Upslope sediment-laden flow impacts on ephemeral gully erosion: Evidences from field monitoring and laboratory simulation

Ephemeral gully (EG) is an important linear erosion feature, often occurring at mid-slope position, that can be greatly influenced by upslope inflow and its sediment transport capacity. How sediment concentration in upslope inflow influences downslope EG erosion is still unclear. Thus, field monitoring on a natural hillslope and indoor simulations on a dual-box system were conducted to better understand their relationship. The results showed that both field monitoring and indoor simulation displayed the additional sediment delivery caused by upslope sediment-laden flow, verifying the transport-dominated sediment regime in EG systems. In field observations, the sediment increment coefficient (SIC, ratio of net sediment delivery caused by upslope sediment-laden flow to the total sediment delivery) on an event scale varied from 4.6% to 88.6%. In indoor simulations, the SIC changed from 24.9% to 87.5%. Indoor simulation experiments with a range of 0-378.2 g L-1 sediment concentration supplied by a feeder box, showed that SIC linearly decreased as sediment concentration of upslope inflow increased. Field monitoring showed complicated phenomenon because of natural random variations. For rainfall events with PI30 (the product of precipitation and maximum 30-minute rainfall intensity) larger than 400 mm(2) h(-1) (similar to the designed extreme rainfall in laboratory simulations), SIC also generally decreased as sediment concentration of upslope inflow increased, especially when sediment concentration of upslope inflow was smaller than 400 gL(-1). Thus, laboratory simulations verified the field monitoring results of extreme rainfall events with large rainfall amounts and intensities. This study illustrated the necessity of in-situ field monitoring and indicated that design of indoor simulation experiments needs more comprehensive considerations on complicated hydrological and sediment conditions.