Characteristics of rill morphology evolution in spoil tips under rainfall and inflow

Soil erosion caused by rill development prevents vegetation recovery of the spoil tips and further accelerates soil erosion, which seriously threatens regional ecological stability. The rill morphological characteristics and rill erosion mechanism of spoil tips slopes were investigated with structure from motion (SfM) photogrammetry. Field runoff plots (5 × 1 m) with slopes of 32° and 36° were treated with rainfall-inflow at inflow rates of 8, 10, and 12 L min^-1 and rainfall intensities of 90, 120, and 150 mm h^-1. Results showed that rill development has three stages: formation, development and adjustment. Rill network on spoil tips slopes were generally dendritic rather than parallel forms. The average rill depth was the best geometric morphological indicator to characterize rill erosion, and geomorphologic comentropy was the best derived morphological indicator. Both of them have a highly significant logarithmic relationship with soil loss. Runoff shear stress was considered as the best hydrodynamic parameters to characterize the rill erosion mechanism, with a critical runoff shear stress of 1.53 N m^-2. These results could contribute to further understanding of rill erosion and provide a useful reference for the prediction and control of soil loss from the spoil tips.