Floodplain: Elevation Requirements
What is the Regulatory Flood Protection Elevation (RFPE)?
The regulatory flood protection elevation (RFPE) refers to an elevation 1 foot (minimum) above the 100-year flood plus any stage increase due to the designation of flood fringe areas. In Minnesota, the floodplain management ordinances (local regulations) require that the elevation of the surface of the lowest floor of a dwelling be at or above the regulatory flood protection elevation. Local regulations will also require the top of the access road elevations to be within 2 feet of the flood protection elevation.
100-year flood elevation
+ Stage increase due to filling in the flood fringe
+ 1 foot (minimum) of freeboard
Regulatory Flood Protection Elevation (RFPE)
Minimum Standards for Structures in 100-year floodplain
- RFPE = 100-yr flood level + flood stage increase (of up to ? foot) due to filling in flood fringe + 1' freeboard (min)
- Lowest floor (including basement) is at RFPE or higher
- Fill at 100-year flood level (including stage increase due to filling in flood fringe) or higher extends at least 15 feet in all directions
- NO fill in floodway
- Access road/driveway no lower than 2 feet below RFPE
What is Freeboard?
A factor of safety usually expressed in feet above a certain level. Freeboard compensates for the many unknown factors that may increase flood levels beyond the calculated level, i.e., waves, ice jams, debris clogging culverts or bridges, short history of known water levels, etc. Freeboard also provides a factor of added safety for when the actual flood levels are higher than that calculated for the "100-year flood." The 100-year flood level has a 1-percent chance of being equaled or exceeded every year.
Examples of Freeboard Requirements in Minnesota:
- The floodplain management ordinances of local communities require a structure's lowest floor to be placed 1 foot (at a minimum) above the 100-year flood level. Many communities have chosen to have 2 feet or even 3 feet for their freeboard requirements.
- Levees intended to remove the areas behind them from floodplain designation (i.e., be "certified") are required to have 3 feet (at a minimum) of freeboard above the 100-year flood level.
Does Being Outside the Shaded Area (100-Year Floodplain on FEMA Maps) or Elevated to the 100-year Flood Level Mean You Won't Get Flooded?
Is Freeboard Required?
The Federal Emergency Management Agency (FEMA) requires the lowest floor of a structure to be at the 100-year flood elevation or higher, but does not require freeboard. However, FEMA advises that states may have more stringent requirements, and the most restrictive standards apply.
Minnesota state law has required that the Regulatory Flood Protection Elevation (RFPE) specified in local floodplain ordinances include one foot of freeboard (at a minimum) since 1997. The model floodplain management ordinances prepared by DNR Waters for local units of government have recommended that 1 foot of freeboard be included for over 20 years. Many watershed districts also have freeboard requirements, sometimes more restrictive than the local city or county. Remember that if more than one regulatory authority has regulations covering the same site, the most restrictive requirements apply!