Hydraulic and Water Quality Performance of Urban Storm Water Detention Basin before and after Outlet Modification

Many existing storm water dry detention basins designed solely for storm water runoff peak discharge magnitude abatement as well as detention basins designed both for water quality and quantity control fail to achieve necessary levels of non-point source pollutant loading reduction. This correctable situation has necessitated the demonstration of various retrofitting techniques on existing systems. An easy and relatively inexpensive retrofitting technique is modification of the basin outlet. The area chosen for such a project was a single-family home development community located in Morris Township, New Jersey. The first modification to the existing dry detention basin consisted of the reduction of the pipe outlet diameter, by placing an outlet box in front of the existing outlet pipe. The second modification was attaching a floating riser, a buoyant outlet device, to the outlet box. The floating riser removes only the surface layer of an impounded storm. Either basin modification resulted in a longer detention time than the original basin did for the same volume of runoff inflow. However, no conclusive correlation between the pollutant removal efficiency and the detention time was determined. Rather, it was found that the pollutant removal efficiency in the field was strongly dependent on the inflow concentration. Therefore, the pollutant removal efficiency must be evaluated in the context of variable inflow concentration. The outflow concentration may be a more reliable and useful criterion for evaluating the water quality performance of a detention basin. The outflow concentrations of total phosphorus were found to be significantly reduced due to basin outlet modifications.