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Roaring Fork River Floodplain In/annation Report Town of Basalt, EaRle & Pitkin Counties, Colorado Roaring Fork River Floodplain lnfomwtion Report Town of Basalt, EaRle & Pitkin Counties, Colorado 4400 cfs would split to Ihe South Side in the 100-year flood event. After this flow split determination, a HEC.RAS model was completed on the South Side, the provided work maps, McLaughlin Water Engineers, Ltd, review determined that the model was only approximate because it did not account for the highly complex flow through the buildings, nor did it include a number of complex flow splits thaI occur within the second and third reaches, At most of these flow splits, floodwaters flow from the South Side back over Ihe highway and into the North Channel. J,F. SalO used the most recent version of HEC-RAS, but Ihe software did nol take into account the two,dimensional flow splils over the highway, Highway Spill Model Modeling of the South Side indicated that approximately 600 cfs spills over Highway 82 and back into Ihe main stem floodplain upstream of Emma bridge, To model this flow, seven cross sections were cut between the flow split over the highway and cross section 76,38 on the main stem model. After re-running the main stem model with the splil flow deducted, Ihe water surface elevation of cross section 76,38 was used as the downstream boundary condition on the new highway spill model. The average centerline distance between cross sections was 190 feet. Manning's roughness values ranged from 0,035 to 0,3, The Town of Basalt was concemed aboul mapping quality and the level of detail in hydraulic modeling FEMA used to generate the proposed lOO-year floodplain map, McLaughlin Water Engineers, Ltd, was retained by the Town of Basalt to complete a new floodplain mapping study based upon more detailed hydraulic modeling and new topographic mapping, Trailers and some other obstructions were modeled using higher roughness values, thus the upper limit of 0.3 for Manning's "n" values, Laleral weirs were also used in this model to calculate the amount of flow spilling inlo zones beyond the extenls of each cross section, The elevation of each lateral weir was set as the elevations at which flow would spill wilhout returning to the main flow, Locations on the cross seclions that corresponded to the energy head at critical spill elevations were used to locate the lateral weirs along the cross section ends, It was found that roughly 145 cfs splits away from the downstream spill over Highway 82 and flows west on both the north and south sides of the highway, South Side Floodplain Mapping McLaughlin W'1ter Engineers used the U.S, Army Corps of Engineers' HEC-RAS 3,0 model to delineate the floodplain of the Roaring Fork River on the south side of Highway 82 (South Side) through the town of Basalt. The soflware' s new split flow capabilities allowed McLaughlin Water Engineers to model the numerous flow splits that occur throughout the South Side floodplain, In addition, sophisticated two- dimensional modeling was used to aid in the application of the HEC-RAS models, HEC-RAS Calibration Using Two-Dimensional Hydraulic Analysis The Roaring Fork River split flow at the Highway 82 Upper Basalt Bypass Bridge was previously modeled by J,F. Sato using HEC,RAS, the one-dimensional hydraulic model developed by the U.S, Army Corps of Engineers Hydrologic Engineering Center. The team decided 10 employ a two-dimensional model as a tool for increasing the accuracy of a one-dimensional solution to the flow field for various reasons, First, the complex nalure of the floodplain lends itself to a two-dimensional solution because of number of buildings in the floodplain and the intricate, mountainous topography, Second, shallow flooding and flow splits downstream of the initial split create a complex problem that can affect the accuracy of a one-dimensional model. Third, while the assumption of one-dimensional flow may be a good approximation in many instances, when used by itself it is not very accurate for floodplains with ill- defined, numerous flow paths, numerOus obstructions, and non-parallel flow stream lines, The two- dimensional modeling tool helped McLaughlin Water Engineers make informed decisions when applying the one-dimensional model. New topographic mapping of the Town of Basalt, (available after J,F. Sato's floodplain study), was used to cut cross sections and delineate the regulatory floodplain, The new aerial photography allowed the team to locate buildings, roads, and other features that obstructed the floodplain, South Side HEC-RAS Model McLaughlin Water Engineers cut a total of 53 cross,sections from the split at the Upper Bypass Bridge on Highway 82 to the confluence of the south side split flow and the main stem river. This confluence is located downstream of the wastewaler treatment plant. The average centerline distance between cross sections was roughly 215 feet. All cross sections were cut perpendicular to flow, Flow obstructions (such as buildings and roads) were coded directly into the cross-sectional geometry, Manning's "n" (roughness) values ranged from 0,18 to 0,035 with 0,08 being Ihe most frequent estimate of roughness in the floodplain. Cross section 62 from the main stem model was used as the downstream limit of the south side split flow model. Manning's "n" values were adjusted in certain seclions to account for effective conveyance areas between cross seclions, After initially running backwater calculations on the South Side, it was determined that flow would spill over Highway 82 and into the main stem floodplain at two locations, (These flow splits were not identified on the previous mapping by J ,F. Sato,) The first location was near the downstream limit of the original South Side model and cross sections were cut across the highway to model the flow as it reached the north side of the highway, Due 10 instability, the model was restricted to include only areas that were completely inundated, We estimate that more than 50 iterations were used to generate a model thaI would converge to a solution of the governing flow equations, These iterations were used, in part, to converge on the true extents of flooding in the model. The second flow split was much farther upstream and was modeled using lateral (side-spill) weirs in HEC-RAS 3,0, Subsequently, olher lateral weirs were placed at locations where significant flows would spill over the highway and the crest of each weir was set as the elevation at which flow would begin to spill, Lateral weirs were also placed at the downstream limits of Ihe study where flow can split as it spills over the highway near the wastewater treatment plant. It was found Ihat approximalely 2400 cfs flows out of the extents of the cross sections to Ihe east of the wastewater Ireatment plant, leaving roughly 1200 cfs to join the main stem floodplain west of the treatment plant. Estimates of other spill flows are shown on Once the extent of the model had been defined, Ihe downstream boundary condition was set using the critical flow depth at the location of the downstream spill over Highway 82 (where the south side split and the main stem flow rejoin), Buildings within the wetted area were blocked out so that flow would be forced around those locations, To obtain model convergence in the shallow and high velocily flow, Manning's "n" values, and the eddy viscosity (a measure of energy losses due to turbulence), were increased until stability was reached, South Side Model Results Results from Ihe two-dimensional model were used to build a more accurate one-dimensional model. Flow trace animations and the distribution of velocity magnitudes were the primary results of interest from the SMS model. Figure 6 is a snapshot of the flow trace animation and illustrates the flow around '.'Ii' """''''''''''''''" .. ..--.- == ..' -- Page 16 November 14, 2001 '~;ll~~Ci<uJ'." .' '== Page 17 November 14,2001