Effects of roughness coefficients and complex hillslope morphology on runoffvariables under laboratory conditions

Abstract

The geometry of hillslopes (plan and profile) affects soil erosion under rainfall-runoffprocesses. This issue comprises of several factors, which must be identified and assessed if efficient control measures are to be designed. The main aim of the current research was to investigate the impact of surface Roughness Coefficients (RCs) and Complex Hillslopes (CHs) on runoffvariables viz. time of generation, time of concentration, and peak discharge value. A total of 81 experiments were conducted with a rainfall intensity of 7 L min-1 on three types of soils with different RCs (i.e., low = 0.015, medium = 0.016, and high = 0.018) and CHs (i.e., profile curvature and plan shape). An inclination of 20% was used for three replications. The results indicate a significant difference (p-value ≤ 0.001) in the above-mentioned runoffvariables under different RCs and CHs. Our investigation of the combined effects of RCs and CHs on the runoffvariables shows that the plan and profile impacts are consistent with a variation in RC. This can implicate that at low RC, the effect of the plan shape (i.e., convergent) on runoffvariables increases but at high RC, the impact of the profile curvature overcomes the plan shapes and the profile curvature's changes become the criteria for changing the behavior of the runoffvariables. The lowest mean values of runoffgeneration and time of concentration were obtained in the convex-convergent and the convex-divergent at 1.15 min and 2.68 min, respectively, for the soil with an RC of 0.015. The highest mean of peak discharge was obtained in the concave-divergent CH in the soil with an RC of 0.018. We conclude that these results can be useful in order to design planned soil erosion control measures where the soil roughness and slope morphology play a key role in activating runoffgeneration.