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~~1d Mmix~Gn:ql.. ~_'_"- -- Page 2 July 24, 2000 '..M~ Mmix_' ~~.. 1l""....- --- Page 3 July 24, 2000 Temperature and precipitation varies greatly from location-to-Iocation and season-to-season drainage basin and are important variables in flooding conditions. Above normal cause early and heavy flows on the Roaring Fork River. Records from within the spring temperatures can the Aspen weather station indicate 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. No floodway analysis was completed under this contract. This report was prepared to provide information relative to the occurrence of floods and to guide local officials in planning the use and regulation of the floodplain areas so that flood hazards and future flood damages are minimized. It includes information on historical floods, existing factors, which influence the flood hazards, and the nature and extent of probable future floods. Precipitation varies widely throughout the Roaring Fork Basin above Basalt. On the continental Divide, near Independence Pass, the average annual precipitation is 26.3 inches with 17.5 inches occurring during the winter months (November - April). Near Basalt, the average annual precipitation is approximately 17.2 inches with 8.7 inches occurring during the winter months November through April. Data on precipitation from the National Oceanic and Atmospheric Administration indicate that in the Aspen area, the 100-year 24-hour storm would produce 2.6 inches of precipitation Reconstruction of the Wingo Bridge and Waterman Bridge, and construction of the Upper Basalt Bypass Bridge, Midland Avenue Bridge and Lower Basalt Bypass Bridge since the original studies 2.3 Climate 4. Encroachment of the Roaring Fork River channel was completed for previous studies the base mapping 3 by development since Encroachment of the Roaring Fork River channel by the Colorado Department of Transportation for the construction of the Highway 82 Bypass bifurcated the floodplain, forcing flood flows to split at the Upper Basalt Bypass Bridge. Flood flows will be either be fully routed through the bridge, or breach the non-FEMA compliant levee and enter the region of the Town of Basalt known as "South Side. The Roaring Fork River bank-full channel in the study area has an average range of about 90 to 120 feet wide as it flows through an alluvial valley. The average channel grade is 0.0127 feet per foot upstream of the Town of Basalt, transitioning to 0.0087 feet per foot west of the Town. The lower study reach has an average grade of approximately 0.007 feet per foot. The stream channel is braided, having a \ bed composed mostly of gravel, cobbles, and small round boulders ranging in the 6 to 15-inch diameter size. There are many riffles and rapids with many shallow pools along its course. Several irrigation ditches divert from the Roaring Fork along this reach. In most areas, the riverbanks are low with steep slopes (often over 45 degrees), being composed mostly of sand, gravel, and cobbles, with little or no vegetation below the mean annual high water mark. The steep slopes and lack of vegetation reduce the resistance of he banks to scour. 2. Water Engineers, the following reasons 1. The Roaring Fork River Ltd. of Channel instability caused by floods in 1983, 1984, 1985 and 1995 which have significantly altered the Roaring Fork River channel alignment and grade in and around the Town of Basalt. The Roaring Fork and Fryingpan Rivers Multi-Obiective Planning Study completed by BRW, Inc. in June 1999 for the Colorado Water Conservation Board demonstrated that the low flow channel has even migrated outside of the previously defined FEMA 100-year floodplain in some areas of the Roaring Fork River. floodplain was restudied in 2000 by Matrix Design Group, Inc. Denver, Colorado for the Town of Basalt. A new analysis and McLaughlin was required due to This Floodplain Information Report is prepared for 9.6 miles of the Roaring Fork River beginning at the GarfieldJEagle County Line and continuing upstream through Eagle County, the Town of Basalt and Pitkin County to the confluence with Snowmass Creek. 2.2 Stud The Roaring Fork River is a major tributary to the Colorado River. The headwaters of the Roaring Fork River start above the City of Aspen and continue approximately 60 miles downstream to the confluence at the City of Glenwood Springs. At the confluence with the Colorado River, the Roaring Fork River has a 1,460 square mile drainage basin. Major tributaries to the Roaring Fork are the Crystal River, Fryingpan River, Maroon Creek, Castle Creek and Hunter Creek. Reach Descri tion The Town of Basalt contract with McLaughlin Water Engineers, Ltd. to review and modify the HEC- RAS model in Reach IT (Lower to Upper Basalt Bypass Bridges) as part of a river master plan for the town. The Town of Basalt also contract with Matrix Design Group, Inc. to review and modify the HEC- RAS model in Reach ill (Willits Lane to the Lower Basalt Bypass Bridge, including the River Oaks Subdivision). Reach I (Wingo Bridge to Upper Basalt Bypass Bridge) had been modeled by the Roaring Fork Club for their river restoration project, and the LOMR was accepted by the CWCB and FEMA in 1998. In 2000, Pitkin County contracted with Matrix Design Group to model the reach from Wingo Bridge upstream to the confluence with Snowmass Creek. The Town of Basalt then contracted with Matrix Design Group, Inc. in 2000 to coordinate and finalize the four floodplain studies into one complete Floodplain Information Report for Reaches I, IT and ill, and to model the 10,50 and 500 year floodplains 1 3 Pu ose and Scooe 2.1 Drainal!e Basin Characteristics SECTION 2 A final Town of Basalt Board of Trustees community meeting was held on July 25, 2000 to adopt this Floodplain Information Report. The meeting was attended by representatives of the CWCB, Matrix Design Group, McLaugWin Water Engineers, the Town of Basalt, and Pitkin County. No significant problems were raised at the meeting. STUDY AREA DESCRIPTION The results of this floodplain study for Basalt were reviewed by the Colorado Water Conservation Board and adopted at their Board Meeting in Gunnison, Colorado July 24 & 25, 2000. ort Town 0 le & Pitkin Counties, Colorado 1.4 Coordination or! Town 0 le & Pitkin Counties, Colorado