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<br />. <br /> <br />F, DISCUSSION Al'ID CONCLUSION <br /> <br />To ensure the validity of the results, the study calculated and compared the flow from Vallejo Gulch <br /> <br /> <br />and Spar Gulch using different models and different methodologies, Table 16 presents the results of <br /> <br /> <br />this analysis, Table 17 provides many of the input data values that were used in the analysis, The data <br /> <br /> <br />between models was kept as consistent as possible, although it did vary slightly in some cases due to <br /> <br /> <br />model limitations, <br /> <br />In Vallejo Gulch, the CUHP/SWMrvI model produced a peak flow forthe 1 DO-year, 2-hour storm that <br />was slightly less than the peak flow produced by FLO-2D and by HEC-1 using the Green-Ampt and <br />Holtan loss equations. It appears thatthe uniform loss method produces amuch higher flow rate than <br />the other methods, This is probably due to the other methods having a high initial infiltration rate that <br />decreases over time. <br /> <br />In Spar Gulch, the CUHP/SWNIM 100-year, 2-hour peak flow is very similar to that generated by <br /> <br /> <br />HEC-I, butthe flow generated by FLO-2D using the same infiltration methodology is much less (532 <br /> <br /> <br />cfs versus 340 cfs), This large difference in flow may be due to FLO-2D's interpretation of the <br /> <br /> <br />direction of flow off of the alluvial fan. FLO-2D may route some of the flow into a different <br /> <br /> <br />watershed, <br /> <br />An anaIY6is was also performed to determine the effect that the uniform loss rate has on the. peak flow <br /> <br /> <br />generated by a rain on snow event. If the ground is assumed to be frozen, the infiltration rate would <br /> <br /> <br />be low (i.e. 0,05 inches per hour), Typically though the ground beneath a snow pack is unfrozen. The <br /> <br /> <br />layer of snow provides an insulation layer from the cold air, and the heat from the earth rises to warm <br /> <br /> <br />the ground near the surface, In this situation the infiltration would be higher (i.e, 0,5 to 0,85 inches <br /> <br /> <br />per hour), As would be expected, the results show that the lower the infiltration rate, the greater the <br /> <br /> <br />rate of runoff, although the increase is not large, <br /> <br />It appears that FLO-2D could be an importanttool for the City of As pen to identify "high risk" areas <br />that may be subjectto flooding and/or mudflows and mud floods, The analysis shows the effect that <br />buildings have on the depth of flow of the runoff on themselves and their neighbors, With further <br />refinement, the FLO-2D model may also be useful in designing mitigation measures, <br /> <br />Snowmelt does not appear to pose a risk to property or life in Aspen, although it probably causes a <br /> <br />23 <br />