Coastline Evolution at Esmoriz-Furadouro Stretch ...

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identifying the sand dune cliff, and for visualization of the dune evolution and sediment balance in this period. These models were converted into grids and used ...

Journal of Coastal Research

SI 56

pg - pg

ICS2009 (Proceedings)

Portugal

ISSN

Coastline Evolution at Esmoriz-Furadouro Stretch (Portugal) F. Taveira-Pinto † J. Pais-Barbosa ‡ and F. Veloso-Gomes † † Hydraulics and Water Resources Institute, Faculty of Engineering of Porto University Porto, 4200-465, Portugal [email protected], [email protected]

‡ Centro de Investigação em Ciências Geo-Espaciais, Faculty of Sciences of Porto University Porto, 4169-007, Portugal [email protected]

ABSTRACT TAVEIRA-PINTO, F., PAIS-BARBOSA, J., and VELOSO-GOMES, F., 2009. Coastline Evolution at Esmoriz-Furadouro Stretch (Portugal). Journal of Coastal Research, SI 56 (Proceedings of the 10th International Coastal Symposium), pg – pg. Lisbon, Portugal, ISBN The Portuguese coastal zone stretch (Esmoriz-Mira beach) has faced a number of serious hazards, namely coastal erosion, flooding, destruction of infrastructures and sea defences, dune breaching and overtopping. Based on aerial photographs taken between 1958 and 2002 the coastline evolution is analyzed here in terms of urban development, vegetation line retreat, overwash and overtopping and coastal defences associated with erosion processes and trends. To accomplish the main goal of this work a Geographical Information System (GIS) database called COMODY (Geographical Information System for COastal MOrphoDYnamic) was used. This geodatabase stores aerial photograph datasets, topographic surveys and data resulting from the onscreen analysis in a GIS environment, as well as wave climate, tide and sediment data. Some important conclusions can be drawn from the preliminary results achieved: the number of overwash/overtopping events increased in number, and especially in size, between 1958 and 1998; in the analysis period a high retreat of the foredune/vegetation line was observed at Maceda beach, reaching up to 200 m in some points; in some locations dunes were completely destroyed and artificial dunes had to be built inland from the original location; the number of coastal defence structures have increased from 1967 until now, mainly in urban waterfronts. This stretch is considered highly vulnerable, especially in the southern sand spit of Aveiro Lagoon where the formation of new inlets is expected. If this scenario occurs, the Aveiro Lagoon ecosystem will undergo deep changes; agriculture areas will be lost and infrastructures will be destroyed, causing serious social-economic problems. ADITIONAL INDEX WORDS: Coastal Zone, Retreat, Coastline change, Urban areas

INTRODUCTION Coastal zones change constantly as a result of winds, waves, tides, mean sea level variations, climatic changes, human actions and other factors affecting sediment transport (DONOVAN, et al., 2002). Several studies have been developed to understand the impact of natural actions and human activities on coastline evolution (REGNAULD, et al., 2004, HUANG and XU, 2005, VALDEMORO, et al., 2008). Sea action is the main modelling agent on the Portuguese coast, where the wave regime is one of the most energetic in Europe. The Portuguese coastal zone presents three types of coasts: sandy, rocky and cliffed. Erosion problems are especially important in sandy coastal areas, and have a long record in this coastal zone (PAIS-BARBOSA et al., 2003). Coastal erosion is expressed by coastline and foredune retreat, flooding, destruction of infrastructures and sea defenses, dune breaching, and overtopping events (TAVEIRA-PINTO et al., 2007). The coastal zone is changing over short and long timescales due to both natural and anthropogenic causes such as: the sediment supply reduction as a consequence of dam construction, dredging activities, coastal defences and harbour breakwaters, urban pressure, among others. From analysing the area of natural drainage to the Atlantic Ocean in the XIX century (without dams) and the natural drainage area today (with dams) it is clearly seen that it has been reduced to about 13%.

Dams constructed in Portuguese and Spanish rivers currently constitute an indispensable component for supplying renewable energy. However, the use of rivers for energy production, water supply, industry and agriculture has drastically reduced the volume of sediments transported to the sea, thereby changing the coastal dynamics and worsening coastal erosion. The main objective of this work is to analyse the coastline evolution locally between 1958 and 2002 in terms of vegetation line retreat, urban area evolution and the influence of coastal defences, among others.

