Efficacy of Grass for Mitigating Runoff and Erosion from an Artificial Loessial Earthen Road

Abstract

Runoff and soil losses from earthen hillside roads are a serious problem on the Loess Plateau in China. We constructed artificial road sections (2.0 m long, 0.55 m wide, 0.35 m deep, 15° slope) with loessial soil packed to give a representative bulk density of 1.5 g cm-3 and planted with different degrees of grass cover (0%, 30%, 40%, 50%, 60%, or 70%). Simulated rainfall (120 mm h-1 for 1 h) was applied to the road sections, and we examined the hydraulic characteristics of the overland flow that was generated and the amount of runoff and sediment yield produced over time. Increasing grass cover inhibited overland flow (as indicated by lower Froude and Reynolds numbers), increased friction and surface roughness (as indicated by Manning and Darcy-Weisbach coefficients), and reduced mean flow velocities. Lower impact energies of raindrops intercepted by the grass led to reduced surface seal formation and detachment of soil particles. This in turn resulted in amounts of runoff and sediment from the grassed roads that were 12.4% to 27.9% and 39% to 76% smaller, respectively, than those from the bare roads. Reductions in sediment yields, which were exponentially related to the degree of cover, were also attributable to reduced carrying capacity of the runoff. Therefore, grass growing on loessial earthen roads can mitigate soil losses effectively and, to a lesser degree, runoff amounts.