Two approaches may be used to protect a property from flood damage: structural and nonstructural. Structural methods are intended to prevent flooding by altering the flow of floodwater; these include constructing levees or dams, or modifying a waterway's channel. Nonstructural methods are intended to reduce damage from encroaching floodwater by altering the property; these include acquiring and/or relocating a building, preparing emergency measures, such as sandbagging, and flood proofing structures.
Flood proofing is defined as "any combination of changes or adjustments incorporated in the design, construction, or alteration of individual buildings or properties that will reduce flood damages." Unlike a structural approach, the building site remains subject to flooding; it is the building or the area adjacent to it that is modified to prevent or minimize flood damage.
Some approaches to flood proofing rely on human intervention. "Human intervention" is the need for one or more people to be present to take the right steps to make a flood proofing system work. For example, if a floodwall will provide protection only if someone installs a closure or activates a pump, it is considered to need human intervention. Measures that need human intervention are considered less dependable, especially if little warning of flood conditions can be expected, since failure to perform human intervention tasks can result in flood damage.
The community programs described in this report funded five approaches to flood proofing. They are summarized in the following sections:
2.1 Elevating the building, so that floodwaters do not reach any damageable portions of it.
2.2 Constructing barriers between the building and floodwaters ("barriers").
2.3 Making the building walls and floor watertight so water does not enter ("dry flood proofing").
2.4 Modifying the structure and relocating the contents so that when floodwaters enter the building there is little or no damage ("wet flood proofing").
2.5 Preventing sewer backups and basement flooding.
The best way to protect a house from surface flooding, short of removing it from the flood plain, is to raise it above the design flood level. This allows floodwaters to flow under a building, causing little or no damage. Most communities have building codes for new and substantially improved buildings located in flood plains that require that this method be used. It is commonly practiced in flood prone locations throughout the country.
Many qualified house-moving contractors know the techniques for elevating a building. The structure is jacked up and temporarily set on cribbing while a new foundation is built underneath. The foundation walls are raised to the flood protection level and the house is lowered onto the new foundation. Utility lines are extended and reconnected, steps are built and, in some cases, the perimeter is backfilled or landscaped to mask the change.
The walls of the new foundation must have openings to allow floodwaters to pass under the building. Otherwise, hydrostatic pressure will be placed on the walls and floor, and the foundation would be in danger of cracking or breaking. In areas subject to wave action or higher velocity flooding, elevation on columns or pilings is recommended to minimize the exposure to these hazards.
If the flood protection level is low, the result can be similar to building a house over a 2- or 3-foot crawlspace. If the house is raised 2 feet, the front door would be 3 steps higher than before. Examples of such low elevation projects are shown in Figure 12. If the house is raised 8 feet, the lower area can be wet flood proofed for use as a garage, to provide access to the building, or for storage of items not subject to flood damage (see Figure 1.).
Cost: $8,000 to $50,000, depending primarily on the type of building.
Barriers keep floodwaters from reaching a building. They can be made of earth, concrete, masonry or steel. Large earth barriers are called levees. In shallow flooding areas, a common approach is to construct a berm, which is a small levee, usually built from locally available fill.
Sheer mass gives berms and levees their strength. A typical design has 3 horizontal feet for each vertical foot (3:1 slope), so at least 6 feet of ground is needed for each foot in height. Thus, berms and levees need a lot of room (see Figures 2 and 10).
Where there is not enough room for a berm or levee, concrete, masonry or steel structures are used. Concrete and masonry walls should be built with internal reinforcing bars for strength, and to resist cracking and settling over time. They must be properly anchored to withstand lateral hydrostatic pressure; care must be taken to ensure they are watertight. Figure 3 depicts an example of a design for a concrete floodwall.
This is part of the specifications for a floodwall around a public housing site in Rosemont, Illinois. The project was funded by the Village's general revenues after a 1986 flood caused $1.3 million in damage.
The reinforcing bars and looting are needed to ensure that the wall will resist the hydrostatic pressures of floodwaters.
The wall has openings at sidewalks to allow the elderly residents easy access. Human intervention is required to sandbag the openings. Maintenance crews are on site around the clock and sandbags are stacked on pallets for quick installation.
Providing access into the area protected by a wall or levee can be complicated. If the slope is not too steep, pedestrians and vehicles can go over the wall, as in the case of the levee illustrated in Figure 10. Some barriers have openings for driveways and sidewalks. Closing these openings is dependent on human intervention, so their use is not appropriate where there is little warning time. If the wall is high, a stairway can be built over it to provide access without human intervention. An example of this approach is illustrated in Figure 11.
Barrier design needs to account for leaks, seepage of water under the wall, and drainage of rainwater that falls inside the barrier perimeter. A sump and/or drain tile is needed to collect the internal ground and surface water. A pump and pipe are also needed to pump the internal drainage over the wall as illustrated in Figures 2 and 9.
