Understanding the Limitations of FIRMs and FIS reports

While Flood Insurance Rate Maps (FIRMs) and Flood Insurance Study (FIS) reports provide readily available, baseline information, they have the following limitations for use as the sole means of estimating risk:

  • FIRMs and FIS reports are dated: FIRMs show the estimated flood risk of an area at the time of the FIS, based on the best available science when the original FIS was conducted. Even new FIRMs are often based on older FISs—check the dates on your community’s FIRMs, and note what was changed or restudied the last time the maps were updated (described in the legend panel of the map and in detail in the FIS). Many FIRMs and FISs are decades old. New development can change flood conditions, as can erosion, rising sea levels, or long-term changes in storm frequency. Recent storms have flooded areas outside of mapped flood zones. Additionally, methods for estimating risk have improved dramatically over the last 20 years, so old FIRMs may not provide an accurate measure of the current risk.
  • FIRMs and FIS reports are based on models: Storm events are incredibly complex, and their impacts are impossible to precisely predict with available models. In most communities, FIRMs are based on very course-scale studies interpolated to estimate the risk for the whole community. Such modeling may not properly account for the varied coastlines of many  communities, and can underestimate or overestimate risk.
  • FIRMs and FIS reports do not show worst-case scenarios: Depending on when your community’s FIS was conducted, your FIRMs may or may not show the risk behind structures like seawalls and levees, which may be overtopped during a major event. In addition, storm effects can and do extend further than those projected on FIRMs.

For these reasons, FIRMs and FIS reports should not be used alone, but in conjunction with other methods that better identify the risks of coastal flooding and storm damage. For sources of additional hazard information, see finding and using additional hazard information. For information on how to interpret FIRMs, see interpreting FIRMs and FIS reports.