Monitoring intertidal topography using the future SWOT (Surface Water and Ocean Topography) mission

Abstract

Intertidal flats are very dynamic environments pressured by anthropogenic activities and climate change. There is an increasing need for updated intertidal topography maps for scientific and coastal management purposes. The future wide-swath altimetry mission SWOT (Surface Water and Ocean Topography) will provide valuable data for intertidal topography monitoring. SWOT will use interferometry to measure heights over water surfaces only, which will prevent the use of this method over intertidal areas. Since SWOT is expected to provide highly accurate 2D sea level measurements in coastal areas, it will be very beneficial for the waterline method. The waterline method is one of the most commonly used spaceborne methods for mapping intertidal topography. This method uses a combination between a series of satellite images acquired at different tidal stages (radar or optical) and sea level information (tide gauge records, models output, or satellite measurements). The aim of this study is to investigate the performance of SWOT in generating intertidal Digital Elevation Models (DEMs) using the waterline method (with a brief investigation regarding the interferometric capabilities of SWOT over intertidal areas). To this end, tests were made by generating intertidal DEMs from SWOT-type observables for two intertidal bays situated on the French coast: the Arcachon Bay and the Bay of Veys. SWOT-type observables were simulated using the SWOT large scale simulator developed by the French space agency (CNES - Centre National d’Etudes Spatiales). By comparing SWOT-derived DEMs to the validation DEMs (generated by the waterline method using Sentinel-1 and -2 images), SWOT showed a great potential for monitoring intertidal topographies. Mean Absolute Errors (MAE) and Root Mean Squared Errors (RMSE) reached respectively 5.2 cm and 8.4 cm for the Arcachon Bay and 10.2 cm and 17.3 cm for the Bay of Veys. However, the accuracy of SWOT is dependent on SWOT's orbit. An unfavorable sampling due to tidal aliasing decreases the accuracy of the DEMs. Thus, the period during which the series of satellite images are acquired (the acquisition period) shall be chosen carefully in order to generate accurate SWOT DEMs. SWOT will be able to generate relatively accurate DEMs with period of acquisitions of 3 months. With acquisition periods that provide adequate sampling of the tidal range, SWOT will be able to detect topographic changes between consecutive DEMs. The present work was supported by the CNES in the framework of the future mission SWOT.