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Publication Number: FHWA-RD-99-156
Date: August 2004

Enhanced Abutment Scour Studies for Compound Channels


PUBLICATION NO: FHWA-RD-99-156
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Enhanced Abutment Scour Studies for Compound Channels Cover

FOREWORD

This report describes a laboratory study of abutment scour for compound channels where the experiments simulated floodplains with defined channel and overbank flow areas at different elevations. A new abutment scour prediction equation was derived as part of this study and is presented in an appendix of the fourth edition of the Federal Highway Administration Hydraulic Engineering Circular No. 18 (HEC-18). This report will be of interest to bridge engineers and hydraulic engineers involved in bridge scour evaluations and to researchers involved in developing improved bridge scour evaluation procedures. It is being published as a Web document only.

NOTICE

This document is disseminated under the sponsorship of the U.S. Department of Transportation in the interest of information exchange. The U.S. Government assumes no liability for the contents or the use thereof. This report does not constitute a standard, specification, or regulation.

The U.S. Government does not endorse products or manufacturers. Trade or manufacturers' names appear herein only because they are considered essential to the objective of this document.

T. Paul Teng, P.E.
Director, Office of Infrastructure
Research and Development

Technical Report Documentation Page

Form DOT F 1700.7 (8-72)

1. Report No.
FHWA-RD-99-156		 2. Government Accession No. 3. Recipient's Catalog No.
4. Title and Subtitle
Enhanced Abutment Scour Studies For Compound Channels		 5. Report Date
August 2004
6. Performing Organization Code
7. Author(s)
Terry W. Sturm		 8. Performing Organization Report No.
9. Performing Organization Name and Address
Georgia Institute of Technology
School of Civil and Environmental Engineering
Atlanta, GA 30332		 10. Work Unit No. (TRAIS)
11. Contract or Grant No.
DTFH61-02-C-00007
12. Sponsoring Agency Name and Address
Office of Infrastructure Research and Development
Federal Highway Administration
6300 Georgetown Pike
McLean, VA 22101-2296		 13. Type of Report and Period Covered
Final Report 1994-1999
14. Sponsoring Agency Code
15. Supplementary Notes
Contracting Officer's Technical Representative (COTR): J. Sterling Jones, HRDI-07
16. Abstract
Experimental results and analyses are given in this report on bridge abutment scour in compound channels. Experiments were conducted in a laboratory flume with a cross section consisting of a wide floodplain adjacent to a main channel. The embankment length, discharge, sediment size, and abutment shape were varied, and the resulting equilibrium scour depths were measured. Water-surface profiles, velocities, and scour-hole contours were also measured. In the report, a methodology is developed for estimating abutment scour that takes into account the redistribution of discharge in the bridge contraction, abutment shape, sediment size, and tailwater depth. The independent variables in the proposed scour formula are evaluated at the approach-channel cross section and can be obtained from a one-dimensional water-surface profile computer program such as the Water-Surface Profile Program (WSPRO). The proposed scour evaluation procedure is outlined and ilustrated, including consideration of the time required to reach equilibrium scour. The proposed methodology is applied to two cases of measured scour in the field.
17. Key Words
Bridge scour, abutment scour, compound channels.		 18. Distribution Statement
No restrictions. This document is available to the public through the National Technical Information Service, Springfield, VA 22161.
19. Security Classif. (of this report)
Unclassified		 20. Security Classif. (of this page)
Unclassified		 21. No. of Pages
144		 22. Price

Reproduction of completed page authorized

SI* (Modern Metric) Conversion Factors

TABLE OF CONTENTS


LIST OF FIGURES

  1. Compound-channel configurations used in scour experiments
  2. Abutment shapes used in scour experiments
  3. Sediment grain size distributions
  4. Manning's n in the main channel and floodplain for compound channel B
  5. Measured and computed normal depth for compound channel B
  6. Ratio of main-channel discharge to total discharge as a function of relative normal depth in the floodplain
  7. Dependence and use of discharge distribution factor M
  8. Effect of variable abutment length on water-surface profiles and velocity distributions for constant discharge
  9. Effect of variable discharge on water-surface profiles and velocity distributions for constant abutment length
  10. Effect of tailwater (yftw) on water-surface profiles and velocity distributions for constant discharge
  11. Water-surface profiles before and after scour for La/Bf = (a) 0.44 and (b) 0.88
  12. Water-surface profiles before and after scour for La/Bf = (a) 0.97 and (b) 1.0
  13. Main-channel centerline velocity from approach to contracted section for La/Bf = 0.88
  14. Main-channel centerline velocity from approach to contracted section for La/Bf = 0.97
  15. Main-channel centerline velocity from approach to contracted section for La/Bf = 1.0
  16. Measured and calculated critical velocities at incipient motion
  17. Bed elevations for shorter VW abutments after scour
  18. Bed elevations for longer VW abutments after scour
  19. Bed elevations for shorter ST abutments after scour
  20. Bed elevations for longer ST abutments after scour
  21. Bed elevations for VW, ST, and WW abutments after scour
  22. Bed elevations for VW abutments and sediments A, B, and C
  23. Definition sketch for idealized floodplain contraction scour in a laboratory compound channel, compound channel A
  24. Definition sketch for idealized main-channel contraction scour in a laboratory compound channel, compound channel B
  25. Scour-depth relationship based on approach hydraulic variables for VW abutments with 0.17 < La/Bf < 0.66
  26. Scour-depth relationship based on approach hydraulic variables in main channel for VW and ST abutments with 0.88 < La/Bf < 1.0 and d50 = 3.3 mm in compound channel B
  27. Scour-depth relationship based on approach hydraulic variables in floodplain for all abutments and sediments, compound channel B
  28. Scour-depth relationship based on local hydraulic variables for VW abutments with La/Bf < 0.66 and d50 = 3.3, 2.7, and 1.1 mm
  29. Scour-depth relationship based on local hydraulic variables for VW and ST abutments with La/Bf > 0.88 and d50 = 3.3 mm
  30. Relationship between local and approach hydraulic variables
  31. Live-bed contraction scour coefficient for La/Bf = 1.0
  32. Calculated and measured water-surface profiles for compound channel A
  33. Calculated and measured approach velocity distributions for compound channel A
  34. Calculated and measured resultant velocity distributions in the contracted section for compound channel A
  35. Calculated and measured water-surface profiles for compound channel B
  36. Calculated and measured approach velocity distributions for compound channel B
  37. Comparison of approach floodplain velocities
  38. Comparison of approach floodplain depths
  39. Comparisons of discharge distribution factor M
  40. Comparisons of maximum velocity at abutment face
  41. Dimensionless representation of time development of scour
  42. Comparison of measured and predicted scour depths using the Melville formula
  43. Comparison of measured and predicted scour depths using the Froehlich clear-water scour formula
  44. Comparison of measured and predicted scour depths using the Froehlich live-bed scour formula
  45. Comparison of measured and predicted scour depths using the G. K. Young (GKY) formula
  46. Comparison of measured and predicted scour depths using the Maryland formula
  47. Comparison of measured and predicted scour depths using the formula from the present study
  48. Cross sections and water-surface elevation for Burdell Creek bridge
  49. