Evaluation of Bridge Scour Research: Pier Scour Processes and Predictions (2011) / Chapter Skim
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29 CHAPTER 3 PIER-SCOUR PROCESSES 3.1 Introduction Pier scour processes are intricate and challenging to formulate (even empirically or approximately) , let alone fully comprehend.
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30 Figure 3-1 Sketch showing flow through a bridge site involving complex interactions between the floodplain, the main channel and the piers situated close to the floodplain and main channel, especially during high flow conditions 3.2 Pier Foundation Material Rivers channels and floodplains form in widely varying combinations of rock, sediment, and soils (or clays)
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31 dimensions (for layered sediments and soils) , strength for soils and rocks, and structure of joints and fractures (rocks)
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32 (c) Figure 3-2 Differences in scour form at a cylinder; (a)
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33 In terms of prevailing ranges of pier width, a, and flow depth, y, it is convenient to identify and discuss three categories of pier flow field, which produce significantly different pier scour morphologies: 1. Narrow piers (y/a > 1.4)
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34 Figure 3-4 The main flow features forming the flow field at a narrow pier of circular cylindrical form. Early research focused on flow immediately upstream of the pier (dashed area)
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35 flanks of piers, or between piles of complex pier configurations, are erosive flow mechanisms of primary importance. The turbulence structures are not isolated from each other.
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36 3.3.2 Transition Piers The main flow-field features described for narrow piers exist in the flow field of piers within the transition range of y/a, but the features now begin to alter in response to reductions of y and or increases in a. The closer proximity of the water surface to the foundation boundary (for constant pier width)
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37 Figure 3-5 Variation of flow field with reducing approach flow depth; narrow to transitional pier of constant pier width. The sketches contain the horseshoe vortex, the bow vortex, and the lee-wake vortices.
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38 Figure 3-6 Main features of the flow field at a wide pier (y/a < 0.2) Since 1990, and especially in the recent ten years, major progress has been made with numerical modeling of flow at piers.
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39 down-flow at the upstream face of the pier, drawing it out toward the bed. Another interesting phenomenon is that the capacity of the wake roller vortices to entrain sediment is strongly dependent on the shape of the pier.
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40 Figure 3-8 is a useful illustration of the highly three-dimensional and contorted path of flow and suspended sediment particles through a scour hole. The paths involve extensive rotations and vertical movements.
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41 Figure 3-8 Numerical simulation showing example flow paths (and fine-sediment paths) around a pier during scour; (a)
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42 removal of sediment particles and the growth of the upstream side of the scour hole formed in sediment. Because sediment erodes from all around the pier base, other flow features facilitate scour.
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43 critical for piers of complex geometry. Then, the complexity of the flow and of the dynamic interactions among the main coherent structures is even greater; e.g., the effects of the underflow beneath a pile cap, the vortex shedding taking place in the wake of the exposed foundation elements supporting the main pile.
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