Overland-Flow Resistance Characteristics of Nonsubmerged Vegetation

In order to discover the resistance characteristics of overland flow with vegetation cover, indoor drainage experiments with six slopes, seven flow discharges, and five degrees of vegetation cover are systematically studied based on the basic theories of hydrodynamics and hydraulics. The experimental results show that the overland flow index of nonsubmerged vegetation gradually increased from 0.333 to 0.845 as the degree of vegetation cover increased. Moreover, the relationship between the flow resistance, water depth, and Reynolds number did not present a monotonic increasing or decreasing tendency, but depended on the vegetation cover. When the degree of vegetation cover was lower than the critical value, the flow resistance coefficient decreased with increasing water depth and Reynolds number, whereas the opposite tendency was found when the degree of vegetation cover was greater than the critical value. The resistance coefficient gradually increased with increasing vegetation cover when the flow discharge was constant. Moreover, the increasing rate became steeper as flow discharge increased. The prediction formulae of average velocity and the resistance coefficient are proposed, and predicted results agree with corresponding experimental data. This conclusion is of scientific significance both in understanding the mechanisms of flow retardation and sand fixation by vegetation as well as in guiding the implementation of ecological restoration projects. (C) 2017 American Society of Civil Engineers.