
Figure 3.9: A reconstructed paleo-bathymetry and adapted GBM.
The water levels and current velocities are obtained in successive steps. Simulations begin with the CSM and are carried out for the entire period. Water levels obtained along the open boundaries of the GBM are used to drive the model leading the flow conditions in the study area. Concerning the present situation, information about the tidal flow was obtained directly from the carried out measurements. Simulations were carried out considering the morphologies of different stages of the Holocene evolution as well as the present day one (for details about reconstructed morphologies see the chapter 7). The simulations of tidal conditions for the different reconstructed morphologies cover a month period and use the corresponding sea-level, according to the existing sea-level curves for the study area. The approx. age of each reconstructed morphology was estimated based on the stratigraphy and the developed conceptual evolutionary model. 3.4. Systematic, organization and presentation of results According to the different spatial and temporal scales considered, different trends and cycles of morphological changes can be identified in the area. They are related to different hydrodynamic and meteorological processes that are more or less important in one or the other scale. Although, there is no clear separation between different temporal or spatial scales, a gradual increase of importance of different processes in longer periods and larger areas is visible. Furthermore, all scales are correlated as far as the cumulative effects of short-term processes can often explain long-term trends. 28
Materials and Methods
According to the different processes involved in the different scales of morphological changes in the area and the different limitations associated with the study of each scale, in the present work these morphological changes were divided in short, medium- and long-term temporal scales and were dealt using different approaches. In the following chapters different scales of morphological changes are presented separately or combined according to their correlation, averting the artificial formation of blanks between different scales that in fact do not exist in nature. In chapter 4. the long-term morphodynamics (Millennia to Centuries) of the area is dealt with, where two main aspects are considered: The natural processes involved in the Holocene evolution of the inner German Bight and the anthropogenic influence in the Dithmarschen area by human settlements and land reclamation. In chapter 5. the medium-term morphodynamics (Decades) of the area is evaluated. In that chapter the natural and anthropogenic effects are discussed as well. Recent and very important land reclamation project in the area of Dithmarschen (Meldorf Bight) and the observed trends of morphological changes in the last 30 years are presented. In chapter 6. an extensive analysis of the short-term morphodynamics (few years to months) is presented, where results of investigations at several monitoring areas regarding to morpho- and hydrodynamic are discussed. Further, the observed trends and reasons for the morphological changes are also discussed. Chapter 7. represents an especial part of this study. It summarizes the results of an innovative approach to study long-term morphodynamics combining techniques usually applied in short- to medium-term scales. The main subject of this chapter is the reconstruction of paleo tidal conditions during the Holocene of the inner German Bight. Combining the findings about the Holocene evolution and morphological changes in different time-scales obtained in the scope of this study, several paleo-morphologies were reconstructed and using numerical modelling (successfully used in short- to medium-term morphodynamic) tidal flow was simulated over each paleo-morphology. Chapter 8 summarizes and integrates the results of the whole study and presents the conclusions, as well as suggestions for further studies.
29
Materials and Methods
30
Results and Discussion
4. Long-term morphodynamics (Millennia to Centuries): Holocene evolution of the inner German Bight (natural and anthropogenic processes) 4.1. End of the last Ice Age and sea-level rise The settlement history of the area, based on archeological studies, show that marshes and tidal flats in the study area exist at least since the last 2,000 years, although they have experienced substantial changes during this period. However, the formation of the tidal flats in Dithmarschen extends over almost the whole Holocene. The beginning of its formation is usually dated back to approx. 8,000 y. BP, when the rising sea-level reached the hinterland of Dithmarschen (STREIF & KÖSTER, 1978). The Holocene evolution of the soft sediment coasts in the South Eastern part of the North Sea has been controlled by the Post-Pleistocene sea-level rise (fig. 4.1). The corresponding history of the study area is characterized by a deposition of almost unexceptional silty and sandy silica-clastic sediments. Consolidated mud
Streif & Köster (1978)
Hight to NN (m)
10 m
Muddy sand
Pe at
Pleistocene
Dikes (coastal line)
Outer sandbanks
Menke (1976)
Coarse sand and gravel
20 m
Hinterland
10 m
0m
0m
- 10 m
- 10 m
- 20 m
- 20 m
- 30 m
- 30 m
- 40 m
9
8
7 6 5 4 3 2 1 Time (thousand years BP)
0
a) after Menke (1976) and Streif & Köster (1978)
25
20
15 10 Aprox. distance (km)
5
0
Hight to NN (m)
Fine sand
- 40 m
b) after Dittmer (1938)
Figure 4.1: Sea-level rise (a) and Holocene sediment deposits (b).
Although a number of previous investigations concerning the geological structure have been carried out in the area, the knowledge on the time dependent evolution of the area is less detailed, compared to adjacent areas in the West and in the North. The reasons for that are the lithological composition of almost only siliciclastic sediments, the intensive redeposition processes and the lack of datable organic rich deposits in the region of Dithmarschen. The earliest and most comprehensive investigations about the Holocene evolution of the Dithmarschen area stem from DITTMER (1938). In his studies, on the base of numerous cores, the different Holocene deposits were identified and the principal litho-stratigraphic structure was described. According to DITTMER (1938, 1952), RUCK (1969), LINKE (1979) and TIETZE (1983), the Pre-Holocene landscape of the investigation area is bordered by the meltwater valleys of the river Elbe in the South and river Eider in the North. Additionally, 31
Results and Discussion
DITTMER (1938, 1952) was able to show that the surface of the Pleistocene sediments dips down from East to West (more rapidly in the South) to a depth of approx. 30 m below present sea-level. The higher parts of the Pleistocene morphology are often build up of till and in the deeper parts it consists of glacio-fluvial sands. On these meltwater sands the history of the post-Pleistocene deposition starts with a thin peat layer. Due to a rapid sea-level rise at the beginning of the Holocene transgression the peat was partly eroded and it is almost completely missing on the higher till areas. In the deeper parts of the area the first brackish and marine sediments are composed of different clayey and sandy silt layers with an overall thickness of up to 10 m, forming the Dithmarscher Klei referred in the previous chapters. The in-fauna of this widely spread deposits indicates that the sedimentation of the Dithmarscher Klei happened under permanent submarine conditions. In the following period this early Holocene clayey silt was partly discordant and partly concordant overlaid by a sequence of sandy sediments with some interlayered cohesive, muddy deposits. The facies-change from clay sediments to increasingly sandy deposits indicates a change in the environment from deeper water to intertidal shallow water conditions (DITTMER, 1952; STREIF & KÖSTER, 1978). In the Holocene prograding shoreline itself, a change from beaches to tidal flats can be recognized in the stratigraphic sequence (HUMMEL & CORDES, 1969; ASP ET AL., 2003). The sea-level rise in the North Sea was followed by an increase in the tidal range in the coastal areas (FLEMMING & DAVIS, 1994). The stratigraphic sequence of sediments in the area, as well as numerical model simulations of paleo-tidal conditions support this hypothesis (FLEMMING & DAVIS, 1994). The reason for that, therefore, could be the constricted access of the ocean tides into the Southern Bight of the Norht Sea through the Strait of Dover and into the German Bight through a strait between the exposed Dogger Bank and a land bridge between Great Britain and Belgium during the early Holocene, especially around 8,000 to 6,000 y. BP (VAN DER MOLEN & VAN DIJCK, 2000). The hypothesis of a gradual increase of the tidal range following the sea-level rise is investigated and defended in the present study. In this context, numerical modelling and geological investigations were combined to reconstruct the tidal conditions during Holocene. 4.2. Succession of geomorphologic configurations during Holocene Based on the previous studies in the region, the available sea-level curves and the interpretation of several cores and seismic profiling in the scope of the present study, the long-term (Holocene) geomorphological evolution was evaluated. In a general overview, the morphological evolution of the area can be defined as a gradual infilling of the tidal basin. However, according to the intense migration of tidal 32
Results and Discussion
channels and sandbanks in the area, it is almost impossible to recognize intermediate infilling or geomorphological stages in the stratigraphic register. In this context, theoretical aspects and factual information about the Holocene geomorphological evolution of the area were combined with hypsometric analysis to support the reconstruction of intermediary morphologies and simulate hydrodynamics (for instance tides) in a long-term scale. The beginning of the formation of the Wadden Sea (including the Dithmarschen tidal flats) can be referred as approx. 9,000 to 8,000 y. BP, when the sea-level rose from around 50 to -30 m (NN) and started to affect the hinterland, specially by incursion of marine water through the fluvial channels in the Pleistocene surface (STREIF & KÖSTER, 1978). Around 8,000 to 7,000 y. BP the sea-level rose rapidly till -12 to -10 m (NN), reaching the hinterland (so-called Geest) around 7,000 y. BP, forming scarps and beaches. Due to the presence of an ice-marginal valley (the so-called Elbe-Urstromtal) the Pleistocene surface laid quite deep and horizontal near the present coast. As a consequence, the overrun of the sealevel in the coastal area of Dithmarschen occurred in a period when the sea-level was rising very quickly. So, the erosion induced by the sea-level rise is supposed to be limited in the Dithmarschen area. At the same time the water column became, in a short time, too deep for an effective bedload transport nearshore, and with the increase of water depth in the whole North Sea, tidal range also increased. It led to the deposition of a marine-coastal clay (Dithmarscher Klei). FLEMMING & DAVIS (1994), studing the Holocene evolution of Spiekeroog Island (East Frisian Islands), classified the period from 8,000 to 7,000 y. BP as an estuary phase, according to the formation of several restrict estuaries along the coast by the penetration of the sea in the lower reaches of river valleys incised in the more elevated Pleistocene hinterland. This phase can be also recognized in the Dithmarschen area. Between 7,000 and 6,000 y. BP the sea-level rise decelerated clearly in the Dithmarschen area. Between 6,000 and 5,000 y. BP a fall of about 1 m might have happened (STREIF & KÖSTER, 1978). The sea level rose from around -10 m (7,000 BP) to around -4 m (6,000 y. BP) and stayed around -4 m between 6,000 and 5,000 y. BP. This period led to important sediment accumulations, mainly in form of marine sandy and silty-clayey deposits. During the period from 8,000 to 6,000 y. BP in the area of East Frisian Islands a spitbar system was formed, originating further the barrier island system of the East Frisian Islands (STREIF, 1989). However, in the area of Dithmarschen marine - coastal silt-clay sediments were predominantly deposited, as a result of the deep Pleistocene underground, compared to the adjacent areas of North and East Friesian Islands. 33
Results and Discussion
After the deposition of several meters of sediments in the area near the coast a beach ridge system started to be formed in Dithmarschen (about 5,000 y. BP). The back-barrier sediments there, present ages around 4,000 to 3,500 BP, showing that at this time the barrierlike system was already formed (HUMMEL & CORDES, 1969). Since the Holocene marine incursion in the area, the tidal range would have been increasing (FLEMMING & DAVIS, 1994). The period corresponding to the formation of the beach ridge system in the area of Dithmarschen was probably characterized by a much lower tidal range than today (FLEMMING & DAVIS, 1994), which results in a relative increased relevancy of the waves in the regional hydrodynamic, favoring barrier island formation (ASP ET AL.,
2003). Further, tidal flat sediments have gradually deposited attached in the barrier-like
system, associated with further increase in the tidal range. With the supply of sediments from the North Sea, and a slow rising or even stable sea-level, these tidal flats progradated rapidly. Around 3,000 y. BP the barrier island system would have became inactive. In a general aspect, the following evolution of the area would be characterized by a gradual progradation of the tidal flats, until they reach a configuration similar to the present. As mentioned previously, until the present the reconstruction of the morphological (Holocene) evolution of the Dithmarschen area is quite limited, especially because of the lithological composition, intensive redepositional processes and the lack of datable organic rich deposits in the Dithmarschen area. A reliable partial reconstruction of this morphological evolution is given by WIELAND (1990), who compiled information of several authors and summarized the evolution of the area in the last 7,000 y. BP in figures 4.2 (a-h): The succession of environments from beach ridges to wide tidal flats during the posttransgression evolution of the area was probably substantially influenced by the increase of the tidal range, parallel to the sea-level rise. The period between 4,000 and 2,000 y. BP seems to be especially interesting, regarding to the evolution of the area, because it would represent the transition of wavedominated coast, characterized by beach ridges and barrier islands (HUMMEL & CORDES, 1969), fully developed around 4,000 y. BP, to a tide-dominated coast, characterized by wide tidal flats, already existing around 2,000 y. BP (MENKE, 1976; MEIER, 2001). As indicated by the gap between 3,500 and 600 y. BP (1,400 AD) in figure 4.2, information about this period is rather sparse and attempts to reconstruct this period were usually avoided. Supported by the findings of the present study and compiled information of the existing studies about the Holocene stratigraphy of the area, a schematic representation of the Holocene evolution of the area was handled out and is presented in figure 4.3. The most critical aspect is still the transition of wave to tide-dominated environments. This question is 34
Results and Discussion
going to be handled in the following chapter with the help of numerical modelling on morphodynamics.
a) around 7,000 y. BP
d) around 1750 AD
g) 1932
b) around 3,500 y. BP
c) around 600 y. BP
e) 1846 AD
f) 1889 AD
h) 1988
Figure 4.2: General aspect of the area in several periods (WIELAND, 1990). 35
Results and Discussion
20 m
10 m
10 m
0m
0m
- 10 m
- 10 m
- 20 m
- 20 m
- 30 m
- 30 m - 40 m
- 40 m
15
10
Aprox. distance (km)
5
0
0m - 10 m
- 20 m
- 20 m
- 30 m
- 30 m
?
20 m
20 m
10 m
10 m
0m
0m
- 10 m
- 10 m
- 20 m
- 20 m
- 30 m
- 30 m
- 40 m
- 40 m
- 40 m
20
15
10
Aprox. distance (km)
5
0
20 m
?
10 m
- 20 m
- 20 m
- 30 m - 40 m
10
20 m
?
5
0
- 20 m
- 20 m
- 30 m
- 30 m
25
20
15
10
Aprox. distance (km)
5
0
- 40 m
20 m
Dikes (coastal line)
10 m
0m
0m - 10 m
- 20 m
- 20 m
- 30 m
- 30 m
- 30 m
- 40 m
- 40 m
25
20
f) TODAY Consolidated mud
10 m
- 10 m
c) 4000 - 3500 BP Fine sand
- 40 m
0m
10 m
- 10 m
15
0
- 10 m
10 m
0m
Aprox. distance (km)
5
- 10 m
20 m
- 10 m
20
15 10 Aprox. distance (km)
0m
20 m
0m
25
20
e) 1000 BP
Hight to NN (m)
25
b) 5000 BP
25
d) 2000 BP
20 m 10 m
0m - 10 m
Peat
Muddy sand
15
10
Aprox. distance (km)
Coarse sand and gravel
5
Hight to NN (m)
20
10 m
0
Hight to NN (m)
25
a) 7000 - 6500 BP
Hight to NN (m)
- 40 m
20 m
?
Hight to NN (m)
20 m
10 m
Hight to NN (m)
20 m
- 40 m
Pleistocene
Figure 4.3: Schematic reconstruction of the Holocene evolution of the area, compiled from different authors, as referred above, and complemented with results from this study.
4.3. Beginning of settlement and anthropogenic influence in Dithmarschen Archeological studies (MEIER, 2001) in the investigation area show that the human occupation of the area became expressive around 2,000 y. BP with the construction of small artificial elevations and incipient dikes in the marshes. This would indicate that extensive tidal flats and marshes were already formed around 2,000 y. BP. During the Middle Ages, around the 12th century, the occupation of the marshes was very widespread, and the first dikes extending through the whole coast of Dithmarschen area were constructed. Since this period, the human activities have been strongly influencing the morphological evolution of the area. First references to sandbanks in the outer parts of the tidal flats that stayed dry even during the mean high water stem from the Middle Ages too. Since then, the general structure of the Dithmarschen tidal flat remained similar, but several “internal” changes in the morphology and probably in the hydraulic regime would have happened. These changes have been also influenced by land reclamation. 4.3.1. History of land reclamation in Dithmarschen since Middle Ages The relevant history of land reclamation in Dithmarschen began in the Middle Ages, with an intense settlement and a higher requirement of land for agriculture and raising (fig. 36
Results and Discussion
4.4). Based on historical and archeological studies, PRANGE (1986) reconstructed the spatial and temporal dike development in Dithmarschen, which became substantial in the 11th century. However, information about dike construction during the Middle Ages is deficient (PRANGE, 1986). Figure 4.5 summarizes the history of dike construction in Dithmarschen.
Figure 4.4: Settlement situation in the Dithmarschen area in the 15th century (after MEIER, 2001).
Regarding tidal flats and tidal channels in the context of land reclamation and morphological changes, the disposable information is even sparser and quite inaccurate. However, due to descriptions for navigation purposes several, general information about the Piep channel system can be found dating back to the middle of the 16th century. LANG (1975) carried out a complete review of the existing archives and summarized the evolution of the area of the Piep system in the last four centuries. According to LANG (1975), the first reference of the Piep channel dates back to 1585 and was done by WAGHENAER. His description is very general and incomplete, but it is important because it shows that the Norderpiep was the main navigation channel to reach Büsum from the sea at that time. During the 16th and 17th centuries descriptions about the Piep system as a navigation channel are very sparse. As morphological changes were very intense since that time, most cartographers rejected the challenge to map and describe those channels (LANG, 1975).
37
Results and Discussion
09° 25’ E
08° 35’ E
54° 20’ N
53° 50’ N Figure 4.5: Dike construction in the Dithmarschen area (after PRANGE, 1986).
The supratidal sandbanks Trischen and Blauort are also referred in the literature for the first time in the 16th and 17th centuries. The first reference of Blauort is dated back to 1551 and the first reference of Trischen is given in 1610 (GÖHREN, 1975). Intense morphological changes of these areas are also referred to during that period. Migration rates in the order of 30 m/year could be observed already at that time (GÖHREN, 1975). According to descriptions from 1701, the Norderpiep was already quite narrow in some parts, but it was the main navigation way to and from Büsum, because it represented a almost straight way to the open sea and presented depths similar to those found today (LANG, 1975). The Süderpiep, however, was described as an open and wide channel, with depths of about 9 meters, clearly shallower than today (LANG 1975). This might also explain why the so-called upper Klei can be found in the Süderpiep and not in the Norderpiep nowadays. The combination of depths of 14 to 16 m and high lateral migration rates resulted in the erosion of the upper Klei in the Norderpiep. In contrast to that, the Süderpiep became gradually deeper from the 16th century until today, and consequently eroded the upper Klei more recently and only partially. 38
Results and Discussion
The coastal area of Dithmarschen was first completely measured using modern hydrographic techniques in 1838. It resulted in the publication of the first “scientific measured chart” of the area in 1846 (fig. 4.6). This chart, compared to the first ones of the 16th and 17th century, reveals that Süderpiep and especially the Norderpiep, migrate several kilometers to the North. Besides this lateral migration, since the first descriptions the Norderpiep seems to have kept the same general aspect (LANG, 1975). The tendency of a migration to the North for the channels Süderpiep and Norderpiep could be characterized in the scope of the present study. It is reliable to assume that the migration to the North of both channels since the 16th century is based on the same process observed today (see chapters 5 and 6). Comparing the charts from 1846 (fig. 4.6) and today (fig. 4.7), it can be seen that the Norderpiep was, until that time, the main and more direct navigation channel from the area of Büsum to the sea. Furthermore, it is clear that the sandbank Tertius, separating Süderpiep and Norderpiep, was situated several kilometers eastwards of its present position. 08° 30’ E
08° 40’ E
08° 50’ E
09° E 54° 10’ N
54° N Figure 4.6: Part of the chart from C.C. ZAHRTMANN (1846) extracted from LANG (1975).
08° 30’ E
08° 40’ E
08° 50’ E
09° E 54° 10’ N
54° N Figure 4.7: General aspect of the area today (bathymetry from 1996).
