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<br />In the 1986 CWCB report, a Manning's n of 0.025 was also used in the Army Corps flume <br />entrance, and a value of 0.04 was used in what would currently be cross-sections 2 through 9.7. <br />Roughness values used for hydraulic structures are listed in Table 2.2.6. <br /> <br />Table 2.2.6. Manning's n for hydraulic structures <br /> <br />IStru ctures I Cross-Sections IManning's nl <br />grouted rip-rap flume 1 0.025 <br />corregated metal culverts 45, E6.5 0.025 <br />steel "tank" culverts 26.5, E18 0.014 <br />concrete to steel pipe 36 0.014 <br />wooden flume 14, 37.6 to 37.8 0.013 <br />steel flume 37.4 to 37.5 0.012 <br /> <br />Plow contraction/expansion loss coefficients for the model were set at 0.1 and 0.3, respectively, <br />except at the cross-sections associated with hydraulic structures which were set at 0.3 and 0.5 <br />respectively. When running HEC-RAS, a mixed flow analysis was performed. Therefore, both <br />sub-critical and super-critical flow regimes were considered. <br /> <br />2.2.3.2 HEC-RAS Results <br />A summary of results produced by the HEC-RAS modeling for each flood return interval is <br />provided in the Appendix. The water surface elevation is the main result used from the HEC- <br />RAS model. However, results for other important characteristics of the flow at each cross- <br />section are also provided, including the main channel velocity, critical water surface elevation, <br />energy gradeline and slope, minimum channel elevation, left and right overbank elevation, top <br />width, Proude number, and shear stress. The energy gradeline is equal to the water surface <br />elevation plus the head of water associated with the flow velocity. The energy gradeline slope is <br />related to the headloss between cross-sections. The Proude number indicates whether the flow at <br />that cross-section is subcritical (<1) or supercritical (> 1). This can also be determined by <br />comparing the critical water surface elevation to the water surface elevation. The shear stress in <br />the channel is a measure of the force the water would exert on the channel bed. That parameter <br />was used to examine sediment transport in the channel (Section 2.4). <br /> <br />The HEC-RAS results for large floods in Willow Creek indicated "supercritical" flow at many <br />locations. In the lower section of the study area (XS 1 - XS 19), supercritical flow was indicated <br />over weirs and timber drops and at flow constrictions. Upstream ofXS 20 near the confluence of <br />East and West Willow Creeks, the model indicated supercritical flow throughout many reaches <br />including Willow Creek from XS20 to the confluence, most of West Willow Creek downstream <br />of cross-section 40, and in East Willow Creek between the confluence and the culvert in North <br />Creede and above XS 13 . <br /> <br />One can envision supercritical flow as water flowing over a steep dam spillway prior to the <br />"hydraulic jump" where the flow transitions back to subcritical flow. Supercritical flow has a <br />high velocity and can be highly erosive. During a large flood, supercritical flows could <br />potentially erode and damage channel banks, levees, and hydraulic structures. Debris blockage <br />could potentially increase this damage or cause transition back to subcritical flow and higher <br /> <br />2-16 <br />