During the past year while conducting workshops in different parts of the country, we have run into questions about installing onsite treatment systems in floodplains.
When we talk about floodplains, it’s apparent that many installers don’t know floodplain terminology, don’t know how to identify floodplains, and what a floodplain means for system installation.
Floodplains are normally dry or semi-dry areas to which water naturally flows as water levels rise. Floodplains are typically found near rivers, lakes and low-lying coastal plains, but many flood-prone lands are simply depressions where water naturally collects when it rains.
Identifying a property where water collects is an important aspect of installation. Routing water away from the system and making sure there are no additional water sources is a key to installation and a long-lasting system. Installing in areas toward which surface or subsurface water flows is a recipe for disaster.
Defining the term
Officially, a floodplain is the land adjoining lakes and rivers that is covered by the 100-year flood. This means a flood has a 1 percent chance of occurring in any given year (it is also called a regional flood).
Using sophisticated engineering and meteorological techniques, it is possible to calculate the magnitude of such floods on rivers where long-term flood records have been kept. Various government agencies conduct these studies, and as they become available, state law usually requires local communities to adopt floodplain zoning ordinances.
The natural floodplain is an important part of our water system. It affects storm runoff, water quality, vegetative diversity, wildlife habitat, and the aesthetic qualities of our rivers and lakes. Any alteration of the floodplain should be carefully evaluated.
When buying or managing property on a river in a community that has adopted floodplain zoning, we need to consider these points: floodway location, flood fringe location, flood protection elevation, flood-proofing, and flood insurance.
Accurate floodplain maps are critical for installers who work on property in or next to floodplains. These maps determine whether a structure or onsite system is in a floodplain and whether the owner will need to purchase flood insurance.
Knowing the terms
Unfortunately, many of these maps are outdated or inaccurate. Installers should investigate thoroughly and take care not to make representations or determinations about floodplain property. It is wise to direct buyers and sellers to a local surveyor or the state Depart-ment of Natural Resources (DNR) for more information. Here are the important definitions that relate to floodplains
The floodway refers to land immediately adjoining the river channel that is the natural conduit for flood waters. The floodway must remain open 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. If the property lies within this mapped floodway, the owners will not be permitted to construct a dwelling or other enclosed structure, place fill, or obstruct flood flows in any way.
Since this area must be left open to pass flood waters, only open space uses, such as farmland, 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 the floodway. This area is generally covered by shallow, slow-moving flood waters. This is where an installer typically becomes involved in a floodplain. Development is normally allowed in the flood fringe provided residential buildings are placed on fill so that the lowest floor, including the basement, is above the 100-year flood level.
For onsite systems, generally the elevation of the infiltrative surface needs to be one foot above the 100-year flood elevation. Usually, this elevation has been set by an engineer or surveyor for the residence being constructed, and the installer can obtain that information from them to establish the elevation of the system.
Flood protection elevation refers to an elevation one 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 vary and may require additional protections, such as requiring access road elevation to be within two feet of the flood protection elevation.
Floodproofing includes a variety of construction methods, such as watertight doors, windows, walls, and bulkheads that 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 a building or accessory structure on fill.
Typically, from an onsite system perspective, owners need to recognize that if flooding occurs, they will have to suspend use of the system during the flood. Before the system is used when waters subside, it will need to be inspected for damage and the tank pumped to remove any sediment or solids that may have washed in during the flood. All parts of the system need to be evaluated.
Flood insurance is important for the owner of floodplain property. The National Flood Insurance Program (NFIP) provides flood insurance for structures and their contents in participating communities. In this program, federally insured or regulated institutions must require flood insurance on all new loans for structures in mapped floodplain areas recognized by the Federal Emergency Management Agency (FEMA).
An installer can find out if a piece of property is located in the floodplain by checking with local building or zoning officials. Ask for the Flood Hazard Boundary Map or Flood Insurance Rate Map furnished by FEMA.
If you are working in an area near a lake or stream that may be subject to flooding, but you are unsure, the soil can help indicate whether this may be a problem. Soils in floodplain areas periodically receive fresh deposits of sediment carried by the flood waters. Gen-erally they lack the clear soil horizons because the horizons do not have time to develop before the next deposit comes along.
The soils will have distinctive layers of different materials deposited by successive flooding. If you see these characteristics, and if the property owner or developer has not investigated or has not had the flood elevation established, it is important to raise this concern. Have them investigate whether there is a problem, or do some investigating on your own.
