Phase 2: Completed Embankment Strengthening
Phase 1: Completed Preload
McCorkle Creek Detention Structure Modifications
Client—Lexington Flood Control Zone District, Cowlitz County, Washington
Gibbs & Olson was hired to provide permitting, surveying, design, and construction phase services for modifications to the South Fork McCorkle Creek Detention Structure. The improvements will significantly reduce the flooding potential in the lower McCorkle Creek watershed and to area residents. The modifications involve raising the existing structure by 15 feet as well as constructing new base-flow piping (with inlet and outlet control structures) and a new overflow spillway with downstream energy dissipation. Extensive geotechnical evaluations and permits were required prior to construction. The project also involved providing wetland and fish habitat mitigation. The modifications are being completed in three phases and include:
Phase 1 | Temporary Construction Measures & Material Preload: The first phase involved completing temporary improvements to prepare the site for construction. This included constructing a temporary retaining wall, a 30-foot-wide access road, and a bridge. Existing unsuitable earthen materials were removed, and the site was preloaded with imported compacted crushed surfacing aggregate material for structural fill.
Phase 2 | Embankment Strengthening: An extensive geotechnical evaluation was performed to design improvements to the earthen embankment. The improvements were designed per USACE Levee Design Guidance and Washington Department of Ecology Dam Safety Office Design Requirements. The design included removing existing grass vegetation and topsoil on the downstream side of the detention structure and installing new material into the existing embankment using the benching and keying method. Chimney and filter drains, a filter blanket, and a 12-inch toe drainpipe with manholes that discharge to McCorkle Creek were also installed. Construction was completed in October 2023 for a total cost of $5.7 million.
Phase 3 | Structure Modifications: Phase 3 is currently under design, and construction is expected to start in 2025. A new overflow spillway with energy dissipation will be constructed. Other improvements include constructing an outlet/inlet structure and installing a slide gate and 48-inch low-level outlet pipe. Once all the upgrades are completed, the existing spillway will be removed.