Evaluation of Bridge-Scour Research: Abutment and Contraction Scour Processes and Prediction (2011)

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.



ACKNOWLEDGMENT This work was sponsored by the American Association of State Highway and Transportation Officials (AASHTO), in cooperation with the Federal Highway Administration, and was conducted in the National Cooperative Highway Research Program (NCHRP), which is administered by the Transportation Research Board (TRB) of the National Academies. COPYRIGHT INFORMATION Authors herein are responsible for the authenticity of their materials and for obtaining written permissions from publishers or persons who own the copyright to any previously published or copyrighted material used herein. Cooperative Research Programs (CRP) grants permission to reproduce material in this publication for classroom and not-for-profit purposes. Permission is given with the understanding that none of the material will be used to imply TRB, AASHTO, FAA, FHWA, FMCSA, FTA, Transit Development Corporation, or AOC endorsement of a particular product, method, or practice. It is expected that those reproducing the material in this document for educational and not-for-profit uses will give appropriate acknowledgment of the source of any reprinted or reproduced material. For other uses of the material, request permission from CRP. DISCLAIMER The opinions and conclusions expressed or implied in this report are those of the researchers who performed the research. They are not necessarily those of the Transportation Research Board, the National Research Council, or the program sponsors. The information contained in this document was taken directly from the submission of the author(s). This material has not been edited by TRB. 



CONTENTS Page No. FIGURES………………..…………………………………………………………………….iii TABLES………………………………………………………………………………………vi SYMBOLS……………………………………………………………………………………vii AUTHOR ACKNOWLEDGMENTS…………………………………………………………ix ABSTRACT……………………………………………………………………………………x EXECUTIVE SUMMARY…………………………………………………………………... 1 CHAPTER 1. INTRODUCTION…………………………………………………………….. 9 1.1 Definitions………………………………………………………………………... 9 1.2 Motivation for Review…………………………………………………………... 10 1.3 Objectives………………………………………………………………………...14 1.4 Approach………………………………………………………………………… 14 CHAPTER 2. ABUTMENT FORM AND CONSTRUCTION……………………………...15 2.1 Abutment Form………………………………………………………………….. 15 2.2 Abutment Layout…………………………………………………………………16 2.3 Abutment Construction………………………………………………………….. 17 2.4 Pier Proximity…………………………………………………………………… 19 2.5 Sediment and Soil Boundary Material……………………………………………20 2.6 Flow Field……………………………………………………………………….. 21 CHAPTER 3. ABUTMENT SCOUR AS A DESIGN CONCERN………………………… 25 3.1 Design Scour Depths……………………………………………………………. 25 3.2 Estimation of Scour Depths………………………………………………………25 3.3 An Essential Design Question……………………………………………………26 CHAPTER 4. SCOUR CONDITIONS………………………………………………………28 4.1 Three Common Conditions of Abutment Scour………………………………… 28 4.2 Influence of Pier Proximity……………………………………………………… 32 4.3 Other Scour Processes……………………………………………………………32 CHAPTER 5. ABUTMENT SCOUR DEPTH ESTIMATION FORMULAS………………34 5.1 Parameter Framework…………………………………………………………… 34 5.2 Summary of Abutment Scour Formulas………………………………………… 37 5.3 Classification of Scour Formulas………………………………………………... 42 5.4 Evaluation of Abutment Scour Formulas………………………………………...48 5.5 Geotechnical Approach………………………………………………………….. 58 

CHAPTER 6. CONTRACTION SCOUR FORMULAS…………………………………… 60 6.1 Definition of Contraction Scour………………………………………………….60 6.2 Dimensional Analysis…………………………………………………………… 60 6.3 Idealized Long-Contraction Scour……………………………………………… 62 6.4 Contraction Scour Formulas from Laboratory Data…………………………….. 63 6.5 Field Data on Contraction Scour…………………………………………………64 6.6 Vertical Contraction Scour……………………………………………………….65 CHAPTER 7. RECOMMENDATIONS FOR DESIGN ESTIMATION OF ABUTMENT AND CONTRACTION SCOUR……………………………………..66 7.1 General Recommendations……………………………………………………….66 7.2 Specific Recommendations………………………………………………………67 CHAPTER 8. RESEARCH AND EDUCATION NEEDS…………………………………..70 8.1 Introduction……………………………………………………………………… 70 8.2 Scour Processes………………………………………………………………….. 70 8.3 Design Estimation of Scour………………………………………………………71 8.4 Monitoring and Maintenance……………………………………………………. 73 CHAPTER 9. CONCLUSIONS……………………………………………………………...75 CHAPTER 10. REFERENCES………………………………………………………………78 APPENDICES A. Abutment Scour Equations………………………………………………………. 83 B. Contraction Scour Equations……………………………………………………...85 C. Research Problem Statements……………………………………………………..87 ii 

