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• Runoff Curve numbers assigned to the undisturbed and reclaimed lands in various stages <br />of reclamation have been selected in accordance with Table 1 in "Methdologies and <br />Assumptions for Sedimentation Pond Design Evaluations" in Exhibit 7 of TR -73. It <br />should be noted that for the reclaimed lands, these runoff curve numbers are higher than <br />those previously used on the Colowyo Mine site, generating proportionately greater <br />runoff volumes and flow rates than those used in the original design of East Taylor <br />Sedimentation Pond. Soil erodibility factors and other parameters used in estimating <br />sediment concentration in runoff are also drawn from that exhibit. <br />• <br />For the intermediate (active mining) scenario, with approximately 60% of the watershed <br />intercepted by the active mine pit, the 10 year, 24 hour storm generates 9.7 acre feet of <br />runoff. The peak pool elevation does not engage the emergency spillway, and the <br />settleable solids concentration in the primary spillway effluent from the slotted riser at 0.0 <br />ml /l is well below the 0.5 ml /l standard. The 25 year, 24 hour storm does engage the <br />emergency spillway, but is routed through the structure at a depth of 0.7 feet in the <br />emergency spillway, leaving 1.3 feet of freeboard to the dam crest, which is in accordance <br />with the standards for a dam with this size and hazard rating in accordance with the <br />Colorado SEO regulations cited by reference in Rule 2.05.3. <br />For the full watershed post- mining case, the 10 year, 24 hour storm generates 14.3 acre <br />feet of runoff at the sediment pond location, after accounting for the interception of part <br />of the runoff volume in the upstream sumps previously described. The peak pool <br />elevation does not engage the emergency spillway, and the settleable solids concentration <br />in the primary spillway effluent (slotted riser) is also well below the 0.5 ml /l standard. <br />The 25 year, 24 hour storm does engage the emergency spillway of the as- constructed <br />dam, and would route through the structure at a depth of 1.1 feet in the emergency <br />spillway, leaving 0.9 feet of freeboard. This is slightly less than the 1 foot requirement <br />incorporated by reference from the Colorado SEO standards. However, this condition <br />(full watershed reporting) does not occur until several years in the future. Prior to that <br />time, minor modifications can be made to the spillway system, including but not limited <br />to lowering the overflow of the primary spillway by one foot and lowering the invert of <br />the emergency spillway 0.5 feet as needed to achieve 1 foot of freeboard when passing the <br />design storm runoff. The SEDCAD runs presented herewith are based on the assumption <br />that these modifications will be made to the spillway systems before the entire watershed <br />reports to the structure. With these modifications, as demonstrated in the attached <br />SEDCAD runs, the East Taylor Sedimentation pond meets the settleable solids standard <br />and satisfies the spillway freeboard requirements of the State Engineer's Office standard <br />incorporated by reference from Rule 2.05.3. It may also be possible to re- evaluate the <br />runoff Curve Numbers for this watershed if the response of the watershed to a few large <br />storms can be measured during the interim period. If a reduction in runoff curve numbers <br />can be justified, these physical modifications to the dam would probably not be necessary. <br />Exh. 7 -ET -3 Revision Date: 12/31/10 <br />Revision No.: TR -81 <br />