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facilities will. pass through a sediment pond designed to ensure that all discharges meet effluent limits in Grand Valley Coal Company's NPDES permit for the Munger Canyon Mine. A storm water permit and spill prevention containment and control plan associated with the NPDES permit also dictate preventive and mitigation measures for ensuring that all water leaving the mine site meets applicable effluent limits. The Division proposes to grant a small area exemption from use of sediment ponds due to the limited size of the azea and due to the fact that ponds and treatment facilities aze not necessary for the drainage to meet the effluent limitations of Rule 4.05.2 and applicable State and Federal water quality standards for receiving streams and also due to the fact that no mixing of surface drainage with a dischazge from underground workings will occur. The small area to be exempted is located at the portal mine bench. This sediment trap is adequately sized to control a 10-year, 24-hour storm event. Calculations for this trap are located in Appendix M (viii). The volume of the sediment trap is approximately 0.10 acre-feet. The Division approved the design of this sediment trap in Technical Revision 5, which Grand Valley Coal Company submitted in answer to the 1996 Mid-Term review. A sediment pond has been approved at such a time that the bench area is enlarged (4.05.2(3)(b)(I)). B. Diversions and Conveyance of Overland Flow The Division proposes to approve a permanent diversion of an ephemeral drainage at the reclaimed mine bench azea. This diversion is consistent with applicable state law, will not diminish downstream water rights and meets the requirements of Rule 4.05.4(4). Mine Bench Area Collection Ditches. Culverts and Diversions- The bypass culvert conveying undisturbed flow beneath the mine bench fill is sufficient to pass the required 10-year, 24-hour peak flow. Velocities of water discharged from the culvert during the 10-year, 24-hour event will be moderate. Since the applicant proposes to install a dumped roclt gutter at the discharge end of the culvert, the energy should be properly dissipated. The bypass culvert will discharge into a riprap-lined temporary diversion which will convey the flow into the natural drainage. Due to the high slope of the channel, flow velocities during the 10-year, 24-hour event could be as high as 16 feet per second. The applicant has committed to the installation of adequate channel lining to inhibit erosion of the diversion and appropriate measures to dissipate flow energy prior to discharge into the receiving; drainage. The collection ditches and culverts conveying runoff from the disturbed area at the mine bench will adequately carry the flow resulting from the IO-year, 24-hour event. Ditches collecting disturbed flow and diverting undisturbed flow at the underground development waste dispo:;al site will also be stable under the (0-year, 24-hour peak flow conditions. rr