NPCC

New York City Panel on Climate Change

NPCC4: Climate change and New York City’s flood risk

Flooding Work Group Co-Chairs: Bernice Rosenzwieg and Franco Montalto

New York City (NYC) is at risk from multiple types of flooding hazards, which may be the result of extreme precipitation, coastal storm surges and high tides, high groundwater tables, or their co-occurrence.

This NPCC4 chapter is the first in the NPCC series to focus solely on New York City’s different types of flood risk. It expands on earlier NPCC reports of climate change impacts on New York City flood risks by comprehensively assessing the city’s five types of flood hazards: pluvial (rainfall), fluvial (rivers and streams), coastal (tidal and storm surge), groundwater, and compound flooding (when a storm causes coastal and rain flooding).

NYC Flood Hazards

This chapter describes how climate change worsens flooding in NYC through a rise in precipitation and sea levels and provides a more comprehensive picture of NYC flood risks by expanding the examination of those risks beyond the FEMA’s Special Flood Hazard Areas and accounts for a wider range of flood hazards beyond coastal flooding. Addressing each type of flood hazard, the chapter characterizes climate change impacts per flood type, historical examples of such flooding, exposure, and vulnerability, including ways in which such flooding may be exacerbated, and persistent knowledge gaps related to such flooding.

The chapter emphasizes structural (e.g., elevating buildings and utilities or adding barriers) and non-structural (e.g., early warning, buyouts, flood insurance) approaches to flood risk management (FRM). Importantly, it presents FRM as a pro–active set of strategies to reduce vulnerability and increase the livability of flood prone communities before, during and after flooding.

Finally, this chapter summarizes areas for future research including continued growth of flood monitoring (e.g. the FloodNet program), furthering work on the Flood Susceptibility to Harm and Recovery Index (FSHRI), improving community-driven, neighborhood-scale flood risk management planning, enabling social and ecological restoration, and developing hazard maps that stand for a broader range of hazards and their increase in size in response to climate change.

Photo Credit: Hunter’s Point South Park by Climate Adaptation Partners

Read the Chapter

Authors

Bernice Rosenzweig, Ph.D. Sarah Lawrence College, Bronxville, NY.

Franco Montalto, Ph.D. Drexel University, Philadelphia, PA and eDesign Dynamics LLC, New York, NY.

Philip Orton, Ph.D. Stevens Institute of Technology, Hoboken, NJ.

Joel Kaatz, PE Arcadis of New York, New York, NY

Nicole Maher, Ph.D. The Nature Conservancy, Cold Spring Harbor, NY.

Jerry Kleyman, PE Arcadis of New York, New York, NY

Ziyu Chen, Ph.D. Stevens Institute of Technology, Hoboken, NJ.

Eric Sanderson, Ph.D. New York Botanical Garden, New York, NY.

Nirajan Adhikari, MSc. Drexel University, Philadelphia, PA and eDesign Dynamics LLC, New York, NY.

Timon McPhearson, Ph.D. Urban Systems Lab, The New School, New York, NY, Cary Institute of Ecosystem Studies, Millbrook, NY, Stockholm Resilience Centre, Stockholm University, Stockholm, Sweden

Pablo Herreros-Cantis, Ph.D. Basque Center for Climate Change, Leioa, ES.

 

Summary

Related Links

  • Key Message 1

    NYC faces risks from four types of flood hazards: pluvial, fluvial, coastal, and groundwater, each with a unique geography of exposure that will expand in different ways in the future due to climate change.

    Identifying these four types as separate, but related, hazards is an important step in studying how they impact NYC, what FRM tools are available to address them, and where future research is needed.

  • Key Message 2

    Discussions about flooding often focus on risks within the Special Flood Hazard Areas (SFHA) mapped by the United States Federal Emergency Management Agency (FEMA). However, the FEMA SFHA maps present fluvial and coastal flood hazards only.

