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Flatirons, Inc. — Surveying & Engineering <br />Sediment Areas <br />Three impacted areas will require sediment control for contact storm water. The required <br />volume for sedimentation is equal to the 10 -year, 24 -hour storm. The sedimentation <br />volume was calculated using the NRCS method. <br />The Berm Contact area near the northwest corner of the TSF is completely contained in <br />Basin A. The peak runoff and runoff volume estimates for the Berm Contact channel are <br />derived from Basin a2. Runoff from Basin A will be directed to the Tailings Storage <br />Facility. The TSF will serve as the sedimentation pond for Basin A. The calculated <br />sedimentation volume for Basin A is 11,944 cubic feet. The available volume in the TSF <br />will be controlled by leaving the level of tailings and/or process water in the TSF lower <br />than the ultimate fill level. The 10 -year, 24 -hour produces a runoff volume of 11,944 cubic <br />feet, or 0.274 acre -feet. <br />Sediment control for the for the West TSF Contact area will handled in a series of 4 <br />shallow, flat areas at the toe of the slope on the exterior embankment on the west side of <br />the TSF. The peak runoff and runoff volumes for the West Contact channel are calculated <br />using Basin al. The flat areas are designated Sediment Basins 1 -4 on the drainage map. <br />The four sediment basins cover an area of 3800 square feet. The sediment basins average <br />0.5 feet in depth and create 1900 cubic feet. The required sedimentation volume for the <br />West TSF Contact Area is 153 cubic feet. <br />Sediment control for the South TSF Contact Area is handled in the channel at the toe of the <br />TSF embankment. Peak runoff and runoff volumes are derived from Basin b 1. The <br />channel is oversized to allow for sedimentation storage and pass the 100 -year peak runoff. <br />15 -inch high check dams will be placed in the swale at 25 -foot intervals. Each check dam <br />in the 2% sloping swale will contain 50 cubic feet. 15 check dams will be placed in the <br />swale and create 750 cubic feet of storage. The depth of the 100 -year peak flow over the <br />top of the check dam is 0.16 feet, and when added to the 1.25 -foot high check dam, the <br />water depth in the South TSF Contact channel is 1.41 feet. The freeboard of the channel is <br />0.59 feet. The required sedimentation volume for the South TSF Contact Area is 517 cubic <br />feet. <br />Union Mill 10 March 5, 2014 <br />