39
Results and Discussion
At the same time that new dikes were built, pushing the coastal line seawards, the outer sandbanks migrated continuously, resulting in a landward displacement of the seaborder of the tidal flats. The combination of these to factors resulted in a substantial reduction in the width of the tidal flats in Dithmarschen. The landward displacement of the sea-border of the tidal flats was already referred by SPIEGEL (1997) as a possible adjustment of the tidal basins of the region to the ongoing sealevel rise. This would be expected especially considering a relative small sediment volume in the basin, compared to the water volume and the absence of an appropriate external sediment supply at the present (the case of most tidal basins in the region). Considering the rates of elevation of high water levels found by FÜHRBÖTER & JENSEN (1985), based on water level registers between 1934 and 1983 (0.325 m/100 years), an elevation in the order of 0.5 m would have happen since the elaboration of the first measured nautical chart prepared between 1838 (measurements) and 1846 (publication) (fig. 4.6). The disposable information enables to conclude that the Piep channel system and the adjacent tidal flats and sandbanks already exist in a configuration similar to today at least since the 16th century. Furthermore, it is evident that the system has a very dynamic character since the time it was first referred to and that morphological changes are mainly associated with natural processes, that were also further influenced by land reclamation. The intensification of the dike construction and land reclamation also date back to the 16th century. The land reclamation has modified the natural course of the morphological evolution of the area, but it is not easy to separate the natural morphological changes and those caused or at least influenced by land reclamation. However, with the study and observation of the effects of recent and important land reclamation activities in the Meldorf Bight, it is possible to recognize the general impact of such land reclamation projects in the morphodynamics of the area.
40
Results and Discussion
5. Medium-term morphodynamics (Decades): Land reclamation and natural processes 5.1. Natural processes in medium-term morphodynamics The morphological changes observed in the evaluated areas in medium-term development are caused and influenced by several factors. Some of these factors seem to act on each compartment of the depositional environment, being the main propellant force of morphological changes. These are especially related to the hydrodynamics. Several other factors have a more local character and explain usually particularities of the morphological changes observed in each area.
5.1.1. Meandering and general aspects of the morphological changes As demonstrated by analyzes of the morphological changes at the fieldwork areas (see chapter 6) the channels present a more intense dynamic, compared to the tidal flats. One of the most substantial observed morphological changes is the lateral migration of the channels. This is mainly related to the so-called estuarine meandering or estuarine meander development (AHNERT, 1960). The meandering, resulting in the lateral migration of tidal creeks, is a well-known process and has been studied for a long time (TRUSHEIM, 1929; VAN STRAATEN, 1951; REINECK, 1958; AHNERT, 1960; GÖNNERT, 1995). It can be roughly explained based on the general aspects of formation from fluvial meandering channels, as described by REID & FROSTICK (1994). Nature does not favor straight channels if only because of the inherent tendency for open-channel flows to propagate secondary circulation (FRANCIS & ASFARI, 1970; THORNE ET AL., 1985). As a result, they are unstable, tending to transform themselves towards more sinuous planform with time (REID & FROSTICK, 1994). Since in tidal channels the flow is bi-directional, there is a tendency of flow segmentation and generation of ebb and flood branches in a channel. Secondary circulation would take place in both branches, in both directions, also influenced by Coriolis effect. The mechanism of estuarine meander development was properly described by AHNERT (1960): Ebb- and flood-tide flow paths occur more intensely in opposite sides, generating ebb and flood bends. Both currents are subject to centrifugal forces in their bends; this and the fact that the directions of their forward motions are opposed to each other lead to a horizontal displacement, a spatial shift of phase: the line of highest velocity of each current tends to stay close to the downcurrent edge of the channel (fig. 5.1).
41
Results and Discussion
Furthermore, where the lateral displacement becomes sufficiently great, ebb and flood currents may maintain channels of their own (ebb- and flood-branches), separated by an elongated sandbar in the central part of the channel cross-section (ROBINSON, 1956, 1960 and AHNERT, 1960). As figure 5.1 shows, in the bends, where the lines of highest velocity of ebb and flood intersect, there is no such lateral shift and there is no formation of elongated underwater sandbars (AHNERT, 1960).
Figure 5.1: Schematic representation of estuarine meanders and their development (after AHNERT, 1960).
Such underwater sandbars as described by ROBINSON (1956, 1960) and AHNERT (1960) can be seen often in the main channels of the investigation area (fig. 5.2). Besides, analysis of current velocities at the Piep channel system (see chapter 6.2.) demonstrates flood dominance for outer parts and ebb dominance for inner parts. As stated by LAMBIASE (1980), bedload sediment transport is trapped in such channel systems, because the slow moving grains are subjected to landward residual transport in the outer (flooddominated) parts of the tidal channels and to seaward residual transport in the innermost parts (DRONKERS, 1986). This mechanism delivers the sediments for the formation of the sandbars in the study area. The formation of the sandbars awards the channel cross-sections usually a ”W” form, with ebb and flood branches. This “W” shape of different cross-sections, as well as the meandering, can be distinctly modified according to local particularities, which are going to be also discussed in the next chapters. In the study area, genuine estuarine meanders as described by AHNERT (1960) are not present, however, the process of meandering takes place (REINECK, 1958; GÖNNERT, 1995). This becomes evident due to the curvature of channels and the erosional – depositional processes visualized in the figure 5.2, which shows the comparison of the bathymetries of 1977 and 1996 overlaid by the isobaths from 1996. The arrows indicate the main direction of sediment movement in medium-term time scale and the ellipses indicate the evaluated areas in short-term scale. 42
Results and Discussion
Figure 5.2: Overview of the medium-term morphological changes on the study area.
A large bend, composed by the channels Piep and Sommerkoog-Steertloch, can be visualized in the inner parts of the study area (fig. 5.2). There, the evaluated areas of Sommerkoog-Steertloch, the channel slope across from Büsum and the cross-section C – Piep are suffering the influence of the meandering process. A gradient of the intensity of meandering can be observed, where the meandering is more intense at the area of Sommerkoog-Steertloch and is quite reduced at the cross-section C – Piep. At the evaluated cross-section A – Norderpiep a lateral migration of the channel can be also observed. Meandering is certainly influencing this lateral migration. However, at this area, a migration of an outer sandbank seems to play a major role in the lateral migration of the channel. At the cross-section B – Süderpiep a meandering cannot be clear identified. The lateral migration of channels seems to be usually accompanied by a reduction in the cross-sectional area of the channels, during the period between 1977 and 1996. This, in turn, would be also associated with land reclamation processes in the Meldorf Bight area, where between 1972 and 1978 a tidal flat area of around 48 km2 was isolated and a reduction of around 37x106 m3 in the tidal prism resulted. According to O’BRIEN (1969), with a strong reduction in the water volume that passes through the channel, a reduction in the channel cross-sections would be expected, since there is an equilibrium between the tidal prism and channel cross-sections. The cross-sectional area of the Profile A – Norderpiep was reduced by about 28%. In the cross-section B – Süderpiep this reduction was about 24% and in the crosssection C – Piep about 12% (period between 1977 and 1996 for all three cases). The reduction in the tidal prism resulted from the land reclamation and coastal protection measures in the Meldorf Bight represent a reduction in the order of 8% in the tidal 43
Results and Discussion
prism of the Piep system basin, with a tidal prism in the order of 420x106 m3 (SPIEGEL, 1997). This might have activate or accelerate the constriction of channels as well as their lateral migration. The questions involving land reclamation in the area and its impacts on the medium-term morphodynamics of the study area are going to be discussed further in the chapter 5.2. The outer part of the study area is characterized by mobile sandbanks that also are quite dynamic and usually migrate landwards in rates of about 30 m/year (WIELAND, 1972; GÖHREN, 1975; KESPER, 1992; RAUSCH, 2000). As visualized in the cross-section A – Norderpiep, this migration of sandbanks results in lateral constriction of the channels, which leads to a tendency of channel incision. This development, in turn, leads to the exposition of a consolidated silt-clay layer (the Dithmarscher Klei) at the streambed of channels. The effects of migration of the outer sandbanks in the general morphological development of the study area are going to be discussed in the chapter 5.1.2., and the effects of erosion-resistant sediments are going to be approached in the chapter 5.1.3. Despite the particularities referred above, as well as seasonal effects, the hydrodynamic regime is the main propellant force of morphological changes. Those particularities first affect the hydrodynamic regime that factually interacts with the morphology, resulting then in morphological changes (see also the morphodynamic concept in WRIGHT & THOM, 1977). In this context, special attention was given to the study of current patterns in the area and this is presented in the chapters 6.2.1. and 6.2.2. As the wave action represents also a (secondary) part of the hydrodynamic regime in the area, a brief discussion about waves and their action in the study area is given in chapter 6.2.3. Discussion about seasonal aspects of the morphological changes and their causes is delivered in chapter 6.3.
5.1.2. Migration of outer sandbanks Parallel to the lateral migration and the seasonal cycle, the migration of the outer sandbanks belong to the most important general morphological changes in the area. As referred before, these outer sandbanks usually migrate landwards in rates of around 30 m/year (WIELAND, 1972; GÖHREN, 1975; KESPER, 1992; RAUSCH, 2000). In fact, in the outer part of the study area two of such sandbanks can be identified: DSteert and Tertius. The latter one is composed by three sandbanks that are partially merged (fig. 5.3).
44
Results and Discussion
Figure 5.3: Location of the mobile outer sandbanks.
On the basis of the lithostratigraphic studies, as well as on the investigation of old nautical charts since the 16th and especially of the 19th century, it can be concluded that such sandbanks arise and grow in the so-called outer coast (intermediary zone North Sea – Wadden Sea), which has an intense hydrological load (TAUBERT, 1986), and present, since their formation, a trend to migrate landward. This ongoing process might have contributed distinctly for the sand transport to the area over the Holocene. Due to the comparison of old nautical charts it could be observed that the supratidal shoal Trischen, observed today in the study area, was formed from at least three of such sandbanks that arose in the North Sea and have been migrating landwards until “merged” with the existing tidal flats and with each other, forming the actual Trischen supratidal shoal (LANG, 1975; WIELAND, 2000). As referred by LANG (1975), the sandbank Tertius is recognized as a single bank around 1838, but around 1866 two different sandbanks seawards of the primary Tertius could be already identified. The further landward migration of these banks has resulted in the triple configuration of the Tertius found nowadays. The sandbank D-Steert seems to have emerged, i.e. grown up above the low water level, around the turn of the 20th century. In nautical charts of 1838 and 1866 it could not be identified yet (LANG, 1975), but in the charts of 1936-1937 it was already visible, although its position was several kilometers seawards of the present one and its area was also much reduced (LORENZEN & SCHELLING, 1940).
45
Results and Discussion
Figures 5.4 and 5.5 show the morphological development of the Tertius sandbank between 1977 and 1999. The lines in the figures correspond to the mean low water level and migration rates were estimated based in the position of this line. The continuous landward migration of the northern part of Tertius is distinctive. The impressive landward migration rate of about 130 m/year has been calculated for that period. However, the central and the southern part are not migrating landwards at all. This fact is attributed to the strong ebb currents along the southern flank of Tertius, as showed by current measurements at the crosssection B – Süderpiep (see also chapter 6.2.2.).
Figure 5.4: The morphological development of the northern and central part of Tertius sandbank between 1977 and 1999.
Figure 5.5: The morphological development of the southern part of Tertius sandbank between 1977 and 1999.
The strong ebb currents have been eroding the southern flank of the Tertius, hindering its landward migration. In the case of the southern part of Tertius, a kind of dynamic equilibrium seems to have been reached in the last years, where the landward transport of sediments in the sea-side (west) of the sandbank seems to be compensated by erosion in the land-side (east). The sandbank presents important modifications in its form, but its position 46
Results and Discussion
seems to be stable (fig. 5.5). Even when a landward migration does not take place, a tendency for this could be identified. Figure 5.6 shows the morphological development of the sandbank D-Steert between 1977 and 1999. This bank has been migrating landwards in rates of 56 m/year during this period.
Figure 5.6: The morphological development of the sandbank D-Steert between 1977 and 1999 (the plotted line corresponds to the mean low water - 1.6 m).
As discussed previously, the migration of such sandbanks is influencing distinctly the trend of lateral migration of channels, especially in association with the outcrops of the Dithmarscher Klei, in the Norderpiep and Süderpiep. It can be concluded that the landward migration of the northern part of the Tertius sandbank causes, or at least improves, the lateral migration of the Norderpiep. However, in the case of the Süderpiep tidal channel, the tendency of landward migration of the southern part of Tertius seems to hinder the lateral migration of the Süderpiep, since this channel would also tend to migrate to the north.
5.1.3. Depth limitation by consolidated layers Several studies have shown the importance of the relief of the Pleistocene surface for the morphology of the North Sea and Wadden Sea (DITTMER, 1952; FIGGE, 1980; TIETZE, 1983; STREIF, 1990; ZEILER
ET AL.,
2000). The course of large and deep tidal channels in
several areas along the Wadden Sea is influenced substantially by old channels of the Pleistocene (TIETZE, 1983; STREIF, 1990; RUPRECHT, 1999). Besides, the maximum depths of several tidal channels and estuaries in the region are usually distinctly controlled by the depth of the Pleistocene surface. Erosional processes are usually stopped or at least very reduced when coarse Pleistocene sediments are reached. In this context, the inferior limit of mobile sediments layer in the inner German Bight, within the study area, is usually established at the Pleistocene surface (ZEILER ET AL., 2000). 47
Results and Discussion
Furthermore, in the study area the Pleistocene surface uses to be found at depths of about 30 m or even deeper and is not reached by the tidal channels. However, the Holocene evolution of coastal areas can also result in the formation of erosion resistant sediment layers. A typical example of that is the formation of consolidated clay-silt layers, like the Dithmarscher Klei in the study area. The erosion resistance of a cohesive sediment layer is more or less intense mainly according to the consolidation level (most associated with the age of the layer) and its thickness. In the case of the Dithmarscher Klei, with an age between 5,000 and 7,000 years BP (DITTMER, 1938) and a thickness of up to 10 m (mean 5 m), the erosion resistance is very distinctive. Figure 5.7 shows the lay of the Dithmarscher Klei and the thickness of sediment layers recovering it, obtained by the measurements of the present work. In fact, the Dithmarscher Klei, which is usually reached in depths of 15 to 24 m NN (fig. 5.7), represents an effective limitation for the depths at the tidal channels, as the analyses of morphological changes have shown. Figure 5.8 shows the development of several cross-sections of the Piep channel system in the last years, evidencing the substantial control of the maximum depth of the channels due to the Dithmarscher Klei. In this context, superficial and sub-superficial sediments were investigated in the study area using side-scan sonar, shallow reflection seismic and coring to map the extension, depth and thickness of the Dithmarschen Klei, as showed in the figure 5.7 (see fig. 3.5 for location and coverage of measurements). Discussion about the origin and formation of the Dithmarscher Klei is delivered in chapter 4. Here only the aspects related to the effect of this layer in the present morphological development of the area are relevant. It was possible to identify several outcrops of that consolidated layer in the main channels, especially using side-scan sonar measurements. Figures 5.8 to 5.9 summarize the results of these investigations in the scope of the present study. A second partially consolidated clay-silt layer, the so-called upper Klei, was also identified. Its occurrence seems to be limited to the outer parts, especially in the vicinity of the Tertius sandbank. The top of this layer is usually reached in depths around 12 m NN. Its thickness is in the order of 1 - 2 m. In the figure 5.9 different outcrops of consolidated layers can be seen. Fully unconsolidated clayey sediments in the channels seem to be rather seldom. These outcrops of consolidated layers were mostly classified as Dithmarscher Klei or upper Klei, but in some cases, cohesive sediment layers or sediment patches, which seem to be related to local morphodynamics, were also detected. In contrast to the widespread Dithmarscher Klei, the upper Klei seems to be limited to the vicinity of the Tertius sandbank. It was not possible to 48
Results and Discussion
determine whether some other outcropping silt-clay sediments found landwards of the Tertius sandbank and laying above the Dithmarscher Klei can be assigned to the upper Klei.
Figure 5.7: The Dithmarscher Klei.
Because of its restricted occurrence and reduced consolidation, the upper Klei has a secondary relevance, compared to the Dithmarscher Klei. However in the Süderpiep tidal channel its effect seems to be more substantial. Younger (less consolidated), and thinner clayey layers might also affect the morphological changes observed in the study area. This can be exemplified with the case of the cross-section A – Norderpiep (see chapter 6.1.1.). In the northern part of the cross-section, where usually erosion dominates, some patches of deposition can be observed surrounded by erosional areas, which were more detailed studied by periodic measurements with side-scan sonar. Several sediment slips were observed (fig. 5.9a and b). 49
Results and Discussion
Figure 5.8: Medium-term development at several channel cross-sections with respect to the depth of the Dithmarscher Klei.
They are formed according to the different composition of layers that constitute the sediment package under the tidal flat in the northern flank of the cross-section. Intercalated 50
Results and Discussion
sandy and cohesive layers result in a complex erosional process, where sandy layers are easier eroded than the cohesive layers of silt and clay, resulting in a jagged flank, where steps of consolidated silt-clay layers are generated and later collapse (fig. 5.9b). It produces so-called slump balls that appear as sudden deposition patches in an erosive area.
Figure 5.9: Formation of steps and slump balls during the erosional process in the northern part of the cross-section A – Norderpiep.
The Dithmarscher Klei is usually outcropping at locations where there is a tendency of channel incision, especially in the outer inlets Süderpiep and Norderpiep as shown in figures 5.10 and 5.11. Erosional processes at the streambed of channels exposed those layers. Further, this erosional process would be stopped or at least decelerated once the Dithmarschen Klei is reached. As mentioned previously, the landward migration of outer sandbanks leads to a tendency of channel constriction, that forces channel incision. The Dithmarschen Klei, in turn, hinders this. This situation is observed in the Norderpiep and Süderpiep tidal channels. Assuming that the cross-sectional area of the channels tend to be preserved, two main reasons for a channel constriction, due to deposition in one flank of the channel can be identified, i.e. channel incision (deepening) or lateral erosion of the opposite side.
51
52
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Figure 5.10: Interpretation of the side-scan sonar measurements carried out during July 2000.
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consolidated mud
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Side-Scan Sonar Measurements - 03. - 05.07.00
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Results and Discussion
Results and Discussion
Side-Scan Sonar Measurements - 31.07.01 6006000
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Figure 5.11: Interpretation of the side-scan sonar measurements carried out during July 2001.
In the case of the Norderpiep, due to the extensive exposition of the Dithmarscher Klei in the channel streambed, the erosion in the northern flank of the channel seems to be the most reliable reaction to compensate the deposition in the southern flank. However, in the Süderpiep the outcrop of the Dithmarscher Klei at the cross-section B is not so extensive like in the Norderpiep. Thus, in the Süderpiep the most reliable reaction to a channel constriction was the erosion of unconsolidated sediments in the streambed, corresponding most to an underwater sandbar (fig. 5.8). As it can be seen in the figure 5.12, the Dithmarscher Klei was not outcropping in the cross-section B – Süderpiep during March 2002 (according to side-scan sonar measurements) and August 2002 (according to bathymetric measurements). Besides, the upper Klei could be identified on the channel bed of that cross section due to side-scan measurements (figs. 5.12 and 5.13). As referred previously, with the exposure of the upper Klei in the Süderpiep, the morphological changes seem to be almost limited to the seasonal cycle. Apparently, the upper Klei is already being eroded in the southern part of the cross-section. It can be supposed that after the erosion of the upper Klei the development of the Süderpiep will get more and more similar to the Norderpiep. In the main channel (Piep), in the vicinity of Büsum, a quite extensive outcrop of the Dithmarscher Klei can be seen (figs. 5.7 and 5.10). At this location there is no evidence of a lateral migration of the channel in the last decades. 53
Results and Discussion
Side-Scan Sonar Measurements - 26.03.02 6001000
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Side-Scan Sonar Measurements 26th March 2002
Tertius
? ? ?
Figure 5.12: Interpretation of the side-scan sonar measurements carried out during March 2002 (vicinity of cross-section B – Süderpiep).
