Revised Clear-Water and Live-Bed Contraction Scour Analysis (2021) / Chapter Skim
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From page 11... ...
1 Findings 2.1 Review of Current Practice 2.1.1 Overview This review of current practice describes the flow and scour processes associated with contraction scour at bridge waterways, summarizes what is known about these processes, identifies existing data sources, and determines what needs to be known for accurate estimation of contraction scour at bridge waterways. This review is structured as follows: 1.
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2-2 Revised Clear-Water and Live-Bed Contraction Scour Analysis Both live-bed and clear-water contraction scour can occur. The former scour condition commonly occurs in the main channel of an alluvial river, while the latter is more likely to be found on a floodplain or at a relief bridge located on the floodplain.
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Findings 2-3 The variation of flow depth and velocity through the different reaches of Figure 2-1 are accompanied by changes in specific energy. Specific energy, E, is defined as E y V 2g (2.1)
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2-4 Revised Clear-Water and Live-Bed Contraction Scour Analysis (a) Negligible choking, involving a small increase in flow depth immediately upstream of the contraction.
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Findings 2-5 Because the flow is non-uniform and loses energy along the q2 path, the flow depth at 2 moves from 2 to 2′, as shown in Figure 2-3b. Consequently, specific energy decreases from E2 to E2′.
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2-6 Revised Clear-Water and Live-Bed Contraction Scour Analysis Variable Symbol Upstream width Contracted width Streamwise width of bridge deck Angle of the contraction transition Upstream normal flow depth Bed slope of the approach channel Median diameter of bed sediment Geometric standard deviation of bed-sediment diameter Sediment density Critical bed shear stress for entrainment Density of water Kinematic viscosity of water Gravity acceleration Time W1 W2 Lb α Y1 S1 d σg ρs τc ρ ν g t Table 2-1. Essential independent variables considered when analyzing flow and sediment transport through a contraction.
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Findings 2-7 Long-Contraction Scour Although several "dependent" variables depend on the effects of the "independent" variables listed in Table 2-1, the variable of primary interest is scour depth in the long contraction. Scour depth, Ds, associated with normal (uniform)
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2-8 Revised Clear-Water and Live-Bed Contraction Scour Analysis Several assumptions lead to a simplification of Eq.
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Findings 2-9 of the contraction transition. Additionally, for a short contraction, bridge-deck (Lb)
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2-10 Revised Clear-Water and Live-Bed Contraction Scour Analysis hydraulics or delineation of the variables influencing contraction scour. Also, none considers the time-varying hydraulics associated with flow in open-channel contractions (as in Section 2.1.2)
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Findings 2-11 Author(s) Publication Year Comment Straub 1934 This is the earliest study regarding live-bed contraction scour.
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2-12 Revised Clear-Water and Live-Bed Contraction Scour Analysis Prior studies also can be grouped into three categories: Category 1. Formulation of contraction scour in terms of an idealized, long contraction with uniform flow occurring in the approach section and the contracted section Category 2.
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Straub (1934) Live-Bed Scour; τc = critical shear stress; τ1 = approach flow shear stress.
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2-14 Revised Clear-Water and Live-Bed Contraction Scour Analysis equation considers both bed load and suspended-load transport; where the exponent p varies according to the relative contribution of bed load and suspended load to the total sediment transport rate. Laursen (1963)
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Findings 2-15 2.2 Evaluation of Existing Clear-Water and Live-Bed Contraction Scour Laboratory Data 2.2.1 Overview Existing laboratory data on clear-water and live-bed contraction scour (see Tables 2-2 and 2-3, respectively) were evaluated considering the known flow and scour processes (see Section 2.1.2)
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2-16 Revised Clear-Water and Live-Bed Contraction Scour Analysis The research conducted under NCHRP Project 24-34 demonstrated quite clearly that, in terms of the reliability index β, the HEC-18 contraction scour procedure exhibits the most uncertainty of all the scour equations. Reliability index β is a useful indicator to compute the failure probability.
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Findings 2-17 scour equations, and would either (1) lead to better values of the reliability index β, or (2)
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2-18 Revised Clear-Water and Live-Bed Contraction Scour Analysis where Vc = Critical velocity for particle motion, ft/s y = Approach flow depth, ft Ks = Dimensionless Shields parameter (0.03 for gravel, 0.047 for sand) Ss = Specific gravity of particle (assumed equal to 2.65 unless otherwise indicated)
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Findings 2-19 Center-River Analysis System)
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2-20 Revised Clear-Water and Live-Bed Contraction Scour Analysis 0.0 0.5 1.0 1.5 2.0 2.5 3.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 D im en si on le ss C ho ki ng R ati o Unit Discharge q2 in Contracted Section, ft3/s/ft Choked Not choked Figure 2-7. Dimensionless choking ratio versus unit discharge in the contracted section, 119 laboratory tests.
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Findings 2-21 not developed from laboratory or field data, but instead was derived from sediment transport concepts and theory. It is therefore a predictive equation, not a design equation, and as such does not have built-in conservatism.
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2-22 Revised Clear-Water and Live-Bed Contraction Scour Analysis Some additional aspects of the quality of the data are generally not described for live-bed scour: 1. There is little or no description of the variation in bedform morphology through the contraction and narrowed channel in previous studies.
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Findings 2-23 Mueller 1996)
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2-24 Revised Clear-Water and Live-Bed Contraction Scour Analysis 2.3.3 Evaluation of Field Data There is a lack of reliable field data for comparison with the contraction scour equations in Table 2-6. The preponderance of these data include the influence of abutments and possibly the presence of piers.
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Findings 2-25 and Caldwell (2009) make about the field data.
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2-26 Revised Clear-Water and Live-Bed Contraction Scour Analysis Figure 2-10. Comparison of normalized, measured depths of contraction scour and the scour depth predicted using HEC-18.
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Findings 2-27 In effect, the channel contraction through the bridge site resulted in both vertical and lateral erosion, as shown in Figure 2-12. 2.3.5 Conclusions from Evaluation of Field Data The following conclusions can be drawn from the evaluation of existing field data: 1.
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