Flow characteristics of convex composite slopes of loess under vegetation cover

Abstract: Convex composite slopes are the common and natural slopes in the Loess Plateau. But researches on loess convex composite slope are limited. To explore sediment yield laws of the loess convex composite slopes in the present erosion situations, the field runoff scouring experiments were conducted, and the effects of vegetation cover on the flow pattern and flow resistance in the process of the runoff erosion occurrence of the loess convex composite slopes were studied. Three water inflow rates with 14, 18 and 22 L/min were involved in this scouring experiment. Three vegetation coverage degrees, 30%, 50%, 70% and two kinds of vegetation spatial locations, the bottom of the up-slope of the loess convex composite slopes and the bottom of the down-slope of the loess convex composite slopes were taken into consideration in the experiment design. Results showed that when the vegetation coverage degree was equal to 70%, and the vegetation was laid in the bottom of the up-slope of the loess convex composite slopes, the flow pattern of the loess convex composite slopes belonged to the slow transition flow. In other cases, the flow pattern of the loess convex composite slope belonged to the turbulent transitional flow. When the vegetation coverage was laid in the bottom of the down-slope of the loess convex composite slopes, the values of Reynolds number of runoff and the Froude numbers of the runoff were relatively large compared to the values when the vegetation coverage was laid in the bottom of the up-slope of the loess convex composite slopes. There are two kinds of runoff resistance representations, one is Darcy-Weisbach friction coefficient, and the other is Manning roughness coefficient. Runoff resistance expressed by Darcy-Weisbach friction coefficient was consistent with that expressed by Manning roughness coefficient in this study. When vegetation coverage was laid in the bottom of the up-slope of the loess convex composite slopes, the parameters of Darcy-Weisbach friction coefficient and Manning roughness coefficient were larger than that when vegetation coverage was laid in the bottom of the up-slope of the loess convex composite slopes. The results of this paper can provide a scientific reference for the research on the erosion and sediment yield mechanism of the loess convex composite slopes under vegetation cover as well as the rational allocation of the vegetation cover in the loess convex composite slopes.