Porthole logo


Historically, people at risk from flooding could only hope for help from their neighbors and charitable organizations in the event of a flood. Government assistance varied from community to community, and flood insurance was scarce. During the 1920s, the insurance industry concluded that flood insurance could not be a profitable venture because the only people who would want flood coverage would be those who lived in floodplains. Since they were sure to be flooded, the rates would be too high to attract customers.

During the 1960s, Congress became concerned with problems related to the traditional methods of dealing with floods and flood damage – construction of structural projects and federal disaster assistance. Both were proving to be quite expensive, with no end in sight.

Congress concluded that:

  • Although Federal flood programs were funded by all taxpayers, they primarily helped only residents of floodplains.
  • Flood protection structures were expensive and could not protect everyone.
  • People continued to build and live in floodplains, thus still risking disaster.
  • Disaster relief was both inadequate and expensive.
  • The private insurance industry could not sell affordable flood insurance because only those at high risk would buy it.

In 1968, Congress passed the National Flood Insurance Act to correct some of the shortcomings of the traditional flood control and flood relief programs. The act created the National Flood Insurance Program (NFIP) to:

  • Transfer the costs of private property flood losses from the taxpayers to floodplain property owners through flood insurance premiums. (PLEASE NOTE this special provision, to clarify that the taxpayers are NOT paying for flood area homeowners, which is often a misconception. These property owners are paying their own way for the “privilege” of living near water.)
  • Provide floodplain residents and property owners with financial aid after floods, especially smaller floods that do not warrant federal disaster aid.
  • Guide development away from flood hazard areas.
  • Require that new and substantially improved buildings be constructed in ways that would minimize or prevent damage in a flood.

Participation in the NFIP grew slowly. In 1972, Hurricane Agnes devastated a wide area of the eastern United States. Disaster assistance costs were the highest ever, leading Congress to examine why the NFIP was so little used. Investigators found that few communities had joined the NFIP – there were fewer than 100,000 flood insurance policies in force nationwide.

To remedy this, the Flood Disaster Protection Act was passed in 1973, requiring that buildings located in identified flood hazard areas have flood insurance coverage as a condition of Federal aid or loans from federally – insured banks and savings and loans, and as a condition for receiving federal disaster assistance. These “sanctions” for non – participation, which are detailed later in this unit, make it hard for any community that wants federal assistance for properties in floodplains to avoid joining the NFIP.

The 1973 Act spurred participation in the program dramatically. By the end of the decade, more than 15,000 communities had signed on and about two million flood insurance policies were in effect.

In 1979, the Federal Insurance Administration (FIA) and the NFIP were transferred to the newly created Federal Emergency Management Agency (FEMA). During the early 1980’s, FIA worked to reduce the program’s dependence on its authority to borrow from the Federal Treasury. Through a series of rate increases and other adjustments, the program has been self – supporting since 1986. The NFIP is funded primarily through premium income, which pays all administrative  and mapping costs as well as claims.

Since 1973, the program has been amended several times. The most important changes came under the National Flood Insurance Reform Act of 1994 which fine tuned various aspects of the program, such as authorizing the Community Rating System, increasing the maximum amount of flood insurance coverage, and establishing a grant program for mitigation plans and projects.

The Reform Act and the initiation of a flood insurance advertising campaign known as “Cover America” boosted sales of flood insurance policies again. By the end of 1997, there were 3.8 million flood insurance policies in force. Also by the end of 1997, the number of participating communities exceeded 19,000 out of 22,000 with identified floodplains. As shown in Figure 2 -1, the greatest growth occurred in the late 1970’s, after the provisions of the 1973 amendments took effect.



 Before describing how flood studies are developed, we first need to introduce some of the common terms uses in floodplain analysis and in the National Flood Insurance Program (NFIP). The following terms are integral for understanding the basis for flood studies and flood maps:

  • The base flood,
  • The 100 – year flood,
  • Special Flood Hazard Area, and
  • Base Food Elevation.

The base flood

Floods come in many sizes – with varying degrees of magnitude and frequency. Rivers and coastlines are expected to food, as all bodies of water have floodplains.

But rivers and coastlines are different, as well; each has its own probability of flooding. Probability is a statistical term having to do with the size of a flood and the odds of that size of flood occurring in any year.

In order to have common standards, the NFIP adopted a baseline probability called the base flood. The base flood is the one – percent annual chance flood. The one – percent annual chance flood is the flood that has a one – percent (one out of 100) chance of occurring in any given year. The base flood, which is also informally referred to as the 100 – year flood, is the national standard used by the NFIP and all Federal agencies for the purposes of requiring the purchase of flood insurance and regulating new development. The one – percent annual chance flood was chosen as a compromise between a more frequent flood (such as a 10 – percent chance flood), which would permit excessive exposure to flood risk, and a more infrequent flood (say, a 0.1 – percent chance flood), which would be considered an excessive and unreasonable standard.

The computed elevation to which floodwater is anticipated to rise during the base flood is the Base Flood Elevation (BFE).


Coastal flood studies are conducted for communities along the Atlantic and Pacific Oceans, the Gulf of Mexico, the Great Lakes, and the Caribbean Sea. Coastal studies are used to establish a base flood and an SFHA, but they may also designate a coastal high hazard area (V Zone). Note that coastal communities, particularly counties, may also have riverine floodplains with designated floodways.


Most coastal floods are caused by coastal storms, usually hurricanes and northeasters. Such storms bring air pressure changes and strong winds that “pile” water up against the shore in what is called a storm surge.

A computer simulation of a coastal storm is developed based on data from past storms. Such data include wind speeds, wind direction, and air pressure from historical hurricanes and northeasters. The resulting surge elevations are then calibrated using historical information so the probabilities for each event can be determined.

