Observing Sediment Transport in Taiwan's National Wetlands by Satellite Images

Alluvial estuaries along the west coast of Taiwan play as habitats for several endangered species. Wetlands are widely scattered over the coastline, including two international-level and forty national-level sites. To understand their evolutions of erosion and siltation processes associated with coastal current and hydrological regime, we analyze 21 major wetlands by observing their interannual changes exhibited in the multiyear satellite images. We utilize Sentinel-2 multispectral data for digital elevation model (DEM) inversion. All low cloud-coverage images are first collected, and the intertidal topography is calculated through waterline detection, image stacking, and marking the tidal range by the DTU16 tide model. Temporal changes in topography are revealed by the reconstruction of DEM on a two-year rolling update. The quality of DEM is verified at an accuracy of sub-meter level, by the multibeam echosounder during high tide. We find that the groin effect and artificially nourished beach are the main causes of sedimentation, while the erosions commonly co-exist near the upstream of the ocean current.