Comprehensive Stormwater Management: Combining Flood Mitigation and Water Reuse

Objectives: Urban areas located near shorelines and/or tidally influenced riverbanks face a growing climate threat from the combined effects of intensified rainfall and elevated outfall tailwater conditions due to sea level rise and/or storm surge. These tailwater conditions pose a challenge to historic sewer systems designed to drain by gravity. In addition to adapting these systems to mitigate flooding, adaptations should align with the concept of the Circular Water Economy, finding innovative ways to reclaim and recycle resources in our water systems to maximize benefits. Drainage improvements as part of the Seaport — Financial District Climate Resilience Master Plan aimed to achieve these dual goals of flood mitigation and water reuse, providing a tangible and aspirational example of facilitating Circular Water Economy goals for other communities.

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 experience frequent tidal flooding and will be at risk from more frequent and severe storms, like hurricanes and nor'easters, bringing storm surge and rainfall to this low-lying area. 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) to implement a comprehensive, sustainable, flood risk reduction strategy.

Status: 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 design work.

Methodology: To meet these flood mitigation and sustainability goals, NYC Economic Development Corporation worked with an Arcadis-led consultant team to study climate adaptation strategies. The team first built a 1D-2D hydrologic and hydraulic model of the study area, primarily a combined sewer system. 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 flood protection system projects (Image 1). 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 (image 2) assuming coastal flood protection 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 future climate events and conducted an alternative analysis to identify a suite of proposed drainage solutions (image 3) that could meet the level-of-service goals. The alternatives were evaluated for performance (image 4), constructability (image 5), cost, long-term and emergency-use operational requirements (e.g., operating large interceptor gates and/or pump stations during hurricane conditions), sustainability and water quality metrics (image 6), and alignment with other design components of this large, multi-disciplinary project (image 7). This integration with other design disciplines was critical to site drainage improvements within a multi-level, community-serving waterfront that maintains access for residents, park space, and active ferry terminals (image 8).

Findings: To achieve the goals of the master plan, the use of the sewer system during extreme weather events had to be re-imagined relative to its original design. After the evaluation described above, a subset of stormwater management solutions was carried forward for further design. These included 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 that includes a water reuse program to offset new water demands. The green infrastructure corridor contains a new, entirely separated sewer system to manage rainfall on the shoreline extension, and not further tax the existing combined sewer system.

Significance: 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 tailwater conditions throughout the United States. The project provides insight into challenges with operating combined sewer systems to meet current level-of-service goals under a future climate. It showed how H&H modeling can be used to assess flood risk and develop an alternative analysis to identify mitigation strategies. Further, it shows how the goals of a Circular Water Economy can be achieved through flood mitigation projects. The presentation will provide a case study example on how to build resilience in sewer systems against climate threats in a dense, urban environment, providing lessons learned and sharing innovations with other communities facing similar challenges. It provides an example for how to upgrade aging sewer systems while promoting a Circular Water Economy.