Laserfiche WebLink
<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 />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br />I <br /> <br />~"._, ,~, -'" <br />r', l!:'r~-_H'''- <br />! <br /> <br />-6- <br /> <br />DAMES 8- MOORE <br /> <br />These actual occurrences emphasize that it is extremely hazardous <br /> <br /> <br />to base a prediction of the probable maximu~ flood peak solely on previous <br /> <br /> <br />flood history. So also do the comparisons of peak floods and peak volume <br /> <br /> <br />with historical floods, given later as Plates 111-2 and 111-3. The entire <br /> <br /> <br />range of conditions that affect flood peaks, including basin topography <br /> <br /> <br />and shapet experienced storms, dewpoints, winds, snow accumulations, <br /> <br /> <br />temperatures, and ability of the,soil to absorb water, must be considered <br /> <br /> <br />in the estimation of the probable maximum flood peak. This has been done <br /> <br /> <br />in the analyses both the USBR and we have made. <br /> <br />Thus, while the 'P~W estimate would be an extremely rare event <br /> <br /> <br />(as it should be), it has the potential of occurring. Therefore, if one <br /> <br /> <br />should choose to take risks, he should not start with trying ro reduce <br /> <br /> <br />the PM]". He should, instead, recognize the Pill' as a ceiling and then <br /> <br /> <br />say how far below that ceiling he is willing to venture and take risks. <br /> <br /> <br />In the case of Narrows, because of the risk to human life, we recommend <br /> <br /> <br />no significant risk, choosing an 1DF that is within the confi.dence, band <br /> <br />of PMF. <br /> <br />Risk of Damage to Spillway <br /> <br />Design Philosophies <br /> <br /> <br />-The acceptance of risk of damage to -the structure in1lolves a <br /> <br /> <br />different philosophy than risk of failure of the dam. !loch we and the <br /> <br /> <br />.' USBR are in agreement 0!1 the philosophy of, failure --' it should not be <br /> <br /> <br />accepted as a possibility.' We tend to disagree~ but in der;ree only, on <br /> <br />acceptance ('If damage. to the spilh....ay through floods T:hat do root endanger <br /> <br /> <br />the integrity of the d"am. This difference i.s a matter of cl2sicn <br /> <br /> <br />philosophies, but involves considerable costs. <br /> <br />The USER has accepted in iC3 design an auxili.'1ry overflow <br /> <br /> <br />spilll,ay that wi1.1 oeprate very rarely (perhaps once ill 1000 ye,,[8), in <br /> <br /> <br />conjunc.tion with a gated service spilJ.yay that Hill operate more <br /> <br /> <br />fr.(~quently. Out- discussions with the -USBR indi.cate that -they arc "not <br /> <br />, <br />