Abstract's details
Assessing and Improving SWOT’s Hydraulic Visibility on French narrow rivers (20-100 m): Precision Flow Lines and Slope variation in different flow condition
Event: 2025 SWOT Science Team Meeting
Session: Hydrology: River Science Working Group
Presentation type: Oral
The high accuracy of SWOT in capturing river surface elevations, slopes, and hydraulic dynamics was evaluated using a comprehensive in situ dataset over the Franco-German Rhine, with elevation accuracy exceeding expectations (Ledauphin et al., 2025). The analysis of products from PIXC to reach-averaged scales demonstrates SWOT’s effectiveness in depicting river surface topography and detecting fine spatial hydraulic signatures driven by longitudinal hydraulic controls including meter-scale sequences of riffles and pools, sandbanks, meanders, and man-made structures as well as dynamic phenomena such as flood wave propagation, intumescence, and a rare submersion wave slope.
This study focuses on evaluating SWOT’s accuracy in capturing hydraulic signatures over narrower rivers (20–100 m wide), including the Meurthe (30-40m), Moselle (50-100m), Ill (20-30m), and Sarre (20-30m, major flood in May 2024) in France’s Grand Est Region. By leveraging data from the nominal science orbit and applying advanced hydraulic-preserving filters to improve slope computation (Montazem et al., 2019), we assess SWOT’s performance near the limits of its design specifications, with up to four revisits per 21-day cycle enabling the observation of key hydrological events such as floods.
Each river’s morphology consists in a mix of natural and modified reaches, sampled at a different positions relative to the SWOT swath and in the overpass dates in the 21-days revisit cycle. This dataset enables evaluation of multi-path observations and the impact of cross-track distance on measurement quality.
These rivers benefit from dense in situ gauge networks with long historical records, complemented by field data (drone flights, bathymetric surveys), making them ideal for analysis the capacity of SWOT over narrow rivers. In the absence of full RiverSP coverage (except for parts of the Moselle), we focus on exploiting PIXC pixel cloud classes 3 and 4, corresponding to water-near-land and open water, respectively.
Preliminary results are promising. For example, the absolute water surface height differences between PIXC-derived and gauge data on the Moselle show a 1-sigma of 16 cm for the science phase to date. Continued comparison with high-resolution lidar topo-bathymetric DEMs are underway, aiming to quantify the imprint of riverbed topography on WSE which acts as a low-pass filter for bottom slope breaks (cf. Montazem et al., 2019).
After analyzing SWOT observations for several hydrological events of varying magnitudes, alongside data acquired during normal flow conditions, the initial results confirm that SWOT’s precision is high, even on small rivers. A large number of events have already been observed, including variations in the water surface elevation related to flow changes, their connection with bathymetry, and fluctuations during exceptional flood events. Each time, we have attempted to validate these observations using available exogenous datasets for each site, such as gauge time series, high-resolution bathymetry and drone flights;
For these narrow rivers, the limits of SWOT’s capability for direct use of raw data are approached, as measurement profiles can be noisy depending on acquisition conditions. To address this, a filtering approach for SWOT data that preserves hydrological characteristics has been developed and studied at fine scales to ensuring consistent and accurate computation of local water surface slopes (cf. Montazen et al., 2019). We also evaluate the impact of this filtering on water surface profiles by comparing the results with in situ measurements
The accuracy of SWOT measurements demonstrated in this study, along with the spatially distributed nature of WSE and slopes, highlight the potential of such data for depicting local hydraulic signatures and inform spatially distributed hydraulic models and regional hydrological models (e.g. Larnier et al. 2025). Such accurate data could also be of interest to support hydraulic structures operations and reinforce the inferrence of bathymetry with additional data.
References:
- Ledauphin T., Garambois P.-A., Larnier K., Azzoni M., Emery C., Picot N., Amzil S., Fjortoft R.,Maxant J., Yesou H. Assessing SWOT’s Hydraulic Visibility on the Rhine: Precision Flow Lines and Slope-Based Flood Wave Propagation Signatures. Earth and Space Science (Under publication)
- Montazem, A.-S., Garambois, P.-A., Calmant, S., Finaud-Guyot, P., Monnier, J., Medeiros Moreira, D.,. . . Biancamaria, S. (2019). Wavelet-based river segmentation using hydraulic control-preserving water surface elevation profile properties. Geophysical Research Letters
- Larnier K., Garambois P.-A, Emery C., Pujol L., Monnier J., Gal L., Paris A., Yesou H., Ledauphin T., Calmant S. (2025) Estimating channel parameters and discharge at river network scale using hydrological-hydraulic models, SWOT and multi-satellite data.
