Velocity and suspended sediment concentration profiles in rivers: in situ measurements and flux modelling

摘要

River basin sediment management is increasingly needed to address both sediment quantity and quality studies. In this context, the sampling of suspended sediment through a river cross-section is unavoidable. Rivers have the interesting property of concentrate sediment and processes through a river cross-section are representative of the whole upstream hydro-sedimentary processes. Evaluating sediment budget at the outlet of a river basin is closely related to the experimental method used, as suspended sediment concentration and velocity are non-constant over the water height. This paper presents results obtained from an in situ experimental program, which aims to measure the velocity and suspended sediment concentration profiles in rivers during different hydrological conditions. In particular, the study focuses on the region located near the interface between water and settled sediment. This region is usually neglected as it corresponds to the dead zone of non-intrusive instruments such as Acoustic Doppler Current Profilers. This is achieved by the use of specific instruments. A velocity profiler was used to measure the velocity profile within the few percents of the water height located near the interface between water and settled sediment. In addition, sediment concentration profiles were measured either by a turbidity sensor moved over the water height or by 144 turbidity sensors mounted on a vertical stick, which allows one to record instantaneously the turbidity profile close to the interface between water and settled sediment. In situ measurements are compared to theoretical models commonly used: the Rouse profile for suspended sediment concentrations, and the logarithmic law for velocities. Results notably show that during low water flows, 35% of the total sediment flux may be located within the first 15% of the water height. Then, following the sampling strategy adopted, the error on the mean suspended sediment flux may be up to 50%.