Extreme Rainfall Isn't Coming, It's Here: Understanding Flood Risk and Developing a Stormwater Resiliency Plan in New York City

Background:
Climate change is causing an increase in intensity and frequency of extreme rainfall events. As a result of these events, many communities are experiencing flash flooding, especially in areas where development has reduced the amount of pervious surface and antiquated sewer systems are not designed to convey the runoff associated with intense rainfall. This flooding can result in a loss of service of infrastructure, public and private property damage, and even loss of life. This flood risk impacts communities outside the traditional riverine and coastal floodplain, as well as adding an additional, compounding risk to those within it. As a result, communities across the nation are developing or will need to develop new action plans to increase their understanding of extreme rainfall risk and take actions to become more resilient. New York City ('NYC') took a significant step towards understanding its risk from extreme rainfall events with the release of the NYC Stormwater Resiliency Plan (the 'Plan') and Flood Map. The Plan outlines NYC's approach to managing this risk, and along with the online Flood Map, communicates this risk to the broad public. Together, the Plan and Flood Map provide a foundation for further study and the development of focused interventions, charting NYC's path towards greater resilience. This paper/presentation will describe the innovative approach taken to develop the Plan and Flood Map, providing other communities with a roadmap towards achieving resilience against extreme rainfall. Objectives: Prior to this study, flood risk planning in NYC was largely focused on coastal storm surge and inundation following the catastrophic impact of Hurricane Sandy in 2012. While this risk had been assessed in detail, including Federal Emergency Management Agency (FEMA) floodplain mapping, limited studies focused on purely rainfall related flood risk absent of storm surge. NYC needed to better understand this risk and develop the appropriate tools to do so, taking a major step towards a more resilient city.
Status: The Plan and web-based interactive Flood Map were published in May 2021. The Plan and Flood Map have subsequently been used by NYC to drive further study, secure funding, and identify priority neighborhoods for cloudburst mitigation strategies to reduce flood risk. The extreme rainfall events that occurred shortly thereafter provided an opportunity to further-validate the hydrologic and hydraulic (H&H) models and improve future maps. Data from Hurricane Ida in Fall of 2021 was compared to the mapped flood risk, showing good validation of modeled results at many locations. NYC also released a post-Ida report, titled The New Normal, reinforcing the City's commitment to flash flood resilience and funding.
Methodology:
To facilitate this analysis and assess extreme rainfall flood risk at a citywide scale, NYC's existing H&H models needed to be improved. The existing models, maintained by NYC Department of Environmental Protection (NYC DEP) were developed primarily for CSO LTCP regulatory analyses. Like many communities, these models only represented NYC's trunk sewer system, were 1-dimensional (1D), and were constructed and calibrated with a focus on CSO volume quantification during the typical year precipitation. In this state, the models did not lend themselves for direct use in extreme rainfall flood risk mapping because: - The models had a relatively low-resolution representation of the drainage network and did not contain sewers in many low-lying areas where flooding can occur, making it difficult to assess flood risk in these areas - The 1D models were not capable of routing flood water over the surface and mapping flood extents and depths in 2D - Like many LTCP models, the calibration period did not include extreme precipitation events To overcome the challenge of leveraging the existing models for this study without an entire model rebuild and recalibration, the team developed an innovative modeling approach, combining a 1D-2D model with a rain-on-mesh model for use under extreme precipitation events [Figure 1]. This approach is applicable to other communities facing similar challenges with their existing H&H models and allowed NYC to continue the flood risk assessment without creating entirely new models. The improved models were then simulated for rainfall return periods ranging from 1-100-year storms coincident with tidal conditions ranging from present-day to 2080s projected sea level rise. After developing and simulating the H&H models, the results were post-processed in ArcMap GIS for public consumption as part of the online Flood Map. The process considered feedback from multiple city agencies and included testing of different visualization options. Flood risk results were mapped with two depth categories showing ponding between 4-inches and 1-foot and a separate color for ponding greater than 1-foot. Model results were aggregated and smoothed-out to improve the visualization and interpretation of results. The final public maps were shared using ArcGIS Online Experience Builder to provide a user-friendly way for the public to experience the maps and understand flood risk. The accompanying Plan was developed in coordination with city agencies through workshops and planning sessions. Considerations included how different NYC agencies may use the available data, how funds should be allocated for capital projects, and guidance for operations and maintenance related to stormwater infrastructure. Key agencies included the Office of Emergency Management, leading discussion on emergency response and how the maps can inform these efforts, and NYC Parks, collaborating on how park facilities are used during flash flood emergencies. The Plan also identified areas for future improvement of the H&H models, including higher-resolution models, refinement of model hydrology, and validation based on quantitative surface flooding data.
Findings and Significance:
The H&H models, Flood Map, and Plan have been used to further assess extreme rainfall risk in NYC and chart next steps for the City as they continue to understand and mitigate this risk. As extreme rainfall events become more frequent, tools such as this are a valuable component of capital planning, as well as communicating and educating communities about this risk.