Effect of raindrop splash and transversal width on soil erosion: Laboratory flume experiments and analysis with the Hairsine-Rose model

The parameter consistency of the one-dimensional Hairsine-Rose (H-R) erosion model under conditions of significant rainfall splash was examined. To account for the splash characteristic length scale and its interaction with the transverse erosion width, experiments were carried out using erosion flumes of the same length (6 m), but different widths, with sediment concentrations measured at the flume exits. Total sediment concentration and the concentration of seven size fractions (< 2, 2-20, 20-50, 50-100, 100-315, 315-1000 and > 1000 µm) were measured at high rainfall intensity (60 mm/h) and with a gentle slope (2.2%). The conditions employed ensured that erosion was predominantly precipitation-driven. The experimental results showed that raindrop splash affected particularly the sediment breakthrough from the wider flumes (flumes 1 and 2, 1- and 0.5-m wide, respectively). However, the raindrop splash effect was less significant in observed sediment concentrations from the narrower flumes (flumes 3 and 4, both 0.25-m wide). For these flumes, the detached sediment was affected by the transversal width of the flume in that an amount of detached sediment adhered to the barriers instead of being removed in the overland flow. The one-dimensional H-R model was fitted to the experimental results and good agreement was found, in particular for the finer size classes. The data for the coarser grain sizes were more scattered, suggesting sediment motion by mechanisms other than as a suspension in the overland flow (e.g., rolling along the soil surface). The optimized parameters indicated that the shield layers (where the shield consists of redeposited eroded sediment) of the wider flumes (1 and 2) developed within 5-10 min from the start of the experiment, whereas in the narrower flumes (3 and 4) they never fully developed. The optimized detachment rates were consistent with previous findings, but the estimated thickness of the deposited layer was too small to provide complete protection of the original soil against raindrop detachment, indicating that the shield was not uniform. The experimental design allowed us to investigate directly the effect of flow non-uniformity on soil erosion by inclusion of an offset drainage point in flume 4. The observations taken during and after the experiment, as well as surface elevation data, confirmed the noticeable impact of non-uniform flow on the erosion process.