2016-02-25_PERMIT FILE - C1982056A (4)

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The average annual runoff in Fish Creek was estimated by Brown and Caldwell (Water, Waste & LandBrown and Caldwell, June 1981, A Surface and Ground Water Hydrology Study for The Fish Creek Underground Mine, • Submitted to The Pittsburg & Midway Coal Mining Company, see Exhibit 4e-10, to be 0.27 feet (18.4 cfs) for the whole basin of 77.3 mix and 0.15 feet (5.7 cfs) for the lower portion of the basin (Twentymile Park) of 42.7 mix. Average annual precipitation was estimated to be 16.4 inches. The remaining water of 1.22 feet is used in evapotranspiration. The pieziometric heads in the underlying bedrock aquifers are higher than that in Fish Creek and the surrounding alluvium. The bedrock aquifers therefore provide some additional recharge; however, the amount is small. Primarily the Lewis Shale that is relatively impermeable underlies the portion of the alluvium that is to be flooded. The approximately 80 foot thick Wadge Overburden, which subcrops downstream of this azea, only discharges approximately 0.02 cfs (see PHC, Groundwater, Diminution of Bedrock Discharge to Alluvium and Surface Streams section). There is constant interchange between the alluvium and the stream but over an average year the net interchange is approximately zero. Hydrographs of the alluvial wells (see 1998 Annual Hydrologic Report) show that the alluvium is recharged from early fall to early summer and then rapidly discharges. th many years there is a period with a very small rate of discharge from the alluvium during the winter. The relief and channel slope in the Fish Creek basin are shown on Table l in Exhibit 4e-10. The Fish Creek channel slope in Twentymile Park area is very flat (0.2% to 0.6%) and the land form also has a low average slope of 3% to 5%. Flow has been measured in Fish Creek in Twentymile Park for over ten years. Station l6 is located upstream of the alluvial valley area to be flooded and is at the upstream end of Twentymile Park. Station 1002 is located in Twentymile Pazk and within the alluvial valley area to be flooded. Station 1003 is located near the downstream end of Twentymile Park and approximately two miles downstream of the area to be flooded (Map 13A). Flows were • measured one to four times per month from March to September of each year since 1984. Some additional measurements have been made in October and November of 1984 to 1988. The flow rate measurements aze summarized on Tables 77, 78, and 79. The minimum flow during the series of relatively low to moderate runoff from 1989 to 1994 was approximately 1 cfs at both Stations 1002 and 1003 and was approximately %: cfs at Station 16. This indicates that there is little contribution to the base flow of Fish Creek during low flow periods from the alluvial aquifer. The average flow in Fish Creek through Twentymile Park was 24.5 cfs at Station 1003. The average flows for the overlapping periods of record for the three stations were almost identical. This indicates that most of the runoff comes from the headwaters area and not from the relatively flat areas th Twentymile Park. This compares favorably with the estimates made by Brown and Caldwell . Peak flows for the three stations were also similar. An analysis of the peak flows indicate that at Station 1003 the 10 year storm would have a discharge of 187 cfs and the 25 year storm would have a discharge of 259 cfs. The analysis was performed using the data from Station 1003 for years 1984 to 1996 and the data from Station 1002 for years 1997 and 1998. Due to difficulty in obtaining a stable rating curve in 1997 and 1998, the high flow at Station 1003 where underestimated. The data was analyzed using a Pierson Type III distribution (U.S. Interagency Advisory Committee on Water Data, 1982, Guidelines for determining flood flow frequency, Bulletin 17-B of the Hydrology Subwmmittce: Reston, Virginia, U.S. Geological Survey, Office of Water Data Coordination, 183 p.). • pIP1Pl+3~W~~ JUiv '~ i 2G09 PR 99-05 2.06-13.2 11/17/99