Overtopping Potential for 550 Dams: Considering Storm Events with Varying Duration and Return Period

ABSTRACT ONLY - During both Hurricane Matthew and Hurricane Florence, flooding and damages in North Carolina were exacerbated by the overtopping of multiple dams in the Cape Fear, Neuse, and Lumber River Basin’s. As a result, the Program initiated overtopping studies in the affected basins, which eventually grew to include most of NC’s major river basins. The initial study for the affected basins was funded entirely by the State from its Disaster Relief funds and the additional basins were funded by House Bill 200. In total, 550 dams of various hazard and size classification were selected for evaluation. The results of this study were compiled into an ArcGIS Online WebApp where point features represent dams included in the study and the associated attributes contain the results. Results can be filtered by the hazard classification, predicted rainfall accumulation and amount of freeboard, or overtopping for a selected storm. This study evaluated the potential for overtopping at the selected dams during a series of 36 pre-selected storm events of varying duration and return period. For each selected dam, Hydrologic Engineering Center - Hydrologic Modeling System (HEC-HMS) models were created, and automated reports generated that would eventually be shared to the public for education and outreach. The reports summarized the Hydrologic and Hydraulic (H&H) performance of the dam and evaluated the potential for overtopping with respect to the accumulation and duration of rainfall. Both North Carolina Department of Environmental Quality (NCDEQ) and North Carolina Emergency Management (NCEM) worked together to compile the results of this study and by utilizing weather forecasts for a given hurricane or storm, we can identify dams at risk of overtopping and implement early mitigation measures in an effort to protect life and property downstream. The results of this study were already implemented during Hurricane Ian in 2022 and helped guide NC Dam Safety and the Regional Office Staff to the dams that were of the most concern based on the data models. Thanks to the Overtopping Study, NC Dam Safety was able to prepare for the incoming hurricane days in advance and notify dam owners and engineers of the increased risk, as well as provide details on what they could do in advance of the storm to mitigate the potential for overtopping.