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<br />Table 11: Comparison of general storm PIVIP values based on technique used. <br /> <br />Metbod 1...hr 6-hr 1-2.br 24...hr [ 48-hr 72-br <br />55APlate 7.80" 17 .50" 27.50" 34.00" <br /> f-.:......_. <br />HMS . 1.28" 4.60" -13~59;;-. 20.48" <br />SSBEM. -19~W -- <br />HMS . . 6.68" 10.47" 26.35" <br />SSCOM -- <br />SSCOMI -14% -40% -29% -2:3% <br />Plate -- -- <br /> ---. <br /> 18~65" -- <br />HMS 3.90" 13.25" 16.25" 23.75" 24.30" <br />CSMPMP -- <br />CSM/Plate -50% -24% -3?~. -29o/~_ <br />CSMI -42% +27% -~j% -13% <br />SSCOM ----. <br /> <br />Total <br />34.00" <br />20.48" <br />26.35" <br />-23% <br />-. <br />24.30" <br />-29% <br />-.8% <br /> <br />Table 11 follows the logic of Table 10. The primary differences in Ilhe HMH 55A <br />methodology based PMP values (HMSBEM and HMSSSCOM) OCGur dUl'ing the <br />first six hours of the PMP event with only slight variations after the first six hours. <br />The 1-hour values of the SSCOM PMP are twice the HMSCSM values but the 6- <br />hour HMSCSM values are 27 per cent larger than the SSCOM values. HMS <br />recommends that the most stressing valuEl be adopted by the hydrologist for the <br />general storm PMP based on the first six hours of the event The final values of <br />the two PMP's are only 8 per cent different. More importantly from a design <br />standpoint, the first six hours of the general storm PMP produce more rain <br />than the 6-hour local storm PMP. This factor is consistent with the role of <br />complex convective oeneral storms in producingl the maioI' flash f1oodi!lQ. <br />rains. <br /> <br />The HMS decision to merge storm components is supported by findings of both <br />the historical storm search reported in this study and the results of the CSLJ <br />Extreme Precipitation Study. Both studies found that extreme rain events have <br />not occurred with totally saturated atmospheres but rather ones with very moist <br />conditions into the middle levels overlain by drier air aloft. This procedure is <br />consistent with the "among storms" PMP discussion in Section 10.1.1 in HMR <br />55A. Additionally the use of the updraft warm layer depth used in the CSM is <br />based on an empirical but reliable relationship for estimating the maximum <br />rainfall potential of an atmospheric structure. <br /> <br />The use of enhanced data sets and the improved CSM methodol09Y when used <br />in concert with the HMR standard methodology provides a valid framework for <br /> <br />43 <br />