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2.3-4 <br />2.3.4 Sedimentation Controls <br />• Surface runoff from the areas disturbed by the mine bench and the <br />waste rock disposal area will be routed to sediment control ponds. <br />The ponds and ditches have been designed in accordance with <br />applicable criteria. Discharge of water will be covered by an NPDBS <br />permit prior to construction of the ponds. <br />Mine Bench Area <br />The sedimentation reservoir will be located at the base of the <br />embankment as shown in Figure 2.2-14. A detailed plan view, <br />hydrologic and hydraulic data used for the designs are shown on <br />Figure 2.2-3. All calculations used in the design are contained in <br />Appendix M(vi). <br />As shown in Figures 2.2-3 and 2.2-14, an eight inch diameter <br />corrugated pipe will be used to dewater the pond. The invert of the <br />pipe will be installed at the elevation of the maximum sediment <br />storage level. A gate valve will be placed upon the discharge pipe <br />to allow for controlled discharges. The pond will be capable of <br />storing slightly more than the volume of runoff from one 10 year 24 <br />hour precipitation event above the discharge pipe. A marker will be <br />installed in the pond to indicate the 60X sediment storage level. <br />Waste Rock Disvosal Area <br />The sediment pond is designed to collect the sediment generated from <br />• the disposal area due to runoff. Runoff will be the result of <br />direct precipitation on the site. The estimated annual erosion rate <br />based upon the Universal Soil Loss fiquation is 908 cubic feet per <br />year. The pond has been designed to contain three years of <br />sediment, 2,800 cubic feet. This volume is based on unprotected <br />slopes and is therefore conservative. The pond will be cleaned when <br />the sediment level reaches 60X of maximum storage. A marker will <br />indicate the 60X level. <br />The 10 year 24 hour storm for the site is 1.5 inches and will <br />generate approximately 3,960 cubic feet of runoff. The pond has <br />been designed to contain 2 - ten year events; 7,920 cubic feet. <br />This volume is sufficient to totally contain a 50 year 24 hour <br />storm, 7,700 cubic feet. Details of flow near the pond are <br />described in Section 2.2.5. Calculations and assumptions are <br />provided in Appendix M(vii). <br />Discharges from the pond will be made through an eight inch <br />corrugated pipe with a normally closed valve on the invert. GVC <br />will then be able to hold and sample water in the pond until the <br />quality exceeds NPDBS requirements. The invert will be at the <br />elevation of maximum sediment storage. As requested by CMLR, a flow <br />control orifice of 0.8 inch diameter can be installed on the <br />discharge pipe to allow "unattended" discharge. Water quality <br />samples would then need to be collected and analyzed for each <br />discharge event. <br />C~ <br />~No~ <br />Hit ~ i. ~ ~ ~: <br />