Coupling effects of erosion and surface roughness on colluvial deposits under continuous rainfall

The colluvial deposits, resulting from the collapsing gully erosion, provide a large amount of sediment for subsequent water erosion in southeastern China. Previous studies have shown the apparent effects of rainfall, runoff and slope gradient on erosion of colluvial deposits, but paid little attention to the effect of soil surface roughness. The objective of this study was to investigate the relationship between surface roughness and erosion processes under continuous rainfall conditions. For this purpose, we repeated an indoor rainfall experiment 20 times on a colluvial deposit slope of 30 degrees. The rainfall intensity was 3.33 mm min(-1) and the rainfall lasted 60 min. We measured the sediment yield for each rainfall experiment and the related hydrodynamic parameters, including the flow velocity, Froude number, Reynolds number, Runoff power and Darcy-Weisbach resistance coefficient. We found that the gradual enrichment of coarse particles and gravel on the surface of the colluvial deposits led to an exponential decrease in sediment yield. The water erosion consists of three processes: (1) The emergence of the falling-sill, followed by the formation and rapid expansion of the rill; (2) The frequent expansion of the rill; and (3) The random landslide of the rill wall. The flow velocity, Froude number (Fr), Reynolds number (Re) and runoff power (omega) all showed a power decrease in contrast to a power increase of Darcy-Weisbach resistance coefficient (f) due to the influence of the roughness of colluvial deposits. The roughness and flow velocity can be used as good indicators for characterizing the water erosion intensity of the colluvial deposits. The findings will facilitate further research into the effect of roughness on the water erosion of colluvial deposits.