Evaluation of Bridge Scour Research: Pier Scour Processes and Predictions (2011) / Chapter Skim
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From page 112... ...
110 CHAPTER 7 PROPOSED DESIGN METHODOLOGY 7.1 Introduction This chapter proposes a structured methodology for design estimation of pier-scour depth. The methodology, introduced in Chapter 2, is structured in terms of pier size and shape complexity, as well as site complications.
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111 well," here, implies that a clear quantitative relationship for scour-depth exists. Chapter 6 identifies and discusses the two leading equations for this purpose.
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112 A structured design methodology is not new. It is fairly normal for difficult or complex pier circumstances to receive additional design attention by means of hydraulic modeling, and possibly numerical modeling.
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113 Table 7-1 Structured design approach Pier Form and Situation Estimation Method 1. Simple pier forms in flowfield categories -- • Narrow pier • Transition pier Empirical equation: Transition from Richardson and Davis (2001)
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114 4. Complex or unusual pier forms, including • wide piers Resort to a hydraulic model, with possible use of a numerical model (system simulation)
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115 reliable bedform estimation, and linearly combining bedform amplitude with estimated pier scour depth. For piers in the narrow-pier category, pier flow field scales with pier width, irrespective of waterway boundary material.
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116 7.4 Single-Column Piers in the Narrow- and Transition-Pier Categories Cylindrical pier forms are piers of a single cylindrical body whose flow field comprises the fully developed features illustrated in Figure 3-4 for a single circular cylinder, or a pair of such cylinders as depicted in Table 7-1. This category includes slender elongated piers reasonably well aligned with the flow.
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117 Figure 7-1 Envelope of potential maximum scour depth for clear-water and live-bed scour conditions at piers The evaluation presented in Chapter 6 indicates that two existing rational methods empirically best reflect, at this point in time, the scour-depth variations for the narrow- and transition-pier categories: Richardson and Davis (2001) , the method currently used in HEC-18; and, the Sheppard et al.
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118 This equation can be used for piers founded in sediment or cohesive soil, and subject to the narrow- and transition-pier flow fields. The designer can use Eq.
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119 1. Alteration of the pier flow field by several processes, notably abutment proximity, debris or ice accumulation, bridge over-topping, channel morphology issues; and, 2.
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120 more complicated, or different, than those studied by Lagasse et al.
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121 The ensuing sub-sections briefly evaluate the prospects for improved scour-depth estimation using hydraulic modeling and numerical modeling, or hybrid modeling involving both approaches. 7.7.1 Hydraulic-Modeling For some pier situations there is little alternative other than hydraulic modeling to estimate a design pier scour depth.
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122 associated with a particular situation. Scale effects increase in severity as the ratio of prototype to model size increases or the number of physical processes to be replicated simultaneously increases.
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123 Most current numerical investigations of bridge pier flows used steady RANS models with wall functions (e.g., see Olsen and Melaaen, 1993, Richardson and Panchang, 1998, Olsen and Kjellesvig, 1998, Wang and Jia, 2000, Ali and Karim, 2002, Salaheldin et al., 2004, Roulund et al., 2000)
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124 in front of the pier. Thus, RANS simulations have marginal use for understanding pier flow fields and consequent erosion and deposition processes.
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125 More recent LES simulations using state of the art subgrid-scale models and fine meshes (e.g., Kirkil et al, 2008, Koken and Constantinescu, 2008a-b) have allowed the investigation of the role of coherent structures in the scouring process at bridge piers and abutments of simplified geometry.
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