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Revised, January 5, 1994 <br />TECHNICAL REVISION REQIIEBT FOR THE WEBT PIT POND <br />HYDROLOGY/BEDIMENTOLOGY AND POND DEBIGN IIPDATE <br />• <br />The enclosed SEDCAD Hydrology and Sedimentology modelling <br />run for the West Pit Pond is to demonstrate the adequacy of the <br />pond design during the period of initial topsoil stripping in the <br />watershed above the pond (a worst case scenario). <br />It is intended that these enclosed pages replace the like <br />pages submitted prior in the Technical Revision (TR) request. The <br />current West Pit Pond information can be found in the Colowyo <br />Permit C-81-019, Volume 3, Exhibit 7 Hydrology Information, Section <br />8 Design of Surface Runoff Control Structures Permit Revision 1991. <br />The following is a discussion of changes made in the <br />SEDCAD run and the basis for the Control Practice (CP) Factor. The <br />SEDCAD General Input Table now shows two separate Particle Size <br />Distributions, one for Topsoil (used prior) and a new one for <br />Subsoil. The distribution for Subsoil was derived in part from <br />information contained in the Colowyo Permit, Volume 5, Exhibit 19, <br />Geotechnical Investigations West Pit Fill by CTL/Thompson, Inc. In <br />their report, page 10, test borings for foundation conditions state <br />"Gradation test results indicated the soils classified as a sandy <br />clay." Boring TH-6 Gradation Test Results, Figure B-5, was used in <br />part to develop the Particle Site Distribution used for the SEDCAD <br />Run. <br />Note also that the stripped area acreage (SWS2) was <br />reduced to 124.3 acres (with corresponding change made to <br />hydrological modelling map) to insure that all runoff is contained <br />within the watershed. With this change in acreage, the Runoff <br />Volume is reduced to 10.26 ac-ft. <br />The CP Factor calculation for the topsoil stripped area <br />is part of the enclosed information. The Sediment Volume <br />Calculation, also enclosed, comes from the SEDCAD sediment tonnage <br />run. The calculated sediment volume of 1.18 ac-ft. is extremely <br />conservative especially when compared to site specific data found <br />in the BLM EMRIA Report for this exact site and watershed. <br />The 124 acres of stripped area represents the maximum <br />area that Colowyo will be allowed to have completely stripped of <br />topsoil at any one time during this period of initial topsoil <br />stripping, if the West Pit Pond is the sole structure for water <br />runoff and sediment containment. Topsoil stripping will only occur <br />within the watershed above the West Pit Pond. <br />In accordance with Rule 4.05.9(2), the West Pit Pond will be <br />considered a temporary impoundment and shall be designed and <br />constructed in accordance with the requirements of Rules 4.05.6(2), <br />• (3)(d), (3)(e), (5), (6), and (8) through (12). Upon completion of <br />the construction of the pond, a certified as built drawing will be <br />forwarded to the Division. The pond will be added to the quarterly <br />pond inspection list. <br />. ~ R a~ <br />