Revised Clear-Water and Live-Bed Contraction Scour Analysis (2021) / Chapter Skim
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From page 109... ...
8-1 8.1 Overview Considering the fundamentals of a flow contraction as delineated in Chapter 2, this chapter provides a bridge between the laboratory and computational analyses presented in Chapters 4 through 7 and the suggested revisions to the contraction scour equations. Section 8.2 provides an appraisal of the results of the preceding chapters.
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From page 110... ...
8-2 Revised Clear-Water and Live-Bed Contraction Scour Analysis (abrupt, angled, or conforming to the shape of entrance streamlines)
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From page 111... ...
Revised Contraction Scour Analysis: Appraisal, Results, and Applications 8-3 For mobile-bed channels that are typical of rivers, the trends indicated by the two branches shown in Figure 8-1 may become complicated by several factors, including the spatially non-uniform formation of bedforms along the contraction and the resulting non-uniform deformation of the bed of the contraction. Bedforms may also exist in the approach section to the contraction.
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From page 112... ...
8-4 Revised Clear-Water and Live-Bed Contraction Scour Analysis vena-contracta and at a flow length of about B1, the flow reattached to the full width of the contracted channel.
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From page 113... ...
Revised Contraction Scour Analysis: Appraisal, Results, and Applications 8-5 local maximum depth (the maximum scour depth in the vena-contracta) shifted as the location of the vena-contracta shifted downstream when parameters B2/B1 and τ1/τc increased.
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From page 114... ...
8-6 Revised Clear-Water and Live-Bed Contraction Scour Analysis analyzed for contraction scour: clear-water scour and live-bed scour. As noted, the focus of this study is on scour in the vena-contracta region, though generally the scour depths were greatest at the corners of the entrance to the contracted channel as a result of the substantial turbulence structures generated there (i.e., an abutment effect)
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From page 115... ...
Revised Contraction Scour Analysis: Appraisal, Results, and Applications 8-7 (B2/B1 = 0.25)
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From page 116... ...
8-8 Revised Clear-Water and Live-Bed Contraction Scour Analysis the scour depth in this area was affected by the length of the contraction such that the bed shear stress gradually increased along the contraction (Nowroozpour 2020)
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From page 117... ...
Revised Contraction Scour Analysis: Appraisal, Results, and Applications 8-9 Given a unit discharge, Manning's n, and flow depth prior to scour, the theoretical force decay function can be developed by varying the scour depth in Eq.
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From page 118... ...
8-10 Revised Clear-Water and Live-Bed Contraction Scour Analysis site (from Kerenyi 2018)
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From page 119... ...
Revised Contraction Scour Analysis: Appraisal, Results, and Applications 8-11 The LSPIV technique calculated the movement of paper tracers on the water surface and within the contraction entrance. The Fudaa software involves the following steps to calculate flow velocities: 1.
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From page 120... ...
Figure 8-13. The calculated streamlines obtained for B2/B1 = 0.5 and discharge = 8.12 ft3/s (0.23 m3/s)
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From page 121... ...
Revised Contraction Scour Analysis: Appraisal, Results, and Applications 8-13 The LSPIV technique was useful for measuring values of B2′ for most of the experiments. However, judgment was required for the higher discharges at the smallest value of B2/B1 used (the Severe contraction)
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From page 122... ...
8-14 Revised Clear-Water and Live-Bed Contraction Scour Analysis • Consideration of simplified contraction scour equations developed as part of NCHRP Project 24-20 and applied to abutment scour calculations in HEC-18 • Addition of a vena-contracta width factor, Kv, to the Laursen contraction scour equations to account for observed additional flow contraction immediately downstream of the entrance to a contraction (i.e., in the bridge reach) The vena-contracta factor, Kv, was measured for the flume testing scenarios described in Section 8.2.7 using LSPIV analysis (Fakhri 2020)
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From page 123... ...
