What is the Hazard?
A flood is an overflow of water from oceans, rivers, or rainfall that submerge areas that are usually dry. Flooding is a natural phenomenon that happens due to a combination of weather events and hydrologic processes, but the hazards associated with flooding in New York City are often exacerbated by features of the diverse, densely packed city environment.
According to FEMA, floods occur in the United States more frequently than any other natural disaster. Floods present hazards that damage and destroy property, natural resources, and lives. One in three federal disaster declarations in the United States is related to flooding. Many floods affect heavily populated coastal communities—including New York City.
New York City is subject to four types of flooding:
Because of New York City’s flooding history, some property owners are required to have flood insurance. More information on this federal insurance requirement and how it applies to New York City is included here:
Coastal flooding is primarily caused by the storm surge that generally accompanies a strong coastal storm, such as a tropical storm, hurricane, or nor'easter. The wind field and the coastal storm’s low pressure cause water levels to rise and create a hazardous storm surge.Storm surge can cause "stillwater" flooding -- a rise in water levels without significant waves -- or flooding accompanied by waves. In either case, coastal flooding can cause erosion, structural damage, and other hazardous conditions. The salt or brackish water that accompanies coastal flooding can ruin mechanical and electrical equipment and harm vegetation. The elevation and slope of a shoreline influences how storm surge behaves. For example, low-laying areas both on the coast and inland are at risk of flooding. Additionally, New York City's geography magnifies the effect of storm surge and the likelihood of coastal floods when severe coastal storms funnel ocean water into New York Harbor.
Climate scientists project that coastal flooding hazards will be exacerbated due to sea level rise in the future.
Tidal flooding, also known as “nuisance flooding,” “blue-sky flooding,” or “sunny day flooding,” is caused by normal variations in the lunar cycle and can occur even in the absence of a storm.
Sea levels fluctuate daily due to the gravitational forces and orbital cycles of the moon, sun, and earth. Two high tides and two low tides occur daily. These daily high tides are at their highest twice a month during "spring tides," when the earth, sun, and moon are aligned.
Tidal flooding currently affects some low-lying sections of New York City. Low-lying shoreline neighborhoods are particularly vulnerable.
As climate change causes sea levels to rise, low-lying neighborhoods will experience more frequent flooding than what they are experiencing already. Even low-lying New York City neighborhoods that do not currently experience tidal flooding could experience floods regularly in the future if precautions are not taken in the coming years. Regular tidal flooding could significantly disrupt neighborhoods and gradually erode shorelines.
Inland floods – also known as "flash floods" or “cloudbursts” or “urban flooding” -- can be caused by short-term, high-intensity rainfall that is often associated with sudden small-scale thunderstorms. Different from coastal flooding, inland flooding can occur with little warning and can be highly localized with some areas receiving higher rainfall amounts. Hazards from inland floods can be made worse by several days of moderate rainfall from weaker storms that drift slowly or stall over one area.
Inland flooding can occur if excessive amounts of rainfall upon low-lying areas and overwhelming and exceeding the design capacity of the sewers or stormwater management infrastructure. Improper street grading and blocked outfalls can also contribute to inland flooding.
In New York City, impervious surfaces, such as buildings, streets, sidewalks, and parking lots, reduce the amount of rainfall from being directly absorbed into the ground, thus increasing the volume of surface runoff flowing into the city's sewer infrastructure. Approximately 72% of New York City’s surface is impervious, which means runoff from rainfall is channeled directly to sewers rather than being absorbed by the landscape. This increased runoff increases the chances of inland flooding. This effect is most pronounced in low-lying areas that have limited natural drainage capacity, including parts of New York City that were built upon filled-in or paved-over marshes or creeks.
The standard design criterion for sewage infrastructure in New York City is to use the intensity-duration values based on a storm with a 5-year return period (e.g., 1.75 inches per hour for a one hour storm; 20 percent chance of occurrence in any given year) to calculate how large the sewer pipes need to be sized to appropriately manage stormwater. Certain older areas of the City are designed for a 3-year storm event. These storm events are becoming more and more frequent further stressing the existing sewage infrastructure.
NYC’s sewer system is approximately 60 percent combined, which means it is used to convey both sanitary and storm flows. When the sewer system is at full capacity, a diluted mixture of rainwater and sewage may be released into the local waterways. This is called a combined sewer overflow, or CSO. The potential for a CSO further increases the risks that inland flooding poses to communities to include exposure to sewage in local waterways.
Riverine flooding occurs when the volume of freshwater flowing through a river or stream exceed the holding capacity and water overruns the river or stream’s banks.
