Green Stormwater Infrastructure Use to Control Combined Sewer Overflows

King County's Wastewater Treatment Division plans to control all of its 38 combined sewer overflow (CSO) sites to an average of no more than one overflow per year by 2030. Projects are under way to control four CSO sites near Puget Sound beaches—Barton Pump Station, Murray Pump Station, South Magnolia Overflow Weir, and North Beach Pump Station—all in the City of Seattle. In addition to storage and treatment, King County is investigating demand management options to control the Puget Sound beach CSOs. Demand management techniques reduce the amount of stormwater and groundwater in the combined sewer system. Examples include sewer separation, infiltration and inflow reduction, and stormwater diversion/management. Green stormwater infrastructure (GSI) is a cost-effective, sustainable, and environmentally friendly approach to diverting and managing stormwater. The areas (basins) that send stormwater to the four Puget Sound beach CSOs were evaluated to determine whether GSI, either alone or in combination with other methods, can achieve CSO control. This paper describes the methods and results of each step of the evaluation. The first step was to identify the sources of stormwater entering the combined sewer system. To do this, King County conducted a geographic information system (GIS) analysis to calculate the acreages in assessed properties and in the right-of-way in the four Puget Sound beach CSO basins that contribute flow to the combined sewer system. Information from the GIS analysis was used to identify areas that showed potential for GSI applications. Subbasin 416 in the Barton basin emerged as the most promising area for GSI application. The next step was to determine appropriate GSI techniques for this basin and to estimate sizes of facilities and costs for implementing them. Initial analyses were done for areas with less potential.