Laserfiche WebLink
For the purpose of annual average sediment yields for the subject basins, WWE and Mr. <br />Pemberton have concluded that an appropriate (i.e., conservative) mean annual runoff for the <br />subject basins of 475 AF per year should be adopted, even though site-specific data for the <br />basins would likely indicate a mean annual runoff of less than 475 AF per year per square mile. <br />The adopted value represents a high mean annual runoff for use in conservative sediment and <br />channel stability studies. For water rights purposes, the average year yield estimate of 200 AF <br />per year is suitable, as approved by the Colorado Water Court in 1986. <br />Peak Runoff Rates <br />Peak rates of storm runoff from rainfall and snowmelt events were defined for three return <br />frequencies: 2-, 10-, and 100-year events. <br />These discharges are based on statistical evaluation of peak daily flow data collected by MCC <br />in the permit area since 1978, study of the long-term published records for the North Fork at <br />Somerset, and statistical evaluation of published streamflow data for basins in the vicinity of <br />West Elk Mine. Special evaluations were made of the Lick Creek, Sylvester Gulch, and Horse <br />Creek basins using the USGS peak rate of runoff computational procedure applicable to small <br />basins in the Colorado mountainous area (USGS 1985). <br />Figure 2 provides a semi-logarithmic plot of the peak discharge rates. The semi-logarithmic <br />plotting technique was adopted to analyze the small tributary basins under consideration. It is <br />consistent with the regional analyses for the North Fork. <br />Sediment Yield <br />The mean annual sediment yield for each of the 12 streams is presented, for convenience, in <br />terms of three units of measurement: <br />1. Tons per year <br />2. AF per year <br />3. Cubic yards per year <br />The mean annual sediment yields for each basin were estimated by utilizing a wide range of <br />published and unpublished data combined with site-specific information and basin <br />characteristics. <br />Sediment yield parameters for similar basins were evaluated using the USGS Water Resources <br />Investigation Report 87-4193 by John Elliott entitled Regionalization of Mean Annual <br />Suspended Sediment Loads in Streams, Central, Northwestern, and Southwestern Colorado <br />(1988). In addition, the rate of sediment production was evaluated using field observations of <br />the basin characteristics, photographs, maps, aerial photos, and after making comparisons with <br />other mountainous regions with similar vegetative cover and similar characteristics for which <br />sediment yield data were available. The typical suspended sediment concentrations of similar <br />streams were taken into consideration for comparison and for reasonableness. checks. <br />The adopted sediment yield rate of 0.03 AF per square mile per year for the subject basins, <br />when coupled with annual water yield, results in an average annualized sediment concentration <br />of approximately 70 mg/L. This is consistent with regional analyses for the North Fork. <br />STREAM CHANNEL HYDRAULIC CHARACTERISTICS <br />The stream channel regime and characteristics were defined using still photographs, <br />videotapes, USGS topographic maps, detailed topographic maps prepared for the mine area, <br />soil surveys, geological evaluations prepared by John Rold (consultant to WWE), aerial <br />photographs, and related evaluations. Channel profiles portraying the pre-mining slope and the <br />range of slopes for each stream segment are presen edin Figures 3 roug 147- <br />Tetra Tech - 0,007161P