Limitations of FEMA Flood Maps (Credit: Science)

DISCUSSION: Flooding is a major problem around the world.  For example, flooding along the Missouri River in the north-central U.S. wreaked havoc recently (see article below this one here).  In addition, Cyclone Idai recently made landfall in Mozambique, flooding a very large area and killing hundreds of people.  The picture above illustrates the flooding impacts of Idai in Africa (credit: Mike Hutchings/Reuters).
 
In the U.S., the Federal Emergency Management Agency (FEMA) provides flood risk maps.  These maps not only provide information on locations most at risk for flooding, they also are the basis for National Flood Insurance Program premiums.  These FEMA-produced flood maps focus on the risk from a 100-year flood (i.e., a flood that on average occurs once every century or has a 1% chance of occurring in any given year), and many of these maps haven’t been updated in a long time.  Higher-resolution, more accurate maps of topography (useful for correctly depicting where water will flow) are now available that could improve the accuracy of FEMA flood maps.  In addition, there are several issues with recurrence intervals and associated flood risk maps based on those intervals.  These intervals are highly dependent on observation record.  So, as the record continually gets longer, homes not previously in the 100-year flood plain can find themselves at risk for such a flood.  In addition, recurrence intervals assume that conditions are stationary with time (e.g., no construction has occurred on the flood plain that could restrict water flow) and that all floods are from the same source (e.g., all floods are from heavy rains from a tropical cyclone) both of which are rarely true.
 
FEMA flood maps can be a useful tool to provide some indication whether there is a significant flood risk at your house/business in the U.S.  But, it is important to know the limitations of this tool so that the maps can be applied and used correctly.  While FEMA is limited by funding and Congress to update their flood maps, work is continuing outside FEMA to create updated, more versatile (i.e., provide risk of flooding from a range of flood sizes, not just a 100-year flood) flood maps for not only the U.S., but across the globe.  These maps may be very useful for planning and preparing for floods, thus lessening future flood damages and fatalities.

To learn more about other flooding-related topics, be sure to click here!
 
© 2019 Meteorologist Dr. Ken Leppert II
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