Shinglebolt Slough Restoration Project
June 2023 - The project consultant team at Natural Systems Design completed a report evaluating a suite of feasibility and design concepts for this restoration project.
The Shinglebolt Slough restoration project is the first integrated floodplain management project funded under the Community Floodplain Solutions (CFS) program. This project is made possible through the voluntary sale and acquisition of private land along Shinglebolt Slough (next to the Skykomish River just south of the city of Sultan, near the 311th Ave SE bridge).
Project benefits include:
- Retaining quality agriculture parcels for local farm production
- Reconnecting and enhancing side channel habitat for endangered salmon
- Supporting flood hazard reduction and mitigation efforts
Why is Restoration Needed?
The Skykomish River and floodplain near Sultan is a dynamic reach where the mainstem migrates and splits with actively connected channels across the floodplain. Floodplain side channels in this reach provide key habitat for threatened salmon and steelhead spawning and rearing. Recurring flooding and geomorphic changes also threaten floodplain residents and agricultural productivity.
Historically the Mann Rd. Floodplain was connected through a side channel east of 311th Ave. This side channel was disconnected over time as infrastructure and agricultural use developed and levees and berms were installed to manage flooding. The mainstem and slough channel bridges were constructed in the early 1960s over channels existing at the time. A levee was installed between the historic and current bridge alignment to protect the Bridge and road from river scour. A berm and revetment was installed further east to prevent channel migration as farming of nearby fields continued.
With the ESA listing of Chinook Salmon in 1999, the site was identified as a potential restoration in the 2005 Snohomish Basin Salmon Recovery Plan. Prior to that, floodplain regulations and flood risk reduction efforts prompted restrictions in land use and development in the area. The County also began an effort to acquire properties that had or were at potential risk of flood damage.
More recently, the County has conducted river hydraulic and hydrologic, geomorphic, infrastructure, and habitat assessments of this river reach. These studies will inform project design and help the riverside communities better understand the dynamics of living in the floodplain.
The Shinglebolt Slough project is a mainstem, off-channel, riparian, and floodplain restoration project. The project is located along the Skykomish River mainstem and active floodplain. Several project alternatives will be evaluated – looking at existing flood and infrastructure conditions, modeled river hydrology, and predicted changes in river morphology – to determine ecological and flood benefits, as well as infrastructure resiliency needs.
Snohomish County hired River and Structural consultants, Natural Systems Design and KPFF to analyze project alternatives for feasibility and best multi-benefit outcomes.
The Shinglebolt Slough project goals are to:
- Reconnect the floodplain side channel at average spring flows for salmon out-migration
- Restore off-channel and riparian habitat forming processes
- Reduce flooding impacts in the adjacent area
- Retain viable agricultural land use on the Mann Road floodplain
Desired project outcomes are:
- A self-sustaining side channel with diverse habitat features and developing channel complexity
- A natural, complex mainstem edge where sediment and woody debris can suspend, sort, and erode
- Flows and channel changes that mitigate and do not increase risk to infrastructure -- including 311th Ave roadway and Bridge 151 (Shinglebolt Slough) -- and adjacent agricultural fields and homes
- A healthy, maturing riparian forest that is free of invasive species
- Retention of approximately 10 acres in viable agriculture
- Reduction in overall flood hazards in the nearby and adjacent floodplain and infrastructure
- Recreation access and environmental education opportunities
- Assessment of road and bridge resiliency, and needed upgrades for potentially impacted infrastructure
Public Engagement Timeline
Snohomish County and consultants are meeting with local stakeholders to gather input on the restoration project design.
- A public Design Charette event was held in November 2022. The project team is incorporating ideas from this engagement as they develop initial design elements.
- A public meeting was held March 8, 2023 to share initial design concepts with the community and gather feedback.
- Another public meeting will be held in late winter-early spring 2024 to share preferred concept alternatives for 30% design.
- Final design is expected by summer 2024, with construction projected to begin in summer 2025.
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This project is made possible by state, federal and local funds from the following sources:
- Washington Department of Ecology’s Floodplains by Design (FbD)
- Washington Recreation and Conservation Office (RCO), Salmon Recovery Funding Board (SRFB)
- Snohomish County Surface Water Management (SWM) and other community partners