Hidden Lake Dam Removal

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Status

As of February 2018
Phase: Design

We have started on work to remove Hidden Lake and dam and restore Boeing Creek. The City stopped dredging Hidden Lake in 2013 and it's expected to gradually fill in with sediment. Dam removal is needed before the lake fills in to prevent a dangerous flood risk at the lake outlet.

A plan for improvements was approved by City Council (see May 23, 2016, Staff Report):

  • Remove the dam and lake
  • Restore Boeing Creek within the lake area
  • Possibly replace the large Boeing Creek pipes (culverts) crossing below NW Innis Arden Way

A concept design was completed in February 2018. Project planning is finished and design is starting. Dam removal is planned for 2020, while the culverts may be replaced later.

Right now we are working on:

  • Starting design for Hidden Lake dam removal, culvert replacement, and Boeing Creek restoration
  • Continuing discussions with neighbors, Shoreview Park users, general public, utilities, regulators, and other interested groups

Completed work includes:

 Hidden Lake webpage photo2
Hidden Lake (From the outlet)

Overview and History

Hidden Lake is a man-made waterbody partially within Shoreview Park. The lake was built about 100 years ago when Boeing Creek was dammed to create a private fishing pond and small hatchery. At this time the creek's watershed was largely undeveloped. Since then, major development along Aurora Avenue N has greatly increased storm runoff flows to the creek. Increased flows, in turn, have caused erosion issues within the Boeing Creek ravine. Studies have found that ravine side slope soils washed down into the creek by erosion ends up as sediment in Hidden Lake.

Before Shoreline was founded, King County built projects to address erosion in Boeing Creek. These projects included channel armoring (1974) and the M1 dam (1983) near Shoreline Community College, and the North Boeing Creek Pond in Boeing Creek Park. The original Hidden Lake Dam failed. The former lake was filled with sediment by 1970 and overgrown with trees. In 1996 King County built a new dam and re-established Hidden Lake.

Sediment entered the new lake much faster than expected. From 2002 to 2013, the City spent over $600,000 to dredge the lake seven times to remove sediment. On September 8, 2014, City Council discussed results of a Hidden Lake Management Plan Feasibility Study. They authorized staff to stop dredging the lake and begin looking into removing the Hidden Lake dam. No sediment has been removed since the summer of 2013.

Hidden Lake is expected to fill with sediment within a few years. Storms play a major role in how much sediment arrives. Once the lake fills, the dam cannot be counted upon to safely pass Boeing Creek flows. This scenario would create a dangerous flood risk to NW Innis Arden Way and nearby properties. A program to watch sediment levels in the lake was started in 2016.

On May 23, 2016, City Council discussed the results of an Alternatives Analysis. They supported a proposed plan to remove the dam, replace the NW Innis Arden Way culverts, and restore Boeing Creek in the lake, dam, and culvert area.

The Hidden Lake dam and the NW Innis Arden Way culverts are barriers to fish movement. Native cutthroat trout and (planted) salmon species are present in Boeing Creek upstream of Hidden Lake. The proposed plan also added a separate but related Boeing Creek Restoration (BCR) Project. The new BCR project looked downstream of NW Innis Arden Way to possibly remove other fish barriers, such as the Seattle Golf Club dam. In 2017, analysis indicated fish passage improvements in lower Boeing Creek would not be feasible as a City project.

Removal of the dam will allow sediment currently trapped in Hidden Lake to move downstream. Analysis showed that this freed sediment is expected to improve fish habitat along lower Boeing Creek and at the Puget Sound beach. In March 2017, this project was added to the 2017 WRIA 8 Salmon Habitat Project List for Puget Sound Nearshore (as Project Number PS-24).

In August 2017, King County Flood Control District awarded a $300,000 Flood Reduction Grant to the project. This grant will cover design of Hidden Lake dam removal and NW Innis Arden Way culvert replacement. This grant is expected to fund about half of all design costs.

On October 2, 2017, City Council was updated on the project. They supported moving forward with the proposed plan to remove the dam, replace the NW Innis Arden Way culverts, and restore Boeing Creek in the lake, dam, and culvert area. They also supported not moving forward with the Boeing Creek Restoration project in the lower creek.

In February 2018, the final report for Hidden Lake Dam Removal Phase 1 Pre-Design was completed. This report presented design concepts for removing the Hidden Lake dam, replacing the NW Innis Arden Way culverts, and restoring Boeing Creek throughout the existing lake, dam, and culvert areas. These concepts will be used to move forward into the project design phase.

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