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In accordance with Section 4.05.3 of the Colorado Mined Land <br />Reclamation Board (CMLRB) rules and regulations, the spillways for check <br />• dam types 1 through 4 have been designed to safely pass the peak flow from <br />the 10-y r, 24-hr storm event. These structures are considered permanent, <br />the watercourse is ephemeral and has a drainage area above the lowermost <br />structure is less than one square mile. The spillways for check dam types <br />5 through 7 have been designed to safely pass the peak flow from the 100- <br />yr, 24-hr storm event in accordance with Section 4.05.4 of the CMLRB rules <br />and regulations. These structures are considered permanent, the watercourse <br />is ephemeral and has a drainage area greater than one square mile. <br />Figure 4.6-M1 shows a typical design for a check dam, check dam weir <br />cross section, and check dam exit channel cross section. Table 4.6-M1 <br />provides the design specifications for each of the types of check dams and <br />check dam weirs. Table 4.6-M2 provides the design specification for each <br />of the types of check dam exit channels. Appendix 4.6-M(a) contains a <br />description of the various SEDIMOT II input parameters, the SEDIMOT II <br />worksheets for hydrograph routing between structures and time of <br />concentration within a watershed, and the computer simulation output <br />summary sheets. <br />• TEARACB PLACEM$NT AND D89ICN9 <br />A total of 14 draining terraces have been designed for placement <br />across the West Ridge mining area. The terraces are spaced up and down the <br />slope at intervals varying from about 300 to 600 feet, with spacing being <br />based upon both field reconnaissance of West Ridge and topography maps. <br />The terraces will consist of two configurations, those having <br />triangular cross sections and those having trapezoidal cross sections. All <br />of the terraces are designed to have a flow gradient of 1.5 percent. At <br />the confluence of the terrace outfall and the watercourse, the terrace <br />gradient may be increased to five percent to ensure flushing of the terrace <br />from potential sediment accumulation. The triangular shaped terraces are <br />used to convey runoff in areas where the flow velocity at peak flow is less <br />the four feet per second. The trapezoidal shaped terraces are used to <br />convey runoff in areas where the triangular shaped terraces are of <br />insufficient size to convey the runoff in a non-erosive fashion. As such, <br /> terraces 7 through 14 use a combination of triangular and trapezoidal <br />• configurations (see Exhibit 4.6-M3 and Table 4.6-M3). <br />3 <br />March 31,1994 <br />