Abstract
Abstract. Slope–velocity equilibrium is hypothesized as a state that evolves naturally over time due to the interaction between overland flow and surface morphology, wherein steeper areas develop a relative increase in physical and hydraulic roughness such that flow velocity is a unique function of overland flow rate independent of slope gradient. This study tests this hypothesis under controlled conditions. Artificial rainfall was applied to 2 m by 6 m plots at 5, 12, and 20 % slope gradients. A series of simulations were made with two replications for each treatment with measurements of runoff rate, velocity, rock cover, and surface roughness. Velocities measured at the end of each experiment were a unique function of discharge rates, independent of slope gradient or rainfall intensity. Physical surface roughness was greater at steeper slopes. The data clearly showed that there was no unique hydraulic coefficient for a given slope, surface condition, or rainfall rate, with hydraulic roughness greater at steeper slopes and lower intensities. This study supports the hypothesis of slope–velocity equilibrium, implying that use of hydraulic equations, such as Chezy and Manning, in hillslope-scale runoff models is problematic because the coefficients vary with both slope and rainfall intensity.
Highlights
Hillslopes in semi-arid landscapes evolve in various ways, one of which is the formation of surface roughness through soil erosion
Surficial material is preferentially detached and transported according to particle size, detachability, and transportability. This process results in a hillslope with an erosion pavement that is characterized by greater surface rock cover than that of the original soil
We define slope–velocity equilibrium as a state that evolves naturally over time due to the interactions between overland flow, erosion, and bed surface morphology, wherein steeper areas develop a relative increase in physical and hydraulic roughness such that flow velocity is a unique function of overland flow rate independent of slope gradient
Summary
Hillslopes in semi-arid landscapes evolve in various ways, one of which is the formation of surface roughness through soil erosion. Surficial material is preferentially detached and transported according to particle size, detachability, and transportability. This process results in a hillslope with an erosion pavement that is characterized by greater surface rock cover than that of the original soil. An erosion pavement is “a surface covering of stone, gravel, or coarse soil particles accumulated as the residue left after sheet or rill erosion has removed the finer soil” (Shaw, 1929). Once formed, the erosion pavement acts as a protective cover against erosive forces, which reduces subsequent rates of soil erosion. Erosion pavements are analogous to desert pavements formed in arid regions by wind erosion
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