Modelling of wind-driven rain and its soil detachment effect on hill slopes

The impact of raindrop is an agent causing soil detachment leading to soil erosion. The effect of the impact is a function of the impact velocity. This paper examines the wind-driven rain (WDR) process over hills of varying heights, the impact velocity on the hill slope and the effect of detachment on the soil. WDR falling onto hill slopes is a complex phenomenon. This paper describes a numerical approach to calculate the WDR intensity and impact velocity on the hill slope of an isolated hill. This method first obtains the wind flow pattern over a two-dimensional hill by using computational fluid dynamics technique. It then solves for the raindrop trajectories for the different raindrop sizes, and lastly, the intensity of WDR and the velocities of raindrops impinging on the hill slope are calculated. This method is used to study the WDR phenomenon and its effect on soil detachment on hill slopes. It is found that wind speed and hill geometry are the major controlling factors governing the amount of soil detachment. The raindrop detachment factor on the slope of a taller hill under high wind conditions can be 2–3 times that on the horizontal ground. Close to the top of the hill, this effect is much more severe; it increases to more than 12 times.