McCorkle Creek Drainage Evaluation & Detention Structure Modifications

Client—Lexington Flood Control Zone District, Cowlitz County, Washington

Gibbs & Olson performed a hydrologic/hydraulic drainage evaluation to determine if raising the height of the existing South Fork McCorkle Creek Detention Structure would provide improved flood protection at a reasonable cost. The hydrologic/hydraulic modeling results showed that raising the detention structure will detain enough water to allow the downstream McCorkle Creek Pump Station to keep up with the runoff from the remainder of the McCorkle Creek watershed during the 24-hour, 100-year design storm. The detained water can then be slowly released after the runoff from the rest of the watershed has been pumped to the Cowlitz River. We are providing design and permitting services for the modifications to the detention structure. The modifications involve raising the existing structure by 15 feet, new base-flow piping with inlet and outlet control structures, a new overflow spillway with energy dissipation; and wetland and fish habitat mitigation. Increased detention in the upper portion of the south fork’s upper watershed will significantly reduce the potential for flooding in the residential areas of the lower watershed. Gibbs & Olson is currently working on final design documents for the detention structure modifications. The modifications will be completed in two construction phases. The first phase will occur in the summer and fall of 2021 and consists mainly of earthwork to increase the cross-section of the detention structure up to the top of the existing structure. The second phase will occur in 2022 and consists of removing an existing spillway and outlet piping, construction of a new outlet pipe and a new spillway, and completing earthwork to raise the detention structure’s crest elevation from 55 feet to 70 feet.