Many heavily urbanized areas within communities rely on closed drainage systems. Many (if not most) of these systems are not designed for the 1% annual chance event and many have not been previously mapped; sometimes even avoided. Capacity limitations of urban closed system can result in heavy flood losses and are a high risk to the public which may not be aware of the risk that lies beneath them. This presentation will discuss recent flood risk mapping activities within the City of St. Louis and other communities that highlight heavily urbanized areas that were previously unmapped. These areas rely primarily on the efficiency and capacity of the sub-surface drainage and/or CSO systems which requires special closed system modeling methods to analyze the subsurface capacity as well as the 2-D overland flow and flood risk areas analyses performed on the surface. This presentation will highlight the technical methods including development of custom rainfall distributions, critical duration methods, historical NEXRAD calibration, coincident frequency analyses of levee components, detailed hydraulic 2-D SWMM modeling, and integrating CSO system components to identify flood risk areas. This presentation will discuss the challenges & overall lessons learned (both technically & policy) and the potential benefits of the project far into the future.
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