Revised Contraction Scour Analysis: Appraisal, Results, and Applications 8-15 environment when evaluating equilibrium scour conditions, as the sediment supply and the bed will interact with the underlying hydraulic controls to produce equilibrium post-scour bed conditions. Table 8-2 presents a partial summary of the experimental results and the reported theoretical normal-depth hydraulics (V1n, y1n)
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From page 124... ...
8-16 Revised Clear-Water and Live-Bed Contraction Scour Analysis Figure 8-18. Example topographic surface for an existing-conditions hydraulic analysis.
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From page 125... ...
Revised Contraction Scour Analysis: Appraisal, Results, and Applications 8-17 supply and pre-scour contracted-section hydraulic conditions. This approach using a 2D model has the advantage of being analogous to the uncontracted hydraulic assumptions associated with the original Laursen equations and with the experimental conditions of the present study.
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From page 126... ...
8-18 Revised Clear-Water and Live-Bed Contraction Scour Analysis If the average velocity upstream of the contraction (either at the approach section or calculated from a theoretical normal depth) is less than Vc, then the flow is assumed to transport little to no bed material into the flow contraction and is, therefore, evaluated using clear-water contraction scour equations.
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From page 127... ...
Revised Contraction Scour Analysis: Appraisal, Results, and Applications 8-19 Simulation Approach Sediment Supply HEC-18, Using Approach Variables HEC-18 +Kv, Using Approach Variables NCHRP Project 24-20, Using Approach Variables NCHRP Project 24-20 +Kv, Using Approach Variables K1a Y2a Ysa Y2a Ysa q2/q1 Y2a Ysa q2/q1 Y2a Ysa CW_0.25-0.55 CW n/a 0.97 0.19 1.33 0.55 4.00 1.05 0.27 5.80 1.44 0.67 CW_0.25-0.65 CW n/a 1.33 0.54 n/a n/a 4.00 1.45 0.66 n/a n/a n/a CW_0.25-0.75 CW n/a 1.26 0.38 1.94 1.05 4.00 1.37 0.48 6.61 2.10 1.22 CW_0.25-0.80 CW n/a 1.33 0.47 n/a n/a 4.00 1.45 0.58 n/a n/a n/a CW_0.50-0.55 CW n/a 0.54 -0.12 0.66 0.00 2.00 0.58 -0.08 2.56 0.72 0.06 CW_0.50-0.65 CW n/a 0.62 -0.10 0.82 0.09 2.00 0.67 -0.05 2.76 0.89 0.16 CW_0.50-0.75 CW n/a 0.70 -0.02 0.95 0.23 2.00 0.76 0.04 2.85 1.03 0.31 CW_0.75-0.75 CW n/a 0.50 -0.16 n/a n/a 1.33 0.54 -0.11 n/a n/a n/a CW_0.75-0.85 CW n/a 0.55 -0.14 0.62 -0.08 1.33 0.60 -0.10 1.52 0.67 -0.03 CW_0.75-0.95 CW n/a 0.61 -0.09 0.72 0.02 1.33 0.66 -0.04 1.62 0.78 0.08 LB_0.50-1.2 CW n/a 1.04 0.26 1.45 0.66 2.00 1.13 0.35 2.93 1.57 0.78 LB_0.50-1.4 CW n/a 1.19 0.43 1.69 0.94 2.00 1.29 0.53 3.02 1.84 1.08 LB_0.50-1.65 CW n/a 1.37 0.48 2.03 1.14 2.00 1.48 0.59 3.16 2.20 1.31 LB_0.50-2.0 CW n/a 1.61 0.72 2.37 1.47 2.00 1.75 0.85 3.13 2.57 1.67 LB_0.75-1.2 CW n/a 0.74 0.03 0.90 0.20 1.33 0.80 0.09 1.69 0.98 0.27 LB_0.75-1.4 LB 0.64 0.85 0.09 0.98 0.22 1.33 0.90 0.15 1.67 1.09 0.34 LB_0.75-1.65 LB 0.64 0.91 0.02 1.03 0.14 1.33 0.97 0.08 1.61 1.14 0.25 LB_0.75-2.0 LB 0.64 0.95 0.17 1.12 0.34 1.33 1.01 0.23 1.72 1.26 0.48 Note: See Section 8.3.5 for a discussion of the vena-contracta coefficient, Kv. Table 8-3.