Back to the list of abstractThis study focuses on evaluating SWOT’s accuracy in capturing hydraulic signatures over narrower rivers (20–100 m wide), including the Meurthe (30-40m), Moselle (50-100m), Ill (20-30m), and Sarre (20-30m, major flood in May 2024) in France’s Grand Est Region. By leveraging data from the nominal science orbit and applying advanced hydraulic-preserving filters to improve slope computation (Montazem et al., 2019), we assess SWOT’s performance near the limits of its design specifications, with up to four revisits per 21-day cycle enabling the observation of key hydrological events such as floods.
Each river’s morphology consists in a mix of natural and modified reaches, sampled at a different positions relative to the SWOT swath and in the overpass dates in the 21-days revisit cycle. This dataset enables evaluation of multi-path observations and the impact of cross-track distance on measurement quality.
These rivers benefit from dense in situ gauge networks with long historical records, complemented by field data (drone flights, bathymetric surveys), making them ideal for analysis the capacity of SWOT over narrow rivers. In the absence of full RiverSP coverage (except for parts of the Moselle), we focus on exploiting PIXC pixel cloud classes 3 and 4, corresponding to water-near-land and open water, respectively.
Preliminary results are promising. For example, the absolute water surface height differences between PIXC-derived and gauge data on the Moselle show a 1-sigma of 16 cm for the science phase to date. Continued comparison with high-resolution lidar topo-bathymetric DEMs are underway, aiming to quantify the imprint of riverbed topography on WSE which acts as a low-pass filter for bottom slope breaks (cf. Montazem et al., 2019).
After analyzing SWOT observations for several hydrological events of varying magnitudes, alongside data acquired during normal flow conditions, the initial results confirm that SWOT’s precision is high, even on small rivers. A large number of events have already been observed, including variations in the water surface elevation related to flow changes, their connection with bathymetry, and fluctuations during exceptional flood events. Each time, we have attempted to validate these observations using available exogenous datasets for each site, such as gauge time series, high-resolution bathymetry and drone flights;
For these narrow rivers, the limits of SWOT’s capability for direct use of raw data are approached, as measurement profiles can be noisy depending on acquisition conditions. To address this, a filtering approach for SWOT data that preserves hydrological characteristics has been developed and studied at fine scales to ensuring consistent and accurate computation of local water surface slopes (cf. Montazen et al., 2019). We also evaluate the impact of this filtering on water surface profiles by comparing the results with in situ measurements
The accuracy of SWOT measurements demonstrated in this study, along with the spatially distributed nature of WSE and slopes, highlight the potential of such data for depicting local hydraulic signatures and inform spatially distributed hydraulic models and regional hydrological models (e.g. Larnier et al. 2025). Such accurate data could also be of interest to support hydraulic structures operations and reinforce the inferrence of bathymetry with additional data.
References:
- Ledauphin T., Garambois P.-A., Larnier K., Azzoni M., Emery C., Picot N., Amzil S., Fjortoft R.,Maxant J., Yesou H. Assessing SWOT’s Hydraulic Visibility on the Rhine: Precision Flow Lines and Slope-Based Flood Wave Propagation Signatures. Earth and Space Science (Under publication)
- Montazem, A.-S., Garambois, P.-A., Calmant, S., Finaud-Guyot, P., Monnier, J., Medeiros Moreira, D.,. . . Biancamaria, S. (2019). Wavelet-based river segmentation using hydraulic control-preserving water surface elevation profile properties. Geophysical Research Letters
- Larnier K., Garambois P.-A, Emery C., Pujol L., Monnier J., Gal L., Paris A., Yesou H., Ledauphin T., Calmant S. (2025) Estimating channel parameters and discharge at river network scale using hydrological-hydraulic models, SWOT and multi-satellite data.