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Twentymile Coal Company 6RT Borehole Pad & Access Road Drainage and Sediment Control System Design <br />resulting from the 10 -year, 24 -hour storm. Sediment logs or straw wattles are modeled as a silt fence in <br />SEDCAD. SEDCAD's silt fence routine requires as input the flow rate through the sediment log or straw <br />wattle (37.5 gpm/ft the height and length of the sediment log or straw wattle (1.5 ft high x 145.9 ft <br />long), and the slope of existing ground that the log or wattle shall be placed on (8.6 %). Flow rate through <br />the wattle was reported by American Excelsior Company for their 20 -inch Curlex sediment log as <br />determined by ASTM D5141. American Excelsior Company also offers a 20 -inch straw wattle. <br />Currently, flow rate information for the 20 -inch straw wattle is unavailable; however, flow rates through <br />straw wattles are available for the 12 -inch and 9 -inch straw wattles, and have a substantially lower flow <br />rate than Curlex sediment logs for the same product size. Because straw wattles have a lower flow rate <br />than the comparable Curlex product, there will be a greater reduction in sediment concentration of <br />stormwater treated by straw wattles than Curlex wattles due to the increased detention time. Thus, either <br />a Curlex type or straw type wattle can be used since the modeling demonstrates that the Curlex sediment <br />log will be adequate, and that a straw watlle will be even more efficient. Hydrology parameters were <br />selected based on existing site conditions and mapping, aerial photo coverage, and NOAA Atlas 2 — <br />Precipitation- Frequency Atlas of the Western United States 1973. A detailed SEDCAD modeling report <br />is attached. The locations of the access road, borehole pad, and straw wattle (or sediment log) are shown <br />in Exhibit 1. <br />SEDCAD hydrologic input variables include the 10 -yr, 24 -hr rainfall amount (1.8 inches), the surface <br />area of the access road, borehole pad, and their contributing watershed (0.15 acres for the access road, <br />0.11 acres for the borehole pad, and 0.02 acres of Routt County Road 33 and 0.19 acres of <br />undisturbed/reclaimed watershed that runs onto the site). The soil -cover values (curve numbers, CNs) <br />that determine the amount of water that runs off vary for the different areas. The undisturbed/reclaimed <br />area was modeled with a CN of 81. The access road and borehole pad (that will be surfaced with coarse <br />gravel) was modeled with a CN of 85. <br />SEDCAD input variables for sedimentology include the soil erodibility factor (K), the soil cover factor <br />(C), the cover practice factor (P), and the soil particle size distribution. The soil type for the <br />undisturbed/reclaimed area is predominantly Coutis fine sandy loam, and the access road and borehole <br />pad will have a gravel soil type. The K, C, and P factors used in the SEDCAD model for the <br />undisturbed/reclaimed area and the access road and borehole pad are shown in Table 1. <br />TABLE 1. <br />K, C, AND P FACTORS FOR SOILS PRESENT AT 611T <br />Soil Type <br />K <br />C <br />P, <br />Undisturbed/Reclaimed <br />0.1 <br />0.03 <br />1 <br />Gravel <br />0.1 <br />0.08 <br />1 <br />SEDCAD was also used to predict surface water velocities over the disturbed borehole pad during the 10- <br />year, 24 -hour storm. The peak discharge from the total watershed was input into SEDCAD's Channel <br />Utility and given a bottom width of 70 ft (the width of the borehole pad) with a channel material of coarse <br />Water & Earth Technologies, Inc. Page 2 of 4 October 23, 2009 <br />