Lesson 4:

Flood Mitigation Measures

After a flood, there are many mitigation measures that can be taken to help reduce the risk of future flood damage. 

This lesson describes mitigation options and the circumstances under which each is used.  Measures to mitigate flood damage include:

• Floodproofing
• Structure Elevation
• Structure Relocation
• Property Acquisition
• Flood Control Barriers

 

At the end of this lesson you will:

• Identify floodproofing measures including the difference between wet and dry floodproofing.
• Identify mitigation measures that can meet local floodplain management requirements for residences, including elevating and relocating structures, and property acquisition.

Floodproofing

“Floodproofing” describes mitigation measures that decrease the risk of flood damage in structures.  Floodproofing measures are generally designed to protect individual components of a structure, not to bring the structure into compliance with a community’s floodplain ordinance.

There are two main types of floodproofing measures:

Wet floodproofing is any measure applied to a structure and/or its contents that prevents or reduces damage from flooding while allowing flood waters to enter the structure.

Dry floodproofing is any measure to make a structure watertight below the level that needs flood protection to prevent floodwaters from entering.

 

Wet Floodproofing

Wet floodproofing measures are typically used to protect electrical, heating, ventilation, and air conditioning (also known as HVAC), and utility components of a structure when flooding occurs.

Many of these mitigation measures involve elevating equipment above the expected flood levels.  FEMA Flood Insurance Rate Maps (FIRMs)indicate these anticipated levels, called Base Flood Elevations (BFE). 

The Base Flood Elevation (BFE) is the anticipated elevation above mean sea level that the “base flood” is expected to reach.  The base flood is a flood having a one percent chance of being equaled or exceeded in any given year.The NFIP uses the BFE to describe the minimum elevation to which new development in a flood hazard area must be regulated

Flood water can easily damage electrical system components including service panels, meters, switches, and outlets.

Even if they are inundated for only short periods, electrical system components usually have to be replaced.

Short circuits in flooded systems pose an increased potential for fire. Raising electrical system components helps avoid such problems.

It makes sense to raise all components of the electrical system, including wiring, above the base flood level identified on the NFIP's Flood Insurance Rate Map for the location.

Additional elevation of electrical components above the recommended level further reduces the risk.

Licensed contractors ensure that electrical system modifications are performed correctly and according to all applicable codes.

 

HVAC equipment, such as furnaces and hot water heaters, can be damaged extensively if inundated by flood waters.

A good way to protect HVAC equipment is to locate it on an upper floor or in the attic rather than in the basement or lower level of a structure.

Ventilation ductwork also can be protected from flooding by locating it above the base flood level.

In addition to elevating large utilities, household appliances such as washing machines and dryers should be elevated above potential flood waters.

Outdoor utilities such as heat pumps or air conditioners can be protected by placing them on an elevated platform above the Base Flood Elevation (BFE).

Unanchored heating oil or propane tanks can be easily moved by flood waters.  Losing these tanks results in major costs and inconvenience to their owners. If dislodged, these tanks also pose serious threats to public safety and to the environment.

An outside tank can be secured by running straps over it and attaching the straps to ground anchors.

 

Dry Floodproofing

Dry floodproofing includes measures that make a structure watertight below the level that needs flood protection to prevent floodwaters from entering.  This type of floodproofing is often used to protect non-residential structures, water supplies, and sewage systems.

Dry floodproofing may NOT be used to bring a Substantially Damaged or Substantially Improved residential structure into compliance with the community’s floodplain ordinance. However it CAN meet requirements for non-residential and commercial structures.

An example of a dry floodproofing measure is to apply a waterproof veneer, such as a layer of brick backed by a waterproof membrane, directly to the outside surface of an existing structure.

Flooding can cause sewage from sewer lines to back up through drain pipes. These backups not only cause damage that is difficult to repair, but they also create health hazards.

One way to protect against this hazard is to install backflow valves, which temporarily block drain pipes if water travels up them the wrong way. If a structure’s lowest level has a floor drain that empties to the exterior of the building, installing a floating floor-drain plug can also prevent flood waters from backing up the drain pipe and entering the structure.

Refer interested individuals to local building officials and plumbing specialists before undertaking any of these measures.

Well contamination, sewer line back up, and mold may occur in any type of flooding.

Flood waters often carry hazardous and toxic materials, including raw sewage, animal waste, oil, gasoline, solvents, and chemicals. 

If a property’s water supply comes from a well, flood waters entering that well can contaminate the groundwater and make the well water unsafe.

Wells suspected of being contaminated should be inspected by a professional.

Proper well construction will protect a well from contamination.  A licensed well-drilling contractor can inspect a well and suggest improvements such as:

• Extending the casing at least 2 feet above the highest known flood elevation
• Installing a sanitary seal or cover on the casing
• Installing a backflow value in the water line

 

Structure Elevation

Elevating a structure reduces the risk of flood damage by raising the entire building above the BFE indicated on the FEMA flood map.

When the lowest floor of a flood-prone structure is raised to or above BFE, it will meet the minimum ordinance standards required by the NFIP for structures located in a Special Flood Hazard Area.

Structures  must be elevated by trained professionals.

Many communities have adopted even stricter standards in their floodplain ordinance and require higher elevation called “freeboard.” Freeboard helps compensate for many unknown factors that can contribute to flood heights greater than those calculated in base flood elevations, such as wave action, bridge openings, and the effects of urbanization of the watershed.

 

Structure Relocation

Relocation is another flood mitigation measure that will comply with most community floodplain ordinances.
Owners can have their buildings moved by professionals to a safer place outside the Special Flood Hazard Area.

This is also an expensive option but it can permanently eliminate the flooding risk to the structure if the building is moved to a safe area.

 

Property Acquisition

A community can permanently eliminate the risk of flood damage to buildings in a floodplain by purchasing the properties from their owners and removing the structures.

Communities generally engage in “acquisition” or “buyout" projects when a grant has been awarded to provide funding.  If a community decides to pursue a property acquisition project, the land cannot be redeveloped and often is preserved as public open space, parks, or recreation areas.  

 

Flood Control Barriers

Flood control barriers protect structures by deflecting the destructive forces of flood waters away from vulnerable structures.

These barriers, including levees, berms, floodwalls, and temporary barriers, may be used to create a protected area for a single structure or for multiple structures.

Barriers may not be used to bring a Substantially Damaged or Substantially Improved residence into compliance with a community’s floodplain management ordinances or laws.

When informing the public about flood control barriers, it is important to point out that this type of mitigation measure can affect local drainage, possibly creating or worsening flood problems for others. It also requires periodic maintenance to remain effective.

This completes this lesson.  In this lesson you learned about the mitigation methods for flooding that included:

• Floodproofing (wet and dry)
• Structure elevation
• Structure relocation
• Property acquisition
• Flood control barriers