Study of pollutant washoff from small impervious experimental plots

An understanding of the pollutant washoff process in urban catchments is essential to develop appropriate models for estimating storm water pollutant washoff loads. This paper investigates the relative importance of the raindrop and runoff energies in the pollutant washoff process by carrying out field and laboratory experiments. Two similar test surfaces are used, with simulated rainfall allowed to fall directly onto one surface, while insect screens are placed above the other surface to remove the rainfall energy. The results indicate that both the turbulence created by falling raindrops and the shear stress imparted by surface runoff are important in loosening the surface particles and suspending them in water, making them available for washoff. The energy of falling raindrops in detaching the surface pollutants is very important at the start of an event and is less dominant as the availability of easily detachable and transportable fractions of the surface pollutant decreases over the storm. Analyses of data from the experimental runs and from actual catchments also suggest that meaningful characteristic curves that relate event total suspended solids (TSS) and total phosphorus (TP) loads to storm durations for specific rainfall intensities can be developed from the experimental data.