Upgrading Historic Sewer Systems to Prevent Flooding from Increasing Rainfall and Elevated Tailwater Conditions - A NYC Case Study

Upgrading Historic Sewer Systems to Prevent Flooding from Increasing Rainfall and Elevated Tailwater Conditions - A NYC Case StudyAbstractPurpose: Urban areas located near shorelines and/or tidally influenced riverbanks face a growing climate threat from the combined effects of increasing and more frequent rainfall and elevated tailwater conditions due to sea level rise and/or storm surge, restricting gravity-driven sewer performance during periods when capacity is needed the most. These tailwater conditions pose a challenge to historic sewer systems designed to drain by gravity, and this challenge is further exacerbated when rainfall rates exceed sewer design capacities. This presentation will share a case study on how to build resilience in sewer systems against these threats in a dense, urban environment, providing lessons learned to other communities facing similar challenges. Overview of Master Plan: Lower Manhattan is at the core of New York City's transportation system, economy, and civic life. Yet by the 2040's, Lower Manhattan's shoreline will begin to experience frequent tidal flooding from sea level rise, impacting streets, sidewalks, buildings, and critical infrastructure. In addition to tidal flooding, Lower Manhattan is at risk from more frequent and severe storms, like hurricanes and nor'easters, bringing storm surge and rainfall to this low-lying area. This has been exemplified by the devastating effects of Hurricane Sandy in 2012 and the record rainfall and widespread pluvial-driven flooding from Tropical Storm Henri (2021), remnants of Hurricane Ida (2021), and remnants of Tropical Storm Ophelia (2023). To reduce both acute and chronic flood risk to the neighborhood, NYC Economic Development Corporation worked with an Arcadis-led consultant team to study climate adaptation strategies and develop a Climate Resilience Master Plan. A major challenge of the master plan was how to upgrade and adapt the city's aging, gravity-driven, combined sewer system to mitigate the combined effects of rainfall, sea-level-rise, and storm surge-driven flooding in this future climate. To do so, the use of the sewer system during extreme weather events had to be re-imagined relative to its original design. This challenge was coupled with designing a coastal flood barrier system in a densely urban area, requiring an assessment of both on-land and in-water solutions (i.e., extending the shoreline of Lower Manhattan via land reclamation) to implement a comprehensive flood risk reduction strategy. To develop the Climate Resilience Master Plan, the team first built a 1D-2D hydrologic and hydraulic (H New York City Economic Development Corporation 2; New York City Mayor's Office of Climate and Environmental Justice 3SourceProceedings of the Water Environment FederationDocument typeConference PaperPublisherWater Environment FederationPrint publication date Apr 2024DOI10.2175/193864718825159342Volume / Issue Content sourceCollection Systems and Stormwater ConferenceCopyright2024Word count20