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b. All sedimentation ponds are designed to provide adequate sediment storage volume in • compliance with the DMG regulations. - c. An appropriate nonclogging dewatering device will be installed to dewater a 10-year, 24- hour storage volume. d. Appropriate combinations of principal and emergency spillways to safely discharge the runoff from a 25-year, 24-hour precipitation or larger event, as required by the DMG's regulations, will be installed in all ponds. The sediment storage capacity of each sedimentation pond will be monitored on an annual basis to ensure that at least one year of sediment storage is available beneath the principal spillway inlet (based on the R.U.S.L.E. or the equivalent sediment storage of one 10-year, 24-hour storm event based on the M.U.S.L.E.1~ When adequate capacity is no longer available, sediment removal or other corrective actions will be taken to restore adequate sediment storage capacity to the pond. Peabody will monitor the sediment storage capacity of each sedimentation pond with one or a combination of the following methods, as applicable: 1. Reference Stake - A reference stake will be set at a predetermined clean-out elevation beneath the principal spillway. When sediment storage reaches this elevation, corrective action will be implemented to restore adequate sediment storage. Reference staking will be utilized in sedimentation ponds that are normally dry and as an indicator for monitoring personnel as to when a more detailed survey of sediment storage capacity is warranted. 2. Cross Sectioning or Grid Method -For the sedimentation ponds that normally contain water year round, or where a reference stake can not be detected below the water surface, a survey cross section or grid method will be used to determine sediment volume using accepted engineering practices. The survey would be performed when needed to determine sediment storage capacity and to determine when corrective action is required to restore adequate sedimentation pond capacity. 3. Aerial Mapping - An alternative method is photogrammetric mapping that would create an accurate 1" = 50' topographic map with 2 ft. contour intervals to determine the remaining storage volume beneath the principal spillway. This method could be used as the sole method for dry sedimentation ponds or used in conjunction with the surveyed cross section method for sedimentation ponds partially full of water. Once the topographic map has been generated, accepted engineering methods will be utilized to perform the volumetric calculations. Corrective action required to restore adequate sediment storage capacity may include the following: 7. Raise the spillways and redesign the sedimentation pond in order to restore adequate • storage capacity and to comply with the applicable regulations. 2. Excavate the ponding area and enlarge the sediment storage capacity. TR-50 4 Revised 11/04