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 (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

Minimum standards for structures in 100 yr 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

block or pilings, filled stem wall and internally-flooded enclosed area diagram

Accessory Structures

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.

flood openings in a structure diagram

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. Beyond the freeboard requirements discussed above, the state has minimum floor elevation standards for those areas just outside of the 100-year floodplain – both within the “Regulatory Floodplain” and within any shoreland district. Though local ordinance standards may be unique - generally, basements within these areas should not be excavated below the RFPE. All utilities and structural service facilities must be protected to an elevation at or above the RFPE. However, local ordinances may have their own unique and specific standards.

The state floodplain rules detail that all buildings within the Regulatory Floodplain to have their lowest floors and service facilities at or above the RFPE. floodplain elevation diagram

 

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. floodplain elevation diagram

Diagram showing three examples of homes and where the bottom-most floor is in relation to the base flood elevation