FIGURES Caption Page No. Figure 1-1. Schematic of long, multi-span bridge over a compound channel………………. 11 Figure 1-2. Schematic of relatively short bridge over narrow main channel ………………..11 Figure 1-3. Abutment scour resulting in embankment failure by collapse due to geotechnical instability……………………………………………………………………………………..12 Figure 1-4. Scour at I-70 bridge over Missouri River from 1993 flood with flow from left to right. (Photo from Parola et al. 1998)………………………………………………… 13 Figure 2-1. Plan views of the two common abutment forms: (a) Wing-wall; (b) Spill-through (Ettema et al. 2010)………………………………………………………………………….. 15 Figure 2-2. Definitions of embankment length, floodplain width, and main channel width (Ettema et al. 2010)………………………………………………………………………….. 16 Figure 2-3. Isometric view of spill-through abutment comprising a standard-stub column located within the end of an earthfill embankment (Ettema et al. 2010)……………………. 17 Figure 2-4. The geometry and dimensions of a standard-stub abutment commonly used for spill-through abutments (prototype scale indicated); design provided by the Iowa DOT (Ettema et al. 2010)………………………………………………………………………….. 18 Figure 2-5. The geometry and dimensions of a wing-wall abutment - compacted earthfill embankment extends back from the abutment structure (prototype scale indicated); design provided by the Iowa DOT (Ettema et al. 2010)……………………………………………..18 Figure 2-6. A spill-through abutment with a pier in close proximity; approximate layout proportions of L/Bf = 1.0; Bf/0.5B ≈ 0.7, and L/W ≈ 1.0, in which W = embankment top width (Ettema et al. 2010)………………………………………………………………..19 Figure 2-7. Variation of soil and sediment types at a bridge crossing (Ettema et al. 2010)….20 Figure 2-8. Flow structure including macro-turbulence generated by flow around abutments in a narrow main channel. (Ettema et al. 2010)…………………………………………….. 21 Figure 2-9. Flow structure including macro-turbulence generated by floodplain/main channel flow interaction, flow separation around abutment, and wake region on the floodplain of a compound channel. (Ettema et al. 2010)……………………………………..22 iii 

Caption Page No. Figure 2-10. Interaction of flow features causing scour and erodibility of boundary (Ettema et al. 2010)………………………………………………………………………….. 23 Figure 2-11. For a spill-through abutment well set back on a flood-plain, deepest scour usually occurs where flow is most contracted through the bridge waterway……………….. 24 Figure 3-1. A common situation of abutment failure; scour has led to failure and partial washout of the earthfill spill-slope at this abutment. A basic question arises as to how abutment design should take scour into account……………………………………………..27 Figure 3-2. Failure of abutment fill in September 2009 Georgia flood accompanied by failure of approach roadway (Hong and Sturm 2010)………………………………………. 27 Figure 4-1. Abutment-scour conditions: Scour Condition A - hydraulic scour of the main channel bed causes bank failure, which causes a failure of the face of the abutment embankment (a); Scour Condition B - hydraulic scour of the floodplain causes failure of the face of the abutment embankment (b); and, Scour Condition C - breaching of the approach embankment exposes the abutment column so that scour progresses as if the abutment were a form of pier (c) (Ettema et al. 2010)………………………………………………………………………….. 29 Figure 4-2. Field example of Scour Condition A……………………………………………31 Figure 4-3. Scour Condition B………………………………………………………………. 31 Figure 4-4. Scour condition C for a wing-wall abutment…………………………………….32 Figure 5-1. Definition sketch for abutment terminating in a compound channel…………….35 Figure 5-2. Bankline abutment in a narrow channel………………………………………… 45 Figure 5-3. Bridge crossing for a compound channel……………………………………….. 45 Figure 5-4. Bridge crossing of a braided channel…………………………………………… 46 Figure 5-5. Comparison between scour data at a spill-through abutment (with riprap protection extended below the surface of the floodplain) and the formula by Sturm and Chrisochoides 1998 (see also Sturm 2004, 2006). Reproduced from NCHRP Report 587 by Barkdoll et al. (2007)………………………………………………. 52 Figure 5-6. Comparison between scour data at a spill-through abutment (with riprap protection extended below the surface of the floodplain) and the formula by Melville (1997). Reproduced from NCHRP Report 587 by Barkdoll et al. (2007)………….52 iv 

