Permeability experiments of the crust layer, channel formation experiments by rainfalls, and numerical simulations focusing on the formation and destruction of crust were conducted to investigate the influence of the formation and destruction of crust on the formation mechanism of gullies. In the Arimura River basin, southwest of Sakurajima in Japan, crusts were formed on volcanic ash deposits. Even within the same watershed area, the degree of crust development differs depending on the intensity of surface disturbance, and the permeability coefficient varies significantly. In the flume experiment considering crusts, the erosion area developed upstream, starting at the location where the crusts were destroyed, and a gully was gradually formed. Compared to the results of the experiment without considering crusts, the channel spacing in the transverse direction was wide and the number of channels was small. A numerical simulation model was developed to reproduce the development process of a gully by changing the angle of repose. In the numerical simulations, the erosion areas developed upstream, and a gully was gradually formed. In a gully with two crust destruction zones in the longitudinal direction, the scour area gradually expanded, and when the two scour holes were connected, the rate of gully development increased rapidly, and the amount of sediment discharged increased.

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