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All drainage basins are shown on the Drainage Map. <br /> 6.1.1. Mining <br /> During all phases of mining, and for each drainage basin, surface water runoff will drain to the <br /> active mining pod, reclaimed reservoir, or the processing area. Water collected in the active <br /> mining pod will be allowed to evaporate or will be discharge via the approved CDPHE outfall <br /> once sediment has settled out. Water collected within the processing area will be allowed to <br /> evaporate or discharged via the approved CDPHE outfall once sediment has settled out. <br /> 6.1.2. Post Reclamation <br /> The drainage patterns during mining will be retained following reclamation of the site. Any <br /> surface water runoff will collect in the reclaimed reservoirs. There is enough storage capacity <br /> above the anticipated reservoir levels and the top of the shore to store the 100-year storm <br /> events. More on those calculations is provided below. <br /> 6.1.3. Flood Protection <br /> Mining Pod 1 will extend to within 150 feet from the South Platte River in two locations as shown <br /> on the Drainage Map. Due to this proximity to the river, measures will be taken to protect the <br /> riverbank from erosion during a flood event. An inflow and outflow structure will be constructed <br /> between the River and the mining pod once mining is within 300 feet of the river. These <br /> structures will allow for the safe exchange of flood waters between the pit and river which <br /> prevents erosion of the riverbank and pitside slope during flood events. These structures are of <br /> a design approved for use by the Mile High Flood District. Details of the inflow/outflow structure <br /> are shown on the Drainage Map. <br /> 6.2. Disturbed Area Runoff <br /> During all stages of mining, there is enough water storage capacity to contain the 5-year and <br /> 100-year 24-hour storm events and prevent erosion from surface water discharge. The <br /> expected rainfall from these events at the Ewing Site is provided in Table G-2 below. <br /> Table G-2. Area Storm Events (from NOAA2) <br /> Event Probability Event Rainfall <br /> 5-YR 24-HR 2.28 <br /> 100-YR 24-HR 4.64 <br /> The peak runoff was generated from these values for the three drainage basins during all <br /> stages of mining. Pre-mine, mining, and reclamation conditions are delineated on the Drainage <br /> Map. The discharge volumes from these storm events are calculated in Appendix G-1 at the end <br /> of this exhibit. Table G-3 summarizes the runoff volumes and storage volumes for each <br /> 2 National Oceanic and Atmospheric Administration <br /> Ewing Gravel Pit <br /> August 2023 G-4 Lewicki&Associates <br />