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<br />For each reach, and for the entire stream, construct a graph of total <br /> <br /> <br />flood damage versus probability of occurrence in' any given'year. The <br /> <br /> <br />graph will be similar toF.igure III-I. From this graph, it is possible <br /> <br /> <br />to interpolate flood damages for flood events other than the ones investi- <br /> <br /> <br />gated. A damage versus probability curve will be required for the <br /> <br /> <br />existing development situation (if a comparison is desired between the <br /> <br /> <br />existing and future flood damages), and for each flood control alter- <br /> <br /> <br />native. Compute the area under the curves, which is the average annual <br /> <br /> <br />flood damage potential in dollars per year. <br /> <br /> <br />Figure 111-1 has a great deal of utility for calculating flood <br /> <br /> <br />damages of numerous alternative solutions. If a peak discharge scale <br /> <br /> <br />is constructed corresponding to the probability scale, flood damages <br /> <br /> <br />can be computed quickly for any sized detention facility. For example, <br /> <br /> <br />assume that a certain sized detention reservoir is being considered <br /> <br /> <br />just upstream of the reach under study, and that hydraulic studies have <br /> <br /> <br />determined that it will reduce the peak 100-year discharge from 20,000 cfs <br /> <br /> <br />to 14,000 cfs, the 2S-year discharge from 10,000 cfs to 7,000 cfs and <br /> <br /> <br />the 10-year discharge from 7,000 cfs to 3,000 cfs. By entering the <br /> <br /> <br />curve on the peak discharge scale we can determine the expected flood <br /> <br /> <br />damages in the reach downstream of the dam. The damages would be $90,000 <br /> <br /> <br />for the 100-year event, $65,000 for the 2S-year event, and $30,000 for the <br /> <br /> <br />10-year event. A new damage versus probability curve can be constructed, <br /> <br /> <br />and a new equivalent annual flood damage potential calculated. <br /> <br /> <br />If flood damages within the existing channel are small compared to <br /> <br /> <br />the total flood damages, then Figure 111-1 can be used to compute flood <br /> <br /> <br />damages of channelization alternatives. Basically, it is only necessary <br /> <br /> <br />to know the carrying capacity of the stream without channelization and <br /> <br />75 <br />