Abstract:

Overland flow is the main carrier for sediment. Understanding hydraulic characteristics of overland flow is of great importance to clarify soil erosion and sediment yield mechanism. The study used a bed to perform indoor scouring test under the condition of different flow rate（3.2 L·min^-1， 5.2 L·min^-1） with different coverages （0， 30%， 50%， 70% and 90%） for grass surface. The parameters of hydraulic characteristics were measured and the correlations between those parameters with sediment yield amount were analyzed by gray relational analysis. The results showed that the discharge and grass coverage both exert great impacts on erosion process. （1） A larger scouring discharge will cause a faster velocity， smaller resistance and roughness coefficient and bigger shear stress and flow kinetic energy and thus larger sediment yield amount. （2）With the increase of grass coverage， the mean flow velocity， Fr and unit stream power decreased exponentially， and shear stress， resistance coefficient and roughness coefficient increased exponentially. However， the runoff kinetic energy showed a linear decrease with the increase of grass coverage. For 1% increase in grass coverage， runoff velocity decreases by an average 1%~3% and the sediment reduction benefit reaches from 15% to 89% at different coverage. （3） The gray correlation analysis found that the correlation coefficients between the above parameters and sediment yield amounts are smaller under the discharge of 3.2 L·min^-1 because of the smaller turbulence. Fr number， unite stream power are the most correlated two parameters with sediment yield， and then is the runoff kinetic energy. Sediment yield amounts decreased exponentially with the increase of unit stream power， which indicated that unit stream power can be a good hydraulic parameter for predicting sediment yield. The stream power is a suitable factor to express the influence of underlying surface on flow dynamic characteristic and erosion process.

Key words: overland flow, hydrodynamic characteristics, grass coverage, gray correlation analysis, unit stream power