Presenter/s: Steve Winter
Symposium Session: 2023 - 07 - Restoration as Risk Reduction: Flooding
Topics covered: flood, floodplain, risk and resilience, sediment transport, and urban
ABSTRACT
The City of Sumner is advancing a 200-acre floodplain restoration project as the first step to create a new river corridor through the heart of the City, reversing a century-long trend of channel simplification and floodplain development. The Lower White River is one of the most modified in western Washington, including an avulsion to a different watershed, widespread channelization and bank hardening, numerous flow diversions, and gravel mining to address rapid aggradation.
The Lower White River is home to many aquatic species, including three salmonid species on the Endangered Species List. The Lower White’s floodplain has been designated one of the state’s Manufacturing and Industrial Center to focus development in this area to take advantage of key shipping corridors. These divergent characteristics lead to the City’s overall goal to “Provide enduring flood protection for Sumner, while creating sustainable instream, floodplain, and wetland habitats in the Lower White River.”
This is a story of the City’s dedication to building partnerships across Tribal and non-tribal governments, and national transportation and business sectors to embrace a different future for the Lower White River. The City set out an intentional process to build a collective vision for the White River by forming a Dialogue Group to bring together stakeholders and provide a venue to work out differences of opinion. Luckily, the early meetings happened in the police station, which helped with dispute resolution.
Geomorphic and hydraulic analysis identified all the pitfalls of urban restoration – old floodplain maps, persistent overbank flow paths, complex property ownership, profound scour depths, water rights, and shallow utilities. In addition, the City is advancing the project in partnership with BNSF railroad, which added momentum to the project and increased the scope of the earthwork but added complexity to coordination and permitting. Working through these elements takes perseverance and a positive and forward thinking group dynamic in the Dialogue Group as fostered by the City.
A general lesson from the design effort is that expectations for urban floodplain restoration need to be developed with a clear view of how the site is situated in the landscape and therefore its potential to influence floods. Floodplains and channels are the cumulative product that everything that has ever happened to them natural or anthropogenic, so it is incredibly difficult to apply general rules of thumb for the influence of floodplain restoration on flooding. The White River Restoration Project will lower water surface elevations, but does not solve all flooding issues off into the future. Understanding and embracing the limitations of the site are a key aspect of this work. By working together, the Dialogue Group was able to acknowledge and accept that the project will not solve every problem, but will establish a much more functional river corridor with space to adapt to future conditions and will meet multiple objectives.