Section 13: Floodplain Management
Also see the Floodplain Management Program.
What is a floodplain and what should I know about buying property within it?
Under state law, the floodplain is considered to be the land adjoining lakes and rivers that is covered by the "100-year" or "regional" flood. This flood is considered to be flood that has a 1 percent chance of occurring in any given year. Using sophisticated engineering and meteorological techniques, it is possible to calculate the magnitude of such a flood along those rivers where long-term flood records have been kept. Various government agencies conduct these studies, and as they become available, local communities are required by state law to adopt floodplains zoning ordinances.
The natural floodplain is an important part of our water system. It 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 intended use should be appropriate to the site selected. The following information about floodplains and local zoning codes deals with restrictions on developing in or near floodplains. Always remember that the least amount of alteration to the natural system is usually the safest and most ecologically sound development decision.
When buying or managing property on a river in a community that has adopted floodplain zoning, consider the following points: floodway location, flood fringe location, flood protection elevation, flood proofing, and flood insurance.
- 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 upstream. Large portions of the floodplain store and later release flood waters, which reduce river flood stages. Development of these areas can result in increased flooding. Under the statewide floodplain management standards, local communities cannot allow development in the floodplain that would cumulatively cause more than a 1/2 foot (6 inches) increase in the height of the 100-year flood. Many communities have delineated the boundary of the floodway on zoning maps based on this 1/2 foot (6 inches) increase in flooding. If the property you own or are interested in buying lies within this mapped floodway, you 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 100-year flood level. In communities that have not delineated separate floodway and flood fringe areas on their zoning map, you will likely be asked to provide certain engineering information before you can place a structure in the floodplain. An engineer or surveyor will have to evaluate the proposed building site and furnish local officials with the necessary data to determine your property's flood protection elevation and whether your proposed building is in the floodway. Professional services and special construction methods can be a substantial expense so you should always check with the local zoning official before you but property on a floodplain.
- Flood Protection Elevation refers to an elevation 1 foot above the 100-year flood. The elevation of the lowest floor of a dwelling must be at or above the flood protection level. Local regulations will also require your access road elevation to be within 2 feet of the flood protection elevation.
- Floodproofing 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 is not a sure deterrent to flooding and is used only in very special circumstances where it may not be possible to place your building or accessory structure on fill. Furthermore, the State Building Code requires all flood proofed structures to be designed by a registered architect or engineer.
- Flood Insurance is important when buying floodplain property. You should be aware of the National Flood Insurance Program (NFIP), which provides flood insurance coverage for structures and their contents in communities participating in the NFIP. Under this program, federally insured or regulated institutions must require flood insurance policies on all new loans for structures in mapped floodplain areas recognized by the Federal Emergency Management Agency (FEMA). You can find out if a piece of property is located in the floodplain by checking with the local building or zoning officials (ask for the "Flood Hazard Boundary Map" or "Flood Insurance Rate Map" furnished by FEMA). If you are considering the purchase or development of floodplain property, your insurance agent can provide information on the cost and availability of flood insurance. It should be noted that flood insurance is available for structures in communities participating in NFIP. The structure does not have to be located in the floodplain in order to obtain insurance.