2016-08-08_PERMIT FILE - C1981019A (116)

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EXHIBIT 7, ITEM 23, PART G Small Area Exemptions Three small areas planned for disturbance cannot practicably be captured within the sediment control system of ditches and ponds, and are therefore treated here via Small Area Exemptions. The areas so classified are depicted on Map 41 B, and are described as follows: • a 1.4 acre zone along the east side of the pit in Section 1 • a similar 7.66 acre area along the east side of the pit in Section 2, and • a 2.6 acre area on the west side of the pit in Section 34. The largest of these areas, the 7.66 acre area is also the steepest, and has been used as the representative case for all of these areas. SEDCAD's grass filter routine has been used to demonstrate that runoff from this are will meet the effluent standards equivalent to the sediment pond system. The SEDCAD output is included in this section. A 7.66 acre area with ground slope of 15%, which matches the representative slope for this area, with CN set at 85 to reflect a topsoil stripped condition, was used to generate the runoff hydrograph from a 10 year 24 hour storm. Sedimentology parameters as defined in TR -73 were used to generate the accompanying sediment graph. The default Manning's roughness coefficient of 0.0121 was substantially reduced to 0.001, and the default stem spacing of 0.67 inches was expanded to 3 inches. Both of these revisions are intended to reflect that the native grass cover density at the mine is less than the values incorporated in SEDCAD's grass filter subroutine drop-down menus. These are both conservative revisions, as they tend to increase the calculated velocity of the overland flow passing through the grass filter, thereby reducing the filter's calculated trap efficiency. This 5.6 cfs flow was then run through a native grass filter approximately 600 feet wide and only 20 feet long (in the direction of the flow). In reality the exposed perimeter of the tract in question is about 1,300 feet, making the 600 foot width assumption a conservative assumption. That is, the 5.6 cfs of 10 -year runoff flow is assumed to be routed over an area less than half of the actual perimeter. As shown in the first attached SEDCAD output file (SAE1), the calculated resulting peak settleable concentration released is 2.54 ml/1, and the 24 -hr weighted settleable solids concentration in the SEDCAD output is 1.16 ml/1, which does not satisfy the intended 24 -hr weighted average of 0.5 ml/l. The second model retains all of the conservative assumptions of the first except that the length of the filter zone is increased to 70 feet. This is the length of the filter zone in the direction of downslope flow. The width is conservatively retained at 600 feet in this case. This case (SAE2) produces a peak settleable solids concentration of 0.01 ml/1, and does meet the target settleable solids concentration of 0.5 ml/1. Finally, the model was run with the grass filter width set at 1,200 feet. In reality, this area would appear to have a length approaching 2,000 feet. In this model (SAE3) the settleable solids are entirely consumed in the vegetative filter, producing peak and 24-hour weighted concentration of 0.00 ml/l. These results indicate that the native vegetation will be able to consume all released settleable solids within the first 70 feet or less of the exposed perimeter, well before release to any offsite receiving stream. Exh. 7-23G-1 Revision Date: 7/31/15 Revision No.: PR -04