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<br />,~ ( c!"... <br /> <br />Information on the Big Elk Meadows and Big Thompson storms is taken from HMR 55A. <br />Discussions are provided in Section 2.4.1.8, page 42, for Big Elk Meadow and in Section 2.4,1.9, <br />page 44, for Big Thompson. HMR 55A in Table 2.2, page 19, lists the elevations ofthe two <br />storms at 8,000 and 8,300 feet respectively and both storm discussions refer to rainfall over the <br />first upslopes ofthe Rocky Mountains. A W A has concluded that neither of these storms are <br />transpositionable to the Cherry Creek drainage basin because the terrain characteristics of the <br />regions where the storms occurred are dissimilar to the topography associated with the Cherry <br />Creek drainage basin. <br /> <br />The following statement was included in the Final Report PowerPoint presentation: <br /> <br />Both the Gibson Dam, MT storm and the Big Elk Meadows, CO storm occurred over and west of <br />the first ups lopes and are not transpositionable to the Cherry Creek basin. <br /> <br />29. Appendix G. Response to Doeskens Comment 7. Has the response been prepared? <br /> <br />A W A response to Ouestion 29. <br /> <br />It appears that this response was never completed. Mr, Doeskens' comment is provided below: <br /> <br />7) Can you offer some sort of confidence in the accuracy of the time distribution and hourly <br />rainfall rates you show in section 4,1. I don't recall being as confident as you about hourly <br />rates, particularly from the 1935 storms. Wefind that eye witnesses, without the benefit of <br />recording gauges, often unintentionally exaggerate hourly rates. Even an error of just 10 <br />minutes in how long very heavy rains lasted could be a 17% overestimate of hourly rainfall. <br />Since we did have pretty good estimates of the observed rainfall rates with the Fort Collins and <br />the Pawnee Creek Floods, we learned that those events did not have extraordinarily high rainfall <br />rates and yet produced major flooding. While I agree it is possible, we've never actually seen <br />hourly rainfall rates exceed 6" per hour for an entire hour. I guess this may help argue that rate <br />and timing information that you show may already be hedged on the conservative side. <br /> <br />A W A included the mass curves for all of the analyzed storms in the study. The most extreme <br />were used to construct the design storm mass curve, To our knowledge, there is not additional <br />information on extreme rainfall rain rates in Colorado. By using the most extreme rain rates (as <br />reflected in the storm mass curves), the rain rates provided in the study represent our best <br />estimate ofthe most extreme rain rates possible for Colorado storms and are consistent with the <br />storm based site-specific PMP approach used in this study. <br /> <br />30. Appendix H, A W A Presentation, page 8 & 12. Why is the temporal envelopment curve <br />different from that shown in the final report? <br /> <br />A W A response to Ouestion 30. <br /> <br />The composite storm mass curves shown provide the shape ofthe most extreme rainfall rates for <br />the historic storms analyzed. Figure 10.1 in the final report scales this mass curve shape to the <br />largest of the adjusted rainfall values from Table 10.1. Figure 10,2 was used to determine a <br />spatial envelopment factor and Figure 10.3 provides the Depth-Duration Envelopment Curve <br />