Controls of Upstream Sediment Erosion on the Deposit Characteristics of Debris Flows

Debris flows can erode mountainsides, cover alluvial fans, and bury people and property by rapid deposition. The deposition characteristics of debris flows are strongly affected by their dynamics and composition, which depend on upstream sediment erosion, but how is still under scientific debate. Here, we conduct a series of experiments to analyze the effects of debris flow grain-size gradation and eroded bed sediment on deposition characteristics. Debris flows deposit on a gentle runout zone and form coarse-grained lateral levees and front lobes and a finer-grained channelized interior due to grain segregation. We show that affected by a high basal pore-fluid pressure, released mud-sand-gravel flows present much flatter deposits than sand-gravel flows. Runout distance, width and inundated area increase with higher bed water content due to the growths of flow volume and momentum. Inundated area correlates to deposition volume with a power relation for all experiments. Savage number shows the greatest positive correlation with runout and inundated area among all factors, suggesting that potential energy of debris flow is more strongly consumed by grain collision stress than by basal friction stress. Debris flows can deposit as a single nose or multiple fingers depending on the relative magnitude between the friction force at the flow front balanced by downslope gravity and the thrust force of the following channelized flow with a higher speed. Our results facilitate the mapping of debris-flow impact zones and provide a mechanistic model for predicting deposit shape in debris flows and other geophysical flows like pyroclastic flows.