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Exh. 7-23I-5 Revision Date: 4/28/22 <br /> Revision No.: TR-154 <br />Terrace Ditches <br /> <br />Two terrace ditches will be constructed across the face of the Little Collom Valley Fill to route <br />surface water runoff to the reconstructed Little Collom Gulch. The locations of both terrace ditches <br />are shown on Map 41B and Figure 7-23I-5. The terrace ditches on the Little Collom Valley Fill <br />will originate near the post mining contours of 7200’ and 7300’ elevations on the fill face. Actual <br />on the ground topography will dictate the final placement and profile of the terrace ditches to <br />ensure proper water flow. <br /> <br />The function of the terrace ditches is to collect surface flows and direct the flows to Little Collom <br />Gulch. 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 />approach. <br /> <br />A SEDCADTM model (100 Year 24 Hour Channel Demonstration model) has been included which <br />evaluates the peak flow for each terrace ditch. The minimum ditch configurations are summarized <br />below <br /> <br />Terrace Ditch <br />Terrace Ditch <br />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 />7200’ (Lower <br />Terrace Ditch 4.7 2.0 Triangular Left - 1.5H:1V <br />Right - 3.0H:1V 3.0 Vegetation <br />7300’ (Upper <br />Terrace Ditch) 9.78 2.0 Triangular Left - 1.5H:1V <br />Right - 3.0H:1V 3.0 Vegetation <br /> <br />