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<br />Rockcastle Company, Grassy Creek Mine Adequacy Responeses • 3 <br />3) Please co»rntit to providing apost-gnrding certification of the pond and the over!}ping slope upon <br />completion of the project. <br />Resoonse: In compliance with applicable provisions of Rule 4.05.9(10) and 4.05.9(8), Rockcastle <br />will certify modification of Pond 4 with respect [o coosis[enry with design specifications. Given that <br />there are no regulatory requirements for certification of backfilling and grading activities, <br />certification of the backsloping component of the proposed work is not appropriate. <br />4) 77te open channel emergency spillway s/tows ripmp along the crest. The operator must provide <br />documentation that flow from the 25 year•24 hour event on the ourslope is non-erosive, or submit <br />designs to armor the spillway to protect the integrity of the embankment during a flood. (Rule <br />4.OS.tS(3)(d)). <br />Response: As shown on [he Pond 4 Design Map, (Map R-3), [he emergency spillway daylights to <br />natural ground. In this area, natural ground slopes off at approximately 2H:1V and there is heavy <br />ground cover consisting of existing natural vegetation. Any discharge through the Pond 4 <br />emergency spillway will be quickly dispersed by [he natural vegetation, moving downslope as <br />overland flow. <br />The calculated emergency spillway discharge velocity as shown on Map R-3 is 4.7 feet/second. <br />Given [he existing vegetative cover at the point where the emergency spillway dayGgh[s [o natural <br />ground and [he fact that flows beyond this point will be dispersed over the slope rather than <br />confined [o a Flow channel, [he discharge velocity should not be erosive and the need for armoring <br />or other protective measures is not indicated. <br />5) Provide designs for the Pit No. 4 irrtemtediate di[clt discussed on page 4 or commit to its removal <br />during the upcoming constrtalion season to show compliance with Rules 4.05.3(4) m:d 4.05.36). <br />Resoonse: Response 1C (Page 7) of the November, 1991 Adequacy Response submittal addresses <br />[he Pi[ 4 intermediate ditch and commits to it's removal. <br />6) Ba~eld, Warner, and Haarr's redbook indicates [hat fine sandy looms can tolerate velocities of 2.5 <br />fps from sediment laden waters. Therefore, al! portions of the ditches except 4-] A must 6e riprapped <br />to prevent erosion. <br />Resoonse: Given the relatively steep natural terrain in [his area, riprapping of essentially the entire <br />Ditch 41 and 4-2 channels would be very difficult and may no[ be necessary. Portions of Ditches <br />4-1 and 42 offer some natural armoring, with exposed cobble and gravel materials within the <br />existing Bow channels. There are also significant portions of Ditch 4-2 which have well established <br />vegetative cover. Flow velocities of up [0 5 fps are allowable for gravel and cobble materials <br />(Table 3-2, Barfield, et. al., 1987) and allowable flow velocities for vegetated channels range from <br />3-5 fps (Table 3-4, Barfield, et. al., 1987). <br />In conjunction with modification of Pond 4 and final redama[ion work in [he Pi[ 4 area, Rockcastle <br />proposes to evaluate current conditions for both Ditches 4-1 and 42 for individual channel <br />segments. Based on this information, Rockcas[le will determine appropriate erosion control <br />measures for each segment and present a proposed erosion control plan and supporting <br />documentation for [he Division's review and approval. Upon receipt of Division approvals, <br />appropriate erosion control measures will be implemented along with the required clean-up and <br />repair previously identified. <br />ACZ Inc. ' P.C. Box 774018 • Steamboat Springs, Colorado 80477 • (303)8736260 <br />