Overview

The National Flood Hazard Layer (NFHL) data incorporates all Digital Flood Insurance Rate Map (DFIRM) databases published by the Federal Emergency Management Agency (FEMA), and any Letters Of Map Revision (LOMRs) that have been issued against those databases since their publication date. The DFIRM Database is the digital, geospatial version of the flood hazard information (location and attributes for boundaries of flood insurance risk zones) shown on the published paper Flood Insurance Rate Maps (FIRMs). The primary risk classifications used are the 1-percent-annual-chance flood event, the 0.2-percent-annual-chance flood event, and areas of minimal flood risk. The NFHL data are derived from Flood Insurance Studies (FISs), previously published Flood Insurance Rate Maps (FIRMs), flood hazard analyses performed in support of the FISs and FIRMs, and new mapping data where available. The FISs and FIRMs are published by FEMA. The specifications for the horizontal control of DFIRM data are consistent with those required for mapping at a scale of 1:12,000. The FIRM is the basis for floodplain management, mitigation, and insurance activities for the National Flood Insurance Program (NFIP). Insurance applications include enforcement of the mandatory purchase requirement of the Flood Disaster Protection Act, which "... requires the purchase of flood insurance by property owners who are being assisted by Federal programs or by Federally supervised, regulated or insured agencies or institutions in the acquisition or improvement of land facilities" (Section 2 (b) (4) of the Flood Disaster Protection Act of 1973). In addition to the identification of Special Flood Hazard Areas (SFHAs), the risk zones shown on the FIRMs are the basis for the establishment of premium rates for flood coverage offered through the NFIP. The NFHL data present the flood risk information depicted on the FIRM in a digital format suitable for use in electronic mapping applications. The NFHL database is a subset of the information created for the FIS and serves as a means to archive a portion of the information collected during the FIS. As distributed by FEMA, the NFHL layer is named "S_Fld_Haz_Ln. Currently this dataset is available for portions of eastern Massachusetts. See the Status Map (PDF). In ArcSDE the layer is named FEMA_NFHL_POLY.

The individual components of the National Flood Hazard Layer data are compiled in conjunction with the hardcopy FIRM and the final FIS report. The specifics of the hydrologic and hydraulic analyses performed are detailed in the FIS reports available for each community. The results of these studies are submitted in digital format to FEMA. These data and unrevised data from effective FIRMs are compiled onto the base map used for DFIRM publication and checked for accuracy and compliance with FEMA standards. As new DFIRM data sets are received the individual DFIRM layers are sewn into the nationwide layers of the NFHL. LOMRs for the DFIRMs in the NFHL are cut directly into the NFHL data layers as they are being produced and finalized.

For more information on this dataset and its creation process, please see the FGDC-compliant and ArcGIS-formatted metadata attached to the downloadable data and FEMA's Guidelines and Specifications for Flood Hazard Mapping Partners page (specifically Appendix L: Guidance for Preparing Draft Digital Data and DFIRM Databases. Also see FEMA's page on Map Modernization.

The polygon attribute table contains the following fields:

