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JD-8 Mine—Drainage Design Plan 7 <br /> 41110 All riprap will be installed with a granular bedding to protect underlying soils per the attached Urban <br /> Drainage and Flood Control District specification for gradation and thickness (UDFCD 2011) in <br /> Attachment 2. <br /> Gravel mulch will be"composed of coarse to very coarse gravel, 16 mm to 64 mm(0.6 to 2.5 inch), similar <br /> to an AASHTO No. 3 coarse aggregate" (FWHA 2005). No bedding will be placed between the gravel <br /> mulch and the bare soil. <br /> 3. DIVERSION DRAINAGE <br /> 3.1 Storm Water Routing from Upland Areas <br /> Runoff from the upland areas directly above the site is intercepted and controlled by borrow2 ditches, cutoff <br /> ditches, culverts, and topography associated with the existing county and access roads as indicated in <br /> Figure 2. <br /> Upland flow from higher elevations on the escarpment above County Road DD19 is ultimately intercepted <br /> by the lower County Road DD 19. Storm water that reaches the DD 19 road ditches is either directed <br /> northwest of the mine site through the existing County Road DD19 culverts shown in Figure 2 (sub-basins <br /> N-A and N-E) or directed to the southeast along the DD19 road borrow ditch to the natural drainage <br /> beyond the map boundary (sub-basins N-B, N-C, and N-D). Although the topography and road locations <br /> shown in Figure 2 are derived from USGS DEM files and US Census Bureau Tiger files, respectively, and <br /> are therefore approximate, a site inspection of the topography indicates that a continuous grade exists <br /> towards the south east along County Road DD 19 which routes water southeast along the borrow ditch. <br /> Given the maximum estimated discharge from Sub-Basins N-B, N-C, and N-D (Section 3.2), no <br /> overtopping of county ditches is anticipated, although any excess flow would be expected to remain within <br /> the county road corridor due to the road surface draining toward the ditch. <br /> Storm water runoff below the county road DD19 will either bypass the mine site (Sub-Basin N-E), or be <br /> intercepted by the existing water tank access road borrow ditch directly upgradient of the mine area and <br /> routed around the site via the culvert in the mine access road(for Sub-Basin N-F). <br /> Storm water and snowmelt from the mine upland areas located south east of the mine are routed toward a <br /> natural channel, away from the mine site, by the existing berm and cutoff ditch on the access road to the <br /> mine(Photo 1). The photograph was taken on the access road southeast of the mine permit boundary, near <br /> the intersection of the haul road, lower access road and road leading to the water tank above the JD-8 Mine <br /> (Figure 2). <br /> 3.2 100-year 24-hour Routing Capacity for Upland Areas <br /> Based on the areas shown in Table 2 and the design storms shown in Table 3, the peak discharge for the <br /> 100-year 24-hr peak flow rate(Qloo)generated by the non-contributing sub-basins are shown in Table 4. <br /> 2 For the purpose of this document,a borrow ditch(or"bar ditch")is a ditch constructed parallel to a road from which material was <br /> S <br /> { excavated (borrowed) to build up the road, and in which runoff water from the road or surrounding area may collect and be <br /> conveyed. <br /> 4122A.120702 Whetstone Associates <br />