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Ms. Anne Baldrige Page 3 November 5, 1985 Actual annual rate of sediment yield 0.53 ac: ft./yr. (3.01 ac.-ft. sediment in pond divided by 5.7 yr. life of pond = 0.53 ac.-ft./yr.) There remains 5.13 ac.-ft. of available storage (estimated to the bottom of the principal spillway) in pond 003 at this time. Using 60% of total estimated storage capacity, pond 003 should have sediment removed when sediment volumes reach 4.88 ac.-ft., or 1.87 ac.- ft. of sediment in addition to the present volume of 3.01 ac.-ft. of sediment. At an annual sediment rate of 0.53 ac.-ft./yr., pond 003 should be cleaned out in 3.5 years. Because of a surveying and plotting error in the 1980 pond survey, the actual elevation of the principal spillway is approximately one (I) foot higher than calculated in 1980. The elevation has recently been rechecked and verified in the field; thus, the pond is 8.14 ac.- ft. as opposed to 7.07 ac.-ft. as originally calculated. Pond 004 (Southeast Areo Pond) calculations are as follows: 1q,~b"I As-built sediment pond storage volume 11.39 ac.-ft. including water and sediment (based on 7/82 and 10/85 surveys) Present remaining pond storage volume 10.20 ac.-ft. considering present sediment levels (from 10/85 sediment survey) Present estimated volume of sediment 1.19 ac: ft. (as-built volume minus remaining surveyed volume) USLE estimated annual rate of sediment yield 0.67 ac.-ft. Estimated actual annual rate of sediment yield 0.38 ac: ft. (1.19 ac.-ft. sediment in pond divided by 3.1 yr life of pond = 0.38 ac.-ft./yr.) There remains 10.20 ac: ft. of available storage (estimated to the bottom of the principal spillway) in pond 004 at this time. Using 609'0 of total estimated storage capacity, pond 004 should have sediment removed when sediment volume reaches 6.83 ac: ft. or 5.64 ac: ft. of sediment in addition to the present volume of 1.19 ac.-f t. of sediment. At an annual sedimentation rate of 0.38 ac.-ft./yr., pond 004 should be cleaned out in 14.8 years. In addition to concerns for sediment levels in the sediment ponds, there is a question as to whether these ponds can store the 10-year, 24-hour storm event without discharging through the emergency spillway. Based on PCC's review of Rule 4.05.6 and subsequent conversations between PCC and CMLRD staff, water can be discharging through the principal spillway during the 10-year, 24-hour storm event as long as there is no dis- charge through the emergency spillway and required effluent standards are met. CaICu- lations by PCC hydrologists show that the 10-year, 24-hour storm event can be stored in each of the three sediment ponds without discharging through the emergency spillway. Thus precluding the need for activating the 004 pond dewatering device.