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Upper Pit 6900 Access Road I Design Summary American Gypsum <br /> determined a total capacity of 0.43 acre-feet is required to contain the sediment and runoff from <br /> a 10-year, 24-hour event. The target capacity of 0.43 acre-feet includes the increased sediment <br /> load from the proposed road disturbance (0.0006 acre-feet of additional sediment storage is <br /> required to accommodate the proposed road disturbance). Attachment E contains the SEDCAD <br /> model output. <br /> The pond will be cleaned out when approximately half full of sediment and limits will be marked <br /> with survey stakes and visual depth indicators to ensure the pond is adequately cleaned out in <br /> the future without over-excavating the pond. The pond will be cleaned out to maintain a crest <br /> elevation of 6705.9 feet amsl, a maximum depth of 15 feet, 2.5H:1V side slopes within the dam <br /> area, and 1.5H:1V side slopes within the native stream channel (to mimic native topography). <br /> 5.0 Cultural Resources <br /> SWCA reviewed previous cultural resources surveys and performed a field reconnaissance <br /> survey to determine there are no cultural resources within the proposed project area. The cultural <br /> resources report was submitted directly to the BLM archaeologist. <br /> 6.0 Post-Mining Reclamation <br /> For this submittal, Tierra Group assumed reclamation activities will be independent from other on- <br /> site reclamation efforts. Due to the proximity of other mining structures, such as the existing road, <br /> existing culverts, and Upper Pit, reclamation activities may be combined during implementation <br /> to reduce surface disturbance and minimize haul distances. <br /> Following cessation of mining activities, the culvert will be plugged at both ends and the riprap <br /> collar (culvert inlet) and riprap apron (culvert outlet) will be removed. The road surface will be <br /> regraded to 3H:1 V slopes or flatter using cut-to-fill methods, covered with 6 to 12 inches of topsoil <br /> or suitable growth media, and revegetated using a previously-approved seed mixture. <br /> A surface water diversion channel will be constructed over the regraded surface, with a <br /> meandering alignment to mimic naturally-occurring streams in the area. The channel was sized <br /> to accommodate flows from the 100-year, 24-hour event using the upland area (Attachment D3) <br /> and HEC-HMS (USACE, 2010). The channel will be lined with 6-inch D50 riprap for erosion <br /> protection and a riprap apron (similar to that constructed at the culvert outlet) will be constructed <br /> where the channel transitions back to the natural stream channel. <br /> 7.0 Conclusion <br /> Tierra Group designed a road to access the Upper Pit at an elevation of 6900 feet amsl. The road <br /> will disturb approximately 5.8 acres of BLM land after reclamation activities, all of which are within <br /> the 830-acre permitted disturbance area shown on Drawing 03 (Attachment A). Slope stability <br /> analyses confirmed the road can be constructed with 1.5H:1V cut and fill slopes to meet minimum <br /> static and pseudo-static FOS requirements as outlined by CO DRMS (CO DRMS, 2016). Upland <br /> surface water runoff will be conveyed under the road via an 18-inch corrugated steel culvert. <br /> Surface water runoff from the disturbed road area will report to the existing sediment control pond <br /> M501, which has sufficient capacity to contain the additional 0.0006 acre-feet of sediment <br /> generated by the new road disturbance area. A licensed archaeologist determined there were no <br /> AG Upper Pit 6900 Access Rd 20181015 October 2018 Page 7 of 9 <br />