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... . ~ LI .,~.,y,,ak; .. }Ivdrologic Balance - Apex No. 2 ~ , Plinor Modification ~~"E~`~ I. Constructior. of Setting Pond Sunland has committed to construct a sedimentation pond large enough to ronta i.n a total volume, including sediment, of 32,700 cubic feet. F;u no Ef Erom the 10-year, 24-hour precipitation event from the 5.6 acres contributing to the pond is 15,246 cubic feet. This was calculated using the procedures outlined in '"Procedures for Determining Peak Flows in Colorado" and "Urban Hydrology for Smai.l [Jatersheds", both published by Che Soil Conservation Service. The sediment volume required is 17,424 cubic fee[, determined using 0. 1 acre-feet per acre of disturbed area. The arr_a to be disturbed totals four acres, and the calculations are therefore correct. The [oral volume of runoff and sediment totals 32,670 r_ubic feet. The pond is designed for 32,700 cubic Ecet, thus making its size aden~iate to contain [he total runoff. fro{n Che 10-year, 24-hour storm. II. Construction of Ditches into and Discharge Facilities From Settling Pond Collector ditches !dos. D1L and D1~ cuncentr.ate runoff from the disturbed area ^nd route it to the sedimentation pond. Both are designed to convey the calculated runoff volumes to the pond w~i.th velocities of three feet per second or less. These calculations and design criteria have been checked and found to be correct. Dive n;ion ditch Nc. D10 is designed Co route runoff from Basin No. 1 (an undisCucbed area} around the disturbed area and cventua.l.ly .into Trout Creek. 1'he to[a.1 surface runoff Erom the basin has been calculated to be 12.5 cfs, which is correct. However, giver, this volume and the design criteria presented in the mine plan, the velocities calculated are too exit@me and will probably cause excessive channel erosion. A velocity of. 10.4~feet per second is way too high [n establish a stable channel. Therefore, this diversion ditch, No. D10, needs to be redesigned to transport the calculated runoff at an acceptable velocity. The publication to reference is Open-Channel Hydraulics by Ven Te Chow, 1959, pgs. 21 and 165. The material in which the ditch is to be constructed must he identified to help determine the maximum permi.sia6le velocity. All of this i.nformatinn needs to he snbmitte_d [o OSi~f fur review. The sedimentation pond dewatering system, .including the emergency spillway design, has been found to he adequate. The four inch E.iherglass riewatering pipe will. safely pass the 10-year, 24-hoar runoff. volume while achieving the minimum theoretical detention time. The emergency spillway is designed to pass 9. .6 cEs, while the ,equi.red dis4harge for peak flow from the 25-year, 24-iieur storm is 9.0 cfs. ..-,~y~,. .. ' y~ - , - - -_ - ~.y - - ..~~...ti~.~._~.wr~-~:f~9 ~ .~ ~. ,:L_ v iA:wi'~~.3J~CW.it'!~