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le County, Colorado Ea Eagle and Colorado Rivers Floodplain Information Report le County, Colorado Ea Eagle and Colorado Rivers Floodvlain Information Report HYDROLOGIC AND HYDRAULIC ANALYSIS The hydrologic analysis for this study of this Eagle River floodplain study was authorized by the Colorado Water Conservation Board and completed by Water Resource Consultants, LLC of Rifle. The purpose of the study was to update flood hydrology on the main stem of the Eagle River. Previous studies published by FEMA in May 1980 did not cover all the reaches of the Eagle River. A regression analysis of tributary area above 10,000 feet elevation was performed where gage data was lacking. Stream data taken at gages for the limited years of existence in the Eagle Basin was analyzed and natural flow frequency curves were developed. The peak flows determined for the 10, 50, 100 and 500-year floods were used to determine the flood profiles and the 100-year floodplain for this report. Table 2 lists the peak discharges for these floods on the Eagle and Colorado Rivers TABLE 2 DESIGN FLOOD FLOWS FOR THE EAGLE AND COLORADO RIVERS River and the portion of the but are no longer active, are listed in order from There are five active USGS gages located on the main stem of the Eagle Colorado River in this study area. Two other gages have existed in the past, although records from that gages are useful in statistical gage analysis. The gages upstream to downstream. Flood Peaks in cfs For Different Return Intervals Area Above 10,000 ft (sq. mi.) Total Drainage Area (sq. mi.) Stream Reach TABLE 1 USGS GAGING STATIONS Peak Discharge (cfs) 1,010 Status Period of Record Gage Elevation (feet) 8654 Drainage Area (s~i.) 70 Station Name Station Number 500-yr 100-yr 50-yr 10-yr 3,980 5,940 6,210 7,060 8,400 9,330 3,490 5,190 5,430 6,170 7,230 8,030 3,290 4,790 5,010 5,710 6,690 7,430 2,520 3,800 3,980 4,530 5,300 5.890 "1 /'1"\,., lOU.1 243.2 254.7 290.6 333.3 364.4 ""-A ":'ou 361* 402* 658 808 944 EAGLE RIVER T"\......Wy_~.._~~~ _.I: '-6-=_6.____ UVWll1>UCOd111 Vi IViUHUUI Downstream of Gore Creek Downstream of Beaver Creek Downstream of Lake Creek Downstream of Brush Creek Downstream of Gypsum Creek COLORADO RIVER 25,500 23,600 21,600 16,400 N/A 4,394 Downstream of Eagle River** are approximate. **Cross sections 44 through 46 of the Colorado River floodplain model assumes the same hydrology upstream of the confluence with the Eagle River as below. Since the use of these cross sections are primarily to transition the model beyond the confluence, a change in hydrology was not warranted for this short reach. sis The water surface elevations for floods of the selected recurrence intervals were computed through use of the Corps of Engineers' HEC-RAS backwater computer program, version 3.0.1 dated March 2001. A total of 471 cross sections were analyzed for the hydraulic analysis of the Eagle and Colorado Rivers and were secured from topographic mapping and field surveys The locations of these cross sections are August 22, 2003 Page 10 iM1 MuixU.~Gtql", ~o....-'_~ ____ August 22, 2003 Page 9 iM1M1t~~c.u,>l< ~\J __ 1 sis SECTION 4 H 4.1 FLOOD HISTORY There is not a long history of stream gage records on the Eagle River. The gage on the Eagle River with the longest period of record is located outside this study area. The Eagle River at Reddiff gage is located upstream from this floodplain study area as has 73 years of record. The peak flood of record occurred in 1912. The Eagle River below Gypsum gage has the next longest period of record and as been in existence since 1947. This gage has a drainage basin of 944 square miles. The USGS gage records do not show a significant flood during this period of record. However, high flows occurred on the Eagle River in the years 1952, 1957, 1983, 1984, 1995, 1997 and 2003. 3.1 Gal!e Records SECTION 3 1,810 3,930 3,640 6,760 7,020 22,200 Active Active Inactive Active Active Active Inactive 1911-1925 1 OAA~-n.roC"ont .L'/"'""TP.l.VovJ..u.. 1990- present 1989-1999 1 999-present 1911-1924 1947 -present 1941- present 8078 7410 7380 6560 6275 6130 186 395 402 629 944 4394 Reddiff Eagle River near Minturn Eagle River at Avon Eagle River WWTP at Avon Eagle River at Eagle Eagle River below Gypsum Colorado River near Dotsero Eagle River at 09063000 09064600 09067005 09067020 09067500 09070000 09070500 drainage area above in italics Values for * 3.2 Flood Protection Measures Homestake Reservoir dam constructed on Homestake Creek in 1967 for water diversion to Colorado Springs and Aurora has acted to inadvertently reduce the peak flood discharges on the lower Eagle River. The dam was not constructed for flood control, but acts to fill during the spring runoff and has reduced peak flooding on the Eagle River. 4.2 Hvdraulic Anal