Modeling Green Infrastructure Components in a Combined Sewer Area

The on-going Kansas City demonstration project on the use of green infrastructure to minimize combined sewer overflows (funded by the US EPA and supported by a wide range of national and local agencies) will use a variety of integrated practices and modeling approaches. This extensive project will collect data before, during, and after implementation of a variety of control practices in a 100 acre test watershed, and in a parallel control site. The reduction of discharges to the drainage system during wet weather will be calculated using models and verified through field monitoring. The continuous models will determine the decreased amount of stormwater discharged for each event as the storage and infiltration facilities dynamically fill and drain over an extended period of time.

Traditional CSO control practices were originally designed for this area. However, several years ago, Kansas City municipal officials, in conjunction with local citizen groups, started exploring how “low impact development” concepts could be used in the area instead of traditional very large storage tanks. The city is applying many CSO controls listed on the Nine Minimum Control list, such as by making necessary repairs to the sewerage to minimize I&I (infiltration and inflow). The use of bioretention controls has been shown to be promising in meeting the CSO control requirements, with less cost, while providing needed community benefits. Initial modeling is being conducted in conjunction with the design efforts to illustrate the levels of control that can be achieved. With the monitoring results, the models will be verified and then used throughout the city to identify and investigate other retrofit opportunities. In addition, the long duration project will also accumulate much needed information concerning actual costs and maintenance for these controls.

Initial modeling results using WinSLAMM indicate that the use of bioretention facilities in the test area (which has poor soils with very limited infiltration capacities) can still be effective in storage of the peak flows during critical events, significantly decreasing overflows. The use of large water storage tanks to allow on-site beneficial uses of the runoff, in contrast, has limited benefits, and small rain barrels are even less effective. Research is also being conducted showing how newly available drainage controls can be used in the bioretention facilities to maximize their storage potentials.