Assessment of SWOT water surface elevations for flood monitoring of a narrow river (< 50 m width)

Abstract. Floods are among the most frequent and damaging natural hazards worldwide, and reliable observations of water surface elevation (WSE) are essential for improving flood modelling and risk management. The Surface Water and Ocean Topography (SWOT) satellite, launched in 2022, offers new opportunities to monitor river hydrodynamics from space, but its performance in relatively narrow rivers (< 50 m width) remains poorly documented. This study evaluates the potential of SWOT WSEs for flood monitoring by comparing them with in situ observations as well as simulations from an existing large-scale hydraulic model (LISFLOOD-FP) on the Du Gouffre River (width ≈ 40 m), located in Quebec, Canada. The L2_HR_RiverSP (RiverSP) SWOT product Version D, derived from a priori database (SWORD), was first compared with one-minute WSE measurements from a tidal gauge located downstream the Du Gouffre River in the St. Lawrence River. This comparison confirmed the overall quality of the SWOT data in this area, with a root mean square error (RMSE) of 0.24 m. Then, a major flood event (with a return period of about 60 years) which occurred on May 1, 2023, during the SWOT’s calibration orbit, was used to conduct a daily analysis of the entire flood event. Eleven observation cycles, covering the period from April 25 to May 7, 2023, were analysed. Limited ground-based observations were available along the studied reach during the flood, highlighting the value of SWOT data. The 1D/2D hydraulic model LISFLOOD-FP was run for the discharges corresponding to eleven SWOT cycles. Overall, there was good agreement with SWOT WSEs, with biases ranging from -0.30 to 0.44 m and RMSEs between 0.14 and 0.54 m. For the peak-flood cycle (May 1), upstream discharges were initially underestimated, and an adjusted LISFLOOD-FP simulation constrained by SWOT observations resulted in a bias of -0.30 m and an RMSE of 0.54 m. This study confirms that SWOT WSEs can provide relevant hydraulic information during flood events in a river below the mission’s detection limit, thereby opening the way for a broader use in flood monitoring and modelling.