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Once mining and reclamation is completed 16 months after the operation commences, 7.5 acres of <br />the original permit area will remain as a lake and the remainder of the land will be restored to dry <br />rangeland. <br />3. Drainase Facilities and Hvdrolo~ic Impacts <br />The overall design of the sediment control system is to do the following: <br />1) ensure that water pumped from the pit into the pond to the south will be clean. <br />2) contain the runoff from all disturbed areas using ditches around stockpile areas and the excava- <br />tionlake to hold runoff. <br />3) see that no damage is done for the 100 year flood from inundation of the site. <br />If a 100 year flood occurs in the Colorado River, the 1929 datum shows that the water level at this <br />site will be 4442'. Using the correction of 1 meter to convert to the 19$8 datum, the actual <br />floodwater elevation is approximately 4495'. This level has been mapped on Exhibit B-1. It is seen <br />that a large portion of the site will be within the 100 year floodplain. Since there are no structures <br />within the mine area, the flood would simply be allowed to enter the pit. Once the flood subsides <br />and the water can be pumped cleanly, dewatering of the pit would resume. Any storage provided by <br />the pit would actually serve to lower the peak flow downstream and possibly prevent damage to <br />properties. <br />'The lake created from the mining operation is not in any danger of being captured by the River dur- <br />ing a flood for two main reasons: <br />1) the distance to the normal bank edge of the River at its closest point to the lake is 420 feet <br />2) this section of River is not eroding the north bank, It is a straight stretch of the River to the north- <br />west. <br />As described in the general discussion of the mine plan, a 1' ditch will be used along the base of the <br />stockpiles to isolate runoff water in these areas. The slopes•of these ditches will be 2H:1V. Once <br />19 Road Pit 10)02 14 <br />