Flooding Overview

Description

Flooding is one of the most common disasters in the United States. Flooding has resulted in billions of dollars worth of property damage, as well as hundreds of lost lives across the country (Figure 1). Whether due to heavy rains, high rivers, coastal storm surges, or broken plumbing, floods can impact homes anywhere in the United States causing damages ranging from minimal to catastrophic(see Figure 2).

The National Weather Service (NWS) reports that between 1984 and 2013, flood losses in the United States from freshwater sources were estimated to be $238 billion. This estimate does not include damages from coastal storm surge events (e.g., Sandy and Katrina). In 2017, New York University’s Fuhrman Center reported that “an average of 15 million people nationwide lived in the 100-year floodplain in 2011-2015, representing nearly 5% of the nation’s population. More than 30 million people—nearly 10% of the nation’s population—lived in the combined 100- and 500-year floodplain during this period.

National Flood Insurance Program Claims Payouts by County, 1974-2014.
Figure 1. National Flood Insurance Program Claims Payouts by County, 1974-2014. (Source: Texas A&M University 2018)

Flooding is the most common disaster type, occurring throughout the United States.
Figure 2. Flooding is the most common disaster type, occurring throughout the United States. (Source: NOAA 2014)

To minimize flood risks with new construction, homes should be built on high ground outside of high-risk flood areas and above the base flood elevation (BFE) and should be constructed in conformance with the National Flood Insurance Program (NFIP) and local floodplain management regulations. Existing homes could be elevated above the BFE or relocated to higher ground, although this is often impractical.

The Base Flood Elevation is the height to which floodwater is expected to rise during the base flood—the flood having a one percent chance of being equaled or exceeded in any given year. Areas affected by the base flood are shown as Special Flood Hazard Areas (SFHAs) on FEMA flood maps (FEMA P-1037, 2015).

National Flood Insurance Program

As described in FEMA P-1037, 2015, Congress passed the Homeowner Flood Insurance Affordability Act (HFIAA) in 2014. In fulfilment of Section 26 of that act, the Federal Emergency Management Agency (FEMA) established guidelines for property owners that:

  • Provide alternative mitigation methods, other than building elevation, to reduce flood risk to residential buildings that cannot be elevated due to their structural characteristics.
  • Inform property owners about how these alternative mitigation methods may affect flood insurance premium rates under the NFIP. NFIP flood insurance premiums are based on a number of factors, including flood risk zone, elevation of the lowest floor relative to the BFE, the type of building and foundation, the number of floors, and whether there is a basement or enclosure below an elevated building. The “lowest floor” is the lowest enclosed area (including a basement). An unfinished or flood-resistant enclosure, usable solely for parking of vehicles, building access, or storage, and having proper openings, is not considered a building’s lowest floor. Note that in V Zones, if the area under an elevated building is enclosed, the bottom of the enclosure would be considered the lowest floor.

As described in FEMA P-1037, existing homes that are retrofitted but are not Substantially Damaged or Substantially Improved may be eligible for NFIP flood insurance premium reductions. According to FEMA, a building is Substantially Damaged or Substantially Improved if it has sustained damage or undergone improvement (i.e., reconstruction, rehabilitation, addition) where the cost of the damage or improvement exceeds 50% of the market value of the building before the damage occurred or improvement began. As with new construction, Substantially Damaged or Substantially Improved structures must be re-built in conformance with NFIP and local floodplain management regulations.

While all of the measures described below can be effective at reducing flood damage, the current flood insurance rating framework does not provide premium reductions for all of these. Even if flood insurance premium reductions are not available, flood mitigation measures should still be considered to reduce damages and financial losses.

The following flood protection measures can be rated under the existing NFIP framework such that implementation of these measures may result in flood insurance premium reductions. The amount of the premium reduction will vary on a case-by-case basis.

All Interior Modification/Retrofit Measures (Basement Infill, Abandon Lowest Floor, and Elevate Lowest Interior Floor) and Wet Floodproofing using Flood Openings:  These measures can moderately to significantly reduce flood risk, and the flood insurance rating framework is presently in place to allow homeowners to receive flood insurance premiums that reflect any flood damage reduction protection provided by these measures. Use of these measures may result in buildings that meet current NFIP minimum requirements and the local floodplain ordinance, if the lowest floor is elevated to or above the BFE or locally adopted regulatory flood elevation.

The following flood mitigation measures can be used to decrease flood losses and damages. However, FEMA will need to undertake further analysis to determine whether it is appropriate to offer flood insurance premium discounts for undertaking such measures, and if so, what level of discount is appropriate for each measure.