5998500
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Bathymetric Measurements 21st Aug. 2002
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slips, slump marks
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load casts, stream marks
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sand
Figure 5.13: Sediments distribution in the cross-section B – Süderpiep based in side-scan sonar measurements in comparison with a bathymetric measurement and the depth of the consolidated siltclay layers. 54
Results and Discussion
However, historical sources indicate that between the second half of the 19th century and the first half of the 20th century the channel seems to have migrated laterally there (LANG, 1975). This migration seems to be reduced or even stopped later on, due to several phases of expansion of the harbor in Büsum, especially in combination with construction of jetties, how 1922 - 1926, and 1938 - 1941. This might have hindered the erosion in the northeast side of the channel in this section, resulting in a more intense channel incision and subsequent exposition of the Dithmarscher Klei in that area. Furthermore, the narrowing of the channel might have been so intense that currents there were strongly increased, resulting even in the partial erosion of the Dithmarscher Klei. LÜNEBURG (1969) found in that area depths of up to 30 m, where Pleistocene sediments were reached. In that part of the study area the top of the Dithmarscher Klei lays usually around 21 m. The study of LÜNEBURG indicated that depths around 30 m could be found only in a very restricted area, characterizing a kind of hole. This structure had dimensions around 500 by 200 m with oval aspect and steep flanks. It is still unclear when and how this hole-like structure was formed. According to LÜNEBURG (1969) the structure identified using echograph measurements in the autumn of 1966, was unknown to the local authorities and fisherman’s community. In this context he concluded that this structure must have been formed recently. Due to sub-bottom profiles (July 2000) and shallow reflection seismic measurements (September 2000) this structure could be also investigated in the scope of the present study. Figure 5.14 shows a section of the seismic over this structure (the location of the structure can be seen in the figure 5.15b). An irregular horizon can be seen in a depth around 30 m, that was interpreted as the Pleistocene surface, which would correspond to the base of the hole described by LÜNEBURG (1969). On the left side of the picture, a regular laminar stratification is visible. This would correspond to the partial infilling of the hole in the last decades. On the right side it can be seen that the Pleistocene surface seems to go upwards reaching a depth of around 20 m. At the central part a kind of secondary depression can be seen. According to some characteristics referred for this structure by LÜNERBURG (1969), an artificial origin (dredge activities) would be cogitated, but LÜNEBURG rule out this possibility due to extensive inquire of local responsible authorities. The same author attributed the origin of this hole to the currents (scour). However, some characteristics of that structure are setting the scour origin as unlikely. This includes the absence of a physical obstruction to the tidal flow, the dimensions of the structure and the steep flanks of the hole. 55
Results and Discussion
In this context, another origin of this structure might be pondered. This can be associated with the geology of the abundant salt structures in the underground of the German Bight (fig. 5.15a), within the study area, where two relevant salt structures (salt domes) can be found (fig. 5.15b). The Büsum salt dome is, in this context, of especial interest. The approximate limits of this structure coincide with the location where the hole near Büsum was found. Associated with the Büsum salt dome, petroleum fields are found and explored. It is possible that the hole-like structure mentioned above was originated in connection with a sagging, associated with secondary peripheral sinks of the Büsum salt dome. The section in the figure 5.14 where the Pleistocene horizon seems to go upwards suddenly, might characterize a kind of fissure. The left part of the section would have sagged resulting in the former hole-like structure referred by LÜNEBURG (1969). The depression (concavity) in the central part of the picture, just attached to the structure interpreted as a fissure in the Pleistocene sediments, would represent a second smaller sagging. This would have been generated later, after an initial phase of infilling of the initial hole, indicated by the regular and thin laminar stratification on the left side. This explanation for the origin of the structure seems to be plausible, especially on the basis of the seismic profiles and the position of the Büsum salt dome. However, more detailed measurements must be carried out at this area to determine the origin of the structure with certainty. According to a sagging origin, sediments corresponding to the Dithmarscher Klei should be found in the streambed of the hole-like structure. These were not found yet. Furthermore, such salt structures might have an important influence on the course of rivers and on the surface morphology (SIROCKO ET AL., 2002). Even in the course of such big tidal channels like the Piep, this influence could be important, considering their long-term (Holocene) evolution and formation. The present course of the channels Sommerkoog-Steertloch and Piep seem to follow the shape of the Büsum salt dome, suggesting that the salt structure might have an influence in the course of these channels. However, according to the depth and the possible inaccuracy of the determination of boundaries of the salt structure, such discussion is rather speculative. As referred by SIROCKO ET AL. (2002), postglacial movements in the upper crust of the North German Basin, within the study area, were most likely caused by active diapirism. ROSS (1998) studying the diapir Bad Segberg (around 75 km eastwards of the diapir Büsum) referred that apparently the caprock of this diapir penetrates the glacial sands and moraines with a modern uplift in rates of at least 0.5 mm/year. 56
Results and Discussion
Figure 5.14: A section of a seismic profile over a structure interpreted as the scour referred by LÜNEBURG (1969).
If such rates of modern uplift could be transferred to the diapirs in the study area (Oldenswort and Büsum, fig. 5.15), this uplift certainly would have affected the Holocene morphological evolution of the area. The course of the channel, as well as the hole-like structure found in the Piep channel (fig. 5.14) might be, theoretically, but coherently, associated to secondary peripheral sinks of the Büsum diapir, as the example of the river Weser (also in the North German basin, around 120 km south of the study area), where depressions in the fluvial plain are associated with the graben system of a diapir and have an important influence in the course of that river (SIROCKO ET AL., 2002). In the area of the Oldenswort diapir, a modern uplift might be also ongoing. In the area of Tertius, above the Oldenswort diapir, the top of the Dithmarscher Klei seems to be around 2 to 3 m higher than the easterly part (fig. 5.7). This layer is supposed to have been deposited mainly between 7,000 and 5,000 y BP (DITTMER, 1938). If an uplift rate of around 0.5 57
Results and Discussion
mm/year is considered (see example above from the Bad Segeberg diapir) an uplift of around 2.5 m would have resulted in the last 5,000 y BP. However, it is more reliable to assume that this difference in the depth of the top of that layer is associated with sedimentation – erosion processes during the Holocene. In the case of the hole near Büsum, on the basis of the disposable data, the scour formation hypothesis due to current action cannot be ruled out.
a) Distribution of diapirs in the inner German Bight
b) Diapirs in the study area in relation to the seismic measurements Figure 5.15: A - General distribution of salt domes in the German Bight (after WALTER, 1992) and B - in detail at the study area with the position of the seismic profiles. The number 1 indicates the localization of the section of seismic profile shown in the figure 5.14.
58
Results and Discussion
5.2. Recent land reclamation in the Meldorf Bight The intense storm of February 1962 demonstrated the vulnerability of the existent dikes until that time in the west coast of Germany, especially related to the storm waves. It unleashed the elaboration of the general plan for reinforcement and shorten of dikes and coastal protection of Schleswig-Holstein State at 1963 (“Deichverstärkung, Deichverkürzung und Küstenschutz in Schleswig-Holstein“). In the scope of this plan, several new dikes were constructed in the study area and have influenced its morphodynamics. Regarding to land reclamation, the construction of the new dikes in the Speicherkoog area, in the inner Meldorf Bight were for sure the most relevant project in the study area. These dikes, with an approx. length of 15 km, resulted in the isolation of almost 50 km2 of tidal flats and represented a reduction of about 37 x 106 m3 in the tidal prism of the area (TARNOW ET AL., 1978). Figure 5.16 shows aerial photographs of the study area some years before and after the dike construction in the Speicherkoog. It gives an idea of the scale of this project, as well as of the morphological changes that have taken place after the conclusion of this project. Several important modifications can be observed, by comparing the aerial photographs of 1966 and 1994. It can be supposed that only the intense modifications in the tidal channels in the Meldorf Bight itself would be a direct result of that land reclamation project of Speicherkoog. The relative importance of the reduction of the tidal prism decreases gradually seawards, according to the increase of the water volume circulation involved. Other modifications would follow trends observed previously. These include the landward migration of supratidal sandbanks (Trischen and Blauort) with reduction of their area, migration of the channels, vanishing of tidal creeks and arise of new ones. It is reliable to propose that the general influence of land reclamation in the morphology of the tidal channels and flats is mainly related to the reduction in the tidal prism. A general reduction of the cross-sectional areas of the channels after the land reclamation in the Meldorf Bight between 1972 and 1979 supports this conclusion, based on the relationship between tidal prism and channel cross-sectional area proposed by O’BRIEN (1969). As referred previously, a general reduction in the cross-sectional area of the main channel was observed between 1977 and 1996, which was attributed to the reduction of the tidal prism resulted from the land reclamation of Speicherkoog (ASP, 2002).
59
Results and Discussion
A
B
Figure 5.16: Aerial photographs of the area in 1966 (A - from KÖNIG, 1972) and in 1994 (B Gewässerkunde Büsum /ALR – Husum).
A second important effect is the general and gradual reduction of current velocities in direction to the new dikes, as a result of the cut off of the innermost part of the tidal flats and the channels as well. This results in increased sedimentation rates in the areas near the new dikes, especially of fine sediments (FIGGE ET AL., 1980; GAST ET AL., 1984). Due to the removal of a part of the intertidal area by land reclamation, the flooddominated sediment transport would be also favored, until a new equilibrium is reached. Considering this process, the land reclamation of the area of Speicherkoog in the Meldorf Bight might have favored the migration of outer sandbanks, like the D-Steert or the northern part of Tertius.
60
Results and Discussion
6. Short-term morphodynamics (few years to months) 6.1. Morphological changes at evaluated areas 6.1.1. Cross-section “A” - Norderpiep The Norderpiep is the northern inlet of the Piep channel system. It shows NorthwestSoutheast orientation and is bordered in the Southwest by the Tertius sandbank and in the Northeast by the Blauortsand flat (figs. 3.2. and 6.1.). In this channel, a cross-section in its narrowest part was chosen for detailed analysis (A – Norderpiep, fig. 6.1.). Here the channel cross-section is “U” shaped and is presenting no significant morphological segmentation in ebb- and flood-branches. However, old nautical charts of the 19th and beginning of the 20th century show a much wider Norderpiep channel and the presence of a sandbar in the position of the selected cross-section (LANG, 1975). Since that period this channel shows narrowing and erosion of the sandbar. The Norderpiep, within the cross-section A, shows presently a dynamic stability. This is associated with the almost absence of the sandbar in the profile A – Norderpiep, because such sandbars in the middle of the channels are usually very mobile and often induce substantial morphological changes. Stability denotes here a regularity in the morphological changes identified there. The figure 6.2 shows the changes in the cross-section A – Norderpiep, in the scale of months (a) and years (b).
Blauortsand Tertius
Figure 6.1: Local morphology and position of the cross-section A – Norderpiep.
The measurements of the last 3 years show seasonal depth variations of up to 4 meters in the cross-section A (figs. 6.2 and 6.3). For example, between December 2000 and July 2001, a sediment sheet of about 2 meters thick was deposited in the streambed. Even with 61
Results and Discussion
such variations, a relative stability in the cross-section can be identified. The sediments deposited in summer seem to be eroded again in the following winter and the erosion during the winter used to be compensated in the next summer (fig. 6.3). Despite this seasonal cycle, the general form of the cross-section seems to be preserved. A lateral migration is observed. During the autumn/winter the erosion seems to be more intense at one side (northern side) and during the spring/summer the deposition seems to be more intense at the opposite side (southern side), as observed in figure 6.3.
Figure 6.2: Bathymetric changes in the cross-section A – Norderpiep.
In the middle of the cross-section (deeper part) the cycle of erosion from summer to winter and deposition from winter to summer could be identified clearly. In the scope of short-term time scale, data of a single winter were collected and results in terms of seasonal variations have to be cautiously interpreted. In the shallower parts, at the flanks of the channel, this cycle is superimposed by another trend, i.e. the lateral migration of the channel, usually from South to North in the Norderpiep. In contrast with the substantial seasonal variations observed in short-term analyses, the lateral migration is the more evident morphological change in the medium-term development of the area (fig. 6.2b).
62
Results and Discussion
Figure 6.3: Comparison of the bathymetric measurements at cross-section A – Norderpiep.
6.1.2. Cross-section “B” - Süderpiep The Süderpiep is the southern inlet of the study area, with a Southwest-Northeast orientation in its inner portion and an approximate West-East orientation in its outer part. According to its dimensions (cross-sectional area about of 3 times larger than the crosssectional area of the Norderpiep) and its orientation in the outer part, this channel is assumed to be the most important inlet for the Piep channel system. It is limited in the South by the D63
Results and Discussion
Steert sandbank and the conjunction of another tidal channel (Bielshövener Loch, fig. 3.2 and 6.4). In the North and Northeast it is bordered by the Tertius sandbank (figs. 3.2 and 6.4). Cross-section B is located almost at the same longitude as the cross-section A – Norderpiep (fig. 6.4). The bottom profile of the cross-section B – Süderpiep is much more complex than the cross-section A - Norderpiep. This complexity is mainly related to the conjunction of the Bielshövener Loch channel with the Süderpiep channel in the vicinity of the cross-section B. Due to this conjunction of channels, there is a tendency of formation of 2 sandbars in the investigated cross-section. Furthermore, both flanks of the cross-section are composed of mobile sandbanks. The mobility of these sandbanks in the flanks and sandbars in the channel-bed yields complex patterns of morphological changes to the Süderpiep.
Bielshövener Loch
Tertius Blauortsand
Figure 6.4: Local morphology and position of the cross-section B – Süderpiep.
As opposed to the Norderpiep channel, a lateral migration of the channel does not seem to take place here currently. Although an erosional trend can be observed in the northern part, there is no clear depositional tendency in the southern part. Furthermore, the development of the profile during 20 years (1977 to 1997) shows also a clear erosion of the northern part (fig. 6.5b). This medium-term erosional process seems to be rather limited in the last 2-3 years (2000-2003). However, contour maps of the elevation difference in this cross-section show also an intense dynamic at the northern flank of the channel (fig. 6.6). Deposition in the southern side is not substantial, in comparison to the erosion in the southern side.
64
Results and Discussion
Figure 6.5: Modifications of the profile in the cross-section B – Süderpiep.
Figure 6.6: Comparison of the different measurements realized in the cross-section B – Süderpiep.
Besides, intercalated depositional and erosional patches and the formation and migration of megaripples was observed there. The migration of such large bedforms would produce the pattern of alternated erosional and depositional areas observed in the northern
65
Results and Discussion
flank of the cross-section. Figure 6.7 shows these bedforms, as well as an outcrop of the Dithmarscher Klei in the deeper parts of the Süderpiep.
Figure 6.7: Side-scan sonar imageries from the measurements during July 2000.
Despite the complexity of the morphological changes in this cross-section, a tendency of erosion predominance in the winter and deposition predominance in the summer could be recognized. The comparisons of the different measurements (figs. 6.5 and 6.6) show usually erosion during autumn – winter and deposition during spring – summer. However, in some cases, like between August and November 2002, the observed morphological changes do not match with the expected seasonal cycle. This is most associated with meteorological factors. The occurrence of storms, for example, can result in important morphological changes in a short time (few days), and the effects of a storm event before or after a measurement could surpass the effects of a seasonal cycle. In this context, measurements were also carried out in the attempt to evaluate the direct effect of storms on the morphology. This task involves several problems, due to the difficulty to predict storms and the need of calm weather conditions to carry out bathymetric measurements, besides the necessary high water levels. In the case of the storm “Janette” (25th – 26th of October 2002) it was possible to carry out measurements a few days after that event (5th November 2002). This data was compared with results of measurements from the 21st August 2002. The comparison of the bathymetric measurements, before and after the storm shows gentle erosion in the flanks of the channel, but the most distinct morphological change corresponds to the deposition of a sediment sheet of about 1 m thickness in the channel bed, especially in areas deeper than 10 m (figs. 6.5 and 6.6). 66
Results and Discussion
This pattern seems to be typical for the wave action during storms, where increased wave energy results in erosion in shallow parts overall the area and the transported sediment seems to be deposited in the deeper parts of the channels, where the wave action is reduced (under wave basis). According to the intensity and frequency of storms, this mechanism can even result in a general depositional aspect in a deep cross-section during a winter, superimposing and surpassing the “normal” seasonal cycle, i.e. erosion during winter. From the findings of several measurements in the different areas (figs. 6.3 and 6.6) it could be realized that usually during the period of September – October not strong morphological changes are expected and these are usually characterized by a weak erosional tendency. Furthermore, the analyses of wind measurements between the measurements of 21st August 2002 and before the storm of 25th – 26th October 2002 reveal rather calm weather during this period. In this context, the observed morphological changes due to the comparison of measurements of 21st August and 05th November 2002 can be mainly attributed to the storm “Janette”.
6.1.3. Cross-section “C” - Piep The cross-section C – Piep is located in the vicinity of the city of Büsum in the Piep channel – the main channel of the system, connecting the inner parts of the area, e.g. the Meldorf Bight, with the outer inlets Süderpiep and Norderpiep (fig. 6.8). Among the evaluated areas, the cross-section C – Piep seems to be the morphologically most stable. Usually, the bathymetric differences in this cross-section are smaller than 0.5m in short-term time scales (months). However, more significant changes could be related to the occurrence of the storm “Janette” at the end of October 2002. After that event a sediment sheet of up to 3 meters thick was deposited in the deeper parts of the channel (fig. 6.9). As morphological changes observed in this cross-section are not substantial, there is no clear evidence of a lateral migration or a seasonal cycle of morphological changes at this channel. Comparison of measurements (June, September and December 2000) indicates deposition, instead of erosion from summer to winter. Comparison of further measurements (December 2000 and May 2001) indicates erosion from winter to summer, instead of deposition (figs. 6.9 and 6.10).
67
Results and Discussion
Figure 6.8: Local morphology and position of the cross-section C – Piep.
Figure 6.9: Modifications of the profile in the cross-section C - Piep.
Comparing two subsequent summers (June 2000 and July 2001) the general aspect indicates a small overall deposition. If two subsequent winters are compared (December 2000 and December 2001), the general aspect indicates small overall erosion (fig. 6.11). But in both cases the erosion and deposition can be related to differences in climate and hydrodynamic between two subsequent summers or winters.
68
Results and Discussion
Figure 6.10: Comparison of the bathymetric measurements at cross-section C – Piep.
Figure 6.11: Seasonal and annual variations at cross-section C – Piep. 69
Results and Discussion
In the comparison between December 2001 and November 2002 deposition dominates the general aspect of the cross-section, where deposition seems to be especially high in the deeper part of the cross-section. As argued for the other cross-sections, this pattern is related to the occurrence of a storm at the end of October 2002. The deeper part of this cross-section represents also a part of the deepest area of the Piep channel, being substantially deeper than the usual wave basis. Thus, deposition during/after a storm would be favored there, because it would represent a kind of shelter for deposition during such high-energy events.
6.1.4. Channel slope across from Büsum The slope of the Piep channel - tidal flat Bielshövensand, directly across from the city of Büsum, was also investigated (fig. 6.12). This area was evaluated because it seems to be a quite mobile area. Furthermore, this area presents ideal characteristics for the evaluation of the relationships between morphological changes in shallow (flats) and deep parts (channels). This fieldwork area is characterized by a shouldered channel-cross-section, where the sand bar characteristic for the Piep channel is merged in the Bielshövensand flat in the southern part of the cross-section, resulting in the formation of a kind of shoulder, which corresponds to the continuation of one of the branches of the Piep channel. The other branch of the channel is well-developed in this area and presents a “V” shape.
Figure 6.12: Local morphology and position of the field area across from Büsum.
The modifications in the channel itself seem to be not so pronounced and follow the patterns identified in the cross-section C – Piep. The frequency of measurements and the order 70
Results and Discussion
of observed variations in association with the accuracy of used methods did not enable to recognize whether there is a seasonal cycle or not in this area. The difference between 2 subsequent measurements (2 to 4 months periods) used to be less than 1m and some times reach levels near the accuracy of the used measuring techniques. However, after the storm at the end of October 2002, an accretion of 1 to 2 meters in the parts deeper than 5 meters could be measured. Besides, in the shallow part (shoulder), in the southern portion of this area, interesting morphological changes take place. The repeated measurements in this area reveal the presence of large-scale bedforms that seem to migrate in rates of up to 1m/day to East – Southeast (fig. 6.13).
Figure 6.13: Comparison of the bathymetric measurements at the corner across from Büsum. 71
Results and Discussion
According to their dimensions (usually around 300 to 400 m length and usually less than 1m high) they cannot be easily classified. According to the classification proposed by REINECK ET AL., (1971), the term “current megaripples” seems to be the most appropriate. The presence and migration of these bedforms in the southern part of the area over the tidal flat generate the pattern of alternating erosion and deposition areas that can be seen in the comparison of measurements (fig. 6.13). Furthermore, a net migration of these bedforms to SE, can be observed. Migration rates in the order of 185 to 350 m/year were calculated (fig. 6.13). This development would result in a bedload sediment transport to the inner parts of the area, e.g. the Meldorf Bight and the channel Sommerkoog-Steertloch, where another crosssection was evaluated. This might represent an important source of sediments for the depositional processes that happen there (see next chapter).
