Underground storage for CSO control in Murray Basin
Murray 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 pipeline storage facility works to control CSOs (PDF)
King County plans to design and build a storage tank beneath private property across from Seattle’s Lowman Beach Park. This facility will be designed to store approximately 1 million gallons of combined stormwater and wastewater during heavy rain when the Murray Pump Station may reach maximum capacity. After storms have passed, an underground pump will transfer stored flows to the pump station for conveyance to King County’s West Point Treatment Plant. This facility will reduce combined sewer overflows to Puget Sound off Lowman Beach Park.
How community input informed the decision process
From October 2009 to November 2010, King County hosted three public meetings and a full-day technical information session and attended a number of community meetings with Morgan Community Association (MoCA) and other groups to discuss proposals for CSO control in Murray. People provided feedback at public meetings, on the Web, and by email, mail, and phone to project staff. (View meeting calendar page for public meeting agendas, presentations and handouts.)
In response to suggestions from the community, King County formed a Murray Basin Community Advisory Group (CAG) representing diverse interests and needs of the community as they relate to the Murray Basin Combined Sewer Overflow Project. The Murray Basin CAG met nine times between June 9 and September 28, 2010. CAG meetings were open to the public, and summaries of each CAG meeting were made available to the CAG and to the public on the King County website (see meeting calendar). King County and the CAG worked together closely to review technical information and questions about all of the county’s alternatives, as well as additional alternatives proposed by CAG members. Both groups recognized the challenge of siting new facilities in a densely developed, steep-sloped neighborhood. King County evaluated a CAG proposal for storage in nearby Lincoln Park and coordinated a public meeting to give West Seattle residents a chance to weigh in on the proposal.
King County’s project team used input from the community along with a range of selection factors and updated technical information to evaluate all alternatives. Community questions and concerns related to project design and construction will be carried forward as King County continues to work with the Murray community throughout the design and construction phase.
Murray CSO Control Project - underground storage on private property
After considering the range of community considerations and project requirements King County will move forward with a project to locate an underground storage tank on property that is currently in private ownership. The recommended proposal involves an underground diversion structure at the existing Murray Pump Station in Lowman Beach Park that will convey peak flows to an underground storage tank on private property across the street (see map). This option for CSO control provides a number of benefits:
- Fewer construction impacts to neighborhoods and parks.
- The vast majority of the project is underground, which provides opportunities to enhance the surface of the site following construction in a way that benefits the neighborhood (for example, additional green space).
- Allows surface components of the project and related improvements to be constructed outside of Lowman Beach Park.
- Allows for a single, reliable, facility located near the existing pump station.
Project elements (see map) include:
- A 1 million gallon underground storage tank on private property east of Lowman Beach Park
- An underground diversion structure located by the Murray Pump Station area in Lowman Beach Park
- Small underground pumps to discharge stored flows after storms have passed
- A pipe connecting the storage tank to the diversion structure
- Odor control and electrical facilities for both the new facility and the existing Murray Pump Station
- Access for operations and maintenance activities at the tank site
What is next:
- King County will continue to work with the Murray community throughout the design and construction phase
- Exact location on private property site will be determined during the design phase
- Many opportunities to be involved in this project are still to come. In early 2011, the State Environmental Policy Act (SEPA) review of the recommended proposal will begin with a threshold determination and formal comment period.
- Construction of the new system, expected to take approximately two years, is expected to begin by December 2013. After construction, all affected areas will be restored to an approved plan.
What we heard from Murray community members
Community members provided King County with a robust level of input during the alternatives evaluation process. The project team received over 200 communications providing feedback on alternatives.
- Most people expressed support for protecting Puget Sound by controlling CSOs.
- Many people expressed concern about placing underground facilities in parks, including Lowman Beach Park and the south parking lot at Lincoln Park. At Lowman Beach Park, there was particular concern about established large trees. At Lincoln Park there were concerns about access, especially for people with disabilities, parking and traffic impacts and the cumulative impact of multiple infrastructure projects in West Seattle neighborhoods.
- Project neighbors posed questions and concerns about construction impacts and expressed a desire for King County to work closely with them during project design.
- People expressed interest and support for upstream alternatives such as Green Stormwater Infrastructure (GSI) or other alternatives that did not impact parks.
- Community members wanted to understand King County’s plans for the existing Murray Pump Station in Lowman Beach Park.