Flood prone area maps, also know as flood insurance rate maps or flood boundary maps, show areas that are subject to flooding based upon probability. In the United States the Federal Emergency Management Agency (FEMA) produces and distributes these maps for all 50 states.
In the example map above, from Wellsboro, Pennsylvania, the areas in black are within the 100 year flood boundary. This means that they have one chance in 100 of being flooded in any given year. The wavy lines crossing the flood prone areas with numbers on them are base flood elevation lines. These show the elevation of the 100 year flood at that location.
The main use of flood prone area maps is in real estate lending and insurance. Federal law requires that all homes purchased with a mortgage be evaluated for flood hazard. If they are within the 100 year flood boundary then flood insurance is required for the term of the loan. This protects banks and their depositors against large flooding events that could wipe out the savings and economic base of an entire community.