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• 24 <br />At the watershed outlet the peak water discharge is 3156 cfs and the total <br />runoff volume produced is 156 acre-feet. There are two proposed culverts for <br />the post-mining condition which are discussed in Section 6.2. <br />4.2 Newlin Creek Drainage <br />The results of the 100-year design storm are used to determine design <br />criteria for the reclamation of the stream channel which is now diverted <br />around the "Prospecting pit'. The channel design is presented in Section 6.4. <br />The total eater and sediment runoff, peak discharges, peak sediment concentra- <br />Lion, and average sediment concentration for the 100-year design storm are <br />C listed in Table 7. <br />The average peak sediment concentration in the channel units is 56,900 ppm. <br />The largest sediment concentration is found in Channel 2 which has a peak sedi- <br />r <br />went concentration of 73,010 ppm. At the watershed outlet, the peak sediment <br />• concentration is 50,740 ppm. The sediment concentrations decrease going down- <br />stream due to the milder slopes. Aggradation of 0.1 feet and 0.5 feet is <br />predicted for CH-4 and CH-8, respectively. Aqgradation in these channels was <br />verified during a site visit by SLA staff. <br />At the watershed outlet, the total water runoff wlume is 631.9 acre-feet <br />and the peak discharge is 9789 cfs. The peak discharge is less at the water- <br />shed outlet than in CH-3 and CH-4. This is caused by the wider channel cross <br />sections and floodplain along CH-8. <br />4.3 Tipple Azea Drainaga <br />The total rater runoff wlume, total sediment runoff, and peak rater dis- <br />charge are 1.7 acre-feet, seven tons, and 48 cfs, respectively for the 100-year <br />design storm. The peak runoff rate and runoff wlumes for the 100-year design <br />storm and post-mining conditions are less than those for the 10-year design <br />storm and present-mining conditions. Several reasons for this follw: <br />1. The post-mining topography decreases the drainage area by 17 percent <br />and the left overland slope of SW-13 becomes milder. <br />2. Reclamation of the Tipple Area rill leave the area less compacted <br />than the existing conditions, and therefore, there is a higher infil- <br />tration rate. <br />- 3. For present-mining conditions, large amounts of water and sediment run- <br />j off were produced due to the imcerviousness of the roads, buildings, <br />and the steep slopes of the coal stockpiles. For pest-mining conditions, <br />