Floodwalls, levees and berms can either surround the building (as in a ring levee) or connect to high ground. Several communities have funded construction of berms or floodwalls that run from high ground adjacent to one end of a house to high ground at the other end to protect a below-grade patio or walkout basement (see Figures 9 and 13).
Cost: Minimal to $20,000+. The cost can range from practically nothing, as when the homeowner regrades the yard or builds a berm with local fill, to $20,000 or more depending on the complexity of the project and the materials used. Landscaping, backfilling and building brick facades to match the structure can greatly increase costs.
Sealing a building to ensure that floodwaters cannot get inside it is called dry flood proofing. All areas below the flood protection level are made watertight. Walls are coated with a waterproofing compound, or plastic sheeting is placed around the walls and covered. Openings, such as doors, windows, sewer lines and vents, are closed - temporarily, with sandbags or removable closures, or permanently.
Dry flood proofing is only appropriate for buildings on slab foundations that are free of cracks. Because most building walls and floors are not strong enough to withstand the hydrostatic pressure from more than 3 feet of water, the design flood should be less than 3 feet above the slab. The technique is not recommended for houses with floors below grade, such as basements and garden apartments, because hydrostatic pressure can collapse the walls or buckle the floor.
This technique is not as desirable as a barrier, which will keep floodwaters from reaching the building. However, where there is not enough space on the lot for a barrier separate from the building, dry flood proofing may be the only alternative.
Proper maintenance of materials used in dry flood proofing is a concern. Waterproofing compounds can deteriorate over time, especially if they are exposed to sunlight. Removable closures can be misplaced. To be dependable, a dry flood proofed building should be inspected periodically, and its owner should conduct drills to ensure that the closures can be located and put into place in time.
Several communities have funded projects that combine dry flood proofing with a barrier. The building walls are made watertight and small floodwalls are built around the windows and doorways. This provides permanent protection that does not need human intervention to close the openings. An example of this is illustrated in Figure 11.
Cost: Minimal to $20,000+. The cost ranges from $100 for the waterproofing compound/sandbag approach, to $20,000 for a more secure and attractive approach that does not need human intervention and blends in with the building's appearance.
Hydrostatic water pressure increases with the depth of water. Depths over 3 feet have been shown to collapse the walls of a typical house. Basements can be subject to 6 or 7 feet of water pressure when the ground is saturated. As a result, watertight walls and floors may crack, buckle or break from shallow surface flooding.
One way to deal with this is simply to let the water in and remove or protect everything that could be damaged. This approach is called wet flood proofing. It employs several techniques to modify a building to ensure that floodwaters are allowed inside, but damage to the building and contents is minimal. Such techniques range from moving a few valuable items to rebuilding the area that might be subject to flooding.
In the latter case, structural components below the flood level are replaced with materials that are not subject to water damage. For example, concrete block walls are used instead of wooden studs and gypsum wallboard. The furnace, water heater and laundry facilities are permanently relocated to a higher level (see Figure 5). Where flooding is not deep, these items may simply be raised on blocks or platforms.
Wet flood proofing is usually not used for one-story houses because the flooded areas are the living areas. However, many people wet flood proof their basements, garages and accessory buildings simply by relocating all hard-to-move items, such as heavy furniture and electrical outlets. Light or movable items, such as lawn furniture and bicycles, can be relocated after a flood warning is issued.
Cost: Minimal to $20,000. In one hour a property owner can accomplish some wet flood proofing by moving valuables out of the floodable area. Costs can range from $3,000 for relocating the furnace, water heater, etc., to as high as $20,000 to rebuild the floodable area with water-resistant materials and to relocate all utilities to a new addition.
A sewer backup occurs during heavy rains. Stormwater flows into combined or separate sanitary sewers, overloading the system's capacity to carry the water to the sewage treatment plant. The water backs up through house service lines into floor drains and then into basements.
Many basements are protected from groundwater problems by drain files that direct groundwater into sumps. Sump pumps move the water from the sump out to the ground, away from the building. Very heavy rains can overload this system; power outages and maintenance problems may knock out a sump pump. Should this happen, the system designed to keep groundwater out can act as a conduit to bring water into a basement.
In some communities, sewer backup and basement flooding are bigger problems than overbank or surface flooding. Often they occur at the same time, so many property owners are not sure how the water entered their homes. Barriers and dry flood proofing projects need to account for backflows through sewer lines, and seepage of groundwater under the protective barrier during surface flooding.
There are some inexpensive and usually effective measures for sewer backup, such as plugs and standpipes for the basement floor drain. This report, however, covers the more expensive approaches, where the owner may need financial assistance. The two most common measures funded by the communities interviewed are overhead sewers and backup valves.