6.1.5. Cross-section Sommerkoog-Steertloch This area is located in a tidal channel, in a distal part of the main channel at the inner part of the study area (figs. 6.14). There, the channel reaches depths of about 8 meters and the cross-section has a intermediary shape between a “V” and an “U”. Here, the channel has a main South – North orientation, instead of East – West, as the other evaluated cross-sections.
Figure 6.14: Local morphology and position of the cross-section Sommerkoog-Steertloch.
The most conspicuous morphological change in medium-term development at this cross-section is the lateral migration of the channel, which is even more substantial than in the 72
Results and Discussion
other areas. Following the pattern observed in the other evaluated cross-sections, this lateral migration takes place to the right hand-side, regarding to the ebb current direction. Measurements between 1974 and 1999 indicate that the lateral migration of the SommerkoogSteertloch channel was associated with continuous growth of a sand bar at the middle of that channel (fig. 6.15). Over the time span from 1974 to 1999, this leads to the complete separation of the two branches of the initial Sommerkoog-Steertloch channel in two independent channels (fig. 6.15).
Figure 6.15: Morphological changes of the evaluated cross-section in the Sommerkoog-Steertloch.
Regarding the short-term development, the most visible trend of morphological change corresponds to the seasonal cycle, as observed in other areas. Furthermore, the erosion during autumn/winter is distinctly more intense in the east-flank of the channel, while the
73
Results and Discussion
deposition during spring/summer is much more intense in the west-flank. In the depositional side (west) the erosion is even during autumn/winter almost absent. In the same way, the deposition in the erosional side (east) is reduced even during spring/summer. However, in the channel streambed both processes are pronounced. The continuous landward migration of bedforms over the tidal flat Bielshövensand at the bend of the channels Piep and Sommerkoog-Steertloch might have delivered the sandy sediments for the growth of the sand bar in the evaluated area, as mentioned in the previous chapter. The figure 6.16 shows the possible sediment transport directions (arrows) from the channel slope area described previously to the channel Sommerkoog-Steertloch, i.e. to the sandbar found there. Two different mechanisms seem to be suitable to explain this sediment transport: A bypassing of sediments from the Bielshövensand flat, through one branch of the Sommerkoog-Steertloch, to reach the sand bar at that area; and/or a transport of sand along one branch of channel to the inner parts during the flood phase and a transport of these sandy sediments along the other branch of the channel till reach the sand bar during the ebb phase. Any more precise affirmation about this process would be rather speculative.
Figure 6.16: Isobaths of –2 m (NN) showing the supposed direction of bedload sediment transport (bypassing = arrows).
74
Results and Discussion
6.2. Hydrodynamics of the Piep channel system 6.2.1. General patterns of water levels and tidal currents A general description of the hydrodynamic regime in the study area was presented in chapter 2.3. As the hydrodynamics of the study area is clearly dominated by tides, the hydrodynamic analysis focused on water levels (tides) and tidal currents. Tidal currents, as well as the analyzed morphological changes, are especially intense in the tidal channels. In the scope of this study, results of measuring campaigns covering entire tidal cycles at three different cross-sections of the main tidal channels were considered. ADCP measurements were carried out every 3 months and covered a spring and a neap tidal cycle in each measuring campaign for each of the three cross-sections (A – Norderpiep, B – Süderpiep and C – Piep) shown in fig. 3.2. Current measurements were carried out over a period of about 3 years. In the present study, only the data set of 2000 was considered. In addition, water levels were continuously measured at the station Büsum over the last 3-4 years and data from some summer tidal gauges from other locations were also used. The relevant aspects of the tidal currents analysis are the mean maximum ebb and flood currents for each cross-section, differences between cross-sections, spring and neap tidal cycles and between seasons, as well as current segmentation in each cross-section. Regarding to water levels, the duration of ebb and flood, their seasonal and neap – spring variations and comparisons between inner and outer parts were investigated. The concepts and the nomenclature of tides used here are sketched in figure 6.17.
Figure 6.17: Definition of terms (German Industry Norm) involving tidal course and water levels (PN = Gauge Zero level, 5 m below NN).
75
Results and Discussion
Due to the well established importance of asymmetric tidal cycles on the transport and accumulation of sediments in shallow estuaries (POSTMA, 1967; DRONKERS, 1986; FRIEDRICHS & AUBREY, 1988), the water levels and current measurements were used to establish “flood-dominance” (flood phase having shorter duration and higher velocities) and “ebb-dominance” (ebb phase having shorter duration and higher velocities) as defined by FRIEDRICHS & AUBREY (1988). The long-term (three years) analyses of the water levels from the permanent tidal gauge of Büsum and from the summer gauges in the vicinity of the cross-section A – Norderpiep and B – Süderpiep (BSH: Federal Maritime and Hydrographic Agency Hamburg, Germany) show no significant differences for high and low water levels among the three gauges. The referred water level gauges in the outer inlets Süderpiep and Norderpiep are around of 13 km seawards of the water level gauge from Büsum. The localization of these tidal gauges can be seen in the figure 3.1. These analyses reveal a mean tidal range for the area of about 3.2 m, corresponding to values of about 3.5 m during spring tidal cycles, while during neap cycles it is about 2.8 m. Regarding to high water and low water time, just small phase lags were identified. The high water level is reached 10 minutes earlier in the Norderpiep and 12 minutes earlier in the Süderpiep, compared to the gauge in Büsum. The low water level is reached in the Norderpiep about 4 minutes later than in Büsum and in the Süderpiep almost at the same time than in Büsum. An analysis of the duration of tidal phases during the months of June, September and December 2000 was carried out for the three mentioned tidal gauges. The results of this analysis are summarized in figure 6.18.
Figure 6.18: Comparison of ebb and flood phase’s duration during June, September and December 2000, for the cross-sections Norderpiep, Süderpiep and Piep. 76
Results and Discussion
The results show that the duration of ebb and flood phases is usually different for the three gauges. The differences of flood and ebb duration range from about 0 to 0.75 hours. It can be seen that the general pattern seems to be the same for the three considered months as well as for neap and spring tidal cycles. It can be seen that at the outer inlets (Norderpiep and Süderpiep) the flood phase tends to be shorter during spring as well as neap cycles. At Büsum, in the inner part of the study area, the opposite pattern was identified, with the ebb phase resulting shorter than the flood phase. Exceptions of this trend are the neap cycles during June for the Norderpiep and spring cycles during September for Büsum. But in both cases the differences are very small. In this context, the outer parts of the main channel seem to be flood-dominated (shorter flood phase), while the inner part seems to be ebb-dominated (shorter ebb phase). Analysis of sediment transport in the same cross-sections shows also a trend of positive sediment transport in the outer parts and negative sediment transport in the inner parts (POERBANDONO ET AL., 2003). Comparing the months of June and December for the gauges from the Norderpiep and Süderpiep it can be seen that the difference in the duration of the ebb and flood phases is substantially smaller for June (summer). Again, the opposite trend can be identified for the gauge of Büsum. There the differences in the duration of the ebb and flood phases increase in June, compared to December (because of technical problems there is lack of data for spring tidal cycle in December 2000 at the Norderpiep). Comparing neap and spring cycles, there is also a trend of smaller differences between ebb and flood duration for the Norderpiep and for the Süderpiep during neap cycles. For Büsum the differences are bigger during neap cycles. It can be concluded that for all three tidal gauges the ebb phase is favored in the summer and during neap cycles. More intense discussion involving this theme will be delivered thereafter. It is expected that these differences in the duration of ebb and flood phases are reflected in the current velocities. Shorter tidal phases should result in higher current velocities. In these terms the maximum cross-sectionally averaged ebb and flood current velocities from the measurements carried out during 2000 were compared. The mean of the maximum cross-sectionally averaged ebb and flood current velocities for measurements during spring and neap tidal cycles are shown in table 6.1 and figure 6.19.
77
Results and Discussion
Table 6.1: Mean Maximum ebb - flood cross-sectional averaged current velocities. Cross-sections
Mean max. flood currents (m/s) Neap Spring
Mean max. ebb currents (m/s) Neap Spring
A - Norderpiep
0.85
1.16
0.92
1.10
B - Süderpiep
0.84
1.19
0.82
1.05
C - Piep
0.87
1.03
0.91
1.05
As expected, for all three cross-sections the maximum ebb and flood velocities are higher during spring tidal cycles. Asymmetries between ebb and flood can also be observed. The cross-section B – Süderpiep seems to be flood-dominated, independent of neap or spring cycle. However, the difference between maximum flood and ebb currents seems to be much reduced during neap tidal cycles. The cross-section A – Norderpiep presents a similar pattern, but for neap cycles the channel seems to be ebb-dominated, where maximum ebb currents surpass maximum flood currents. For the cross-section C – Piep the ebb currents are higher in neap and spring cycles.
Figure 6.19: Comparison of maximum ebb and flood cross-sectional averaged current velocities.
The comparison of the data shown in figure 6.19 and table 6.1 with the data from figure 6.18 reveals that differences in the duration of tidal phases and the maximum current velocities reached are in a quite good agreement, however the differences in the current velocities seem to be rather small. A discrepancy can be seen only for neap cycles in the Norderpiep. Regarding to the duration of tidal phases, ebb phase during neap cycles was longer in two of three evaluated months, but the average of maximum cross-sectionally averaged current velocities during neap cycles (measurements of 2000) reveals higher values for ebb currents in the Norderpiep. Such discrepancies, among other factors, require a more detailed analysis of the current velocity measurements.
78
Results and Discussion
Besides the astronomic tidal forcing itself, winds can affect substantially water levels and consequently tidal currents in a short time. In the study area the westerly winds predominate and are especially strong during winter. This would explain why flood phase becomes shorter and the ebb phase becomes longer in the winter, e.g. difference between June and December 2000 (fig. 6.18). The westerly winds would dam up partially the water outflow during ebb and would favor the flood due to wind driven currents. This results in a shortening of the flood and prolongs the ebb phase. Flood current velocities are consequently higher and ebb current velocities lower. It is important to remind that the current velocity data used here were collected during low or moderate wind conditions, since strong winds do not enable vessel-based measurements in the study area. As referred by BOON & BYRNE (1981), DRONKERS (1986) and FRIEDRICHS & AUBREY (1988), a further component affecting water levels and duration of tidal phases is the morphology. This effect of morphology on the tidal flow will be discussed in detail throughout this chapter. The instant tidal range, i.e. the water level difference between the high and low water level of a considered tidal cycle, can also vary substantially, most due to wind action, even considering two subsequent high or low waters. This wind effect might also explain discrepancies in the current velocity measurements analyzed here. It makes the evaluation of the correlation between duration of tidal phases and maximum current velocities during single tidal cycles rather complicated. Because higher ranges correspond to bigger water volumes flowing in a tidal phase, the current velocities in this tidal phase certainly would be increased. In the same way, shorter tidal phases would correspond to higher currents, considering the same tidal range, i.e. water volume. In this context, relationships between the tidal ranges and the duration of the tidal phases with the measured cross-sectionally averaged maximum current velocities were investigated. Differences (difference = flood – ebb) in the tidal range, in the duration and in the velocity during ebb and flood phase were calculated and compared. These are summarized in the figures 6.20 and 6.21. It is important to accentuate here that relative small differences between the duration of tidal phases might produce significant differences in the maximum current velocities between flood and ebb phases. This is more applied to the study area, because of its relative big tidal prism. Besides, it can be concluded that, based on the studies of BOON (1975), BOON & BYRNE (1981) and LESSA & MASSELINK (1995), the differences in the duration between ebb and flood tidal phases found in the present study are representative. Furthermore, the bedload sediment transport is proportional to the third (or higher) power of the effective flow 79
Results and Discussion
velocity (DRONKERS, 1986). Thus, small differences between the maximum ebb and flood current velocities might be very important to the morphodynamic of the study area. Figure 6.20 shows the calculated correlation between the difference of the tidal range during the ebb and during the flood, with the difference of maximum cross-sectional averaged current velocity (a), as well as the correlation of the difference in the duration of the ebb and during the flood with the difference of maximum cross-sectional averaged current velocity (b) were not high at all.
a)
Diffe re nce curre nts (m /s)
0.25 0.2
S pring: R 2 = 0.27
0.15 0.1 0.05 0
Neap: R 2 = 0.58
-0.05 -0.1 -0.15 -0.2 -0.4
-0.3
-0.2 -0.1 0 0.1 0.2 Diffe re nce ra nge Flood - Ebb (m )
Flood - E bb (Neap)
0.3
0.4
Flood - E bb (Spring)
b)
Diffe re nce curre nts (m /s)
0.25 0.2 0.15
S pring: R 2 = 0.41
0.1 0.05 0 -0.05
Neap: R 2 = 0.12
-0.1 -0.15 -0.2 -1.5
-1 -0.5 0 0.5 1 Diffe re nce dura tion Flood - Ebb (hrs.)
Flood - E bb (Neap)
1.5
2
Flood - E bb (S pring)
Figure 6.20: Correlation of the difference between the maximum flood and ebb currents with the difference in the corresponding tidal range (a) and with the difference in the duration of the flood and ebb phase (b), separate by neap-spring cycle.
To carry out those analyses, the data was divided in two blocs, corresponding to neap and to spring cycles involving the data of all three cross-sections evaluated. Comparisons show that there were not substantial differences in considering neap and spring cycles together or separately to correlate differences between ebb and flood current velocities with differences between duration of ebb and flood phases as well as with differences between the tidal range between flood and ebb phases. However, for each cross-section the relationship seems to be different. In this context, further analyses were carried out considering each cross-section separately (fig. 6.21). 80
Results and Discussion
Comparing the figures 6.18 (a) and 6.21 (a) it becomes clear the improvement of the correlation between differences of the flood and ebb maximum current velocities and differences of the flood and ebb tidal range (for the tidal cycle corresponding to the current measurements). The stability index (R2) for all three cross-sections is quite high. It is around of 0.98 for the Norderpiep, 0.82 for the Süderpiep and 0.79 for the Piep cross-section. Furthermore, the results of correlating differences of flood and ebb maximum current velocities and differences of the flood and ebb phases duration (for the tidal cycle corresponding to the current measurements) the stability index was unsatisfactory for two of the three cross-sections (Norderpiep and Piep). Only for the cross-section Süderpiep the correlation was substantially high (R2 = 0.90). 0.3
a)
Diffe re nce curre nts (m /s)
0.25 0.2
S üderpiep: R 2 = 0.82
0.15 0.1 0.05 0
P iep: R 2 = 0.79
-0.05 -0.1
Norderpiep: R 2 = 0.9846
-0.15 -0.2 -0.4
-0.3
-0.2
-0 .1
0
0.1
0.2
0.3
0.4
Diffe re nce ra nge Flood - Ebb (m ) N orde rpiep Line ar (N orderpie p)
Süderpiep Lin ear (Süd erpiep )
Piep L inear (Piep )
0.3
b)
Diffe re nce curre nts (m /s)
0.25
S üderpiep: R 2 = 0.90
0.2 0.15 0.1
Norderpiep: R 2 = 0.04
0.05 0 -0.05
P iep: R 2 = 0.03
-0.1 -0.15 -0.2 -1.5
-1
-0.5
0
0.5
1
1.5
2
Diffe re nce dura tion Flood - Ebb (hrs.) N orde rpiep Line ar (N orderpie p)
Süderpiep Lin ear (Süd erpiep )
Piep L inear (Piep )
Figure 6.21: Correlation of the difference between the maximum flood and ebb currents with the difference in the corresponding tidal range (a) and with the difference in the duration of the flood and ebb phase (b), separate by cross-section.
For such comparisons a great amount of measurements is eligible. Several other factors might influence the water levels and the water flow in a single tidal cycle, but longer time series of cross-sectional current velocity covering several tidal cycles are very complicated to obtain, beyond the disposable resources of this study. Water level measurements are, however, rather uncomplicated. 81
Results and Discussion
Comparing the graphics of the figure 6.21 (a and b) it might be argued that differences between the maximum velocities during the ebb and during the flood phase are more correlated to the difference of the tidal ranges between the to tidal phases than to the differences between the duration of the tidal phases. The theoretical correlation between the duration of tidal phases and the maximum current velocity could no be well established, on the basis of the disposable data. This is most attributed to the sparse number of “pairs of data” which can be used to correlate duration of tidal phases and the corresponding maximum current velocities, as referred above. Besides, differences in the tidal range between the ebb and flood might also result in differences in the duration of each tidal phase. In this context, an eventual correlation between differences in the tidal range and differences in the duration of tidal phases was evaluated. This is summarized in the figure 6.22. Diffe re nce ra nge Flood - Ebb (m )
0.4 0.3
S üderpiep: R 2 = 0.87
Norderpiep: R 2 = 0.43
0.2 0.1 0
P iep: R 2 = 0.37
-0.1 -0.2 -0.3 -0.4 -1 .5
-1
-0.5
0
0.5
1
1 .5
2
Diffe re nce dura tion Flood - Ebb (hrs.) N o rd erpiep L in ea r (N orde rp ie p)
Sü de rp ie p Line ar (Süd erpiep )
Pie p Line ar (Piep )
Figure 6.22: Correlation of the difference between the flood and ebb tidal range and the difference in the duration of the flood and ebb phase, separate by cross-section.
In the graphic above, it can be seen again that the stability index is unsatisfactory for two of the three cross-sections (Norderpiep and Piep). Only for the cross-section Süderpiep the correlation seems to be satisfactory (R2 = 0.87). Again, more data would be necessary for a better correlation. Time series covering several tidal cycles are eligible, but were not disposable or possible to obtain in the scope of the present work, especially regarding to current measurements. Summarizing the results involving maximum ebb and flood cross-sectionally averaged current velocities and duration of tidal phases, the following topics can be referred: •
82
Based on current velocities, as well as the duration of tidal phases, the cross-section C – Piep can be classified as ebb-dominated, during neap and spring tidal cycles.
Results and Discussion
•
The cross-section B – Süderpiep can be classified as flood-dominated based on the current velocities as well as the duration of tidal phases for neap and spring cycles.
•
However, the cross-section A – Norderpiep is clearly flood-dominated only during spring cycles. During neap cycles the flood phases seem to be shorter, but the current measurements shown a certain ebb-dominance.
•
Generally the ebb-dominance in the cross-section C – Piep (Büsum) is increased during neap cycles, while the flood-dominance in the Süderpiep and in the Norderpiep is increased during spring cycles.
•
Furthermore, comparisons of measurements from June and December show that during summer (June) the ebb-dominance near Büsum (cross-section C – Piep) is especially marked, while in the cross-sections A – Norderpiep and B – Süderpiep the flood-dominance is especially marked during winter (December).
