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1 <br />' PROPOSED HYDRAULIC CONDITIONS <br />Roaring Fork Resources, Inc. proposes to construct a series of four gravel pits along the south <br />side of the Colorado River. The first pit will be the "West property line to Horse fence Pit," <br />which is located at the east end of the current mining plan. The other three pits will progress <br />' from east to west. <br />Per the request of the Division of Fish and Games and the Division of Minerals and Geology, <br />' a berm is proposed around the perimeter of the pits to prevent the 50 year storm from entering <br />the gravel pits. The 100 year storm will be allowed to overtop the pits, but it will be shallow <br />flooding. Figure 5 shows the proposed limits for the 50 year and 100 year floodplains. <br />' The proposed berms are shown at 0.1 feet above the 50 year water surface elevation. <br />Typically, one foot of freeboard would be provided. However, the 100 year water surface <br />' elevation is less than one foot in many of the cross sections. The intent of the berms was to <br />allow the l00 year storm to flood to its historic locations and still restrict the 50 year storm <br />from being introduced into the pit. <br />The berms are located outside the 100 year floodway -which is where the velocities are <br />typically the highest. They range from a maximum height of 5.5 feet at approximately section <br />' 440 to less than a foot at the majority of the cross sections along the pit. The berm will be <br />constructed with 3:1 side slopes with a 5 foot level area along the top of the berm. The berm <br />will need to be constructed and armoring put in place at the beginning of the gravel extraction. <br />' According to Open Channel Hydraulics, Ven Te Chow, the maximum permissible velocity for <br />fine gravel to noncolloidal cobbles is 5 feet per second. The berms will be armored with <br />' riprap where the velocities exceed the 5 feet per second. The riprap sizing is based on the <br />"velocity method" presented in the "HEC-11" manual, U.S. DOT. The applicable charts are <br />provided in the Appendix. <br />' The erosion potential of the 50 year and 100 year flows are provided in the Appendix in <br />Tables 4 and 5. The proposed gravel pit is reflected in cross sections 437 through 443. From <br />' Tables 4 and 5, the velocities in the left overbank region range from 3.2 feet per second (at <br />Section 441.5) to 9.7 feet per second (at Section 442). <br />' A concern of the Division of Minerals and Geology is "pit capture". Pit capture occurs when <br />the natural channel for a river changes courses to the location of the gravel pits. When the <br />proposed pits are being actively mined, the available flow area is the greatest. When the pits <br />' are in the reclamation phase, the pits will be mostly full with groundwater and the available <br />flow area is much less. To reduce the potential for pit capture, the berms around the <br />perimeter of the pit will be armored to prevent them from breaching. Also, a control sill shail <br />' be installed at the downstream end of the active pit to give it a controlled overflow location. <br />(The configuration of the sill is provided in the Appendix, Figure 2-19, Urban Storm Drainage <br />Criteria Manual, June 2001.) <br />1 <br />