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<br />River Mec:henics end Geomorphology <br /> <br />e <br /> <br />TABLE 2. 2 <br /> <br />Example Jlpplication Using Discharge Frequency Data to Evaluate Least Costly <br />Atternativ. , <br /> <br />An.neti". Deolgn F.....uency Aret "nn...' <br />Hum..... Dledl_ (y.....) Coet Coeta <br /> (ct.) ($1000) ($1 000) <br />1 1000 5 100 20 <br />2 3000 25 450 18 <br />3 10000 100 2100 21 <br /> <br />This simp ~fied example uses only the discharge frequency relationship and first cost of <br />the project to detennine the annual cost of each alternative. It assumes that if a llood <br />greater tha design occurred, the project would have to be completely rebuilt Normally, <br />a more de laiIed economic analysis would have to be conducted to include estimated <br />project benefits, inllation, interest, and operation, maintenance, and replacement costs to <br />obtain the optimum alternative. This simplified example iUustrates that the least costly <br />project would be alternative 2, which has a design discharge of 3000 cfs. From the <br />discharge frequency relationship, it was detennined that a discharge of 3000 cfs would <br />be exceed,:d on the average of once every 25 years. Over a 100 year period, the project <br />would ha'le to be reconstructed 4 times, each time at a cost of $450,000 for an <br />annualized cost of $18,000 per year. <br /> <br />2.4.3 Use 01 Flow Duration Inlonnallon . <br /> <br />Row dur:ltion relationships are useful in sediment and erosion studies to estimate <br />sediment yield and average annual transport capacity. Combining the lIow duration <br />curve wit II a water discharge- sediment discharge relationship, yields a sediment <br />discharge duration curve. Integrating the area under a sediment discharge duration curve <br />results in the average annual sediment that is transported by the stream. A sample <br />applicatiOil is presented in Table 2-3. <br /> <br />As showr in the preceding table, columns (1) and (2) represent the lIow duration <br />relationsh 'p, while columns (2) and (3) represent the water discharge-sediment <br />discharge relationship. Column (4) is the incremental area under the sediment duration <br />curve which is computed as the incremental percent of time multiplied by the average <br />sediment ,lischarge over that interval. As shown in table 2, the average annual sediment <br />discharge is 559.691 tons per year. It also shows that for this example only 3 percent of <br />the sediml:nt is transported by Dows exceeded less than 50 percent of the time, while 62 <br />percent of the sediment is transported by flows exceeded only 5 percent of the time. <br /> <br />. <br /> <br />12 <br /> <br />Coloradl) Erosion Control Manual <br />