Comparison of AIR Flood Hazard Maps and FEMA Maps

This topic provides a brief comparison of AIR Flood Hazard maps for the United States and FEMA maps.

•       AIR Flood Hazard Maps provide consistent coverage across the entire contiguous United States, using a consistent high-resolution coverage of 2.2 million kilometers (1.4 million miles) of rivers. By covering the entire area, the maps are consistent in their portrayal of the hazard at any locality. The flood extents are independent of administrative boundaries. The maps include all stream links with a drainage area of at least 10 km2 (3.9 square miles).

•       FEMA maps are developed for specific locations. They are developed using a different methodology that depends on the type of exposures at specific locations. They do not cover sparsely populated areas. The coverage includes several different types of maps, including the FEMA digital Flood Insurance Rate Maps (DFIRMS), scanned FIRMs, Flood Insurance Study (FIS) areas, and others. A closer look at an area with coverage shows that it includes subareas where the flood hazard is undetermined and where maps are not available (not printed) as well as areas that are not mapped.

•       Not all FEMA maps have been digitized. FEMA DFIRMs are currently available for 2,199 out of 3,108 counties (and county equivalents) in the contiguous United States. Many DFIRM-covered counties include unmapped areas within the county.

•       AIR Flood Hazard maps are not limited by administrative boundaries. The difference in coverage between the AIR Flood Hazard maps and FEMA DFIRMs is quite apparent when analyzing flood extents at a specific location. FEMA maps end at the county line along several county borders whereas AIR flood extents reach across administrative boundaries.

•       AIR Hazard maps account for flood risk in levee-protected areas. Levees are designed to reduce flood risk, not eliminate it, and levee failure is a concern, particularly during large floods. AIR’s flood hazard maps show the full risk for a specified return period, including any levee-protected areas. While FEMA's maps include a statement regarding the possibility of levee failure or overtopping, their DFIRM maps for a specific return period assume levees do not fail during floods of that same return period.