Laboratory Investigation of Surface Rock Fragment Cover Effects on Roughness Coefficient, Shear Stress and Soil Loss

Soil surface conditions such as roughness, structure, vegetation and rock fragment cover have very important effects on infiltration, run-off and soil erosion. Rock fragments at the soil surface directly affects soil erosion processes, particularly in arid and semiarid environments where vegetation cover is poor. The objective of the present research was to study the influence of rock fragment cover on the rate of soil loss and the hydraulic properties of flow such as roughness coefficient and shear stress in a loess soil sample from Golestan province. The investigation was conducted using a flume with 6 m length, 0.5 m width, and 3% gradient. The treatments included rock fragment covers (0, 10, 20 and 30%), and flow discharges of 0.5, 1.0 and 1.5 (10-4 m3 s-1). With increasing of rock fragment cover from 0 to 30% in flow discharges of 0.5, 1.0 and 1.5 (10-4 m3 s-1), relative velocity of water flow decreased 68.9, 67.7 and 70.9%, respectively. The value of relative roughness coefficient and shear stress increased linearly (R2=0.99) with increasing of rock fragment covers. In addition, with increasing of rock fragment cover from 0 to 30%, the rate of relative soil loss in flow discharges of 0.5, 1.0 and 1.5 (10-4 m3 s-1) decreased 85.0, 83.7 and 73.6%, respectively. Decreasing soil loss rate was related to Rock fragment cover by an exponential function (R2=0.98).