Why do FEMA and Flood Factor have different flood risk assessments for my property?

Learn about and compare flood risk between FEMA Special Flood Hazard Areas and the First Street Foundation Flood Model.


Your FEMA zone and Flood Factor are two independent risk assessments. FEMA flood maps are used to determine insurance and building code requirements, whereas the Flood Factor is used to determine flood risk to the specific home today and into the future. 

While the findings of these models do not diverge starkly across the US, in some areas Flood Factor may show more or less flood risk than FEMA, simply because of differences in the methodologies employed. Flood Factor calculates flood probabilities on the property-level and accounts for changing climate conditions. A core component of the model is its ability to include rainfall events as probabilistic flood risks. 

Flood risk on Flood Factor

The methods used to create the First Street Foundation Flood Model bring together a number of resources and techniques already developed as inputs to understanding one’s flood risk. By combining these resources, the model represents flooding from multiple risks, such as rain, river, tidal and storm surge flooding, while also integrating current and future environmental considerations all at a property level.  Adding to the full picture of flood risk, the model also recreated more than one hundred coastal and inland flood events over a 20 year period (2000-2019) to provide estimates of past flooding likelihoods.  This combination of precision and national scope allow for a more comprehensive flood risk tool than currently available. 

FEMA flood risk

To date, the standard national comparison for flood risk is the Federal Emergency Management Agency’s (FEMA) Special Flood Hazard Areas (SFHA).  The FEMA SFHA models are currently the most popular flood-risk identification tool and are widely used throughout government, research, and private companies as a way to understand flood risk, price insurance premiums, and prepare for potential hazards. A property's FEMA zone designation is listed on Flood Factor and is provided by MassiveCert Inc. 

The initial FEMA SFHA models are made for individual communities and are generally very high-quality, have been built to explicitly understand the risks to a standard 1-in-100 or 1-in-500 year flood event, and have been developed over many years to identify community flood risk based on local historical context. However, these models do not exist for the entire country, and their output also undergoes a series of adjustments and revisions based on local stakeholder feedback.  That means that the FEMA SFHA maps that get instituted are very rarely the same as the initial risk identified in the area. Couple that with the fact that the model is created to capture risk from a single 1-in-100 or 1-in-500 event, reliance on historical trends, and the lack of national scale, and you have opportunities for improvement in the communication of national flood risk.

Flood Factor vs FEMA

In comparison, the method used to create the flood risk visualized on Flood Factor expands flood risk mapping to the entire country by relying on an innovative Regionalized Flood Frequency Analysis (RFFA) approach. This approach makes use of traditional statistical matching techniques to model the characteristics of ungauged streams and river reaches across the country with known gauged characteristics to produce likely flow conditions with high confidence. Additionally, a core component of the model is the ability to also include pluvial (rainfall) events as probabilistic flood risks with depths and associated return periods in areas that are outside the modeled extents of FEMA studies.  Both the RFFA and pluvial flooding integrations have allowed for a model that captures risk that is generally not captured in FEMA SFHAs. 

To highlight the additional coverage of the risk identified on Flood Factor, the 1-in-100 hazard layer, representing a 1% annual risk of occurrence, was compared to the same probability zones outlined by the FEMA SFHA zones. The results indicate that the Flood Model generally captures about 3 times the risk as the FEMA SFHA.  Interestingly, the two models align well along gauged river channels and FEMA SFHAs help provide a source of validation for the fluvial risk identified in the model.  However, the discrepancies indicate the practical need for the more comprehensive approach that was used to produce the statistics in this report.

The alignment of the Flood Model results with SFHAs in areas that are gauged and mapped by FEMA is important. This highlights the utility of the Flood Model as being in agreement with standard indicators of flood risk by building on trusted models, reports, and open data resources. To that point, this model relies heavily on data from the National Oceanic and Atmospheric Administration (NOAA), FEMA, the United States Geological Survey (USGS), the World Climate Research Programme (WCRP), and hundreds of local government resources.  As such, the Flood Factor risk results should be seen as an extension of existing resources developed with innovative and creative methods to comprehensively include geographic areas that may have been left out of alternative-risk models.  


Learn more

Understand the differences between FEMA Flood Zones

How is my Flood Factor calculated?

How are different flood events created for the maps shown on Flood Factor?

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