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Roaring Fork River Floodplain Information Report Town of Basalt, Eagle & Pitkin Counties, Colorado Roaring Fork River Floodplain Information Report Town of Basalt, Eagle & Pitkin Counties, Colorado The river in the problem area is characterized by low banks, braided channels, and a considerable amount of gravel, cobbles, and snags deposited on gravel bars, The river carries a large bedload of gravel and cobbles, some of the latter being more than 6 inches in diameter, The capacity of the channel has been reduced by this sediment, In some instances, where channel changes took place during the flood, the original channels were so filled with sediment that nearly all of the present flows are discharging through the "new" channels, Generally, however, the "new" channels are old watercourses abandoned by the river in previous year, The littered condition of the channels, and the increased danger of bank erosion, inundation, and additional channel changes resulting from this condition, constitutes the present flood problem, SECTION 4 - HYDROLOGIC AND HYDRAULIC ANALYSIS 4.1 Hvdrolol!ic Analvsis No urban areas are affected by floods, The principal items damaged by the 1957 flood were the agricultural lands and roads and bridges adjacent to, or over, the river, A few farm buildings were flooded. The total known damages in all categories in this reach amounted to slightly more than $45,000, The hydrologic analysis for Ihis study of the Roaring Fork drainage basin was compleled by the U,S, Anny Corps of Engineers, A regional analysis of stream data taken at gages in the Roaring Fork Basin was performed 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 Ihe peak discharges for these floods on the Roaring Fork River. TABLE 2 DESIGN FLOOD FLOWS FOR THE ROARING FORK RIVER A common type of damage from the 1957 flood was bank erosion, The slope of the stream ranges from 65 feet per mile above Basalt to 40 feet per mile below Basalt, Velocities probably in excess of IO feet per second occurred during the 1957 flood. The banks are composed largely of sand, gravel, and cobbles overlain by a comparatively thin mantle of soil, and are quite erosible, Spoil-type dikes, constructed by local interests with material bulldozed in the process of channel clearing, proved to be very erosible, A substantial percentage of the total damages were due to overtopping of the low banks and the inundation of pasture mui croplands, The banks in most instances range form 2 to 5 feet in height, In two locations, where old channel areas have been reclaimed, considerable volumes of overflow and widespread flooding resulted from bank overtopping at the upper ends of the areas, Stream Reach Cumulative Flood Peaks in cfs Drainage For Different Return Intervals Area (Sq. mi.) 10-vr 50-vr 100-vr 500-vr Frvinl!Pan River, above Basalt 298 2,250 3,300 3,950 7.150 Roaring Fork River, above 510 6,100 8,500 9,400 12,200 Frvinl!Pan River* Roaring Fork River, above 850 7,100 9,400 10,400 14,300 Sopris Creek, below Fryingpan River Roaring Fork river, above 870 7,300 9,800, 10,800 14,700 Garfield County Line, below Sopris Creek Source: U.S, Anny Corps of Engineers study values published in Flood Hazard Reports, More than one-half of the known damages in the 1957 flood were caused to roads and bridges in the area, with nearly $24,000 of damages being caused to three bridges and their abutments, Two of these were public bridges and have since been restored, A third bridge, which was private was completely destroyed and has not been replaced, The 18,700 cfs instantaneous peak discharge referred to in the Corps of Engineers letter report has been revised to 19,000 cfs by the U,S, Geological Survey, This is the maximum instantaneous peak discharge recorded at the Glenwood Springs gage on the Roaring Fork Ri ver through 2000, * Peak discharges were taken from Floodplain Information Report, Roaring Fork River, Wright- McLaughlin Engineers, 1976, More recent flooding on the Roaring Fork River has occurred in 1983, 1984, 1985 and 1995, According to USGS records from the Roaring Fork River at Glenwood Springs gage, the flood peaks occurred on: June 25, 1983; July I, 1984; June 9, 1985; and July 13, 1995, These floods, although less significant than the 1957 flood in magnitude, impacled the residents of the Roaring Fork Valley, Flooding was marked by dramatic lateral shifts in the low flow channel, mobilization of the cobble bed and bank material, floating debris from fallen Irees, changes in channel form to a braided pattern and localized flood damage, In 1995, a portion of the Two Rivers Road in the Town of Basalt was eroded by the floodwaters, Isolated sand bagging occurred along the banks of the Roaring Fork River in Lazy Glen subdivision, the Roaring Fork Mobile Home Park, the Pan & Fork Mobile Home Park and River Oaks subdivision to prolect property from flood damage, Work crewS from the Colorado Department of Transportation (CDOT) were stationed on the Upper Basalt Bypass Bridge 10 remove trees and other debris from the bridge piers, U.S, Anny Corps of Engineers completed hydrologic analyses of the Roaring Fork River, and the published information is presenled graphically in Figure 5, loe., M"''''1....""""" 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