Caption Page No. Figure 5-7. Comparison of Briaud et al. (2009) formula with experimental results of Ettema et al. (2010) for Scour Condition B. [Reproduced from Briaud et al. (2009). Final design curves are Figs. 12.3 and 12.4 of the NCHRP 24-20 report by Ettema et al. (2010)]………………………………………………………………………….54 Figure 5-8. Comparison of Melville (1992, 1997) formula and Sturm (2004, 2006) data for rigid abutments with Ettema et al. (2010) data for erodible abutments and Scour Condition B………………………………………………………………………………….. 55 Figure 5-9. Scour depth trends for Scour Condition B. (Ettema et al. 2010)………………...56 Figure 5-10. Minnesota River near Belle Plaine, MN for 2001 flood. (Wagner et al. 2006)...57 Figure 5-11. Scour depth estimation based on geotechnical stability of embankment; (a) variables, (b) failure of embankment past abutment column relieves flow so that maximum scour depth is attained (Ettema et al. 2010)…………………………………………………..59 Figure 6-1. Definition sketch for idealized long contraction scour (Q1 = main channel flowrate for live-bed scour; Q2 = total flowrate in channel at contracted section; dsc = contraction scour depth…………………………………………………………………………………………. 61 v 

TABLES Caption Page No. Table 5-1. Classification of abutment scour parameters…………………………………….. 36 Table 5-2. Formulas categorized by parameter groups……………………………………... 43 Table 5-3. Limitations and experimental databases of abutment scour formula……………. 50 Table 8-1. Prioritized list of research and education needs addressing improved understanding of abutment-scour processes………………………………………………….71 Table 8-2. List of design-related research tasks addressing improved design estimation of abutment scour depth coupled to research needs in Table 8-1………………...73 Table 8-3. Prioritized list of research and education needs addressing improved methods for monitoring and maintenance (needs I1, I2, and I3 can be combined)…………..74 Table A-1. A selection of abutment scour equations (revised and extended from Melville and Coleman 2000)………………………………………………………………….83 Table B-1. A selection of contraction scour formulas (B1 = approach flow channel width; B2 = contracted channel width; Y1 = approach flow channel depth; Y2 = contracted channel depth after scour)……………………………………………………………………………. 85 vi 

SYMBOLS B = width of total flow cross section at the bridge crossing Bf = width of the floodplain Bm1 = width of the main channel in the approach flow section Bm2 = width of the main channel in the bridge section ds = scour depth at the bridge section d = some measure of the sediment size such as the median size by weight, d50 F = flow Froude number Fc = critical flow Froude number when sediment motion begins Fd = densimetric grain Froude number = V / [(ρs/ρ−1)gd]1/2 g = acceleration of gravity HE = height of the embankment kF = roughness height of the floodplain km = roughness height of the main channel Ks = shape factor of the abutment as it affects scour by the flow field Kθ = embankment skewness factor as it affects scour Kf = spiral flow factor in Maryland formula Kv = velocity adjustment factor in Maryland formula Kp = pressure flow coefficient in Maryland formula L = length of the abutment/embankment Lc = length of contraction transition m = geometric contraction ratio = (B – 2L)/L M = discharge contraction ratio = (Q – Qobst) /Q Q = total discharge going through the bridge Qobst = discharge in the approach flow obstructed by the bridge embankment q1 = discharge per unit width in approach flow cross section q2 = discharge per unit width in contracted bridge section u*1 = shear velocity of the approach flow u*c = critical value of shear velocity for initiation of sediment motion V1 = approach flow velocity Vc = critical velocity for initiation of sediment motion W = width of the embankment in the flow direction Y1 = upstream approach flow depth in main channel Y2 = maximum depth of flow after scour at the bridge in main channel or floodplain YC = mean flow depth at the bridge due to contraction scour YF = upstream approach flow depth in the floodplain YMAX = maximum flow depth at the bridge after scour Greek symbols α = scour amplification factor ρ = density of the fluid ρs = sediment density µ = viscosity of the fluid, respectively σg = geometric standard deviation of grain size distribution σ = bulk shear strength of the embankment fill vii 

