North Henderson Basins
The North Henderson CSO Reduction Project (view overview map of North Henderson CSO improvements) will reduce the amount of untreated sewage and stormwater that overflows into Lake Washington at two outfalls located offshore of:
- Seward Park
(view map of Basin 44 CSO improvements)
- 57th Ave. S. near Martha Washington Park
(view map of Basin 45 CSO improvements)
Based on flow data, SPU determined that combined sewer overflows (CSO) occur 17 times per year, totaling approximately 2 million gallons. The North Henderson Project will reduce this frequency to no more than one overflow per year per outfall, on average, to meet regulatory requirements. We will do this by adding underground storage at two sites within Seward Park and in the 57th Ave. S. right-of-way near Martha Washington Park. The proposed underground storage facilities would store excess combined sewage and stormwater during major rain events. SPU plans to complete the two new storage facilities in 2018.
Seward Park (Basin 44)
Basin 44 drains toward Seward Park and then south to King County’s Pump Station #10, located at the eastern end of 57th Ave. S. The proposed underground storage facility there will consist of a 2.65 million gallon tank, plus facility vaults to house odor control, mechanical and electrical equipment. The project includes additional piping, outfall replacement, with shoreline work and landscape improvements as determined working with Seattle Department of Parks and Recreation, which owns and manages Seward Park. SPU is recommending to City Council that the underground storage facility be located under the existing tennis courts, which would be rebuilt once the facility is constructed.
57th Ave. S (Basin 45)
Basin 45 also drains to Pump Station 10, which then pumps the combined flows south to King County’s Henderson Pump Station near the corner of Seward Park Ave. S. and S. Henderson St. in Rainier Beach. The Henderson Pump Station ultimately conveys the flows to a King County wastewater treatment plant. The 57th Ave. S. right-of-way is managed by the Seattle Department of Transportation. The proposed underground facility for Basin 45 consists of a 16,000 gallon pipe and piping improvements.
Upcoming meetings will be announced on this page and through our mailing list. Sign up for project updates.
January 28, 2014 Update
The stakes are in place for the corners of tennis courts, tank, and limits of construction in Seward Park. The tree flagging indicates trees currently planned to be removed for construction. Parks surveyors planned to remove the tree flagging by Friday, February 7, 2014. The corners were marked using duct tape, paint, and stakes depending on the location. View boundary locations (pdf) marked in the field.
November 2013 - Frequently Asked Questions (pdf)
September 2013 - Revised Final Environmental Impact Statement Addendum (pdf)
Site Selection and Public Process
Through of series of meetings, workshops, email updates, one-on-one conversations, briefings and direct mailings starting in 2010, we have engaged the community and kept stakeholders informed about the North Henderson Project throughout the site selection process. See Project Background [link to new page] for more details.
The process resulted in these outcomes:
- 2.65 million gallon underground storage facility beneath the tennis courts in Seward Park
- 16,000 gallon storage pipe in 57th Ave. S. near SPU’s Pump Station 10, next to Martha Washington Park
In a series of four public workshops, SPU sought input on its evaluation of types of control options and then on four conceptual solutions. Key workshops and decision points were:
- sewer separation/green infrastructure
- tunnel storage
- pump and storage in Martha Washington Park
- separate storage for each basin, considering street right-of-way, private property and park property
Separate storage facilities in Seward Park and in or near Martha Washington Park were selected as:
- the least disruptive to the communities during construction
- the best fit to meet technical requirements
- having the least environmental impacts, and
- the most likely to meet the regulatory schedule mandated by the Washington Dept. of Ecology and the U.S. EPA.
Storage Facility Beneath the Tennis Courts In Seward Park (Basin 44)
In the June 7, 2011 Draft EIS Scoping meeting and the October 8, 2012 public hearing, SPU sought input on two site-specific alternatives for storage in Seward Park (Basin 44): beneath the tennis courts or beneath Parking Lot #2. SPU analyzed the environmental impacts of each alternative through the EIS process and is recommending building the underground storage facility at the tennis court location because:
- Seward Park is a destination park, and storage under the tennis courts would have less short-term (construction) and long-term impacts on recreation in Seward Park than storage under Parking Lot #2, and
- Other environmental impacts were not significant as defined by State Environmental Policy Act (SEPA), i.e., there was not a reasonable likelihood of more than a moderate adverse impact on environmental quality.
The City Council must still approve the selected alternative in Seward Park. This decision is expected in 2014.
