South Henderson Basins
The Henderson projects cover two areas: North Henderson and South Henderson. Basins 46, 47, 49 and 171 are defined as South Henderson. The purpose of the South Henderson area projects are to reduce the amount of untreated sewage and polluted stormwater that is overflowing into Lake Washington at combined sewer overflow outfalls. Improving water quality in the Henderson Basins will benefit the environment and recreation.
North and South Henderson combined sewer overflow (CSO) reduction projects incorporate additional storage, transmission pipeline and system upgrades to control about 3 million gallons that currently overflow into Lake Washington during storm events. The projects are planned for completion in 2018. When finished, the Henderson projects will bring six combined sewer basins into control. Improving water quality in the Henderson Basins will benefit the environment and recreation.
Because of topography and other factors, Green Stormwater Infrastructure (GSI) opportunities are limited in South Henderson Basins. Some opportunity may exist for small-scale projects, such as the RainWise Program.
Lower Mapes Creek Restoration Project
In July 2011, SPU found that it could reduce costs of the South Henderson Combined Sewer Overflow Basin 47B/171 Project and ensure the project would be constructed by completing the Lower Mapes Creek Restoration Project in combination with the CSO project. For more information, view Lower Mapes Creek Restoration Project.
Multiple alternatives to reduce CSOs in South Henderson basins are being analyzed in order to meet the regulatory requirement. Current alternatives being researched include system retrofits and upgrades. In order to understand the feasibility of specific alternatives, SPU needs to study the conditions of the basin. As a result, you may notice SPU conducting field work in South Henderson Basins. Check back for the latest project information.
May 26, 2011 Project Meeting
The State Department of Ecology has set the following schedule for South Henderson CSO Reduction Projects:
- Complete and submit a draft Engineering Report by August 30, 2012
- Complete and submit for approval a final Engineering Report by January 31, 2013
- Complete and submit draft plans and specifications by October 31, 2014
- Complete and submit for review and approval final plans and specifications by March 30, 2015
- Begin construction by May 31, 2015
- 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)
Contact the Henderson Basin project team.
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