Assessing Flood Attenuation Potential through Reservoir Storage: A Case Study of the 2015 Gills Creek Watershed Flooding

Between October 2 and 5, 2015, central and coastal South Carolina endured intense precipitation, which led to extensive flooding, 36 dam failures, 19 fatalities, and substantial infrastructure damage. Within the 190 km2 Gills Creek Watershed, an unprecedented 424 mm of rain fell in a 24-h span, exceeding the 1,000-year precipitation depth of 319 mm. This phenomenon resulted in three breached dams and 13 dams being overtopped. This research investigates downstream flooding resulting from multiple dam failures and assesses the potential of upstream reservoir storage to mitigate floods across different return periods. Through the use of the Hydrologic Engineering Center’s Hydrologic Modeling System (HEC-HMS) and River Analysis System (HEC-RAS) software, extensive hydrologic and two-dimensional (2D) hydrodynamic analyses were conducted. The propagation of flood waves across five interconnected dam reservoirs, incorporating three breached and two overtopped dams during the October 2015 storm, was investigated. The shapes of dam breaches were established based on postflood field measurements, and breach formation time was computed employing Von Thun and Gillette’s parametric equation. The model results show a satisfactory match with observed water levels at Forest Lake Dam and visual observations of flooded storefronts in the City of Forest Acres. The simulations encompassed various storm events—10, 100, 500, and 1,000 years—while considering various combinations of initial pool levels. The results indicated that the prestorm drainage of water from smaller reservoirs, which accounted for 12% of the total reservoir capacities, led to a 24% reduction in peak flood flow during a 1,000-year storm event. However, increasing storage capacity by emptying reservoirs in advance would have a minimal impact on the flooding caused by the October 2015 storm.