American Society of Civil Engineers

Lakewood RainCatchers Pilot Project for Reducing Combined Sewer Overflows

by R. Kirschbaum, (PE, LEED, Senior Engineer, Herrera Environmental Consultants, Inc., 2200 Sixth Avenue, Suite 1100, Seattle WA 98121; E-mail: and B. Spencer, (Drainage & Wastewater Division, Seattle Public Utilities, 700 Fifth Avenue, Suite 4900, Seattle, WA 98124; E-mail: Bob.Spencer@Seattle.Gov)
Section: Special LID Applications, pp. 1542-1556, (doi:

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Document type: Conference Proceeding Paper
Part of: Low Impact Development 2010: Redefining Water in the City
Abstract: Like many cities in the United States, a large portion of Seattle’s underground drainage pipe networks consist of combined stormwater/sewer systems that were designed to convey both sewage and rainfall runoff from paved surfaces, such as rooftops and roadways. These systems were not designed with adequate capacity for the demands placed on them today. With population and development in Seattle already beyond the designed system capacity in many areas, the combined sewer systems are frequently overwhelmed during large rain storms, resulting in combined sewer overflows (CSOs) into local lakes and Puget Sound. Seattle Public Utilities (SPU) is investigating various strategies for controlling these CSO events. Traditionally, large centralized detention facilities have been used to store high flow volumes during the peak of a storm, which are then released back to the system after the storm has subsided. Currently SPU is conducting a pilot project in the Lakewood neighborhood to evaluate the use of decentralized (customer-based) strategies for reducing CSOs to Lake Washington. These strategies include rain gardens and cisterns installed on single family residential sites to capture and control rainwater on-site. Hydrologic and hydraulic modeling performed for this project using InfoWorks Collection System (CS) and Western Washington Hydrology Model (WWHM) indicate that the pilot project alone would not achieve the regulatory goal of one CSO event per year in the basin. However, widespread use of cisterns and rain gardens by Lakewood residents could significantly reduce the required volume of other traditional CSO infrastructure by as much as 25 percent. This paper documents the development and evaluation of alternatives for decentralized strategy pilot studies in the Lakewood neighborhood of Seattle, Washington. To support alternative development, an array of decentralized strategies was evaluated for CSO reduction benefits, as well as potential water quality impacts to Lake Washington.

ASCE Subject Headings:
Combined sewers
Best Management Practice
Urban areas