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Water and Land Resources Division

King County, Washington

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Water and Land Resources Division, King County, Washington

Willowmoor Floodplain Restoration Project 

Please click below to provide us your feedback on the design alternatives:
Willowmoor on OneKingCounty

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Please follow the link to the right to take our survey about the Willowmoor Project on OneKingCounty (external link).

Public involvement

  • Jun. 2013 - First public meeting was held in Marymoor Park to kick-off the Willowmoor Project. View the presentation (PDF, 2MB).
  • Aug. 2013 - King County convened a Stakeholder Advisory Committee (SAC) to provide input through selection of a preferred design alternative.
  • Mar. 2015 - A second public meeting was held Mar. 14 at the Redmond Senior Center. The presentation and the materials will be posted shortly. 
  • Next steps: Additional public meetings will be scheduled following approval of a preferred alternative and as the project progresses from preliminary through final design.

Project area

The project proposes to reconfigure the Sammamish River Transition Zone, which is at River Mile 13.0-13.5 of the Sammamish River in Marymoor Park. The project area includes the river channel and adjacent undeveloped land to the southwest of the river channel.

Accomplishments to-date

The project began in early 2013. Accomplishments to-date include:

  • hosted a project kick-off meeting, Jan. 27, 2013;
  • conducted several technical studies;
  • convened a Stakeholder Advisory Committee and conducted eight meetings;
  • developed a comprehensive suite of design objectives and corresponding quantitative criteria; and
  • produced multiple iterations of conceptual project design alternatives along with corresponding cost estimates.

Project timeline

Project Start May 2013
Stakeholder Advisory Committee Process Aug. 2013 – Apr. 2015 (est.)
Technical Studies
Jun. 2013 – Early 2016 (est.)
Design Objectives & Criteria Development
Oct. 2013 – May 2014
Draft Alternatives Development
Apr. 2014 – Feb. 2015
Preferred Alternative Selection and Approval
Feb. 2015 – Apr. 2015 (est.)
Preliminary (30%) Design Mid 2015 – Early 2016 (est.)
Final Design and Permitting To be determined
Construction To be determined

Overview

In the mid-1960s, the U.S. Army Corps of Engineers constructed the Sammamish River Flood Control Project – an almost 14-mile long project from Lake Sammamish to Lake Washington to provide flood protection to the Sammamish River valley. The project effectively lowered the bottom of the Sammamish River by over 6-feet. The Transition Zone, a key component of this project, is essentially a ramp near the Lake Sammamish outlet that connects the old river channel to the deepened river channel downstream. This straight, rock-armored channel is approximately 200 feet wide and drops over six feet along its 1,500 foot length. During low to moderate lake levels, lake outflow is contained within a 30-foot wide low-flow channel running down the center of the Transition Zone. Ten-foot wide willow buffers line both banks of this low flow channel. During higher lake levels, lake outflow occupies the entire width of the Transition Zone.

At the upstream end of the Transition Zone, a concrete weir controls lake outflow maintaining minimum summer lake levels. A narrow notch in the middle of the weir provides both up and downstream passage for Chinook salmon and other fish species, and allows downstream passage for small boats such as canoes and kayaks. During high lake outflows, flow control shifts from the weir to the Transition Zone.

King County crews annually maintain the Transition Zone, including trimming the willow buffer and mowing the grass. Though currently required, these maintenance practices harm habitat and water quality and conflict with federal, state and local goals to protect ecological functions and species listed under the Endangered Species Act.  

Overall, these efforts, known as the Sammamish River Flood Control Project, have been very successful in limiting flooding in the Sammamish River Valley. However, its construction and corresponding development has drastically impacted habitat conditions in the river channel and adjacent floodplains. In the vicinity of the Transition Zone, the poor physical habitat conditions are compounded during the summer by higher temperature water from the Lake Sammamish outflow that is harmful to salmon.

Project goals

The project proposes to reconfigure the Sammamish River Transition Zone and adjacent undeveloped King County property in a manner that meets the following three project goals:

  1. Ensure the Transition Zone’s capability to provide necessary lake level control, flow conveyance, and downstream flood control.
  2. Enhance habitat conditions in the river channel, floodplain, buffers, associated tributaries and adjacent wetlands for ESA-listed Chinook, steelhead, and other fish and wildlife species.
  3. Reduce costs, complexity, and ecological impacts of construction, operation and maintenance.

A suite of design objectives has been developed for each of the three goals along with supporting quantitative criteria. Additionally, several objectives outside of the three goals have also been developed, primarily associated with recreational opportunities and constraints.

    Design alternatives development process

    The project design team has developed a series of design alternatives to address the project goals. The identification, development, refinement and elimination of alternatives were based on their performance relative to the suite of design objectives, the results of various technical studies, and guidance and feedback from the Stakeholder Advisory Committee.

    The design alternatives were initially developed as two separate categories:

    • Channel reconfiguration – Alternative that would provide better physical habitat conditions while still maintaining essential flow conveyance through the Transition Zone.
    • Cold water supplementation – Alternatives that would supplement warm summer lake outflow with cold water to help lower temperatures to levels more suitable for migrating salmon.

    During several iterations of review and refinement, some alternatives from each category were eliminated from consideration, while those determined to have the best potential were carried forward. Ultimately the two most promising channel reconfiguration alternatives were combined with the two most promising cold water supplementation alternatives resulting in two integrated alternatives.

    These two integrated alternatives along with a “No Action” alternative (continue the ongoing maintenance) are now being reviewed to determine a preferred alternative that will ultimately be advanced for final design and construction.

    Following are links to graphical depictions of each of these design alternatives (for help with PDF files, visit our Acrobat Help page):

    The alternatives and their development process will be discussed in greater detail in the Concept Design Summary Report which is expected to be published in April 2015.

    Project documents

    (For help viewing PDF files, see Acrobat Help page.)

    For more information about the Willowmoor Floodplain Restoration Project, please contact Craig Garric, Project Manager, King County River and Floodplain Management Section.