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e e e .""'.-.. '. File: PMPtopu:s799swnmary.doc J:>'724F-r July 21, 1999 This referred to items done in Hydro 45 that were not needed for the antecedent study. The time constraints did' not affect the quality of the study. Group agreed this was satisfactory answer. ,I 1 j. The region within which storms should have meteorologically homogeneous characteristics with storms that could occut over the Cherry Creek drainage basin is defined on p2 of the study. The eastern mo$t boundary of the region is 1 OJ. 9 degrees West longitude. In comparing this eastern limit with the 29 storms in deriving the within/without storm curves in HNfR J2, there is no overlap. This appears to be stating that any storms east of 10j.9 de"grees West are not to be considered to have meteorologically homogeneous characteristics with the Cherry Creek drainage basin location. The only Colorado storm used in within/without curves derivation in HNfR j2 (Hale, 193j) does not fall within this region. This indicates an inconsistency between this study and the site-specific Pll-fP study. (Tomlinson) Correct that the boundary is 104.3 degrees longitude. No consensus is reached by the group. The NWS will examine a way to verify this assumption v 16. NWS HYDRO 4j, dated January 1995, states on page 7j, Finding Jj that "By logical deduction and extension, the conclusion is that a reasonable and prudent antecedent precipitation associated with a 3- to j-day PMP event in the region of study would be 10-20 percent of the PMP withill a 31-day period centered on the day of maximum precipitation in the PMP storm for the region of study." Explicitly what caused the difference between this 10-20% and 32% provided in the Cherry Creek antecedent precipitation study ? (Tomlinson) We (US ACE) will investigate the possiblity of and impacts from revising the antecedent study. 17. Often dam regulators u~ the 1 ~O-year event, e.g. rainfall or snowmelt, as the antecedent condition. What is the return frequency of the 32% PMP event for the Cherry Creek drainage basin? If the PMP is seven times the 1 DO-year rainfaIl, that should make the antecedent rainfall 32% of seven or 2.24 times the 1 DO-year rainfall values. How does this compare to the antecedent rainfaIl used for other Corp Projects, in particular Cochise and Elephant Butte in New Mexico? (Tomlinson) The USACE guidelines state an antecedent flood of 50% of the PMF with a 5-day drawdown or a half full flood control pool (which ever is more appropriate) is ," . . . . T0pb:31'?21.JIIiy l?99TeduricaI M<<tbIg .:"'.: .~~.~~~~iM~~;~~,.:..~-.~~: -: . .:';r::" 4 --.......'.-