DFIRM_ID		Study Identifier. For a single jurisdiction flood risk project, the value is composed of the two-digit State FIPS code and the four-digit FEMA CID code (e.g., 480001). For a countywide flood risk project, the value is composed of the two-digit State FIPS code, the three-digit county FIPS code, and the letter “C” (e.g., 48107C). Within each FIRM Database, the DFIRM_ID value will be identical.
FLD_AR_ID		Primary key for table lookup. Assigned by table creator.
FLD_ZONE		Flood Zone. This is a flood zone designation. These zones are used by FEMA to designate the SFHAs and for insurance rating purposes. NOTE: The symbol ‘%’ is a reserved symbol in most software packages, so the word ‘percent’ was abbreviated to ‘PCT.’ Acceptable values for this field are listed in the D_Zone table. NOTE: AR Zones are now stored in the S_Zone_AR table and include the zone formerly stored in the AR_REVERT field in the 2003 DFIRM Database specification.
FLOODWAY		Types of floodways are stored in this field. Floodways are designated by FEMA and adopted by communities to provide an area that will remain free of development to moderate increases in flood heights due to encroachment on the floodplain. Normal floodways are specified as ‘FLOODWAY.’ Special cases will have a more specific term for the designation (such as COLORADO RIVER) and will appear as a note on the hardcopy FIRM. See Appendix K of the Guidelines and Standards for available floodway notes. Acceptable values for this field are listed in the D_Zone_Subtype table.
SFHA_TF		Special Flood Hazard Area. If the area is within a SFHA this field would be true. This field will be true for any area coded as an A or V flood zone area. It should be false for any X or D flood areas. Acceptable values for this field are listed in the D_TrueFalse table.
STATIC_BFE		Static Base Flood Elevation. This field will be populated for areas that have been determined to have a constant Base Flood Elevation (BFE) over a flood zone. The BFE value will be shown beneath the zone label. In this situation the same BFE applies to the entire polygon. This normally occurs in lakes or coastal zones.
V_DATUM		Vertical Datum. The vertical datum indicates the reference surface from which the flood elevations are measured. Normally this would be North American Vertical Datum of 1988 for new studies. This field is only populated if the STATIC_BFE field is populated. Acceptable values for this field are listed in the D_V_Datum table.
DEPTH		This is the depth for Zone AO areas. This value is shown beneath the zone label on the FIRM. This field is only populated if a depth is shown on the FIRM.
LEN_UNIT		Length Units. This unit indicates the measurement system used for the BFEs and/or depths. Normally this would be feet. This field is only populated if the STATIC_BFE or DEPTH field is populated. Acceptable values for this field are listed in the D_Length_Units table.
VELOCITY		The velocity measurement of the flood flow in the area. Normally this is applicable to alluvial fan areas (certain Zone AO areas). This value is shown beneath the zone label on the FIRM. This field is only populated when a velocity is associated with the flood zone area.
VEL_UNIT		Velocity Unit. This is the unit of measurement for the velocity. This field is populated when the VELOCITY field is populated. Acceptable values for this field are listed in the D_Velocity_Units table.
SOURCE_CIT		Source Citation. Abbreviation used in the metadata file when describing the source information for the feature. The abbreviation must match a value in L_Source_Cit.
HYDRO_ID		Hydrologic Model Identification. A code that provides a link to the now-removed hydrologic model table, L_Hydro_Model. The L_Hydro_Model table provides information on the methods used to determine the flood hazard for areas of shallow flooding and ponding. This ID field must contain a number that matches the HYDRO_ID field for a record in the L_Hydro_Model table. This attribute is only required for flood hazard areas with AO and AH zone designations or ponding associated with AE zones.
CST_MDL_ID		Coastal Model Identification. This field is populated by a linking element to the L_Cst_Model table. The L_Cst_Model table contains detailed information about the coastal models that were used to determine the coastal flood hazard for this area. This ID field must contain a number that matches the CST_MDL_ID field for a record in the L_Cst_Model table that documents coastal model information. This attribute is only required in areas for which flood hazard determinations do not consider wave hazards. The static BFE values in these areas are based on surge modeling and/or tidal analyses. When wave hazards are considered modeling information is linked to the transects used to complete the wave analysis.
LABELING		Text description of flod zones for map legends

As FEMA makes more NFHL data available MassGIS will incorporate updates and modifications into the dataset available for download from this page. Questions about this dataset should be directed to FEMA at 1-877-FEMA MAP (336-2627). Users may also visit FEMA's Map Service Center.

To view the NFHL data online with other features developed by FEMA, see the FEMA's Flood Map Viewer.

Older Q3 FEMA Flood data exists for all areas of the state except Franklin County.

Last Updated 1/24/2012
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