PERMFILE71533 - Laserfiche WebLink

Home Browse Search

~~ • • flooding depends on the amount of snow melt and; or thunderstor- activity. Historically measured high discharge or. the U..^.compahgre River occur during the months of May and; or June and ranee from 200 cfs to 1740 cfs. Spring runoff occurs fro:,. as early as May 1 to as late as September 10, dependinc on the s.^.owpac'.: in the area. High flocs resulting fr.,,., thunderstorms car. occur any time frcm April 1 to October 15. Site measurements over the past two high flow seasons (1990 and 1991; indicated a flow rate on the Uncospahare of 220 cfs results in minor flooding of the gravel bar to be mined. Floodinc frcm these flows is confined tc the three high flow charnels shown on Exhibit E-1. Water in these charnels ranged from O.5 feet to 1.5 feet deep and were fror.. 2 to SO feet wide. Flood season flow rates for the individual channels ranted from 0.5 cfs to 1.2 cfs. The most recent extreme high flow of 1740 cfs occurred on June 24, 1983 and resulted is the gravel bar to be mined being exposed. During that flood year the River charrel expanded east cf the .. r...-1 cha..ael by 300 feet (Plate 12, Curay County 1980 Flood Plain Study). Associated sheet flows extended several hundred feet east of this boundary. This flood year .,,ay represent a 50 year or a lOC year flood event. The Ouray County Flood Plain Study of 1978 indicates that a SOO year flood would raise the water levels is the River in this area by 6 1;2 to 7 feet. It would also spread sheet flows betvreen 1500 and 2500 feet beyond the main River channel (see Plates 12 and 27 of the Ouray County Flood Plain Study}. While this mining operation will help reduce the potential of flooding or. the easterr. and northerr. bcundaries durinc average runoff high flow conditions, the helpful effects would be minimal or non-existent during a 5O or 10C year flood. Peck Inc. makes ao promises coacerninc increased flood protection during such events; but, by lesseninc flood channel strea,:, discharge and removing gravels frcm the bar adjacent to the main River channel. erosion on the eastern and norther.^. banks will be lessened. It would take a flood flow o: 2 feet deep spread over a 300 foot wide bar to erode the banks when mining is completed. This would be a similar flow to that of the 1983 high flow event. 2. HYDROLOGIC IMPACTS: The proposed berm 10 feet wide and 2 to 5 feet high left adjacent to the eastern edge of the River will keep the River out of the active pit during pit operations. All mir.irg will occur at least one foot above the ground water table. To enable Peck to determine if ground water is present, monitoring pipes will be installed--at least three for each acre to be mined. These will be installed prior to ..~.irg in a particular area of the pit. These will be installed to a depth of 3 feet below the low 0