Hidden Lake Dam Removal and Boeing Creek Restoration Projects

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As of October 2017
Phase: Pre-design

The City is in the early stages of implementing a phased approach to restore Boeing Creek at the Hidden Lake site. This work follows the 2014 City Council direction to cease dredging the lake and May 2016 selection of the preferred alternative.

The long-term approach is divided into two complementary conceptual projects:

HLDR and BCR project areas thumbnail

The project will implement improvements located within Shoreview Park, including removal of Hidden Lake Dam and waterbody (Hidden Lake), Boeing Creek channel restoration, trail restoration, and native plantings. This phase is currently expected to address the flood hazard due to sediment loading prior to 2020.

Obtaining a majority of project funding through sources other than utility funds is a critical component of this approach. Due to the need to address the flood hazard from sediment loading in a timely fashion, the City is seeking to secure sufficient funding for the Hidden Lake Dam Removal Project by 2018. If this does not occur, staff will provide Council with an updated recommendation considering status of sediment infilling of lake with a revised array of options to address the flood hazard in a timely manner utilizing available funding. 

Ongoing Efforts

  • Develop Hidden Lake Dam Removal design and Boeing Creek Restoration concepts, including stakeholder outreach.

  • Pursue grants and other funding sources. Grants pursued include: Washington State Recreation and Conservation Office Land and Water Conservation Fund (2016); King County Flood Control District Flood Reduction Grant (2017).

  • Sediment Monitoring Plan.  Monitor ongoing sediment accumulation in Hidden Lake. (Initiated October 2016) First sediment measuring update completed July 2017.

  • Gather Boeing Creek flow data. (Flow meter installed October 2016)

  • Nearshore Habitat Gains Analysis. (Completed February 2017)

  • Survey downstream of NW Innis Arden Way. (Completed December 2016)

The Boeing Creek Restoration Project will evaluate potential long-term, large-scale fish passage and habitat improvements, particularly removal of major downstream fish passage barriers: including the NW Innis Arden Way culverts, riprap cascade, and Seattle Golf Club diversion dam. These improvements would also increase flood protection and address aging infrastructure by replacing the NW Innis Arden Way culverts. This phase is less time sensitive than Hidden Lake Dam Removal, and successful implementation could potentially occur several years later and be divided into separate phases as needed. While the City’s primary interest and responsibility are limited to Boeing Creek sections within areas of public ownership (such as Shoreview/Boeing Creek Park and the NW Innis Arden Way culverts), conceptual improvements to downstream reaches are being evaluated in the interest of the holistic wellness of the stream as well as more specific possible types of downstream improvements (such as potential fish passage restoration) which might be leveraged to maximize benefits of improvements within the upstream areas of City ownership. The City intends to assist to the maximum extent feasible the downstream property owners and other stakeholders along lower Boeing Creek in stewardship of this reach.

Hidden Lake webpage photo2
Hidden Lake (From the outlet)

Overview and History (Up to 2013)

Hidden Lake is an artificial waterbody located east of the intersection of NW Innis Arden Way and 10th Avenue NW, partially within Shoreview Park. The lake originated in the early 20th Century when Boeing Creek was dammed to create a private fishing pond and small hatchery at a time when the creek's watershed was largely undeveloped. Since then, the land around Hidden Lake and Boeing Creek has been dedicated to parks, Shoreline Community College, and residential neighborhoods. Major development along Aurora Avenue N in the 1950s through 1970s greatly increased stormwater peak flows, leading to persistent erosion issues within the Boeing Creek ravine upstream of Hidden Lake.

Prior to Shoreline’s incorporation in 1995, King County implemented multiple projects to mitigate stormwater impacts to Boeing Creek, including construction of the M1 dam, channel hardening located in the vicinity of the M1 Dam, and the North Boeing Creek Detention Pond (near NW 175th St and 6th Ave NW). The original version of Hidden Lake Dam had failed and the lake was filled with sediment by 1970, eventually overgrown with trees and vegetation. In 1996, King County constructed the present Hidden Lake Dam, re-establishing the pond as an environmental mitigation project and installing a forebay to collect sediment to be periodically removed by dredging.

More recently City of Shoreline completed two upstream stormwater system improvement projects to provide detention and water quality benefits: Boeing Creek Park Improvements (2008) and Pan Terra Regional Stormwater Facility Improvements  (2009).

