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<br />Usefulness of Risk Analysis <br /> <br />Risk analysis provides a means to quantify in dollars the flood <br />hazard to the highway and the effect of the proposed highway to lives, <br />property, and stream stability. Since both risk and construction costs <br />depend upon the bridge and embankment, the sum of risk and the annual <br />construction cost (total construction cost multiplied by the capital <br />recovery factor) measures the true cost of the flood hazard to the <br />highway and the effect of the proposed highway to lives, property, and <br />stream stability. (Other costs such as land acquisition and engineering <br />design have not been inc1uded.in the analysis because they are assumed <br />to be constant one-time costs which are not usually subject to the flood <br />hazard.) The most economical design would theoretically be the bridge <br />and embankment with the lowest cost. <br /> <br />A risk analysis is a useful design analysis tool in that it focuses <br />the designer's attention on the risk to the structure caused by flowing <br />water and the risk to items such as traffic, property, and the river <br />channel caused by the structure in relation to the actual construction <br />cost. The designer may find that the least costly structure is one that <br />just spans the river at valley level and may be occasionally overtopped <br />by a low frequency flood. <br /> <br />Risk may be reduced by spending additional money on the highway <br />structure. For example, a longer bridge which spans the valley and <br />contracts the flood less will reduce the amount of backwater caused by <br />the bridge. This in turn will reduce the flooding of property which may <br />be located on the flood plain and which would not be flooded had the <br />bridge not been there. Risk analysis allows the designer to determine <br />the significance of such damages. When these damages are significant, <br />the designer may choose to adjust -the fill height, add spans to the <br />bridge, or perhaps use channel protection. The annual cost of such <br />additions should be computed and compared to the risk that they are <br />intended to prevent. The annual cost of additions in general, should <br />not exceed the expected cost dde to risk. For example, adding spur <br />dikes to a bridge may cost $5,000 or $354 per year (5 percent interest, <br />25 years life). Assume that an abutment washout would be anticipated <br />with a 5 percent (20 yr.) flood. The spur dike itself might fail in <br />a higher flood (say a 0.5 percent flood) but it effectively reduces <br />the probability of a washout from 5 percent to 0.5 percent (Ap = .045). <br />Furthermore the spur dikes will probably require repairs (say 0 loss <br />for a 5-year flood, 10 percent loss for a 10-year flood, and 100 percent <br />loss for a 200-year flood) which are equivalent to $312 year. <br />($5000 ((0 + .1)/2 (.2 - .1) + (.1 + 1.)/2 (.1 - .005) + 1.0(.005)) = <br />$312). The total annual cost of spur dikes is $354 + $312 = $666 which <br />is justified if the expected damages - cost of repairs, traffic losses, <br />and potential loss of lives - from an a abutment washout (without spur <br />dikes) exceeds $14,800 since $14,800(0.045) = $666. <br /> <br />9 <br />