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<br />Thc concurrent peak flow on Lost Canyon Creek, about 820 cfs. is more frequent than a <br /> <br />5% chance event and not in conflict with the rainflood frequcncy curve on Plate 22. <br /> <br />'Ilte IIMS model with thc 1% West Fork centering produces reasonable results that are in <br /> <br />agrecment with thc flow frequency curves. The now frequcncy analysis validates the rainfall <br /> <br />runon- model. <br /> <br />9, I % Peak Flo\\o' Comparison <br /> <br />Tablc 18 prcsents a comparison bctwecn the 1 % pcak flow frcqucncy values devclopcd for the <br /> <br />Rico and Dolores gage locations for this flood insurance study, for the 1976 flood hazard <br /> <br />infomlation study, and flow frequency analyses done for othcr federal agencies (Reference 4-m <br /> <br />and 4-0). Thc 1% pcak llow value of 14.500 cfs uscd in the 1976 hydrology report for Dolores <br /> <br />River below LosI Canyon Creek was computcd by combining the rainflood and snowmelt <br /> <br />frcqucncy curves statistically. using thc Addition Rule for indcpcndent cvcnts: <br /> <br />Pa = Pr+ Ps- (Pr '" Ps) <br />where Pa = annual excecdcncc probability ofpcak flow for combined populations, <br /> <br />I)r = annual exccedencc probability of peak flow in rainflood season. <br /> <br />Ps = annual excccdence probability of peak flO\v in snowmelt scason, <br /> <br />and thc rainllood and snowmelt peaks arc indepcndent of each other, that is. neither c\"ent <br /> <br />influences the other. Ilov..'c\"cr. combining !low frequency statistics is not appropriate for the <br /> <br />2004 study. becausc the pcak rainllood curve incorporates rain-on-snow events that have occurred <br /> <br />during the sno\vmelt season of April through July. Most of the peak flows during sno\\1llclt <br /> <br />season occur in ~lay. Se\"craltimes in the record. a rainfall cvent toward the end ofsnowmclt <br /> <br />season has caused a high peak l1ow. with snowmelt comprising a small part of it, as in 1927, <br /> <br />35 <br />