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Annual sediment rate = Vs = (18,008 tons/yr)(2,000 Ibs/ton) = 444,494 ft3/yr <br />• BI Ibs/ft3 <br />Vs = 10.2 ac-ft/yr <br />Because these sediment yield calculations are conservative, Peabody Coal Company will <br />reevaluate these calculations in Their 1986 Annual Hydrology Report for the Seneca II <br />Mine. <br />COMMENT 2 <br />My calculations show a slight deviation from The ones presented by Peabody for the shop <br />area pond (003). Sediment levels for this pond do not require cleanout at present, <br />however, my calculations show 1.56 ac-ft of storage remaining before the 60 percent <br />level is reached. Please evaluate and correct if necessary. Peabody may want to <br />re-evaluate annual sediment yield calculations for this pond also. <br />RESPONSE: <br />Anne Baldrige's calculations are as follows: <br />• Shop Area Pond (003 Pond) <br />3.39 ac-ft Runoff <br />I I.00 ac-ft total volume <br />I I.00 - 3.39 = 7.61 ac-ft sediment <br />60% level is 4.57 ac-ft <br />Current sediment level is 3.01 ac-ft <br />These calculations are not in agreement with Those presented in PCC's II/5/85 letter <br />because of disagreement over what the initial storage volume for sediment in the pond <br />is. The peak inflow from the 10-yr, 24-hr storm event is 34.9 cfs (see attached runoff <br />hydrograph, Table I, at end of narrative). The 36 inch CMP principal spillway with a 2.9 <br />foot headwater depth can discharge 35 cfs without discharging through the emergency <br />spillway. As a result, PCC used the capacity of the pond from the principal spillway, <br />8.14 ac-ft, as the initial sediment storage capacity of the pond. The calculations in the <br />I I /5/85 letter are therefore still valid. PCC will reevaluate annual sediment yield <br />calculations in Their 1986 Annual Hydrology Report. <br />• <br />2 <br />