Distributed Stormwater Controls for Flood Mitigation within Urbanized Watersheds: Case Study of Rocky Branch Watershed in Columbia, South Carolina

For highly urbanized watersheds where space is limited, distributed low impact development (LID) stormwater controls could offer an effective retrofit to address flooding problems. The goal of this study is to determine the feasibility of using distributed LID controls within an urbanized watershed for flood mitigation. The Rocky Branch Watershed in Columbia, South Carolina, is an excellent case study because it experiences flash floods almost annually and has limited space for traditional, centralized stormwater controls to mitigate these floods. The Storm Water Management Model (SWMM) was used to model flooding with rain gardens as the LID approach for flood mitigation owing to their storage potential. The results of the study suggest that rain gardens with 30-cm berm heights and a total area equal to 20% of the impervious surfaces within the watershed should provide sufficient storage to mitigate flooding for rain events up to and including a 10-year storm event. Once sufficient storage is available, the challenge is diverting runoff generated from impervious surfaces into rain gardens. The results of this study suggest approximately 15, 27, and 38% of the runoff generated from impervious surfaces should be diverted to rain gardens to mitigate flooding from 2-, 5-, and 10-year storm events, respectively. Given prior work on the adoption of LID approaches for other watersheds, rain gardens could be effective at mitigating up to a 5-year storm event within the watershed, although further research on possible adoption rates in the study watershed is needed to more fully support this conclusion.