Evaluation of Bridge-Scour Research: Abutment and Contraction Scour Processes and Prediction (2011) / Chapter Skim
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5 SCOUR DEPTH ESTIMATION FORMULAS
5 SCOUR DEPTH ESTIMATION FORMULAS
Pages 34-59
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From page 34... ...
Furthermore, comparisons of abutment scour depth estimations from existing formulas with field data and with engineering experience can produce mixed results partly because of the misperception that abutment scour formulas apply to all types of abutment scour, even to the most complicated field situations, and partly due to the difficulty of estimating the flow and sediment parameters required in existing scour formulas. It is the purpose of this chapter to present several leading abutment scour formulas, classify them, assess their limitations, and evaluate their usefulness in various types of abutment scour cases.
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From page 36... ...
Under these circumstances, a two-dimensional model for setback abutments on wide floodplains, or even a three-dimensional model for bankline abutments may be necessary to quantify the flow distribution at the bridge section. Finally, it is paramount to have as much information as possible on the sediment itself both in the floodplain and in the main channel, and over soil formation depths commensurate with the abutment foundation depth.
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From page 37... ...
Prevention of abutment failures due to these morphologic changes requires close consultation between the hydraulic engineer and a fluvial geomorphologist such that bridge abutments are located well outside any meander belts or braided stream paths. 5.2 SUMMARY OF ABUTMENT SCOUR FORMULAS Table A-1 of Appendix A presents several abutment scour formulas in an approximately chronological order of publication.
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From page 38... ...
resulted in a comprehensive approach to estimating abutment scour that was integrated with earlier pier scour formulas. The Melville approach accounts not only for abutment length in ratio to flow depth but also for effects of flow intensity, bed armouring, abutment shape and orientation, relative sediment size, and channel geometry as separate multiplicative factors obtained from experiments that were conducted primarily in rectangular channels.
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From page 39... ...
have derived abutment scour formulas for clear-water and live-bed scour, respectively, in a rectangular channel. They assume that the flow rate through the scour hole area is the same before and after scour.
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From page 40... ...
A further consideration in choosing abutment scour formulas is the case of predicting scour depth and possible embankment failure when armor protection and an apron have been applied to an embankment. Van Ballegooy (2005)
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From page 41... ...
The scour formula developed from the data indicates that the maximum abutment scour depth depends on the difference between 1.57 times a flow Froude number near the toe of the abutment and a critical value of the flow Froude number, where "critical" refers to initiation of scour. The reference velocity in the Froude number is determined as the mean velocity in the bridge opening for bankline abutments, and as the mean velocity of the upstream floodplain flow if it were to pass entirely through the floodplain in the contracted section for set-back abutments.
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From page 42... ...
5.3 CLASSIFICATION OF SCOUR FORMULAS Because existing scour formulas apply to different types of abutment scour situations and rely on different classes of basic parameters on the one hand, and may not apply at all to some cases of abutment scour such as those due to stream morphology changes on the other hand, it is worthwhile to classify the formulas in several different ways. Furthermore, it is imperative that abutment scour formulas not be applied outside the range of variability of the basic dimensionless parameters for which they were derived.
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From page 43... ...
(2009) for maximum scour depth might also be placed in this second category only because it is based on an excess value of the flow Froude number relative to a critical value in the contracted section.
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From page 44... ...
2. Class II: Wider crossings over compound river channels Class II refers to wider bridge crossings, where the channel is typically compound, comprising a main channel and wide flood channels.
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From page 45... ...
The bridge abutments are usually located on the flood channels, and may be near to the main channel bank or set back from it. From the perspective of abutment scour analysis, such sites exhibit significant flow diverted from the floodplain towards and into the main channel at the bridge section.
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From page 46... ...
Class I and Class III are the simplest situations to model in the laboratory and many of the existing laboratory data apply to these two classes which can be modeled approximately as rectangular channels. It is important to recognize, however, that nearly all known data in these two classes were collected using rigid abutment models extending below the maximum measured scour depth.
