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,,. /anuary 1996 TR~65 Riprap Evaluation "Hayden Gulch Mine Permit No. C-80.003 • 1 HG COAL COMPANY RIPRAP VIABILITY EVALUATION The Colorado Division of Minerals and Geology (Division) requested additional information for the existing riprap evaluation in a letter dated December 28, 1995. Specifically, the Division questioned why the "western ditch° and Pond HG8's emergency spillway were omitted from the riprap evaluation. This supplemental submittal includes the analysis information requested by the Division which should allow a completeness determination for Technical Revision No. 65. Western Ditch Field observations conduced by TerraMatrix personnel during 1995 indicated that the existing riprap in the western ditch show some signs of weathering. However, the western ditch does not display any signs of recent erosion. The original design (see MR-O5, approved 8/21/87) specified an average Dso of six inches for the lower channel segment. The lower segment remains armored with a layer of rock revetment and a thick, healthy stand of perennial grasses growing in the channel. To ensure that the existing riprap will retain its energy dissipation funcion in the future, four riprap Dso scenarios were evaluated: • That the Dso of the riprap is six inches; • That the Dso of the riprap is three inches; • That the Dso of the riprap is one inch; • That the Dso of the riprap is one inch, and the channel contains perennial grasses SEDCAD+ was used to alculate peak flow. The total contributing drainage area to the western ditch is approximately 41 acres. Peak discharge from the 10-yeaz, 24-hour storm event is calculated at 2.4 cfs. The western ditch terminates at Pond HG8. Flow velocities were calculated using the FlowMaster hydraulic calculation model, which uses Manning's equation for open channel flow. The results of the F1owMaster modeling are included with this submittal. Table 1, Western Ditch Flow Chazaceristics, summarizes flow conditions for the four riprap scenarios: The results of the FlowMaster modeling indicate that the existing riprap will provide adequate channel protecion, both presently and into the future, against erosive flow under all four channel protection scenarios in the lower channel segment. Under each scenario, flow velocities were calculated to be significantly less than 5.0 fps. Consequently, the western ditch does not require additional riprap. TenaMatrix Inc. " P.O. Boz 774018 • Steamboat Spring; Colorado 80477 " (970J 879-6260