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JD-8 Mine—Drainage Design Plan 14 <br /> anticipated peak flow rate (Qloo) generated by the operational and post mining site and entering the <br /> retention pond (Pond 3) are shown in Table 14. Post mining runoff will reach the retention pond via two <br /> different flow paths. Flow from the upper areas of the site (sub-basins 1 and 4 and the Reclaimed Waste <br /> Rock behind the upper interceptor ditch) will be dominated by sheet flow and concentrated overland flow, <br /> while channel flow through the lower interceptor ditch will control runoff time of travel for the eastern and <br /> lower areas of the Reclaimed Waste Rock pile. <br /> Table 14. Operational and Post-Mining Drainage—Peak Discharge(Q,ov) for Pond 3 <br /> Parameter Upper Flow Lower Flow Catchment <br /> Total <br /> Tsh(sheet flow)(hr) 0.07 0.11 <br /> Tsc(shallow concentrated)(hr) 0.13 0.01 <br /> Tch(channel)(hr) 0.002 0.06 <br /> Tc(hr) 0.2 0.18 0.18 <br /> Q„(cfs) 490 520 520 <br /> Q100(cfs) 3.1 2.6 5.9 <br /> The interceptor ditch design flow is generated from approximately 49% of the reclaimed waste rock area <br /> and is shown as Lower Flow in Table 14. In consideration of the steep longitudinal channel slope of the <br /> ditch (Table 15), a riprap liner(D50 = 6", see section 2)with 4"type I and 4"type II granular bedding(see <br /> Attachment 2)will be installed. Though channel velocity is relatively low, this is due largely to the effect <br /> of the coarse liner and the associated Manning roughness coefficient of 0.045 (HEC 15, eqn. 6.2, FWHA <br /> 2005). In absence of this liner, channel velocities would increase to approximately 6 ft/second. <br /> The slightly lower time of concentration (TO value from the two flow paths in Table 14 has been used to <br /> calculate a conservative estimate for total peak discharge from the site for the 100-year 24-hr event(Qloo). <br /> This flow will be conveyed across the spillway shown in Figure 3. The spillway will be constructed to the <br /> design specifications in Table 15 and will generally follow the schematic provided in Attachment 3. The <br /> spillway crest elevation will be identical to the 10-year 24-hour storage elevation for the retention pond <br /> (Pond 3). <br /> (1/4.• <br /> 4122A.120702 Whetstone Associates <br />