The role of detention storage of sediment on erosion by rain-impacted flows.

• Amount & composition of stored sediment in rain-impacted flows affect erosion. • Amount & composition of stored sediment varies until the steady state occurs. • Compositions of stored and discharge sediment differ from each other. • Compositions of soil and sediment discharge is the same at the steady state. Many experiments on erosion by rainfall involve applying artificial rainfall on inclined planar soil surfaces. When sediment is discharged by rain-impacted flows, the transport of silt and sand-sized particles is dominated by raindrop-driven transport processes such as raindrop-induced saltation and rolling. Raindrop-induced saltation is a process which stores detached material on the soil surface as it transports detached material downstream. The amount stored increases with time until it reaches a steady state. If all particles transported downstream travel at the same rate, then the composition of the stored material is the same as that of the original soil. However, particles transported by raindrop-induced saltation travel at rates that vary with their size a density. This results in the composition of the sediment discharged to vary with time until the steady state is reached. In this paper, a qualitative mathematical model is used to demonstrate how the composition of the stored material varies with slope length, rainfall intensity, flow depth and the particle sizes being transported. The results produced by the model demonstrate that fine material is enriched at the expense of coarse material in the sediment discharge while being depleted in the stored material until the steady state is reached. The buildup of detained material on the soil surface also influences the amount of soil material discharge as time progresses during an erosion event. Consequently, the erosion rate will vary with time until the steady state is reached. Many experiments involve applying rain for a fixed time and consider the average erosion rate as the independent variable is regression analyses to determine the effect of factors such rainfall intensity, slope length, slope gradient and soil type on erosion. The inclusion of erosion when the steady state does not exist means that the results of these experiments have limited values beyond the experimental conditions used in the experiment. [ABSTRACT FROM AUTHOR]