Storage Pipe in 57th Ave. S. (Basin 45)
In the June 9, 2011 workshop, SPU sought input on alternatives for locating Basin 45 storage in the 57th Ave. S. right-of-way, nearby private property and in Martha Washington Park. SPU followed the community’s suggestion to contact private property owners, located a willing seller and purchased the home at 5560 S. Holly St. for the proposed 200,000 gallon storage facility. SPU received additional input from the community on the conceptual storage design in the February 26, 2012 open house.
Subsequent analysis demonstrated that CSO control could be achieved by simplifying the Basin 45 project from a 200,000 gallon storage tank to a smaller, 16,000 gallon storage pipe, some repiping in the 57th Ave. S. right-of-way and retaining more of Basin 44 flows in Basin 44. This solution results in a slight increase (from 2.4 to 2.65 million gallons) in storage under the Seward Park tennis courts without increasing the outside dimensions of the storage tank. This change eliminates the need for the storage facility at 5560 S. Holly St. and will be significantly less disruptive to the community. SPU completed a Final EIS Addendum process in October 2013 that documents the environmental analysis associated with this change.
Past Meetings and Materials
- November 13, 2104 Basin 44 (Seward Park) Open House – Same materials as Nov. 4
- November 4, 2013 Friends of Seattle's Olmsted Parks Briefing – project map, Seward Park shoreline treatment, Seward Park landscape plan, UPARR designs (pdf)
- November 2, 2013 Friends of Seward Park Briefing – Meeting Notes and Responses (pdf)
- October 23, 2013 Basin 45 (Martha Washington Park vicinity) Open House – presented updated project design and schedule
- September 2013 – Revised Final Environmental Impact Statement Addendum (pdf)
- February 26, 2012 Basin 45 (Martha Washington vicinity) Staff-Recommended Alternative Open House – possible restoration concepts and conceptual layouts, comment summary (pdf)
- June 9, 2011 Basin 45 (Martha Washington Park Vicinity) Alternatives Workshop – meeting summary, alternatives comparison, display boards, comment form (pdf)
- June 7, 2011 Basin 44 (Seward Park) Open House – public scoping summary, project guide, presentation, display boards, comment form (pdf)
- March 10, 2011 North Henderson Workshop #4 – presentation, summary of project cost estimates (pdf)
- January 19, 2011 North Henderson Workshop #3 – agenda, presentation, meeting summary, meeting summary appendices, evaluation criteria, scoring, weighting sheet, cost summary (pdf)
- December 14, 2010 North Henderson Workshop #2 – meeting summary, presentation, display boards (pdf)
- November 18, 2010 North Henderson Workshop #1 – meeting summary, presentation, display boards( pdf)
- October 4, 2010 Friends of Seattle's Olmsted Parks Briefing - presentation (pdf)
- October 2, 2010 Friends of Seward Park Briefing – presentation pdf)
- September 16, 2010 North Henderson Basin Community Briefing – notice, presentation (pdf)
- June 23, 2010 Southeast District Council Meeting – presentation (pdf)
References and Final Reports
- North Henderson (Basin 45) Determination of Non-Significance (pdf)
- North Henderson (Basin 45) SEPA Checklist (pdf)
- North Henderson (Basin 44) Geotechnical Evaluation (pdf)
- North Henderson (Basin 44) Determination of Significance (pdf)
- Genesee & Henderson - Initial Estimation of Overflow Control Volumes (pdf)
- Genesee & Henderson - Preliminary Geotechnical Evaluation (pdf)
- Henderson - CSO Flow Monitoring Report - October 2009 (pdf)
- Henderson - Flow Monitoring Study (pdf)
- Henderson - Modeling Alternatives (pdf)
- Henderson - Modeling Plan (pdf)
- July 15, 2013
30% Design Completion (Complete)
- Issued September 5, 2013
Revised Final Environmental Impact Statement (Complete)
- Issued September 30, 2013
Revised Final Environmental Impact Statement Addendum (Complete)
- December 2013
60% Design Completion (In Progress)
- June 2014
90% Design Completion
- November 2014
Final Design Completion
- 1st Quarter 2015
Begin Construction (Basin 44)*
- 4th Quarter 2017
End Construction (Basin 44)*
* Construction of Basin 45 improvements will require approximately 6 months and will occur during the construction of Basin 44.
- 4th Quarter 2018
Commissioning and Testing of Facilities Completion
Your continued input is important to us. To help the project team best understand your concerns and interests, please specify the project site with your comments: Seward Park (Basin 44) or 57th Ave. S. right-of-way (Basin 45).
Sign up for project updates at seattle.gov/lists/cso-northhenderson.htm.
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