Analysis of urban storm-runoff data and the effects on the South Platte River, Denver metropolitan area, Colorado

Abstract

The Denver metropolitan area has been the site of urban-runoff studies for several years. Denver was selected for inclusion in the Nationwide Urban Runoff Program (NURP), a program sponsored by the U.S. Environmental Protection Agency and the U.S. Geological Survey. This report, prepared in cooperation with the Denver Regional Council of Governments, is based on data from nine small basins, two of which are at the outlets of detention ponds, and six tributary basins and presents regression analyses of selected small basins and selected combined small and tributary basins, and the effects of urban storm runoff on the South Platte River in the Denver metropolitan area.The data from six of the small and five of the tributary basins were analyzed using regression analysis, resulting in two sets of regression equations to predict the storm-runoff volume and selected constituent loads. The first set of equations may be used for basins with drainage areas of 15 to 600 acres in size, with 15-to 90-percent effective impervious area. This set of equations was derived from the six small basins. The second set of regression equations, derived from the combined small and tributary basins, is to be used for basins from 600 to 16,000 acres in size, with 15-to 90-percent effective impervious area.A comparison was made between the mean seasonal storm-runoff volumes and selected constituent loads estimated by deterministic and regression models. The estimated mean seasonal values were equivalent for three of four basins compared. One basin, Villa Italia, where the results differed by as much as 400 percent, probably had better results using deterministic models. Due to the costs of using deterministic models and the uncertainty in their use on unmonitored basins, regression models probably are sufficient for planning purposes on most unmonitored basins.The effects of urban runoff on the South Platte River in the Denver area were compared in three ways. The first comparison was between selected constituent concentrations during dry-weather flow and storm-runoff periods. Dry-weather flow concentrations were larger for total nitrite, total nitrate, and total manganese.The concentrations were larger during storm-runoff periods for total organic carbon, total iron, total manganese, total lead, and total zinc. Concentrations were about the same for both flow periods for total nitrogen, total ammonia, total phosphorus, total cadmium, and total copper.The second comparison was between the percent exceedance of streamquality standards during dry-weather flow and storm-runoff periods. Constituents that exceeded the standards more than 50 percent of the time during storm-runoff periods were total cadmium, total copper, total iron, total manganese, dissolved manganese, total lead, and total zinc. Constituents that exceeded the standards more than 50 percent of the time in dry-weather flows were total copper, total iron, total manganese, and dissolved manganese.The third comparison was between the percentage of total flow volume and constituen...