Streamflow Monitoring at High Temporal Resolution Based on Non‐Contact Instruments and Manually Surveyed Bathymetry in a River Prone to Bathymetric Shifts

Abstract This study presents a proof of concept of a reliable methodology for monitoring streamflow in a dynamic river of the Alps prone to bathymetric changes using non‐contact instruments. The method relies on water level and surface velocity radar monitoring, discharge measurements by Large‐Scale Particle Image Velocimetry (LSPIV), and topographic surveys. A single proportional relation, stable under bathymetric changes, is established between maximum surface velocity ( V s,max ) and bulk velocity ( U mean ) using LSPIV measurements. The location of the maximum surface velocity is also shown to be relatively stable under bathymetric changes. The Isovel model, a theoretical approach which requires minimal information (i.e., bathymetry, water level and bed roughness) is also used to assess its capacity to predict the V s,max – U mean relation and the location of the maximum surface velocity. Such model could be useful for applying the method in the absence of LSPIV measurements in the future. The applicability of the method is finally tested over a 2.5‐year data set. Discharge is calculated at a time step of 10 min by multiplying the bulk velocity and the wetted cross‐sectional area. The results are compared to the specific discharge time series at the historical station located 2.5 km further upstream, which has a stage‐discharge rating curve, to assess the credibility of the proposed method. Good agreement is generally observed when surface velocity is above 0.7 m/s, but accuracy decreases for lower velocities. A simplified uncertainty analysis estimates a 25% relative error on discharge calculated with the presented method.