Modelo conceptual de factores de controlo no

7º Simpósio sobre a Margem Ibérica Atlântica - MIA12

16-20 de Dezembro de 2012, Lisboa

Modelo conceptual de factores de controlo no assoreamento da baía de Atouguia da Baleia, do último milénio ao século XXI: um projecto de investigação Conceptual model on the control factors in the infilling of the Atouguia da Baleia bay, from the last millennium to the XXI century: a research project André Torres (1) (1)

Bolseiro de Doutoramento da FCT, SLIF - Centro de Estudos Geográficos, IGOT/ Universidade de Lisboa. E -mail: [email protected]

Abstract: This paper presents a conceptual model that aims to reconstruct the evolution of the coastline near the city of Peniche, in the Portuguese Western Coast, over the last millennium. The selected time span was chosen because it encompasses the period when the human activities had the most significant impact in the coastline evolution, especially after the 15th Century AD. The Atouguia da Baleia bay shows a fast sedimentary infilling that was conditioned by control factors, both allogenic (external) and autogenic (internal, intrinsic processes), and mechanisms/processes (natural and anthropogenic) behind the most significant environmental changes. The model seeks to determine and quantify these factors and processes, by building a process/response numerical model that recreates and predicts the environmental changes in the study area. Palavras-chave: enchimento estuarino, factores de controlo, alterações climáticas, impacto antrópico Keywords: estuarine infill, control factors, climate change, anthropic impact

1. INTRODUCTION The coastline evolution at a millennial scale is the result of the interaction of an intricate network of factors, such as: sedimentary budget, (including fluvial input and coastal erosion), coastal drift, local sedimentation rates, climate change, anthropic impacts and eustatic sea-level fluctuations (Dias et al., 2000). The sea-level eustatic rise seems to have been the main driven force in the evolution of the Portuguese coastline until the Middle Holocene, when the mean sea-level stabilized (Dias et al., 2000; Bao et al., 2007; Dias, 2009). From that time on, and until present, the evolution of the Portuguese coastline was conditioned by natural factors, assuming particular relevance those related to climate change (Dias, 2009). Nevertheless, during the last millennium, the expansion of agriculture and deforestation, using technological innovations, motivated a greater impact of human activities in the coastal zone with and increasing role since the 15th Century (Dinis et al., 2006). Several studies were conducted in alluvial plains and estuaries along

the Portuguese coast (Bao, 1999; Boski, 2002; Freitas et al., 2002; Ramos-Pereira, 2002; Ramos-Pereira et al., 2011; Drago, 2004) which established a correlation between the sedimentary record, geomorphology and the environmental changes. In this research, a multidisciplinary analysis of cores will make it possible to identify the main trend lines of the Atouguia da Baleia bay millennial evolution as well as the relative role of the allogenic and autogenic control factors in the dynamics of the area (Blum et al., 2000; Bao et al., 2007). The research is developed in the context of a PhDthesis (FCT grant: SFRH/BD/80905/2011).

2. OBJECTIVES The main goal of this paper is to present a conceptual model for studying the infilling of the Atouguia da Baleia bay, identifying base lines and changing trajectories over the last one thousand years. Thus, it is crucial to establish correlations between the local sedimentary deposits and the available historical data in order to understand in which way the


environmental dynamics interlocked with the human activities in the area. For the conceptual model it is fundamental to determine the control factors (allogenic and autogenic) as well as the mechanisms/processes that influence the evolution of the coastline in each moment. In this scope it is necessary to understand the balance between the natural factors, especially those linked to climate change, and the humaninduced impact in the millennial-scale coastline evolution. To model the input/output sedimentary fluxes and sedimentation/erosion rhythms over time for the study area, the sedimentation rates and sediment budgets must be calculated by time step. By cross-checking the sedimentary timefluxes with the climate change and land use data, at a local and regional scale, it will be possible to determine the causes behind the sediment budget fluctuations and the changes in sedimentation rates. The integration of the available data will make it possible to build a GIS model that simulates the infilling of the Atouguia da Baleia bay and will contribute to predict future lines of change, thus providing an important decision making tool in the context of spatial planning and the sea level rise.

3. METHODS AND APPROACH 3.1. Geomorphological framework and coring In a first stage, the former embayment and the estuarine morphology of the study area must be thoroughly analyzed and the coring sites selected. The location of the cores must allow the projection of the individual cores in transects that produce an orthogonal framework of the Atouguia da Baleia bay. Mechanical (using pneumatic rotary hammer drills) and manual coring must be performed in the present day alluvial plain to sample nondisturbed sediments, so that it will be possible to obtain the last millennium stratigraphic sequence. Further drilling must be performed with hand-driven Edelman corers to calculate the lateral extension of the deposits and to have an overview of the study area sedimentary architecture. The sampled sediments will be treated in the laboratory in order to: i) evaluate the changes


in the sedimentary structure, ii) analyze the grain-sizes (statistical parameters and correlation diagrams), study the micromorphology and imbrications of the larger clasts and pebbles, and analyze the organic composition of the deposits (Freitas et al., 2002; Drago, et al., 2006; Bao et al., 2007). The changes in the sedimentary structure of the deposits together with the 14C dating of organic levels will allow the evaluation of the environmental changes, flooding frequency and recurrence time, and to define the wet and drought periods that occurred during the time frame of the research. The palynological analysis of the sedimentary record may be used to reconstruct the paleoenvironmental local and regional conditions and assess the role of climate change and anthropic impact in the evolution of the bay.

