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Parameter <br />Upper Flow <br />Lower Flow <br />Catchment <br />Total <br />T (sheet flow) (hr) <br />0.07 <br />0.11 <br />T„ (shallow concentrated) (hr) <br />0.13 <br />0.01 <br />Tch (channel) (hr) <br />0.002 <br />0.06 <br />Tc (hr) <br />0.2 <br />0.18 <br />0.18 <br />Q„ (cfs) <br />490 <br />520 <br />520 <br />Q100 (cfs) <br />3.1 <br />2.6 <br />5.9 <br />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 (Q100) for Pond 3 <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 (D = 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 />4122A.120702 Whetstone Associates <br />