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© Mr. Tom Kaldenbach <br />October 30, 1997 <br />Page 3 <br />embankment shall be 1.0 above the water surface in the pond with the emergency spillway <br />flowing at design depth. <br />Exhibit 12-4 in the existing Permit shows that the combination spillway invert is at 6,383.3 <br />feet above mean sea level (msl). The top of the embankment is approximately 6,384 fee[ <br />above msl. Field observations by HGTI personnel during spring pond inspections have <br />noted that during the spring snowmelt season, the water elevation of the discharge in the <br />combination spillway is less than 1.0 below the top of embankment. <br />SEDCAD+ modeling of the 25-year, 24-hour storm event suggests that the discharge water <br />elevation would be approximately 6,383.5 feet. <br />In order to meet the intentions of Rule 4.05.6(11)(e), HGTI proposes to raise the <br />embankment elevation approximately 1.0 feet. Construction shall follow accepted <br />engineering practices and meet the maximum design requirements of U.S. Soil Conservation <br />Service Public Standard 378, "Ponds", Colorado, fanuary, 1989. <br />Upon completion of the pond embankment modification, HGTI will submit an as-built <br />drawing of the Rail Loop Pond to the DMG. <br />4.0 CONCLUSIONS <br />The SEDCAD modeling demonstrates that the existing pond configuration contains <br />sufficient storage capacity to treat runoff From the 10-year, 24-hour storm event. In <br />addition, the pond has, at a minimum, 10 years of available sediment storage. <br />A copy of the proposed public notice, to be published in the S[eamboat Pilot is included as <br />Attachment B. The attached table, Rail Loop Pond Technical Revision Table, summarizes <br />how this TR needs to be inserted in the to existing HGTI Permit. <br />If you have any questions, please don't hesitate to contact either of the below undersigned. <br />Sincerely, <br />TerraMatrix/Montgomery Watson <br />n C. W~ m~~- C <br />Project Hydrologist _~ <br />~~¢( /G <br />Thomas E. Leidich, P.E. <br />Senior Engineer <br />encl. <br />cc: Thomas B. Petrosky, HGTI (3 copies) <br />Project File 883 <br />w.\88]\Rail Loop Pond <br />le/JC/97 e!w <br />