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<br />25 <br /> <br />1, Natura I type channel (soft channel) <br /> <br />2, Lined channel (hard channel) <br /> <br />3, New Condu it <br /> <br />4, No Change <br /> <br />Each of the alternatives (except No, 4) can be installed for <br /> <br />any design frequency, The lowest cost system for the range of <br /> <br />desired design frequencies can be identified by the following <br /> <br />analysis, <br /> <br />Soft channel, hard channel or conduit designs increase in cost <br /> <br />with design frequency, The variation in annual cost, including <br /> <br />maintenance, might be as shown in Figure []-5. <br /> <br />100 <br /> <br /> <br />PO <br />o <br /> <br />400 <br /> <br />, <br />-- <br /> <br />300 <br /> <br />I- <br />en <br />o <br />'-' <br /> <br />..J <br /><t <br />::l <br />Z <br />z <br /><t <br /> <br />200 <br /> <br />o <br />o 0.1 0.2 0,3 0.4 0.5 0,6 <br /> <br />EXCEEDANCE PROBA131 LIT Y <br /> <br />Figure 11- 5 <br /> <br />This shows that the soft channel affords lowest annual cost* <br /> <br />protection below a 0,27 exceedance probability whereas hard channel <br /> <br />is lowest for all other cases, <br /> <br />*Determining annual cost involved applying the appropriate discount <br />factor to the present cost of the improvements, See any engineering <br />economics text for details, <br />