Using additional roughness to characterize erosion control treatment effectiveness in roadside ditch lines

Abstract Forest roadside ditch lines capture and redirect road runoff and typically have erosion control treatments installed therein. Existing methods used to determine the effectiveness of roadside ditch line erosion control treatments estimate fixed fractional reductions in sediment yield. However, fixed fractional reductions do not describe dependence on any measurable physical property of treatment, climate, and the environment. Here, we use additional flow roughness induced by erosion control treatments as a metric that can be used as the basis of estimating treatment effectiveness in varying contexts. We investigate its utility in small‐scale field experiments in western Washington. We measured the physical characteristics of each ditch (e.g., shape, soil texture, and slope) and flow velocities and sediment concentrations for each treatment under multiple experimental discharges. We then used the concept of shear stress partitioning to relate sediment yield from the ditch line erosion treatments to grain shear stress, which is a function of flow roughness (Manning's n) of the respective treatment. We found that (1) a given erosion control treatment produced consistent Manning's n values across multiple replications and sites, with a bare ditch (no treatment) yielding the lowest roughness (n = 0.05) and a densely wattled ditch yielding the highest roughness (n = 0.75); (2) sediment load and calculated grain shear stress data yielded a single positive relationship when data from each experiment were combined, which suggests the effect of additional roughness on grain shear stress is a main driver in the reduction of ditch line sediment load; and (3) in our dataset, fractional erosion reduction had a variable and nonlinear sensitivity to low flow rates (99% of observed flows) for lower roughnesses. Our results demonstrate how additional flow roughness can be used as a general metric to help evaluate the effectiveness of ditch line erosion control treatments for a variety of physical conditions.