Various 100-year floodplain maps are a product of the U.S. Department of Homeland Security's Federal Emergency Management Agency and are available on its website at msc.fema.gov/portal and at its regional offices. These maps are also available at county clerk offices.
This type of map shows the potential for flood zones on a particular piece of land during a 100-year flood, one predicted to only happen once a century, on average. FEMA creates the map by using future probabilities based on historical floods. The agency uses the map to administer the federal flood insurance program. It is also used by insurance companies to limit flood coverage in high-risk areas and by local government and developers who seek to determine the viability of locations to build on.
To find a particular map, go to FEMA's website and enter an address, the name of the location, or a set of longitude and latitude coordinates. The site also has a map that you can pan through and pinpoint your location for most places in the United States. In addition to the floodplain map, your location may have received a Letter of Map Change, which is also available from FEMA and should be checked against the map itself. The LOMC has important differences that could affect your plans, including the purchase of flood insurance.