By Steven Schauer, Deputy Executive Director and King County Staff

The King County Flood Control District (KCFCD) has been working diligently since its creation in 2007 to safeguard our communities from flooding. The KCFCD, in collaboration with King County as service provider, has spearheaded over 370 capital projects aimed at mitigating the risk of levee and revetment failures. These projects, along with the operation and maintenance of over 500 flood protection facilities, cover 118 linear miles and approximately 800 acres of land crucial for flood mitigation.

While the KCFCD’s weekly blog typically shares information about new and ongoing projects, each month, we will shine a spotlight on a completed project that is quietly doing its job to safeguard our community against the threat of flooding. The project we’re covering this month is the Shake Mill Left Bank Project near North Bend.

Protecting Infrastructure While Preserving Habitat: The Shake Mill Left Bank Project

In the picturesque Snoqualmie River Basin near North Bend, the Shake Mill Left Bank Project stands as a testament to an innovative approach to flood risk management and environmental stewardship. Completed in 2019, this project not only addresses the significant challenges posed by channel migration but also reflects a balance between infrastructure protection and the preservation of natural habitats.

The Challenge: Channel Migration Threatens Infrastructure

The Shake Mill Left Revetment on the North Fork Snoqualmie River incurred significant damage in 2009, which posed a critical threat to the North Fork Bridge and the adjacent 428th Ave SE roadway. Channel migration, a natural process where a river’s course changes over time, was rapidly advancing toward the county’s vital transportation infrastructure. Without intervention, the continued erosion could have led to severe damage or even the loss of the bridge and roadway, disrupting the local community and economy.

Faced with this pressing challenge, the KCFCD, together with King County as Service Provider, needed a solution that not only safeguarded infrastructure, but also supported the health of the river’s ecosystem. This required balancing flood risk reduction natural river processes, and fish habitat preservation — no easy task!

The Solution: A Buried Revetment and Natural River Processes

Rather than opting for a traditional revetment rebuild, a more sustainable approach was embraced. The solution? With the revetment already naturally eroding, construct a buried setback revetment, with a creative design that consists of strategically placed rock and anchored logs that are buried underground and set back from the original revetment’s location.

This buried setback revetment allowed the river to continue its natural geomorphic processes, such as channel migration, without threatening roads and properties. The innovative structure was designed to interact with the river during flood events — as the river eroded its way toward the revetment, the logs and other natural materials were gradually exposed, providing essential habitat for fish and other aquatic species while protecting the land behind it.

Project Success: A Resilient System

Completed just in time for the first flood of the 2019 season, the Shake Mill Left Bank Project performed exactly as anticipated. As the Snoqualmie River’s winter floods arrived, the river began to erode toward the revetment. Sections of the buried structure were exposed, allowing the anchored logs and other habitat features to come into contact with the river’s flow. This not only reinforced the bank but also created valuable in-stream habitat for local fish populations.

The photos of the project’s evolution illustrate how the design allowed for a harmonious interaction between flood protection measures and the river’s natural processes. With the land behind the revetment protected, the project has successfully balanced human infrastructure needs with ecological priorities.

A Model for Future Projects

The Shake Mill Left Bank Project demonstrates how creative engineering solutions can address flood risks while promoting environmental sustainability. By embracing natural processes and designing projects that work with, rather than against, the river, the KCFCD and King County continue to lead the way in innovative flood risk management.

As we look toward future projects, the lessons learned from this project will inform ongoing efforts to protect communities while enhancing the natural beauty and ecological richness of the Snoqualmie River Valley and the other river basins we serve throughout King County.