Hydraulic characteristic of overland flow under different vegetation coverage

Determining hydraulic characteristics of overland flow under the condition of vegetation coverage is a theoretical foundation of studying flow retardation and sand fixation by vegetation. By a simulated vegetation experiment, the variations of hydraulic parameters of overland flow were systematically investigated at ten flow discharges, six slope gradients and five vegetation coverage degrees so as to expect to disclose the inherent law of resistance of vegetation cover for overland flow. Results showed that under the experimental condition, the flow-state indicators increased as the vegetation cover increase; under the same vegetation cover, the slope under the experiment had no apparent influence on it, and the flow regimes mainly ranged from "virtual laminar flow" to transition states, not reaching turbulent state. The overland flow developed from subcritical flow to supercritical flow with increasing slope gradient, but extended in opposite direction with increasing vegetation coverage. Under the condition of vegetation cover, the change law of overland flow resistance and Reynolds number was not the one of monotonical increase or decrease, but was highly related to the vegetation coverage. As Reynolds number increased, the resistance coefficient decreased at low vegetation coverage and however showed a linearly increasing trend at high vegetation coverage. The study is of scientific significance in understanding the mechanisms of regulating slope runoff and sediment yields by vegetation and may provide a scientific basis for the implementation of ecological restoration projects.