    The recently released NYC Stormwater Flood Maps represent the city’s first attempt to map pluvial and some compound flood hazards, with risks spread out over a much larger faction of NYC. In the coming year, the USGS and NYCDEP will be embarking on a study to investigate and model groundwater flooding in Queens and Staten Island. In this chapter, we present a preliminary assessment of pluvial and groundwater flood hazard exposure areas that can be utilized to support FRM. Additional research is necessary to develop hazard maps that represent a broader range of flooding hazards and their increase in magnitude in response to anthropogenic climate change.

  • Key Message 3

    In NYC, fluvial flood risks are spatially localized to areas of the Bronx and Staten Island where surface stream channels remain. In the remainder of the city, historical surface streams were filled and replaced, with their flow routed to the sewer system.

    As a result, fluvial flood hazard has largely been replaced by pluvial flood hazard in most of the city. Both fluvial and pluvial flood hazards will increase due to climate-change driven intensification of precipitation and elevation of sea level. While traditional floodplain management can be an effective strategy in reducing exposure to fluvial floods, a broader, watershed-scale approach that retains, detains, and redirects stormwater is needed to jointly manage pluvial and fluvial flood risks.

  • Key Message 4

    Current and future coastal flood risks are caused by high storm tides, rising sea levels, and historical development on landfill over tidal marshes and nearshore areas.

    In Jamaica Bay, tides and storm surges have also been significantly elevated by historical dredging and landfilling, worsening chronic and extreme flooding. On December 23rd, 2022, a major flood event around Jamaica Bay was caused, in part, by dredging that has led to amplified storm tides which were nearly a foot higher there than elsewhere in the harbor. Further improvement of our understanding of future coastal flood hazard is possible through downscaling of climate model data and modeling of multiple compounding flood drivers.

  • Key Message 5

    Many NYC neighborhoods have very shallow groundwater tables and already experience groundwater flooding. These areas include parts of the city that were developed when groundwater levels were substantially lower due to historical pumping of groundwater for municipal water supply.

    Groundwater flood risk has the potential to be particularly significant in NYC because of the prevalence of subterranean infrastructure. Groundwater flood hazards have not yet been assessed citywide, but preliminary efforts are underway. Sea level rise could cause groundwater levels to rise, resulting in inflow and infiltration of groundwater into separate and combined sewer pipes and subterranean spaces. Higher water tables can also lead to waterlogged soils. Improved characterization of spatially heterogenous aquifer hydraulic properties and sustained monitoring of ground water levels will be necessary to develop projections for future groundwater flooding.

  • Key Message 6

    Climate change is worsening extreme precipitation events and elevating sea levels, increasing the likelihood of compounding either one of these flood drivers by the other.

    In addition, tropical and post-tropical cyclones (TCs) have caused severe storm surges and extreme rainfall to occur simultaneously. While assessment is limited by the small number of historical TC events, the limited evidence suggests that TCs can cause low-probability, dangerous compound flooding. Given the importance of TCs and limited historical data, a deeper understanding of compound flood hazard likely requires detailed modeling and downscaling to simulate such storms under the present and future climate.

  • Key Message 7

    New York City’s natural and nature-based systems (NNBS), provide many valuable ecosystem services, including critical water regulation services that can play a role in Flood Reduction Management (FRM).

    However, many of these systems are themselves vulnerable to different flood hazards, especially along the coast. Research into how different types of NNBS are impacted by flood/storm surge events, hydroperiod changes, rising water tables and salinization is needed to better evaluate future changes in ecosystem services. Opportunities for designing NNBS to mitigate the impacts of various flood hazards need to be further explored.

  • Key Message 8

    Comprehensive FRM plans must be designed to address the full range of flood hazards faced by individual communities. Planning must begin with participatory decision-making processes that establish neighborhood-specific levels of acceptable future flood risk.

    To reduce risks from current levels, FRM tailored to each community will include combinations of structural and non-structural approaches, including NNBS, that are implemented in ways that reduce social vulnerability and are also synergistic with community histories, needs, and goals.