Underground storage for CSO control in South Magnolia

South Magnolia CSO basin project area


Underground storage tank facility concept (PDF).
This is a typical profile of an underground storage tank facility used for controlling flows in a wastewater conveyance system. Tank size will vary depending on target storage volume. The depth of the tank below the surface depends on topography and elevations of the associated conveyance pipelines. 


How the underground storage facility works to control CSOs (PDF)

King County proposes to design and build an underground diversion structure and tank to store approximately 1.8 million gallons of peak flows during large storm events. A diversion structure at 32nd Avenue West will transfer flows through a gravity sewer line in the right-of-way to an underground storage tank located in the Smith Cove Park/Port of Seattle West Yard area. After storms have passed, in-tank pumps and force main will send flows back to the wastewater treatment system for conveyance to West Point Treatment Plant in Discovery Park. No treatment facilities will be built at the project site.

King County is working closely with Port of Seattle and the City of Seattle to determine final tank location.

How community input informed the decision process

In March 2010, King County presented three proposals for CSO control in South Magnolia. The county’s project team hosted public meetings in fall 2009 and spring 2010 to inform people about the project, the decision process, and proposed alternatives, and to get feedback.

King County’s project team used input from the community along with a range of selection factors and updated technical information to evaluate the three alternatives. Community questions and concerns related to project design and construction will be carried forward as King County continues to work with the community throughout the project.

South Magnolia CSO Control Project - underground diversion structure and storage tank

The recommended proposal involves an underground diversion structure in 32nd Avenue West that will convey peak flows to an underground storage tank located in the Smith Cove Park/Port of Seattle West Yard area (see map). This option for CSO control provides a number of benefits:

• A single facility similar to other King County operating facilities
• Largest facilities are located south of the Magnolia Bridge, limiting temporary construction impacts on narrow residential streets in Magnolia
• Limits facilities in steep slope areas around current CSO control location at 32nd Avenue West

Project elements (see map) include:

• Underground diversion structure installed under 32nd Avenue West  right-of-way
• Approximately 2,700 lineal feet of gravity sewer installed in the right-of-way from 32nd Avenue West  to the tank storage area
• Concrete underground storage tank
• Small underground pumps and a force main to convey stored flows back to the county’s wastewater conveyance system
• Odor control and electrical facilities

What is next

• King County will work closely with the Port of Seattle and the City of Seattle to identify an exact site where these facilities would be built.
• King County will continue to work with the community throughout design and construction.
• Many opportunities to be involved in this project are still to come. In early 2011, the State Environmental Policy Review Act (SEPA) of the recommended proposal will begin with a threshold determination and formal comment period.
• Construction of the new system, expected to take up to two years, is expected to begin by December 2013.
• After construction, all affected areas will be restored.

What we heard from South Magnolia stakeholders

Community members provided King County with input during the alternatives evaluation process at meetings, by email and mail, and by phone.

We heard:

• Strong concern about temporary construction impacts and steep slopes if tank storage were located on Seattle Parks property adjacent to 32nd Avenue West
• Questions about and support for King County’s analysis of green stormwater infrastructure for CSO control in Magnolia
• Recommendations for considering opportunities for public recreation and/or habitat area as part of final restoration
• Request for King County to continue working closely with project neighbors and the community during construction