Study Area The study area is located in the central region of Portugal, between Esmoriz and Mira beach (58 km). It is dominated by the Aveiro Lagoon (Figure 1), which is a very dynamic and fragile physical and biological system, constantly changing in response to natural processes as well as human activities. In this paper essentially the results obtained for the Esmoriz-Furadouro sub stretch (11.4 km) are presented. This coastal stretch can be classified in two distinct time periods. The first is denominated by a strong sediment supply and the second by an enhanced erosion process. In accordance with Submarine Geology, 3rd ed. of Francis P. Shepard, mentioned by MORANG et al., (2002), the study area can be classified as follows (VELOSO-GOMES et al., 2006):

Journal of Coastal Research, Special Issue 56, 2009

Coastline Evolution at Esmoriz-Furadouro Stretch (Portugal)

The longshore transport is predominantly from north to south. A natural transposition on reefs and shallow external bars through the Aveiro lagoon inlet occurs. It is important to note that because of access requirements to the Aveiro harbour two breakwaters were built for inlet control, thereby provoking erosion and moving the natural sand transposition to deeper depths. Dredging is necessary for navigational purposes but the material is taken out of the sediment transport system, thereby worsening the negative mass balance downdrift of the breakwaters.

METHOD GIS Database To accomplish the main goal of this work, data/information stored in a Geographical Information System (GIS) database called COMODY (Geographical Information System for COastal MOrphoDYnamic) was used (PAIS-BARBOSA, et al., 2005a, 2005b, 2007 and PAIS-BARBOSA, 2007). The GIS database stores vector and raster datasets as well as alphanumeric information: fourteen aerial photographic surveys, topographic maps obtained from the photographic surveys of 1996 and 2001 at 1:2,000 scale; oblique photographs from 2001 (two surveys) and 2002; tide data; deepwater wave climate data (with gaps); sediments grain sizes; beach slopes from 1996, 2001 and 2002; nautical and military charts from 1974/1975 and 1998 (1:25,000). Based on COMODY it was possible to carry out several spatial analyses such as: vegetation line retreat; urban settlement evolution; overwash and overtopping locations over time; 3D modelling and coastline evolution from 1958 until 2002 (Figure 3 and Figure 4). Figure 1. Study area. •

Vegetation Line Retreat

Secondary coasts: Shaped primary by marine agents or by marine organisms, that could have been (or not) a primary coast before being shaped by the sea. • Marine deposition coast: Coasts prograded by waves and currents; • Barrier coasts (Aveiro lagoon); • Bay barrier – Sand spits that have completely blocked a bay (before 1808); • Barrier spits – Connected to mainland (after 1808 with the artificial inlet opening); • Beach plains – Sand plains differing from barriers by having no lagoon inside (Espinho - Furadouro and Mira – Cape Mondego). From a geomorphologic perspective, this stretch has a NNESSW general orientation, being mostly a low-lying coastal plain. It is formed by a sandy coast throughout its length, composed of alluvium sands and having dunes which extend into inland elevations. This coastal area also includes sandy dunes that form small elevations, particularly in the zone of Maceda and Cortegaça. In the inland areas of Esmoriz and Mira beach are two small lagoons, the Barrinha of Esmoriz and Barrinha of Mira. The wave climate in this coastal zone is characterized by a mean significant wave height ranging from 2 to 3 m, with a mean period ranging from 8 to 12 s. In storm conditions the significant wave heights exceed 8 m (once per year), with periods reaching 16 to 18 s. Wave direction exhibits the higher frequencies in the NW quadrant with 43.8 % of occurrences, WNW with 28.0% and NNW with 21.2% (VELOSO-GOMES et al., 2006). Tides are of the semidiurnal type, with amplitude of 2 to 4 m in spring tides.