The coastal storm surge computer program produces stillwater flood elevations – the elevations of various coastal floods, not including waves. The computer model is calibrated by reproducing the observed historical stillwater elevations. The program determines the stillwater elevation from these historical data.


In addition to storm surge, wave action is an important aspect of coastal storms. Wind – driven waves produce velocities and impacts that may cause significant structural damage. The coastal flood study analyzes how high the wave crest elevation will be above the stillwater elevation as water is driven onshore.

When waves hit the shore, water is moving with such force that it keeps traveling inland. This is called wave runup, when land areas that are higher than the stillwater elevation are flooded (Figure 3 -8). Wave setup is defined as the additional elevation of the water surface over normal surge elevation caused by onshore mass transport of the water by wave action. Wave set – up is a function of deepwater wave height and duration.


Waves pack a lot of power. Much more destructive than standing or slow – moving water, their power increases dramatically  with their height. For the purposes of the NFIP, the flood study identifies the coastal high hazard area as that most hazardous part of the coastal floodplain, due to its exposure to wave effects. This is typically the area between the shoreline and the most landward of the following points:

  • where the computed wave heights for the base flood are three feet or more,
  • the inland limit of the primary frontal dune, or
  • where the eroded ground profile is three feet below the computed runup elevation.

The three – foot wave height threshold was selected because a three – foot wave  generally carries enough energy to break a wall panel away from a floor to which it has been nailed. These areas are designated as V Zones, where the “V” stands for “velocity wave action”. V Zones are subject to more stringent regulatory requirements and a different flood insurance rate structure because they are exposed to an increased degree of risk.

Coastal flood areas not within the coastal high hazard area are mapped as A Zones (see figure 3 – 9).

figure 3 9

 FIGURE 3 – 9

The second method is to have the design meet or exceed the following three criteria:

  1. The bottom of the openings must be no higher than one foot above grade (see Figure 5 – 12)
  2. The openings should be installed on at least two walls of the enclosure to ensure that at least one will work if others get blocked or plugged.
  3. Provide a minimum of two openings having a net area of not less than one square inch for every square foot of enclosed area that is subject to flooding.

If the area of the enclosure is 1,000 square feet, the area of the openings combined must total at least 1,000 square inches. A standard crawlspace vent for block walls is 8″ x 16″ or 128 square inches (see Figure 5 – 12). To determine how many would be needed, divide the square footage of the floor area by 128.

Example 1: 1,280 square foot house = 10 10 vents will be needed 128 square inches/ vent

Example: 2,000 square foot house = 15.62 16 vents will be needed 128 square inches/ vent

Openings may be equipped with screens, louvers, valves or other coverings or devices to keep animals out of the enclosure. However, any covering must permit the automatic flow of floodwater in both directions.

The opening sizes in the previous examples and in Figure 5 – 12 are based on standard crawlspace vents, which most building codes require to be installed in a crawlspace for ventilation purposes. Often these are located close to the floor in order to circulate air around the floor joists.


floodproof buildingThe minimum NFIP requirement is to floodproof a building to the BFE. However, when it is rated for flood insurance, one foot is subtracted from the floodproofed elevation. Therefore, a building has to be floodproofed to one foot above the BFE to receive the same favorable insurance rates as a building elevated to the BFE. Unit 9, Section B, discusses this in more detail.


Zones V1 – 30, VE and/or V identified on FIRMs designate high hazard areas along coastlines that are subject to high water levels and wave action from strong storms and hurricanes. The winds, waves and tidal surges associated with these storms cause water of high velocity to sweep over nearby land. Many V Zones are also subject to erosion and scour which can undercut building foundations.


All new construction and substantial improvements to buildings in V Zones must be elevated on pilings, posts, piers or columns. Elevation – on fill, solid walls or crawlspaces – and flood proofing are prohibited because these techniques present obstructions to wave action. The force of a breaking wave is so great that these types of foundations would be severely damaged, resulting in collapse of the building.

Construction on piles or columns allows waves to pass under the building without transmitting the full force of the waves to the  building’s foundation. A special case is made for installing breakaway walls between the pilings or columns, but such walls are not supporting foundation walls.

While fill is not allowed for structural support for buildings within V Zones because of the severe erosion potential of such locations, limited fill is allowed for landscaping, local drainage needs, and to smooth out a site for an unreinforced concrete pad.

How high? Within V Zones, the controlling elevation is the bottom of the lowest horizontal structural member of the lowest floor. (In comparison, within A Zones, the controlling elevation is the top of the lowest floor.) This is to keep the entire building above the anticipated breaking wave height of a base flood storm surge.

Posts of wood, steel, or pre – cast concrete are preferred over block columns and similar foundations that are less resistant to lateral forces. Pilings are preferred in areas subject to erosion and scour, but it is critical that they be embedded deep enough.


A registered professional engineer or architect must develop or review the structural design, specifications and plans for the construction, and certify that the design and planned methods of construction are in accordance with accepted standards of practice for meeting the above provisions.

You must maintain a copy of the engineer’s or architect’s certification in the permit file for all structures built or substantially  improved in the V Zone.

The North Carolina Division of Emergency Management has prepared a V Zone certification form to ensure that these requirements are met. Check with your state NFIP coordinator to see if your state has developed a V Zone certification form.


Any walls below the lowest floor in a building in a V Zone should give way under wind and water loads without causing collapse, displacement or other damage to the elevated portion of the building or the supporting pilings or columns.

Any enclosed space below the lowest floor must be free of obstruction, or constructed with non – supporting breakaway materials such as open wood latticework or insect screening. Just as in A Zones, this space is to be used solely for parking of vehicles, building access or storage, and must be constructed of flood resistant material.