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2 Permits Sediment retention structures will be constructed prior to mining related disturbance in the • overlying watershed. These structures are designed to have adequate sediment storage, to effectively treat the 10-yeaz 24-hour storm for settleable solids, and to safely pass the 25-year 24- hour storm event through the discharge structures. Sediment control reservoirs will be operated to maintain their effective design capacity. Sediment will be cleaned out at approximately 60 percent of the three-year sediment storage level. The sediment level in ponds will be visuaity approximated during the quarterly pond inspections, as water level conditions permit with support from a staff gage placed at the base of the pond on the inlet side. Gradations wilt be marked on the staff gage and the maximum storage level will be marked on the riser pipe, to allow sightings during dry conditions. Cleaning will be done during dry periods of year, typically between fall and mid-winter or in eazly summer when the pond sediments have had an opportunity to dry out. Depending on their portability, cleanings may be stacked upstream yet adjacent to the pond to dry out, or they may be immediately taken to the surface mine For disposal in the pit. The ponds are designed to prevent diminution to downstream water users. Each structure has been designed under the supervision of a professional engineer. Ponds are shown on Map 2.05.3-2 and design documentation is presented in Exhibit 15. All impoundments are designed to have embankments approximately 10 ft high and storage capacity less than 20 acre feet. In those locations where the topography is too steep, storage capacity is achieved by excavation. Excavated impoundments aze incised no deeper than 5 ft below the bottom of the embankment, and the volume in the lower 5 ft is used for sediment storage and dead pool. Exhibit 15-14 illustrates typical design parameters for both totally impounded and excavated ponds. By using excavated impoundments, LCC anticipates that • MSHA and State Engineer's approvals will not be required. Excavated material will be used for embankment construction and placed in the mine backfill or in the excess spoil fill areas. Topsoil will be stockpiled near the pond sites. Small azea exemption (SAE) designs are included in Exhibit 15. These provide documentation that the effluent draining from small disturbances at auxiliary facilities or adjacent to topsoil piles outside the major disturbance areas will meet effluent limitations following the use of one or several sediment control measures. These include but aze not restricted to the use of rock, mulch, straw bale dikes, silt fencing, rock check dams and vegetative filters. A small (approximately one acre) service azea will be constructed to the north of the Jeff Canyon Haul Road, as shown on Figure 2.05.3-14b. This area will be used for placement of portable fuel Tanks, ANFO prep bins, a cap house, a miscellaneous equipment. The site will be leveled and graded to drain to a bermed drainage collection area on the south east side, where storm water will be treated with a silt fence or gravel drain before discharge. SEDCAD calculations (Exhibit IS) estimate the runoff from a 10-yeaz, 24-hour, storm event will be approximately 0.2 acre-ft which can be effectively treated with a 100-ft silt fence. The mine office SAE is located in an area previously disturbed by ranching and oil and gas operations. Effluent limits will be met by applying gravel to traffic areas and constructing a containment berm along the north edge of the site. • This bean will include a gravel drain structure at its lowest elevation. Page 2.05-33 Rule 2 Permits (Apr.4.02).doc 2.05-33 (Revised 04/04/02)