Impacts of raindrops increase particle sedimentation in a sheet flow

International audience; Interrill erosion is driven by raindrops and sheet flow. Raindrop impacts cause sediment detachment and splash, but can also affect flow transport. Even if these processes have been studied for long, the actual effect of raindrop impacts on particle settling velocities has not been experimentally assessed. This leads to unconstrained adjustments in the soil erosion models, the settling velocity of particles being a freely adjustable parameter allowing for better fitting the particle flux measured at the outlet. To address the effect of raindrop impacts on the settling of particles in sheet flow, a laboratory flume experiment was designed, using an upstream feeder of sediment (100–200 μm) and simulated rainfalls. It reproduced conditions close to sheet flow, while not allowing for the detachment of particles from the flume bottom. Two series of experiments were run: a series with a high rainfall intensity (175 mm h−1) generated by an oscillating-nozzle rainfall simulator, and a series with lower rainfall intensities (10, 15, 25, 35, 55 mm h−1) generated by a drop-former rainfall simulator. When a rainfall was applied, it systematically decreased the sediment concentration at the outflow compared to the no rain condition, however no obvious relationship was found with the rainfall intensity. This shows that raindrop impacts increase particle settling velocities in sheet flow. Two underlying mechanisms are suggested, related to the momentum of the raindrops or to the turbulence caused by the raindrops into the flow. Further studies should be carried out, using computational fluid dynamics and collaboration with the fluid mechanics community.