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The results of this pre-mining runoff prediction are summarized in the pre-mining plan <br />maps. The notes, worksheets, and Secad input/output tables for the runoff analysis are <br />included in Appendix B of this report. <br />Post-mining <br />To limit the potential erosion and sedimentation impacts of releasing stormwater from the <br />' mined areas to the minor drainageways, CMI proposes to retain all storrnwater on the site <br />for 8.8" of rainfall The amount of runoff was set equal to the rainfall. <br />' This plan simplified the modeling of the post-mining upland areas. The retention <br />capacities were calculated for post-mining upland areas and compared to the storage <br />volumes required for back-to-back 100-year 24-hour storm events (8.8" of rain). The <br />' results are summarized on the post-mining plan maps. <br />The runoff volumes from the sub-basins below the mine site were predicted using Sedcad <br />to compaze to the pre-mining conditions. The results aze summarized on the post-mining <br />drainage plan maps. The Sedcad program results aze included in Appendix B. <br />3. Storage calculation method. A conservative approach was taken to predict the storage <br />volume requirements. The following assumptions were made for the upland post-mining <br />sub-basins: <br />a) The entire Castle Rock Quarry site would be covered by the same depth of <br />' precipitation. <br />b) The mining area is impermeable with no evaporation, infiltration, or any other <br />potential loss of runoff from the upland area during storm events, which is <br />conservative. <br />i c) The unmined, undisturbed areas of the upland sub-basins tributary to the mine <br />azea are also completely impermeable, which is very conservative. <br />d} All stormwater from the upland sub-basins will accumulate at the edge, or "rim" <br />areas of the proposed quames. The projected quarry floors aze based upon post- <br />' mining relief and slopes being similar to pre-mining topography. This is CMI's <br />best estimate from test holes drilled to the bottom of the rhyolite deposit. <br />However, mining rhyolite at the Ditmar Quarry has proven that there are local variations <br />in the thickness of the rhyolite deposit. This means there might be more undulating or <br />uneven final topography than shown in the post-mining plan maps. This in turn would <br />' provide more available runoff storage volume than included in our estimates for this <br />report. Again, the approach used for our computations was conservative. <br /> <br /> <br />