National Flood Hazard Layer (FEMA)

FEMA builds the National Flood Hazard Layer (NFHL) from the effective Flood Insurance Rate Maps (FIRMs) delivered to communities and from Letters of Map Change.¹ The result is one geospatial database of current flood hazard, kept to support the National Flood Insurance Program.¹ With appropriate care it can be used in place of the FIRM for the program's purposes.¹ Its digital data covers over 90 per cent of the US population, and new and revised data are added continuously.¹

The zones on the map carry legal weight. A Special Flood Hazard Area is land that a flood with a 1 per cent chance of being equalled or exceeded in any year, the "100-year flood", would cover.² It is labelled Zone A or Zone V and their variants, such as AE and VE.^2,3 Inside it, the program's floodplain management rules must be enforced and the mandatory purchase of flood insurance applies.³ FEMA's insurance page says that homes and businesses in high-risk flood areas with mortgages from government-backed lenders are required to have flood insurance.⁴ Zone B or shaded Zone X marks moderate hazard, between the 100-year and the 500-year flood, and Zone C or unshaded Zone X marks minimal hazard.² Where an effective map exists and provides data, that data must be used to set the minimum requirements of the program.¹

In a US model of a place the NFHL is part of the rules layer. For whether a property will flood, FEMA names the NFHL Viewer and the Map Service Center as the best place to start on the official determinations.¹ Map updates on the way show in the Flood Map Changes Viewer as preliminary or pending data.¹ The Flood Hazard and Risk Data Viewer adds data beyond the regulatory maps, including current and possible future flooding and sea level rise on the coast.¹

The NFHL Viewer finds an address and prints a full FIRM or a FIRMette, a smaller printable extract.¹ The Flood Map Service Center offers the same address search and downloads of the data for a county or state as shapefiles.¹ For developers there are GIS web services and a KMZ file for Google Earth.¹ Official use needs a base map that meets FEMA's accuracy standards, and the viewer and Map Service Center FIRMettes come with a default base map that meets the minimum.¹