A physical model of particulate wash-off from rough impervious surfaces

Current urban water quality models rely on empirical, catchment-scale functions of particulate wash-off that do not clearly represent the governing physical erosion and transport processes. We proposed a saltation-type wash-off model in which particles are repeatedly ejected from an impervious surface by raindrop impacts and are transported laterally by overland flow while settling back to the surface. The rate that particles are ejected is proportional to rain intensity and the spatial density of particles on the surface. We found that for low particle spatial densities, ejection is proportional to spatial density, but for high spatial densities, the ejection rate is independent of spatial density. We tested our model against data from small (10.5 × 80 cm) flume experiments in which rain intensity and upslope overland flow could be independently controlled. We used a rough flume surface (∼1 mm roughness element height) and sand particles (diameter = 500-590 μm). Rainfall rates, upslope overland flow rates, and initial particle amounts were varied among experiments with ranges of, 4.2-7.8 cm h^-1, 300-565 mL min^-1, and 0.01-0.07 g cm^-2, respectively. The model predictions agreed well with the experiments (R² > 0.85). This work provides a basis for developing new, uniquely mechanistic models for predicting "wash-off" from urban surfaces.