2011-06-06_REVISION - M1984014 (7)

Home Browse Search

JD-8 Mine - Drainage Design Plan 7 • 2.4 Existing Stormwater Retention and Routing Capacity Runoff from the eastern areas of the mine is currently routed to the constructed catchment (Zone 6 in Figure 2). The current working capacity of the constructed catchment is approximately 0.08 acre-ft (3,400 ft3, 25,000 gallons). The constructed catchment can be expanded in the existing area to a volume of 0.32 acre-ft (14,080 ft, 105,000 gallons) using the available 8,050 ft2 footprint shown in Figure 2 with 5H: IV side slopes and a 2.5 ft total depth (Table 6). The 14,080 ft3 volume would contain 86% of the 100-year storm. The remaining storm volume would be conveyed across the preexisting roadway or under the road way through a culvert. Table 6. Planned Dimensions for the Expanded East Stormwater Retention Pond Area of Stormwater Pond 8,047 ft Area of Floor (5H:1 V slopes) 3,766 ft2 Perimeter of Floor 299 ft Depth 2.5 ft Volume 4,320 ft3 Volume 0.323 acre-ft Volume 105,000 gallons Runoff from the western areas of the pile is currently routed across the waste rock dump and into the ephemeral drainage near the soils stockpile. Water pools temporarily between the toe of the waste rock pile and the soil stockpile (Zone 3 in Figure 2) before evaporating or infiltrating. Much of the water evaporates because infiltration capacity of site soils is low (NRCS, 2011) and potential evapotranspiration rates exceed precipitation rates in the region. The existing temporary pool was not engineered to contain the 10-year 24- hour storm or to convey the 100-year 24-hour storm. The existing pool will be expanded to a footprint of approximately 3,600 ft2 with a planned maximum depth of 2 feet and slopes of 5H:1 V. This retention pond will then hold 0.099 acre-ft (4,320 ft2) of water without freeboard (Table 7) which is 40% more than the 10-year 24-hour storm runoff to receiving area 3 (Table 4). If the excavation encounters bedrock and the pond cannot be constructed as planned, the pond will be excavated to the depth practicable and silt fences will be installed on the downslope side to retain any entrained sediment should the retention pond overtop. Table 7. Planned Dimensions for the West Stormwater Retention Pond Area of Stormwater Pond 3,652 ft Area of Floor (5H:1 V slopes) 1,037 ft2 Perimeter of floor 224.2 ft Depth 2.0 ft Volume 4,320 ft3 Volume 0.099 acre-ft Volume 32,310 gallons 3. OPERATIONAL AND POST-MINING DRAINAGE CONDITIONS When mining resumes, the existing waste rock pile will be built up to house additional mine facilities including a new maintenance shop and office. During the build-out period, drainage will be modified to 18 prevent water from accumulating in the facilities area and to prevent excessive erosion and sedimentation during operations. For the post-mining reclamation period, drainage will be engineered and constructed to reduce erosion and protect reclaimed surfaces. 4122A.110601 Whetstone Associates