Floodplain

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Canning River fork in Alaska with floodplain.

Floodplain management is a community-based effort to prevent or reduce the risk of flooding. The Federal Emergency Management Agency administers the National Flood Insurance Program (NFIP) and sets minimum floodplain management standards for participating communities. Floodplain regulations and ordinances are typically enforced on a local level.

Dams are commonly located within floodplains, and are therefore subject to floodplain regulations. When located within or upstream of a regulatory 100-year floodplain or floodway as defined by FEMA, dam modification projects are subject to limitations on the amount of flood elevation rise that is allowable. Constriction of the floodplain can result in increases to flooding elevations upstream of a dam, while expansion of a spillway can result in an increase in outflows during the 100-year base flow event and subsequent increased impacts to the downstream floodplain. Even in areas where a regulatory floodplain is not defined, local regulations may apply.

In addition to impacts during large flood events, consideration should be given to the downstream impact of releases from dams during normal operation. Upexpected changes in water levels can overwhelm unsuspecting people recreating or working within the downstream floodplain. "Channel capacity depends on the long-term trends in downstream floodplain land use. While it is not a hydrologic problem, channel capacity studies should recognize the impact of floodplain encroachments on what is considered the nondamaging channel capacity. Anecdotal history has shown that many projects are not able to make planned channel-capacity releases due to development and encroachment downstream."

Best Practices Resources

Hydrologic Engineering Requirements for Reservoirs (EM 1110-2-1420), USACE

Floodplain Management Website (Federal Emergency Management Agency)

Citations:

Revision ID: 7205
Revision Date: 07/11/2023