Abstract
Large-scale coastal bathymetry is paramount to understand natural and human-induced coastal behaviour and plays a vital role in coastal research and governance. Here, we use a recently developed algorithm, S2Shores (Satellite to Shores), to invert coastal bathymetry from wave kinematics, based on the linear dispersion relation. Wave numbers and celerity are extracted from optical Sentinel-2 imagery, by exploiting the small temporal offset between the image bands of its Multi-Spectral Instrument. Inverted depths are output at 200 m resolution, and individual depth points are merged to create a composite bathymetry using a weighted average of images from 10 different dates. The resulting bathymetry mosaic spans 4000 km along the West African coast. S2Shores is able to detect depths up to 35 m, depending on mean incident wave conditions and cloud cover, which varies by location. Underwater features are well reproduced by S2Shores, such as flow channels in Guinea, the St. Ann's Shoal in Sierra Leone, and ebb delta lobes at several outlets along the Niger River Delta. S2Shores results match well (r2 = 0.76, RMSE = 4.9 m) with a bathymetry survey along the Senegalese coast. As a difference with traditional satellite-derived bathymetry methods based on water colour, a wave-based approach allows estimations in turbid areas and relatively deep waters, which suggest that the two approaches are complementary and should be used in combination to cover coastal environments in their diversity. The new possibility offered by this regional coastal atlas opens the door to increased research and planning capabilities and sets an example that can be applied to the rest of the world.
•S2Shores SDB algorithm used to estimate bathymetry from Sentinel-2 MSI image bands.•Depth estimates spanning 4000 km along the West African coast compared to GEBCO.•Maximum detecable depth of 35 m is 2.5 times deeper than colour-based SDB methods.•Shallow features are well reproduced, such as underwater dunes and ebb delta lobes.•S2Shores opens the door to increased research and planning capabilities in the region.