Particulate matter "washed-off" of impervious surfaces constitutes a large portion of urban nonpoint source pollution. However, current water quality models rely on empirical functions of particulate wash-off that do not meaningfully describe the physical mechanisms involved. In this paper, we investigate the physical mechanisms of rain-flow transportation (Moss et al. 1979), raindrop induced particle ejection that occurs in shallow flows on moderate slope. Rain-flow transport involves the interaction of both rainfall impact and overland flow, in contrast to the overland flow-dominated, shear-driven particle entrainment that may occur on steep slopes.

We propose a saltation model in which particles are ejected from an impervious surface by raindrop impacts and are translated laterally while settling-out of overland flow. Particles are assumed to be ejected in proportion to rain intensity and the spatial density of particles on the surface. Once ejected, we propose that particles move laterally at the flow velocity and settle according to Stoke's Law. We tested our conceptual model against laboratory flume experiments (10.5 cm wide, 80 cm long) in which rain intensity and upslope overland flow could be independently controlled. The surface of the flume was rough (~1 mm roughness element height) and the particles were 545 mm diameter sand grains. Rainfall rates were between 4.5 and 12.1 cm/hr and overland flow rates were between 150 and 420 mL/min. The conceptual model agreed well with observed data, R2 > 0.85 and was best at the higher overland flows. At low flows the particles spread-out across the surface more than the model predicted. We hypothesize that at low flows lateral movement arising during raindrop impact may be greater than the translation due to overland flow; more research is needed to develop a way to simulate this process. These model results provide a basis for developing a mechanistic wash-off model for spatially distributed urban water quality models.