Coastal modelling topography data by waterline retrieval from Sentinel-1 satellite acquisitions: from method development to a Near Real Time service

The automatic daily processing of coastlines from Sentinel-1 satellite data for updating the bathymetry to support modelling as part of a Near Real Time chain is being developed. In shallow waters, the physical processes caused by the interaction between waves, currents and the sea bottom become important and are crucial for modelling, requiring current and accurate bathymetry data. The spatial properties of sea state and currents change strongly in coastal areas and especially in the Wadden Sea at the Danish, German and Dutch North Sea coast with large areas of intertidal flats. The morphodynamics of seabed structures are significant in such littoral zones; the soft seabed can change within days during severe storms. Data from past measurement campaigns, which are very expensive using ship soundings and airborne LIDAR scanning, deprecates quickly as the positions of islands, sandbanks or tidal inlets change. Acquisitions from the Sentinel-1 (S1) Synthetic Aperture Radar (SAR) satellites cover most coasts worldwide. With two satellites currently in orbit, the acquisition interval is at most 6 days at the equator and almost daily in our latitudes. The Interferometric Wide Swath (IW) mode covers lands and coastal waters with a swath width of 250km and an image resolution of 10m. In contrast to optical satellites, SAR acquisitions are independent of illumination and cloud cover; hence, they deliver new images very reliably. The DLR satellite ground station in Neustrelitz receives these data and processes and delivers them in Near Real Time (NRT), usually within 20 minutes after acquisition. We have developed automated algorithms to retrieve the waterline from SAR images. In the tidal flat areas of the Wadden Sea, analyzing time series of acquisitions at similar tidal states allows estimating rates of change and sediment transport; a combination of several images acquired within a short time frame at different tidal states allows estimating the topography. Our algorithms for Sentinel-1 SAR images offer a quick and cheap way to spot ongoing processes, verify modelling results and identify regions of major change for future measurement campaigns.