Detention Basin Retrofit: Optimization of Water Quality Improvement and Flood Control

Retrofitting existing stormwater facilities is of critical importance to restoring impaired watersheds. Many existing storm water detention basins were designed solely for stormwater runoff peak discharge magnitude abatement or were under-designed for current water quality control standards. These basins can be retrofitted to trap additional amounts of non-point source pollutants. This study analyzed a detention basin in Morris Township, NJ as an example to demonstrate the tradeoff between water quality benefits and increases in flooding problems resulting from retrofit. As part of the first retrofit, an outlet structure with a small-diameter circular orifice at the bottom and a rectangular weir on the top was placed in front of the existing large diameter outlet pipe. Diameter of the orifice and height of the weir were varied during hydraulic routing. Quantitative relationships were obtained between the increase in stormwater detention time (pollutant removal efficiency) for a defined, small, infrequent water quality storm and the maximum increase in peak outflow rate (flooding impact) for a full range of storms. It was found that beyond a specific weir height, the detention time no longer increased whereas the maximum peak outflow rate increased greatly. The longest detention time that could be achieved was also calculated for different orifice diameters. In addition, effect of different definitions of the water quality storm on the achievable detention time was quantified.