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Cedar River gravel removal

Protecting both aircraft and fish

City of Renton Department of Public Works / Renton, Washington

Opportunity

The City of Renton, the eighth largest community in the state of Washington, is located at the mouth of the Cedar River, which flows from the Cascade Range to Lake Washington. The last 1.4 miles of the river’s course takes it through a channel constructed in 1912.

Flood protection along the lower Cedar River is provided by a combination of flood walls and levees designed and constructed by the Seattle District of the Corps in 1997. The channel now flows through downtown Renton and past the airport and a Boeing plant where final assembly of 737 jet liners takes place. Along the left bank of the river is the Cedar River trail, a popular walking path in the community.

The very low gradient of the channel causes sediment to build up, reducing the channel’s capacity and leading to flooding. As the channel continues to fill with sediment, the ability of the floodwalls and levees to protect against major storm events decreases and more frequent flooding occurs.

Solution

In 2011, Mott MacDonald was retained to provide engineering services for the Cedar River Gravel Removal project, aimed at dredging gravel from the channel and improving flood conveyance and stream banks in the area near the confluence with Lake Washington.

Mott MacDonald conducted a field reconnaissance to review biological conditions and the potential use of the river by species listed under the Endangered Species Act. We used hydraulic numerical modeling to evaluate the effect on the channel bed of the City’s preferred dredging concept, as well as effects on riverbanks and local habitat.

Mott MacDonald updated the preferred alternative concept to 30% for preliminary design, summarized dredging methods, rehandling areas, and proposed reuse and stockpile areas, and assembled permit drawings and plans in JARPA format.

We provided final design at 65% and 95% levels, including a cost estimate, drawings, and technical specifications for bidding and construction. We coordinated with regulatory agencies to prepare a permit application package, including plans for drainage, conceptual mitigation, clearing of vegetation, and analysis of alternatives.

Outcome

The Cedar River Gravel Removal project, completed in 2016, will help prevent expensive and destructive flooding of the lower river. An estimated 100,000 cubic yards (76,455 cubic meters) of gravel was removed from the river lowering the 100-year flood level by an average of 2.1 feet (0.6 meters) throughout the project extent.

The bank stabilization, mitigation plantings, and outfall repair work along the river will improve the functionality and aesthetics of the river for years to come. In addition, the construction of a new spawning channel will mitigate the environmental impact of the dredging, allowing the river to continue to provide habitat for fish including chinook salmon, coho salmon, sockeye salmon, and steelhead trout.

100,000
cubic yards (76,455 cubic meters) of gravel removed

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