Know Your Risk

Flooding is the most common natural disaster in the United States and it can happen anywhere at any time. Informed communities can take the steps necessary to prepare for and mitigate hazards, but the first step is to know your risk. That’s why FEMA provides communities with credible, accessible flood risk products to enhance decision making and improve mitigation planning. Use the points below to help discuss these tools with your community.

Interact with Flood Risk Data – GeoSpatial PDF

  • A GeoSpatial PDF is a mapping tool that helps identify flood hazards without needing Geographic Information System software or an internet connection.
  • This tool will be helpful as you investigate areas to reduce flood risk in your community.
  • Some layers will be unique depending on the specific flood risk datasets developed for your community.

Changes Since Last FIRM Data (CSLF)

  • Changes Since Last FIRM (CSLF) shows areas where flood risk has increased or decreased since the last Flood Insurance Rate Map was issued.
  • As development within the community increases, storm water flow can change and impact flooding.
  • The CSLF helps assist community officials in the review of engineering data prior to the release of the Preliminary FIRMs.
  • Property owners can use the CSLF to determine if their flood zone designation is likely to change and if they should be proactive to take advantage of certain flood insurance discounts.
  • Understanding areas with identified changes in risk can help communities develop targeted outreach and communication approaches.

Depth Grids

  • Flood Depth Grids calculate the difference between the ground surface elevation and the calculated water surface.
  • Depth Grids are commonly prepared for the 1-percent annual chance flood event, but can be prepared for a variety of flood frequencies.
  • Local officials can use depth grids to prioritize flood mitigation activities and advance recovery planning and disaster preparedness.
  • Depth grids can support requirements for sustainable building practices and adoption of higher floodplain standards.

Water Surface Elevation Grids (WSEL)

  • Water Surface Elevation Grids can be used to estimate base flood elevation in Approximate Zone A areas.
  • Homebuilders can work with local officials to review grids to estimate base flood elevation in Approximate Zone A areas and promote building above the expected 1-percent annual chance flood.
  • Local officials can determine if flood risk has changed due to development, screen potential projects for cost effectiveness, and discourage infrastructure investment in high risk areas.

Average Annualized Loss (AAL)

  • Hazus is a GIS-based planning tool that estimates structural damage, as well as economic and social losses.
  • In 2009, a National Study on flood loss was developed using Hazus Multi-hazard, as a part of FEMA’s Risk Mapping, Assessment and Planning program, or Risk MAP. This study was the first national Average Annualized Loss (AAL) study for flood losses of the contiguous states ever performed.
  • The AAL only provides a coarse estimate of annualized losses. When paired with local data, the AAL can be used to draw conclusions regarding your community’s risk.
  • The information contained in the Average Annualized Loss (AAL) can be used to identify flood-prone areas, communicate relative flood risk, provide potential damage severity information for different flood frequencies, and identify possible locations for mitigation options.

Percent Annual Chance Grids

  • Across the 1-percent annual chance floodplain extent, there is a variety of flood depth.
  • Two gridded datasets will be available for each stream studied:
    • The Percent Annual Chance grid describes the chance of flooding each year
    • The Percent 30-Year Chance grid describes the probability of flooding in a 30-year period, typical
  • Percent Annual Chance Grids can assist local community officials to determine areas that are more susceptible to flooding for project prioritization and targeted areas for outreach.
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