West African Coastal Bathymetry Derived from Wave Kinematics in Sentinel-2 Satellite Imagery

Abstract:

Much of the world’s oceans and coastal waters remain unmapped using traditional ship-based bathymetric survey techniques because of the enormous amount of time it takes to collect and process such data. Satellite-based missions, on the other hand, offer the possibility to collect data at a global scale with higher revisit times. Satellite-derived bathymetry (SDB) techniques using water colour as a proxy for depth have been developed over recent decades to try to fill the void in coastal bathymetry; however, it is generally limited to 10 m depth and depends heavily on water clarity. Here, we use a novel wave-kinematics-based SDB technique, S2Shores (Satellite to Shores), with image products of the European Space Agency's Sentinel-2 mission. Small temporal offsets between the image bands of the Sentinel-2 Multi-Spectral Instrument allow phase shifts in the wave field to be detected using a localized Radon Transform combined with a DFFT technique. From this, wave celerity is obtained, which is then inverted for depth using the linear dispersion relation. After cloud removal, multiple bathymetry estimates are used to create a single composite for specific Sentinel-2 tiles, which are then merged to create a mosaic of depth estimates spanning 4000 km along the West African coast. Data is output at a resolution of 100 m at the regional scale, and may be further refined in local areas. S2Shores is able to detect depths up to 45 m (an area 2.5 times greater than colour-based techniques), depending on mean incident wave conditions (the longer the wave period, the deeper the estimate) and cloud cover. Shallow water features between 2-15 m are well reproduced by S2Shores, such as the Senegalese barrier island coast, tidal flow channels in Guinea, the St. Ann's Shoal in Sierra Leone, and ebb delta lobes at several outlets along the Niger River Delta. This new Coastal Atlas of West Africa opens the door to increased research and planning capabilities for the region, such as monitoring the dynamics of shallow coastal features on a seasonal to annual basis, and it also sets an example that can be upscaled to the rest of the world.