3.2. Historical changes)

database

(and

land

use

The results from the coring and laboratory sedimentological analysis must be correlated to the archaeological database and historical documents (at a local and regional level). The data related to flooding/drought periods, and changes in land use, namely deforestation and drainage of wetlands records, is particularly relevant. Historical maps and records of old ship routes may also provide important information about environmental changes and human activities in the area. It is fundamental to establish a time-correlation between the sedimentary record and the historical database so that it will be possible to understand, for each time-step, the significance of the human activities in the coastal morphodynamics (Blot, 2003).

3.3. Allogenic and autogenic control factors This stage emphasizes in assessing the allogenic and autogenic control factors associated to the millennial coastal dynamics. The main premise of this research stage is to understand in which way the forcing of the allogenic factors (climate change, neotectonics, anthropogenic impact and sea-level changes) conditioned the autogenic evolution (intrinsic behaviour and complex feedback of the system) of the former estuary and embayment (Hoogendoorn et al., 2002; Stouthamer et al., 2007).


TASKS Field survey and geomorphological framework


OBJECTIVES Estuarine geomorphological analysis

Selection of coring sites

Mechanical drilling and manual coring

Coring and sedimentological analysis

Laboratory sedimentological analysis

RESULTS Field survey with dGPS

Transects that produce an orthogonal framework of the basin Calculate the lateral extension of the deposits Textural analysis based on statistical parameters

Micro-morphology of the grains Reconstruct paleoenvironments and human impacts

Pollen and npp analysis

Historical and archaeological database

Archaeological database

Detection of human induced changes Detection of land use and coastal changes

Historical documents analysis

Determining the system control factors

Assess the allogenic and autogenic control factors

Determine the system sensitivity and geomorphological response

Detection of flood and drought events

Assess the process/response mechanisms and patterns of the system

14

Quantification of intra-millennial sediment delivery

Dating of the estuary milestones evolution

C dating for organic levels

OSL dating for sandy deposits Define time lines

Calculate sediment volumes by time step

Data integration in a GIS framework

Data intergration and GIS modelling

Sedimentation rates and sedimentary budgets GIS mapping of land use changes

Statistical study of evolution trends

CA-model of the system key components Fig. 1 – Conceptual model: tasks, objectives and results. Adapted from Ramos Pereira et al. (2011).

GIS model for future trends


It will be fundamental to determine the sensitivity and geomorphological response of the system to the climate change and human impact (Overeem et al., 2003; Hoofman et al., 2010). In this sense, a spatial model that contains the specific characteristics of the study area (topography, edaphic properties, lithology and land use) must be built and set in a GIS database. As the connections between this database and the allogenic factors, namely climate variability and land use changes, are set, it will be possible to calculate the response of the coastline and Atouguia da Baleia bay to the climate and anthropogenic forcings.

3.4. Quantification sediment delivery

of

intra-millennial

The 14C dating for organic beds in the deposits and the optically stimulated luminescence (OSL) dating for sandy deposits will provide a time control and the means to define time lines that can be extended into adjacent areas using stratigraphic correlations between deposits (Erkens et al., 2006; Erkens, 2009). Using the data obtained in the high-resolution transects (thickness of the deposits, isochrones based on the 14C/OSL dating and inter-core correlations) and the lateral extension of the deposits, it will be possible to calculate the century-scale sedimentation rates and sedimentary budgets. The starting step will be to quantify the total volume of the sediments in the area for the selected time span (if possible differentiating between fluvial, marine and brackish-water deposits) and to classify the results by clastic fractions that may correspond to distinct depositional environments in the system. The average thickness of each sedimentary time-segment will be calculated by dividing the transversal area of the deposit, given in m2 in the Y/Z plane, by its length (m) in the longitudinal plane. By multiplying each sedimentary segment by its extension area (m3) it will be obtained the total sediment volume for each time interval. The sedimentation rate will be calculated by establishing a relationship between the thickness of segments and the time interval between the isochrones that bound them.


3.5. Data integration and GIS modeling After assessing the process/response mechanisms and patterns identified in the previous stages of this research, it will be built a quantitative model for evaluating the intrafactor sensitivity and the feedback of the estuary and the embayment to the variability of the constraining factors of the system (Bloom et al., 2000; Brown, 2006). In this model the factors linked to climate change and land use changes will assume a highly relevant role because of the great impact they had in the evolution of the Atouguia da Baleia bay/basin over the last millennium. In order to eliminate the uncertainties associated to a predicting model, the chosen approach will focus on previously determining base rules that will define the limits of environmental changes. The model will be developed in a GIS framework that will provide the necessary structure to integrate and analyze the different components in successive degrees of abstraction from the real data (Ward, 2009). Cellular automata (CA) frameworks can be used to model key components of the estuarine sediment system because the CA-models can capture the complex interactions between decadal and centennial time-scale processes through fundamental rules, catching the feedbacks, time-lags and leads while providing a nonlinear dynamical behaviour (Dearing et al., 2006). The prediction time scope for the model will be the end of the 21st Century because there is available data referring to the mean sea-level fluctuations in the Western Portuguese Coast for this period (Antunes & Taborda, 2009). Nevertheless, the influence of the spatial planning policies can only be reliably inferred until the year 2050 (Brown, 2006).

4. FINAL REMARKS Unraveling the control factors by time step for the Atouguia da Baleia bay/basin is the main goal of this research. The understanding of the relative role of the conditioning/triggering factors and the process/response mechanisms of the system, will provide a detailed insight of estuarine system dynamics and infilling of this coastal basin.


The purposed conceptual model was built to accomplish this task in the most straight forward and effective way.

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