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