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Aquitaine Coast Observatory

Context

The Aquitaine coast is a straight shore spreading over 240 kilometres. The Arcachon basin marks an inflection point halfway in the morphology of the coast and in the associated sedimentary transits.

This tourist built-up coast is constantly undergoing a strong sedimentary dynamics (sand stock movement, impact of swell, wind, tide or storms...). Socioeconomic issues are so important that the marine erosion hazard must be taken into account in the land planning policy, turning into a Coastal Risk Prevention Plan on the ocean front. The mechanisms which govern these dynamics must therefore be understood in order to anticipate and prevent their consequences whenever possible.

In order to match with these objectives of knowledge, anticipation and quantification of the movements of the Aquitaine coast - and more particularly of the Arcachon basin -, analysis and study programs have been placed to the last State-Region Contracts (1996-2000, 2000-2006) and the 2007-2013 Project Contract involving the Government, the Aquitaine Regional Council, the Gironde General Council, the Syndicat mixte du Bassin d’Arcachon., BRGM and ONF. This project, entitled “Aquitaine Coast Observatory”, offers in addition permanent expertise related to events (coastal instabilities or installations) which makes it possible to apply the results of these analysis to public policies requirements.

Activities currently carried out by scientific teams

A first cooperation between the EPOC Research Unit of the University Bordeaux 1 and BRGM allowed to appraise Spot 5 multi-thematic interest over the Arcachon basin. This assessment concerned the mapping of coastal entities located in lagoon areas as well as in maritime ones.

The used method includes three steps:

- Field surveys on training areas, completed by photo-interpretation. These areas of limited extent are selected in order to present a wide diversity on the morphological and bio-sedimentary plans;

- An exhaustive validation of the outlines of the entities resulting from the processing of Spot 5 images with those identified on-the-spot in those training areas.

- Applying validated Spot 5 processing to the whole study site. Results are then assessed on control areas different from the training ones.

The first results show that Spot 5 proves to be efficient to follow key coastal indicators from seasonal to long-term scale. This concerns coastal morphological changes (erosion, equilibrium, spreading) thanks to the monitoring of the dune foot, of dune facies and of erosion signs.

Generally speaking, Spot 5 interest is significant for mapping and monitoring bare soil and vegetation (ocean and lagoon vegetation, slikken and vegetation limits next to salt marshes). However, even if various vegetation areas can be clearly separated, it is not always possible to identify their type reliably and on-the-spot spectral measurements are requested over determined vegetation. Preliminary results allow to draw a reliable map of the observed geomorphological indicators, to study the changes of the ocean front of the Arcachon basin, such as white and grey dune, dune foot, high water mark, berm or sand-water limit.

At short term, Spot 5 offers interesting applications to monitor shoreline changes from seasonal to long-term scale. Improving the detection and the extraction of high water marks and of embryonic dunes should allow to better quantify these changes. Work is currently going on in the frame of the Aquitaine Coast Observatory to improve the thematic classification of the Arcachon basin coast.

Relevance of a reference remote sensing database

The Kalideos Littoral database offers the opportunity to use a large number of Spot data covering a long period (i.e. in many conditions) and at various resolutions. This makes it possible:

- To better characterise the relationship between reflectance and the different observed geomorphological indicators (sandbars, tidal pools, dune foot limit, dune and lagoon vegetation, channels, sandbanks...) in order to improve the thematic classification approach;

- To study temporal changes of these indicators.

Contact point

This activity is coordinated by Cyril MALLET from the Aquitaine region Geological Division of BRGM.