Using COMODY and image interpretation results, overtopping and overwash locations, dune cliffs, and vegetation lines were identified and digitalized. Differences between the identified historical vegetation lines were measured in an attempt to quantify the shoreline change. The methodology used for the shoreline evolution quantification was based on the (x,y) coordinates. These coordinates were obtained based on a reference line parallel to the coastline, and on crossshore lines perpendicular to the reference line with 50 m spacing. Points were added and referenced in the intersection between cross-shore lines and historical vegetation lines.

Digital Terrain Models Two topographic surveys, resulting from the aerial surveys of 1996 and 2001, were used. The first step was the creation of digital terrain models for both surveys, using a Triangular Irregular Network (TIN). These models were important to identifying the sand dune cliff, and for visualization of the dune evolution and sediment balance in this period. These models were converted into grids and used to elaborate the cross and longitudinal sections of the coastal zone.

Profile Estimation Two different methods were used for profile estimation. One was recurring to Spatial Tools that permits the collection of point series of the grids to dbf files. Afterwards, graphics with these groups of points were analyzed and created. The second method used Profile and Profile Extractor 6.0, extensions of ArcView that permit obtaining the graphic profile directly in ArcView or exporting the values to other applications such as Microsoft Excel.

Journal of Coastal Research, Special Issue 56, 2009

Taveira-Pinto et al.

Figure 3. Coastline evolution from 1958 until 1998 – Esmoriz-Furadouro stretch.

RESULTS AND ANALYSIS The study area located between Esmoriz and Mira beach has severe coastal erosion problems. The following results and analysis are essentially related with the sub stretch EzmorizFuradouro. Some important conclusions can be drawn from the results achieved.

Coastal erosion is a very serious problem and will probably increase as a response to the continuous weakening of river sediment sources, harbour works, mean sea-level rise, human waterfront settlements and other causes (VELOSO-GOMES et al., 2006). New protection systems for urban waterfronts will be required. This will result in a further imbalance in the physical dynamics and will lead to constraints and increasing vulnerability of some areas.

Journal of Coastal Research, Special Issue 56, 2009

Coastline Evolution at Esmoriz-Furadouro Stretch (Portugal)

Figure 4. Coastline evolution from 2001 to 2002 –Esmoriz-Furadouro stretch. Figure 3 and Figure 4 illustrate the foredune occupation, the settlements and the evolution of coastal defences for the EsmorizFuradouro stretch from 1958 until 2002. This representation shows the increasing urban area, number of groins and seawalls length. For example, the Furadouro urban area doubled in 30 years (Figure 5). It is important to recall that in 1958 coastal defence structures weren’t present. Their number has increased since then to protect various urban waterfronts. The analysis performed to quantify the vegetation line retreat, between Cortegaça and Furadouro settlements shows that in some coastal points the vegetation line 400000 350000

m2

300000 250000 200000 150000 1958

1970

1991 Year

Figure 5. Furadouro urban area evolution (1958-1998).

1998

moved inland between 1958 and 2002, by up to 200 m (Figure 6). However, it seems that the coastline is undergoing a rotation between two fixed points, Cortegaça in the north and Furadouro in the south, where a groin field exists. According to the overwash/overtopping maps obtained for a partial area between Furadouro-Torreira for 1958, 1970 and 1998, the number of overwash/overtopping increased, as well as their size (Figure 7). This fact could be related with the foredune retreat, as well as the decreasing dune elevation. The comparison of cross sections from 1996 and 2001 topographic surveys clearly shows that the dune was overwashed, and in some locations the overwash/overtopping was so extreme and intense that the dune was completely destroyed. An artificial dune was constructed inland from the natural dune as an emergency intervention after some storm events, mainly at Aveiro Lagoon southern sand spit. Analysing two longitudinal sections (at the southern sand spit of Aveiro Lagoon, obtained from the 1996 topographic survey), one located over the existing dune and the other 50 m inland, it can be observed that the average elevation of the inland profile was lower (2 m MSL) than that of the dune (4 m MSL). This fact shows the importance of the foredune for protecting this lowland area from sea actions. If the existent foredune breaches new inlets could be opened, thereby changing the Aveiro Lagoon ecosystem, flooding agriculture areas, destroying infrastructures (roads, houses), and causing serious socialeconomic problems.

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Taveira-Pinto et al.

