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<br /> <br />London Mountain Clean Up <br />July 2025 G-2 <br /> <br /> <br />altitude of the waste piles. Unlikely operations at lower altitudes, all of the waste piles are fairly close to <br />the top of mountains that act as the top of drainage basins. Appendix G-1 lists the hydrologic <br />calculations of the 10-YR, 24-HR event for each of the standardized drainage basins. The standard <br />basins are: <br />• 5-acre • 30-acre <br />• 10-acre • 40-acre <br />• 20-acre • 50-acre <br />Time of concentration for each basin ranges from 1500 to 2500 ft, based on the range of distances from <br />the top of area ridges to the bottom of the gulch. Based on these standard drainage basins, the <br />following 10-YR, 24-HR event peak flows were determined using the Rational Method. <br />Table G-1. Standard Basins and Peak Flows <br />Standard Basin Size 10-YR, 24-HR Peak Flow (CFS) <br />5-acre 9.85 <br />10-acre 19.70 <br />20-acre 39.40 <br />30-acre 56.58 <br />40-acre 75.44 <br />50-acre 90.48 <br />The standardized drainage basins provide peak flows that standard diversion structures can be <br />designed around. Each waste pile will have a diversion structure installed uphill of it to route local runoff <br />around it while mining and reclamation is occurring. Map C-2 and C-2A detail these structures. The <br />hydrologic design basis can be found in Appendix G-1. Diversion structures of the following depths will <br />handle a corresponding flow: <br />Table G-2. Uphill Diversion Structure Depth vs. Flow Capacity <br />Depth (ft) Flow Capacity (CFS) <br />1.5 23.2 <br />2.0 50.0 <br />2.5 90.6 <br />3.0 147.4 <br />Note: Design figures in Appendix G-1 show diversion structure sizing calculations. <br />When a waste pile is selected for cleanup, its uphill drainage basin size will be determined and it will be <br />assigned to a standard basin size at least as big as the drainage measured. Using this standard basin <br />size, the peak flow can be used to determine the depth of the diversion structure that said waste pile <br />requires. Therefore, the diversion structures shown on Map C-2 and C-2A will have varying depths <br />depending on the waste pile being mined. <br />Stormwater control structures will be installed on the downhill end of the waste pile and at the crest of <br />the waste pile slope. Mining will typically be conducted from the top of a waste pile downward, to <br />minimize the amount of waste pile disturbed at any time, and to provide a working surface for