Study on the dynamic conditions of initial generation of gully in loess plateau

Based on the fact that severe water erosion is mainly caused by a few torrential rains of the year, this paper derives relationship between runoff velocity variations, energy increase in per width runoff and runoff concentration caused by nonuniformity of slope, with mechanical analysis of the main dynamic, surface runoff on the slope. The result shows that the value of energy increase in per width runoff is proportional to (B/b)^8/3 and inversely proportional to H^1/3, (in which the B is the initial slope width; the b is the slope width after runoff concentration; the H is runoff depth). When the runoff depth is 1cm, with the increase of B/b, the E which is the energy in per width runoff after runoff concentration increases significantly compared with E′ which is the energy in per width runoff on the uniform slope. Particularly, the E can increase 2156.30 times for the E′ in the case of the B/b is 10. Because of this situation is very normal in the nature, this conclusion has important practical significance. Compared with the intent runoff, the runoff on the uniform slope can greatly increase the probability of the scouring force rolling in the unit area of surface, which exceeds the soil shear strength. That leads to the soil shear failure in the wetted perimeter of flow section, and the probability of forming the waterway and erosion rill. With the development of this process, the erosion rill extends and develops gradually. And this process could be longer for the uniform slope.