Rock Hill, SC—1-D and 2-D Hydraulic Modeling to Support Flood Mitigation

Originally presented at StormCon 2022 on September 28, 2022, 8:30-9:00 AM

Rock Hill, SC, completed a city-wide watershed modeling initiative with the primary goals of identifying flood-prone areas and creating a prioritized Capital Improvement Project program. The watershed modeling efforts include the development of hydraulic and hydrologic models for over 36,000 acres and hydraulic models for over 76 miles of stream. Data used included aerial photography, LIDAR topography, as-built plans, and field surveying. Hydrologic models were based on the NRCS methodologies within the Pond Pack platform. Hydraulic models (one-dimensional computations) were based on standard-step backwater computations within the HEC-RAS model. Floodplain mapping was performed using the HEC-GeoRAS platform.

The Capital Improvement Project evaluation resulted in over 100 projects. Typical solutions included infrastructure improvements such as pipe/culvert replacement and stream restoration/enhancement. Some project solutions included the conventional mitigation strategy of building removal from flood-prone areas. For example, the solution recommendations for the College Downs Stormwater Improvement project included a combination of pipe/culvert improvements, floodplain improvements, stream enhancements, roadway removal and re-routing, and building purchase/removal.

One-dimensional HEC-RAS model results were used initially to estimate flood depth and velocity and also define the floodway limits. Because the floodplain/floodway is relatively wide and flat, a two-dimensional model was developed to provide more detailed results. A comparison of the one- and two-dimensional modeling for flood depth, velocity, and floodway limit was completed. A building purchase/removal priority list was developed using both sets of model data. The model results were validated during discussions with property owners that recorded the severity of flood damage and the willingness of the property owner to participate in Rock Hill’s building removal mitigation program.

Presented by:

Stephen Sands – Hazen & Sawyer P.C.
David Dickson – City of Rock Hill, SC