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• EXHIBIT 7, ITEM 20, PART G <br />TERRACE DITCH DESIGN <br />A series of terrace ditches will be constructed across the face of the permanent fills. The location <br />of these are shown on Map 41 A. These ditches will be approximately located along the 100 foot <br />postmining contours. The function of the ditches is to direct surface water runoff from the face <br />towards the riprap channels of the groin ditches denoted as the East and West Tributaries of the <br />East Taylor Ditch and the West Taylor Fill Ditch. <br />The longest segment of the proposed Terrace Ditches is a westward draining branch of the ditch <br />originating at the 8100 foot elevation; this segment is approximately 3,862 feet in length. Using <br />this length as a design basis, and assuming that the contributory drainage azea to the ditch is <br />composed of a 300 foot wide upstream area (100 feet of elevation change at 3H:1 V postmining <br />slope), the maximum drainage area for a ditch segment would be 300 feet times 3,862 feet (26.6 <br />acres). Most of the terrace ditches would have somewhat less contributory drainage area, but <br />this value was used for all segments. <br />SEDCAD4 was used to determine the peak flow from this drainage area for a representative <br />ditch segment as detailed on the following pages. Two separate SEDCAD4 runs aze provided to <br />• model a 100 year, 24 hour design event. One run assumes a Curve Number of 86 which would <br />be representative of a short term condition where the upstream area has recently been topsoiled <br />and seeded but which is without substantial vegetative cover. The second run assumes a Curve <br />Number of 60 that would be representative of the 1-2 year growth and would be more <br />appropriate for the long term condition. The first case produced a peak flow of about 39 cfs and <br />the second (long term) case generated about 6 cfs under the 100 yeaz event. As shown, a <br />triangular ditch cross section is assumed, with a channel slope of 0.015 ft/ft and 3H:1 V slopes. <br />Modeling indicates that a ditch with a depth of 1.80 feet would be required. <br />For a conservative design, the following ditch specifications will be used: <br />• a triangular ditch cross-section with laterals provided every 100 feet of elevation. <br />• a total depth of 4.0 feet to insure freeboard adequacy, <br />• 3H:1 V sideslopes, and <br />• a maximum channel slope of 0.015 ft/ft (1.5%) <br />No erosion protection is necessary for these lateral ditches. The above design criteria are similar <br />to the lateral ditch designs used for the Section 16 Fill, Streeter Fill, and West Pit Fills. The <br />Colowyo Mine has a demonstrated history of these designs functioning properly. <br />• <br />Exh. 7-20G-1 Revision Date: 3/29/07 <br />Revision No.: PR-02 <br />