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-3- Sediment Ponds Sediment ponds at Pits 1, 2, 3, and 4 are located on hill slopes, and as a result, discharge could potentially result in significant erosion if not properly controlled. Rule 4.U5.7 requires that discharge be controlled by energy dissipators where necessary to reduce erosion and minimize disturbance to the hydrologic balance. To date, ponds at pits 1, 2, and 3 have hot discharged. Pond sizing calculations have not been submitted for these ponds and it may be that the ponds are of sufficient volume to contain the entire runoff from the 25 year, 24 hour event. As previously approved existing structures they may be acceptable. However, design drawings and certification that the ponds have been constructed as designed (Rule 4.05.6(7)) must be included in the permanent program permit application. 2. The design of the recently constructed pond at Pit 4 has not yet been approved under the permanent program by the Division. Water would be discharged from a 3U" diameter culvert primary spill~aay onto a 600 foot slope mapped as a landslide deposit with slope gradients approaching 50'x. Figure 9 indicates riprap placement 'for a short distance below both the emergency and primary spillways, but no further detail is provided. As required by Rule 4.05.7 a plan for a discharge structure designed according to standard engineering design procedures should be submitted to the Division. Energy dissipation should be provided wherever design flow velocities would result in erasion. Diversion of discharge flows along the contour to the adjacent stream channel may be an acceptable approach. 3. Statements in the text and Table 4 seem to indicate that the pond at Pit 4 has been sized to store the volume of a 10 year, 24 hour precipitation event together with a 1 year accumulation of sediment without discharging. However, Figure 9, the Detail Drawing, indicates the water level in the pond resulting from a 10 year, 24 hour storm would be 2 feet above the level of the primary spillway riser. The applicant should clarify whether any water would be discharged from the primary spillway as a result of a 10 year, 24 hour or smaller runoff event. 4. Rule 4.05.6(3)(c) requires a non clogging dewateriny device at the 100% sediment storage level to remove water storage rsulting from inflow. The opening should provide for at least 24 hours detention time. Designs do not indicate a dewatering device for existing or proposed sediment ponds. 5. The pond at Pit 4 and the pond at Pits 5 and 6 store water in excess of 2 acre feet. To be in compliance with requirements of the State Engineer, the applicant must obtain water rights or provide for discharge of water stored in excess of 2 acre feet within 36 hours.