Laserfiche WebLink
<br />The base flood should be within the range of discharges selected to <br />make the risk analysis, but it will not necessarily be the final design <br />discharge in any case. The base flood will not necessarily be the <br />largest flood considered in the r~sk analysis. <br /> <br />Freeboard <br /> <br />Freeboard is a common practice of adding elevation to the embankment. <br />Embankment freeboard is perceived as a safety factor to prevent occasional <br />saturation of the base course and to reduce expected traffic interruption. <br />While embankment freeboard does reduce expected traffic related losses <br />and perhaps expected highway damages, it increases expected losses due <br />to additional flooding of the upstream flood plain. <br /> <br />While freeboard is commonly added at the end in traditional design, <br />it is included in the trial designs for a risk analysis. In risk analysis, <br />expected traffic losses play a predominate part in the optimization <br />procedure; therefore, applying freeboard to the embankment would simply <br />be a means of introducing another trial design for consideration. <br />Occasional saturation of the base course, furthermore, is just another <br />component of expected highway damages. Although the occasional saturation <br />component was ignored for the examples in this report, it could certainly <br />be included at a designers discretion. <br /> <br />In summary, then, embankment freeboard has no meaning in risk <br />analysis since embankment elevation is one of the variables in the <br />analysis. Risk analysis provides the basis for selecting the most <br />economical elevation, and it provides the basis for evaluating the cost <br />of selecting an elevation other than the optimum. A designer can certainly <br />select an embankment elevation that is greater than the most economical <br />one. The biggest deviation from tradition is the design discharge which <br />is unique to the elevation that is finally selected. <br /> <br />Economic Analysis <br /> <br />Every assessable cost or damage is included in the economic <br />analysis. Individual costs can be examined in the decision making <br />process to determine whether they are reasonable or whether minor design <br />changes might be made to reduce risk further. An assumption that <br />influences the economic analysis is that damages will be repaired so <br />that they have the same opportunity to recur year after year. This <br />assumption means that the probability of a given risk is the probability, <br />p, that a given flood will occur in a year. Actually, there is chance <br />that a given flood may occur several times during the life of a structure. <br />The probability that a given flood occurs at least once during the life <br />of a structure is [1 - (1 - p)n] where n is the service life of the <br />structure. <br /> <br />6 <br /> <br />I <br />~~ I <br />