Methods & Sources

How the NYC Rainfall Flood Hazard Exposure map is built

What the map shows

This map measures how much each New York City neighborhood is affected by stormwater flooding. Stormwater flooding happens when rainfall overwhelms the city's drainage systems and water pools on streets and low-lying areas. Unlike coastal surge, this type of flooding can happen anywhere in the city, often far from the waterfront.

The map offers three ways to look at the problem:

• Exposure — What percentage of each neighborhood's land area falls within a flood zone?
• Risk to People — Roughly how many residents live in the flooded portion of each neighborhood?
• Building Risk — What share of a neighborhood's buildings are both structurally vulnerable to flooding and located inside a flood zone?

Neighborhoods

The map uses Neighborhood Tabulation Areas (NTAs), geographic boundaries defined by the NYC Department of City Planning. There are roughly 200 residential NTAs across the five boroughs. Parks, cemeteries, and airports are excluded.

Flood scenarios

Three stormwater flood scenarios are available, each modeled by the NYC Department of Environmental Protection (DEP):

• Current Sea Levels — A moderate rainfall event (2.13 inches per hour) with today's sea levels.
• 2050 Sea Level Rise — The same moderate rainfall with projected sea level rise by 2050.
• 2080 Extreme — A more intense storm (3.66 inches per hour) combined with projected 2080 sea level rise. This represents a worst-case planning scenario.

Each scenario is a set of polygons showing where water is expected to collect on the surface. The map overlays these polygons with neighborhood boundaries to measure how much of each neighborhood would be affected.

Exposure (% area)

For each neighborhood, the analysis calculates the percentage of land area that overlaps with the flood layer. A neighborhood with 20% exposure has one-fifth of its area inside a modeled flood zone.

Risk to People

Population estimates come from the U.S. Census Bureau's American Community Survey (2023, 5-year estimates) at the census-tract level, then aggregated to NTAs using an official crosswalk table. The number of people at risk is estimated by assuming that residents are evenly distributed across each neighborhood. For example, if 30% of a neighborhood is flooded and 10,000 people live there, roughly 3,000 are estimated to be in the flood zone.

Building Risk

This view uses the Building Elevation and Subgrade (BES) dataset, which includes elevation measurements for nearly every building in New York City (about 862,000 buildings).

A building is considered vulnerable if either:

• Its lowest actively-used floor is below the surrounding ground level (the floor elevation is lower than the grade elevation), or
• A subgrade space (such as a basement or cellar) was observed during the survey.

A vulnerable building is considered at risk if it also falls within a modeled stormwater flood zone. The map shows what percentage of each neighborhood's buildings are at risk, combining structural vulnerability with flood exposure.

Limitations

• Population distribution within neighborhoods is assumed to be uniform, which may over- or under-count people in unevenly developed areas.
• The BES dataset reflects exterior observations and cannot measure interior conditions like actual basement depth.
• Flood models represent planning-level estimates, not precise predictions for any single storm event.
• The analysis does not account for local drainage improvements, green infrastructure, or other mitigation measures that may reduce actual flood risk.

Data sources