All other Wet Floodproofing Measures (Elevate Building Utilities, Floodproof Building Utilities, and Use of Flood Damage-Resistant Materials, see Figure 3): These measures can moderately reduce flood risk and damage to utilities, floors, walls, and other areas subject to flooding.

All Dry Floodproofing Measures (Passive Dry Floodproofing System) and Barrier Measures (Floodwall with Gates and Floodwall without Gates, Levee with Gates and Levee without Gates): These measures can moderately to significantly reduce flood risk in areas subject to shallow flooding.

Moisture-resistant plastic and fiber cement exterior trim and cladding are indistinguishable from wood building elements.
Figure 3. Moisture-resistant plastic and fiber cement exterior trim and cladding are indistinguishable from wood building elements. (Source: Building Science Corporation 2006)

Retrofit Approaches

Retrofit approaches described below include basement infill, abandoning the lowest floor, elevating the lowest interior floor, wet floodproofing, and dry floodproofing.

Basement Infill

This involves filling in a basement located below the BFE to grade (ground level) (Figure 4). Sections of the basement walls that remain above ground must be retrofitted with flood openings that allow automatic entry and/or exit of floodwaters. Any basement utility systems and associated equipment must be elevated to protect utilities from damage or loss of function from flooding.

Pros – This technique has proven effective at reducing damages to building elements located below the BFE.

Cons – Livable square footage is lost. Costs are considerable and may trigger a Substantial Improvement declaration.

The basement can be filled in below the base flood elevation (BFE) to mitigate flood impacts.
Figure 4. The basement can be filled in below the base flood elevation (BFE) to mitigate flood impacts. (Source: FEMA P-1037, 2015)

Abandon Lowest Floor

This approach involves abandoning the lowest floor of a two- or more story slab-on-grade residential building (Figure 5). The lowest floor walls must be retrofitted with flood openings that allow automatic entry and exit of floodwaters. Additionally, any utility systems and associated equipment on the lowest floor must be elevated to protect utilities from damage or loss of function from flooding.

Pros – This technique has proven effective at reducing damages to building elements located below the BFE.

Cons – Livable square footage is lost. Costs are considerable and may trigger a Substantial Improvement declaration.

Abandon the lowest floor below the base flood elevation (BFE) to mitigate flood impacts
Figure 5. Abandon the lowest floor below the base flood elevation (BFE) to mitigate flood impacts. (Source: FEMA P-1037, 2015)

Elevate Lowest Interior Floor

This approach involves elevating the lowest interior floor within a residential building (Figure 6). The home must have high ceilings for this approach to work. The space below the lowest elevated interior floor must be either filled to create a stem wall or retrofitted with flood openings that allow automatic entry and/or exit of floodwaters. Additionally, any utility systems and associated equipment located below the lowest interior floor must be elevated to protect utilities from damage or loss of function from flooding.

Pros – This technique has proven effective at reducing damages to building elements located below the BFE.

Cons – Livable square footage is lost. Costs are considerable and may trigger a Substantial Improvement declaration.

Elevate the lowest interior floor above the base flood elevation (BFE) to mitigate flood impacts
Figure 6. Elevate the lowest interior floor above the base flood elevation (BFE) to mitigate flood impacts. (Source: FEMA P-1037, 2015)

Wet Floodproofing

Wet floodproofing is a design strategy that includes choosing flood-resistant materials and allowing flood waters to enter the enclosed areas of a house at nearly the same rate at which the water level is rising outside of the house. Wet floodproofing is used for spaces that will be minimally damaged by flood water such as enclosures below elevated buildings, walkout-on-grade basements, below-grade basements, crawlspaces, or attached garages. Wet floodproofing is not a viable strategy to use for living spaces such as living rooms and bedrooms because all affected parts of the space must be resistant to damage from direct and possibly prolonged flood water contact. Many elements common in living spaces, like floor and wall coverings, finishing materials, furniture, appliances, and other items, are easily damaged by flood waters. Since these common elements are so easily damaged, it is imperative to install materials that will resist the bulk of the damage caused by flooding.

With wet floodproofing, flood water is allowed into the house through openings in the foundation level of the home to ensure that floodwaters can enter and exit the house, rising and falling at the same rate inside as outside the house, as shown in Figure 7. This equal rising and falling ensures hydrostatic pressure differences will be minimal, reducing the likelihood of damage to the structure. Openings must allow the flow of water both ways while not letting in debris or pests. They should not require manual operation to be opened.