However, sediment deposition occurred within Hidden Lake at a rate six times greater than estimated by the facility’s designers; in order to maintain the lake as an open water feature from 2002 to 2013, the City’s Surface Water Utility spent over $600,000 to implement seven separate dredging projects which removed a total of nearly 13,000 cubic yards of material.

Feasibility Study (2014)

In 2014, the City completed the Hidden Lake Management Plan Feasibility Study to identify surface water management alternative approaches for the Hidden Lake facility in order to reduce costs while meeting other general goals. The study determined that upstream Boeing Creek ravine slope failure is the primary cause of Hidden Lake sedimentation issue, and analyzed a number of potential solutions. 

On September 8, 2014, City Council discussed the results of the study and authorized staff to cease dredging the lake and begin a phased approach to remove Hidden Lake Dam and re-establish Boeing Creek at Hidden Lake. No sediment removal has occurred since the summer of 2013; without dredging, Hidden Lake is expected to fill with sediment by 2020 to 2025. Once the lake fills, the existing outlet system is vulnerable to being blocked or bypassed by natural processes, which could cause flood flows to overtop and erode the existing dam spillway and block the NW Innis Arden Way culvert inlets. This scenario could lead to a sudden washout of the road embankment and/or other major flooding damage. Accordingly, a “no action” alternative is not viable for consideration.

A program to monitor ongoing sedimentation was launched in October 2016, in order to track natural in-filling of the lake and adjust dam removal timeframe as needed.

Alternatives Analysis (2015-2016)

In 2016, the City completed the Hidden Lake Dam Removal Design Alternatives Analysis which developed and compared multiple design alternatives for alteration or removal of Hidden Lake Dam based on the 2014 Council decision to cease dredging and remove the dam. Each design alternative would modify the existing dam and lake configuration to responsibly manage sediment transport and safely convey flood flows. Multiple methods of detailed analysis used to create and evaluate the alternatives.

On May 23, 2016, City Council discussed the results of the Alternatives Analysis and authorized staff to pursue an approach to maximize funding opportunities and minimize time-sensitive flood hazard risks, while potentially offering the best array of benefits, for fish passage and habitat, park improvements, roadway infrastructure protection and the greatest overall reduction of flood risk. This combination of benefits was favored in stakeholder outreach efforts. (See Alternative Selection Memorandum) This approach (Alternative 4) was developed after preparation of the Draft Alternatives Analysis report as a phased and expanded version of Alternative 3.

Boeing Creek Fish Passability and Habitat

Hidden Lake Dam is a complete barrier to fish passage, one of four major barriers on lower Boeing Creek identified in the 2013 Boeing Creek Basin Plan (the other three barriers are the Seattle Golf Club diversion dam, the riprap cascades and manmade waterfall below NW Innis Arden Way, and the NW Innis Arden Way culverts themselves). The basin plan recommended taking advantage of any opportunities to improve Boeing Creek fish passage as related to future Hidden Lake-related work (under Recommended Project BC-Hab-1).

Downstream of the Seattle Golf Club diversion dam is a half-mile-long continuous reach of Boeing Creek running to the Puget Sound without major barriers. Anadromous salmonids currently using this reach of lower Boeing Creek consist primarily of coho and chum salmon and sea-run cutthroat trout, though occasional Chinook salmon have also been observed. The project is currently analyzing feasibility to restore and/or enhance fish passage along Boeing Creek between the Seattle Golf Club Dam and NW Innis Arden Way.

Removal of the Hidden Lake Dam will also improve downstream sediment transport, which is expected to improve fish habitat along the lower reaches of Boeing Creek and at the Puget Sound Creek delta where critical new forage fish habitat can be restored. See Nearshore Habitat Gains Analysis for a detailed explanation of potential nearshore benefits. In March 2017 the Hidden Lake Dam Removal Project elements which will provide nearshore habitat benefits by means of restoring sediment transport to downstream areas were proposed to be added to the WRIA8 Chinook Salmon Conservation Plan Project List - Nearshore. Confirmation of the project's WRIA listing will occur in November 2017; Hidden Lake Dam Removal nearshore beneficial elements are proposed under the following project name:  Boeing Creek Mouth and Delta Restoration.

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