Depending on how much rain there’s been, Austin’s creeks may be bone dry, gently flowing with water or a raging torrent. The floodplain is the area of land that is likely to be under water when the creek rushes over its banks. In a sense, the floodplain is the full extension of the creek.
The 100-year floodplain is the land that is predicted to flood during a 100-year storm, which has a 1% chance of occurring in any given year. You may also hear the 100-year floodplain called the 1% annual chance floodplain or base flood. Areas within the 100-year floodplain may flood in much smaller storms as well. The 100-year floodplain is used by FEMA to administer the federal flood insurance program and the City of Austin to regulate development.