When wet floodproofing a home, install openings in the foundation wall to allow water levels to rise equally within and outside of the structure to eliminate hydrostatic pressure differences
Figure 7. When wet floodproofing a home, install openings in the foundation wall to allow water levels to rise equally within and outside of the structure to eliminate hydrostatic pressure differences. (Source: FEMA 551, 2007)

See FEMA’s Flood Damage-Resistant Home Requirements for detailed information on whether a home  is eligible for wet floodproofing. Complying with these requirements when retrofitting an existing home or designing a new home may make homeowners eligible for national flood program funding and reduced NFIP flood insurance rates. The document also classifies building materials as either acceptable or unacceptable on a 1 to 5 scale, with 4 and 5 being acceptable materials and 1 to 3 being unacceptable.

FEMA document 551, Selecting Appropriate Mitigation Measures for Flood-Prone Structures (FEMA 551, 2007), describes wet floodproofing in Chapter 6.

Strategies for Wet Floodproofing

The following strategies for wet floodproofing are recommended by the Louisiana State University AgCenter. (See LSU AgCenter’s Wet Floodproofing video for additional tips.)

Raise Appliances:

If this is not possible, build individual flood walls around the appliances.

Use Flood-Resistant Building Materials:
  • Use flood-resistant building materials, such as  concrete, stone, masonry block, ceramic and clay tile, pressure-treated and naturally decay-resistant lumber, epoxy paints, and metal (FEMA P-312, 2009).
    • Figure 5 illustrates this concept. The first level of the home uses masonry construction, a flood-resistant building material, while the second level is wood frame. (BSC 2006 Press)
  • Avoid vulnerable materials including drywall, blown-in and fiberglass batt insulation, carpeting, and non-pressure-treated wood and plywood (FEMA P-312, 2009).
  • Both paper-faced gypsum board and fiberglass cavity insulation are common building interior liners; however, both will be ruined by flood waters. Instead, use non-paper-faced gypsum board for interior lining purposes (BSC 2006 Press).
  • If using non-flood-resistant materials, make sure they are easily removable if they do get wet.
Make Wall Cavities Accessible:
  • Provide a way for walls or other building cavities to be drained, cleaned, and dried post-flood.

Pros and Cons of Wet Floodproofing (FEMA 551, 2007)

Pros:
  • Hydrostatic loads from water on the house over the course of a flood event are greatly reduced, reducing the threat of structural damage to the house.
  • It is not an all-or-nothing system, contrary to dry floodproofing which relies on everything staying completely dry and structurally sound.
  • It can be done in stages such as replacing flooring, elevating appliances, retrofitting walls, etc., based on time before the flood and budget.
  • It can be a relatively inexpensive option to implement.
  • It is a good option when exterior walls are not strong enough to withstand flood forces, in older homes especially.
  • It is a good option if you can’t build a reliable floodwall or elevate the home.
Cons:
  • Contents that can be damaged must be moved before flood water enters the space. Sometimes floods will strike quickly or when no one is around, thus minimizing the effectiveness of the wet floodproofing measures.
  • The process of cleaning up post-flood can be time-consuming, expensive, and difficult.
  • There is the potential for damage and health concerns from hazardous materials within the flood waters, such as sewage, chemicals, fertilizers, etc.
  • It does not protect against the hydrodynamic force of flowing water, erosion and scour, or the impact of flood-borne debris.

Dry Floodproofing

A dry floodproofed home is a watertight space designed to keep out all flood waters and to keep inside contents dry. This is accomplished by blocking all of the pathways where water can enter a building during a flood, such as doors, windows, and unsealed walls. Typically, dry floodproofing is used to protect living spaces such as kitchens, living rooms, and bedrooms, where flood water would damage or destroy the contents. Dry floodproofing is used as a retrofitting strategy for existing homes. It can also be used as a building strategy for new low-risk homes that may fall in the 100- to 500-year floodplains. Dry floodproofing can be considered permanent or temporary. Permanent dry floodproofing is done by constructing the wall with waterproof materials and ensuring the structural integrity of the wall. Temporary dry floodproofing involves installing an external waterproof barrier around the home to stop water before it touches the walls of the home. Dry floodproofing tactics that involve waterproofing the home’s walls can result in the home’s walls having to bear considerable hydrostatic force in the event of a flood, so ensuring that the walls are capable of handling this stress is of utmost importance.

