Plant a Rain Garden
Rain gardens are shallow depressions (3-4 inches to two feet) planted with native wildflowers and other plants that soak up rainwater or melted snow from your rooftop, driveway and lawn. Planting a rain garden helps improve drinking water quality by catching and filtering rainwater through the soil and preventing runoff from getting to the storm drains. Rain gardens can also help reduce local flooding. Some homeowners have found that flooding in their basements was eliminated after they planted a rain garden.
Dane County Office of Lakes and Watersheds: Rain Garden
Install Sewer Backflows
In some floodprone areas, flooding can cause sewage from sanitary sewer lines to back up into houses through drain pipes. These backups not only cause damage that is difficult to repair, but also create health hazards. A good way to protect your house from sewage backups is to install backflow valves, which are designed to block drain pipes temporarily and prevent flow into the house. Backflow valves are available in a variety of designs that range from the simple to the complex. One of the more complex designs is a gate valve. It provides a strong seal, but must be operated by hand. So the effectiveness of a gate valve will depend on how much warning you have of impending flooding. Among the simpler valves are a flap or check valves, which open to allow flow out of the house but close when the flow reverses. These valves operate automatically but do not provide as strong a seal as a gate valve.
Anchor Fuel Tanks
Unanchored fuel tanks can be easily moved by flood waters. These tanks pose serious threats not only to you, your family, and your house, but also to public safety and the environment. An unanchored tank outside your house can be driven into your walls by flood waters, and it can be swept downstream, where it can damage other houses. When an unanchored tank in your basement is moved by flood waters, the supply line can tear free and your basement can be contaminated by oil. Even a buried tank can be pushed to the surface by the buoyant effect of soil saturated by water. One way to anchor a tank is to attach it to a large concrete slab whose weight is great enough to resist the force of flood waters. This method can be used for all tanks, both inside and outside your house. You can also anchor an outside tank by running straps over it and attaching them to ground anchors.
Raise Electrical System Components
Electrical system components, including service panels (fuse and circuit breaker boxes), meters, switches, and outlets, are easily damaged by flood water. If they are inundated for even short periods, they will probably have to be replaced. Another serious problem is the potential for fires caused by short circuits in flooded systems. Raising electrical system components helps you avoid those problems. Also, having an undamaged, operating electrical system after a flood will help you clean up, make repairs, and return to your home with fewer delays. All components of the electrical system, including the wiring, should be raised at least 1 foot above Regional Flood Elevations (found by consulting your community’s floodplain maps or contacting your community’s zoning department). In an existing house, this work will require the removal of some interior wall sheathing (drywall, for example). If you are repairing a flood-damaged house or building a new house, elevating the electrical system will be easier.
Protection of HVAC Equipment
Heating, ventilating, and cooling (HVAC) equipment, such as a furnace or hot water heater, can be damaged extensively if it is inundated by flood waters. The amount of damage will depend partly on the depth of flooding and the amount of time the equipment remains under water. Often, the damage is so great that the only solution is replacement. In floodprone houses, a good way to protect HVAC equipment is to move it from the basement or lower level of the house to an upper floor or even to the attic. A less desirable method is to leave the equipment where it is and build a concrete or masonry block floodwall around it. Relocation can involve plumbing and electrical changes, and floodwalls must be adequately designed and constructed so that they are strong enough and high enough to provide the necessary level of protection.
Wet floodproofing a house is modifying the uninhabited portions of the house (such as a crawlspace, unfinished basement, or attached garage) so that flood waters will enter but not cause significant damage to either the house or its contents. This includes adding openings to allow water to enter the structure, using water resistant paints and other materials to allow for easy cleanup after floodwater exposure, and moving all utilities to parts of the house that are above Regional Flood Elevation. Wet floodproofing may be preferable to attempting to keep waters out completely, because it allows for controlled flooding to balance interior and exterior water forces and discourage structural collapse. Wet floodproofing is often used when all other retrofitting methods are either too costly or are not feasible. However, wet floodproofing does require extensive cleanup after a flood.
