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Exh. 7-20B-4 Revision Date: 10/23/23 <br /> Revision No.: TR-161 <br />Terrace Ditches <br /> <br />A series of terrace ditches will be constructed across the face of the West Taylor Fill which <br />reports surface water runoff to the West Taylor Pond. The locations of terrace ditches are <br />shown on Map 12. The terrace ditches on the West Taylor Fill will originate near the even 100- <br />foot post mining contours of 7600’, 7700’, and 7800’ elevations on the fill face. Actual on the <br />ground topography will dictate the final placement and profile of the terrace ditches to ensure <br />proper water flow. <br /> <br />The function of the terrace ditches is collect surface flows and direct the flow to the permanent <br />post mine channels (East and West Fork Taylor Ditches) on the outer edges of the West Taylor <br />Fill. The design basis for these terrace ditches is the 100-year, 24-hours event as they will be <br />permanent in the post mine topography, and the 100-year 24-hour storm provides a conservative <br />design. <br /> <br />A summary of the configurations for each terrace ditch is provided on the table below. <br /> <br />Terrace Ditch <br />Terrance <br />Ditch Name <br />Peak <br />Flow <br />(CFS) <br />Average <br />Slope <br />(%) <br />Channel <br />Type Side Slopes <br />Minimum <br />Depth <br />(Feet) <br />Erosion <br />Protection <br />7600’ 1.41 1.5 Triangular Left - 1.5H:1V <br />Right - 3.0H:1V 3.0 Vegetation <br />7700’ 2.58 1.5 Triangular Left - 1.5H:1V <br />Right - 3.0H:1V 3.0 Vegetation <br />West 7800’ 7.49 1.5 Triangular Left - 1.5H:1V <br />Right - 3.0H:1V 3.0 Vegetation <br />East 7800’ 12.51 1.5 Triangular Left - 1.5H:1V <br />Right - 3.0H:1V 3.0 Vegetation <br />