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the restored arroyos, resulting in approximately the same watershed area. This approach will provide <br />reasonable assurances that peak flows and runoff volumes will not be increased over existing conditions. <br />In addition, because the final reclamation surface is flatter than existing topography, runoff peaks and <br />sediment loads will likely be less than existing conditions. <br />Additional Protection Measures Will Be Provided For The Critical Area Where The Natural Arroyos <br />Discharge Into The Reclaimed Mining Area <br />Since the reclaimed mining surface will be lower than natural topography at the highwalls, additional <br />protection measures are required. These measures include a rock - lined, sloping drop structure, which can <br />convey the 100 -year flood down the steeper slopes (4H:1 V). The drop structure, limited to a drop height <br />of 10 feet, can be formed into exposed rock or created using mined rock. The structure is "V- shaped" to <br />concentrate flows in the center and includes a scour basin, energy dissipator, which transitions the flow to <br />the reconstructed arroyo. This approach will allow existing arroyos to be transitioned into the <br />reconstructed drainage at the upper end of the mined area. <br />If the drop height exceeds 10 feet, then an optional method to control flow down the restored highwall is <br />to construct a detention pond above the slope. The pond would reduce the peak runoff rates for up to the <br />100 -year event and allow the flow to be conveyed in a pipe with a preformed, scour -basin energy <br />dissipator at the outlet. <br />Existing Stock Ponds Will Be Preserved Or Reconstructed As Part Of Reclamation <br />The existing stock pond in Arroyo "A," where a topographic depression can trap water that supports tree <br />and other vegetative growth, will be preserved throughout the mining operation. Activities within Arroyo <br />"A" watershed are limited to constructing the access road railroad spur, and culverts, and some mining in <br />a relatively small area (that is, 24- acres) to the east of the arroyo drainage. Activities will not disturb <br />upgradient water flow to the area. Refer to Exhibit D -1 and D -2 for more detail of the relationship <br />between Arroyo "A" and the mining activities. <br />The existing stock pond in Arroyo "E" will be preserved through approximately block year 10, after <br />which mining will progress through the pond. The stock pond will be restored to its existing capacity by <br />excavation and construction of a small embankment during reclamation. <br />Technical Rev No. 4 <br />April 26, 2013 <br />D -12 <br />