Unsteady soil erosion due to rainfall impact: a model of sediment sorting on the hillslope

A new method is presented for predicting sediment sorting associated with s oil erosion by raindrop impact for non-equilibrium conditions. The form of soil erosion considered is that which results from raindrop impact in the p resence of shallow overland flow itself where the flow is not capable of er oding sediment. The method specifically considers early time runoff and ero sion when sediment leaving an eroding area is generally finer and thus may have a higher potential for transport of sorbed pollutants. The new mechani sm described is the formation of a deposited layer on the soil surface, whi ch is shown to lead to sediment sorting during an erosion event. The deposi ted layer is taken to have two roles in this process: to temporarily store sediment on the surface between successive trajectories, and to shield the underlying soil from erosive stresses. Equations describing the dynamics of the suspended sediment mixture and the deposited layer are developed. By i ntegrating these equations over the length of eroding land element and over the duration of the erosion event, an event-based solution is proposed whi ch predicts total sediment sorting over the event. This solution is shown t o be consistent with experimentally observed trends in enrichment of fine s ediment. Predictions using this approach are found to only partly explain m easured enrichment for sets of experimental data for two quite different so ils, but to be in poor agreement for an aridsol of dispersive character. It is concluded that the formation of the deposited layer is a significant me chanism in the enrichment of fine sediment and associated sorbed pollutants , but that processes in the dispersive soil are not as well described by th e theory presented. (C) 1999 Elsevier Science B.V. All rights reserved.