Combining Individual Scour Components to Determine Total Scour (2018) / Chapter Skim
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From page 81... ...
81 CHAPTER 4. Research Findings 4.1 Introduction Organization of combined scour into interaction categories as described in the previous chapter provided the framework for explaining how complex scour interactions could be captured and predicted using a practical but effective methodology.
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82 Table 4-1. Experimental maximum abutment scour depth results for CWS and LSA.
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83 For each scour experiment at Georgia Tech, the right floodplain flow was blocked by a bankline abutment (BLA) for varying abutment lengths on the left floodplain which resulted in scour depth data for the BLA in the main channel as well as for the LSA in the left floodplain.
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84 Table 4-2. Experimental maximum abutment scour depth results for CWS around BLA.
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85 Table 4-3. Experimental maximum scour depth results for CWS around SSA.
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86 Table 4-4. Experimental maximum scour depth results for CWS with piers in place.
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87 orifice such that at the same tailwater elevation, the ratio of overtopping discharge to total discharge, Qot/Q, increased from 0.30 to 0.41 with larger values of headwater elevation and Q
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88 in compound channel flow. The relative magnitude of velocities for different flow types is determined by the tailwater-discharge setting and the resultant upstream flow depth caused by the bridge backwater.
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89 As discussed in Chapter 2 (see Eqs.
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90 Given the distribution of q established between floodplain and main channel in the approach flow section, the spatial alteration of that distribution at the bridge is illustrated in Figure 4-4. The redistribution is the result of lateral width contraction of the bridge opening in addition to vertical contraction for SO and OT flow, and the flow separation around the abutment.
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From page 91... ...
91 0.0 0.3 0.5 0.8 1.0 1.3 1.5 1.8 0.8 1.2 1.6 2.0 2.4 2.8 0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 U ni t D isc ha rg e (ft 2 /s ec )
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92 1 ft/sec (a) Free flow 1 ft/sec (b)
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93 The connection between spatial scour distribution and the properties of the flow field is illustrated in Figure 4-6 for La/Bf = 0.41 and 0.77 for free, submerged orifice and overtopping flows. In each figure plate, cross-section distributions at the downstream face of the bridge (subscript "2")
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From page 94... ...
94 0 5 10 15 20 25 30 0 0.3 0.6 0.9 1.2 1.5 1.8 0 2 4 6 8 10 12 14 Di m en sio nl es s Va ria bl e El ev at io n , z (f t) Lateral distance, Y (ft)
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95 -6 -4 -2 0 2 4 x (ft)
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96 -6 -4 -2 0 2 4 x (ft)
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97 An exception to this behavior is Run 11 for SO flow with La/Bf = 0.77 in which the transition from the linear range to the asymptotic range is rather abrupt. In this case, two scour holes developed initially (see Figure 4-7)
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From page 99... ...
99 4.2.5 Organization of Scour Prediction Methodology Scour components of abutment/lateral contraction scour, vertical contraction scour, and pier scour were found to interact in specific combinations and were organized into categories such that a scour prediction formula could be developed from experimental data obtained for each category. Abutment and lateral contraction scour were treated in the same category because they are a function of the same independent variables, albeit in different proportions, as justified previously in Chapter 2.
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100 -6 -4 -2 0 2 4 X (ft)
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101 An alternative approach for categorizing abutment lengths as LSA or SSA is suggested in Figure 4-11 using the same experimental data as in Figure 4-10. The criterion is given as the product of La/Bf and Yf1/Yfo in which the former is representative of the degree of geometric contraction and the latter indicates the degree of flow contraction indirectly through the backwater ratio.
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102 physical mechanisms and parameters that influence those mechanisms, and because of its very large database, particularly covering piers of various width classes (Ettema et al.
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103 4.2.7 Prediction of Vertical Contraction Scour Alone In contrast to pier scour, relatively few studies have been completed for vertical contraction scour which occurs for SO and OT flows. This study added to the database of vertical contraction scour results which are shown in Figure 4-13 along with those of a few other investigators.
