Laserfiche WebLink
JD -8 Mine — Drainage Design Plan 7 <br />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 borrow 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 DD 19 is ultimately intercepted <br />by the lower County Road DD19. Storm water that reaches the DD19 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 DD 19 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 DD19 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 />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 />