Laserfiche WebLink
<br />~,.~~ !'~~'" ' <br />#'''' ."",," <br />.'1 <br /> <br />I <br /> <br />DAMES 8- MOORE <br /> <br />-3- <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />,I <br /> <br />I <br />II <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />The Inflow Design Flood (IDFJc <br /> <br /> <br />With the above policy in mind, we'consider that the IDF should <br /> <br /> <br />be near the maximum flood calculated as being probable, that is, the <br /> <br /> <br />probable maximum flood (PMF). <br /> <br />We have checked the rationale of the USBR probable maximum <br /> <br /> <br />precipitation (PMP) together with their estimates of PMF. We conclude <br /> <br /> <br />that the basic approaches are correct; the principal differences between <br /> <br /> <br />our estimate and theirs are, we believe, due to the greater amount of <br /> <br /> <br />time we have had in reviewing the details. With less time available to <br /> <br /> <br />them, the necessary approximatiousby the USBR have been on the con- <br /> <br /> <br />servative side. The details of these differences are documented in the <br /> <br />attached report. <br /> <br />In considering the appropriate IDF for the Narrows Dam, all <br /> <br /> <br />of the facets involved should be considered in a systems context, i.e. <br /> <br /> <br />PMP, PMF, desi.gn of the dam and spillway, the risks to life through <br /> <br />fai.lure, and the safety factors ,inherent in the calculations. Included <br /> <br /> <br />in the design aspects is the possi.bility of use of a parapet wall to <br /> <br /> <br />serve as part of the freeboard. Each factor needs to be considered in <br /> <br /> <br />relation to the others, rather than in a step-by-step process i.n which <br /> <br /> <br />each value is determined separately. We know of no mathematical pro- <br /> <br /> <br />cedure that will allow an optimization of all of the factors involved <br /> <br /> <br />as tradeoffs bet"ecn costs and risks. Instead, we consi.der that only <br /> <br /> <br />judgment can be uS'ed to appropriately weigh the factors involved. <br /> <br />Taking i.nto account these factors with respect to safety of <br />the dam, we ~onclude: <br /> <br />1) <br /> <br />The clam should be designed for an IDF having a <br />volume of ],960,000 acre-feet (75 percent of <br />USER t s IDF). Thi..s corres!,onds to a PXP of 94 <br />percent of USER PHP, as given on Table 111-15 <br />in the bocly of the report. <br />