Fall City Floodplain Restoration Project
King County retained WSE and Herrera Environmental Consultants to evaluate the hydraulic and geomorphic effects of proposed levee and revetment setback along the Snoqualmie River near Fall City, WA that is aimed at restoring salmonid habitat and maintaining or improving existing flood hazard protection. The proposed restoration design involves allowing the river to migrate, reconnecting floodplains and side channels, thus creating a more dynamic and diverse fluvial environment.
Role/Services:
- Predicting the geomorphic trajectory of the reach following project completion
- 2D modeling of the hydraulics of the proposed project design
- Ensuring that the proposed restoration design both generates diverse riverine habitat and protects surrounding infrastructure and property.
WSE conducted studies of hydraulics, bank migration, wood dynamics, and historical channel evolution along the Lower Snoqualmie. This research has informed predictions and identified key uncertainties regarding geomorphic change expected to result from the proposed design and has iteratively informed revisions to that design. Combining geomorphic analyses and hydraulic modeling, WSE helped evaluate how proposed design elements influence both habitat gains and potential risks.
Outcome: The project finished construction in Fall 2023. During the first flood season, the project saw some changes at higher flows but behaved similarly to predictions from WSE's hydraulic and geomorphic analysis. The project was successful at re-connecting side channels and floodplain and increasing the quality and diversity of riverine habitat.