•
A direct correlation between the duration of the single tidal cycles during the current measurements and the measured maximum cross-sectional averaged current velocities could not be properly established, but considering the monthly averaged duration of tidal phases and the current velocities the results are coherent. Despite of the absence of genuine barrier islands, the channels Süderpiep and
Norderpiep are tidal inlets of the Piep channel system and the area near Büsum, i.e. the crosssection C – Piep, can be roughly considered as a backbarrier environment. In these terms, considering exclusively the channels (not the intertidal areas) the Piep system would be characterized by flood-dominated inlets and ebb-dominated backbarrier environment. Besides, this general pattern seems to oscillate between summer and winter and also varies between neap and spring cycles. Both, summer and neap cycles seem to favor ebbdominance, while winter and spring cycles seem to favor flood-dominance. This process would be associated with an effect of the morphology combined with the variation in the mean water level. The elevation of the mean water level caused by strong westerly winds and the difference of the high and low water levels between neap and spring tides would change the effect of the morphology in the tidal flow. Two main effects of the tidal basin’s morphology in the tidal development can be quoted: The topography-induced generation of overtides, corresponding to the so-called hypsometric effect (BOON & BYRNE, 1981; SPEER & AUBREY, 1985) and the topographyinduced rectification (BAKKER & DE VRIEND, 1995). With respect to the first one (hypsometry effect), the most relevant aspect is the generation of overtides inside the basin. Especially important are the harmonic overtides (M4, M6, M8 constituents) of the principal lunar tidal constituent (M2). BOON (1975) notes that the relative amplitude and phase of the so-called overtides cause rise and fall duration differences in the vertical tide. Further studies (BOON & BYRNE, 1981; SPEER & AUBREY, 1985) demonstrate that usually sediment-filled basins produce overtides favoring ebb-dominance, 83
Results and Discussion
while not sediment-filled basins tend to result in flood-dominance, according to the overtides generated there. Basin
hypsometry
is
actually
an
approach
to
evaluate
or
quantify
a
morphology/topography in numerical terms, or in other words, the infilling grade of a tidal basin (BOON & BYRNE, 1981). Detailed explanation about the concept of basin hypsometry is supplied in the chapter 3.3.2. The second relevant topography-induced effect in the tidal process, the rectification, results mainly from a residual circulation between the shoals and the channels (BAKKER & DE VRIEND, 1995). According to those authors, the residual flux of water (mass flux) over the intertidal area to the channels during the ebb phase would result in another residual current in the channels, in order to satisfy the overall mass balance. This effect would be directly proportional to the “water storage capacity” of the intertidal areas, which can be very increased due to vegetation (salt marshes or mangroves). LESSA & MASSELINK (1996) found out that extensive mangrove areas in a macrotidal barrier estuary (large “water storage capacity”) resulted in ebb-dominated current patterns during spring tides, where the spring high tides reach most of those mangrove areas, incrementing the water storage capacity. In the tidal flats of Dithmarschen vegetation is almost absent, being only present associated with two supratidal sand shoals (Blauort and Trischen) and as fringes, bordering the coastal line at several places. Here, spring tidal cycles seem to favor flood-dominance, instead ebb-dominance, as referred by LESSA & MASSELINK (1996) for the Louisa Creek estuary (Queensland, Australia). The patterns found in the study area for duration differences of the tidal phases as well as for current velocities might indicate that, with respect to topography induced effects, the hypsometry effect is more important than the rectification effect. This hypothesis is mainly supported by the increment of flood-dominance during spring tides and during winter (mean water level elevation). For a better comprehension of these processes data sets and measurements covering longer periods are necessary. For the area of Büsum, long-term water level data (tides) are disposable. Here a data set of around three years was used for a more detailed evaluation of the differences in the duration of ebb and flood phases. Wind data for the same period was also available and as wind is one of the most important factor affecting water levels and duration of tidal phases in the study area, this data was analyzed and correlated to the water levels, i.e. with the duration of tidal phases based on the tidal gauge of Büsum. Figure 6.23 summarizes the calculation of tidal phases duration and of wind speed and direction of the analyzed period (August 1999 to August 2002). 84
Results and Discussion
In figure 6.23 (a) differences in the duration of the tidal phases and a seasonal variation can be seen. In a “normal” situation, with weak or moderate winds and corresponding “normal” water levels (approximate to the mean level) there is a clear tendency for shorter ebb phases. During winter, with stronger and frequent westerly winds, the mean water level increases and the difference between ebb and flood phases duration almost vanishes. 6.6
0.75
Flood
Mean level (monthly-avereged)
6.4
0.5
6.2
0.25
6
0
5.8
W ater level (m to N N )
D uratio n (hrs.)
E bb
-0.25
a) Ebb - flood duration and mean water level. 9
270
D ir. (30 days-avereged)
7.5
225 180
6
135 4.5
W in d d irection (°N )
Wind velocity (m/s)
V el.(30 days-avereged)
90
02 02 02 02 02 Apr Mai Jun Jul Aug
Feb 02 Mrz 02
Aug 01 Sep 01 Okt 01 N ov 01 D ez 01 Jan 02
M onth
Jun 01 Jul 01
D ez 00 Jan 01 Feb 01 Mrz 01 Apr 01 Mai 01
Mai 00 Jun 00 Jul 00 Aug 00 Sep 00 Okt 00 N ov 00
Jan 00 Feb 00 Mrz 00 Apr 00
N ov 99 D ez 99
Aug 99
45 Sep 99 Okt 99
3
b) Wind velocity and direction. Figure 6.23: Ebb and flood phase duration, compared with mean water level at gauge Büsum and wind measurements (30-days averaged) at station Büsum.
Considering the monthly-averaged values plotted in the figure 6.23a, the difference between the duration of ebb and flood phases follow a gradual seasonal pattern, reaching a maximum around July – August. The overall maximum was about 0.75 hours. The minimum difference does not follow a clear pattern. More than that, a relative minimum seems to be reached around October and use to oscillate more or less around this minimum (according to variations in the wind, among other factors) until around March, when the ebb phase becomes gradually shorter.
85
Results and Discussion
Furthermore, the good correlation between strong westerly winds (fig. 6.23b) and the subsequent elevation of the mean water level can be seen (fig. 6.23a). Observations of the mean water levels at Büsum compared with the duration of tidal phases indicate that there is a certain critical elevation of the mean level, which causes a substantial reduction of the difference between the duration of ebb and flood phases. This critical elevation level seems to be about 0.2 to 0.3 m for the study area (fig. 6.23a). With an increase of the mean level, the effect of the morphology, for instance hypsometry effect, would be reduced, especially due to the increase of the cross-sectional area of the channels or actually because the tidal flow would not be so confined in the channels. The facts discussed above also support the hypothesis that the ebb-dominance in the inner parts of the study area is more associated with hypsometry effects. According to BOON & BYRNE (1981) values of the parameter gamma (γ) around 1.8 and 2.5 would characterize a mature or sediment-filled basin, which has an adequate communication with the sea and would produce, or at least favor, ebb-dominance (see chapter 3.3.2. for explanations about hypsometric calculations). Hypsometric calculations of the tidal basin of the Piep system shown values corresponding to a sediment-filled basin. SPIEGEL (1997), using bathymetric and topography data of the area from 1976, found γ = 2.5. In the present study further calculations were carried out and on the basis of a bathymetric data set covering the area from 1996 it was found a value of gamma γ = 1.2. Such a small value of gamma corresponds to a quite sediment-filled basin (BOON & BYRNE, 1981) and the difference (reduction) of the values of gamma between 1976 and 1996 would result from the land reclamation projects between 1972 and 1979, that would have result in a kind of “rejuvenation” of the basin, favoring flooddominance and leading to further sediment-fill (or internal reorganization of the morphology) until an equilibrium will or have been reached. This further sediment-fill had just been started in 1976, after the first phase of land reclamation in the Meldorf Bight, explaining the relative elevated value of gamma (2.5) found by SPIEGEL, in contrast to the value of 1.2 found in the present study for the morphology of 1996 (see also chapter 6.2.1). AUBREY & SPEER (1985) and SPEER & AUBREY (1985) also demonstrate that wide tidal flats as well as deep channels favor ebb-dominance. Both characteristics are present in the study area. Parallel to the asymmetric patterns in the duration and maximum velocities, the analysis of the measurements of tidal current velocities show also important lateral distortion (lateral asymmetry), i.e. more intense ebb and flood currents in opposite sides of cross86
Results and Discussion
sections, in the velocity field for each of the three evaluated cross-sections. This also stress the importance of consider cross-sectional averaged velocities for evaluations in terms of ebb or flood-dominance. In the following section, a detailed analysis of the current velocity patterns in the three cross-sections is presented, with main focus on the so-called lateral asymmetry.
6.2.2. Lateral tidal asymmetry To evaluate the lateral asymmetric current pattern, several points (cells) and vertical ensembles of cells from ADCP current measurements were chosen and analyzed. The results were further compared to the cross-sectional averaged velocity of each measured profile, as well as with the water level development of the corresponding tidal cycle (fig. 4.24).
Figure 6.24: Schematic representation of the location of cells and ensembles of cells used in the evaluation of lateral asymmetry for each cross-section.
This lateral asymmetry is supposed to be the result of the Coriolis effect in the tidal currents. The deviation to the right hand-side of the movement direction would result in the segmentation of the bi-directional currents in different branches. As the channels are usually characterized by two branches, two vertical ensembles were evaluated for each cross-section, corresponding to the south (flood) and north (ebb) branches of each cross-section. For the cross-section B – Süderpiep 3 branches can be identified (see also chapter 6.1.2) and consequently three ensembles were selected. The data from each ensemble resulted in depth-integrated velocities for each chosen location. Furthermore, time series of the current velocity in the bottom and surface cells were also evaluated for each location.
87
Results and Discussion
In general, the analyses of the data show the same pattern for the three cross-sections. Flood currents are more intense at the south side of the cross-sections (at the right of the main current direction) and ebb currents at the north side, at the right side of the main current direction as well. The following graphics summarize the results concerning the lateral asymmetry of tidal currents at the three cross-sections. According to the quality of the obtained data, the calm weather conditions during the measurements and the existence of measurements from complete tidal cycles for the three cross-sections, an ensemble of results from March 2000 (a spring cycle) and another from December 2000 (a neap cycle) were chosen. The results of the analysis of lateral asymmetry based on the other measurements can be found in the appendix. The figures 6.25 to 6.28 show the current pattern in the cross-sections during a spring tidal cycle in March (measurements of 21st to 23rd of March 2000). In the cross-section A – Norderpiep (fig. 6.25) it can be seen that there is no significant asymmetry between ebb and flood, considering cross-sectional averaged velocities. However, if the north and south ensembles are compared, a lateral asymmetry becomes obvious. The asymmetry is more evident in the current velocities at the surface, where the Coriolis effect should be more intense. In the depth-integrated velocities the asymmetry is still clear, but attenuated, if compared with surface velocities (fig. 6.25). Here it is important to remain that the Norderpiep channel is quite narrow at the evaluated cross-section and the distance between the two ensembles is just about 300 m.
88
Results and Discussion
Norderpiep - South (22.03.00) 1.8 1.6
Velocity (m/s)
1.4 1.2 1 0.8 0.6 0.4 0.2 0 7
9
11
13
15
17
19
21
Time (hrs.) Surface Velocity
Bottom Velocity
2
1.25
1
1
0
0.75
7
9
11
13
15
17
19
21
-1
0.5
-2
0.25
-3
Velocity (m/s)
Water level (m)
Deepth-Integrated Velocity
0
Time (hrs.) W ater level
Vel. cross-sectional. averaged
Norderpiep - North (22.03.00) 1.8 1.6
Velocity (m/s)
1.4 1.2 1 0.8 0.6 0.4 0.2 0 7
9
11
13
15
17
19
21
Time (hrs.) Deepth-Integrated Velocity
Surface Velocity
Bottom Velocity
Figure 6.25: Ebb and flood current pattern in the cross-section A – Norderpiep (spring cycle, 22nd March 2000).
For the cross-section B – Süderpiep the lateral asymmetry is substantial (fig. 6.26). A gentle asymmetry in the cross-sectional averaged velocities can be also seen, indicating flooddominance. For this cross-section a third ensemble was chosen, because of the existence of three branches (fig. 6.24). In the so-called middle ensemble the current pattern is very similar to the south ensemble, showing also a tendency to flood-dominance (fig. 6.27).
89
Results and Discussion
2 1.8 1.6 Velocity (m/s)
1.4 1.2 1 0.8 0.6 0.4 0.2 0 6
8
10
12
14
16
18
20
Time (hrs.)
Surface Velocity
Bottom Velocity
2
1.5
1
1.2
0
0.9
6
8
10
12
14
16
18
20
-1
0.6
-2
0.3
-3
Velocity (m/s)
Water level (m)
Deepth-Integrated Velocity
0
Time (hrs.)
W ater level
vel. cross-sectional. averaged
Süderpiep - North (21.03.00) 2 1.8
Velocity (m/s)
1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0 6
8
10
12
14
16
18
20
Time (hrs.)
Deepth-Integrated Velocity
Surface Velocity
Bottom Velocity
Figure 6.26: Ebb and flood current pattern in the cross-section B – Süderpiep (spring cycle, 21st March 2000). Süderpiep - Middle (21.03.00) 2 1.8
Velocity (m/s)
1.6 1.4
Ebb
1.2 1
Flood
0.8 0.6 0.4 0.2 0 6
8
10
12
14
16
18
20
Time (hrs.)
Deepth-Integrated Velocity
Surface Velocity
Bottom Velocity
Figure 6.27: Ebb and flood current pattern in the middle ensemble of the cross-section B – Süderpiep (spring cycle, 21st March 2000).
90
Results and Discussion
In the cross-section C – Piep, regarding to the cross-sectional averaged velocities the pattern is quite symmetric. However, considering the south and north ensemble, the lateral asymmetry is as evident as in the other cross-sections, following the same pattern (fig. 6.28). Piep - South (23.03.00) 1.6 1.4
Velocity (m/s)
1.2 1 0.8 0.6 0.4 0.2 0 7
9
11
13
15
17
19
21
Time (hrs.)
Surface Velocity
Bottom Velocity
2
1.25
1
1
0
0.75
7
9
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15
17
19
21
-1
0.5
-2
0.25
-3
Velocity (m/s)
Water level (m)
Deepth-Integrated Velocity
0
Time (hrs.)
W ater level
Vel. cross-sectional. averaged
Piep - North (23.03.00) 1.6 1.4 Velocity (m/s)
1.2 1 0.8 0.6 0.4 0.2 0 7
9
11
13
15
17
19
21
Time (hrs.)
Deepth-Integrated Velocity
Surface Velocity
Bottom Velocity
Figure 6.28: Ebb and flood current pattern in the cross-section C – Piep (spring cycle, 23rd March 2000).
Concerning neap tidal cycles, the same pattern than in spring cycles would be expected, however the velocities evolved are substantially reduced. The measurements in the cross-section A – Norderpiep during 5th December 2000 (neap cycle, fig. 6.29) show an apparent strong asymmetry in the cross-sectional averaged velocities. However, the detailed observation of the water levels shows that the tidal range corresponding to the ebb phase was around 0.35 m greater than the tidal range during the flood. This difference corresponds to approx. 15% of the total range and would explain the differences observed in the crosssectional averaged current velocities. 91
Results and Discussion
Norderpiep - South (05.12.00) 1
Velocity (m/s)
0.8 0.6 0.4 0.2 0 5
7
9
11
13
15
17
19
Time (hrs.)
Deepth-Integrated Velocity
Surface Velocity
Bottom Velocity
2
0.8 0.7 0.6 0.5
0
0.4
5
7
9
11
13
15
17
19 0.3
-1
0.2
Velocity (m/s)
Water level (m)
1
0.1
-2
0
Time (hrs.)
W ater level
Vel. cross-sectional. averaged
Norderpiep - North (05.12.00) 1
Velocity (m/s)
0.8 0.6 0.4 0.2 0 5
7
9
11
13
15
17
19
Time (hrs.)
Deepth-Integrated Velocity
Surface Velocity
Bottom Velocity
Figure 6.29: Ebb and flood current pattern in the cross-section A – Norderpiep (neap cycle, 5th December 2000).
As a result of this difference of the tidal range between the ebb and flood phases, the expected asymmetry favoring flood currents in the south ensemble practically vanished and the ebb-dominance in the north ensemble was incremented (fig. 6.29). The measurements in the cross-section B – Süderpiep were also carried out during 5th December 2000, so the difference in the tidal range can also be visualized in this case. The observed cross-sectionally averaged velocities show the corresponding asymmetric pattern (stronger ebb). Despite to this overall dominance of the ebb for this cross-section in the considered tidal cycle, the lateral asymmetry is present and follows the expected pattern for the north and south ensemble (fig. 6.30). For the middle ensemble however, there is no significant asymmetry between ebb and flood currents. If the depth-integrated velocities are considered, then a gentle difference favoring the ebb currents can be observed (fig. 6.31). 92
Results and Discussion
1.2
Velocity (m/s)
1 0.8 0.6 0.4 0.2 0 5
7
9
11
13
15
17
19
Time (hrs.)
Deepth-Integrated Velocity
Surface Velocity
Bottom Velocity
2
0.8 0.7 0.6 0.5
0
0.4
5
7
9
11
13
15
17
19 0.3
-1
0.2
Velocity (m/s)
Water level (m)
1
0.1
-2
0
Time (hrs.)
Water level
Vel cross-sectional. averaged
Süderpiep - North (05.12.00) 1.2
Velocity (m/s)
1 0.8 0.6 0.4 0.2 0 5
7
9
11
13
15
17
19
Time (hrs.)
Deepth-Integrated Velocity
Surface Velocity
Bottom Velocity
Figure 6.30: Ebb and flood current pattern in the cross-section B – Süderpiep (neap cycle, 5th December 2000). Süderpiep - Middle (05.12.00) 1
Velocity (m/s)
0.8 0.6
Ebb
Flood
0.4 0.2 0 5
7
9
11
13
15
17
19
Time (hrs.) Deepth-Integrated Velocity
Surface Velocity
Bottom Velocity
Figure 6.31: Ebb and flood current pattern in the middle ensemble of the cross-section B – Süderpiep (neap cycle, 5th December 2000).
The following measurements were carried out during the 6th December 2000 in the cross-section C – Piep (fig. 6.32). In this case, no significant differences of the tidal rage 93
Results and Discussion
between the ebb and flood phase were detected, but the cross-sectional averaged velocities show a light ebb-dominance. The lateral asymmetry seems to be evident only in the south ensemble. In the north ensemble, velocities reached during ebb and flood are quite similar. Piep - South (06.12.00) 1
Velocity (m/s)
0.8 0.6 0.4 0.2
0 6
8
10
12
14
16
18
20
Time (hrs.) Deepth-Integrated Velocity
Surface Velocity
Bottom Velocity
2
0.8 0.7 0.6 0.5
0
0.4
6
8
10
12
14
16
18
20 0.3
-1
0.2
Velocity (m/s)
Water level (m)
1
0.1
-2
0
Time (hrs.) W ater level
Vel. cross-sectional. averaged
Piep - North (06.12.00) 1
Velocity (m/s)
0.8 0.6 0.4
0.2 0 6
8
10
12
14
16
18
20
Time (hrs.) Deepth-Integrated Velocity
Surface Velocity
Bottom Velocity
Figure 6.32: Ebb and flood current pattern in the cross-section C – Piep (neap cycle, 6th December 2000).
In an overview, it can be stated that the lateral asymmetry is substantial for all three evaluated cross-sections during spring tidal cycles, when the ebb current velocities used to be approx. 30% higher in the north ensemble and the flood current velocities in the south ensemble used to be also approx. 30% higher. During neap tidal cycles the lateral asymmetry can be also observed, but it is not so marked as during spring cycles. In several cases, even during neap cycles (north and south ensembles in the cross-section B – Süderpiep, south ensemble in the cross-section C – Piep) a 94
Results and Discussion
difference of approx. 30% is still present. A general reduction of current velocities in the order of 40% during neap cycles, in comparison to spring cycles, can be also observed. Correlating the findings of hydrodynamics and morphological changes, it can be assumed that the lateral migration of the channels is associated with the detected lateral asymmetry of the currents in the cross-sectional velocity field. This assumption argues also with the mechanism suggested by AHNERT (1960) to explain the formation of estuarine meanders. Deposition (associated mainly to the flood?) would be favored in summer, especially in the southern part of the cross-sections. Besides, erosion (associated mainly to the ebb?) would be favored in the winter, especially in the northern part of the cross-sections. Furthermore, the lateral asymmetry of the tidal currents explains also the formation of the sandbars observed in the study area. Since the ebb and flood currents are more intense in opposite sides, substantial net sediment transport, in opposite directions, would result in both sides. Besides, there would be a part approx. in the middle of the cross-section where ebb and flood currents are approx. equivalent. Hence, the net bedload sediment transport would be much reduced, resulting in the accumulation of sandy sediments.
6.2.3. General aspects of the wave action As referred in the previous chapters, the wave action is supposed to be substantial only in the outer parts of the study area. Numerous hydrodynamic model simulations carried out for the area (WILKENS, 2000; WILKENS & MAYERLE, 2002; WILKENS, 2003) support this affirmation. Figure 6.33 shows wave heights in the study area calculated by a numerical model, using a conventional setup corresponding to a normal hydrodynamic situation (moderate winds, without storm surge) and input waves from the west, with a height of 2 m and a period of 6 seconds, representing to the most common wave regime in this area. Due to the extensive tidal flats the swell penetration in the area is hindered. However, according to the west – east orientation of the wide tidal channel Süderpiep, waves are partially conveyed through the main channel Piep. During high water levels, wave heights of up to about 0.5 m can be found in the vicinity of Büsum (fig. 6.33a). The orientation of the tidal channel Süderpiep also favor the wave propagation over the tidal flat Bielshövensand during high water levels. This mechanism might be associated with the morphological changes observed in the inner parts of the system, like the evaluated areas of the channel slope across from Büsum (chapter 6.1.4.) and the cross-section Sommerkoog-Steertloch (chapter 6.1.5.). The wave action over the tidal flat Bielshövensand would favor the sand transport to those areas (see also fig. 6.16). 95
Results and Discussion
During low water levels, the wave propagation is reduced, but in the main channel (Piep) wave heights around 0.4 m are still present near Büsum (fig. 6.33a). Wave refraction in the channels might also have an important influence. The simulations indicate that during high water levels the wave height might increase locally along the main channel, probably as a result of wave shoaling and refraction.
