Slope erosion induced by surges of debris flow: insights from field experiments

We conducted field observations and experiments to explore debris flow dynamics, sediment transportation and slope erosion at an active natural debris flow gully in the headwaters of Jiangjia Ravine (Dongchuan region, Southwest China). In this region, the hillslopes were heavily jointed, weathered and sparsely vegetated, providing continuous and rich sediment supplies for initiating debris flows. The debris flow propagated in the channel as a sequence of surges, with periodical changes of flow flux, velocity, water content, and viscosity as controlled mainly by the conditions of erodible sediments and water supplies from the upstream. The water content of bank sediments ranged from 5 to 8%, while it was 16 to 26% for debris surges in the channel. The particle size distribution of sediments on the alluvial fan followed the Weibull's cumulative distribution and the mean size was in the range of 2 similar to 4 mm. The coarse particles were primarily elongated or prismoidal and aspect ratios followed well a normal distribution with the mean value of 0.4. The angular particles entrained in dense viscous debris flow surges could effectively abrade and groove the channel bed and banks, increasing the intensity of slope erosion. The incised slope had a sequence of terraced depositional layers on both banks. The layer thickness decreased as the erosion depth moved deeper into the stratum where hard bed soil/rock layers existed. The water-soil mixture of debris flow exhibited a clear shear-thinning behavior with its viscosity decreasing gradually with the increase of shear rate following the widely accepted power-law model. The dense viscous debris flow can facilitate the transportation of coarse gravels in channel and contribute to slope erosion.