Physical Characteristics of Urban Roadway Solids Transported during Rain Events

Urban stormwater runoff from paved surfaces transports a wide gradation of solids ranging in size from smaller than 1 μm to greater than 10,000 μm. This study measured physical characteristics of solids transported in lateral pavement sheet flow from a heavily traveled roadway in Cincinnati. Particles smaller than 25 μm were counted and sized using a light obscuration particle counter. Particles larger than 25 μm were separated mechanically to generate particle size distributions. Solids in the 2–8 μm range generated the largest counts and were rapidly washed from the pavement. LPSF rate and duration controlled yield and size of transported solids. Particle transport was mass limited during long duration high intensity events, but flow limited during intermittent low intensity events with high traffic. Particle counts exhibited a first flush from the pavement. Specific surface area generally increased with decreasing particle size, but measured values deviated from the monotonic pattern expected for spherical particles. Particles 425 to 850 μm in size contributed the greatest total surface area. Results provide guidance for assessment of the impact of urban runoff water quality and for design of in situ treatment strategies.