The FEMA floodplain maps are used for administering flood insurance. FEMA floodplains are based the conditions that existed at the time of the floodplain study, including buildings, parking lots, driveways, bridges, culverts and channel geometry.
The City of Austin regulatory floodplain is used for development and building permits. Its floodplains are based on fully developed conditions. In other words, all of the land area is assumed to be fully built out with the maximum impervious cover currently allowed by zoning. Generally, the regulatory floodplains cover a greater area of land than the FEMA floodplains. In some cases, for example in the urban watersheds, the FEMA and City of Austin floodplains are exactly the same.