Vegetation has significant effects on overland flow velocity and plays an important role in sediment transportation on a slope. The land coverage is an important factor to affect the roughness of the slope. The previous researches have focused on a relatively small range of vegetation coverage. To reveal the relationship between vegetation coverage and overland flow velocity, a series of flume experiments are carried out by using artificial grass to study quantitatively the characteristics of the overland flow velocity under different vegetation coverage. The experiments are set under the conditions of 9 different vegetation coverage from 0 to 80%, 3 slope gradients (10 °,20 °,25 °) and 6 flow discharge (0.286~1.058 L·s-1), which are 162 groups in total. The result shows that the velocity along the slop changes from increase to decrease with the increase of the vegetation coverage. The discharge has a great effect on velocity when the coverage is less than 30% and has small effect when the coverage is more than 30%. The vegetation coverage is introduced in the velocity formula of overland flow, using the common form e-xin the growth model to reflect the influence of vegetation on slope roughness. Applying the nonlinear regression to the experiment data, the parametersk,a,b,care determined as 10.06, -1.71, 0.22, 0.4 respectively. The formula obtained matches well with the measurement (R2=0.96). Applying the formula to the lab experiments in other literatures, the results agreed with the experiment data reasonably well (R2=0.73). It seems that the formula has good adaptability to laboratory tests, although its applicability to field conditions needs further testing.
