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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 STUDY AREA DESCRIPTION SECTION 2 1.4 Purpose Prior to this study, the Eagle River and Colorado River floodplain was either not published or delineated as approximate Zone A. The JF Sato study was never published. This floodplain analysis was required due to the following reasons The Eagle River is a major tributary to the Colorado River. The headwaters of the Eagle River start above the City of Aspen and continue approximately 60 miles downstream to the confluence with the Colorado River near the beginning of Glenwood Canyon. At the confluence with the Colorado River, the Eagle River has a 990 square mile drainage basin. Major tributaries to the Eagle are Homestake Creek, Gore Creek, Beaver Creek, Lake Creek, Brush Creek, and Gypsum Creek. Other smaller drainages such as Stone, Eby, Squaw, Berry, Metcalf, Alkali and Turkey Creeks are tributary to the Eagle River. Drainae:e Basin Characteristics 2.1 the river corridor requiring better hazard mapping Intense development pressure along and floodplain elevations. 1 the Eagle over Proposed reconstruciion of several bridges and proposed new bridges River requiring a better assessment of the floodplain in these areas 2 The topography is characterized by rolling hills and wide valley floors vary from 6,120 feet at the mouth of the Eagle River to 11,785 feet on Red Table Mountain. Slopes the lower Eagle River basin vary from 40 to 65 feet per mile. Vegetation at lower altitudes is dominated by desert and scrub flora, while higher altitudes are mostly aspen and fir forests Elevations in to channel instability and development. information relative to the occurrence of floods and to guide local use an regulation of the floodplain areas so that flood hazards and future flood It includes information on historical floods, existing factors. which influence the nature and extent of probable future floods The upper Eagle River basin is bounded on the east-southeast by the Sawatch Range and to the north by the Gore Range. The high point of the basin is Mount of the Holy Cross with an elevation of 14,005 feet. Two major basins form the upper portion of the watershed, the Gore Creek basin and the Eagle River basin. The Gore Creek basin is 16 miles long and averages seven miles in width. Slopes vary from 80 feet per mile near Vail to 900 feet per mile in the upper areas of the Gore Range. The upper Eagle r basin is 19 miles long and is approximately 11 miles wide. Slopes vary from 65 feet per mile near to 500 feet per mile in the upper areas of the basin. Rive Minturn The report data includes flooded area maps delineating the 100 and 500-year flood boundaries, flood profiles and floodwater surface elevations for the 10, 50, 100 and 500-year floods at selected reference points. The floodway analysis is based upon the standard FEMA one-foot rise concept or limits of the channel to determine the floodway delineation. 1.5 Coordination tion This Floodplain Information Report is prepared for 4.29 miles of the Colorado River beginning at the GarfieldlEagle County Line and continuing upstream through Eagle County to a point upstream of the confluence with the Eagle River and downstream of the 1-70 bridges. The Eagle River was studied for 43.77 miles beginning at the confluence, and extending past the confluence with Gore Creek to the arch concrete bridge located in the north portion of the Town of Minturn. The results of this floodplain study has been reviewed and supported by the Colorado Water Conservation Board for future adoption. The Eagle River bank-full channel width varies from 80 feet up to 200 feet in the study area. The average channel grade is 0.0070 feet per foot. The channel slope ranges from 0.002 feet per foot in the broad, flat reach near Edwards at the confluence with Lake Creek to 0.01 feet per foot in the steep reaches around Eagle-Vail. The stream channel is generally incised single-thread meandering channel in the upper reaches, having a bed composed mostly of gravel, cobbles, and small round boulders. There are many riffles and rapids with many shallow pools along its course. Several irrigation ditches divert from the Eagle along this reach. In most areas, the riverbanks are low with steep slopes, being composed mostly of sand, gravel, and cobbles. The lower valley reaches are wider meandering reaches with ox-bow cut-offs and wide floodplains on terraced pastureland. The Colorado River bank-full channel width is generally 200 to 300 feet wide and includes some well vegetated islands and mid-channel bars. The stream channel for the study reach has an average grade of 0.0010 feet per foot, which is seven times more flat than the average slope of the Eagle River. August 22, 2003 Page 6 ~ MIIix[>"~~", '\,'lJ --- -- and topography due Changes to the river channel This report was prepared to provide officials in planning the d damages are minimized. flood hazards, and the Reach Descri 2.2 Stud Eagle County authorized Matrix Design Group, Inc. to begin the floodplain delineation of the Eagle and Colorado Rivers J"vlay 7,2002. Numerous coordination medings were conducted throughout the duration of this project with Eagle County and the Colorado Water Conservation Board. Draft reports were released to the towns of Gypsum, Eagle, Avon and Minturn at the meeting with FEMA on June 19, 2003. August 22, 2003 Page 5 001 ~u.~c.u,>1< "'V' __ 3