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Phase 1 Scope of Work <br />Design Analysis: <br />To determine the amount of run -of -mine rock that was placed within the drainage way of the LDG that <br />will be removed, an analysis was completed of the slopes and cross sections of the Gulch upstream and <br />downstream of the disturbed area. Cross sections were cut on a 25 -foot interval to determine <br />approximate side slopes of the Gulch in an undisturbed state as well as to determine the approximate <br />longitudinal grade prior to rock being placed within the Gulch. The existing profile of the Gulch and the <br />cross sections are shown in Appendix B, Figures 1, 2, 2A and 2B. <br />Based on this analysis, it was determined that the side slope on the west side of the Gulch was <br />approximately 4:1 and the side slope on the east side of the drainage way varies from approximately 2:1 <br />to approximately 8:1. The longitudinal slope of the Gulch flattens in front of the Chief Portal to about <br />15 %, with longitudinal slopes of approximately 25% upstream and 30% downstream of the portal area. <br />Using these parameters, the approximate configuration of the LDG would be as shown in Figure 4 — <br />Proposed Little Deadwood Gulch Grading (see Appendix B). It will be necessary to cut the area along <br />the channel per Figure 4 to remove the debris from the channel prior to placing the culvert in the channel <br />and grading the bench over the culvert. <br />A drainage analysis was completed for the LDG (at this point in the drainage way) to determine the <br />maximum flows that could be conveyed by a 24" culvert (vertical constraints prohibit a larger pipe). The <br />Little Deadwood Gulch has a tributary area of approximately 311 acres and a 10 -year and 100 -year, 24 <br />hour flow of 19.5 and 87 cubic feet per second (cfs), respectively. Using these numbers, a Culvert Master <br />calculation was completed and it was determined that a 24" RCP could convey 29.4 cfs. The remaining <br />run -off will overtop the pipe and flow across the bench. The bench area has been graded as a <br />trapezoidal channel with a 1 -foot depth and a minimum bottom width of 10 feet. Calculations were run on <br />a channel of these dimensions and it was determined that 113 cfs could be conveyed. During a 100 -year <br />event, 57.6 cfs would overtop the culvert, so there will be approximately 0.3 feet of freeboard during this <br />storm event. Drainage calculations are shown in Appendix A. <br />Construction of Improvements: <br />Prior to the start of construction, wetlands within the project area will be delineated and clearly marked to <br />meet the EPA order and remain in compliance with the Clean Water Act. After wetlands have been <br />delineated, construction will begin with installation of necessary runoff and erosion BMP controls as <br />shown on Figure 3 — Initial SWMP in Appendix B. Following installation of these controls, grading <br />operations will begin by removing debris from the LDG to meet the proposed grading shown in Figure 4. <br />It is estimated that approximately 550 cubic yards will be excavated from the LDG. As shown in Figure 5 <br />— Stockpile Areas, all of the excavated material to be stockpiled will be transported to May Day 3 for <br />storage and to construct the bench and to repair the highwall above the Chief Portal. The stockpile will <br />be surrounded with silt fence and, if it will remain undisturbed for more than 30 days, seeded and <br />mulched. <br />After the material has been removed from the LDG, a 24" RCP culvert will be installed as shown in <br />Figure 6 — Little Deadwood Gulch Culvert Plan & Profile. A bench will be graded over the top of the pipe <br />to provide emergency egress from the Chief portal. The proposed grading of this bench is also shown on <br />Figure 6. <br />After completion of grading activities, all disturbed areas will need to be stabilized to prevent erosion. <br />Rock Check Dams will be placed along the flow line of the channel above and below the culvert to slow <br />the velocity of run -off and prevent scouring. Seeding and mulch will be placed on all disturbed areas. <br />Due to the high probability that water will flow through this area, it is recommended that slopes greater <br />than 3:1 disturbed during grading activities be covered with erosion control blankets. Erosion control <br />measures are shown on Figure 7 — Interim SWMP. <br />Chief Portal Work Plan Page 3 <br />P: \Wmcduc \dwg \Eng \Final Drawings \DRMS TRs \b - Chief Portal TR \Chief Portal Work Plan- Rev1.doc <br />