Rill erosion in post-seismic watershed – A non-negligible transporting way of fluvial sediment

Abstract

Mass wasting caused by large-magnitude earthquakes supplies suspended sediment. Landslides and debris flows are commonly considered major transport pathways by which mass wasting is conveyed to streams. However, a contradiction exists in that fluvial sediment remains high years after the earthquake, when landslides and debris flows rarely occur. Our study assumes that water erosion is a major process releasing fine grains and estimate sediment from landslide, debris flow and water erosion. When calculating water erosion, we simulate two states of rill and sheet erosion, respectively. By comparing observed sediments to simulated sheet and rill erosion, our results verify that alteration from sheet to rill erosion is major primary for fine sediment transportation in post-seismic watershed. Changes in the erosion state increase the sediment supply to streams, turn the sediment regime from “supply-limited” to “transport-limited”, and explain the high fluvial sediment amount under well-recovered vegetation conditions in post-seismic watersheds. Water erosion is demonstrated to be as important as debris flows in exporting suspended sediments and is suggested to be included in the mass balance calculations of earthquakes.