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 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 in Massachusetts 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 Massachusetts 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 Assessing Vulnerability of Coastal Properties.
Communities should be aware that MassGIS's Federal Emergency Management Agency (FEMA) Flood Map layer, also known as the Q3 data layer, is NOT an appropriate substitute for actual FIRMs for site-specific delineations of FEMA floodplain boundaries. For accurate, fine-scale determinations of FEMA floodplain boundaries, communities must use the hard copy FEMA FIRM maps until FEMA produces new FIRMs for each community, which will be available in digital format (DFIRMs). As of August 2007, digital FIRMs have been produced for Revere, Scituate, Marshfield, Duxbury, Quincy, and Plymouth. They are available through FEMA's DFIRM Database. For all other communities, to zoom in to a specific property on a current FIRM, try making a FIRMette. For information on how to interpret FIRMs, see the interpreting FIRMs and FIS reports.