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1 <br />1 <br />The calculated capacities of the storm drain culverts under I-70 are: <br />Description Capacity, cfs <br />North Culvert 56.55 <br />South Culvert 15.18 <br />These culvert capacities compare to the following estimated runoff flows to each culvert: <br /> <br />Description Runoff, cfs <br />North Culvert 15.47 <br />South Culvert 18.02 <br />The runoff for the surface facility area prior to the pit being established in the Dry Creek mining <br />area will be collected by the road borrow ditches and directed to a detention pond on the east end <br />' of the surface facilities. (See Exhibit SF-1) The cross sectional area for the borrow ditch on the <br />south side of the road is: <br /> <br /> <br />1 <br />Description Feet <br />Bottom of Ditch 2.00 <br />Depth of Flow 0.78 <br />Ditch Width - Top of Flow 5.90 <br />Ditch Width -Top of Ditch 7.40 <br />Freeboard 0.30 <br />Ditch Side Slope 2.5:1 <br />Velocity, fps 3.2 <br />' The ditch should be lined with riprap with a Dso = 6 inches. The dimensions above are inside <br />dimensions from the riprap surface. An alternate erosion control method would be the use of <br />' check dams on 150 feet centers. <br />The cross sectional azea for the borrow ditch on the north side of the road is expected to be a <br />V-Ditch that is 1 foot deep to handle runotTfrom the north halfof the surface facilities road. <br />1 <br /> <br /> <br /> <br /> <br />The calculated capacity of the detention pond for the three storm events is: <br /> Volume, cu. fr. <br /> 10 yr - 24 hr 25 yr - 24 hr 100 yr - 24 hr <br />Storage 5,587 7,988 11,361 <br />Runoff 15,383 24,800 35,340 <br /> <br />