Evaluation of Bridge Scour Research: Pier Scour Processes and Predictions (2011) / Chapter Skim
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From page 74... ...
72 CHAPTER 5 PIER SITE COMPLICATIONS 5.1 Introduction Several factors complicate pier sites at bridge waterways, and thereby introduce additional processes and parameters to be considered when estimating pier scour depth. The additional processes of practical importance can be grouped in two categories: 1.
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73 ݕ௦ ൌ ݂ݑ݊ܿݐ݅݊ ۏ ێێ ێێ ێێ ێ ۍ݂݈ݓ ሺߩ, ߤ, ܸ, ݕ, ݃ሻ, ܾ݁݀ ݉ܽݐ݁ݎ݈݅ܽ ൫ܦ, ߪ, ߩ௦, ܿ, ܸ , ൯, ݅݁ݎ ሺܽ, Ω, ߠሻ,ݐ݅݉݁ ሺݐሻ, ܾݎ݅݀݃݁ ݏݐݎݑܿݐݑݎ݁, ܾ݀݁ݎ݅ݏ ݎ ݅ܿ݁ ܽܿܿݑ݉ݑ݈ܽݐ݅݊, ݄݈ܿܽ݊݊݁ ݃݁݉݁ݐݎݕ, ݂ݑ݊݀ܽݐ݅݊ ݉ܽݐ݁ݎ݈݅ܽ ݈ܿ݉݁ݔ݅ݐ݅݁ݏ, ݓܽݏ݄݈ܽ݀ ݅݊ ܽݎ݄ܽܿ ݂݈ݓ ے ۑۑ ۑۑ ۑۑ ۑ ې (5.1) Design estimation to account for these possible complications can be difficult when the pier is of unusually large size, especially relative to flow depth, or when the pier is of an unusual form.
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74 Figure 5-1 Common pier structures comprise a column on a slab footing, or column on pile cap with underpinning piles (Melville and Coleman 2000) For piers tapered on the upstream and downstream faces, the slope, in elevation, of the leading edge of the pier affects the local scour depth.
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75 similarly has been extensively studied (early studies by Hannah 1978, Raudkivi and Sutherland 1981, Jones 1989) , with more recent publications by Richardson and Davis 1995, Sheppard et al.
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76 Figure 5-2 Scour depth variation for four cases of non-uniform pier shape (Melville and Coleman, 2000)
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77 Figure 5-3 Influence of non-uniform shape on local scour depth at piers (Melville and Raudkivi 1996) For scour at piled foundations where the pile cap is clear of the water surface (Case V)
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78 subsequently by Nazariha and Townsend (1997)
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79 occurs within the region of abutment scour. Because an abutment may develop a much larger scour than usually occurs at a pier, scour depth at the pier is dominated by abutment scour.
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80 examined. A significant parameter is Lp/yf, where Lp = pier distance from the abutment toe, and yf = flow depth at the abutment toe.
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81 was at the toe of an erodible spill-through abutment on a floodplain, pier presence reduced scour depth by about 10 to 20%.
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82 Figure 5-7 Normalized scour depth at pier relative to scour depth at a spill-through abutment, and wing-wall abutment, on an erosion resistant or fixed flood plain (F) or an erodible flood plain (E)
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83 Figure 5-8 Bridge beams submerged by flood water Scour associated with a submerged bridge superstructure is attributable to flow contraction under the bridge. The misnomer "pressure scour" is often used for this scour process.
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84 6. As expected, the scour depth increases with greater difference in water levels either side of the bridge deck, and decreases with reduced erodibility of the boundary material; 7.
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85 1. Accumulation occurs at a single pier, modifying the pier's effective width and form; and, 2.
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87 Figure 5-11 Woody debris accumulation at a single pier (Lagasse et al.
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88 laterally as well as downwards, producing a shallower but wider scour hole than if no debris were present.
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89 Figure 5-12 Rectangular accumulation of debris at a pier (Lagasse et al.
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90 Figure 5-13 Local scour-depth variation with quantity of floating debris (Melville and Coleman 2000) The literature on pier scour at bridge piers subject to ice accumulation is quite sparse.
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91 themselves are well understood, there can be considerable uncertainty as to how they impact pier scour. Some influences can be difficult to take into account, especially when they trigger unplanned changes in channel alignment and bathymetry.
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92 One set of questions still to be addressed adequately concerns the combination of pier scour and some other scour forms, notably contraction scour, or scour at some other channel feature such as a confluence of approach flows (at a confluence of two channels, or behind an island or large bar)
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93 to the upstream coarse surface particles and the underlying surface fine particles, respectively. Ettema (1980)
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94 Figure 5-15 Local scour in layered sediments (Breusers and Raudkivi, 1991) ; yc = depth to top of coarse layer, ys = depth to top of fine layer 5.8 Scour of Cohesive Soil Since 1990, insight regarding pier scour in cohesive soils or clay has advanced substantially.
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95 and Chaudhuri (2010)
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96 been conducted. Hopkins and Beckham (1999)
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97 turbulence. Larger concentrations of sediment near the bed may cause flow density stratification near the bed, if the concentration of sediment is sufficiently great.
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98 suspended sediment was present. The mixing induced by wall turbulence is inhibited by the flow stratification in the lower part of the channel where the concentration levels are relatively large.
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99 As the increase in water turbidity was gradual, Sheppard et al.
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