Development within the floodplain is generally allowed, but proper floodplain management aims to minimize risk from potential flood hazards. Landowners should work with their local government for floodplain-related development. The Minnesota DNR offers assistance to local floodplain administrators.
Communities use their floodplain ordinance in conjunction with FEMA-approved maps to guide land use decisions. The DNR assists local communities by providing general regulatory assistance.
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Under state law, the floodplain is the land adjoining lakes and rivers that is covered by the "100-year" or "regional" flood. This flood is considered to be a flood that has a 1 percent chance of occurring in any given year. Floods of this magnitude occurred throughout the state in 1965, 1969, 1997 and 2001, and in various parts of the state in 1972, 1975, 1978, 1979, 1987 and 1993. It is possible to estimate the magnitude of such a flood along those rivers where long-term flood records have been kept. Local communities are required by state law to adopt technical data in their floodplain zoning ordinances.
The natural floodplain affects storm runoff, water quality, vegetative diversity, wildlife habitat, and aesthetic qualities of our rivers and lakes. Any alteration of the floodplain should be carefully evaluated.
The floodway is the land immediately adjoining the river channel that is the natural conduit for flood waters. The floodway must remain open in order to allow flood waters to pass. When the floodway is obstructed by buildings, structures, or debris, flood waters will be dammed up and will flood even greater areas. Large portions of the floodplain store and later release flood waters, which reduce river flood stages.
Under statewide floodplain management standards, communities can designate areas for development in the floodplain, called flood fringe areas, that would cumulatively cause no more than a six inche stage increase in the 100-year flood. A lesser stage increase than six inches would be appropriate where filling/development of proposed flood fringe areas would increase flood damage potential to nearby properties.
Many communities have delineated the boundary of the floodway and flood fringe on zoning maps. If the property a person owns or is interested in buying lies within this mapped floodway, they will not be permitted to construct a dwelling or other enclosed structure, place fill material, or obstruct flood flows in any other way. Since this area must be left open to pass flood waters, only open space uses, such as farm land, residential yards or gardens, golf courses, parks, playgrounds, or parking areas, are normally allowed in the floodway.
The flood fringe is the remainder of the floodplain lying outside of the floodway. This area is generally covered by shallow, slow moving flood waters. Development is normally allowed in the flood fringe provided that residential buildings are placed on fill so that the lowest floor, including the basement, is above the flood protection elevation. In communities that have not delineated separate floodway and flood fringe areas on their zoning map, a permit applicant will likely be asked to provide certain engineering information before they could build a structure in the floodplain. An engineer/surveyor will have to evaluate the proposed building site and furnish local officials with the necessary data to determine the property's flood protection elevation and whether the proposed structure is in the floodway. Professional services and special construction methods can be a substantial expense so a person should always check with the local zoning official before they buy property in a floodplain.
The flood protection elevation refers to an elevation 1 foot above the 100-year flood plus any stage increase due to the designation of flood fringe areas. The elevation of the lowest floor of a dwelling must be at or above the 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.
FEMA works with the State of Minnesota and its communities to conduct flood studies and identify special flood hazard areas (SFHAs) on flood maps, known as Flood Insurance Rate Maps. They incorporate a variety of information, are publicly available, and are most often used by planning professionals, engineers, lenders, and insurance agents.
New data and modern hydraulic and hydrologic engineering methods are now being used to map floodplains more accurately than they once were. Additionally, newer maps are much more readily accessible in a digital format.
Property owners are encouraged to contact their local community official to get floodplain maps information.
Once the 100-year flood elevation is determined, the boundaries of the floodplain are delineated using a topographic map. The property owner may file a Letter of Map Amendment (LOMA) with FEMA to appeal a boundary determination. Local communities will be able to help property owners through this process.
Construction and fill is generally permitted in portions of the floodplain, provided they adhere to their community’s floodplain management ordinance. Such projects as building a house, placement of fill, or installing a culvert or bridge would all require a floodplain permit.
Flood proofing includes a variety of construction methods, such as watertight doors, windows, walls, and bulkheads, which can be used to prevent flood waters from entering a structure. This method of flood protection, called "dry" flood proofing is not a sure deterrent to flooding and is used only in very special circumstances where it may not be possible to place the building or accessory structure on fill. Local floodplain regulations restrict dry flood proofing to non-residential structures. New residential basements are prohibited unless the community has been granted a residential basement exemption from the Federal Emergency Management Agency (FEMA). "Wet" flood proofing, which involves intentional internal flooding of areas constructed of flood resistant materials, may be allowed for minor additions to structures and certain accessory structures that constitute a minimal investment. Furthermore, state and federal floodplain management standards require all flood proofed structures to be designed and certified by a registered architect or engineer.