The New Era of Regional Coastal Bathymetry from Space: A Showcase for West Africa using Sentinel-2 Imagery

Christopher J Daly, Wassim Baba, Erwin W. J. Bergsma, Rafael Almar, Thierry Garlan

Large-scale coastal bathymetry is an essential data product for use in coastal research and governance. Traditional methods of measuring bathymetry, using sonar deployed from ships, take an enormous amount of time to collect and process, and thus prevent the gathering of data at high spatial and temporal resolution at a regional scale. Space-borne missions, such as the European Space Agencys Sentinel-2 mission, offer a solution to this limitation by frequent sensing at a global scale, with repeat observations every 5 days at the equator or less at higher latitudes. Satellite-derived bathymetry has often been associated with methods which exploit water colour to invert depth. However, depth can also be inverted from wave celerity using the linear dispersion relation. Here, we use a recently developed algorithm, S2Shores (Satellite to Shores), to extract wave celerity from the Sentinel-2 dataset by exploiting the small temporal offset between the image bands of its Multi-Spectral Instrument. The resulting bathymetry estimates are then merged to create a mosaic of composite depth estimates spanning 4000 km along the West African coast. Given the scarcity of in-situ measured bathymetry in the region, the resulting atlas is compared to GEBCO, a global bathymetry dataset created by assimilating ship soundings with altimeter data. S2Shores is able to detect depths up to 20-40 m for West Africa, depending on mean incident wave conditions and cloud cover, which varies by location. Shallow water features between 2-15 m are well reproduced by S2Shores, such as flow channels in Guinea, the St. Anns Shoal in Sierra Leone, and ebb delta lobes at several outlets along the Niger River Delta. Bathymetry data for the Senegalese coast is used to validate the S2Shores results, with very good results in shallow water. This new Coastal Atlas of West Africa opens the door to increased research and planning capabilities for the region, and sets an example that can be applied to the rest of the world.