Why a property’s FEMA zone does NOT impact its Flood Factor

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FEMA and Flood Factor are different risk assessments. Flood Factor may show a higher risk because it accounts for flooding from rain and the effects of climate change.


Your FEMA zone and Flood Factor® are independent risk assessments. 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. FEMA determines flood risk on the community level, their risk projections capture risk from a single 1-in-100 or 1-in-500 event from storm surges and overflowing rivers and streams. Flood Factor calculates flood risk on the property level, accounts for changing climate conditions, and considers the risk of flooding due to high-intensity rainfall.  

As a result, Flood Factor shows a more nuanced, property-specific flood risk, as opposed to a binary in-or-out of floodplain analysis from FEMA maps. Flood Factor uses publicly available flood risk information that shows how risk will accumulate and change over time. It is most powerful when used in conjunction with FEMA flood maps and other available state and local flood risk resources, and should be viewed as complementary to FEMA flood maps, which need to be used for building and permitting purposes. Individuals can search their address on Risk Factor to find their property’s flood risk as well as their FEMA zone. 

Flood risk on Risk 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 allows for a more comprehensive flood risk tool than is 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 by 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 estimated on Flood Factor. 

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 Risk 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 by the First Street Foundation Flood Model, the 1-in-100 hazard layer, represents a 1% annual risk of occurrence, as 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 a 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.  

Use Risk Factor to understand your risk

FEMA flood maps identify over 1.1 million miles of flood hazard areas, and while the maps can provide detailed information for homeowners on their flood risks, they are not available everywhere. Risk Factor’s national flood model shows that flood risk is more widespread in the United States, with over 26 million properties at risk over the next 30 years. Risk Factor™ also includes flood risk from extended periods of high-intensity rainfall, leading to urban stormwater flooding and storm surge. Additionally, future conditions are considered, such as sea level rise.

Without free access to accurate flood information, more Americans will unknowingly be at risk from the nation’s costliest natural disaster. Risk Factor aims to quantify and communicate America’s flood risk. By making flood risk data freely available for all, individuals and communities can prepare for and mitigate risks before they become a reality. 

Press Coverage

CNN: Flood risk is growing for US homeowners due to climate change. Current insurance rates greatly underestimate the threat, a new report finds

The New York Times: Climate Threats Could Mean Big Jumps in Insurance Costs This Year

Bloomberg: Most Americans Don’t Have Enough Flood Insurance for Climate Change

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