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104 4.2.8 Category I Scour Interactions Category I scour interactions incorporate combined abutment/lateral contraction scour with or without vertical contraction scour for a LSA. Scour contours for Category I can be observed for two different abutment lengths subjected to F, SO, and OT flow as shown in Figure 4-7, except for panel (d)
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105 For equilibrium scour depth, the time parameter can be removed. The Froude number (F1)
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106 0.0 1.0 2.0 3.0 4.0 5.0 0.0 1.0 2.0 3.0 4.0 5.0 Y f 2m ax /Y fc (qf2/qf1) Hong F Flow Hong SO Flow Hong OT Flow GT F Flow GT SO Flow GT OT Flow UoA F Flow UoA SO Flow UoA OT Flow Ettema F Flow Ettema Envelope Figure 4-14.
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107 The two relationships suggested by Hong (2013) for rT were applied to the full complement of available data in Figure 4-14 including the CWS data from Georgia Tech and UoA in the present study, Hong's data, and the data from Ettema et al.
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From page 108... ...
108 50.0 1 2 1 1 50.1 1max2 363.2 f f fc f fo f fo f q q V V Y Y Y Y (4-8) with R2 = 0.82 and SEE = 0.230.
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109 0.0 1.0 2.0 3.0 4.0 5.0 0.0 1.0 2.0 3.0 4.0 5.0 Y f 2m ax /Y f0 [(Yf1/Yf0)
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From page 110... ...
110 0.0 1.0 2.0 3.0 4.0 5.0 0.0 1.0 2.0 3.0 4.0 5.0 Y f 2m ax /Y f0 [(Yf1/Yf0)
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111 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 Y f 2m ax /Y f0 [(Yf1/Yf0)
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112 upper limit. For further context, dimensionless scour depth (ds/Yfo)
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113 0.0 1.0 2.0 3.0 4.0 5.0 6.0 0.0 1.0 2.0 3.0 4.0 5.0 6.0 (Y f2 m ax /Y fo )
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114 for the BLA (R2=0.70 and SEE = 0.153) is given by 50.0 1 2 1 1max2 *
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115 0.0 1.0 2.0 3.0 4.0 5.0 0.0 1.0 2.0 3.0 4.0 5.0 Y m 2m ax /Y m o [(Vm1/Vmc1)
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116 0.0 1.0 2.0 3.0 4.0 5.0 0.0 1.0 2.0 3.0 4.0 5.0 Y m 2m ax /Y m 0 [(Vm1/Vmc1)
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117 As discussed in Chapter 2, adequate field measurements at the peak of the flood event are rarely available for bridge scour; however, three documented flood events were found in the literature for Category II scour. The Towaliga River flood event caused by Tropical Storm Alberto, which occurred in Georgia in 1994, was reproduced in the hydraulics laboratory of the Georgia Institute of Technology.
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118 0.0 1.0 2.0 3.0 4.0 5.0 0.0 1.0 2.0 3.0 4.0 5.0 Y m 2m ax /Y m o [(Vm1/Vmc1)
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119 0.0 1.0 2.0 3.0 4.0 0.0 1.0 2.0 3.0 4.0 (Y m 2m ax /Y m o)
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120 4.2.10 Category III Scour Interactions Pier scour interactions between abutment/lateral contraction scour, with or without vertical contraction scour, define Category III scour interactions. Experiments were conducted without the pier and then repeated with different pier locations for all three types of flows (F, SO, and OT flows)
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121 0.3 0.5 0.7 0.9 1.1 1.3 1.5 Elevation Z (ft)
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122 Figure 4-26(b) Run 28 SO flow, Lp/W = 0.18.
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123 4.2.10.1 Effect of Pier on Abutment/Contraction Scour (Category III) Table 4-4 was presented previously to show the scour parameters measured for the experiments conducted with the pier placed within the influence of the abutment scour hole along with the pier location, type of flow, abutment length, and the maximum equilibrium scour depth measured at the upstream front of the pier (Yf2max/ Yo (Pier)
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124 0.0 0.5 1.0 1.5 2.0 0.00 3.00 6.00 9.00 12.00 Y f 2m ax /Y f2 m ax o Lp/Yf1 Pier Affected Abutment Scour La/Bf=0.41 Pier Affected Abutment Scour La/Bf=0.77 Pier Affected Abutment Scour La/Bf=0.53 +15% -15% (a) Rectangular column piers 0.0 0.5 1.0 1.5 2.0 0.00 3.00 6.00 9.00 12.00 Y f 2m ax /Y f2 m ax o Lp/Yf1 Pier Affected Abutment Scour La/Bf=0.41 Wall Pier +15% -15% (b)
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From page 125... ...
