Coastal Flooding & Solutions, Workshop Case Studies
Compounding effects of Storm Surge, Sea Level Rise, extreme Rainfall and Water Table on Urban Flooding in Southeast Coast of Florida
Location: Miami Dade County, FL
Submitted By: Thomas Wahl - Civil, Environmental and Construction Engineering & National Center for Integrated Coastal Research, University of Central Florida, Assistant Professor
Miami-Dade County (south-east Florida) is among the United States’ most vulnerable regions to sea-level rise (SLR), due to its gently sloped low-lying topography, densely populated urban areas, and economic importance, among other factors. The region is also susceptible to compound flooding, where multiple flooding drivers including precipitation, storm surge and a high groundwater level co-occur, typically exacerbating the impacts of a flood compared with their isolated occurrence. The flooding drivers are likely to be statistically dependent as they often result from the same meteorological forcing. Consequently, ignoring dependencies between the drivers will potentially lead to underestimation of flood risk and under-design of flood defense structures. South Florida Water Management District (SFWMD) is responsible for managing and protecting the counties’ water resources. When undertaking design assessments of flood defense structures, rather than ignoring the dependence, the current approach used by SFWMD assumes that rainfall and water level downstream of a structure [referred to as Ocean-side Water Level (O-sWL)] of equal severity, in terms of occurrence frequency, always coincide. For example, the rainfall expected once every 10 years on average is paired with the O-sWL expected once every 10 years on average to comprise an event expected once every 10 years on average. The conservative approach has the potential to induce large safety factors, i.e. structures provide protection against events more severe than previously assumed. This project quantifies the conservative nature of this assumption at locations in the region and demonstrates how it becomes ever more conservative in terms of O-sWL as the severity of the event under consideration increases. The overall magnitude of the conservative assumption is found to be highly dependent on the future regional SLR projections considered. At the three locations considered in the project the groundwater levels exhibit small but statistically significant correlations with rainfall and O-sWL over a range of time lags (i.e., one of the variables peaks before or after the other one(s)). Miami-Dade County is underlined by the highly transmissive and porous (predominantly limestone) Biscayne aquifer which is also the region’s main source of potable freshwater. SLR enhances the risk of contamination of the water supply and flooding that is at least partially induced by groundwater. The ability of three state-of-the-art statistical models to capture dependence between the three flooding drivers is assessed and the limitations of extending the existing approach to include groundwater level are demonstrated. This leads to recommendations for revised future design frameworks able to capture and represent dependencies between different flood drivers.
To include compound flooding into future design and risk analysis conducted by SFWMD.
Making sure that the research outcomes will lead to updated risk analysis and design guidelines
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