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Storm water will be temporarily stored for up to 3 days within the in -pit detention /sediment pond. Stored <br />runoff will be tested for as required by the Colorado Discharge Permit before being discharged. Runoff <br />will be pumped into reconstructed Arroyo `B" for mining block years 1 through 25 and reconstructed <br />Arroyo "I" for block years 26 through 50. These arroyos will be reconstructed by excavating a new <br />channel beginning at an elevation coincident with the planned final reclamation surface and extending <br />downstream until the excavation daylights. To minimize stockpiling requirements for overburden <br />materials, a temporary arroyo configuration will be constructed for Arroyo `B ", which is narrower than <br />the final arroyo cross section. The final arroyo cross - section will be constructed as part of reclamation <br />through block year 25. <br />Post Reclamation <br />As part of reclamation, the mining area surface will be back - filled and contoured, the reconstructed <br />arroyos completed, the drainage patterns restored to a more natural state, and the upper arroyos protected <br />from erosion. The existing drainage conditions were evaluated for arroyos impacted by the project, and <br />peak flows, runoff volumes, and sediment loads were calculated for the 5 -, 25- and 100 -year, 24 -hour <br />storm events. Required drainage facilities were developed and sized, and values for the key drainage <br />parameters were calculated for post - reclamation conditions. The surface -water management facilities for <br />this phase have been designed to meet the following goals and objectives: <br />• Restored arroyos will discharge into the same alignment as existing arroyos. <br />• Arroyos will be restored to approximately the same existing configuration. <br />• Arroyo watersheds will be restored to approximately the same areas. <br />• Additional protection measures will be provided for critical areas. <br />Restored Arroyos Will Discharge Into The Same Alignment As Existing Arroyos <br />This objective is achieved primarily by the excavating new arroyos `B," "E," "G," and "I" beginning at <br />the final reclamation surface and extending downstream until the excavation daylights. This approach <br />provides a means to passively discharge each arroyo's runoff , which can flow unimpeded back to the St. <br />Charles River. <br />Arroyos Will Be Restored To Approximately The Same Existing Configuration <br />Several arroyos were sampled to identify cross sections, reflecting the broad, shallow area, typical in the <br />upper watersheds. From these samples, a typical cross section was identified as having a trapezoidal <br />shape with a 5 -foot bottom and nearly flat slopes (that is, 20H:1 V). This cross section will convey the <br />100 -year flood with depths no greater than 3 feet, which reduces flow velocities, shear stresses, and <br />therefore erosive forces. Depending on flow velocity, erosion control measures vary from vegetation for <br />flat slopes (up to 0.5 %), to vegetation and rock for intermediate slopes (from 0.5 % to 2.0 %), and <br />vegetation and turf reinforcement mat for higher velocities. Specifications for the reconstructed arroyos <br />are in Appendix F "Surface Water Management Plan." This approach will provide an erosion resistant <br />channel not exceeding existing conditions and providing adequate protection until vegetation is <br />established. <br />Arroyo Watersheds Will Be Restored To Approximately The Same Areas <br />When establishing the final reclamation surface, consideration was given to the existing arroyo watershed <br />boundaries. The final reclamation surface includes a small "ridge" to allow surface runoff to flow towards <br />Technical Rev No. 4 <br />April 26, 2013 <br />D -11 <br />