What Is a Flood Insurance Study (FIS)?

Every Flood Insurance Rate Map (FIRM) is backed by a Flood Insurance Study. The FIS is the technical report that documents how FEMA determined the flood zone boundaries, Base Flood Elevations, and floodway limits for a community. While the FIRM shows the results on a map, the FIS explains how those results were calculated and provides data at a much higher level of precision.

The FIS is primarily used by floodplain managers, engineers, surveyors, and anyone involved in permitting or construction in the floodplain. However, property owners benefit from understanding what the FIS contains, especially when it comes to accurate BFE values and the engineering basis for their flood zone designation.

A standard FIS report is organized into 10 numbered sections plus supporting data tables and maps.

Beyond the narrative sections, the FIS includes supporting data that provides the quantitative engineering basis for the flood map.

• Summary of Discharges Table: Drainage areas and peak flood flows at locations along studied streams for the 10%, 4%, 2%, 1%, and 0.2% annual chance floods. Measured in cubic feet per second (cfs).
• Floodway Data Table: A 9-column table with data at each cross section including floodway width, cross-sectional area, mean velocity, and the regulatory BFE. This is where the precise BFE for each cross section is recorded.
• Flood Profiles: Graphs of flood elevation along the centerline of each studied stream. These profiles show the 1% annual chance flood elevation (BFE) at every point along the stream, not just at the rounded values printed on the FIRM.
• Stillwater Elevations Table: For coastal and lake communities, lists stillwater flood elevations (without waves) at selected locations for each recurrence interval.
• Transect Data: For coastal areas, data along lines perpendicular to the shoreline showing wave heights, runup elevations, zone designations, and BFEs.

This is one of the most important practical points about the FIS. Base Flood Elevations shown on the FIRM are rounded to the nearest foot. That means a FIRM BFE is only accurate to plus or minus 6 inches (0.5 feet). In many cases, the difference between the rounded FIRM value and the precise FIS value can be 2 feet or more.

The NFIP requires using the FIS flood profile, not the rounded FIRM value, to determine the accurate BFE for permitting and construction purposes. If you are building, renovating, or applying for a LOMA, ask your surveyor or floodplain administrator for the FIS-derived BFE at your specific location.

Flood profiles are one of the most useful parts of the FIS for property owners. They are graphs that show the flood elevation along a studied stream, and they provide BFE data that is far more precise than what the FIRM shows.

• X-axis (horizontal): Distance along the stream in feet, measured from the downstream end of the study reach. Cross section letters (A, B, C, etc.) are marked along this axis. These correspond to the labeled cross sections on the FIRM panel.
• Y-axis (vertical): Elevation in feet, referenced to the vertical datum (NAVD 1988 or NGVD 1929). The scale shows the range of elevations relevant to that stream reach.
• Profile lines: Multiple curves are plotted, each representing a different flood recurrence interval: 10-year (10%), 50-year (2%), 100-year (1%), and 500-year (0.2%). The 1% annual chance line is the BFE. The stream bed (channel invert) may also be shown as the lowest line.
• Floodway profile: A separate line showing the water surface elevation within the regulatory floodway. This is typically slightly higher than the base flood elevation because the floodway calculation assumes encroachment from both sides of the floodplain.

To find the precise BFE at your property, locate the stream cross section closest to your property on the FIRM, then find that same cross section letter on the flood profile. Read the 1% annual chance elevation at that point. This value, not the rounded number on the FIRM, is the regulatory BFE for permitting purposes.

The floodway data table is the most data-dense part of the FIS. Each row represents a cross section along the studied stream, and the columns provide the engineering data that defines the regulatory floodway at that location.

The surcharge column is key for floodway management. NFIP regulations (44 CFR 60.3) prohibit development in the floodway unless a no-rise certification or hydraulic analysis demonstrates that the project will not increase the BFE. Some communities adopt a zero-rise standard, which is stricter than the federal 1-foot allowance.

Most property owners will never need to open an FIS. But there are specific situations where the FIS data is essential and cannot be replaced by the FIRM alone.

• Building permit applications: Your floodplain administrator needs the FIS BFE (not the FIRM BFE) to verify that your structure meets the elevation requirement. The rounded FIRM BFE can differ by 2 or more feet from the FIS value.
• LOMA applications: A LOMA compares your property's elevation to the BFE. Using the wrong BFE (from the FIRM instead of the FIS) can result in a denial. Your surveyor should reference the FIS flood profile to extract the precise BFE.
• Floodway development: If your property is in or near the floodway, the floodway data table provides the baseline conditions your engineer must match in a no-rise certification.
• Insurance disputes: If you believe your flood insurance premium is based on an incorrect BFE, the FIS flood profile can provide the evidence to support a correction. This situation can arise when a property falls between two FIRM cross sections.
• Datum conversions: The FIS identifies which vertical datum was used for the study. If your Elevation Certificate uses a different datum, you need the FIS to perform the correct conversion using the appropriate VERTCON correction factor.

FIS reports are available for free from FEMA's Map Service Center.

Your local floodplain administrator or community planning office may also have copies of the FIS. If you need help finding or interpreting the data, a licensed surveyor or professional engineer can assist.