The soil configuration on granite residuals affects Benggang erosion by altering the soil water regime on the slope

A permanent collapsing gully, locally called Benggang, formed on slopes with deep granite red soil and is a type of unique gully erosion widely prevalent in southern China. Three different soil configurations (SC), ie, red-transition-sandy (SC Ⅰ, the transition is the soil layer between the red soil and the sandy soil layer), transition-sandy (SC Ⅱ) or sandy (SC Ⅲ) are usually present in the soil profile of the Benggang slope. However, little attention has been paid to impacts of SCs on the triggering of Benggang erosion. In this study, we aimed to explore the relationships between soil water content (SWC) and triggering of Benggang erosion under different SC conditions. The soil properties of different soil layers were measured and the SWC at depths of 20, 40, 60, and 80 cm were monitored at 5-min intervals along a typical Benggang (SC Ⅰ) during 2016–2018. The SWC of Benggang slopes with different SCs were simulated by VADOSE/W model. Results showed that the red soil layer had a higher water retention capacity and shear strength than the sandy soil layer. Even if the SWC is higher (e.g., 0.42 cm3/cm3) at red soil layer or transition layer, the corresponding shear strength is greater than that of sandy soil layer with a lower SWC (e.g., 0.32 cm3/cm3). Relationships between shear strength and SWC of different soil layers indicate that Benggang erosion is triggered by an increase in the SWC in the deep sandy layer. Results also showed that differences exist in the SWC distribution among the different SCs. The SWC is higher in topsoil than in deeper soil in SC Ⅰ and SC Ⅱ, while in SC Ⅲ, the opposite trend is observed. These results revealed that the presence of the red soil or transition layer can reduce the infiltration of rainwater into the deep sandy layer, thus can reduce the possibility of collapse. Our results show that the SC affects the stability of the headwall, and results provide great significances to guide the mitigation of Benggang erosion.