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Stream Channel Parameters and Changes <br />Exhibit 55B Due to Longwall Mining-Induced Subsidence Page 8 <br /> <br /> <br /> <br />831-032.923 Wright Water Engineers, Inc. <br />December 2021 <br />plan (Case No. 86CW38), indicated that typical annual water yields for tributaries of Dry Fork <br />were approximately 200 AF per square mile. This value compares favorably with the yield <br />estimate of 160 AF per square mile per year based on the U.S. Geological Survey (USGS) regional <br />regression equations (USGS 1985). <br />The 1936 through 1947 and 1985 through 2002 periods of gaging flow on Minnesota Creek show <br />an average annual yield for 41.3 square miles of 385 AF per square mile. <br />For the purpose of annual average sediment yields for the subject basins, WWE has concluded that <br />an appropriate (i.e., conservative) mean annual runoff for the subject basins of 475 AF per square <br />mile should be adopted, even though site-specific data for the basins would likely indicate a mean <br />annual runoff of less than 475 AF 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 AF per year is suitable, as approved by the <br />Colorado Water Court in 1986. <br />The adopted mean annual runoff for the subject basins is used for sediment and channel stability <br />purposes only. It is not proposed for use in water rights studies or for site-specific water budgets. <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 in <br />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 West <br />Elk Mine. Special evaluations were made of the Lick Creek, Sylvester Gulch, and Horse Creek <br />basins using the USGS peak rate of runoff computational procedure applicable to small basins in <br />the Colorado mountainous area (USGS 1985). <br />Figure 2 provides a semi-logarithmic plot of the peak discharge rates. The semi-logarithmic