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 often dated: FIRMs show estimated flood risk at the time the detailed engineering studies were conducted. Even new FIRMs are often based on older analyses—check the dates on your community's FIRMs and the description in the FIS to determine what was changed or restudied the last time the maps were updated. Some engineering studies are decades old and new analyses are often only conducted on portions of each community. In addition, new development can change flood conditions, as can erosion, rising sea levels, and long-term changes in storm frequency. Recent storms in Massachusetts have flooded areas beyond mapped flood zones and methods for estimating risk have improved dramatically over the last 20 years, so old FIRMs may not provide an accurate measure of 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: The FIRMs depict the areas that are predicted to flood in a storm having a 1% chance of occurring in a given year (also called the 100-year storm). Larger storm events do occur with impacts beyond the mapped floodplain. Depending on when your community's FIS was conducted, your FIRMs may or may not show the full 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 identify 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, known as the Q3 data layer, is NOT appropriate for site-specific delineations of FEMA floodplain boundaries. For accurate, fine-scale determinations of FEMA floodplain boundaries, communities must use hard copy FIRMs or the new digital FIRMs. All flood maps are available for viewing or downloading through FEMA's Flood Map Service Center. MassGIS has added a new digital floodplain layer called the National Flood Hazard Layer (NFHL), which is more current. Depending on when it was last updated, the NFHL in MassGIS may not be the most current source of data depicting flood zones. Therefore, the use of digital FIRMs available from the FEMA Flood Map Service Center is strongly advised. For information on how to interpret FIRMs, see Tools to Assist in Interpreting Flood Insurance Rate Maps and Flood Insurance Study Reports.