Effects of gravel on concentrated flow hydraulics and erosion in simulated landslide deposits

•Field investigation and laboratory scouring simulations of landslide deposits were performed.•Gravel contents had significant effects on flow hydraulics.•A threshold of 25% gravel content was the turning point for hydraulics and erosion.•The rate of soil and water loss had a significant logarithmic relationship with Re.

Gravel has an important influence on concentrated flow hydraulics and soil and water loss. We conducted a field investigation and a subsequent simulated scouring experiment with four gravel contents (0, 25, 33.3, 50%) and three inflow rates (4, 8, 12 L/min) under two steep slopes (67.5, 72.7%). We assessed the characteristics of concentrated flow hydraulics, including flow velocity, flow depth, Reynolds number (Re), Froude number (Fr), Darcy-Weisbach friction coefficient (f), and the rate of soil and water loss (SW). We observed significant effects of gravel and inflow rate on hydraulics and erosion, as well as interactions between gravel and inflow rate. The mean flow velocity reduced as the gravel content increased. The changes in flow depth fluctuated with gravel content in the order 25 > 0 > 50 > 33.3%. Re was larger in 0% gravel than in 25, 33.3 and 50% gravel. Fr was > 1 in 0% gravel but was approximately equal to 1 in gravel-containing soils. Values of f were lowest in 0% gravel. The f values for gravel-containing soils not only decreased as the gravel content increased, but also had a significant relationship with the corresponding Re values. SW was maximal in 25% gravel content and gradually declined for gravel content beyond 25%. SW had a significant logarithmic relationship with Re. The results have significant implications for predictions of soil and water loss steeply sloping, gravel-rich, soils.