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• existing Coal Gulch Diversion will be backfilled as shown on the Post-mining <br />Topography Map (Map 14~ to blend with the existing adjacent undisturbed land. <br />The backfill slopes will be less than or equal to 3H:1V to minimize erosion <br />and to ensure overall slope stability. <br />Peerless has designed the permanent Coal Gulch channel, flood plain and <br />longitudinal profile to provide sufficient capacity to pass the peak flows <br />from a 100 year, 24 hour precipitation event. Given the slope of the channel, <br />armoring has been designed for use in areas where the channel bottom is not in <br />bedrock. Armoring with large riprap wili minimize additional contribution of <br />suspended solids. <br />Large riprap material from the existing Coal Gulch diversion channel will be <br />used to provide velocity control measures in the permanent channel. As shown <br />on the Post-mining Topography Map (Map 14) and the Coal Gulch Permanent <br />Channel Profile Map (Map 161, permanent channel gradient established during <br />backfilling and grading is very similar to estimated pre-disturbance gradient <br />from the existing Coal Gulch upstream from the mine to the State Highway 160 <br />crossing downstream of the mine. The downstream channel has been previously <br />modified and cannot be compared with the existing design. The cross-section <br />of the upstream channel (Figure 5) entering the diversion ditch indicates that <br />the design channel is as large as that previously in place. Therefore, the <br />re-established channel with armoring will pass the 100-year, 24-hour peak flow <br />at conditions similar to the original Coal, Gulch channel. <br />Revised 9/12/86 <br /> <br />10 <br />