Everyone living near a stream needs to understand the frequency with which floods occur. Small floods happen quite frequently, every year or so. Large floods return less often, every score of years, centaury, or longer. Statistically speaking, the larger the floods, the longer are the recurrence times between each. A typical analysis of flood frequency involves a plot of historic flood sizes versus time. Flood-discharge volumes are plotted on the vertical axis, and the recurrence intervals are plotted on the horizontal axis in years between floods of each size. The longer the historical record of floods in an area, the more accurately the curve can be drawn. With a flood-frequency curve, the return times of floods can be estimated. For example, in move upward from 100 years, intercept the curve, then read to the left to obtain the expected flood size. The U.S Federal Emergency Management Agency (FEMA) uses the 100-year flood in establishing regulatory requirements.
Individual flood-frequency curves must be constructed for each stream because each stream has its own characteristic floods. A flood-frequency curve should serve as the basis for designing all structures built on a floodplain and determining where buildings should be located for the highest probability of safety. Planners can decide what size flood (how many years of protection) to accommodate when determining how the land is to be used.
When designing roads, bridges, and buildings, it is seductive to consider only the smaller floods and save large amounts of money on initial construction costs. However, these initial savings are eaten away by higher maintenance and repair costs. In the long run, it is commonly cheaper to build with respect for large floods; this not only saves money in the future but also eliminates much of the human suffering that occurs when homes and other buildings are flooded.
A designer needs to know the likelihood of a given size flood occurring during the expected usage time of a structure. Flood frequency also can be expressed as the statistical probability of stream discharges of given size arriving in any year or number of years. The bigger the flood, the longer the return period and the smaller the probability of experiencing it any one year. Statistically, the 100-year flood will occur once in 100years? The obvious answer of once is wrong; from the probability is only 63 percent. No flood has a 100 percent chance of occurring. By similar analysis, even though a “150-year flood” may occur one year, it is still possible for another of the same size to come again in the following year, or even in the same year. For example, in 1971, the patuxent River between Baltimore and Washington, D.C., had a flood that was 1.6 times bigger than its calculated 100-year flood, or any other size flood, is a statistically average event that occurs by chance, not at regular intervals.
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