2008-03-21_PERMIT FILE - C1980007A (7)

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•? 1. Mean annual runoff 2. Peak discharges for the 2-year, 10-year, and 100-year frequency floods 3. Mean annual sediment yields 4. Average thalweg2 slopes 5. Channel characteristics for the dominant discharge (consisting of width and depth relationships) 6. The range of the channel slopes from near mouth to upper reach Other factors that influence the long-term channel characteristics include the forested area and frequency of landslides. BASIN HYDROLOGIC CHARACTERISTICS WWE and Mr. Pemberton evaluated and studied each of the 12 stream segments and their basins to define basic hydrologic characteristics related to each stream. This information is summarized in Table 2. While there are differences in soils and vegetation within the study areas, for the purpose of this conceptual analysis, conservative estimates of water yield and sediment yield have been adopted which are reasonable for both areas. Average Annual Runoff The mean annual runoff expressed in acre-feet (AF) per year per square mile is a fundamental parameter for determining annual average sediment yield. Total annual precipitation varies • widely throughout the 526-square-mile basin of the North Fork at Somerset. In the higher elevations of the entire North Fork basin, precipitation can total up to 50 inches per year. Annual precipitation for the drainage basins near West Elk Mine typically falls in the 22-inch to 30-inch range. Precipitation in excess of that portion lost to evapotranspiration and deep percolation, and that ultimately becomes streamflow, also varies widely in the basin. Woodward-Clyde conducted a water balance analysis for Horse Creek and Lick Creek for 1978 to 1980. The study, based on the Lick Creek stream gaging station, indicated an average annual runoff of 8.9 inches, representing 475 AF per square mile. MCC's Exhibit 18 indicates an annual runoff approaching 1,000 AF for one square mile for a variety of North Fork watersheds. However, that finding is modified by their reference to much lowered water yields in South Prong and Horse Creek for the 1977 to 1978 period, which were lower than the driest year yields of the regional basins. The North Fork gage near Somerset provides the best long-term runoff data in the region, with 69 years of continuous gaging records. The 69-year period of record identifies a mean annual runoff of 630 AF per square mile; the runoff magnitude is significantly affected by the higher precipitation values at higher elevations in the drainage basin. Analyses of water yield by William Lorah of WWE for the Division No. 4 Water Court approved water augmentation plan, indicated that typical annual water yields for tributaries of Dry Fork were approximately 200 AF per square mile per year. This value compares favorably with the yield estimate of 160 AF per square mile per year based on the U.S. Geological Survey (USGS) regional regression equations (USGS 1985). The 1936 through 1947 and 1985 through 2002 periods of gaging flow on Minnesota Creek 4 show an average annual yield for 41.3 square miles of 385 AF per square mile. 2 Line following lowest part of a valley, i.e., invert of channel. Tetra Tech - 0907161P