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Determining Flow Rates and Runoff Volumes for Various Curve Numbers <br />Using a maximum drainage area of 640 acres, a representative drainage basin was <br />modeled under the assumed designed storm with a curve number of 72. Refer to <br />modeling that is presented in Appendix A. The storm produced a peak flow of 9.3 cfs and <br />a total runoff volume of 3.65 acre-feet. Similarly, Appendix B shows the identical <br />watershed modeled with a curve number of 67, which is representative of reclamation <br />with 2 year growth. This storm produced a peak flow of 1.67 cfs and a total runoff volume <br />of 1.37 acre-feet. Finally, Appendix C shows the same watershed with a curve number <br />of 62. This storm produced a peak flow of 0.45 cfs and a total runoff volume of 0.20 acre- <br />feet. <br />It can be observed that all of these peak flow rates are below the erosion threshold shown <br />in Figure 1. Accordingly, the check dams are not needed for most conditions of mature <br />and nearly mature reclamation. <br />In early stages of reclamation, before the vegetative cover is mature, the resultant flows <br />are large and the check dams are useful to establish grass in the reclaimed channel. As <br />reclamation from watersheds becomes more mature, the appropriate curve number <br />becomes lower and the resultant peak flow and total runoff volume are decreased to <br />values less than the erosion threshold in a grass -lined channel. Accordingly, by the time <br />reclamation is suitable for bond release, the check dams are no longer needed. <br />Throughout the thirty plus years of its history, Trapper Mine was aware that the <br />establishment of a permanent and self-sustaining vegetative lining in the channels would <br />be enhanced by the use of check dams and other measures, but also has shown a <br />consistently observed history of stable channels by the time the land is ready for bond <br />release. This study, by evaluating this process on a generic 640 acre watershed at <br />various channel gradients and for reclamation conditions somewhat before bond release <br />(i.e., 2 years growth and CN=67) provides the analytical support for what Trapper has <br />consistently observed over the years. <br />Part 2 <br />Determining Suggested Number of Check dams for Various Watersheds <br />The final step in this process is to recommend a method to specify the number of check <br />dams required during the initial period of reclamation. As the channel slopes increase, <br />the temporary storage capacity decreases. See Figures 2, 3, 4, and 5. For example, the <br />storage capacities achieved for a 3 -foot high check dam constructed in a 20 foot wide <br />channel vary as shown below. <br />Channel slope <br />Storage Capacity <br />5% <br />0.0435 ac -ft <br />10% <br />0.0225 ac -ft <br />15% <br />0.0137 ac -ft <br />20% <br />0.0129 ac -ft <br />25% <br />0.0033 ac -ft <br />