470 - Flood Protection Assessment for Water Treatment Plant – Chesterfield County, VA

The Addison-Evans Water Production and Laboratory Facilities (AEWPLF) is a water treatment plant located in Chesterfield County, Virginia, approximately 15 miles southwest of Richmond, downstream of the Swift Creek Reservoir Dam. The facility has experienced flooding from Swift Creek, including in August 2020 when a massive flooding event shut the plant down for over eight months. Chesterfield County Utilities retained Arcadis to design, model, and draft a conditional letter of map revision (CLOMR) for a floodwall proposed to be built on County property along Swift Creek protecting the AEWPLF from future flooding. The hydraulic modeling performed as part of this CLOMR application utilized 1D/2D HEC-RAS modeling capabilities and incorporated hydrologic data from a preliminary (unpublished) FEMA model, topography and structural data from survey, and proposed floodwall designs from other members of the Arcadis team. An updated Existing Conditions model was created as a basis of comparison, as the preliminary model provided by FEMA did not employ the 2D modeling capabilities that would have allowed for a more detailed analysis of the conditions. The Proposed Conditions model was used to assess the performance and potential impacts of the proposed floodwall. The 2D section of the model, per FEMA documentation, required the determination of evaluation lines, using unit discharge weighted surcharge calculations, as well as the creation of a topographic work map and an annotated FIRM using ArcGIS. Once this CLOMR has been approved by FEMA, Chesterfield County will be able to begin construction on this much needed flood protection structure.