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• ^ Restore existing arroyo cross section. <br />^ Restore arroyo watershed boundaries. <br />^ Provide additional protection measures. <br />^ Preserve and restore existing stock ponds. <br />Restore Existing Arroyo Alignment. Restored arroyos will dischazge into the same <br />alignment as existing arroyos. This objective is achieved primarily by the excavating new <br />arroyos "B," "E," "G," and "I" beginning at the final reclamation surface and extending <br />downstream until the excavation daylights (see Figure 3). This approach will provide reasonable <br />assurances that runoff is passively dischazged and flows unimpeded back to the St. Charles <br />River. <br />Restore Existing Arroyo Cross Section. Arroyos will be restored to approximately the <br />same existing configuration. Several arroyos were sampled to identify cross sections, reflecting <br />the broad, shallow azea, typical in the upper watersheds. From these samples, a typical cross <br />section was identified as having a trapezoidal shape with a 5-foot bottom and neazly flat slopes <br />(20H:1 V). This cross section wil] convey the 100-yeaz flood with depths no greater than 3 feet. <br />Depending on the flow velocity, erosion control measures vary from vegetation for flat slopes <br />up to 0.5%) to vegetation and rock for intermediate slopes (from 0.5 % to 2.0%), and vegetation <br />and turf reinforcement mat for higher velocities. <br />Calculations of arroyo capacities and flow velocities were performed using Manning's <br />• equation with variable n-values depending on depth and hydraulic radius. Velocity and sheaz <br />stress results were compazed to acceptable limits, which establishes the depth and slope limit for <br />the arroyo (see Part B Calculation Appendix). This information was then combined into a design <br />chart for arroyos (see Figure 9) which shows the slope and flow limitations for vegetation lining, <br />vegetation and rock lining, and vegetation and turf reinforcement mat lining. This approach will <br />result in a channel not exceeding existing erosion conditions and providing adequate protection <br />until vegetation is established. <br />Restore Arroyo Watershed Boundaries. Arroyo watersheds will be restored to <br />approximately the same azeas. When establishing the final reclamation surface, consideration <br />was given to the existing arroyo watershed boundaries. The final reclamation surface includes a <br />small "ridge" to allow surface runoff to flow towards the restored arroyos, resulting in <br />approximately the same watershed azea. This approach will provide reasonable assurances that <br />peak flows and runoff volumes will not be increased over existing conditions. In addition, <br />because the final reclamation surface is flatter than existing topography, runoff peaks, and <br />sediment loads will likely be less than existing conditions. <br />Provide Additional Protection Measures. Additional protection measures will be <br />provided for the critical area where the natural arroyos discharge into the reclaimed mining area. <br />Since the reclaimed mining surface will be lower than natural topography at the highwalls, <br />additional protection measures will be provided. These measures include arock-lined, sloping <br />drop structure, which can convey the 100-yeaz flood down the steeper slopes (that is, 4H:1 V). <br />The drop structure, limited to a drop height of 10-feet, can be formed into exposed rock or <br />created using mined rock. The structure (see Figure 10) is "V-shaped" to concentrate flows in <br />SWMP BC June 28 2002.doc, O6R8/02 10 <br />