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From page 128... ...
8-20 Revised Clear-Water and Live-Bed Contraction Scour Analysis and predicted scour depths using the HEC-18 prediction methods for upstream approach hydraulics. See Section 8.2.
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From page 129... ...
Revised Contraction Scour Analysis: Appraisal, Results, and Applications 8-21 In Figure 8-21, note that the HEC-18 scour equation still substantially underpredicts scour. However, a comparison of post-scour equilibrium flow depths presents a more conservative prediction and more conservative results.
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From page 130... ...
8-22 Revised Clear-Water and Live-Bed Contraction Scour Analysis pre-scour conditions. This degree of prediction variability will change depending on the individual case modeled and is not easily generalized using the findings of this study.
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From page 131... ...
Revised Contraction Scour Analysis: Appraisal, Results, and Applications 8-23 Ku = 11.17 U.S. customary units d50 = Particle size with 50% finer, ft NCHRP Project 24-20 Live-Bed Post-Scour Flow Depth Estimation y y q q (8.6)
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From page 132... ...
8-24 Revised Clear-Water and Live-Bed Contraction Scour Analysis presented in HEC-18 only consider physical (geometric) flow contraction.
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From page 133... ...
Revised Contraction Scour Analysis: Appraisal, Results, and Applications 8-25 Figure 8-26. Vena-contracta and effective width adjustment factor, Kv.
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From page 134... ...
8-26 Revised Clear-Water and Live-Bed Contraction Scour Analysis This effective width adjustment factor, Kv, can be incorporated into the HEC-18 and NCHRP Project 24-20 post-contraction scour flow depth equations as follows: HEC-18 clear-water post-contraction scour flow depth equation with vena-contracta modification.
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From page 135... ...
Revised Contraction Scour Analysis: Appraisal, Results, and Applications 8-27 database. Consequently, the width adjustment factor, if adopted, would have significant direct application for the practitioner.
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From page 136... ...
8-28 Revised Clear-Water and Live-Bed Contraction Scour Analysis In the interim, for bridges that do not meet these criteria, the application of the existing best-practice modeling methods (see, for example, Robinson et al.
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From page 137... ...
Revised Contraction Scour Analysis: Appraisal, Results, and Applications 8-29 of scour, the water surface within the bridge opening is controlled by the water surface elevation at the fully expanded downstream section and the head losses associated with the region of flow expansion. As scour progresses, the shear stresses in the contraction and the resulting changes in water surface elevation tend to reduce the water surface elevation toward a "no-embankment" or "natural conditions" condition.
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From page 138... ...
8-30 Revised Clear-Water and Live-Bed Contraction Scour Analysis 8.4 Application Example 8.4.1 Introduction In this section, the existing and revised live-bed contraction scour equations are applied to a field case study. The bridge selected for this case study falls into the FHWA-defined "hydraulically narrow" category for which the revised contraction scour equations are considered directly applicable.
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From page 139... ...
Revised Contraction Scour Analysis: Appraisal, Results, and Applications 8-31 Figure 8-32. Numerical mesh and terrain dataset for the U.S.
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From page 140... ...
8-32 Revised Clear-Water and Live-Bed Contraction Scour Analysis 8.4.3 HEC-18 Method The recommended existing method for calculating contraction scour is described in Chapter 6 of the HEC-18. The first step is to determine if the bridge is being supplied with a sediment from upstream.
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From page 141... ...
Revised Contraction Scour Analysis: Appraisal, Results, and Applications 8-33 y y Q Q W W y 8.7 ft 1819.3 ft s 602.0 ft s 11.9 ft 20.0 ft 15.69 ft 2 1 2 1 6 7 1 2 k 2 3 3 6 7 0.69 1 ( ) = = = Equation 6.3 from HEC-18 allows the conversion from the post-scour flow depth (y2)
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From page 142... ...
