America’s Flood Maps Have Left Millions Out for Decades—Until Now

FEMA has detailed studies for only about one-third of U.S. rivers. A national model identifies 11 million more people and 4.1 million more buildings within 100-year flood extents.

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Compare FEMA coverage with the national model

The light layer shows FEMA’s published 100-year flood extent; blue shows the model-estimated extent.

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Animated map showing FEMA flood patterns in pale blue and model-generated flood extents in royal blue across Indiana terrain
FEMA pattern Model-generated extent

How the model fills the gaps

The model learns from FEMA’s current flood patterns and the surrounding terrain, then generates flood estimates for places without detailed maps.

Read the method in the paper →

The national map reveals substantially more exposure

Estimated exposure rises from 16.06 to 27.09 million people, with 4.12 million additional buildings beyond FEMA’s published extent.

FIG. 01 — EXPOSURE

Largest population gaps

Miami and New York have the largest additional exposed populations; McAllen and Houma have almost no FEMA-mapped exposure.

FIG. 02 — POPULATION GAPS

Most undermapped metropolitan areas

Ranked by the share of exposed residents outside FEMA’s extent, among metropolitan areas with at least 10,000 exposed people.

FIG. 03 — UNDERMAPPED METROS

What FEMA flood maps leave out

Compare FEMA and model-estimated extents across three cities.

Limitations
  • Shows fluvial and coastal 100-year flood extents; largely excludes rainfall-driven flooding.
  • This is a research product, not a regulatory map. For official decisions, consult the FEMA Map Service Center.
  • Accuracy varies by terrain (validation mIoU 0.48–0.54).