Overhead Sewer: An overhead sewer restricts backed-up sewer water to the plumbing system. A sump is installed under the basement floor to intercept sewage flowing from basement fixtures and the basement floor drain. The sewage is pumped out by an ejector pump in the sump (see Figure 6). Plumbing fixtures on the first floor are not affected. They continue to drain by gravity to the sewer service line.
It is unlikely that sewers will back up above the level of the overhead sewer line. If water does go higher, a check valve in the pipe from the ejector pump keeps it in the pipes. Backed up sewage is contained in the sewer pipes so there is no worry about overflowing laundry tubs or basement toilets. An overhead sewer is also illustrated in Figure 14.
Cost: $2,000 to $5,000 for a plumbing contractor to reconstruct the pipes in the basement and install the ejector pump.
Backup Valve: A backup valve stops the water inside the sewer pipes. Older versions of this approach were located in the basement and relied on gravity to close the valve. If debris caught in the flapper, the valve did not close securely. Because of their unreliability, use of valves was discouraged and even prohibited in some communities.
The "balanced valve" has corrected these design shortcomings. A system of counterweights keeps it open all the time so debris won't catch and clog it. When the sewer backs up, instead of relying on gravity, floats force the valve closed. A balanced valve is usually installed in a manhole in the yard so there is less disruption during construction. This is also preferred for older buildings which may not have cast iron sewer pipes under the basement. An outside location prevents water pressure from breaking the pipes under the basement floor.
As with overhead sewers, a valve is fully automatic. It can even work when there is surface flooding. The installation is outdoors, so there is minimal disruption in the basement during construction. The owner still can use the sanitary sewers during flooding, as long as there is power to run the ejector pump which ejects wastewater when the valve is closed.
Cost: $3,000 to $4,500 for an automatic backup valve located outside the building.
Basement Flooding: Basement flooding caused by saturated ground can be corrected by installing drain tile around the foundation. The tile drains into a sump and water is pumped out, usually onto the ground away from the building. Depending on local conditions, the pipe and pumping system may have to handle large volumes of water. In some parts of the country, all new houses with basements are built with drain tile and sump pumps.
As noted in Section 2.5, if the sump pump does not operate, this system can be a conduit directing water into the basement. On the other hand, a properly installed system can protect basements during surface flooding. As long as there is a barrier that keeps surface water from reaching the building, a drain tile and sump pump should keep the underseepage from building up water pressure on the basement walls and floor.
Installing or correcting these drain tile systems and sump pumps is considered flood proofing in many communities. Correction projects include larger or additional sump pumps, backup sources of electricity for the pumps during power outages, and redirecting the outfall.
Cost: $50- $15,000. Lower cost projects include adding a second sump pump or power backup source. The most expensive approach would entail retrofitting a house with a drain tile and sump pump and waterproofing basement walls.
Several good references on flood proofing techniques are available from the U.S. Army Corps of Engineers. The following may be obtained free from:
Flood Proofing Techniques Programs and References, February 1991: an introduction to flood proofing and the government programs and manuals that can help a community.
Flood Proofing Systems & Techniques, December 1984: a well-illustrated review of different techniques that have been built around the country.
Flood Proofing - How to Evaluate Your Options, July 1993: a guidebook on selecting the most appropriate and cost effective protection measure.
Flood-Proofing Regulations, EP 1165-3-314, March 1992: detailed specifications for various dry and wet flood proofing measures.
Flood Proofing Technology in the Tug Fork Valley, April 1994: a detailed report on the flood proofing techniques used to protect many homes in this Corps project in West Virginia and Kentucky.
A Flood Proofing Success Story along Dry Creek at Goodlettsville, Tennessee, September 1993: a review of a Corps project to elevate homes which includes contracting documents and other guides for financing flood proofing.
Raising and Moving the Slab-on-Grade House, 1990: an illustrated review of the techniques used to elevate and relocate houses on slab foundations, keeping the slab attached.
Flood Proofing Tests, August 1988: a review of tests of waterproofing materials and dry flood proofing systems conducted by the Corps' National Flood Proofing Committee.
Except as noted, the following documents are available free from:
Design Manual for Retrofitting Flood-prone Residential Structures, FEMA-55, February 1986: an extensive review that discusses all aspects of protecting an existing house from flood damage, including technical design criteria.
Elevated Residential Structures, FEMA-54, March 1984: includes concepts, examples, performance criteria, architectural drawings, photographs, cost calculation forms and sources of information and assistance.
Floodproofing Non-residential Structures, FEMA-102, May 1986: an overview of flood proofing new and existing buildings, designed to familiarize the reader with a variety of techniques.
Best Build 3: Protecting a Flood-Prone Home, FEMA/National Association of Home Builders: a 30-minute video overview of flood proofing techniques ($5).