8-34 Revised Clear-Water and Live-Bed Contraction Scour Analysis With this result, the vena-contracta coefficient (Kv) can be calculated as follows (see Section 8.3.5)
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From page 143... ...
Revised Contraction Scour Analysis: Appraisal, Results, and Applications 8-35 q Q W 602 ft s 11.9 ft 50.59 ft s q Q W 1819.3 ft s 20.0 ft 90.97 ft s y y q q 8.7 ft 90.97 ft s 50.59 ft s 14.38 ft 1 1 1 3 2 2 2 2 3 2 2 1 2 1 6 7 2 2 6 7 = = = = = = = = = This method produces a post-scour flow depth of 14.4 ft. Equation 6.3 from HEC-18 allows the conversion from the post-scour flow depth (y2)
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From page 144... ...
8-36 Revised Clear-Water and Live-Bed Contraction Scour Analysis ( ) = = =y y q K q 8.7 ft 90.97 ft s 0.63 50.59 ft s 21.38 ft2 1 2 v 1 6 7 2 2 6 7 This method produces a post-scour flow depth of 21.4 ft.
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From page 145... ...
Revised Contraction Scour Analysis: Appraisal, Results, and Applications 8-37 ratio of observed post-scour flow depth to predicted post-scour flow depth for a given scour equation. This definition is dependent on a common zero value.
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From page 146... ...
8-38 Revised Clear-Water and Live-Bed Contraction Scour Analysis the 19 flume experiments conducted by CSU as part of this study. The reliability indices (β)
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From page 147... ...
Revised Contraction Scour Analysis: Appraisal, Results, and Applications 8-39 CSU Test No. Approach-Section Hydraulics (Calibrated HEC-RAS)
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From page 148... ...
8-40 Revised Clear-Water and Live-Bed Contraction Scour Analysis HEC-18 calculations using a theoretical normal depth were found to have a lower CV and substantially underpredicted post-scour depth, even when Kv was applied. This result combined with the difficulty and subjectivity of selecting a representative theoretical normal depth (or calculating it from an assumed friction slope using the Manning's n equation)
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From page 149... ...
Revised Contraction Scour Analysis: Appraisal, Results, and Applications 8-41 post-scour change in ground elevation (needed for design or stability analysis) , an improved estimate of the post-scour water surface elevation is needed.
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From page 150... ...
8-42 Revised Clear-Water and Live-Bed Contraction Scour Analysis in a positive statistical reliability factor for a lognormally distributed parameter for both the HEC-18 and NCHRP Project 24-20 methods. The NCHRP Project 24-20 method with Kv applied and using downstream (y3)
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From page 151... ...
Revised Contraction Scour Analysis: Appraisal, Results, and Applications 8-43 8.6.3 The Market The market or audience for the results of this research includes bridge engineers, hydraulic engineers, and bridge maintenance and inspection personnel in state, federal, and local agencies with a bridge-related responsibility for design, installation, evaluation, and maintenance of bridge foundations considering the impacts of scour, including contraction scour and stream instability at bridges and other highway facilities. These would include the following: State Highway Agencies FHWA City/County Bridge Engineers Railroad Bridge Engineers U.S.
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From page 152... ...
8-44 Revised Clear-Water and Live-Bed Contraction Scour Analysis the AASHTO process. As a collective representation of individual state DOTs, AASHTO can also suggest any needed training to be developed by FHWA or others.
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From page 153... ...
Revised Contraction Scour Analysis: Appraisal, Results, and Applications 8-45 Highway bridge failures caused by scour and stream instability account for most of the bridge failures in this country, resulting in substantial direct cost for repair and replacement. This cost does not include the additional indirect costs to highway users for fuel and operating costs resulting from temporary closure and detours and to the public for costs associated with higher tariffs, freight rates, additional labor costs, and time.
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From page 154... ...
8-46 Revised Clear-Water and Live-Bed Contraction Scour Analysis Learning Outcomes At the end of Part I, Participants will be able to • Describe contraction hydraulics and scour, and current evaluation techniques. • Identify the quality and shortcomings of existing laboratory and field data.
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