DISCUSSION Retreat (m) -125

-75

-25

25

75

125

175

225 9000

Cortegaça

9500

10500

Distance (m)

10000

The results presented show that the Portuguese west coast (Esmoriz – Mira) has serious erosion problems, dune overwash and overtopping events. The COMODY GIS database allowed a comparison and quantification of the evolution of overwash and overtopping events from different years. Overwash/overtopping events seem to be increasing in number and size. It was also observed that the dune front retreated from 1958 until 2002, and at some points this retreat was up to 200 m. These retreat trends are a challenge at all administrative levels (national, regional and local) as well as for managers and engineers. It is necessary to define policies and plans to minimize the losses of land, infrastructures and dune destruction. If nothing is done the foredune could breach, new inlets could be formed, drastically changing the Aveiro Lagoon ecosystem and having serious socioeconomic consequences. The next step of this work is to extend it to new areas of analysis using other aerial photographic datasets and field work, and to improve the methodology so as to present more results and with greater accuracy.

LITERATURE CITED 11000

11500

1958-1967 1967-1996 1996-2002a Groin

12000

Furadouro

Figure 6. Vegetation line retreat between Cortegaça and Furadouro.

Figure 7. Overwash/overtopping maps for a partial stretch between Furadouro-Torreira.

DONOVAN, A., HANEY, R. and MAGUE, S., 2002. Massachusetts Shifting Shorelines. New Data on Shoreline Change, Massachusetts Office of Coastal Zone Management, www.state.ma.us/CZM/shorelinechangefactsheet.pdf. HUANG, J. and XU L., 2005. Remote sensing monitoring on coastline evolution in Yellow river delta since 1976, IEEE, pp 2161-2164. MORANG, A., GORMAN, L., KING, D. and MEISBURGUER, E., 2002. Coastal classification and morphology, Coastal Engineering Manual. Parte IV, Capítulo IV-2, Ed. Andrew Morang. Washington, DC, U. S. Army Corps of Engineers, 83 p. PAIS-BARBOSA, J., 2007. Hidroformas e Hidromorfologias costeiras locais (in Portuguese). Faculty of Engineering of Porto University, PhD Thesis, 2 Vol., 780 p. PAIS-BARBOSA, J., VELOSO-GOMES, F. and TAVEIRA-PINTO, F., 2005a. GIS tool for coastal morphodynamics analysis. Proceedings of the CoastGIS 2005, Aberdeen, Scotland, CD. PAIS-BARBOSA, J., VELOSO-GOMES, F. and TAVEIRA-PINTO, F., 2005b. Analysis of the Portuguese west coast morphology and morphodynamics, based on aerial images and GIS tools. Proceedings of the 2nd EARSel Workshop - Remote Sensing of the Coastal Zone (Porto, Portugal), pp 809-818. PAIS-BARBOSA, J.; VELOSO-GOMES, F., and TAVEIRA-PINTO, F., 2007. Coastal features in the energetic and mesotidal west coast of Portugal, Journal of Coastal Research, Special Issue No. 50, pp 459-463. REGNAULD, H.; PIRAZZOLI, P.; MORVAN, G. and RUZ, M., 2004. Impacts of storms and evolution of the coastline in western France, Marine Geology, 210, pp 325-337. TAVEIRA-PINTO, F.; VELOSO-GOMES, F; and PAIS-BARBOSA, J., 2007. Evolution of Coastal Dune’ Fields in the West Coast of Portugal, in relation to Erosion Phenomena, ICCD2007. VALDEMORO, H.; JIMÉNEZ, J.; GRACIA, V.; SOLE, F.; MENDOZA, E.; ARIZA, E. and SÁNCHEZ-ARCILLA, A., 2008. Coastline evolution at northern Barcelona: natural vs human factors, ICCE2008, Hamburg. VELOSO-GOMES, F.; TAVEIRA-PINTO, F.; das NEVES, L., and PAISBARBOSA, J., 2006. EUrosion - A European Initiative for Sustainable Coastal Erosion. Pilot Site of River Douro - Cape Mondego and Case Studies of Estela, Aveiro, Caparica, Vale do Lobo and Azores, Porto, Portugal, 317 p.

Journal of Coastal Research, Special Issue 56, 2009

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