125 0.0 1.0 2.0 3.0 4.0 5.0 6.0 0.0 1.0 2.0 3.0 4.0 5.0 6.0 (Y f2 m ax /Y fo )
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126 The data in Figure 4-29 show considerable variability with respect to La/Bf and the type of flow so that a single relationship for the pier amplification ratio was difficult to develop. However, the figure was used to delineate two zones of influence of abutment/contraction scour on pier scour in terms of magnitude: (1)
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127 fo focalculatedpiersmeasuredpiers excessfo f Y Ydd Y Y )
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128 0.0 1.0 2.0 3.0 4.0 5.0 0.0 1.0 2.0 3.0 4.0 5.0 (Y f2 m ax /Y fo )
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129 Measured vs. calculated Category III pier scour depths are shown in Figure 4-32 using the HEC18 procedure in which the calculated scour depth is the simple addition of results from the abutment scour and pier scour equations.
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130 0.0 1.0 2.0 3.0 4.0 5.0 6.0 0.0 1.0 2.0 3.0 4.0 5.0 6.0 (Y f2 m ax /Y fo )
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131 approach flow velocity is likely to be small so that their interaction is relatively weak. In this case, the pier scour depth is added to the vertical contraction scour depth to obtain the combined scour.
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132 0 0.3 0.6 0.9 1.2 1.5 1.8 0 0.3 0.6 0.9 1.2 1.5 1.8 d s e/Y 1(C al cu la te d) dse/Y1 (Measured)
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133 Table 4-7. Summary of proposed combined scour equations and prediction errors.
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134 Table 4-7 continued Note: * These values are for ±20 % from line of agreement, *
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135 4.3 Live-Bed Scour Experimental Results 4.3.1 Summary of Results The relative depth to the point of maximum scour below the water surface at the bridge, Ym2max/Ymo, is shown for three relative abutment lengths subject to LBS in Table 4-8. (For definition of parameters, see Table 4-2)
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136 4.3.2 Water Surface Profiles Water surface profiles for F, SO and OT flows are compared in Figure 4-36 for two different abutment lengths of La/Bf =0.80 and 0.50. All water surface elevations shown are relative to the floodplain elevation of 1.30 ft.
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137 1.2 1.4 1.6 1.8 2 -30 -20 -10 0 10 20 30 El ev at io n (ft ) Longitudnal Direction (ft)
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138 0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 0.80 1.20 1.60 2.00 2.40 2.80 0.0 1.0 2.0 3.0 4.0 5.0 Ve lo ci ty (f t/s ec )
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139 0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 0.80 1.20 1.60 2.00 2.40 2.80 0.0 1.0 2.0 3.0 4.0 5.0 U ni t D is ch ar ge (f t2 / se c)
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From page 140... ...
140 0 10 20 30 40 50 60 70 0 0.5 1 1.5 2 2.5 3 0 1 2 3 4 5 D im en sio nl es s Va ri ab le El ev at io n , z (f t) Lateral distance, y (ft)
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141 increased resulting from the increase in abutment length, while the scour depth at the bridge section increased to a lesser degree. In addition, the magnitude of the transverse gradients of V2R/u*
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144 0.0 0.5 1.0 1.5 2.0 2.5 0.0 0.5 1.0 1.5 2.0 2.5 Y m 2m ax /Y m c (qm2/qm1) UoA LBS F Flow UoA LBS SO Flow UoA LBS OT Flow Ettema et al LBS(BLA/SSA)
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145 0.00 0.20 0.40 0.60 0.80 1.00 0.6 0.8 1 1.2 Q ot /Q , Q ob st /Q qm2/qm1 Qobst/Q Qot/Q Figure 4-43. Illustration of Qot/Q > Qobst/Q as a criterion for determining the condition qm2/qm1 < 1.
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From page 146... ...
