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IV DRAINAGE FACILITY DESIGN A. General Concept 1. Concept and typical drainage patterns. The result of mining the Castle Rock Quarry site is not expected to significantly alter the drainage pattems of the upland azea. The final topography of the site is expected to be very similar to the current topography. The rhyolite deposit has a rather uniform thickness and dips in the same direction as the current ground surface. During large storm events Stormwater is assumed to accumulate near the edge or "rim" the post-mining sub-basins. However, there could be dips, or ' depressions, in the final quarry floors that would provide for additional runoff storage. 2. Compliance with off-site runoff considerations. Drainage will be confined to the upland areas and will not increase runoff off-site. ' 3. Tables, charts, figures, or drawings. Enclosed with this report aze the pre-mining and post-mining drainage plan maps for the Castle Rock Quarry site (Appendix A). A brief itemized list of some of the drainage data on the pre-mining maps include sub- basin delineations and azeas, basin area, design points, Vavel paths, and SCS weighted curve numbers. Also included on these maps are Runoff Summary Tables. The tables include volumes and peak flows predicted by the Sedcad program for the 10-year and 100-year 24-hour storm events. ' The post-mining maps contain most of the same information as the pre-mining maps. Sub-basins were delineated for the mined upland areas and sepazately for those areas below the mine. ' The available storage in each upland post-mining sub-basin was compared graphically to the azea required for back-to-back 100-year 24-hour storm events on the post-mining plan maps. The shaded area is the available storage area and the hatched pattern depicts the area required for consecutive 100-year storm events. The Post-Mining Storage Volume ' Tables also compaze the available and required storage volumes, as well as the available freeboard. ' The Runoff Summary Tables on the post-mining plan maps list the predicted runoff for 10- and 100-yeaz 24-hour storm events from the undistwbed sub-basins below the mined areas. The post-mining design points are the same as for pre-mining, that is at the ' Winkler lease boundary, for each of these basins. Appendix B includes the Sedcad modeling results. ' 4. Discussion of anticipated and proposed drainage patterns. Discussed in Section IV,A,1. ' S. Stormwater quality. A Stormwater discharge permit has been obtained from the Colorado Department of Health and a Stormwater Management Plan (SWMP) prepared. An updated, current copy of the SWMP will be a kept on-site by the operator throughout the project. 10