Countermeasures to Protect Bridge Abutments from Scour (2007) / Chapter Skim
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From page 118... ...
118 8.1 Experimental Work 8.1.1 Introduction The aim of the experiments reported in this chapter was to investigate the use of riprap and cable-tied blocks as bridge abutment scour countermeasures. Both riprap and cable-tied block aprons were placed around spill-through bridge abutments to protect them from scour, which could otherwise potentially undermine the abutments.
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119 ter pipes. The discharge from each pipe is regulated by butterfly valves, and the flow rate is measured by measuring the pressure difference across an orifice plate in each pipeline.
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120 mold was split into a 0.4-m frontal section and several extension sections with varying lengths, as shown in Figure 8-6. 8.1.3 Bridge Abutment Flow Field Measurements A particle-tracking velocimetry (PTV)
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diagram of the cross section of the corresponding compound channel is shown under each velocity distribution. The critical velocity for sediment entrainment Vc, determined using Shields criterion adjusted for lateral slope, is shown on each velocity distribution.
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122 (Figure 8-8)
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The experiments were carried out with a geotextile placed underneath the countermeasure apron to eliminate the winnowing of sand from between the riprap stones or cable-tied blocks. The geotextile used for testing (shown in Table 7-3)
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124 of the velocity vector field around a 0.8-m long spill-through abutment situated on a 1.6-m wide floodplain is shown in Figure 8-13. The associated vorticity, , for each flow field was calculated by using the time-averaged velocity measurements and adopting a central-difference approximation for the following vorticity expression: (8-1)
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Figure 8-12. Scour hole parameters measured for each experiment in the 2.4-m wide flume.
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126 stress fields at the spill-through abutment is shown in Figure 8-15. 8.2.3 Results Figures 8-16 and 8-17 give lateral distributions of flow velocity.
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There is a small counterclockwise rotation in the flow field at the upstream corner of the abutment (also observed by Kwan, 1984) and a larger counterclockwise rotation in the flow field downstream of the abutment, which extends out past the end of the abutment (see Figure 8-16)
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128 stream of the abutment end, irrespective of abutment length and proximity to the main channel. The zone of strongest vorticity at the abutment corresponds to the zone where riprap shear failure occurred in the study by Atayee, suggesting that the vorticity strength may be the dominant parameter initiating riprap shear failure in an apron around an abutment.
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129 assume a logarithmic velocity distribution to calculate the bed shear stress. Although the normalized shear stress plots must be interpreted with caution, they can effectively indicate zones where the shear stress increases relative to other areas in the flow field.
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130 length because of a greater contraction at the bridge section. Figure 8-16 shows that the velocity component across the flume in the y-direction increases with increasing abutment length as the flow is diverted around the abutment.
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131 Figures 8-16 and 8-17 show that, for most cases, the velocity, vorticity strength, and shear stress at the abutment end increase slightly with increasing floodplain width. These trends are explained as follows.
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and the development of scour at the abutments. These sections also describe how the flow fields can be used to determine the zones around the abutment that need to be protected from scour.
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with the scour formation was difficult, because the scour hole development occurred away from the abutment because of the presence of the apron. Figure 8-22 shows an example of the equilibrium scour hole at a 0.8-m long abutment situated on a 1.6-m wide floodplain, with spill slope protection extended below the surface of the floodplain.
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If the flow fields at the abutment had been remeasured after the development of a scour hole at the abutment, the velocity around the abutment would have decreased because the flow depth would have been deeper as a result of the scour at the abutment. Consequently, the bed shear stresses around the abutment would also have decreased.
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stresses would decrease to the critical bed shear stress level, at which point equilibrium scour conditions would be attained. The problem with remeasuring the flow fields around the abutment after the development of a scour hole at the abutment is that the measured flow fields would not be representative of the actual flow fields at the abutment.
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forms beyond the floodplain (αe > Bf) , the data points lie above the envelope suggested by Melville and Coleman (2000)
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• The scour hole formed in the main channel (e Bf)
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The lines in Figure 8-34 represent Equation 8-13 for the four different Bf/yf values used in the experimental study. As in Figure 8-33, the solid and hollow symbols in Figure 8-34 signify scour data where e Bf and e Bf, respectively.
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Minimum Apron Extent The apron extent Wo, for which Wmin 0, was measured for each experimental configuration -- that is, for every combination of L/yf and Bf/yf. The data are plotted in Figure 8-36, where the encircled data represent values for which Wo was measured directly.
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predicted scour depths of Equation 8-13. Figure 8-37 also includes the data given in Figure 8-35.
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Figure 8-41 compares scour depth predictions using Equation 8-13 with those using Equation 8-18 in terms of the flow field parameters listed in Table 8-4. Figure 8-41 shows that there is a good agreement between Equations 8-13 and 8-18.
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configurations. The physical model's configuration of abutment, channel, and floodplain, diagrammed in Figure 8-42, was chosen to evaluate the numerical model.
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the low-velocity regions of the wake. No further calibration was performed.
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the depth-averaged velocity at this location is likely to be inaccurate. Depth-averaged velocity and computed boundary shear stress were plotted for four cross-flume transects, as shown in Figures 8-47 and 8-48.
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147 Figure 8-47. Depth-averaged velocity along transects across flume.
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148 Note: two-dimensional model velocities are depth-averaged, and PTV velocities are from the water surface only. Figure 8-49.
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mind that the PTV data represent flow velocity at the water surface in the physical model, while two-dimensional model data represent computed depth-averaged flow velocity. Comparisons of local peak velocity and average velocity over the floodplain and main channel are provided below.
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of flow over the abutment slope and over the sloping channel bank were not included in the averages for the floodplain and channel, respectively. The average velocity in the channel is well predicted by the two-dimensional model.
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and the stone size conform in scale to the values mentioned in Section 8.1. Also, the clear-water approach-flow condition matched those for the experiments described in Section 8.1.
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abutment. Downstream of the roughened entry was a 4.87-m (16-ft)
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An initial experiment was conducted with an abutment formed of loose sand embankment placed around a standard stub abutment structure, as shown in Figure 8-58. This experiment sought to illustrate scour development around an unprotected abutment formed of an erodible earthfill embankment.
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The estimated critical shear velocity for this stone u* C was calculated to be 0.26 m/s.
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8.5.3 Results The results presented herein include data on the maximum depth and location of the consequent region of scour, along with information illustrating the variations in flow field corresponding to the scour region. As clear-water scour asymptotically approached an equilibrium condition over time, each experiment was run for about 72 to 80 hours until negligible change was observed in the scour hole dimensions.
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chapter (and earlier chapters) , the apron did not prevent scour development, but shifted its position away from the abutment.
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Figure 8-64 plots maximum scour depth, dsmax, versus apron width, W This figure indicates three regions of scour depth trends: • Scour attributable to abutment form and erodibility, • Scour development attributable to the combined structural form of abutment and apron, and • Scour attributable to flow over apron.
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Flow Field Over Apron The trends for scour depth and location are explainable in terms of the flow field around the abutment and over the apron, as well as in terms of apron extent. In this regard, the flow field insights provided by the LSPIV measurements, along with the findings from the two-dimensional numerical simulation, usefully show the following trends in flow field as apron width increases: • The median mean value of depth-averaged velocity of the approach flow to the abutment, with the 0.40-m wide apron, is 0.45 m/s.As the flow passes around the abutment, the flow contracts, producing an overall depth-averaged velocity of 0.55 m/s at the plane extending through the center of the abutment.
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lengths. The scour deepening of the bed drew more flow to the scour region, thereby locally increasing unit discharge of water in the region of scour.
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