A Green Approach to Combined Sewer Overflow Control: Source Control Implementation on a Watershed Scale

A green approach to combined sewer overflow control promotes reduction of stormwater at the source through the application of low impact development and low impact redevelopment techniques. The approach is supported by innovations such as stormwater regulations and other progressive practices like street tree planting and riparian buffer creation and restoration. These practices are designed to improve conditions in both the combined and separate sewered areas and to better manage the water resources, to protect and ensure beneficial uses of water bodies, to restore stream habitat, and to meet the priorities of residents for cleaner and safer streams. The practices are intended to optimize the use of infrastructure planned and constructed in the past for larger populations than those realized today. This paper describes approaches to and benefits expected from low impact development and redevelopment in terms of projected reductions in sewage discharges from a highly urbanized area served by combined sewers.The portion of impervious surfaces in the study area subjected to stormwater control is related to the urban water balance and to combined sewer overflow volume, duration, and frequency. Runoff resulting from small, frequent storms is infiltrated into native soils to the maximum extent possible. Where soil conditions do not allow the full volume to be infiltrated, the volume can be detained and released at a prescribed rate, typically 24 hours or more.Relationships between impervious cover and stormwater management benefits are investigated numerically using the USEPA Storm Water Management Model (SWMM). Model representations such as depression storage, directly connected impervious area, soil properties, combined sewer regulator settings, head loss, and wastewater treatment plant capacity, are validated using observed sewer flow and stream discharge records. Model elements representing stormwater management facilities are added between the hydrologic model representing the precipitationrunoff processes from land surfaces, and the hydraulic model representing the pipes and related appurtenances of the combined sewer collection system. These additional model elements represent physical processes occurring in stormwater management facilities. Infiltration into native soil is represented using soil moisture accounting and a modified Green-Ampt model of infiltration processes. Evaporation of ponded water, storage, and controlled release are simulated as they would occur in facilities meeting the performance requirements of stormwater control regulations.Hydrologic and hydraulic model simulations indicate that significant reductions in annual combined sewer overflow volume will result from implementation of low impact development and low impact redevelopment on a watershed scale. Estimates are made of annual combined sewer overflow volumes for various levels of implementation of control practices.A detailed analysis of impervious surfaces by type and by ownership leads to conclusions regarding the potential for implementing these types of controls in such a highly urbanized area. The study finds that these significant levels of combined sewer overflow reduction can be achieved by requiring stormwater control for redevelopment, by providing incentives for control on private lands, and through large-scale implementation on public lands, especially streets and sidewalks.