When large rivers flood, it is usually due to prolonged rainfall from a large-scale weather system over a significant area. During these types of weather events, rainfall can flood smaller basins that drain into major rivers, contributing to riverine flooding.
Narrow rivers and streams are more susceptible to flooding if a local weather system pours rain intensely over a smaller, more concentrated area.
Across New York State, riverine flooding is the most common type of flood event. In New York City, however, riverine flooding is less common and less severe, because most freshwater rivers and streams within the city are short and drain only small areas. Nevertheless, in parts of Staten Island and the Bronx, riverine flooding can still pose a hazard.
Flood severity depends on the type of flood, its cause, its duration, and other existing conditions, such as the capacity of sewers and other pathways that allow water to exit. As shown below, the National Weather Service (NWS) categorizes the severity of flooding into three categories -- minor, moderate, and major.
National Weather Service Flood Categories
Flood probability projections in New York City are based upon the frequency of past flood events of different intensities and the intervals of time between them. Flood probability classifications range from low probability high impact (e.g., Hurricane Sandy in 2012) to high probability low impact (e.g., more frequent, smaller flood events).
However, determining the likelihood of a flood and the potential impact it could have upon an area of the city is very complex. To provide more understanding, this report presents a detailed discussion and probability analyses for three very different types of floods that put New York City at risk:
“100-Year” and “500-Year” Flood Terminology
Using the terms "100-year" and "500-year" when discussing the hazards associated with coastal and riverine flooding can be misleading and can possibly convey a false sense of security about flooding hazards.
Referring to a 100-year flood does not mean that the flood happens once every 100 years. Rather, it means that the flood has a 1 percent chance of being met or exceeded in any given year. If a community experiences a 100-year flood, it does not decrease the chance of a second 100-year flood occurring that same year or in any following year.
Even the 1 percent concept can be misleading because of the way probability is calculated. The 1 percent chance refers to the probability of a flood in any given year. But if you look at the probability of one flood occurring in a longer time frame, the probability is compound. That means that, as you increase the number of years considered, the probability of a flood occurring is much higher. So while a building located in the 1 percent annual chance floodplain has a 1 percent chance of experiencing a flood in any given year, it has a 26 percent chance of experiencing a flood during the lifespan of a 30-year mortgage.
Similarly a 500-year flood is not a flood that happens once every 500 years. Rather it has a 0.2 percent (1 in 500) chance of occurring in any given year.
The probability of coastal and riverine flood hazards in New York City is determined by several factors, including severe weather events, such as hurricanes, and regular high tide.
New York City consults the Federal Emergency Management Agency (FEMA) Flood Insurance Rate Maps (FIRMs) to determine the flood risk probability for coastal and riverine flooding. The FIRMs, which are used to determine flood insurance rates, graphically represent the federal government's official assessment of flood risk in specific areas of the city.
The flood zones shown on the FIRMs are geographic areas classified according to levels of flood risk, with each zone reflecting the severity and/or type of flooding.
The 1 percent annual chance floodplain, commonly called the “100-year floodplain,” indicates an area that has a one-percent-or-greater chance of experiencing coastal or riverine flooding in any given year. Floodplain management standards apply to land located within the 100-year floodplain. Owners of property located within the zone are mandated to purchase flood insurance if they have a federally-backed mortgage.
FIRMs also show projected flood heights. Additionally, in their next revision, FIRMs will look to include the 500-year floodplain for New York City -- an area with a 0.2 percent or greater annual chance of flooding.
The FIRMs classify different geographic areas into flood zones according to their levels of flood risk. As shown here, each of the four zones reflect the severity and/or type of flooding.
|Flood Zone Category||Description|
*E indicates that base flood elevation is determined on the FIRMs
Initially created for New York City in 1983, FEMA’s maps received a minor update in 2007 but remained largely unchanged until recently. In 2009, FEMA initiated a new coastal flood study and released new Preliminary Flood Insurance Rate Maps (PFIRMs) in January 2015. This update to the New York City’s 1 percent annual chance floodplain encompassed an area with approximately 400,000 residents, 71,500 buildings, and 532 million square feet of floor area.
In reviewing the maps, New York City discovered that FEMA overestimated the size of its 1 percent annual chance floodplain. In June 2015, New York City filed an appeal to FEMA to request revision of its floodplain estimate. In October 2016, FEMA announced its agreement with the City’s findings, and its commitment to work with the City to revise the 2015 Preliminary FIRMs and issue new maps in the coming years that reflect local flood risk more accurately. In December 2017, remapping kicked off, with revised PFIRMs expected in 2021.