γE = bulk density of the embankment τ1 = mean boundary shear stress in approach flow τ2 = mean boundary shear stress in contracted flow section τc = critical shear stress for initiation of sediment motion viii 

ACKNOWLEDGMENTS The research reported herein was performed under NCHRP Project 24-27(02) by the Georgia Institute of Technology, School of Civil and Environmental Engineering, Atlanta, GA. Georgia Tech was the primary contractor for this study with subcontracts issued to the University of Wyoming, Laramie, WY and the University of Auckland, Auckland, New Zealand. Dr. Terry W. Sturm, Ph.D., P.E. , Professor of Civil and Environmental Engineering, was the Project Director and Co-Principal Investigator. The other authors of this report are Dr. Robert Ettema, Ph.D., P.E., Professor and Dean, College of Engineering, University of Wyoming, and Dr. Bruce W. Melville, Professor, Department of Civil and Environmental Engineering, University of Auckland, both of whom were members of the research team serving as Co-Principal Investigators. The research team is very grateful for the technical comments provided by an expert team of researchers and consultants: The expert team members were: • Mr. David Andres, Northwest Hydraulic Consultants, Edmonton, Alberta • Mr. Sterling Jones, formerly with the Federal Highway Administration, Turner-Fairbanks Hydraulic Laboratory, McLean, Virginia • Dr. D. Max Sheppard, Ocean Engineering Associates, Gainesville, Florida • Dr. Mutlu Sumer, Technical University of Denmark, Lyngby, Denmark • Dr. Lyle Zevenbergen, Ayres Associates, Inc., Fort Collins, Colorado Many useful suggestions, which are gratefully acknowledged by the research team, were provided by the NCHRP Panel for this project. ix 

ABSTRACT This report reviews the present state of knowledge regarding bridge-abutment scour and the veracity of the leading methods currently used for estimating design scour depth. It focuses on research information obtained since 1990, which is to be considered in updating the scour estimation methods that are recommended by AASHTO, and used generally by engineering practitioners. Though considerable further progress has been made since 1990, the findings indicate that several important aspects of abutment scour processes remain inadequately understood and therefore, are not included in current methods for scour depth estimation. The state-of-art for abutment scour estimation is considerably less advanced than for pier scour. Moreover, there is a need for design practice to consider how abutment design should best take scour into account, as scour typically results in the geotechnical failure of an abutment’s earthfill embankment, possibly before a maximum potential scour depth is attained hydraulically. Abutment scour herein is taken to be scour at the bridge-opening end of an abutment, and directly attributable to the flow field developed by flow passing around an abutment. This definition excludes other flow and channel-erosion processes such as lateral geomorphic shifting of the bridge approach channel but includes contraction and abutment scour as part of the same physical processes that should be treated together rather than separately in their estimation. The review shows that, since 1990, advances have been made in understanding abutment-scour processes, and in (1) estimating scour depth at abutments with erodible compacted earthfill embankments, and at those with solid-body (caisson-like) foundations; (2) identifying the occurrence of at least three distinct abutment scour conditions depending on abutment location and construction; (3) utilizing the capacity of numerical modeling to reveal the flow field at abutments in ways that laboratory work heretofore has been unable to provide. The review identifies and evaluates leading scour formulas and suggests a framework for developing a unified abutment scour formula that depends on satisfying several targeted future research needs. x