The Chehalis River Basin has suffered six either major or catastrophic floods in the last 40 years causing hundreds of millions of dollars. HDR has been working in the basin for decades, supporting the cities and counties in project planning and implementation to reduce flood damage and restore aquatic species habitat. This work has included completion of a feasibility study to develop flood risk reduction strategies. Goals of the feasibility study were developing alternatives that would protect flood risk reduction to the public and property within the basin while also enhancing fish habitat. The study provided the State of Washington with information to inform capital project planning and emergency and flood hazard planning efforts.

Following the initial feasibility study, HDR developed a conceptual design for a proposed flow-through dam for flood control called the Flood Retention Expandable, or FRE. This structure is proposed on the main stem of the Chehalis River near the town of Pe Ell. The project would provide flood retention and fish passage while enhancing water quality and aquatic habitat in the basin. This analysis provided the information the State needed to support its decision on whether to advance the project to the next phase of feasibility studies and project permitting.

For the most recent phase of the project, the Chehalis River Basin Flood Control Zone District directly retained HDR to further refine and advance development of a proposed FRE structure, permanent and temporary fish passage, and levee improvements around the Chehalis-Centralia Airport. The U.S. Army Corps of Engineers and the Washington State Department of Ecology are preparing environmental impact statements under the National Environmental Policy Act and the Washington State Environmental Policy Act, respectively, for the proposed project. HDR is providing engineering and environmental services in support of the district for refinement and progression of the proposed project; avoidance, minimization and mitigation measures proposed by the district; and permitting and regulatory compliance.

Our team has a thorough understanding of the hydrologic and hydraulic complexities of the Chehalis River Basin. Our team developed a new state-of-the-art hydrologic model, including updated climate change scenarios for flood frequency analyses incorporating the FRE. The new H&H modeling for the FRE project will represent the newest and best available on the Chehalis River upstream of Grand Mound.