Figure 6.33: Simulated wave heights based on an input of waves from west, with 2 m height and 6s period (after WILKENS, 2003).
Analysis of sediment cores taken in the area delivered also interesting information about the role of waves in the evolving of the morphology in the area (RAMLI, 2002). It was found out that muddy sediments are almost absent in the outer sandbanks in the first 3 to 4 m of the cores. It results probably from the more intense wave action, keeping fine particles in suspension even during slack water periods (high water) thus, hindering their deposition (see also fig. 6.34). The wave action in the outer parts of the tidal flats would be also very important for the bedload transport, especially due to resuspention of sand in the outer parts (e.g. Tertius) 96
Results and Discussion
that would get available for the transport by tidal currents. Since the outer parts of the channels tend to be flood-dominated and the wave action in the inner parts is substantially reduced, a net landward bedload sediment transport would result. During storms, wave heights and mean water levels increase and the wave penetration in the entire area is increased as well. In the deeper parts (channels), their action would decrease, but in the tidal flats, the wind driven currents (also waves) might be substantially increased and modify significantly the regular tidal current pattern.
6.3. Seasonality of morphological changes To explain the seasonal cycle, i.e. predominance of erosion during autumn/winter and deposition during spring/summer observed in the analyses of morphological changes (chapter 6.1.), two general mechanisms can be proposed as follows: The first one involves the physical characteristics of the water, which varies distinctly between summer and winter in the study area. Important variations in the water density are caused by changes in the salinity and especially in the water temperature. Due to eventual strong rainfalls or ice formation during the winter, as well as eventual strong evaporation during the summer, the salinity can oscillate in a range of more than 10‰. Besides, the difference of the water temperature in the tidal channels between summer and winter can exceed 20 °C. In the study area the lowest water temperatures are reached in February, which has a monthly average of around 2 °C. The highest water temperatures are reached usually in August, when the monthly average uses to be 17 °C (BECKER, 1998). With the significantly lower water temperatures, the water density is much higher during the winter and the settling velocities of the sediment particles are consequently much lower during the winter. FLEMMING & BARTHOLOMÄ (1997) pointed out that a difference of 15 °C in the water temperature (20 to 5 °C) results in a reduction of around 25% in the settling velocity of particles, due to a higher kinematic viscosity of the water1. An important conclusion based on the analysis of morphological changes in the scope of seasonal cycles is the finding that erosion seems to happen mainly in the transition between summer and winter and not in the winter itself. A maximum of erosion seems to be reached usually in December, at the beginning of the winter. However, it must be considered that in the period of January and February, it was not possible to carry out measurements, due to ice formation and frequent storms. It means that a maximum of erosion could be reached during January or February and would not have been detected. Besides, the maximum density of the
1
Kinematic viscosity of pure water = 1 x 10-6 m2/s (T = 20 °C) and 1.505 x 10-6 m2/s (T = 5 °C). 97
Results and Discussion
water (also kinematic viscosity) is reached at temperatures of about 4 °C (WEAST & ASTLE, 1980), which is also the mean water temperature in the study area during December (BECKER, 1998). The reverse process seems to happen from winter to summer. Important accretion can usually be identified between winter and summer. However, a maximum of accretion is often detected in July-August, around the middle summer, when the water temperature uses to reach its annual maximum. In this context, the mechanism associated with the erosion between summer and winter, as well as deposition from winter to summer, would be mainly related to the seasonal and gradual variation of the kinematic viscosity of the water, as a consequence of the variation in the water temperature. With the gradual decrease of kinematic viscosity of the water, finer sediments will be deposited gradually from winter to summer and in the summer itself. These sediments would be gradually eroded again with the gradual increase of kinematic viscosity of the water from the end of summer to winter. Based on theoretical assumptions and the results of this study, ebb currents are more related to erosion and flood currents more related to deposition in the study area. Studies of DIECKMANN (1985) already argued that ebb channels are associated with erosion (negative sediment transport) and flood channels are associated with positive sediment transport into the tidal flat areas of the Wadden Sea. In the study area (Piep channel system) this conclusion is also supported by the almost absence of direct fluvial transport from the mainland and the placing of the open sea (North Sea) as main source of sediments for the Piep system. This is the basis for the formulation of another factor of second order, which could explain partially the seasonal cycle, also associated with the physical properties of the water masses involved in the sediment transport in the area. During the winter, the coastal water is colder than the North Sea water and the ebb currents would have more capability to transport sediments than the flood currents, since the water leaving the tidal flats during the ebb would have lower temperature (higher kinematic viscosity) than the water that flooded the area in the flood phase (winter). During the summer there is an inversion in this pattern and the North Sea water is usually colder than the coastal water, favoring sediment transport during the flood, because the water flooding the area would have higher kinematic viscosity than the water leaving the area during the ebb phase. Measurements and water circulation simulations carried out in the North Sea (BSH) show a difference in the surface water temperature, between the open sea and the coastline, in 98
Results and Discussion
summer and winter, in the order of 4 to 5 °C. A gradient of around +1.5 °C/50 km from the coast to the open sea in the winter (3rd week December) and of around -1.15 °C/50 km in the summer (3rd week June) can be recognized (LOEWE, 1998). In the scope of the Promorph project, measurements of water temperature and salinity were also carried out. These were combined with current velocity measurements in campaigns every 3 months from 2000 to 2002, mainly in the cross-sections A – Norderpiep, B – Süderpiep and C – Piep. At those cross-sections, so-called CTD (Conductivity, Temperature and Depth) vertical profiles (with resolution of 0.2 m) were carried out approx. every 200 m along the cross-sections covering entire tidal cycles. These measurements shown usually differences in the order of 0.1 – 0.2 °C in the water temperature at the same location between ebb and flood. But a certain vertical stratification and especially the interaction of the water heating – cooling daily cycle with the course of the tide complicates any analysis of the differences in the water temperature, i.e. kinematic viscosity, between ebb and flood phases. WIELAND (1982) and WIELAND
ET AL.
(1984) investigated variations in the water
temperature considering the problem of superimposition of day-night and tidal cycle. The authors found out that between a low water level in the morning (6 to 10 a.m.) and a high water level in the afternoon (2 to 6 p.m.) the water temperature difference is in the order of 0.5 °C for a sunny day (more than 8 sunny hours). For a cloudy day (up to 4 sunny hours) the difference is around 0.32°C. For the opposite situation, i.e. high water level in the morning and low water level in the afternoon, the difference reaches values of about 1.28°C in a sunny day and values of about 0.46 °C in a cloudy day. In this context, it becomes evident the relevancy of the combination of the daily, the tidal and the seasonal cycle in the temperature, i.e. kinematic viscosity, of the water during ebb and flood flow. However, as a result of the slight differences in the water temperature expected for ebb and flood currents, this effect in short- to medium-term sediment transport processes would be nearly negligible. A temperature difference in the order of 1° C would result in a difference in the settling velocity of particles in the order of 1 to 2 %. The second factor to explain the identified seasonal cycle implies that the wave action and the variations in the wave regime during winter and summer would force erosion in the winter, due to an increased wave action and more frequent storms (higher energy level). During the summer, calm weather would result in more sedimentation. It is well known that in supratidal sand shoals (e.g. Trischen and Blauort) at the outer parts of the study area the increased wave energy during the winter (i.e. storms) results in
99
Results and Discussion
erosion (TAUBERT, 1986; KESPER, 1992). The same behavior is expected for the intertidal sandbanks of the outer part (Tertius and D-Steert). However, this is still unsubstantiated. Besides, field observations indicate that the erosional-depositional processes at the outer sandbanks seem to be more related to wave action (RAMLI, 2002). The comparison of sediment cores taken in the outer, central and inner parts of the study area shows for the outer parts almost pure sandy sediments. In contrast, the deposits from the inner parts consist of sand intercalated with clay-silt (cohesive) layers, characteristic for the tidal-dominated sediment transport. Central parts show an intermediary pattern (fig. 6.34).
1.6m MHW
Station KE 8 0.0m
Station KE 2
Station KE 1
Station KE 10 0.0m
0.0m
Station KE 9
water level
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Station KE 7 0.0m
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1.0m 1.0m 2.0m 1.m 1.0m
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1.0m 1.0m 1.0m 3.0m 2.0m 2.0m 2.0m 2.0m
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3.0m
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3.0m 5.0m
3.0m 3.65m 3.5m
3.0m
5.5m
Legend
4.0m 4.0m 3.0m
Massive sand
Wavy bedding
Channel bed deposit
Cross bedding
Interlayer sand-mud
Shell fragment
Laminated sand
Lenticular bedding
Shell fragment-incomplete
Flaser bedding
Mud layer
Bioturbation
4.0m 3.15m
3.51m
Seaward
4.82m
4.75m 4.6m
Landward
Figure 6.34: Description of cores taken in the study area (after RAMLI, 2002).
However, the seasonal variations are more distinct in the deeper parts of the evaluated cross-sections, in depths of about 15 m, usually not reached by the wave basis. Although the area of Sommerkoog-Steertloch is located in a quite protected inner part of the area with respect to the wave action, the seasonal cycle could be clearly identified there.
100
Results and Discussion
It is reliable to propose that if the seasonal cycle of erosion-deposition occurs in different parts (inner and outer parts) of the study area, the main propellant force should be the same at inner and outer parts. So it seems to be unlikely that the seasonal variation in the wave regime is the main cause of that seasonal cycle of morphological changes, because wave action would be really effective in the outer parts. Storms might also result in increased current velocities in the channels due to the storm surge effect, which might produce exceptional water level variations (tidal ranges), with corresponding higher current velocities. This mechanism would act over channels in the whole area, but the evaluation of effects from a storm in October 2002 actually indicates deposition in the channels as a result of a storm, instead of erosion, disqualifying increased storm frequency (energy level) as a cause of increased erosion during autumn and winter for the channels (deep parts) of the study area. In short, storms or general increased wave energy, common in autumn/winter, results in erosion only in quite shallow areas, but results in deposition in deep areas and channels, that are usually deeper than the wave base. However, the seasonal variation in the kinematic viscosity of the water seems to be the most suitable explanation for the observed seasonal cycle of erosion during autumn/winter and predominantly deposition during spring/summer in the channels.
101
Results and Discussion
102
Results and Discussion
7. Reconstruction of paleo-morphologies and tidal conditions during Holocene: An innovative approach 7.1. Reconstruction of paleo-morphologies As already mentioned in the chapter 3, the topography of Pleistocene-Holocene boundary is used as model bathymetry for the simulation of the tidal conditions of about 7,000 y. BP, when the sea level was approximately 10 m lower than today. Based on the geological results we can assume that at this period large scale morphological changes like the grading of the ancient coastline and the deposition of massive intertidal sediment layers did not take place still. So in a first and simplified approach we used the Pleistocene surface for the model bathymetry. For a reconstruction of the Pleistocene surface all existing data from previous studies were used. Due to the kind cooperation of Dr. Reiner Schmidt (LANU – Schleswig-Holstein), the descriptions of a set of more than 30 cores collected in the area between 1936 and 1987 reaching the Pleistocene were available for this study. The information from the cores was essential for the validation and complementation of the seismic data. Helpful information about the depth of the limit Pleistocene – Holocene, especially in the area nowadays covered by the tidal flats, were extracted from DITTMER (1938, 1952) and FISCHER (1955). In the region of the river Eider’s mouth, the detailed work of RUCK (1969), combining shallow reflection seismic and cores, enabled a perfect recognition of the limit Pleistocene – Holocene and also the depth and thickness of the Dithmarscher Klei for that area. TIETZE (1983) and RUPRECHT (1999) carried out seismic surveys and coring in the area adjacent to the mouth of the Eider River, investigating its course (the so-called Ur-Eider) before the last transgression and found two different Pleistocene channels in the area. Both could represent the old course of the Eider. The work of HUMMEL & CORDES (1969) was essential for the choice of the most adequate channel corresponding to the course of that river just before and at the early phase of the transgression. This work, as well as the studies of LINKE (1979) in the area of Cuxhaven, and MENKE (1976) in Dithmarschen, provided important information about the formation of the coastal line and beaches as the sea level reached the moraines. It is also the case of the studies from HOFFMANN (1998) and MEIER (2001), which enabled more detailed reconstruction of the “Geest” border (Pleistocene reached by the sea, i.e. hinterland), using geological and archeological data, respectively. In the area of the East Friesian Islands and the river Weser, the works of GWINNER (1954), LANG (1959) and especially the extensive work of STREIF (1990) were used to reconstruct the paleo-morphology. SCHMIDT (1975) carried out a detailed work in the inner 103
Results and Discussion
Meldorf Bight area determining Pleistocene surface as well. Here it was even possible to identify some small drainage channels from the moraines running to west and southwest. FIGGE (1980) in the effort of map the so-called Elbe-Urstromtal delivered important information about the thickness of the Holocene layer and the depth of Pleistocene in a large area between the island Helgoland and the so-called Weiße Bank, about 160 km northwest of Helgoland. In this area it was possible to reconstruct the course of the Elbe-Urstromtal (ancient Elbe) and the Ur-Eider (ancient Eider). From the vicinities of the Elbe’s mouth to the area near Helgoland Island, the gas effect hindered an appropriate mapping of the sub-bottom. Further work carried out by ZEILER ET AL. (2000) in the attempt of mapping the thickness of mobile sands in the North Sea, solve this problem partially with the combination of several cores in the area. In this way ZEILER
ET AL.
(2000) provided also information about the
Holocene – Pleistocene limit. A general overview of the location and coverage of each study mentioned above can be seen in figure 7.1. These data, as well as relevant information from the sedimentological chart of the German Bight (FIGGE, 1981) and available bathymetric charts, were combined in a digital elevation model (DEM) with a grid spacing of 600 m. In this DEM the HolocenePleistocene boundary in a first approach is presented as a relative smooth plain with some rudimentary major channels, covering the whole inner German Bight (fig. 7.2).
Figure 7.1: Location of considered previous studies (a), cores and measurements (b).
Furthermore, two other intermediate topographies were developed representing intermediate stages around 5,000 and 4,000 y. BP (fig. 7.2). These two topographies have a very simplified character and are mainly based on theoretical assumptions and indications of disposable stratigraphic information, also supported by hypsometric calculations. Because of the spatial scale considered (grid spacing 600 m), local and small uncertainties in the reconstructed morphologies would not compromise the results of the simulations. 104
Results and Discussion
In contrast to the reconstructed morphology of the stage of approx. 7,000 y. BP, the developed morphologies for stages around 5,000 and 4,000 y. BP already present a relative fill of Holocene sediments. The period between 7,000 and 5,000 y. BP was characterised by a deceleration of the sea-level rise and a supposed relative small tidal range, resulting first in a small accumulation of sand and further in the formation of the Dithmarscher Klei in the study area. The developed morphology for the 5,000 y. BP stage corresponds to the end of this phase. The period between 5,000 and 4,000 y. BP was characterised by a slow rising sealevel, as well as by a reduced tidal range. In this situation, currents and waves would have transported more sandy sediments to the coast. The slow rising sea-level also enabled the reworking of the sediments of the hinterland due to wave action and leading further to the formation of beach ridges and barrier-like landforms. The developed morphology for the period around 4,000 y. BP corresponds to the phase where this barrier system was fully developed in Dithmarschen area.
Figure 7.2: Reconstructed morphologies corresponding to the periods around 7,000 y. BP (A), 5,000 y. BP (B) and 4,000 y. BP (C) and sea-levels around – 10 m (A), –5 m (B) and –3 m (C), compared to the present morphology (D). 105
Results and Discussion
7.2. Reconstruction of early tidal conditions As explained in chapter 3, two numerical model settings were used to reconstruct ancient tidal conditions in the study area. The first model setting uses only the large-scale model (Continental Shelf Model - CSM), which covers the northwest European continental shelf area with grid spacing of about 9 km (VERBOOM ET AL., 1992). The second model setting uses a nested model. Here the CSM is nested with a model of the German Bight area, the socalled German Bight Model (GBM) with grid spacing ranging from 0.5 km to 1.9 km. Initial simulations were carried out using the large-scale model (CSM) and the current morphology with four different sea-levels corresponding to -15, -10, -5 and –3 m. Further simulations were carried out using the reconstructed morphologies with the corresponding sea-levels (-10, -5 and –3 m) using both model settings, the CSM and the nested model. Figure 7.3 shows the results of these simulations using the large-scale model (CSM) and the current morphology with different sea-levels for two points (Spiekeroog and Cuxhaven) to enable comparisons with the results of previous studies. The results show a clear increase in the tidal range following the sea-level rise, which agrees quite well with the previous studies from POST (1976) and especially FRANKEN (1987). These show a steep increase of the tidal range accompanying the steep rise of the sea-level, especially before 7,000 y. BP. After this period the sea-level, as well as the tidal range, increases gently. However, the simulations using the GBM nested in the CSM and the developed morphologies show a quite different pattern. The calculated tidal ranges are in general reduced substantially, compared to the results from the use of the CSM alone, using the present morphology. Also the main increase of the tidal range is indicated to be after 5,000 y. BP (fig. 7.4). Using the developed morphologies and the GBM the results indicate almost the same tidal range for the morphologies of 7,000 y. BP (-10 m) and 5,000 y. BP (-5 m). Only for the morphology corresponding to 4,000 y. BP (-3 m) a substantial increase in the tidal range can be observed. At first, these results indicate the relevance of the morphology for the tidal regime in areas near the coastline. Results using a finer grid spacing (GBM) and approximate topographies of the corresponding sea-levels are expected to be more reliable. In fact, the results obtained with the simulations using the GBM and the developed morphologies harmonise better with the disposable information about the Holocene evolution of the Dithmarschen area. 106
Results and Discussion
Furthermore, with grid cells of around 9 km, the CSM is supposed to reduce the effect of the coastal morphology in the tidal regime substantially. However, the GBM, with a grid spacing ranging from 0.5 km to 1.9 km, seems to have the capability to reproduce the expected regional effects of the morphology in the tidal range. An almost constant and small tidal range for the phase corresponding to the formation of the barrier-like system in Dithmarschen (6,000 to 4,000 y. BP) seems to be quite reasonable. Besides, about 2,000 y. BP the Dithmarschen area was already characterised by relative wide tidal flats (HUMMEL & CORDES, 1969; MENKE, 1976; MEIER, 2001). In this context, the main phase of tidal flats formation in the region must be placed between 4,000 and 2,000 y. BP. It is in good agreement with the indication from the simulations with developed morphologies that the main phase of increase of tidal range started between 5,000 and 4,000 y. BP in Dithmarschen. 3.5
-3m
2.5
-10m -15m
2 1.5 1 0.5
Tida l ra nge (m )
3
-5m
0 -16
-14
-12
-10
-8
-6
-4
-2
0
S e a -le ve l (m )
P os t (1976) - Cux haven
Frank en (1987) - S piek eroog
Cux haven (this s tudy )
S piek eroog (this s tudy )
3.5
2.5 2 1.5 1 0.5
Tida l ra nge (m )
3
0 800 0
700 0
600 0
500 0
400 0
300 0
200 0
100 0
0
Ye a rs BP
P os t (1976) - Cux haven
Frank en (1987) - S piek eroog
Cux haven (this s tudy )
S piek eroog (this s tudy )
Figure 7.3: Calculated tidal ranges at selected points using the CSM over the current morphology with respect to the sea-level and the corresponding age.
107
Results and Discussion
3.5
2.5
-3m -10m
2
-5m
1.5 1 0.5
Tida l ra nge (m )
3
0 -12
-10
-8
-6
-4
-2
0
S e a -le ve l (m )
B üs um
S piek eroog
3.5
2.5 2 1.5 1
Tida l ra nge (m )
3
0.5 0 8000
7000 6000
5000 4000
3000 2000
1000
0
Ye a rs BP
B üs um
S piek eroog
Figure 7.4: The tidal ranges using the modified GBM (nested in the CSM) with the reconstructed morphologies.
