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Channel mapping

Context

The Arcachon basin includes a lagoon area and a mouth with channels and sandbanks moving under the influence of swell and currents. This mouth stretches over around 10 kilometres from North to South and more than 6 kilometres from West to East and includes two channels, the Northern Channel and the Southern Channel, separated by the Arguin bank. The channels end in a scalloped barrier where the strong western swell breaks on.

Natural evolution strives for the shift of these channels to the South. This movement leads to a very strong erosion of the southern coast, which recedes of more than 10 m a year, but also of the northern coast, where the Cap-Ferret peninsula undergoes a significant erosion worrying for the coastal houses.

Activities currently carried out by scientific teams

Under the leadership of the Syndicat mixte du Bassin d’Arcachon (SIBA), various measurements concerning the channels are regularly made: sounder-based hydrographic surveys carried out by the Maritime and Shipping Division of the Gironde department and channel mapping through remote sensing carried by the EPOC research unit of the Bordeaux-1 university.

Measurements using Spot images started in 1997, at the instigation of SIBA and the Maritime and Shipping Division. The Spot based bathymetric method, developed more than 10 years ago, has been improved and adapted to operational requirements, allowing especially:

- To map shallow seabeds (higher than 8 m under sea level),

- To assess yearly change of banks and channels,

- To carry out long-term comparison.

Space based bathymetry supposes a relationship between the intensity of backscattered light and sea depth: for a homogeneous coloured seabed, the higher the intensity backscattered by the seabed and the thinner the water layer. Survey data are used to set up a function between depth and Spot reflectance. This calibration is then extrapolated to the mouth basing upon two assumptions:

- Seabed colour is identical on the whole site, which is close to reality, except in some areas around the Arguin bank or the Bernet one, where seabed lower backscattering leads to an overestimation of depth;

- Water turbidity is homogeneous for the whole site, which is the case at calm sea.

Spot data are recorded in summer because water is clearer at that time. The XS1 red band is mainly used because it is the one which reduces light the less. The XS3 near infrared band ensures an accurate location of the land-sea line.

A methodology has been developed to achieve this mapping rapidly. Sea depths have been mapped down to - 8 m in 2000, - 15 m in 2001 and - 10 m in 2002, but only to - 3 m in 2006 because of water high turbidity. Results are all the more accurate as depth is shallow (for instance ± 20 cm between 0 and - 2 m and ± 1,5 m between - 8 and - 12 m).

This approach, which makes it possible to derive a morphological map of shallow seabeds, can be applied to every place of low and homogeneous water turbidity and uniformly coloured seabed.

Relevance of a reference remote sensing database

Through the use of a great amount of Spot data collected over a long time and at different resolutions, the Kalideos Littoral database offers the opportunity to better characterise the link between reflectance and seabed depth in various conditions, and thus to improve the calibration scheme at the core of the morphological mapping process.

Contact point

This activity is coordinated by Jean-Marie FROIDEFOND from the CNRS Research Unit “Oceanic Environments and Paleoenvironments” (UMR EPOC).