146 0.0 1.0 2.0 3.0 4.0 5.0 0.0 1.0 2.0 3.0 4.0 5.0 Y m 2m ax /Y m 0 (Vm1/Vmc*
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147 0.0 1.0 2.0 3.0 4.0 5.0 0.0 1.0 2.0 3.0 4.0 5.0 Y m 2m ax /Y m 0 (Vm1/Vmc*
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148 0.0 1.0 2.0 3.0 4.0 5.0 0.0 1.0 2.0 3.0 4.0 5.0 (Y m 2m ax /Y m o)
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149 4.4 CFD Model Results This section reports on the analysis of the computational fluid dynamics (CFD) simulations.
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150 Figure 4-48. Schematic top view of the simulation domain, with cross sections highlighted where experimental measurements were taken.
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151 Figure 4-50. Profiles of the normalized streamwise time-averaged velocity for Run 1 (free flow, long setback abutment)
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152 for the downstream locations of down_bridge, down_toe and down_further cross sections. Profiles (i)
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153 Figure 4-52. Profiles of the normalized streamwise time-averaged velocity for Run 10 (free flow short setback abutment)
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154 Figure 4-53. Profiles of the normalized water surface elevation for Run 10 at three cross sections Figure 4-53 presents profiles of the normalized water surface elevation as computed via LES (solid line)
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155 Figure 4-54. Profiles of the normalized streamwise time-averaged velocity for Run 2 (submerged orifice flow and long setback abutment)
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156 than the experiments (there are some experimental data points above the dashed line which indicates LES-predicted water surface) , and hence the LES underestimates the velocity near the water surface.
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157 Figure 4-56. Profiles of the normalized streamwise time-averaged velocity for Run 11 (submerged orifice flow and short setback abutment)
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158 Figure 4-58. Profiles of the normalized streamwise time-averaged velocity for Run 3 (overtopping flow and long setback abutment)
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159 Figure 4-59. Profiles of the normalized water surface elevation for Run 3 at three cross sections.
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160 Simulated and measured profiles of the normalized streamwise time-averaged velocity for Run 12 at three cross sections are presented in Figure 4-60. As for the other overtopping run, the results from the LES are in fairly good agreement with the experimental data.
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161 Figure 4-62. Streamlines of the mean flow of Run 1 (free flow, long setback abutment)
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162 Figure 4-63. Streamlines of the mean flow of Run 10 (free flow, short setback abutment)
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163 Figure 4-64. Streamlines of the mean flow of Run 2 (submerged orifice, long setback abutment)
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164 Figure 4-65. Streamlines of the mean flow of Run 11 (submerged orifice, short setback abutment)
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165 Figure 4-66. Streamlines of the mean flow of Run 2 (overtopping, long setback abutment)
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166 Figure 4-67. Streamlines of the mean flow of Run 12 (overtopping, short setback abutment)
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167 4.4.3 Instantaneous Flow and Turbulence Figure 4-68 presents an instantaneous snapshot of the water surface for the free flow, long setback run (Run 1)
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168 Turbulence structures are educed by means of the Q-criterion (related to rate of rotation of turbulent vortex filaments) , and these are plotted for Run 1; that is, free flow and long setback abutment, in Figure 4-69.
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169 Figure 4-70 shows contour lines of normalized near-bed turbulent kinetic energy (TKE) overlaid onto contours of the scoured experimentally observed bathymetry (a)
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171 Figure 4-71 presents a snapshot of the instantaneous water surface for the free flow, short setback run (Run 10)
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172 Figure 4-72 presents visualised turbulence structures educed by isosurfaces of the Q-criterion for free flow around the short setback abutment. The turbulence structures are highlighted via black lines and they are labelled A to F
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173 Figure 4-73 presents contour lines of normalized near-bed turbulent kinetic (TKE) overlaid onto contours of the scoured experimentally observed bathymetry (a)
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174 For submerged orifice flows, the instantaneous water surfaces were similar for the long setback and short setback abutments, so Figure 4-74 presents a snapshot of the instantaneous water surface for the submerged orifice flow, short setback run (Run 11)
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175 Isosurfaces of the Q-criterion of the instantaneous flow for the submerged orifice flow with a long setback abutment are plotted in Figure 4-75. In this flow not so many prominent turbulence structures are found, except for some small roller-type vortices near the water surface, which are a result of the negative velocity gradient near the surface.
