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West Elk Mine <br />The existing stream channel characteristics have :been defined for various parameters, including <br />channel slopes, peak flow rates for a variety of frequencies of occurrence, flow velocities for each <br />of the frequencies, sediment transport regime, stage-discharge relationships, channel profiles, and <br />channel and over-bank stability. <br />For each of the sixteen existing stream channels, many hydrologic and stream channel parameters <br />were defined and evaluated by Wright Water Engineers and Mr. Pemberton, including: <br />1. Mean annual runoff <br />2. Peak discharges for the 2-year, 10-year, and 100-year frequency floods. <br />3. Mean annual sediment yields. <br />4. Average thalweg' (Line following lowest part of a valley, i.e., invert to channel) slopes. <br />Note: 'Line following lowest part of a valley, i.e., invert to channel. <br />5. Channel characteristics for the dominant discharge (consisting of width and depth <br />relationships). <br />6. The range of the channel slopes from near mouth to upper reach. <br />The mean annual runoff expressed in acre-feet per year per square mile represents a fundamental <br />input parameter in the determination of annual average sediment yield. For the purpose of annual <br />average sediment yields for the subject basins, WWE has concluded that an appropriate (i.e., <br />conservative) mean annual runoff for the subject basins of 475 acre-feet per square mile per year <br />should be adopted. <br />Even though site-specific data for the basins would likely indicate that a smaller mean annual <br />runoff of less than 475 acre-feet per year per square mile, the adopted value represents a high mean <br />annual runoff for use in conservative sediment and channel stability studies. For water rights <br />purposes, the average year yield estimate of 200 acre-feet per square mile per year is suitable, as <br />approved by the Colorado Water Court in 1986. This 200 acre-feet per square mile per year <br />projection was confiinned by WNZ T based on recent gage data which indicate an average amlual <br />yield of Lower Dry Fork of approximately 190 acre-feet persquare mile. <br />The dominant discharge, is that flow in the stream which tends to shape the channel and help <br />establish the width and depth of the defined channel. The dominant discharge flow in cubic feet per <br />second for each basin is the peak flog, expected to occur there once every two years. <br />Figure 2 in Exhibit 55A and 55B provides a semi-logarithmic plot of the peak discharge rates. The figure <br />was adopted for use in the small tributary basins under consideration. It is consistent with the reuional <br />analyses for the North Fork. <br />40 Engineering evaluation and study of each of the sixteen stream segments and their basins has <br />resulted in defining basic hydrologic characteristics related to each stream. This infonnation is <br />2.04-100 Revised November 1004 PRIG; Rev..4pri11006 PRIO; Sep. 2007 PR12; Feb. 2008 PR12