Upgrading Historic Sewer Systems to Prevent Flooding from Increasing Rainfall and Elevated Tailwater Conditions – An NYC Case Study
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. 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. To reduce both acute and chronic flood risk to the neighborhood, a Climate Resilience Master Plan was developed. A major challenge of the master plan was how to upgrade and adapt the city’s aging, gravity-driven, combined sewer system. 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 Master Plan, the team built a 1D-2D hydrologic and hydraulic model of the study area and used it to identify flood depths and extents under a variety of existing and future extreme weather events. In collaboration with multiple city agencies, an alternative analysis was conducted to evaluate performance, constructability, cost, and maintenance requirements. After identifying a proposed solution, a conceptual design was advanced, which includes proposed pumping, green infrastructure, and water reuse applications. 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.
Author Bio
Mr. Kaatz is a National Technical Manager in Stormwater Resilience and co-lead of the Global Flood Risk and Stormwater Management Community of Practice for Arcadis. As a technical advisor and project manager, he specializes in stormwater resiliency assessments, using hydrologic and hydraulic models to adapt communities to increasing precipitation, both inland and as part of coastal flood protection.
