Recreating Historic Floods

Millions of homes across the United States have already experienced flooding.

Flood Factor uses data from First Street Foundation, which has modeled 55 major river and storm surge floods in the U.S. since 2000, and is continually adding more. The floods are modeled simulations recreated and calibrated using recorded flood data and validated against known government flood claims. By recreating the observed impacts of these floods, scientists are able to identify where, and to what depth, flooding occurred.

How historic floods were chosen

The floods recreated by First Street Foundation broadly fall into two categories: coastal and inland. Coastal floods refer to floods caused by the storm surge of tropical storms, hurricanes, and nor’easters. Inland floods refer to floods caused by overflowing rivers. Heavy rains are often associated with both types of flood events, but are not modeled in the floods shown on this tool directly.

Factors Considered

  1. Storm Event Database
    Both coastal and inland flood events were selected from the National Oceanic and Atmospheric Administration’s (NOAA) storm event database, which contains archived National Weather Service (NWS) storm data
  2. Claims Data
    Floods were then narrowed down to only include events that have National Flood Insurance Program (NFIP) and Individual Assistance (IA) claims, which helps validate where and to what depth flooding occurred.
  3. Water Levels
    Inland floods were further limited to events where rivers reached or exceeded their record-high water levels.

Recreating coastal and inland floods

Historic floods in the First Street Foundation database are modelled simulations recreated and calibrated using recorded flood data and validated against known government flood claims.

Recreated coastal and inland floods are informed by the U.S. Geological Survey (USGS) high water mark data, which indicate how high water reached after a flood. Coastal floods also use data from recorded hurricane wind fields, which can affect storm surge levels, as well as data from NOAA tide gauges, which measure the height of ocean levels every 6 minutes.

Inland floods similarly incorporate data from stream gauges, which measure and record water levels every 15-60 minutes. Events may be made up of a single gauge reading or the combination of multiple gauges for larger events.

Property-level claims associated with each flood are then used to ensure the accuracy of the recreated floods. A conservative estimate is used to then calculate the number of properties where a flood reaches either a building or the center of an empty lot. As such, the recreated floods may underreport the number of impacted properties.

Coastal floods are representations of flooding from the surge of the storm only, with the exceptions of Hurricanes Harvey, Florence, and Matthew that also include flooding from heavy rainfall. Inland floods are limited to river over-topping from the event only.

What can be done to prevent future flooding?

Although flood risk can never be completely eliminated, there are a wide range of flood protection measures that reduce risk.

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