Analytical and experimental study on sediment wash-off from urban road surface under rainfall runoff

Accurately modeling sediment wash-off from urban roads is critical for stormwater pollution control but remains challenging due to the limited physical basis of existing transport models. This study addresses this gap by developing a new physically based transport model that integrates the “control layer” theory with a newly defined “detachment coefficient” ( c ) to explicitly represent raindrop-driven sediment detachment into the surface runoff water layer. The model was rigorously tested through a comprehensive set of laboratory experiments, systematically varying key factors: particle size (13.2–524.8 μm), rainfall intensity (20–78 mm/h), surface slope (2–4%), and initial sediment load (10–100 g/m 2 ). Results demonstrate that the model accurately reproduces the temporal dynamics of sediment concentration and transport rate at the outlet, with Nash–Sutcliffe efficiency exceeding 0.9 for all test conditions. The detachment coefficient c exhibits an inverse exponential relationship with particle size, decreasing sharply for finer particles and stabilizing for particles larger than approximately 150 μm. Furthermore, the depth of the control layer decreased with increasing rainfall intensity and slope. These results quantify the dominance of raindrop impact in the wash-off process and provide a mechanistic explanation for the preferential transport of fine particles. By successfully extending the control-layer concept from solute to sediment transport and introducing a quantifiable detachment parameter, this work provides a more predictive and physically sound framework for advancing urban stormwater quality modeling. This study entitled “Analytical and experimental study on sediment wash-off from urban road surface under rainfall runoff”, and it has four research highlights: • Novel “control layer” model for physically based sediment wash-off on urban roads. • Detachment coefficient ( c ) quantifies raindrop-driven sediment detachment. • Model accurately predicts sediment dynamics (NSE > 0.9) in lab experiments. • Coefficient c inversely related to particle size, stable above 150 μm.