You may not have heard these terms before, but they describe important characteristics of flooding: how rapidly the elevation (and therefore the depth) of water increases and decreases during a flood. These rates are usually expressed in terms of feet or inches per hour. Floodwaters with high flow velocities, such as those in areas of steep terrain, and water released by the failure of a dam or levee, usually rise and fall more rapidly than slower-moving floodwaters, such as those in more gently sloping floodplains.
Rate of rise is important because it affects how much warning you will have of an impending flood. For example, homeowners in the floodplains of large rivers like the Mississippi and Missouri may know days in advance that flooding is occurring upstream and will eventually reach their homes. But in the floodplains of streams with rapid rates of rise, homeowners may have only a few hours’ notice of a coming flood or perhaps none at all. With adequate warning, you will be better prepared to take steps to protect yourself and your property. Warning time is particularly important for flood protection methods that depend on action you must take.
Rates of rise and fall are important also because of their effect on hydrostatic pressure. As explained in the discussion of flood depth/elevation, hydrostatic pressure is most dangerous for a home when the internal and external pressures are not equalized. This situation occurs when the level of water inside the home is significantly higher or lower than the level outside. When floodwaters rise rapidly, water may not be able to flow into a home quickly enough for the level inside the home to rise as rapidly as the level outside. Conversely, when floodwaters fall rapidly, water that has filled a home may not be able to flow out quickly enough, and the level inside will be higher than the level outside. In either situation, the unequalized hydrostatic pressures can cause serious structural damage, possibly to the extent that the home collapses.
Duration is how long a flood lasts. One of the meanings of duration is how long it takes for the creek, river, bay, or ocean to return to its normal level. As a homeowner, you may be more interested in how long floodwaters remain in or around your home or perhaps how long they block nearby streets. In many floodplains, duration is related to rates of rise and fall. Generally, water that rises and falls rapidly will recede more rapidly, and water that rises and falls slowly will recede more slowly. An example of this relationship is the extensive flooding that occurred in the broad, flat floodplains of the Midwest in 2008. In those areas, floodwaters rose slowly and remained high for many weeks or longer.
If your home is flooded, duration is important because it determines how long the structural members (such as the foundation, floor joists, and wall studs), interior finishes (such as drywall and paneling), service equipment (such as furnaces and hot water heaters), and building contents will be affected by floodwaters. Long periods of inundation are more likely to cause greater damage than short periods. Duration can also determine how long your home remains uninhabitable.