**Abstract Text**
The understanding of sand dynamics in rivers is a scientific challenge. Sand loads in rivers have been reduced due mostly to dam construction and changes in watershed management. Severe impacts due to erosion and deposition can be observed in some rivers and coastal areas. It remains very difficult to measure vertical and horizontal suspended-sand gradients in rivers because existing sampling methods are either time-consuming or incomplete. The classical approach by solid gauging consists of taking several samples throughout the river cross-sections, which results in a limited spatial and temporal resolution. However, solid gauging is still the only method to estimate sand fluxes and to validate other methods such as hydroacoustic methods.
In this study, we deployed several samplers and we compare measured concentrations in different configurations. These comparisons are helpful to identify and evaluate uncertainty sources and to improve our field protocols. As from 2021 we have been intensified sampling campaigns at various hydro-sedimentary conditions in the Isère River including during a dam flush and snowmelt season experiments. Isere River is an Alpine river in which the suspended sediment load is characterized by a bimodal sediment particle distribution. Several comparisons were conducted including pumping samplers, isokinetic samplers (US-P72, US-P6, Delft Bottle, Nilsson sampler). We observed variability in concentration between the different samplers, even in short time intervals. Solid gauging with distributed samples highlight the spatial variability of the suspended sand concentration in the cross-sections. Other experiments with repetitions at a same location with different samplers indicate that the temporal variability of suspended sand concentration is one of the main uncertainties. Finally, the computation method used to calculate the total flux also has an impact, especially concentration extrapolations in the unmeasured parts of the cross section.



Comparing suspended sand samplers and evaluating the uncertainty (Room 322, Wednesday June 28th, 10:30am-12:00pm)