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RULE 4 PERFORMANCE STANDARDS <br />The sedimentation ponds at Colowyo are designed to treat the 10 -year, 24-hour storm event in accordance <br />with Rule 4.05.6(3)(a). As such, the general operation of the ponds will be a passive discharge system <br />where water is allowed to discharge automatically as necessary. Colowyo will sample discharges as <br />appropriate to remain in compliance with applicable CDPS Permit requirements. Pond dewatering <br />through a manual headgate may be performed as necessary to lower the water level depending on <br />operational requirements. Manual dewatering of ponds will meet applicable CDPS Permit standards. <br />Discharge from sedimentation ponds will be controlled by energy dissipaters and flow check devices <br />where necessary. All embankment ponds utilize separate principal and emergency spillways with the <br />emergency spillway located at a minimum of 1 foot above the elevation of the maximum water surface <br />during the discharge of the 10 -year, 24-hour storm event through the principal spillway. The principal <br />spillways are designed for the 10 -year, 24-hour storm event and the emergency spillways are designed to <br />pass the 25 -year, 24-hour storm event in accordance with Rule 4.05.9(2)(c)(i). The design requirements <br />for existing ponds can be found on each of the pond as -built drawings or in Exhibit 7, Item 15. All <br />sedimentation ponds will provide a non -clogging dewatering device or conduit spillway to remove water <br />storage from inflow. Design requirements for all sediment ponds associate with the Collom Pit be found <br />in Exhibit 7, Item 23. <br />4.05.8 Acid-forming and Toxic -forming Spoil <br />Acid forming materials do not exist within the overburden to be removed by the mining operations. A <br />discussion on the overburden at the Colowyo operation has been conducted as set forth in Section 2.04.6. <br />A discussion of the overburden monitoring plan is set forth in Section 2.05. <br />4.05.9 Permanent and Temporary Impoundments <br />Colowyo constructs small impoundments on reclaimed areas in accordance with Rule 4.05.9. These <br />small impoundments are essential and basic to the management of the rangeland post -mining land use of <br />livestock grazing and wildlife habitat. The design of post -mining impoundments provides for structures <br />having a vertical height less than five feet from the bottom of the channel to the bottom of the spillway <br />and impound less than two acre-feet of water. As such they are exempt from Division of Water <br />Resources, Office of State Engineer requirements. Water harvesting ditches may also be used to enhance <br />the function of the impoundments, which is consistent with practices employed on adjacent rangelands. <br />The impoundments collect surface runoff from precipitation events and snowmelt from reclaimed areas. <br />The impoundments do not result in the diminution of the quality or quantity of water for downstream <br />water users. Colowyo is the holder of water rights immediately downstream. During periods of low <br />precipitation, the impoundments may be dry, which is consistent with regional practices on similar <br />rangelands. Since the source of water is surface runoff from revegetated areas the quality of the water <br />will meet the requirements of the intended use. <br />The post -mining impoundments have slopes of 3h:Iv or less to provide easy access to both livestock and <br />wildlife. These impoundments and any associated ditches, while intended to be permanent, will be <br />classified as temporary until the requirements of Rule 4.05.9 are met. Prior to construction, all designs <br />are submitted to the Division. A copy of the as -built design information will be submitted after <br />construction for inclusion into the permit document. In addition, sedimentation ponds that are <br />subsequently approved as part of the post -mining land use, as shown on the hydrology Maps, 41A and <br />4113, will remain as permanent impoundments after the requirements of Rule 4.05.9 have been met. <br />Please refer to Section 4.05.6 for additional information regarding compliance with Rule 4.05.9(7)(a thru <br />e). <br />Rule 4 Performance Standards 4-12 Revision Date: 8/28/17 <br />Revision No.: TR -115 <br />