Influence of combined stem vegetation distribution and discretization on the hydraulic characteristics of overland flow

Vegetation has important functions of conserving water sources, reducing surface sediment transport, slowing down the process of soil erosion, purifying soil quality and filtering pollutants, and plays an important role in soil and water conservation and environmental protection governance. Accurate simulation of the slope runoff process is important in the research of hydrological models. Existing distributed hydrological models use a single factor, slope vegetation, to determine the vegetation distribution pattern between slope grid units; thus, the homogenization process does not consider the influence of slope vegetation on the hydraulic characteristics. To clarify the effects of the combination and discrete distribution of stem-type vegetation on the hydraulic characteristics of overland flow, this study conducted 768 sets of indoor flume tests through six combined distributions, six discrete distributions, four slopes, and 16 flow discharges. The results show that: 1) Under the experimental conditions, overland flow is located in the transition and turbulent flow regions, and the flow pattern is mutually restricted by the combined distribution of vegetation, discrete distribution of vegetation, and slope. 2) The Darcy–Weisbach resistance coefficient increases with the increase in the combined diameter and dispersion of vegetative stems, which verifies that the results of the study on complex and varied vegetation overland flows obtained via generalized simulations of homogeneous vegetation deviate from the physical reality of overland flow. 3) An increase in the thickness of the integrated vegetation stems and the increase in the degree of dispersion of vegetation both lead to an increase in the vegetation coverage density. The change in vegetation density initiates the change in the flow pattern and resistance of the overland flow. 4) The total resistance under the combined vegetation cover is divided into morphological resistance and particle resistance. The proportion of particle resistance is smaller than that of morphological resistance and decreases with an increase in flow. 5) The influence of stem diameter, Reynolds number, and slope of the combined vegetation was considered comprehensively, and a flow-resistance calculation model was established.