A floodplain is the lowland adjacent to a river, lake, or ocean. Floodplains are designated by the rarity of the flood that is large enough to inundate them. For example, a 10-year floodplain is likely to be inundated by a 10-year flood and a 100-year floodplain by a 100-year flood. Flood frequencies, such as the "100-year flood," are determined by plotting a graph of the size of all known floods for an area and determining how often floods of a particular size occur. Most known floodplains in the U.S. have been mapped by the Federal Emergency Management Agency’s Flood Insurance Administration.
Executive Order 11988 (PDF), "Floodplain Management," places special importance on floodplains and directs federal agencies to avoid conducting, allowing or supporting actions on a floodplain. When contemplating a mass transportation project, maps of the Federal Insurance Administration should be consulted to determine if the proposed project site is located within the 100-year floodplain. Flood Insurance Rate Maps (FIRMs) are available for review at local zoning or planning commission offices. Regional offices of Federal Emergency Management Agency (FEMA) can be contacted for assistance but do not maintain these maps. If a FIRM is not available for a particular area, a flood hazard boundary map should be reviewed to get an indication of whether the project site is clearly out of the floodplain or whether it may be located in a flood prone area.
If the proposed project is located within a floodplain, a detailed analysis should be included in the environmental document, as specified in US Department of Transportation Order 5650.2 (PDF) , "Floodplain Management and Protection," April 23, 1979. The analysis should discuss any risk to, or resulting from, the action, the impacts on natural and beneficial floodplain values, the degree to which the action provides direct or indirect support for development in the floodplain and measures to minimize harm or to restore or preserve the natural and beneficial floodplain values affected by the project.
If the preferred alternative involves significant encroachment of the floodplain the final environmental document must include:
Significant encroachment would involve one or more of the following impacts:
Expansion of a facility already located within a floodplain usually would not be considered a significant encroachment.
|National Environmental Policy Act of 1969 (42 U.S.C. 4321-4347)||Requires the preparation of an environmental impact statement (EIS) for any major federal action significantly affecting the quality of the human environment. USDOT Order 5610.1C, Procedures for Considering Environmental Impacts, September 18, 1979, attachment 2, paragraph requires that information on floodplains, if relevant, included in the EIS.|
|Executive Order 11988, Floodplain Management||Directs federal agencies to avoid conducting, allowing, or supporting actions in a floodplain.|
|USDOT Order 5650.2, "Floodplain Management and Protection," April 23, 1979 [PDF].||Prescribes policies and procedures for ensuring that proper consideration is given to the avoidance and mitigation adverse flooodplain impacts in agency actions, planning programs, and budget requests.|