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29-hour event). The annual sediment yield was calculated with the <br />• Universal Soil Loss Equation (Haan & Barfield, 1978). The pond is <br />designed to store two years of sediment below the volume of the 10- <br />year, 29-hour event. Sediment from the pond will be cleaned when <br />60 percent of the allocated volume has accumulated. A staff gauge, <br />located in the pond, will be used to monitor sediment accumulation. <br />The 25-year, 29-hour capacity spillway will be located one foot <br />above the design runoff and sediment volume. <br />There will be no operational spillway, only an emergency spillway. <br />Water collected in the pond during runoff events will be pumped <br />from the pond into the hydraulic mining process water as part of <br />the regular makeup water consumed in the operations. Pump capacity <br />will allow dewatering the pond within 98 hours after the 10-year <br />event. Design features of Pond A are summarized in the table <br />below. As-constructed details of Pond A and associated facilities <br />are shown in Figure 9.3-6. <br />• <br /> Table 4.3-6. Summary of Sediment Pond A <br /> DS st urbed Area Und, srurbed <br /> Design Feature (Surface fa c.~ Arca 'oral <br /> Drainage Area 78.1 ac 125 ac. 163.1 ac <br /> 10-y r., 24-h c. Runoff 2.4 ac- R .7 ac-R 3.l ac-fc <br /> 10-y r., 24-hr. Runoff 37 cfs 11 c(s 44 cfs <br /> 25-y r., 24-h r. Runoff 52 cfs 25 cfs 77 cfs <br /> Annual Sedi me n[ Load 1.45 ac-ft .6 ac-ft 2.05 ac-ft <br /> Pond Surface Area - - 0.8 ac <br /> To[al Pond Sed. Ca paci[y 2.9 ac-([ 1.2 ac-ft 4.1 arE[ <br /> Total Pond Storage 5.3 ac-f[ 1.9 ac-E[ 7.2 a<-ft <br /> Slope of Pond Embankment 2:1 2:1 - <br />Sediment cleaned from Pond A, either after unusual precipitation <br />events, during annual cleaning, or after emergency slurry system <br />shutdown and diversion to the pond, will be added to the refuse <br />slurry system for disposal underground. However, immediately after <br />an emergency slurry system shutdown situation has been corrected, <br />the sediment will be pumped back to the process building for coal <br />separation, should the sediment contain coal fines. The maximum <br />. NCIG - Coal Ridge #1 4-39 ABC/1145/910509 <br />'rR- ~ <br />