Ravenna Creek transfer pipe extension

The King County Wastewater Treatment Division is extending the pipeline that conveys Ravenna Creek downstream from Ravenna Park. To protect public health and the environment, a section of pipeline will be added to isolate the stream and prevent potential sewer overflows into University Slough.

Need for project

Project vicinity map - click for larger version (PDF)

 
Project diagram - click for larger version (PDF)

A 2004 King County daylighting project diverted Ravenna Creek flows from the county wastewater treatment plant in Magnolia to the University Slough. During investigation of a May 2008 sewer spill into University Slough (PDF), King County determined that installing an additional section of pipe to the stream transfer pipeline will prevent flows from the sewer system from combining with creek flows during heavy rains.

King County and Seattle Public Utilities worked together to evaluate a range of solutions. Together the county and SPU identified this project as the most cost-effective way to isolate stream flows and maintain system capacity.

Project scope

King County proposes to isolate Ravenna Creek flows within a pipe that will travel through the box culvert that currently conveys creek flows. The box culvert may receive sewer overflows during heavy storms. After this project is completed, normal creek flows will be able to travel through the system to University Slough without risk of cross-contamination with sewage during periods of heavy rain. No change in normal stream flows would result from this project.

Sewer overflows would continue downstream in the box culvert to the county's Laurelhurst Trunk and on to the county's West Point Treatment Plant.

This project is currently in early stages of design. Construction is expected to occur during the summer of 2010 to coincide with lower sewer and stream flows.

What can you expect?

• King County will maintain an inflatable plug in the Ravenna system from fall 2009 until construction is completed in 2010. The plug diverts sewer and creek flows to West Point Treatment Plant during the wet weather season. King County installed the plug after identifying the problem in late 2008 and receiving an hydraulic permit approval (HPA) from Washington Department of Fish and Wildlife (WDFW).
• Updates to agencies and community members will be provided as project design advances and construction plans are developed. Some impacts can be expected during construction.
• People will have opportunities to ask questions and provide comments. Refer to the staff contact under More Information in the right column of this page.

Schedule

• Project design - 2009
• Procurement (contracting) - late 2009 / early 2010
• Construction - summer 2010