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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />1 <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />Engineers' Phase I Dam Inspection Reports for Comanche and Peterson Reservoirs <br />stated that studies on mountainous and forested areas in Colorado have used an <br />infiltration rate of 1 inch per hour. A slightly more conservative value of <br />0.6 inches per hour, in the moderately slow category, was selected for this <br />investigation. The final infiltration curve is shown in Figure V;3, along <br />with the remaining selected soil parameters. <br /> <br />The probable maximum precipitation (PMP) for Twin Lakes Reservoir was obtained <br />from NOAA Hydrometeorological Report Number 55. The Twin Lake Reservoir <br />drainage basin lies just outside the area shown to potentially be controlled <br />by the local (thunderstorm) PMP. Consequently, both the general and local <br />storms were included in the analysis. The local storm one-hour rainfall value <br />of 9.9 inches exceeded the general storm value of 9.5 inches. Although the <br />six-hour general storm PMP estimate of 14.4 inches exceeded the local stOnl) <br />value of 13.4 inches, for a small basin of less than one square mile, the one- <br />hour rainfall will likely determine the peak discharge. Consequently; the <br />local storm PMP was used to simulate the probable maximum flood (PMFl. The <br />PMF runoff hydrograph had a peak discharge of 8,420 cfs with a volume of 465 <br />acre-feet. The HEC-l computer pri ntout for the PMF runoff hydrograph is <br />contained in Appendix D. <br /> <br />An Interagency Hydrometeorological Study Team is in the process of updating <br />HMR 55. Preliminary PMP maps have been prepared, and values taken from these <br />match the base values used to obtain the local storm figures given above; It <br />is expected that there will be changes to the fi gure in HMR 55 used to make <br />the elevation adjustments. Because all the maps, figures, and procedures have <br />not been finalized, it is not possible to determine how the updated <br />information will affect the PMP and PMF. From the magnitude of the PMP values <br />obtained from the preliminary maps, it appears that there will be little <br />change in the local storm PMP estimate or the PMF. <br /> <br />3;2 Flood Routing <br /> <br />The exi sting condi ti ons at Twi n Lakes Reservoi r i ncl ude a 11 O-foot natural <br />overflow section at elevation 9295 gage height 19.1). The dam crest, <br /> <br />-57- <br />