Subsurface Lateral Flow in Steep Granite Slope and Its Relation to Benggang Erosion

Rainfall-induced soil slope failures represent a widespread issue, with subsurface lateral flow (SLF) playing a pivotal role in slope hydrological processes and stability. Monitoring SLF remains challenging owing to its spatially heterogeneous distribution and complex pathways. Through continuous soil moisture monitoring on steep granite slopes from 2016 to 2022, this study estimated SLF dynamics using the Soil Moisture Mass Balance (SWMB) method and SEEP/W numerical modelling. The results demonstrated that SLF frequency exhibited a positive correlation with rainfall frequency, with the highest annual mean SLF (1811.0 mm) recorded at the downslope position near the Benggang wall, followed by the midslope (1605.6 mm) and upslope (1211.3 mm). Saturated SLF developed when antecedent soil water content reached the field capacity on the upslope (0.38 cm³/cm³) and midslope (0.36 cm³/cm³) and approached the field capacity on the downslope (0.31 cm³/cm³), with the precipitation exceeding 20 mm. Benggang wall collapses were observed during heavy rainfall events, with cumulative SLF at the downslope position before the collapse of the Benggang wall reaching more than 238.4 mm. The SWMB method failed to estimate the SLF during long-duration or heavy rainfall events. Compared to the SEEP/W model, the SWMB method underestimated the SLF, exhibiting an average discrepancy of 527.1 mm during collapse events. The findings confirm that SLF is an important reason for the instability of the steep slope, and the saturated SLF can indicate the Benggang wall collapse.