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~~ <br />III IIIIIIIIlIIIIIII <br />MINED LAND RECLAMATION DIVISION <br />Department of Natural Resources <br />1313 Sherman St., Room 215 <br />Denver, CO 60203 <br />303 866-3567 <br />Fnx 303 6328106 <br />OF COO <br />R~ ,9 <br />~e%`~ <br />• '~~r <br />s <br />3 re]6 ~ <br />Roy Romer, <br />Governor <br />Fred R. Banta. <br />Division Director <br />DATE: April 16, 1991 <br />T0: Grassy Gap File <br />FROM: Cathy Begej C~~ <br />RE: Grassy Gap Drainage Plan, February 6, 1991 <br />The Grassy Gap Drainage Plan submittal consists of a response to questions <br />3 and 7 of the December 19, 1990 adequacy letter and two attachments covering <br />remediation of access road drainage at Pit 5 and documentation of hydraulic <br />calculations. Many of the following adequacy questions result from <br />Rockcastle's failure to provide post-mining topography mapping of Pits 5 and ~ <br />and their failure to design the hydraulics to handle the 100-year 24-hour <br />event. <br />Questions derived from Response No. 3: <br />1. Rockcastle/ACZ is proposing to allow the upper diversion ditches on <br />Pits 4, 5 and 6 to silt in. As the Division indicated in the October 23, <br />1990, engineering study review, this proposal is unacceptable per <br />Rule 4.05,3(5), The Division does not agree to the concept associated <br />with such reclamation activities 'significant, unwarranted disturbance', <br />Rather, we feel that directed reclamation activities will accelerate the <br />revegetation at the site. <br />2. There is no discussion about the fate of the lower diversion ditch <br />immediately above Pit 6 which was noted as part of NOV C-89-034, item 1A. <br />Please provide complete documentation. <br />3. The contour ditch (No, 5, Figure 1, Attachment 1) is not mapped on the <br />Reclaimed Topography Map. Please map this ditch on a representation of <br />the topography at the site and size it for a 100-year 24-hour event per <br />Rule 4.05.4(2)(b). Revise its orientation as shown on Figure 2 of <br />Attachment 1 to avoid a right angled turn to the east as such a sharp <br />angle would probably blow out prematurely. <br />4. Please provide designs for the rock check dams proposed for the Pit 5 <br />access road ditch (Rule 4.05,3(6)(a)). Provide information on the <br />structure, the size of the rock to be used, its porosity and an analysis <br />of the flow velocities through the drainage. <br />5, The Division would prefer to see Ditches 2-1 and 2-2 reclaimed, but we <br />recognize that paragraph IV (3) of the June 26, 1990 Agreement allows the <br />retention of drainage structures supporting the pond. However, all <br />permanent drainages must be sized to handle the ]00-year 24-hour event <br />per Rule 4.05,4(2)(b), <br />STATE OF COLORADO <br />