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 (one-percent annual chance) flood, plus any stage increase due to the designation of flood fringe areas. These standards will be further detailed in your local government's floodplain ordinance.
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 the Regulatory Floodplain
Minnesota development standards for structures will typically require:
- Lowest floor (including basement) is at RFPE or higher
- Fill at 100-year flood level (including stage increase due to designation of the flood fringe areas) or higher extends at least 15 feet in all directions
- No structures allowed in floodway
- No fill in floodway
- Access road/driveway no lower than 2 feet below RFPE
- All electrical, HVAC, ductwork must be elevated above the RFPE
Accessory Structures would typically have their own unique standards. Sheds and stand-alone garages are most commonly constructed in a way that accommodates internal flooding, similar to the image below. With a few exceptions, accessory structures are not allowed in floodway.
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.
What if I'm Outside the 100-Year Floodplain?
Floods bigger than the 100-year (or “base”) flood can occur, and flood risk is not restricted to the mapped floodplain. We've experienced bigger floods in many areas of Minnesota in recent history, and the modeled risk is not always consistent with the actual risk. Additionally, areas near floodplains are not always suitable for development. Beyond the freeboard requirements discussed above, many local governments may designate a “regulatory floodplain” that extends beyond the typical 100-year floodplain. This may include areas just outside the mapped floodplain, or areas where localized stormwater issues have been identified.
Local ordinances commonly extend the regulatory reach of their floodplain ordinance to the 500-year floodplain, areas with poor drainage, or the horizontal extent of the Regulatory Flood Protection Elevation (as shown below).
Within shoreland districts, the lowest floor must be as high as the RFPE, when this elevation is known. Where this elevation is unavailable, lowest floor elevations must be at least 3 feet above the ordinary high water level (OHWL), or highest known water level, whichever is greater. This standard applies both in and outside of the Regulatory Floodplain.