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

Purpose: 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&H) model of the study area. One complexity of the model-build was the necessity to include upstream drainage areas and downstream discharge areas along the interceptor system so that operation of storm surge isolation components could be assessed across neighboring coastal flood protection system projects. After developing the model, the team identified the depth and extents of surface flooding that would occur under a variety of existing and future extreme weather events assuming a coastal flood protection barrier was implemented without upgrading the existing sewer system. The team then worked across multiple city agencies and stakeholders to develop level-of-service goals for the master plan under these future climate events and developed an alternative analysis to identify a suite of proposed drainage solutions that could meet the level-of-service goals under future sea level rise, storm surge, and rainfall conditions. The alternatives were evaluated for performance, constructability, cost, long-term and emergency-use operational requirements (e.g., operating large interceptor gates and/or pump stations during hurricane conditions), and alignment with other design components of this large, multi-disciplinary project. After evaluation, a subset of stormwater management solutions was carried forward for further development in the master plan, including an innovative approach to reverse the flow in a large-diameter existing interceptor sewer during extreme weather events, a large emergency-use combined sewage pump station, sewer conveyance improvements, and a green infrastructure corridor along the proposed extended shoreline. These solutions needed to fit within a multi-level, community-serving waterfront that maintains access for residents, park space, and active ferry terminals. Benefits of Presentation: Benefits of the presentation include the lessons-learned for many coastal and riverine communities that will face similar climate challenges in coming years, and sharing the innovative solutions employed here that may be considered on similar projects. The presentation will highlight the challenges of designing and siting new drainage infrastructure in dense, space-limited neighborhoods, and considerations for how to integrate this new infrastructure in an existing, aging sewer system. It will discuss considerations regarding coordination with stakeholders on adjacent, ongoing coastal resilience projects. Finally, it will share lessons learned and recommendations to other communities facing similar challenges. Status of Completion: The Master Plan was completed and released in December 2021 and is shared with the public at: [https://fidiseaportclimate.nyc]. The team is currently advancing the early design and engineering of the master plan, which will be completed in 2025. The presentation will cover the completed master plan and new components incorporated in the ongoing work. Conclusion: The employed methodology, proposed solutions, and lessons-learned provide an example to address similar issues in urban areas impacted by the combined effects of increasing rainfall and coastal or tidal riverine tailwater conditions throughout the United States. The presentation will explain the challenges with operating combined sewer systems to meet current level-of-service goals under a future climate and then describe the H&H modeling approach to assess flood risk and alternative analysis to identify mitigation strategies to build a more resilient Lower Manhattan.