What are boundaries and footprints and how are they used to calculate my Flood Factor?

Learn about how boundaries and footprints are used to determine the likelihood of water reaching the lowest point of a property’s footprint.

Boundaries and Footprints

The model used by Flood Factor includes publicly available and 3rd party data to identify property boundaries and building footprints. A property boundary can be thought of as an outline of the individual lot of land, while building footprints refer to the outline(s) of building structures on a property. Along with publicly available elevation data, this information is used to determine the likelihood of water on the ground reaching the lowest point of a building’s footprint.

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The black box indicates the home’s footprint, while the red outline signifies the property’s boundaries. A Flood Factor score is analyzed based on the depth of likelihood flooding will reach any point of the black box, or the home’s footprint.

Where possible, the First Street Foundation Flood Model calculates flood statistics based on the footprint of the largest building on a property. For this reason, the map pin is placed in the center of the largest building. In some cases, that may be a barn or a large garage.

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While this home has two large structures on the property, the pin is located on the larger building. Therefore, the Flood Factor for this property is based on the larger structure, where the pin is located. 

It’s important to note that water reaching this boundary doesn't necessarily equate to water entering the home. How quickly the water reaches the building, how long it stays, and the building’s foundation type have a lot to do with the actual damage the home may experience. 

For the majority of homes in the US, the elevation of the first floor of living space in a home corresponds to its footprint, but this varies from home to home. Homes with basements may experience more flooding at lower depths, while raised homes may be better protected from higher depths of flooding. Because the First Street Foundation Flood Model looks at risk to the building footprint, not the first floor elevation, elevated homes may still have high Flood Factors, and homes with basements may still have low Flood Factors.

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Homes with basements may experience more flooding at lower depths, while raised homes may be better protected from higher depths of flooding.

Flood Factor also takes into account how flood risks are likely to change over time with sea level rise and atmospheric changes over the next 30 years, which means even a property that hasn’t flooded in the recent past could still face flood risk in the future. 

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Properties that have not experienced flooding previously, may flood over the next 30 years due to the changing climate.

Even a property located high atop a hill could have a relatively high Flood Factor if it’s in an area where heavy rain events are expected to become worse, and more frequent, over the next three decades.

 

Learn more

How will different flood depths affect my property?

What are probabilities, and how do they affect my Flood Factor?

How is my Flood Factor calculated?

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