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management practices by directing and controlling surface water runoff that enters the disturbed <br /> areas. More information on sediment and surface water control is provided below. <br /> 3.6. Surface Water Handling <br /> Two drainage basins collect all stormwater runoff on and around the London Borrow Pit site. <br /> This is shown on Map E-5. Both drainage basins consist almost entirely of highly permeable <br /> talus rock overlaying the mountain bedrock. The processing area has seen activity historically, <br /> and thus is assumed to have a lower infiltration capability than the natural talus fields. All curve <br /> numbers are based on NRCS TR-55 values. The composite curve numbers are shown on Map <br /> E-5 for each basin in baseline, mining, and reclamation conditions. Runoff calculations using <br /> TR-55 can be seen in Appendix B-1. <br /> 3.6.1. Mining <br /> During all phases of mining, surface water runoff will drain to the active pit and processing area. <br /> Water collected in these areas will be allowed to evaporate or infiltrate. In the event that <br /> infiltration and evaporation has not occurred after 24-hours, either area will be pumped out via <br /> the approved CDPHE outfall once sediment has settled out. See Map E-2 for the stormwater <br /> control berms. These berms provide enough capacity for the 100-YR 24-HR flood event. <br /> 3.6.2. Post Reclamation <br /> The baseline drainage conditions will be restored following reclamation of the site. Topsoil and <br /> revegetation of the pit bottom and processing area will reduce the runoff quantity from the site. <br /> See Map E-5 for drainage basin details. <br /> 3.T Disturbed Area Runoff <br /> During all stages of mining, there is enough water storage capacity to contain the 100-year 24- <br /> hour storm events and prevent erosion from surface water discharge. The expected rainfall from <br /> these events at the site is provided in Table G-2 below. <br /> Table B-2. Area Storm Events (from NOAA1) <br /> Event Probability Event Rainfall <br /> (inches) <br /> 100-YR 24-HR 3.53 <br /> The peak runoff was generated from these values for the two drainage basins during all stages <br /> of mining. Pre-mine, mining, and reclamation conditions are delineated on Map E-5. The <br /> discharge volumes from these storm events are calculated in Appendix B-1 at the end of this <br /> exhibit. Map E-5 shows all drainage volumes and runoff storage capacity. <br /> National Oceanic and Atmospheric Administration <br /> London Borrow Pit <br /> April 2024 8-5 Lewicki&Associates <br />