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corresponds strongly with greater tree density. Therefore, in drainage areas where the Tahana- <br /> Legault-Complex is the majority soil, the hydrologic soil group will be assumed to be B. Based <br /> on the NRCS method of calculating the surface water runoff volume and peak discharge for the <br /> existing areas, the curve numbers of the baseline drainages are either 79 (hydro soil group D) or <br /> 70 (hydro soil group B). Appendix 4 lists the curve numbers for each area in the pre mine <br /> conditions. Appendix 4 lists all storm event data for each area. The basin/area numbering on that <br /> table correspond to the numbering found on Map G-1 — Surface Hydrology: Baseline Conditions. <br /> During Mining Surface Hydrology <br /> During Phase 1, the entire processing area will act as a `sump' that allows stormwater to <br /> infiltrate into the porous processing area pad, see Map C-3. Fine sediment will be removed from <br /> the lowest settling area of the pad, as needed. Surface water within the disturbed area of the site <br /> will be controlled by perimeter ditches and berms which surround the disturbed area. When <br /> paired with small perimeter berms, the designed pad elevation (-3 foot lower in elevation than <br /> the current CCP) of the processing area will have the capacity to contain 100 year 24-hour rain <br /> events. No stormwater, process water, or sediment will be allowed to leave the site. All runoff <br /> from a 100 year 24-hour rain event will be trapped on site and waters will infiltrate the <br /> processing area floor within 72 hours. The calculated available storage volume, >30 acre-feet, of <br /> the processing area is able to hold>2 back-to-back 100-year storm events (4.44 inches/24 hours; <br /> Appendix 4—Hydrology). <br /> The typical 24-HR events for this area are listed below: <br /> Table G-1. Area Storm Events <br /> Event Probability Event Rainfall (inches) <br /> 100-YR 24-HR 4.44 <br /> 25-YR 24-HR 3.26 <br /> 10-YR 24-HR 2.60 <br /> Surface water within the disturbed mining area will be trapped within the quarry following storm <br /> events and will coalesce in the lowest area of the processing area pad, aka the sump. This <br /> Young Ranch Resource i:nX IAwicks and Aswcialct YI.Lf <br /> February 2020 G-2 <br />