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Effects of vegetation lodging on overland runoff flow regime and resistance

Vegetation is a vital part of the natural environment. Variations in vegetation morphology produce changes in the mechanical and fluid characteristics of overland flow. Determining the effects of vegetation lodging on the overland runoff flow regime and resistance is a prerequisite for accurately simulating overland runoff and convergence, revealing the mechanism of overland flow propagation, and the design and management of vegetation protection, soil consolidation, and ecological slope engineering. To systematically study the effects of vegetation lodging on overland runoff, four planting vegetation lodging angles (α) and 10 test water depths were used to simulate experimental research with a 1.0% slope ratio. Experimental results show that the depth and state of vegetation inundation and the degree of lodging significantly influence the flow regime and resistance. Under the same water depth, higher values of α are associated with higher values of the flow velocity, Reynolds number, Froude number, and Darcy–Weisbach resistance coefficient (f), and lower values of the drag coefficient (CD). The overall result is enhanced turbulence in the flow field and weaker flow resistance. Numerical statistics and difference analysis indicate that, when the vegetation is non-submerged, a 10° increase in α produces a 9.30% decrease in f. In the submerged state, a 10° increase in α causes a 26.70% decrease in f. CD is greatly affected by the boundary water depth. Below some critical water depths, an increase of 10° in α reduces CD by 8.48%. Above the critical depth, a 10° increase in α decreases CD by 41.10%.