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W A'TE~2 IMF O I~1~VI~'~'I~1~! <br />LJ <br />1. SURFACE WATER GENERAL DISCUSSION <br />E~IBIT G <br />It is seen from Map C-1 that a lazge portion of the permit azea is within the 100 year floodplain of <br />the Colorado River. If a 100 year flood occurs in the River, the 1929 datum shows that the water <br />level at this site will be 4462' at the western end of the permit area and 4466' at the upstream east <br />end. Using the correction of 0.99 meter to convert to the 1988 datum, the actual floodwater elevation <br />is approximately 4465' at the west end and 4469' at the east end. This level has been mapped on <br />Exhibit C-1 and should be more accurate than any regional floodplain map. Since there are no <br />structures within the mine pit area, the flood would simply be allowed to enter the pit. Once the <br />flood subsides and the water can be pumped cleanly, dewatering of the pit would resume. Any <br />storage provided by the pit would actually serve to lower the peak flow downstream and possibly <br />prevent damage to properties. <br />. The lakes created from the mining operation are not in any danger of being captured by the River <br />during a flood for two main reasons: <br />1) The Interstate is located between the main river channel and the site. <br />2) This section of River is not eroding the north bank at the west edge of the property. It is a straight <br />stretch of the River to the north until the river toms west at the section line between Sections 11 and <br />12. <br />The Mine Plan section describes how surface water runoff from the site will be handled. Basically, a <br />topsoil berm will be placed azound the south perimeter of the site to prevent any runoff from exiting <br />the site and to provide a visual berm to Interstate 70 .Once excavation of the lake is started, all <br />runoff will enter the excavation. <br />• <br />15 Road Pit 10/02 32 <br />