FloodRisk

Methodology

How FloodRisk risk scores are computed: data sources, modeling approach, and limitations.

FloodRisk publishes flood risk scores at neighborhood resolution across US flood-prone regions. We model both river (fluvial) and rainfall (pluvial) flooding so we surface real exposure beyond regulatory boundaries. Every score on this site is computed by PerilScore using the same data layer used by insurance and risk management professionals.

Data sources

We start from public scientific records. Every input is auditable, and we don’t use proprietary or paywalled data.

  • USGS stream gauge network: long-run discharge and high-water records for riverine flood modeling.
  • NOAA Atlas 14 precipitation atlas: rainfall frequency for the 100 and 500 year return periods at 1-hour and 24-hour durations.
  • USGS 3DEP National Elevation Dataset: high-resolution terrain for watershed delineation and flow accumulation.
  • NLCD land cover and curve number layers: runoff potential per land-use type.
  • FEMA NFHL flood zones: used as a reference layer for regulatory context.

Modeling approach

We combine hydrologic models (watershed runoff response) and hydraulic flow models (where water actually goes when it rains) with the precipitation and discharge records to estimate annual flood probability per location. The result is a single 0 to 10 probability score at each neighborhood-scale sample point (about 5 km²).

Output metrics include long-run flood frequency, the 100 and 500 year annual probability, separate riverine, coastal, and pluvial component scores, a compound-flood index where multiple drivers can occur together, and the FEMA flood zone for reference.

Resolution

Scores are computed at neighborhood resolution: approximately 5 km² sample points across the contiguous US. This is much finer than the county-level averages most public flood data provides, and substantially more granular than FEMA’s discrete-boundary flood zones.

Update cadence

The model is refreshed annually as USGS publishes updated gauge records and as NOAA Atlas 14 / Atlas 15 precipitation atlases are published. FEMA zone references are updated as new Flood Insurance Rate Maps are released.

Validation

Models are evaluated against held-out historical flood events including major presidential disaster declarations. The exact validation protocol is documented in the PerilScore technical papers.

Limitations

  • Forecast boundary. FloodRisk reflects long-run probability across many seasons. For active flood guidance, use National Weather Service and local emergency sources.
  • Property-specific detail. Scores reflect a neighborhood-scale sample point. For a property-specific score that incorporates first-floor elevation, foundation type, and flood-vent compliance, use the free PerilScore app.
  • Regulatory context. Use FEMA Flood Insurance Rate Maps for regulatory flood-insurance status. FloodRisk surfaces actuarial exposure beyond the FEMA boundary for research and planning.

Attribution

Risk scores powered by PerilScore. Visit perilscore.com for the full platform, API access, and commercial-use licensing.

Methodology

Public data. Real science. No black boxes.

Every score is computed from decades of public weather records using physics-based probability modeling. It's the same approach used by insurance and risk management professionals.

Decades of public weather data

Hurricane tracks, storm intensities, fire perimeters, hail reports, all drawn from public scientific archives. We don't use proprietary data. You can audit every input.

Physics-based probability modeling

Scores reflect how the actual peril behaves: wind fields, fire spread, ground shaking, and storm tracks. The model keeps the physics visible instead of flattening every place into a broad average.

Used by professionals

The same PerilScore data layer is used by insurance and risk management professionals. We publish it here so anyone can find authoritative risk numbers for their location.

Frequently asked questions

Where does the flood risk score come from?
Every score is computed from decades of public weather data (gauge records, precipitation history, terrain, and watershed modeling) using physics-based probability modeling for both river and rainfall flooding.
How is this different from a FEMA flood zone?
FEMA flood zones are regulatory boundaries used to set flood insurance requirements. Our score is a continuous probability designed for actual risk modeling. Many properties outside FEMA zones still carry meaningful flood probability.
Does this cover pluvial (rainfall) flooding?
Yes. Pluvial flooding from intense rainfall is one of the largest uninsured losses in the U.S. Our score accounts for both pluvial and fluvial sources.

Want the full picture for a specific property?

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