Countermeasures to Protect Bridge Abutments from Scour (2007) / Chapter Skim
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Pages 161-190
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From page 161... ...
161 This chapter reports laboratory results of three flow modification countermeasures: parallel walls, spur dikes, and abutment collars. Section 9.1 describes the laboratory equipment and procedure, including a description of the flume, the abutment model, the velocity ration, the sediment characteristics, instrumentation, and the experimental procedure employed.
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the critical shear velocity of the bed sediment is 1.995 cm/s. In experiments under live-bed conditions, the sediment was recirculated with the water.
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For the experiments of this research project, u*
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3. Wet and drained the flume completely; 4.
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of the abutment. This scour zone may pose a threat to the main channel bank immediately downstream of the abutment.
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of the abutment. The reason why the maximum scour depth was not located right at the upstream corner of the abutment was that the velocity ratio between the floodplain flow and the main channel flow was so high that the floodplain flow was able to jet into the main channel a distance away from the bank.
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profile measurements turned out to be difficult. Therefore, the flow was mainly controlled by the discharge and the average water surface profile and the average bed profile along a 12-m transect in the middle of the approach channel, where each bed profile was monitored.
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From Figure 9-9, it can be seen that the maximum timeaveraged local scour still took place at the upstream corner of the abutment, with a scour depth of 7.23 cm. This finding agreed with the baseline scour pattern in the clear-water scour condition.
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between values near zero and values nearly twice the maximum scour under clear-water flows because of the superposition of the trough of bed forms (see Figure 9-10)
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of guidebanks. The work fills a need for low-cost countermeasures for small bridges with wing-wall abutments.
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results of the solid-wall experiments under clear-water conditions, and Table 9-4 gives results under live-bed conditions. Figure 9-12 is a string contour of the 1.2L solid wall run in clear-water scour conditions, where ym is 132 mm, yf is 52 mm, Q is 0.0379 m3/s, and te is 80 hours.
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abutment was able to move the scour hole upstream from the abutment corner and, therefore, was effective as a scour countermeasure. It was also found that, for clear-water scour conditions, as the length of the wall increased, the scour at the abutment declined.
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173 Flow Floodplain A A1 Abutment ym Floodplain Section B-B1 Abutment Flow Riverbed Riverbed Wa Bm B1B Floodplain Parallel Wall V H LpW Parallel Wall Apron Apron Parallel Wall Lw Top View Main Channel La Section A-A1 Abutment Top and Bottom of Bridge Deck Figure 9-14. Parallel wall (aprons were present only in live-bed experiments)
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not pose a direct threat to the abutment. However, when the length of the wall was 0.5La, the scour hole was 12.96 cm and the abutment was highly threatened.
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Discussion Figure 9-18 shows the scour depth at the bridge abutment versus rock wall length for different wall protrusion amounts for clear-water scour conditions. It can be seen that, for protrusion lengths, Lp, of 0.25W and 0.5W, increases in wall lengths can reduce scour at the abutment significantly.
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However, the increases in wall lengths only reduced the maximum instantaneous scour from 51.3 to 47.3 mm (an 8-percent reduction)
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was observed to flow through the rocks. This indicated that there is no significant flow transfer through the wall.
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wall should be 1.6La to obtain acceptable scour reduction rate at the abutment for the conditions tried in this study. • A parallel solid wall attached at the upstream corner of the abutment parallel with the flow may or may not be able to reduce the amplitude of the bed forms that pass through the bridge opening, depending on the changes of the flow parameters from the approaching channel after entering the bridge crossing.
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direct the flow into the main channel. It may create wake vortices behind itself.
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180 Figure 9-23. Definition sketch for spur dike scour countermeasure.
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was 1.5La (66 cm)
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contraction scour. In relatively wide bridge crossings, this may not cause a significant problem, but it may be a problem for narrower ones.
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From the clear-water experimental data, the configurations of Test Sp-8 and Sp-10 were considered to have the most potential for protecting the abutment and were tested further under live-bed flows. No further experiments were conducted with the configurations used in Sp-7 or Sp-9.
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dikes at the abutment and reduced the scour depth at the upstream side of the first spur dike, as shown in Table 9-10. However, the top of the first spur dike still sank 50.0 mm.
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In addition to the variables directly tested in the laboratory experiments described above, the following design guidelines are offered. The top height of each spur dike should be high enough so that it is not overtopped by the flow during flooding, since all of the experiments described here are for emergent spur dikes and the flow patterns will be greatly altered if the spur dikes were overtopped.
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conditions to determine the best configuration for a collar to be a successful countermeasure. Figure 9-35 shows the collar countermeasure.
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each of these collars had a similar magnitude as the scour depth at the same location in the baseline case with no countermeasures. Figure 9-38 shows the transverse bed profile in the bridge crossing of the baseline case and the scour profile formed by the maximum local scour depth values under the edge of the various collars of different widths.
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188 -80 -70 -60 -50 -40 -30 -20 -10 0 0 50 100 150 200 250 300 350 400 Transverse Collar Width (mm)
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threaten the abutment. In Tests T5 and T6, the upstream end of the collars was still buried in the sand at the end of the experiments and, therefore, no scour was found.
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The scour became insignificant as the main channel edge of the collar was extended beyond the local scour hole area measured in the baseline case without countermeasures. The trailing edge of the collar should extend to a location downstream of the abutment.
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