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APPBNDIX M viii <br />SMALL AREA SXHMPTION # 3 - 5A& 3 <br />Containment structures are in place to control the runoff of water. <br />and sediment from the pile of dirt on the mine bench. The bulk of <br />the runoff from the pile flows toward the existing sediment trap <br />near the mine fan, (See SAB-1). Water from the south and west sides <br />of the pile has been directed to another sediment trap at the <br />southwest corner of the pile. The sides of the pile that drain into <br />• the trap are approximately 150 feet long and 40 feet wide. The <br />trap has functioned well but has increased the saturation of the <br />soil at the crest of the outalope. This increase in soil <br />saturation has resulted in localized slumping of the west side of <br />the containment structure. <br />In accordance with applicable regulations, these calculations will <br />show that the revegetation of this portion of the dirt pile has <br />been successful in reducing the amount of sediment in surface water <br />runoff to below that for the undisturbed area near the pile. The <br />southern portion of the pile now exhibits a cover of over 80 ~. The <br />west side of the pile has over 60 k cover with a small area of about <br />20~ cover. These estimates of cover were discussed during a DGM <br />inspection of the pile. <br />SPBCIFICATIONS <br />Area = 10,400 agft = 0.24 acres Ave Slope = 40~ <br />Soil Type = mixed ; use Group B <br />Length = 150 ft Veg. cover = 45X <br />P$AH FLOW CALCULATIONS <br />From NOAA Atlas Pio = 1.7" Pzs - 2.1" <br />From "Peak Flows In Colorado" Figure S-3 for Sage-grass CN = 54 <br />RUNOFF 10 yr Pe= 0.00" 25 yr Pe= 0.09 <br />Since the runoff from the 10 year storm has been reduced to 0.00" <br />there will be no runoff from the area. For the adjacent Juniper - <br />grass undisturbed area with a measured cover of 43.3X ( see Volume <br />1; page 4-112) the CN = 61 and Pe = 0.03. <br />2. Sedimentation Controls <br />The existing drainage pattern to the sediment trap will be <br />maintained to direct the flow to the area of the existing <br />sediment trap. These drainage controls are well vegetated (>70X <br />cover) and will allow deposition of sediment from areas upslope. <br />Rather than totally containing the runoff from storms greater <br />than the 10 year - 24 hour event, runoff from the area will be <br />passed through a small depression and excess water will be passed <br />a silt fence. The sediment will be trapped behind the fence but <br />runoff will be allowed to drain. The controls will be more <br />effective since the outalope will remain intact and not be <br />subject to further slumping due to saturation of the soils. <br />M(viii) - 3 <br />6/29/95 <br />