Currently, New York City applies whichever map (2015 PFIRMs and 2007 FIRMs) is the more restrictive of the two for Building Code and zoning purposes. FEMA uses the 2007 FIRMs for the National Flood Insurance Program (NFIP).
To update FEMA’s FIRMs for New York City, the City participates in a Coordinating Technical Partnership (CTP) with FEMA Region II, managed by the Mayor's Office of Resiliency (MOR).To see a map of the most current 2015 PFIRMs, please see the location section within the Flooding Profile.
The probability that specific parts of New York City will experience tidal flooding depends on the area’s proximity to a tidal area and the lunar cycle. New York City’s low-lying coastal sections experience regular floods from high tides. Many of these areas regularly experience tidal flooding today.
As sea levels rise, it is likely that the lowest-lying areas of the New York City will gradually become more vulnerable to regular flooding from daily and monthly high tides. Parts of New York City that currently do not flood may begin to do so.
Unlike coastal and tidal floods, inland floods can occur with little or no warning. The probability of inland flooding is hard to predict because the heavy rainfall from storms and other weather events that cause inland flooding are often localized and can occur at any time of the year.
Inland flooding in New York City is created or exacerbated by the degree to which an area has impervious ground coverage or by the area’s local topography. These local features can make it harder to determine exactly when or where an inland flood might occur, even if the forecast predicts storm or rain events.
In the future, climate trends project that New York City will experience heavier rainfall. If this occurs, the sewer system is likely to become overburdened more regularly and outfalls— drainage infrastructure for stormwater runoff and CSOs — are likely to be inundated more frequently.
Inland flooding is unrelated to the 1 percent annual chance floodplain designation, and FEMA does not map it in the floodplain on the Flood Insurance Rate Map. However, a study conducted by the Mayor’s Office of Climate Resiliency (MOCR) and Department of Environmental Protection (DEP) developed models based on rainfall and future sea-level rise that indicate the probability of inland flooding. They include a 10-year event with an increase of 2.5 feet in mean higher high water (moderate), and a 100-year event with an increase of 4.8 feet in mean higher high water (extreme).
|Map||Rainfall||Sea Level Rise|
|Moderate||10-yr||MHHW + 2.5 feet|
|Extreme||100-yr||MHHW + 4.8 feet|
New York’s unique topography and variations in its built environment greatly affect the degree to which different parts of the city will experience flooding.
New York City consults the Federal Emergency Management Agency (FEMA) Flood Insurance Rate Maps (FIRMs) to determine the location of coastal and riverine flooding.FIRMs are important reference documents because flood insurance rates are affected by the degree of flood risk, and property owners with federally-backed mortgages in high-risk areas are required to have flood insurance.
Coastal flooding can occur anywhere along New York City's 520-mile shoreline, but the severity of the hazard varies widely across the city. The extent of inland flood zones and the elevation of the 1 percent annual chance flood event is not uniform across all neighborhoods.
The shoreline’s elevation and slope influence how a storm surge behaves and how sea level rise will affect an area. New York City’s shoreline conditions are diverse, ranging from soft marshy or sandy areas of coastline to waterfront that is reinforced with hard rock and/or concrete.
New York City’s proximity to the open ocean and long shoreline exposes it to risks from wave action. Low-lying areas, whether on the coast or inland, are at risk of flooding. The geography of the New York Bight – the right angle formed by Long Island and New Jersey –increases the effect of storm surge by funneling the water into New York Harbor, as described in more detail in the Coastal Storms Hazard Profile:
New York City’s incremental development over the centuries has resulted in a mosaic of building types, infrastructure networks, and land uses, so the risks from coastal floods are diverse. Floods can potentially affect stretches of coastal marshland in Jamaica Bay, residential neighborhoods with small wood-frame bungalows along oceanfront beaches, Manhattan’s dense commercial districts, and industrial and post-industrial areas along the New York Harbor.
Since many critical infrastructure systems and services are located along the waterfront, there are additional risks to New York City’s continued operation when it experiences coastal flooding.
Riverine flooding affects only a very small portion of flood-prone areas in New York City. Staten Island and the Bronx are the boroughs most vulnerable to riverine flooding due to the watershed characteristics in those boroughs. The Bronx and Hutchinson Rivers can create floods in the Bronx. On Staten Island, flooding can come from streams and river networks along the south shore and mid-island.
Some low-lying neighborhoods throughout New York City are currently vulnerable to flooding from lunar or seasonal high tides. As sea levels rise over time, the vulnerability of these low-lying neighborhoods will gradually increase due to more frequent flooding from daily and monthly high tides.