Since the area of Büsum becomes to shallow using the current morphology interpolated with the CSM and lower sea-levels, another point was chosen for a more detailed comparison of results using one morphology (the present one) and varying the sea-level with the results of simulations considering the developed morphologies. The location of Spiekeroog was selected, enabling also correlation with previous studies. Figure 7.5 shows in case of the use of the same morphology (1996) for all simulations (CSM) that the increase of tidal range, as well as maximum current velocities (ebb and flood) are proportional and directly correlated to the sea-level increase. Using the developed ancient morphologies (GBM) the relationship between ebb and flood current velocities is similar to the results of using the same morphology and varying only the sea-level (CSM), however, due to the formation of the barrier island system of the East Frisian Islands (including Spiekeroog) both, ebb and flood current velocities seem to be reduced and increase again with the further increase of tidal range. Results from the CSM using the same morphology indicate a continuous and almost linear increase of current velocities. 108
Results and Discussion
0.5
-10m -15m
-5m
0.4
-3m
1.5
0.3
1
0.2
0.5
0.1
0 7000
6000
5000
4000
GB M - N ested m o d el, reco n st. m o rp h o lo g ies
0.4
-10m
1.5
-5m
-3m
0.3
1
0.2
0.5
0.1
0
3000
0
8000
7000
6000
Ye a rs BP
flood m ax . vel.
0.6 0.5
2
0
8000
3
2.5 Tida l ra nge (m )
Tida l ra nge (m )
2.5 2
b)
0.6
V e l m a g. (m /s)
C S M - p resen t m o rp h o lo g y (1996)
3
V e l m a g. (m /s)
a)
5000
4000
3000
Ye a rs BP
ebb m ax . vel.
tidal range
flood m ax . vel.
ebb m ax . vel.
tidal range
Figure 7.5: Tidal range and maximum tidal currents for the area of Spiekeroog using the same morphology with different sea-levels (a) and using the developed ancient morphologies (b). B ü su m
1.5
0.6
-3m -10m
-5m
0.45
1
0.3
0.5
0.15
0 8000
Ve l. mag . (m/s)
Tid al ran g e (m)
2
0 7000
6000
5000
4000
3000
Ye a rs BP
flood m ax. vel.
ebb m ax. vel.
tidal range
Figure 7.6: Tidal ranges and corresponding maximum current velocities for the location Büsum using the developed ancient morphologies.
For the area of Dithmarschen (Büsum) the results regarding tidal range are very similar to the results of the location Spiekeroog (see also fig. 7.4). However, at Büsum a gradual reduction of the early overall flood-dominance due to a relative increase of the ebbcurrent velocities can be observed, parallel to the gradual increase of tidal range and absolute maximum current velocities (figs. 7.6 and 7.7). The reduction of the flood-dominance due to a relative increase of the ebb-current velocities would be more related to the increase of sediment fill of the basin after 7,000 y. BP, especially after 5,000 y. BP. In this context, the results of the simulations from the Dithmarschen area are in agreement with the study of BOON & BYRNE (1981), which postulates that basin infilling favours ebb-dominance. Two main factors can be referred to explain the increase of the tidal range associated with the sea-level rise: 1. The increasing water depth propitiated a better penetration of the tidal signal from North Atlantic into the North Sea (VAN
DER
MOLEN & VAN DIJCK, 2000)
and: 2. caused a general reduction of the friction effect in the tidal propagation in the North Sea as well (POST, 1976). For the German Bight the reduction of the friction effect seems to 109
Results and Discussion
be more substantial in the increase in the tidal range, compared with adjacent areas, e.g. southernmost North Sea (POST, 1976). -10m (around 7,000 B P )
0.9
0.3
0.6
0.2
0.3
0.1
0
0 552
576
600
624
648
672
696
-0.6
-0.2
-0.9
-0.3
-1.2
-0.4 water level
veloc ity
Wate r le v e l (m)
Time (h r s .)
-5m (around 5,000 B P )
1.2
b)
0.4
0.9
0.3
0.6
0.2
0.3
0.1
0 -0.3 528
0 552
576
600
624
648
672
696
-0.2
-0.9
-0.3 -0.4 water level
velocity
Time (h r s .)
-3m (around 4,000 B P )
1.2
Wate r le v e l (m)
720-0.1
-0.6
-1.2
c)
720 -0.1
0.4
0.9
0.3
0.6
0.2
0.3
0.1
0 -0.3 528
V e l. m a g. (m /s)
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V e l. m a g. (m /s)
0.4
0 552
576
600
624
648
672
696
720-0.1
-0.6
-0.2
-0.9
-0.3
-1.2
Ve l. mag. (m/s)
a)
Wate r le v e l (m)
1.2
-0.4 water level
velocity
Time (h r s .)
Figure 7.7: Tidal course for the location Büsum using the reconstructed ancient morphologies.
Despite the reduction of the friction effect and a better penetration of the tidal signal from the North Atlantic into the North Sea, the results using the developed ancient morphologies and the German Bight Model (GBM) indicate that the regional and local effects of the morphology in the tidal flow might have effects of primary order. An elevation of 5 meters in the sea-level (-10 to –5 m) caused only very slightly variation in the tidal range and current velocities. However, an elevation of 2 meters (-5 to –3 m) resulted in a more significant increase of the tidal range and tidal currents. This can be explained by the fact that the used morphologies of 7,000 y. BP (-10 m) and 5,000 y. BP (-5 m) do not differ substantially in the general aspect. The morphology of 5,000 y. BP in the region of Dithmarschen is characterised by the deposition of an almost regular sediment sheet, which would have compensated the elevation of the sea-level in the area and result in the constancy of almost the same water depth in the region of Dithmarschen, 110
Results and Discussion
comparing the developed morphologies of 7,000 and 5,000 y. BP. This would have resulted in the maintenance of elevated friction effect in the study area and consequently stable and low tidal range. The sea-level difference of 2 meters between the morphologies of 5,000 and 4,000 y. BP would be more significant for the area of Dithmarschen, because it represents an effective increase of the water depth in the off and nearshore area of Dithmarschen, once no significant sedimentation in the off and nearshore area of the region is assumed for that period. Furthermore, the period between 5,000 and 4,000 y. BP is characterised by important modification in the coastal configuration of the region, because of the development of the beach ridge system. Based on the results of the developed simulations it can be assumed that for the coastal area of the inner German Bight, the evolution of the tidal range during the Holocene was at first controlled by the overall increase of the tidal range in the North Sea, but especially in the nearshore area of the Dithmarschen area the tidal conditions were substantially influenced by the local and regional morphology, which was also continuously evolving and interacting with the changing hydrodynamic conditions during the Holocene.
111
Results and Discussion
112
Summary and Conclusions
8. Summary and Conclusions 8.1. Summary and correlation of the morphological processes of the different evaluated temporal-spatial scales Based on the previous studies and the findings of the present study, it can be concluded that the Holocene evolution of the study area was characterised by an initial phase of inundation of reaches from hinterland and by a relative moderate influence of tides, compared to the tide-dominance in the hydrodynamic processes that characterise the study area today. According to the results of the analyses combining numerical modelling in morphodynamics and geological findings in the Dithmarschen area, the initial phase of inundation of the area can be set around 8,500 to 7,000 y. BP. The rapid sea-level rise during this period did not result in a significant sedimentation in the off- and nearshore of the study area. As a consequence, the water depth increased rapidly, reaching values of about 15 to 20 meters in the area of Büsum. In this first phase, the friction effect in the water flow decreased rapidly with the increasing water depth, at the same time that the penetration of the tidal signal from the North Atlantic in the North Sea was improved (VAN
DER
MOLEN & VAN DIJCK, 2000). The tidal
range increased and reached values of around 1.5 m about 7,000 y. BP, in the area occupied by Büsum nowadays (figs. 7.6 and 7.7). In this period the sea-level rise started to decelerate substantially at the same time the sedimentation rates increased distinctly. These two characteristics mark the beginning of a second phase. In this second phase the sea-level rise decelerated from rates of about 2 m/100 years to rates of about 0.2 m/100 years in the inner German Bight, between 7,000 and 6,000 y. BP. Between 6,000 and 5,000 y BP the sea-level is supposed to have been almost stable (STREIF & KÖSTER, 1978; STREIF, 1986). After this period, the sedimentation rates started to surpass the creation of accommodation space in the study area, because of the deceleration of the sealevel rise, as happen in the coastal areas of Belgium and The Netherlands (BEETS & VAN DER SPECK, 2000). This resulted in a maintenance or even reduction of the water depths in the nearshore area of Dithmarschen, that in turn resulted in conservation or even increase of the friction effect in the tidal flow and result in a subsequent relative stabilization of the tidal range. A third phase started about 5,000 y. BP, with a moderate sea-level rise, but important sedimentation and especially with important geomorphological changes in the coastal area of 113
Summary and Conclusions
Dithmarschen. The results of numerical simulations indicate that a further substantial increase of the tidal range started in this period. This is supposed to be related to the important modification of the geomorphological configuration of the region during this period, which includes, among others, the formation of a beach ridge system. The next phase would be marked by the rapid formation of tidal flats in the region, between 4,000 and 2,000 y. BP, associated with a rapid increase of the tidal range. The morphological modifications of the basin due to the increasing tidal range would have acted in a synergetic way resulting in further increase of the tidal range. The last phase of the Dithmarschen coastal evolution is characterized by the continuous modification of the existing tidal flats and channels. These modifications have been increased later by the human activities, especially by land reclamation. Due to the findings in geology, the results of the simulations and the actual morphodynamics of the Dithmarschen area, its long- and medium-term morphodynamic evolution can be correlated. It was demonstrated by the simulations that increased infilling levels of the basin reduce flood-dominance and may lead to ebb-dominance, as described by previous studies (BOON & BYRNE, 1981; SPEER & AUBREY, 1985; AUBREY & SPEER, 1985). The results of the simulation corresponding to the situation around 4,000 y. BP shows almost equivalent ebb and flood current velocities for the location of Büsum (figs. 7.6 and 7.7). Recent measurements of currents and tides at Büsum indicate a tendency for weak ebbdominance. Once the morphodynamic evolution of the area from 4,000 y. BP up to now would be characterized by a gradual process of infilling of the basin and slow rising sea-level, a gradual evolution of the tidal range, as well as only small variations in the differences between ebb and flood current velocities, are expected for that period. Further, the results of measurements and analyses on short- to medium-term morphodynamics indicate a weak ebb-dominance in the inner parts of the Piep channel system, but the outer inlets seem to be flood-dominated. This situation might indicate that the system is in a dynamic equilibrium, which seems to be very sensitive to variations of the basin geometry and sea-level (see also results of medium-term morphological changes). The analysis of water levels (tides) at Büsum over 3 years demonstrates that moderate elevations in the mean water level in the order of 0.3 m result in the vanishing of the weak ebb-dominance usually observed there, showing the important effect of the basin geometry in the hydrodynamic as well. The interaction between sea-level rise (accommodation space generation), sediment supply and the morphology is expressed in the morphodynamics of the area. Assuming that 114
Summary and Conclusions
there is, at any time, a satisfactory sediment source, an accommodation space generation, resulting from a sea-level rise, would change the basin geometry and would favor flooddominance, improving the further infilling of the basin, which would head for a new equilibrium, which might be represented by a situation like found today in the Piep channel system, with weak ebb-dominated inner parts and weak flood-dominated outer inlets. In the modern evolution of the area, new aspects were introduced. These are mainly represented by large-scale land reclamation projects since the Middle Ages, the supposed ongoing sea-level rise and the fact referred by SPIEGEL (1997) that the existing external sediment sources would not be able to compensate the accommodation space resulting from the sea-level rise. This scenario would implicate that the system has to respond to the modifications of the hydrodynamic regime (sea-level rise, favor of flood-dominance) with an internal reorganization, which takes place due to important morphological changes. Land reclamation projects, like that carried out in the study area between 1972 and 1979, cut off proportionally much more inter- and supratidal areas than subtidal areas, representing a relative reduction of the sediment volume of the basin. This can be approached as a weak rejuvenation of the system (accommodation space generation) that would favor flood-dominance. Besides, it represents at the same time a reduction of the tidal prism. A sea-level rise would also act favoring flood-dominance, since it represents a generation of accommodation space to be filled with sediment (BOON & BYRNE, 1981). SPIEGEL (1997) already discussed the eventual morphological changes that might be generated by a rising sea-level in the tidal basins of the Wadden Sea. The author concluded that the sediment deficit generated by an elevation of high water levels (for instance sea-level rise) in the order of values found by FÜHRBÖTER & JENSEN (1985) could not be supplied by an external sediment source. The authors, based on water level registers between 1934 and 1983, found out an elevation in the order of 0.325 m/100 years. This would represent an elevation in the order of 0.5 m since the elaboration of the first measured nautical chart prepared between 1838 (measurements) and 1846 (publication) (fig. 4.6). Comparing the chart of 1846 with the present morphology (1996) the landward displacement of the sea border of the tidal flats is evident (chapter 4). This process was already referred by SPIEGEL (1997) as a possible adjustment strategy of the tidal basins of the region to a sea-level rise. This would be expected especially considering a relative small sediment volume in the basin, compared to the water volume and the absence of an appropriate external sediment supply, as the case of the most tidal basins in the region SPIEGEL (1997). 115
Summary and Conclusions
The studies of DIECKMANN (1985) and DIECKMANN ET AL. (1987) also indicate an area reduction of the tidal flats in the Wadden Sea. This reduction was referred as a consequence of sea-level rise, associated with the fact that the shoreline is fixed by dikes. Since the system as a whole cannot migrate landwards as expected for a slow and continuos transgression, substantial morphological changes would result in the tidal basins (coastal squeeze). Beside the reduction in area due to landward displacement of the sea border of the tidal flats (migration of outer sandbanks, barrier island erosion) the deepening of channels by erosion (channel incision) is often cited as an expected morphological “reaction” to the sea-level rise (DIECKMANN ET AL., 1987; SPIEGEL, 1997). In this context, the investigated consolidated mud layer (Dithmarscher Klei) plays a very relevant role, because of its effect hindering the deepening of the main and deeper channels. In this way, a compensation of sea-level rise by erosion of subtidal areas to supply sediments for the intertidal areas, as proposed by SPIEGEL (1997), cannot take place due to deepening of channels. The characteristic deep channels of the study area would be a consequence of several interconnected factors. A relative ebb-dominance in the inner parts of the tidal flats of the study area is supposed to take place since the tidal basin reached a mature stage (sediment filled basin) that, in turn, might be the case for the period around 2,000 y. BP. This ebbdominance would have favoured channel incision since then. The continuous landward migration of sandbanks was for sure an important process over the last 2 – 4 millennia. This leads to channel constriction and subsequently deepening of channels. It can be supposed that channels would be even much more incised and deeper, if the Dithmarscher Klei would not hinder this process. The two main trends of observed morphological changes, i.e. seasonal cycle and lateral migration of channels, are also supposed to be ongoing since long periods. The variations in the kinematic viscosity, resulting from the seasonal variations of water temperature, as well as an increased energy level during the winter, represent a constant since the North Sea waters start to inundate the study area in the early Holocene. Hence, the seasonal cycle must be also ongoing since the early Holocene. The lateral migration of channels is mainly attributed to the lateral asymmetry of the tidal currents in the channels that, in turn, are more related to the Coriolis effect combined with the restriction of the tidal flow in channels. In this context, the lateral asymmetry of the flow and subsequently lateral migration should be ongoing since the channels themselves exist.
116
Summary and Conclusions
8.2. Conclusions Due to the periodic measurements of morphology and hydrodynamic during the last 3 years, it was possible evaluate the short-term morphological changes in the Piep tidal channel system. These results were complemented by the analyses of bathymetric measurements from the last 30 years, carried out by the German Federal Agency for Navigation and Hydrography (BSH). The results delivered very interesting results about the morphodynamics of the study area. It was possible to identify the main morphological changes and morphodynamic trends that are taken place and, due to the study of the hydrodynamics, the mechanisms forcing the observed changes. Two main trends of morphological changes were identified. These comprise a lateral migration of channels and a seasonal cycle of predominant erosion in the autumn to winter and predominant deposition in the spring to summer. The lateral migration of the channels can be referred as the major general morphological change for the Dithmarschen area. This process represents the estuarine meandering, as referred by AHNERT (1960), where flow segmentation in the channels associated with Coriolis effect results in the formation of ebb and flood branches. The flow in both branches is subject to centrifugal forces, resulting in the formation of bends. Particularities of the tidal flat area of Dithmarschen modify distinctly this process. The absence of lateral restriction for the tidal channels and tidal asymmetry are, in this context, very important. The analyses of current velocities and water levels in three different cross-sections of the main channels of the Piep system (A – Norderpiep and B – Süderpiep in the outer part; C – Piep in the inner part) over entire tidal cycles in different situations enabled the recognition of the characteristics of the hydrodynamic regime in the study area. The results show a similar pattern for all evaluated cross-sections, regarding to current velocities. The max. crosssectional averaged current velocities during spring tidal cycles are usually between 1 and 1.2 m/s, while during neap tidal cycles they are between 0.6 and 0.8 m/s. Measured tidal ranges are in the order of 3.5 to 3.8 m for spring and in the order of 2.6 to 2.8 m for neap tidal cycles (despite storm surges). No important differences between water levels or phase lags are referred between the outer (Norderpiep and Süderpiep) and the inner parts, e.g. Piep/Büsum (BSH – German Federal Agency for Navigation and Hydrography). However, the analysis of water levels carried out in the present study shows differences in the ebb and flood duration for the three areas, where in the outer parts the flood phase tends to be shorter than the ebb phase, while in the inner part the ebb phase tends to be shorter. 117
Summary and Conclusions
The general current pattern follows the pattern of tidal phase’s duration, where the max. current velocities tend to be reached in the shorter tidal phase, i.e. flood in the outer parts and ebb phase in the inner parts. Based on the obtained results comparing neap and spring cycles, there is also a trend for smaller differences between ebb and flood duration for the Norderpiep and for the Süderpiep during neap cycles. For Büsum (Piep) the differences are bigger during neap cycles. It could be also concluded that for all three tidal gauges the ebb phase is favored in the summer and during neap cycles, while flood-dominance is favored during in the winter and during spring tides. It could be also determined that a substantial lateral distortion (asymmetry) in the tidal flow takes place in all evaluated cross-sections. This lateral asymmetry is attributed to the Coriolis effect, in connection with the constriction of tidal flow in the channels. The flood currents are more associated with positive sediment transport (deposition) and the ebb currents with negative sediment transport (erosion). This pattern would favor the lateral migration to the ebb-side. Furthermore, ebb- or flood-dominance might also lead to the predominance of ebb or flood bends. In the case of the inner parts of the area, ebb-dominance in the tidal flow could be determined and due to the analyses of the morphological changes it could be determined that, in fact, lateral migration of the channels is widely predominant to the ebb-side. The seasonal erosion – sedimentation cycle observed in the channels would be mainly associated with the seasonal variation in the kinematic viscosity of the water mass involved in the tidal flow, resulting from important differences in the average water temperature during summer and winter. An increased energy level resulting from a more intense wind and wave action during the autumn and winter seems to have a secondary role in the seasonal cycle, regarding to the channels. Measurements of channel cross-sections before and after a storm have shown that increased wave action result in fact in substantial deposition in the channels, which seem to be deeper enough to prevent erosion due to wave action. During storms, sediment would be eroded from shallow parts and transported / deposited in the channels, as indicated in the present study. Furthermore, it can be concluded that the two main identified processes of morphological changes are closely related. In general, the lateral migration of the main channels is the result of combination of tidal asymmetry with the seasonal cycle in terms of substantial differences in the kinematic viscosity of the water between summer and winter. Erosion prevails in the winter, especially in the flood-side, while deposition prevails in the summer, especially in the ebb-side of the channels, resulting in their general lateral migration. 118
Summary and Conclusions
Geological measurements involving shallow reflection seismic, side-scan sonar imagery and cores, enabled the study the Holocene stratigraphy of the Dithmarschen area. One of the most important results of this topic of the present study was the mapping of the widespread consolidated silt-clay layer, namely the Dithmarscher Klei, which represents an important tool for the study of the morphological-sedimentological evolution of the Dithmarschen area. Besides, the combination of the mapping of the Dithmarscher Klei with the analyses of periodic bathymetric measurements, enabled the establishment of the role of that layer in the morphological changes in the study area, hindering erosion (channel incision) that, in turn, affect other ongoing morphological changes like lateral migration of channels, erosion in the winter and migration of outer sandbanks. The landward migration of the outer sandbanks represents also an important general morphological change. It represents in medium- to long-term scales an important coarse sediment source for the tidal flats of Dithmarschen. Impressive landward migration rates of 130 m/year (northern part of Tertius) and 56 m/year (D-Steert) were obtained for the period from 1977 to 1999. Based on the geological findings of the present study, combined with modern numerical modeling in morphodynamics, it was possible to reconstruct partially the Holocene evolution of the area. The obtained geological information was used to develop ancient morphological stages of the Holocene evolution of the area, that were further use in numerical models to simulate the corresponding ancient tidal conditions. Based on the extensive measurements involving the present hydrodynamic conditions the used numerical model was setup, validated and calibrated, and further adapted to the ancient morphologies. This approach resulted in a reliable reconstruction of tidal conditions in the area. The results show that in association with the lower sea-levels during the early Holocene, the tidal range in the area was much more reduced than today. With the sea-level rise, the tidal range has increased substantially. The results of the numerical simulations show that for a period between 7,000 to 5,000 y. BP the calculated tidal range at the observation point Büsum reached values of about 1.5 m. The formation of a beach ridge system in the study area is also attributed to this period (DITTMER, 1938, 1952). A stable tidal range of about 1.5 m, in contrast to the tidal range of about 3.2 m found today in the area, would be in good agreement with this geomorphological prospect, because a reduced importance of the tides in the hydrodynamic resulted in a relative
119
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increase of importance of the wave regime for the geomorphology of the area, supporting the formation of beach ridges for that period. For the following phase of the Holocene evolution of the area, the simulation results indicate that the tidal range increased rapidly. This also agrees with the observed stratigraphic features of the Dithmarschen, where tidal flat sediments and structures seem to have been deposited and formed after 5,000 y. BP (HUMMEL & CORDES, 1969; MENKE, 1976). For the area of Dithmarschen (Büsum) a gradual reduction of the early overall flooddominance due to a relative increase of the ebb-current velocities can be observed, parallel to the gradual increase of tidal range and absolute maximum current velocities. The reduction of the flood-dominance due to a relative increase of the ebb-current velocities would be more related to the increase of sediment fill of the basin after 7,000 y. BP, especially after 5,000 y. BP. In this context, the results of the simulations from the Dithmarschen area are in agreement with the study of BOON & BYRNE (1981), which postulates that basin infilling favours ebb-dominance. Furthermore, simulations demonstrate that the morphology has a very important effect in the tidal regime in the coastal area. Simulations using a large-scale model, without the corresponding morphological changes, indicate for the inner German Bight by a rising sealevel, a gradual increase of tidal range, which is directly proportional to the sea-level rise. These results are very similar to the results from previous studies (POST, 1976; FRANKEN, 1987). Using a refined model with the corresponding morphological changes for each simulated sea-level, the results are quite different, showing the effect of the morphology in the tidal regime (tidal range and tidal currents). With the analysis of data from historical sources, as well as numerous previous studies, it was possible to evaluate the medium- to long-term evolution of the coastal area of Dithmarschen and correlate the morphodynamic processes involved in the different timespace scales, delivering also a good overview of the effects of long-term land reclamation in the morphodynamic evolution of the study area.
