Advantages of a mobile LSPIV method for measuring flood discharges and improving stage–discharge curves

Abstract This paper investigates the potential of fast flood discharge measurements conducted with a mobile LSPIV device. LSPIV discharge measurements were performed during two hydrological events on the Arc River, a gravel-bed river in the French Alps: a flood greater than the 10-year return period flood in May, 2008, and a reservoir flushing release in June, 2009. The mobile LSPIV device consists of a telescopic mast with a remotely controlled platform equipped with a video camera. The digital video camera acquired sequences of images of the surface flow velocities. Ground Reference Points (GRPs) were positioned using a total station, for further geometrical correction of the images. During the flood peak, surface flow velocities up to 7 m/s and large floating objects prevented any kind of intrusive flow measurements. For the computation of discharge, the velocity coefficient was derived from available vertical velocity profiles measured by current meter. The obtained value range (0.72–0.79) is consistent with previous observations at this site and smaller than the usual default value (0.85) or values observed for deeper river sections (0.90 typically). Practical recommendations are drawn. Estimating stream discharge in high flow conditions from LSPIV measurements entails a complex measurement process since many parameters (water level, surface velocities, bathymetry, velocity coefficient, etc.) are affected by uncertainties and can change during the experiment. Sensitivity tests, comparisons and theoretical considerations are reported to assess the dominant sources of error in such measurements. The multiplicative error induced by the velocity coefficient was confirmed to be a major source of error compared with estimated errors due to water level uncertainty, free-surface deformations, number of image pairs, absence or presence of artificial tracers, and cross-section bathymetry profiles. All these errors are estimated to range from 1% to 5% whereas the velocity coefficient variability may be 10%–15% according to the site and the flow characteristics. The analysis of 36 LSPIV sequences during both events allowed the assessment of the flood discharges with an overall uncertainty less than 10%. A simple hydraulic law based on the geometry of the three sills of the Pontamafrey gauging station was proposed instead of the existing curve that is fitted on available gauging data. The high flow LSPIV discharge measurements indicated that this new curve is more accurate for high discharges since they are evenly distributed in a ±10% interval around it. These results demonstrate the interest of the remote stream gauging techniques together with hydraulic analysis for improving stage–discharge relationships and reducing uncertainties associated with fast flood discharges.