Comparisons and water level analyses using Sentinel-6MF satellite altimetry data with 1D Mascaret and 2D Telemac models.

Remote sensing products provided by satellite missions, airborne and unmanned aerial vehicle (UAV) campaigns have tremendously developed over the last decade. They undoubtedly offer opportunities to improve our ability to monitor and forecast flooding. The observation of inland waters benefits from several altimetry missions that provide along-track water surface elevation observation from nadir (e.g. TOPEX/Poseidon, Jason, SARAL/AltiKa, Sentinel-6) or large-swath altimeters (e.g. SWOT launched in December 2022), as well as other radar/optical missions (Sentinel-1, Sentinel-2) that provide high-resolution water masks. The limitations of each type of sensor are potentially circumvented when data from different satellite sensors are combined; the fusion of multi-source data has thus become one of the mainstream research topics in the remote-sensing community nowadays. Such fusion can be achieved with data assimilation algorithms applied to hydrodynamics models, namely MASCARET-1D and TELEMAC-2D. 

The present work focuses on the validation of water surface height (WSH) data from Sentinel-6MF with respect to in-situ gauge data, UAV and 1D/2D-hydrodynamics model outputs as shown in Figure 1. This work participates in a global effort that aims at combining various remote sensing products to represent and forecast flooding. The study is carried out over a dry period in June 2022 and over a flood event that occurred in December 2021-January 2022 over the Garonne catchment near Marmande, in the southwest of France. The WSH of the river is retrieved from Sentinel-6MF high-resolution fully-focused SAR data with an algorithm that relies on the estimation of the river width and the positioning of the river center line. The impact of these a priori data is investigated and the Sentinel-6MF-derived WSH observations are compared to the WSH simulated with TELEMAC-2D. It should be noted that due to the defection of Sentinel-1B (one of the two satellites in the Sentinel-1 constellation) in mid-December 2021, this flood event is only partly observed by Sentinel-1A and that the additional data from Sentinel-6MF with a 10-day revisit period are of great use. This study shows that hydrodynamic simulations and satellite altimetry time-series compare particularly well during the dry period whereas flooding events are often underestimated by satellite altimetry data. We believe that the combined use of satellite altimetry, hydrodynamics model simulations and independent UAV-borne data is key to a better understanding of the processes involved in the Garonne river flow dynamics and flooding events. We also take advantage of this study to improve our Sentinel-6MF data processing techniques.

Figure 1- Representation of water level height and flood extent with remote sensing data and hydrodynamic models.