Strategies for Dry Floodproofing:

Dry floodproofing requires the following three steps (see Figure 9). Dry floodproofing measures can be temporary or permanent as shown in Figures 10 and 11.

  • Seal the walls with a waterproof coating that is sprayed or painted on, or install an impermeable membrane such as a roof deck adhesive membrane, or construct an additional layer of masonry or concrete
  • Install a backflow valve to prevent sewer and drain backup.
  • Install shields to cover openings such as doors and windows.

Dry floodproofing requires installing an impervious coating or covering over walls below the BFE, covering openings, and installing a sewer backflow prevention valve.
Figure 8. Dry floodproofing requires installing an impervious coating or covering over walls below the BFE, covering openings, and installing a sewer backflow prevention valve. (Source: FEMA 551, 2007)

 

Dry Floodproofing Constraints:

  • Properly constructed walls can typically withstand the pressure of up to 3 feet of water outside the house. For floods where water levels reach further up the walls, dry floodproofing is not effective.
  • Because of the buoyant force under the house from water in the soil, dry floodproofing should only be used for floods lasting 1 to 2 days. Any longer can risk damage to the home’s foundation as shown in Figure 9.

 

Temporary dry floodproofing of a wall.
Figure 9. Temporary dry floodproofing of a wall. (Source: FEMA 551, 2007)

 

Permanent dry floodproofing of an exterior brick wall by adding a second layer of brick.
Figure 10. Permanent dry floodproofing of an exterior brick wall by adding a second layer of brick. (Source: FEMA 551, 2007)

 

Buoyancy forces from water in the soil can act on dry floodproofed homes to cause foundation damage, especially homes with deep basements.
Figure 11. Buoyancy forces from water in the soil can act on dry floodproofed homes to cause foundation damage, especially homes with deep basements. (Source: FEMA 551, 2007)

Pros and Cons of Dry Floodproofing (FEMA 551, 2007)

Pros:
  • Less costly than elevating a home.
  • Doesn’t require additional land that may be needed for levees or floodwalls.
  • May be fundable under FEMA mitigation grant programs.
  • Some measures can be temporary, eliminating unsightly barrier walls.
Cons:
  • May not be used to bring a substantially damaged or substantially improved residential structure into compliance with the community’s floodplain management ordinances. (See FEMA Floodproofing.)
  • Requires human intervention and adequate warning to install temporary protective measures.
  • May not minimize potential damage from high-velocity flood flow and wave action.>
  • Ongoing maintenance is required.
  • Flood shields are typically not considered aesthetically pleasing.
  • Not recommended for floods lasting more than 1 to 2 days.

 

Flooding Guides

The following guides provide information on making the components and assemblies of a home more resistant to floods.

Roof:

Walls/Windows/Doors:

Building Attachments:

Foundation/Site:

  • Pier foundation
  • Crawlspace foundation
  • Raised slab foundation
  • Water-resistant flooring
  • Wet floodproof lowest level ->info guide: wet proofing vs dry proofing homes in flood-prone areas
  • Dry floodproof lowest level->info guide: wet proofing vs dry proofing homes in flood-prone areas
  • Retrofit - Relocate house
  • Retrofit - Elevate house
  • Site selection outside of Coastal V Zones and known erosion areas
  • Vegetation and Dunes for Site Protection
  • Retrofit - Install floodwall barrier.

Operations/Equipment:

  • Storage space for emergency supplies
  • Outdoor HVAC equipment elevated and secured
  • Electrical outlets located/relocated higher in wall
  • Ventilation
  • Gas shutoff - Retitled "Automatic gas shutoff valve installed at meter"
  • Sewage backflow prevention device.

Design:

  • Design for coastal flooding
  • Design house 3 ft. above BFE in flood zones
  • Consider siting for disaster resistance.

In addition to these guides, the Solution Center contains an extensive collection of references, training videos, images, and webinars on disaster-resistant construction related to flooding. These can be found by searching “flood” in the Solution Center.

More Info

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References and Resources*
Author(s)
Louisiana State University College of Agriculture
Organization(s)
LSU AgCenter
Publication Date
Description
Webpage from LSU AgCenter answering frequently asked questions after gutting a flooded home, including mitigation strategies for future floods.
Author(s)
Joseph Lstiburek
Organization(s)
Building Science Corporation
Publication Date
Description
Guide from Building Science Corporation on how to construct a flood and hurricane resistant home.
*For non-dated media, such as websites, the date listed is the date accessed.
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