The depth and elevation of flooding are so closely related that, for the purposes of this discussion, they can be viewed as a single characteristic. Flood depth is the height of the floodwater above the surface of the ground or other feature at a specific point. Flood elevation is the height of the floodwater above an established reference datum. The standard datums used by most Federal agencies and many State and local agencies are the National Geodetic Vertical Datum (NGVD) and the North American Vertical Datum (NAVD); however, other datums are also in use. The use of other datums is important because elevations of the ground, floodwaters, and other features cannot be meaningfully compared with one another unless they are based on the same datum.
When the elevation of the ground (or another surface such as the lowest floor of your home) and the elevation of the floodwaters are both based on the same datum, the flood depth at any point is equal to the flood elevation at that point minus the elevation of the ground (or other surface) at that point. Figure 2-6 illustrates this relationship. One more thing you should know: ground elevations are established by surveys; flood elevations may be calculated or they may be known from water marks left by past floods.
The depth of flooding at your home is important primarily because floodwaters, even when they are not moving, exert pressure on structural components such as walls and concrete floor
An elevation datum is an arbitrary surface that serves as a common reference for the elevations of points above or below it. Elevations are expressed in terms of feet, meters, or other units of measure and are identified as negative or positive, depending on whether they are above or below the datum. Three common elevation datums are mean sea level (msl), NGVD, and NAVD.
Under the National Flood Insurance Program, the lowest floor of a building is the floor of the lowest enclosed area within the building, including the basement. The only exception is an enclosed area below an elevated building, but only when the enclosed area is used solely for parking, building access, or storage and is compliant with relevant regulations. The elevation of the lowest floor can be very important in retrofitting.
Buoyancy refers to the upward hydrostatic force that floodwater exerts on the floors of homes with enclosed spaces below the flood level.
slabs. The pressure exerted by still water is called “hydrostatic pressure.” It is caused by the weight of the water, so it increases as the depth of the water increases. As shown in Figure 2-7, floodwater, including water that has saturated the soil under the home, pushes in on walls and up on floors. The upward force on floors is called “buoyancy.”
As shown in Figure 2-7b, water that has saturated the soil poses a special hazard for basement walls. Because hydrostatic pressure increases with the depth of the water, the pressure on basement walls is greater than the pressure on the walls of the upper floor, as indicated by the arrows in the figure. This pressure is made even greater by the weight of the saturated soil that surrounds the basement.
The walls of homes built using standard wood-frame or masonry construction are not designed to resist this pressure. If the pressure exceeds the strength of the walls (including basement walls), it can push them in or out, cause extensive structural damage, and possibly cause the home to collapse. In some areas, the buoyant force of hydrostatic pressure on basement floors has pushed homes entirely out of the ground.
If water is allowed to enter, the hydrostatic pressures on both sides of the walls and floor become the same, or “equalized” (Figure 2-9), and the walls are much less likely to fail. This is an important consideration in some types of retrofitting methods.