30 Minute Presentation
Outside the Flood Zone: A Different Perspective of Coastal Flood Risk
The Federal Emergency Management Agency (FEMA) currently produces regulatory flood maps for many coastal communities based on surge risk. This approach does not provide regulatory products for coastal communities outside of the surge zone but subject to flood risk from rainfall / precipitation driven events. A community experiencing this deficiency in Charleston, South Carolina is Johns Island, a rapidly developing suburb and the 4th largest barrier island on the east coast.
Flood Risk Assessment: Here, we present the results of two-dimensional (2-D) hydrologic and hydraulic analyses for design rainfall events throughout John’s Island. In this study, water depth thresholds were analyzed and were processed to identify watersheds subject to rainfall flood risks. Inundation limits have been incorporated into a preliminary / planning level flood risk and vulnerability mapping tool. This tool provides a different perspective on potential flood risks and aims to strategically prevent flooding problems in and resulting from new development. This tool also provides a systematic approach for identifying, prioritizing, and completing stormwater/drainage improvement projects.
The City of Charleston adopted a Sea Level Rise Strategy in 2015 and updated it in 2019. Sea level rise (SLR) projections were also incorporated into this risk assessment. The impacts of increased boundary conditions due to projected SLR scenarios are presented and the changes in vulnerabilities are discussed.
Case Study: As a backdrop, we also present the results of a comprehensive case study of neighborhoods within a problematic watershed on John’s Island. This case study outlines the framework used to transition from the flood risk planning tool to a detailed 1-D / 2-D hydraulic model that investigates the performance of the regional stormwater infrastructure. The impacts of developing within a natural yet unmapped / unregulated floodplain are discussed. An improvement investigation, performed as part of this case study, is overviewed, and the challenges of implementing feasible flood mitigation practices are discussed. The City used (and continues to use) the results of this case study to review the stormwater management plans of proposed developments within the problematic watershed to prevent increased flooding and adverse impacts to adjacent properties. The means, methods, and results of this regulatory review effort are presented and discussed.
Ultimately, this project highlights the need for developing multivariate flood models in order to bridge the regulatory gap and identify risk zones in coastal communities that may be subject to storm surge and/or rainfall driven flooding. It also provides an investigation into the impacts of sea level rise on a developing coastal community.