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From page 47... ...
This condition can lead to instability of the main channel bank and the abutment embankment which collapse into the scour hole. This scour condition is usually live-bed scour in the main channel.
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From page 48... ...
EVALUATION OF ABUTMENT SCOUR FORMULAS From the foregoing classifications of existing abutment scour formulas, it appears that no single formula can apply to all possible cases of abutment scour, and in fact, none of the equations apply to the more difficult geomorphic transformations characteristic of meandering and braided streams. Nevertheless, it is useful to evaluate existing formulas in order to identify those that may provide promise and direction or even a framework for future research.
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From page 49... ...
based on L/Y1 is useful in comparing the applicable ranges of different abutment scour formulas: 0 < L/Y1 < 1 Short abutments similar to pier obstructions 1 < L/Y1 < 25 Intermediate length abutments 25 < L/Y1 Long abutments Only a few of the formulas in Table 5-3 include experiments with abutments in the "long" category. This classification scheme should be accompanied by one that measures the flow distribution between floodplain and main channel in compound channels; Bf /Bm and L/Bf are not quite sufficient in this regard as discussed in Section 5.4.1.
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From page 50... ...
lab data in R, C CW (1998, 1999) , HEC-18; SC field data MSHA (2010)
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From page 51... ...
report limits of ±30% for their extensive data set, although their comparisons for dimensionless scour depth include dimensionless time as an additional independent variable. Abutment scour data at a spill-through abutment in a laboratory compound channel are compared in Figures 5-5 and 5-6 with predictions from the formulas proposed by Sturm (2004, 2006)
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From page 52... ...
Comparison between scour data at a spill-through abutment (with riprap protection extended below the surface of the floodplain) and the formula by Melville and Coleman (2000, essentially the same formula as proposed by Melville, 1997)
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From page 53... ...
concluded that there are significant differences in the adjustment factors in the two formulas. ABSCOUR includes a velocity adjustment Kv, which is given as a function of q1/q2 and is derived for potential flow, and a spiral flow adjustment Kf determined as a function of approach flow Froude number.
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From page 54... ...
The Melville formula is plotted on these axes by using the Laursen assumption that the flow contraction takes place at the end of the abutment in a flow width of 2.75ds. Experiments show that this width is actually variable; a flow width of 3.5ds is shown in the figure for a slightly better correspondence with the data, but the main purpose here is only to place into perspective the Melville formula and the Sturm data, which are both for solid abutments with sheet pile foundations, relative to the Ettema et al.
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From page 56... ...
In some cases, the formula by Sturm predicted abutment scour depths that agreed with the field data within the ±25% uncertainty of the formula, most notably on the Pomme de Terre River in Minnesota, while predictions were excessive on the Minnesota River near Belle Plaine, Minnesota. In the latter case, a skewed crossing with two small radius meanders immediately upstream of the bridge resulted in a very complex flow field (see Figure 5-10)
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From page 57... ...
Besides considerations of embankment strength, time of measurement of scour, and location of maximum scour depth, abutment scour often is associated with lateral shifting of the approach channel. Comparisons between measured post-flood scour depths at bridges in South Carolina with several abutment scour formulas have been reported by Benedict et al.
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From page 58... ...
The critical velocity or shear stress is of paramount concern in clear-water scour formulas, while the estimate of flow-field parameters such as velocity and discharge per unit width in the approach flow and in the contracted bridge section are important in both clear-water and live-bed scour. In the former case, relationships for critical velocity may be misapplied, and so further education is needed in this regard; however, the inescapable inference is that better methods are needed with respect to making initial estimates of critical velocity or critical shear stress in the case of fine-grained sediments.
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From page 59... ...
failure of embankment past abutment column relieves flow so that maximum scour depth is attained (Ettema et al.
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