Impact of Planting Grass on Loessial Earthen Road with Different Slope Gradients to Avoid Runoff and Erosion

The transportation and environmental systems are destroyed by runoff and soil losses from unpaved earthen hillside roads on the Loess Plateau in China. We constructed artificial road sections (2.0 m long, 0.55 m wide and 0.35 m deep) with loessial soil packed to give a representative bulk density of 1.5 g/cm³, and with different slope gradients (6°, 9°, 12°, 15° and 18°). Ten sections were planted with 50% grass coverage and another ten were bare as control treatments. Simulated rainfall (120 mm/h for 1 hour) was applied to the road sections and the hydraulic characteristics of the overland flow generated, and the amount of runoff and sediment yield produced over time, were examined. For any given slope gradient, grass inhibited overland flow as indicated by lower Froude and Reynolds numbers, higher friction and surface roughness as indicated by Manning and Darcy-Weisbach coefficients, and lower mean flow velocities. Lower impact energies of raindrops intercepted by the grass led to less surface seal formation and detachment of soil particles. This in turn resulted in amounts of runoff and sediment from the grassed roads with different slope gradients, which were 7.7%~13.1% and 47.3%~60.4% smaller than those from the bare roads, respectively. Reductions in sediment yields, which were linearly related to the slope gradient, 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.