Dry floodproofing means sealing a structure with waterproofing compounds, sheeting, or other impermeable materials to prevent floodwaters from entering. The seals can be either permanently installed, or temporarily put into place during a flood. Doors, windows, and other openings below the flood elevation must be equipped with permanent or removable shields, and backflow valves must be installed in sewer lines and drains. When deciding whether dry floodproofing is appropriate for a structure, it is important to consider how the pressure exerted from typical flood waters will affect the building’s walls. Since floodwaters cannot enter a dry floodproofed building, the waters will exert pressure that could cause walls to collapse. Dry floodproofing is best used on structures in good condition, constructed of concrete blocks or brick veneer on wood frame, and subject to less than three feet of floodwaters. It is inappropriate for wood-frame structures, structures with a crawl space or other elevated foundation (because they are not watertight), and structures located in a high-velocity floodway. Dry floodproofing is not recommended for houses with basements. Saturated soils pressing against basement walls can damage them or cause them to fail. The buoyancy force exerted by saturated soils below the basement can cause the basement floor to fail or even push the entire house up. FEMA does not recommend dry floodproofing for residential structures.
Filling in Subgrade Basement
This is often required to bring buildings that are “substantially damaged” (i.e., damage to the point that repairs would cost 50% or more of the structure’s pre-damage equalized assessed value) or “substantially improved” (i.e., improved by 50% or more of the structure’s present equalized assessed value) into compliance with Wisconsin’s floodplain development requirements. New homes and homes that have been substantially damaged or substantially improved are not permitted by Wisconsin law to have a basement below Regional Flood Elevation.
Barriers protect an individual structure or a small cluster of structures, and include levees and floodwalls. Levees are earthen dikes built around a property. Floodwalls are protective walls made of masonry block, reinforced concrete, or similar impermeable material. Levees usually require more space, but are less expensive and more permanent than floodwalls. Floodwalls usually contain openings that must be closed in the event of a flood and, therefore, must rely on warning systems. Both are feasible protection against flood depths up to 6 feet in the flood fringe. Generally, neither is feasible in a floodway because both obstruct the flood flow. Both are useless against floods that exceed the heights of their protective walls.
Elevation involves raising the first habitable floor of a structure above flood elevations. Steel beams are installed under a structure and hydraulic jacks lift both the beams and structure. A new foundation is then constructed beneath the raised structure. When the foundation is complete, the structure is lowered onto and secured to it. Elevation works well for houses originally built on basement, crawlspace, and open foundations. Elevation is a poor mitigation alternative for slab-on-grade, attached (e.g., row houses), and large brick or masonry structures. However, alternative techniques are available for masonry houses on slab-on-grade foundations. These techniques do not require the lifting of the house. Instead, they involve raising the floor within the house or moving the living space to an upper story. Elevation is not a practical alternative for structures in a high-velocity floodway because swift water and debris can undermine the new foundation.
FEMA’s Above the Flood: Elevating your Floodprone House
Relocating a structure means moving it to high ground, outside the flood hazard area, maybe another location on the same piece of property. Relocating a house usually involves jacking it up and placing it on a wheeled vehicle, which delivers it to the new site. The original foundation cannot be moved, so it is demolished and a new foundation is built at the new site. The house is installed on the new foundation and all utility lines are connected. Relocation is particularly appropriate in areas where the flood hazard is severe. Although similar to elevation, relocation requires additional steps that usually make it more expensive. These include moving the house, buying and preparing a new site (including building the new foundation and providing the necessary utilities), and restoring the old site (including demolishing the old foundation and properly capping and abandoning old utility lines). Houses of all sizes and types can be relocated, either as a unit or in segments. One-story frame houses are usually the easiest to move, particularly if they are built on a crawlspace or basement foundation that provides easy access to the floor framing. Masonry houses can also be moved, but usually with more difficulty and at a higher cost.
Keep in mind that choosing to take no action is a legitimate mitigation alternative that can be used to evaluate the effectiveness of other alternatives by comparing and contrasting them. In some cases, for example where the costs outweigh the benefits regardless of the mitigation option, no action might be the appropriate alternative.