Storm runoff and erosion control measures in agricultural fields and construction sites must properly account for concentrated flows, which may be responsible for a considerable fraction of a field’s soil loss. Runoff accumulation and concentration are primarily driven by topography but strongly affected by features that impart oriented roughness to the soil surface, such as tillage corrugations, crop rows, vegetated strips, and field berms. Flow directions are not necessarily along topographic slopes but follow a direction that results from the interaction of local terrain steepness, flow depth, and hydraulic resistance, with runoff following the direction of least resistance when water depths are small and gradually aligning with the direction of the steepest slope when depths increase. A grid-based, two-dimensional overland flow model was developed to dynamically consider the effects of oriented roughness on runoff direction and accumulation over complex terrain topography. Anisotropic hydraulic resistance coefficients are dynamically calculated as a function of local flow depth, considering soil clod sizes, oriented roughness created by tillage or crop rows, and the drag created by crop residue and live vegetation at different degrees of submergence. The model serves as the dynamic routing component in analyses for the planning and performance evaluation of runoff control and soil conservation measures. It has been applied to quantify the benefits of the introduction of grass hedges, contour planting and grassed waterways on steep topography, and to provide detailed data on the location and magnitude of erosive forces leading to the initiation of ephemeral gullies.
