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General Concepts

Base Flood (1% Chance Flood)
The definition of base flood is a flood that has a 1% or 1 out of 100 chance of occurring in any given year. Over a long period of time, base floods will occur on the average once every 100 years. For this reason, the base flood used to be commonly referred to as the "100-year flood." An item related to the base flood is the Base Flood Elevation (BFE). The BFE is defined as the elevation to which a body of water could be expected to rise during a base flood.

Special Flood Hazard Areas (SFHA)

Vickie Davis,
former Town Planner, Lyme, on A Zones

The area that would be inundated by the base flood is called a "Special Flood Hazard Area" (SFHA). The SFHA is the land in the floodplain subject to a one percent or greater chance of flooding in any given year (also known as the 100 year floodplain) and is designated as Zone AE or VE on the post-1986 flood maps and as Zone A and V on the pre-1986 flood maps. In order to minimize the potential for flooding, the NFIP program regulates development and reconstruction in the SFHA.

The SFHA is composed of the floodway and the flood fringe. The floodway is the stream channel and that portion of the adjacent floodplain that must remain open to permit passage of the base flood. Floodwaters generally are deepest and swiftest in the floodway, and anything in this area is in the greatest danger during a flood. The remainder of the floodplain is called the flood fringe, where water may be shallower and slower. The following figure illustrates the relationship between the floodway and flood fringe.

Hazard Area

Cross-section showing the Floodway and Flood Fringe


NFIP minimum standards provide that other areas outside the boundaries of the floodway can be developed without further analysis. Consequently, most communities permit development in the flood fringe if the development is elevated or otherwise protected to the base flood level (or any higher state or local standards). Development in the floodway is allowed if it can be demonstrated that no rise in the BFE will occur.

The area between the 100 and 500 year flood boundaries is the "Moderate Flood Hazard Area", known as the B zone on the pre-1986 maps or Shaded X zone on the post-1986 maps. The remaining area above the 500-year flood level is termed the "Minimal Flood Hazard Area" and is known as the C zone on the pre-1986 maps or X zone on the post-1986 maps. However, this is not to say that areas outside of the SFHA are not subject to flooding. As the following figure shows, the water level for the 500-year flood is higher than that for a 100-year storm so consequently more land is inundated.

100 Year

Cross Section with Flood Elevations for Different Flood Events


Hydrology and Hydraulics

For regulating floodplain development, a map developed through hydrologic and hydraulic analysis (H&H) is used. The science of hydrology is used to determine the amount of water that a river or stream must convey for a given storm. This involves calculating the amount of runoff that can be expected to drain from the surrounding watershed. The principles of hydraulics are applied to help determine how the river or stream channel will handle the flow and to what extent the excess water will spread over the floodplain when the flood is at its peak. Specialized computer programs are used to perform most hydrologic and hydraulic computations. The advantage of using H&H analyses is that specific frequencies of flooding can be selected for delineating a floodplain. It is often difficult to associate the drawing of a floodplain based on soils, physiography, or vegetation, with a particular frequency of flooding. If applied properly, an H&H analysis provides a sound technical and legal basis for adopting and administering floodplain management regulations.

Flood Damage

Hydrodynamic Forces
Moving water creates a hydrodynamic force which can damage a building's walls in three ways (see figure below):

  • Frontal impact, as water strikes the structure;
  • Drag effect, as water runs along the sides of a structure; and/or
  • Eddies or negative pressures, created as water passes the downstream side.

Paul Larochelle,
Larry's Country Store, Salem, on spring 2006 flood site impacts





The speed of moving water is called velocity, which is measured in feet per second. The faster water moves, the more pressure it puts on a structure and the more it will erode stream banks and scour the earth around a building's foundation. Floodwaters moving faster than 5 feet per second comprise a high-velocity flood, requiring special design considerations for buildings, roads, bridges and other manmade structures in its path.

While velocity is one factor in determining the potential harm of a flood, the total impact of moving water is related to the depth of the flooding. Studies have shown that deep water and low velocities can cause as much damage as shallow water and high velocities. People are more susceptible to damage than buildings. Studies have shown that it doesn't take much depth or velocity to knock a person over. Thus, no areas with moving floodwater can be considered safe for walking. A car will float in only two feet of moving water, which is one reason people trapped in vehicles have died in flooding events. Often victims put themselves in perilous situations by ignoring warnings about travel or mistakenly thinking that a washed-out bridge is still open.

Debris Impact
Debris also increases the hazard posed by moving water. Floodwaters can and will pick up anything that will float - logs, lumber, ice, even propane tanks and vehicles. Moving water will also drag or roll objects that don't float. All of this debris acts as battering rams that can knock holes in walls.


Debris carried by floodwaters along Route 123 in October 2005


Hydrostatic Forces
The weight of standing water puts hydrostatic pressure on a structure. Because water is fluid, it exerts the same amount of pressure sideways (lateral pressure) as it does downward. The deeper the water, the more it weighs, with a greater hydrostatic pressure being exerted in all directions. Most walls are not built to withstand lateral pressure. Studies and tests have shown that the lateral force presented by three feet of standing water can be enough to collapse the walls of a typical frame house. Basement walls and floors are particularly susceptible to damage by hydrostatic pressure. Not only is the water deeper in a basement, it is also subjected to the combined weight of water and saturated earth. Water in the ground underneath a flooded building will seek its own level - resulting in uplift forces that can break a concrete basement floor. Hydrostatic pressure can also cause damage due to floatation or buoyancy. Improperly anchored buildings can float off their foundations and empty, in-ground storage tanks can pop out of the ground even forcing their way through several inches of concrete.


Lateral Hydrostatic Force and Pressure


When soaked, many materials change their composition or shape. Wet wood will swell, and if it is dried too fast it will crack, split or warp. Plywood can come apart. Gypsum wallboard will fall apart if it is bumped before it dries out. The longer these materials are wet, the more moisture they will absorb. Soaking can cause extensive damage to household goods. Wooden furniture may get so badly warped that it can't be used. Other furnishings, such as upholstery, carpeting, mattresses and books, usually are not worth drying out and restoring. Electrical appliances and gasoline engines won't work safely until they are professionally dried and cleaned.

Sediment and Contaminants
Many materials, including wood and fiberglass or cellulose insulation, absorb floodwater and its sediment. Even if allowed to dry out, the materials will still hold the sediment, salt and contaminants brought by the flood. Simply letting a flooded house dry out will not render it clean - and it certainly will not be as healthy a place as it was before the flood. Few floods, especially those that strike inland, have clear floodwater, and so they leave a mess made of natural and man-made debris. Storm water, snow melt and river water pick up whatever was on the ground, such as soil, road oil, and farm and lawn chemicals. If a wastewater treatment plant upstream was inundated, the floodwaters will likely include untreated sewage. Flooding can leave large amounts of sand, sediment and debris that require major cleanup efforts. After the water recedes or evaporates, these sediments are left on and in a building, and its contents.

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