Flow Resistance Coefficient Under Rigid Vegetation Coverage: An Experimental Study

Abstract

Rigid vegetation is prevalent in the Loess Plateau region. Most simulation studies assume that vegetation grows perpendicularly to the slope surface (BS), overlooking the influence of growth perpendicular to the horizontal plane (BH) on the slope flow resistance coefficient. Therefore, this study explores the effects of rigid vegetation on the slope flow resistance coefficient by conducting indoor simulated scouring experiments under different vegetation coverage (0–15.89%), two growth directions (BS and BH), four slopes (5°, 10°, 15° and 20°), and five flow rates (5, 10, 20, 30 and 40 L·min⁻¹). The results showed that the resistance coefficient increased non-linearly with vegetation coverage; however, the growth rate slowed with increasing slope at high coverage, showing a decreasing trend. The resistance coefficient under BS conditions was generally higher than that under BH conditions. As the coverage increased (0–15.89%), the resistance coefficient under BS conditions became 1.51–1.99, 1.1–1.26 and 1.52–1.59 times that under BH conditions. The resistance coefficient ratio $\varphi$ negatively correlated with the flow rate and water depth and positively with the slope. Furthermore, the relationship between the Reynolds number (Re) and resistance coefficient under BS and BH conditions significantly differed, with a negative correlation under BS conditions and a positive one under BH conditions. The Froude number (Fr) exhibited a strong negative correlation with the resistance coefficient, and the impact became more significant as vegetation coverage increased. Through dimensional analysis and nonlinear fitting, a high-precision resistance coefficient prediction model (adjR² = 0.98, NSE = 0.99, RRMSE = 0.04) was developed, which provided theoretical support for engineering designs and enhancing soil and water conservation strategies on the Loess Plateau.