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8.3. Some recommendations for further work To design and carry out measurements focused in the determination of water temperature and salinity, i.e. kinematic viscosity of the water, aiming the investigation of differences between ebb and flood flow, during winter and summer, also comparing outer and inner parts. To continue with periodic bathymetric measurements of the areas evaluated in the present study, aiming to get information about several summers and winters to confirm the apparent seasonal cycle. Carry out bathymetric measurements with the specific objective of investigate the effects of storms on inter and especially subtidal areas, as well as their recovery. To carry on the study of hydrodynamic conditions in the German Bight by combining geology and numerical models on hydrodynamics.
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References
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130
Appendices
10. Appendices
131
Appendices
132
Appendices
10.1 Supplementary results on current velocities from the further measurements of 2000
P iep - S ou th (14.03.00) 1.6
Velocity (m /s)
1.4 1.2 1 0.8 0.6 0.4 0.2 0 6
8
10
12
14
16
18
Tim e (hrs .)
S urfac e V eloc ity
B ottom V eloc ity
3
1.5
2
1.2
1
0.9
0
0.6
6
8
10
12
14
16
18
-1
0.3
-2
V e locity (m /s)
W a te r le ve l (m )
Deepth-Integrated V eloc ity
0
Tim e (hrs.)
W ater level
V el. c ros s -s ec tional. averaged
P iep - N o rth (14.03.00) 1.8
V e locity (m /s)
1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0 6
8
10
12
14
16
18
Tim e (hrs.)
Deepth-Integrated V eloc ity
S urfac e V eloc ity
B ottom V eloc ity
App. a: Current velocities at the cross-section C - Piep at 14th March 2000.
a
Appendices
N o rd erp iep - S o u th (16.03.00) 1.4
V e locity (m /s)
1.2 1 0.8 0.6 0.4 0.2 0 7
9
11
13
15
17
19
21
Tim e (hrs.)
S urfac e V eloc ity
B ottom V eloc ity
2
1.2
1
0.9
0
0.6
7
9
11
13
15
17
19
21
-1
V e locity (m /s )
W a te r le ve l (m )
Deepth-Integrated V eloc ity
0.3
-2
0
Tim e (hrs.)
W ater level
vel. cross -sectional. averaged
N o rd erp iep - N o rth (16.03.00) 1.6
V e locity (m /s)
1.4 1.2 1 0.8 0.6 0.4 0.2 0 7
9
11
13
15
17
19
21
Tim e (hrs.)
Deepth-Integrated V eloc ity
S urfac e V eloc ity
B ottom V eloc ity
App. b: Current velocities at the cross-section A - Norderpiep at 16th March 2000.
b
Appendices
N o rd erp iep - S o u th (05.06.00) 1.8 1.6 V e locity (m /s)
1.4 1.2 1 0.8 0.6 0.4 0.2 0 8
10
12
14
16
18
20
22
Tim e (hrs.)
S urfac e V elocity
B ottom V elocity
2
1.5
1
1.2
0
0.9
8
10
12
14
16
18
20
22
-1
0.6
-2
0.3
-3
0
Tim e (hrs.)
W ater level
V e locity (m /s )
W a te r le ve l (m )
Deepth-Integrated V elocity
vel. c ros s-s ec tional. averaged
N o rd erp iep - N o rth (05.06.00) 2 1.8 V e locity (m /s)
1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0 8
10
12
14
16
18
20
22
Tim e (hrs.)
Deepth-Integrated V eloc ity
S urfac e V elocity
B ottom V elocity
App. c: Current velocities at the cross-section A - Norderpiep at 05th June 2000.
c
Appendices
S ü d erp iep - S o u th (05.06.00) 1.6
Ve locity (m /s)
1.4 1.2 1 0.8 0.6 0.4 0.2 0 9
11
13
15
17
19
21
T im e (hrs.)
S urfac e V eloc ity
B ottom V elocity
2
1.5
1
1.2
0
0.9
9
11
13
15
17
19
21
-1
0.6
-2
0.3
-3
0
T im e (hrs.)
W ater level
V e locity (m /s )
W a te r le ve l (m )
Deepth-Integrated V eloc ity
vel. cros s -s ec tional. averaged
S ü d erp iep - N o rth (05.06.00) 1.6
Ve locity (m /s)
1.4 1.2 1 0.8 0.6 0.4 0.2 0 9
11
13
15
17
19
21
T im e (hrs.)
Deepth-Integrated V eloc ity
S urfac e V eloc ity
B ottom V elocity
S ü d erp iep - M id d le (05.06.00) 1.6
V e locity (m /s)
1.4 1.2 1 0.8 0.6 0.4 0.2 0 9
11
13
15
17
19
21
T im e (h rs.)
Deepth-Integrated V eloc ity
S urfac e V eloc ity
B ottom V eloc ity
App. d: Current velocities at the cross-section B - Süderpiep at 05th June 2000. d
Appendices
P iep - S o u th (06.06.00) 1.6
V e locity (m /s)
1.4 1.2 1 0.8 0.6 0.4 0.2 0 7
9
11
13
15
17
Tim e (hrs.)
Deepth-Integrated V elocity
S urfac e V elocity
B ottom V elocity
3
1.5
1.2
1 0.9
0 -1
7
9
11
13
15
17
0.6
V e locity (m /s )
W a te r le ve l (m )
2
0.3
-2 -3
0
Tim e (hrs.)
W ater level
vel. c ros s-s ec tional. averaged
P iep - N o rth (06.06.00) 1.6
V e locity (m /s)
1.4 1.2 1 0.8 0.6 0.4 0.2 0 7
9
11
13
15
17
Tim e (hrs.)
Deepth-Integrated V elocity
S urfac e V elocity
B ottom V elocity
App. e: Current velocities at the cross-section C - Piep at 06th June 2000.
e
Appendices
P iep - S o u th (14.06.00) 1.2
V e locity (m /s)
1 0.8 0.6 0.4 0.2 0 3
5
7
9
11
13
15
17
Tim e (hrs.)
S urfac e V eloc ity
B ottom V eloc ity
3
1.5
2
1.2
1
0.9
0
0.6
3
5
7
9
11
13
15
17
-1
V e locity (m /s )
W a te r le ve l (m )
Deepth-Integrated V eloc ity
0.3
-2
0
Tim e (hrs.)
W ater level
vel. cross -sectional. averaged
P iep - N o rth (14.06.00) 1.6
V e locity (m /s)
1.4 1.2 1 0.8 0.6 0.4 0.2 0 3
5
7
9
11
13
15
17
Tim e (hrs.)
Deepth-Integrated V eloc ity
S urfac e V eloc ity
B ottom V eloc ity
App. f: Current velocities at the cross-section C - Piep at 14th June 2000.
f
Appendices
N o rd erp iep - S o u th (05.09.00) 1.4
V e locity (m /s)
1.2 1 0.8 0.6 0.4 0.2 0 4
6
8
10
12
14
16
Tim e (hrs.)
S urfac e V elocity
B ottom V elocity
2
1.2
1
0.9
0
0.6
4
6
8
10
12
14
16
-1
V e locity (m /s )
W a te r le ve l (m )
Deepth-Integrated V elocity
0.3
-2
0
Tim e (hrs.)
W ater level
vel. c ros s-s ec tional. averaged
N o rd erp iep - N o rth (05.09.00) 1.6
V e locity (m /s)
1.4 1.2 1 0.8 0.6 0.4 0.2 0 4
6
8
10
12
14
16
18
Tim e (hrs.)
Deepth-Integrated V elocity
S urfac e V elocity
B ottom V elocity
App. g: Current velocities at the cross-section A - Norderpiep at 05th September 2000.
g
Appendices
S ü d erp iep - S o u th (05.09.00) 1.6
V e locity (m /s)
1.4 1.2 1 0.8 0.6 0.4 0.2 0 3
5
7
9
11
13
15
Tim e (hrs.)
S urfac e V eloc ity
B ottom V elocity
2
1.2
1
0.9
0
0.6
3
5
7
9
11
13
15
-1
V e locity (m /s )
W a te r le ve l (m )
Deepth-Integrated V elocity
0.3
-2
0
Tim e (hrs.)
W ater level
vel. cross -sectional. averaged
S ü d erp iep - N o rth (05.09.00) 1.4
V e locity (m /s)
1.2 1 0.8 0.6 0.4 0.2 0 3
5
7
9
11
13
15
Tim e (hrs.)
Deepth-Integrated V elocity
S urfac e V eloc ity
B ottom V elocity
S ü d erp iep - Mid d le (05.09.00) 1.6
V e locity (m /s)
1.4 1.2 1 0.8 0.6 0.4 0.2 0 3
5
7
9
11
13
15
T im e (hrs.)
Deepth-Integrated V eloc ity
S urfac e V eloc ity
B ottom V eloc ity
App. h: Current velocities at the cross-section B - Süderpiep at 05th September 2000.
h
Appendices
P iep - S o u th (06.09.00) 1.2
V e locity (m /s)
1 0.8 0.6 0.4 0.2 0 4
6
8
10
12
14
16
Tim e (hrs.)
S urfac e V eloc ity
B ottom V eloc ity
2
1.2
1
0.9
0
0.6
4
6
8
10
12
14
16
-1
V e lo city (m /s )
W a te r le ve l (m )
Deepth-Integrated V eloc ity
0.3
-2
0
Tim e (hrs.)
W ater level
vel. c ros s -s ec tional. averaged
P iep - N o rth (06.09.00) 1.2
V e locity (m /s)
1 0.8 0.6 0.4 0.2 0 4
6
8
10
12
14
16
Tim e (hrs.)
Deepth-Integrated V eloc ity
S urfac e V eloc ity
B ottom V eloc ity
App. i: Current velocities at the cross-section C - Piep at 06th September 2000.
i
Appendices
N o rd erp iep - S o u th (12.09.00) 1.4
V e locity (m /s)
1.2 1 0.8 0.6 0.4 0.2 0 4
6
8
10
12
14
16
18
Tim e (hrs.)
S urface V elocity
B ottom V elocity
2
1.2
1
0.9
0
0.6
4
6
8
10
12
14
16
18
-1
V e locity (m /s )
W a te r le ve l (m )
Deepth-Integrated V elocity
0.3
-2
0
Tim e (hrs.)
W ater level
vel. c ros s-s ec tional. averaged
N o rd erp iep - N o rth (12.09.00) 1.4
V e locity (m /s)
1.2 1 0.8 0.6 0.4 0.2 0 4
6
8
10
12
14
16
18
Tim e (hrs.)
Deepth-Integrated V elocity
S urface V elocity
B ottom V elocity
App. j: Current velocities at the cross-section A - Norderpiep at 12th September 2000.
j
Appendices
S ü d erp iep - S o u th (12.09.00) 1.6
V e locity (m /s)
1.4 1.2 1 0.8 0.6 0.4 0.2 0 4
6
8
10
12
14
16
18
Tim e (hrs.)
S urface V eloc ity
B ottom V eloc ity
2
1.2
1
0.9
0
0.6
4
6
8
10
12
14
16
18
-1
V e locity (m /s )
W a te r le ve l (m )
Deepth-Integrated V elocity
0.3
-2
0
Tim e (hrs.)
W ater level
vel. c ros s -s ec tional. averaged
S ü d erp iep - N o rth (12.09.00) 1.6
V e locity (m /s)
1.4 1.2 1 0.8 0.6 0.4 0.2 0 4
6
8
10
12
14
16
18
Tim e (hrs.)
Deepth-Integrated V elocity
S urface V eloc ity
B ottom V eloc ity
S ü d erp iep - M id d le (12.09.00) 1.8 1.6
Ve locity (m /s)
1.4 1.2 1 0.8 0.6 0.4 0.2 0 4
6
8
10
12
14
16
18
Tim e (h rs.)
Deepth-Integrated V eloc ity
S urfac e V eloc ity
B ottom V eloc ity
App. k: Current velocities at the cross-section B - Süderpiep at 12th September 2000. k
Appendices
P iep - S o u th (13.09.00) 1.6
V e locity (m /s)
1.4 1.2 1 0.8 0.6 0.4 0.2 0 5
6
7
8
9
10
11
Tim e (hrs.)
Deepth-Integrated V elocity
S urface V elocity
B ottom V elocity
3
1.2
0.9
1 0.6
0 5
7
9
11
-1 -2
0.3
0
Tim e (hrs.)
W ater level
V e locity (m /s )
W a te r le ve l (m )
2
vel. c ros s-s ec tional. averaged
P iep - N o rth (13.09.00) 1.6 1.4 V e locity (m /s)
1.2 1 0.8 0.6 0.4 0.2 0 5
6
7
Deepth-Integrated V elocity
8 Tim e (hrs.)
9
S urface V elocity
10
11
B ottom V elocity
App. l: Current velocities at the cross-section C - Piep at 13th September 2000.
l
Appendices
N o rd erp iep - S o u th (12.12.00) 1.8 1.6 V e locity (m /s)
1.4 1.2 1 0.8 0.6 0.4 0.2 0 7
9
11 Tim e (hrs.)
S urface V elocity
B ottom V elocity
3
1.5
2
1.2
1
0.9
0
0.6
7
9
11
-1
V e locity (m /s )
W a te r le ve l (m )
Deepth-Integrated V elocity
0.3
-2
0
Tim e (hrs.)
W ater level
vel. c ros s-s ec tional. averaged
N o rd erp iep - N o rth (12.12.00) 1.8 1.6 V e locity (m /s)
1.4 1.2 1 0.8 0.6 0.4 0.2 0 7
9
11 Tim e (hrs.)
Deepth-Integrated V elocity
S urface V elocity
B ottom V elocity
App. m: Current velocities at the cross-section A - Norderpiep at 12th December 2000.
m
Appendices
S ü d erp iep - S o u th (12.12.0 0) 1.8 1.6 V e locity (m /s)
1.4 1.2 1 0.8 0.6 0.4 0.2 0 6
8
10
12
Tim e (hrs.)
S urface V eloc ity
B ottom V elocity
3
1.5
2
1.2
1
0.9
0
0.6
6
8
10
12
-1
V e lo city (m /s )
W a te r le ve l (m )
Deepth-Integrated V eloc ity
0.3
-2
0
Tim e (hrs.)
W ater level
vel. cross-sectional. averaged
S ü d erp iep - N o rth (12.12.00) 1.4
V e locity (m /s)
1.2 1 0.8 0.6 0.4 0.2 0 6
8
10
12
Tim e (hrs.)
Deepth-Integrated V eloc ity
S urface V eloc ity
B ottom V elocity
S ü d erp iep - M id d le (12.12.00) 1.6
V e locity (m /s)
1.4 1.2 1 0.8 0.6 0.4 0.2 0 6
8
10
12
Tim e (hrs.)
Deepth-Integrated V eloc ity
S urface V eloc ity
B ottom V elocity
App. n: Current velocities at the cross-section B - Süderpiep at 12th December 2000. n
ERKLÄRUNG
Hiermit erkläre ich an Eides statt, dass ich die vorliegende Dissertation mit dem Titel „Long- to Short-term Morphodynamic Evolution of the Tidal Flats and Channels of the Dithmarschen Bight, German North Sea“ selbständig angefertigt habe und dabei nur Daten und Informationen der genannten Quellen benutzt habe. Weiterhin versichere ich, dass die vorliegende Dissertation weder ganz, noch zum Teil bei einer anderen Stelle im Rahmen eines Prüfungsverfahrens vorgelegt wurde. Kiel, den 16.12.2003.
Nils Edvin Asp Neto
CURRICULUM VITAE Name: Nils Edvin Asp Neto Date and Place of Birth: Porto Alegre, Brazil. 17th of December 1974. Nationality: Brazilian Contact address: Rua Felipe Neri, 129/32 – Bairro Auxiliadora CEP 90440-150 Porto Alegre/RS - BRAZIL Professional Formation: Doctorate Candidate: Geology, at the Kiel University, Germany, from April 2000 up to the present. Ph.D. Thesis: Long to Short-term Morphodynamic Evolution of the Tidal Flats of the Dithmarschen Bight, German North Sea. Master of Science:
Marine Geology, at The Federal University of Rio Grande do Sul (UFRGS), started the curse in March of 1997 and finished in July of 1999. M.Sc. Thesis: Quaternary Sea-levels in the Inner Continental Shelf of Southern Brazil,
Diploma:
Oceanography, Specialisation in Coastal Management, at the Fundação Universidade do Rio Grande (FURG), Rio Grande/RS, Brazil, started the curse in march of 1992 and finished in December of 1996. Graduate thesis: Economic Potential of Seashell Deposits of Rio Grande do Sul Inner Shelf.
Main Professional Experience From April 2000
Scientific Collaborator at the Research and Technology Center Westcoast, University of Kiel, Germany.
November 1996 - March 1997. Technical-Scientific Assistant at the Systemic Ecology Laboratory in the project ”Environmental Impact of Rio Grande Harbour Activities” (SUPRG). May 1994 - October 1996.
Scientific Trainee at the Geological Oceanography Laboratory (DEGEO/FURG), project ”Geofisic and Sedimentologic Studies in the Coastal Zone of Southern Brazil”, with co-ordination of Dr. Lauro Júlio Calliari.
May - December 1994 & 1995. Practical Assistant of Sedimentology Course of Geosciences Department of FURG University, 12 hours weekly. Supervising by Teachers M.Sc. Luciana Slomp Esteves and M.Sc. Marcus Vinicius Berao Ade. May 1993 - April of 1994.
Scientific Trainee at the Geological Oceanography Laboratory (DEGEO/FURG), project ”Palaeontologic Aspects of Mirim Lagoon, RS/Brazil, Based on Ostracodes Communities Studies (CRUSTÁCEA), from Pleistocene to Recent”, with co-ordination of M.Sc. Maria Elizabeth Gomes da Silva Itussarry.
