2002-07-03_PERMIT FILE - M2002004

Home Browse Search

Exhibit D pond. Stored runoff will be tested for pH, TSS, oil and grease, COD, and, potentially, blasting residuals. • Runoff will be pumped into reconstructed Arroyo "B" for mining block years 1 through 25 and reconstructed Arroyo "I" for block years 26 through 50. These arroyos will be reconstructed by excavating a new channel beginning at an elevation coincident with the planned final reclamation surface and extending downstream until the excavation daylights. To minimize stockpiling requirements for overburden materials, a temporary arroyo configuration will be constructed for Arroyo "B", which is narrower than the final arroyo cross section. The final arroyo cross-section will be constructed as part of reclamation through block year 25. Post Reclamation As part of reclamation, the mining area surface will be back-filled and contoured, the reconstructed arroyos completed, the drainage patterns restored to a more natural state, and the upper arroyos protected from erosion. The existing drainage conditions were evaluated for arroyos impacted by the project, and peak flows, runoff volumes, and sediment loads were calculated for the 5-, 25- and 100-year, 24-hour storm events. Required drainage facilities were developed and sized, and values for the key drainage parameters were calculated for post-reclamation conditions. The surface-water management facilities for this phase have been designed to meet the following goals and objectives: ¦ Restored arroyos will discharge into the same alignment as existing arroyos. ¦ Arroyos will be restored to approximately the same existing configuration. ¦ Arroyo watersheds will be restored to approximately the same areas. ¦ Additional protection measures will be provided for critical areas. • Restored Arroyos Will Discharge Into The Same Alignment As Existing Arroyos This objective is achieved primarily by the excavating new arroyos "B," "E," "G," and "I" beginning at the final reclamation surface and extending downstream until the excavation daylights. This approach provides a means to passively discharge each arroyo's runoff , which can flow unimpeded back to the St. Charles River. Arroyos Will Be Restored To Approximately The Same Existing Configuration Several arroyos were sampled to identify cross sections, reflecting the broad, shallow area, typical in the upper watersheds. From these samples, a typical cross section was identified as having a trapezoidal shape with a 5-foot bottom and nearly flat slopes (that is, 20H: IV). This cross section will convey the 100-year flood with depths no greater than 3 feet, which reduces flow velocities, shear stresses, and therefore erosive forces. Depending on flow velocity, erosion control measures vary from vegetation for flat slopes (up to 0.5%), to vegetation and rock for intermediate slopes ( from 0.5 % to 2.0%), and vegetation and turf reinforcement mat for higher velocities. Specifications for the reconstructed arroyos are in Appendix F "Surface Water Management Plan." This approach will provide an erosion resistant channel not exceeding existing conditions and providing adequate protection until vegetation is established. Arroyo Watersheds Will Be Restored To Approximately The Same Areas When establishing the final reclamation surface, consideration was given to the existing arroyo watershed boundaries. The final reclamation surface includes a small "ridge" to allow surface runoff to flow towards • the restored arroyos, resulting in approximately the same watershed area. This approach will provide 1180-Mine Permit Exhibits (Jul.03.02)FINAL.doc D-12 July 3, 2002