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Mountain Region, though a small portion (3-48) lies within the <br />Northwest Region (area below elevation 7,500 ft.). Peak flows <br />• were calculated using a mixed population method which <br />incorporates characteristics from both regions. The effect of <br />the area of watershed below elevation 7,500 ft. was <br />insignificant. The peak flows tabulated and shown in Table H-1 <br />were calculated for the Mountain Region only. These values are <br />conservative because the snowmelt controlled peak flows expressed <br />by the regressions for the Mountain Region in both Method 1 and 2 <br />yield slightly higher peak flow values than the mixed population <br />method. Peak flows ranged from 23 cfs for a 2-year recurrence <br />interval (Method 2) to 135 cfs for a 500-year recurrence interval <br />(Method 2). <br />2.3.2 Portal Subbasins <br />The subbasins affecting the proposed portal bench have been <br />identified on Figure H-1. The subbasin consists of the <br />undisturbed area draining to the diversion ditch, the undisturbed <br />area draining to the portal bench, the cut slopes, portal bench <br />and portion of the haul road draining to the sediment pond and <br />the two road fill slopes. The drainage areas and design points <br />are shown in Table H-2. These areas are based on currently <br />projected bench and fill design parameters and will be slightly <br />varied as a result of final design. <br />• The runoff and sediment calculations used as design <br />parameters for the portal bench and sediment pond design have <br />been described in a separate report (Golder, 1986). This report <br />describes the diversion ditch hydrologic and hydraulic <br />parameters. <br />The diversion ditch location as described in this report is <br />located on the old road grade above (west) of the portal bench to <br />minimize additional disturbance to the natural ground. Selection <br />of this location is preliminary and will be varied slightly in <br />other reports and final design. However, because of the small <br />drainage area involved and the low runoff potential, these <br />variations will cause no significant changes in the runoff values <br />shown below. <br />The method used to calculate a peak flow rate for the area <br />above the proposed diversion ditch is the SCS "Peak Flows in <br />Colorado", 1980. The calculation is summarized below: <br />P10/24 = 1.9 inches; <br />Runoff = 0.04 inches at CN = 60; <br />Area = 13.2 acres; <br />• Runoff = 16.2 cfs/inch at CN = 60; <br />410/24 = 0.65 cfs. <br />3 <br />