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<br />• <br />• <br />profession, the depth of flow for the 25-year flood peak of 600 cfs was <br />estimated to be not more than 4 feet in depth. Pertinent parameter for this <br />calculation are the stream slope of 1.3X, a roughness coefficient o .040 for <br />the main channel and 0.060 for the sparsely vegetated flood plain a ea. At a <br />depth of flow of 4 feet, the flow in the stream will be approximate y 600 cfs. <br />This flow depth is approximately from the bottom of the existing at eam <br />channel, not the current water surface. This value is reasonably i <br />conformance with the existing stream flow noted on June 19, 1990. he flow <br />depth of the stream at that time was a maximum of 2.5 feet, and the <br />approximately 30-foot top width of the water surface was contained 'thin the <br />normal stream channel. According to the city of Aurora, which moni ors flows <br />in the Middle Fork of the South Platte, the flow on that date was <br />approximately 130 cfs. They also noted that the peak of the spring runoff <br />season has just passed and the runoff is beginning to recede. <br />The toe of the pond embankment extends virtually to the water surfs a at only <br />one location. As previously noted, this area has been riprapped to an <br />elevation of from five to six feet above the existing water level, hich means <br />it is protected for a flow depth in the stream of approximately eig t feet. <br />We note that one other location of the berm toe is approximately fo r feet <br />above the current water level, but also is some twenty or more feet away From <br />the bank of the water surface. At four feet above the existing wat r surface, <br />this second location is approximately six feet above the stream bot om, xhich <br />is two feet in excess of the estimated four foot Flood depth. <br />The only portion of the embankments that is within the 25-year floo plain is <br />riprapped to withstand the 25-year flood, and no other areas of the embankment <br />are in danger,by the 25-year flood of the Middle Fork of the South latte <br />River. , <br />POAD'CAPACII'Y <br />As stated in our June 8 submittal, pond capacity is not yet finaliz d since <br />earthmoving operations are still underway. That submittal, however stated <br />that we estimate a water surface area of approximately 800 feet lon by 200 <br />feet wide, with an estimated depth of about eight feet. Conservati ely <br />estimating an average depth of only four feet, rather than the eigh feet, the <br />associated storage volume in the ponds is approximately 15 acre fee . <br />On June 19, 1990, the MLRD staff indicated that they desired a spec'fic <br />operating water volume of at least four hours worth of storage. Pe the <br />original Application, the estimated mine water flow is 7500 gallons per minute <br />(gpm). A Eour hour duration of flow yields 5.5 acre feet of water. <br />Therefore, according to data submitted on June 8, the mining operat'on will <br />have approximately three times the minimum storage desired by the D staff. <br />We note that as of June 19, it appeared that the ponds would be dee er than <br />eight feet, possibly as much as 16 Feet in places, and 12 Eeet in ge eral. <br />This adds even greater storage volume than previously estimated. <br />SEALING OF POND ONE <br />The l4.RD has expressed concerns that outflox seepage from the ponds may be <br />excessive. Leakage from the impoundment was discussed in our Junc <br />submittal. We stated that the pond bottom would be essentially sel -sealing <br />-2- <br />