Characterizing Extreme Rainfall-Induced Culvert Washouts in North Carolina

Recent hurricane and tropical storm-induced extreme rainfall events have caused hundreds of culvert washouts across North Carolina. These failures present serious vulnerabilities in the transportation network and pose a safety risk. As part of this study, the North Carolina Department of Transportation provided records for 1410 locations where hydraulic structures were reported to have washed out during Hurricanes Matthew and Florence and during extreme flooding in 2020. These records were analyzed to characterize the pre-storm properties of the structures and to identify factors that indicate high washout risk based on a remote desktop analysis. Drainage area, watershed characteristics, structure capacity, and physical parameters were calculated and compiled for all washouts. The same data were also compiled for nearby structures that did not wash out in a six-county area. Results indicated washouts were most common in small watersheds (0.5 mi 2 [<1.3 km 2 ]), for secondary roads, and for corrugated metal and reinforced concrete culverts with diameters of <72 in. (1.8 m). While the analysis did not reveal any specific parameters or thresholds that definitively predicted a washout would occur, several factors that appear to increase the potential were identified, including absence of a headwall, high headwater over culvert diameter ratios, and lower culvert cross-sectional area to watershed area ratios. We recommend agencies conduct asset inventories and inspections to document existing conditions, identify structures in poor condition or those that are very under-sized, and implement repair and upgrade efforts to help reduce failure risk.