Erosion thickness distribution and occurrence of rainfall-induced mass wasting in over-steepened sidewalls of permanent gully

Mass wasting is globally recognized as a key geomorphic agent in permanent gully expansion and greatly contributes to watershed sediment losses. Though its formation process has been assessed by some physical models, the occurrence and rainfall threshold have been rarely documented. In this study, rainfall-induced mass wasting events in two permanent gullies located in the Mollisols region of Northeast China, with Mollisols (gully 1) and sandy soil (gully 2) underneath were observed, and their differences were explored based on their soil strengths, hydraulic properties, excess topographies, and theoretical rainfall amounts. The sandy soil had a higher strength, faster pore water pressure dissipation rate, and lower suction stress at a specific soil moisture content compared to the black soil. The erosion thickness of the gully bed and sidewalls in gully 1 was shallower compared to gully 2. This was confirmed by the relationship between the erosion thickness and excess topography. The differences in the mass wasting erosion of the gully bed and sidewalls were due to the higher shear strength and well-drained hydraulic properties of the sandy soil compared to the black-soil. An infinite model was chosen to examine the temporal order of the mass wasting in the two gullies. It was found that the mass wasting in gully 2 occurred earlier than that in gully 1. This was likely due to the occurrence of an intense storm with less rainfall at the location of gully 2, while a light storm with heavier rainfall occurred in the location of gully 1. As Mollisols and sandy soil are the typical soil horizons in the Mollisols region worldwide, the results of this work could provide insightful knowledge for understanding the physical process of permanent gully expansion, which may be helpful for developing prediction models for sediment losses in some watersheds with vast Mollisols and highly developed gully system.