From SWOT Cal/Val to Science Phase : Assessing and Enhancing Multi-Mission Satellite Altimeter Data (SWOT, Sentinel-3) for Hydraulic Visibility of the Natural Upper Rhine

Early analysis conducted on a reach of the Upper Rhine during the daily revisit Calibration/Validation phase of the SWOT mission (Cal/Val 1-day) have demonstrated the very high quality of SWOT observations, both in terms of absolute water levels and the retrieval of fine hydraulic signatures such as riffles, pools, gravel bars, and slope breaks (Ledauphin et al., 2025). SWOT data have notably shown their ability to accurately reflect channel morphology, as well as temporal variations in the longitudinal water surface profile as a function of discharge. However, the strong hydrological and hydro-sedimentary dynamics of the Rhine cannot be fully captured during the SWOT Science phase, characterized by a 21-day revisit period and one or two overpasses, which is likely to miss rapid processes such as floods and related morphological adjustments.

In this context, this study focuses on the long Old Rhine, a 50-km long bypassed reach with a width ranging from 80 to 150 m, resulting of the successive of the Rhine’s course over the past two centuries, including the construction of hydroelectric plants. This reach has a minimum flow of 52 m³/s through it, most of which is diverted into the Grand Canal d’Alsace to supply power plants. The site is subject to pronounced hydro-sedimentary dynamics and benefits from a particularly dense observational framework, including in situ gauging stations, bathymetric surveys, topo-bathymetric LiDAR data, and drone acquisitions. It is also a calibration site for several satellite altimetry missions, including SWOT (Tier-1 site during the Cal/Val phase) and Sentinel-3 (ESA St3TART projects (Da Sylva et al., 2023)), making it a well-known reference site for multi-mission altimetric studies.

The primary objective of this study is to exploit the SWOT Cal/Val 1-day phase to consistently compare and validate observations from different satellite altimetry missions at equivalent dates. This multi-mission approach allows the assessment of data quality, consistency, and complementarity in a dynamic fluvial environment. Hydraulic-preserving filtering methods are applied to improve and homogenize water surface longitudinal profiles and derived slopes, particularly in morphologically complex areas, with their performance evaluated using in situ measurements (Montazem et al. 2025; Larnier et al., 2025). As a result, WSE profiles where longitudinal signatures and non linearities due to morphological variability are well preserved/depicted

In a second step, the complementarity of multi-mission observations is analyzed during the SWOT Science phase to determine to what extent their combination enables a denser temporal sampling and a more detailed monitoring of the long Old Rhine dynamics compared to individual missions. Finally, the combined datasets are used to assess the potential for calibrating and cross-validating hydraulic models, integrating recent topo-bathymetric data. Preliminary results highlight the strong potential of SWOT data and multi-mission altimetry for the dynamic monitoring of large rivers and for improving hydraulic modeling at reach to regional scales.