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176 Figure 4-76 presents contour lines of normalized near-bed turbulent kinetic (TKE) overlaid onto contours of the scoured experimentally observed bathymetry (a)
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177 Isosurfaces of the Q-criterion of the instantaneous flow for Run 11, i.e. submerged orifice flow and short setback abutment, are plotted in Figure 4-77.
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178 Figure 4-78 presents contour lines of normalized near-bed turbulent kinetic (TKE) overlaid onto contours of the scoured experimentally observed bathymetry (a)
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179 Figure 4-79 presents a snapshot of the instantaneous water surface for the overtopping flow, short setback run (Run 12)
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180 Figure 4-80 presents visualised turbulence structures which are educed by isosurfaces of the Qcriterion for the overtopping flow and long setback abutment. Similar to all other runs, interface turbulence is found downstream of both abutments.
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181 Figure 4-81 presents contour lines of normalized near-bed turbulent kinetic (TKE) overlaid onto contours of the scoured experimentally observed bathymetry (a)
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182 Figure 4-81(c) Figure 4-81.
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183 Figure 4-82. Coherent turbulence visualised by isosurfaces of the Q-criterion color-coded with the normalized time-averaged streamwise velocity (U/Ubulk)
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185 Figure 4-84. Contour lines of near-bed turbulent kinetic (TKE)
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186 Figure 4-85. Streamlines of the near-bed flow through the scour hole.
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187 Figure 4-86. Contours of the streamwise velocity together with streamlines (top row)
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188 The RANS-predicted profiles are in reasonably good agreement with the experimental data on the left floodplain, i.e. profiles (a)
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189 induced by the increasing the length of the left abutment in terms of the velocity field is the increasing predominance of the recirculation area behind the abutment. For the MSA, SSA, and BLA runs, the high-velocity, high-momentum flow through the bridge opening is forced towards the right bank of the main channel and onto the right floodplain.
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190 Figure 4-88. (left)
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191 Figure 4-89. Profiles of measured and 3D RANS -predicted scour predictors (q2/q1, V2(R)
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192 4.4.6 Two-Dimensional or Depth-Averaged RANS 4.4.6.1 Validation of the RANS Simulations Figure 4-90 plots profiles of the depth-averaged normalized streamwise velocity as measured (open circles) and predicted by LES (solid line)
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193 Figure 4-91 plots profiles of the depth-averaged normalized lateral velocity as measured (open circles) and predicted by LES (solid line)
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194 4.4.6.2 Effect of Bridge Opening on Hydrodynamics and Scour Predictions Similar to the 3D RANS simulations, Figure 4-92 presents the depth-averaged velocities together with profiles of the scour predictor variables for four different scenarios as predicted by the 2D RANS simulations. A comparable trend to the 3D RANS results is observed in the 2D RANS simulations where the extension of the left abutment's length produced an increasing size of the recirculation area.
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195 Figure 4-92. (left)
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196 Figure 4-93. Profiles of measured and 2D RANS -predicted scour predictors (q2/q1, V2(R)
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197 Figure 4-94. Predicted maximum normalized depth of scour hole for Category I scour as measured in the laboratory (squares)
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198 physical model study. A reach of the Ocmulgee River in Macon, Georgia, which included the Fifth Street Bridge where the drainage area is 2240 mi2 (865 km2)
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199 Three different cross sections are selected to provide a quantitative comparison of the computed 2D results with the prototype data and the physical model data. The cross sections are highlighted in Figure 4-97, where C.S.-3b is a cross section upstream of the bridge, C.S.-4 is at the beginning of the upstream piers and C.S.-5 is immediately after the downstream piers.
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200 Figure 4-98 Velocity vectors around the bridge piers. A more quantitative assessment of the predictive capabilities of the 2D RANS model is possible with Fig.
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201 ' Figure 4-99. Comparison of SRH-2D velocity distribution at C.S.-4 with physical model and prototype data.
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202 The 1D HEC-RAS water surface profile program was also applied to the Ocmulgee River reach for comparison with the physical model study and prototype data. The same Manning's n of 0.025 and flow conditions as set in the SRH-2D CFD model were applied.
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