The risk of regular tidal flooding is most pronounced in the lowest-lying areas in New York City -- in the Broad Channel and Hamilton Beach neighborhoods around Jamaica Bay in southeastern Queens, on portions of the bay side of the Rockaway Peninsula, and in low-lying sections of Staten Island and Red Hook.
New York City areas that are prone to inland flooding are often low-lying, with an abundance of impervious surface cover . Inland flooding may occur when the amount of rainfall received and the total volume of the water exceeds the capacity of the sewer system.
Flooding complaints submitted through New York City’s 311 system between 2004 and 2017 shows that certain neighborhoods report inland floods more frequently than others. The areas at greatest risk from inland flooding are typically those situated on filled-in wetlands or where streets have limited stormwater infrastructure because they are not built out to the city’s sewer system plan.
The following map shows that the areas with the highest 311 complaint rates from flooding include southeastern Queens and the northern and other parts of Staten Island, which were not built out to the city’s sewer system plan.
Map 311 Flooding Complaints from 2004-2017
The following map, produced by the Mayors Office of Climate Resiliency and DEP, depicts areas of predicted rain-driven flooding. It includes two scenarios, moderate stormwater flooding under future conditions and extreme stormwater flooding under future conditions. Incorporating sea-level rise allows the City to better understand the magnitude of the risk in the coming decades as well as preview the performance of the existing drainage system.
FEMA administers NFIP and sets insurance premiums. Flood Insurance purchase requirements and minimum building standards are based on FEMA’s FIRMs. Property owners are required to carry flood insurance if they have a federally-backed mortgage on structures located within the 1 percent annual chance floodplain designated on FIRMs
FEMA collects a vast quantity of information on insured structures in New York City through the NFIP, including the number and location of flood insurance policies; claim payment amounts; and the number of claims per insured structure, including Repetitive Loss and Severe Repetitive Loss structures that are defined as follows:
- Repetitive Loss structures: Structures for which a policyholder receives two or more claim payments of $1,000 or more after flood events within a 10-year period.
- Severe Repetitive Loss structures: Any insured structure that has incurred flood damage for which:
- At least two separate claim payments have been made under a Standard Flood Insurance Policy, with the cumulative amount of such payments exceeds the fair market value of the insured buildings on the day before each loss; or
- At least four or more claim payments over $5,000, and with the cumulative amount of claim payments exceeding $20,000.
Source: FEMA NFIP, 2019
NFIP Premiums, Contracts, Policies, and Coverage for New York City 2010 - 2019
|Year||Borough||Premium||Contracts||Policies||Building Coverage||Content Coverage|
Number of NFIP Contracts in the 1 Percent Annual Chance Floodplain (using 2007 Effective FIRMS)
|Borough||No.# of NFIP Contracts|
The following maps spatially present several types of NFIP insurance data for all of the New York City boroughs – density of NFIP policies, NFIP policy claims, and NFIP repetitive loss properties.
|NFIP Policies, 2018
Source: FEMA; NFIP
|NFIP Repetitive Loss Claims, 2018
Source: FEMA; NFIP
NFIP Claims 1976-2018
Source: FEMA; NFIP
Although each map represents a different variable related to the NFIP, all show a relatively large concentration of claims and policies on Staten Island’s East Shore, in portions of Brooklyn and Queens that face the Atlantic Ocean and Jamaica Bay, and in the low-lying southeastern section of the Bronx.
From November 2017 to October 2018 there were 125 claims filed with a building damage total of $594,000 and content damage total of $35,000. Since 1976, New York City has had almost 42,000 claims. As of January 2019, 16,900 claims had been filed since Hurricane Sandy, totaling $1.4 billion in payouts.
As of September 2018, New York City had 4,188 Repetitive Loss claims amounting to $238 million in payouts. Approximately 57 percent of these structures were located within the Preliminary FIRM 1 percent annual chance floodplain. As shown in the table of repetitive loss claims, 69 percent were for single-family homes and 23 percent were for two-to-four family homes.
Repetitive Loss Claims by Housing Type, 2012 – 2018
|Housing Type||Percentage of Repetitive Loss Claims (4,188 total)|
|Unknown||Less than 1%|
Source: FEMA; NFIP
These claims were concentrated in Howard Beach, Breezy Point, Arverne, Broad Channel, and the Midland Beach areas. As of May 2018, there were 38 Severe Repetitive Loss structures that represented $5.3 million in payouts. All 38 of these areas are a high priority for flood mitigation.