The state of North Carolina has been struck by several extreme rainfall events over the past few years, which have caused failures in stormwater infrastructure (including but not limited to Stormwater Control Measures (SCMs) regulated under the Department’s NPDES stormwater permits (NCS000250)). While SCMs are designed to treat runoff, their principal focus has been treating moderately sized rain storms. How these SCMs fared during larger events, and the restorative maintenance efforts associated with SCM damage is a significant concern for NCDOT.
The Department has a substantial investment in stormwater management assets with over 1900 SCMs having been constructed to treat runoff from roadways, bridges, rest areas, and maintenance yards across the state. Moreover downstream stormwater infrastructure is protected by SCMs (e.g., swales and other conveyance channels). Fortunately, NCDOT has conducted quantitative monitoring of several of these SCMs installed as part of its NPDES permit-required Retrofit Program. NCDOT would benefit understanding at what storm size do typicallydesigned SCMs no longer provided hydrologic mitigation. At what point do SCMs likely fail with significant structural degradation (both to the SCM and downstream) that would lead to costly reconstructive repair? Moreover, are there simple retrofits to existing SCMs (or design features for to-be-built SCMs) that can enhance or extend hydrologic mitigation and reduce the chances of failure?