Regional Stormwater Management Plan

Client—Lewis County Flood Control District 1, Washington

The Lewis County Flood Control District No. 1 (District) was established to provide flood protection for the approximately 500 acres of rural, residential, commercial and industrial property within its boundary. A large portion of the District is comprised of the Chehalis Industrial Park, operated by the Port of Chehalis. Development of the Chehalis Industrial Park began in the mid-1950’s and has continued to grow. Many older industrial sites were not constructed with adequate stormwater facilities nor was land set aside for stormwater treatment and detention. Newer industrial development has had to comply with the stormwater regulations in effect at the time of their development. As a result, additional regional facilities may be needed to address localized flooding issues created by heavy and prolonged storm events. To address on-going flooding issues, the District hired Gibbs & Olson to develop a regional stormwater management plan for the industrial park properties and adjacent affected areas. Three main drainage basins contribute runoff to the District area: Dillenbaugh, Dilly Twig and Berwick.

A Regional Plan was prepared and included: an inventory of existing infrastructure; evaluating the hydraulic capacity of the existing drainage network; recommending maintenance work; recommending a regional detention and treatment facility and associated conveyance system; and preparing a capital improvement plan for implementing improvements. Upon receipt of comments from the district on the preliminary plan, a final Regional Stormwater Management Plan was prepared for the District.