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Roaring Fork River Floodplain Information Report Town of Basalt, Eagle & Pitkin Counties, Colorado Roaring Fork River Floodplain b!formation Report Town of Basalt, EaRle & Pitkin Counties, Colorado The Town of Basalt contract with McLaughlin Water Engineers, Ltd, to review and modify Ihe HEC- RAS model in Reach II (Lower to Upper Basalt Bypass Bridges) as part of a river master plan for the town, The Town of Basalt also conlract with Matrix Design Group, Inc, to review and modify the HEC- RAS model in Reach III (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 resloration project, and the LOMR was accepted by the CWCB and FEMA in 1998, In 2000, Pitkin Counly contracted with Malrix Design Group to model the reach from Wingo Bridge upstream to the confluence with Snowmass Creek, The Town of Basall then contracted wilh Matrix Design Group, Inc, in 2000 to coordinate and finalize the four floodplain studies into one complete Floodplain Information Report for Reaches I, II and III, and to model the 10, 50 and 500 year floodplains, dated July 24, 2000, 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 a "Zero-Rise" concept or "No Adverse Impact" 10 conveyance, whereby only the areas of ineffective flow and shallow flooding are excluded from Ihe 100- year floodplain 10 delermine Ihe flood way, 1.4 Coordination The results of the year 2000 Floodplain Information Report for Basalt were reviewed by the Colorado Water Conservation Board and adopted at their Board Meeting in Gunnison, Colorado July 24 & 25, 2000, This Floodplain Information Report was republished November 14,2001 to include detailed floodplain mapping with base flood elevations in the area south of the Town of Basalt, know as "Soulh Side" where floodwaters split al the Upper Basalt Bypass Bridge and flow overland, This report also includes new topographic mapping obtained for the Town of Basalt area in January 2001 from Aero-Metric, Inc, This new report also includes a Floodway delineation that was not present in the earlier edition, A final Town of Basall Board of Trustees communily meeting was held on July 25, 2000 to adopt the 2000 Floodplain Information Report, The meeting was attended by representatives of the CWCB, Matrix Design Group, McLaughlin Water Engineers, the Town of Basall, and Pitkin Counly, No significant problems were raised at the meeting, 1.3 Purpose and Scope Meetings were held on September 25, 2001 with both the Board of County Commissioners of Pitkin County and the Town of Basalt Board of Trustees 10 present the revised Floodplain Infonnalion Report for acceptance of the more stringent "Zero Rise" floodway delineation for the portions of this Roaring Fork River study within their respective jurisdictions, A similar workshop meeting was held with the Eagle County Board of County Commissioners on November 13, 2001 to review the "Zero Rise" floodway delineation, The Roaring Fork River floodplain was restudied in 2000 to 2001 by Matrix Design Group, Inc, and McLaughlin Water Engineers, Ltd, of Denver, Colorado for the Town of Basalt. A new analysis was required due to the following reasons: I. Channel instability caused by floods in 1983, 1984, 1985 and 1995 which have significantly altered Ihe Roaring Fork River channel alignment and grade in and around the Town of Basalt, The Roaring Fork and Frvingpan Rivers Multi,Obiective Planning Study completed by BRW, Ine. in June 1999 for the Colorado Water Conservation Board demonstrated that the low flow channel has even migrated outside of the previously defined FEMA lOO'year floodplain in some areas of the Roaring Fork River. SECTION 2 - STUDY AREA DESCRIPTION 2.1 Drainae:e Basin Characteristics 2, Encroachment of the Roaring Fork River channel by the Colorado Department of Transportalion 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 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. 2.2 Study Reach Description 3, Encroachment of the Roaring Fork River channel by development since the base mapping was completed for previous studies, This Floodplain Infonnation Report is prepared for 9,6 miles of the Roaring Fork River beginning at the GarfieldfEagle County Line and continuing upstream through Eagle County, the Town of Basalt and Pitkin County to the confluence with Snowmass Creek, 4, Reconstruction of the Wingo Bridge and Waterman Bridge, and construction of the Upper Basalt Bypass Bridge, Midland A venue Bridge and Lower Basalt Bypass Bridge since the original studies, 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 IS-inch diameter size, There are many riffles and rapids with many shallow pools along ils course, Several irrigation ditches divert from the Roaring Fork along this reach, In mosl areas, the riverbanks are low with steep slopes (often over 45 degrees), being composed mostly of sand, gravel, and cobbles, wilh little or no vegetation This report was prepared to provide infonnation relative to the occurrence of floods and to guide local officials in planning the use and regulation of Ihe 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, .t.;~~""l'.~ '~1 "__ Page 2 November 14,2001 ~Ij~~~" Page 3 November 14,2001