Quantity and quality of storm runoff from three urban catchments in Bellevue, Washington

Data on the quantity and quality of urban runoff were collected, analyzed, and used to evaluate the effects of street sweeping and of stormwater detention on quality of runoff. The data included rainfall, runoff discharge, concentrations of selected constituents in discrete samples of runoff, and chemical characteristics of wet- and dry atmospheric deposition. Statistical analyses of runoff loads and of discharge-weighted constituent concentrations in runoff for about 25 different storms showed that, for most constituents, street sweeping had little effect on water quality. One reason is that much of the suspended material in runoff consisted of silt- and clay-size particles, the size classes least affected by street sweeping. That data also show that rainfall is often the source of one-third of the total nitrogen in stormwater runoff. Comparison of discharge-weighted average concentrations of the inflow and outflow of a stormwater detention system for four to seven storms indicated that the detention system did not have a large effect on the average concentrations of constituents in runoff. Regression equations for predicting runoff volumes and peak discharges for individual storms were derived separately for each catchment using data from nearly all storms. Standard errors of estimate for these storms were 21-28% for runoff volume and 22-40% for peak discharge. (Peters-PTT)