Abstract: The appearance of drop pit is an important phenomenon in the process of slope erosion, which indicates that the rill erosion is developing gradually; the rill formation is marked by the connectivity of the drop pit. The development characteristics and distribution of the pit were investigated by the soil flume flushing model test with orthogonal design of flow, slope gradient, scouring time and slope morphology. From primary to secondary, the order of the factors influencing the development of drop pit ranked as: scouring time, flow, slope morphology, and slope gradient. The drop pit was continuously distributed along the slope and the its depth was volatility. The trend of the pit depth firstly increased and then decreased with the downward trend of slope, and its maximum value was generally in the middle of the slope. According to the characteristics of the development of the pit, the rill erosion of the slope was divided into four stages: sheet erosion, rill prototype, rill development and rill adjustment. There was a significant linear relationship between the product of depth and width of the rill erosion and the soil erosion amount. This result had a certain reference value for researching oil vapor pipeline flood damage and controlling water soil erosion.

Key words: alluvial fan, rill erosion, orthogonal experiment, drop pit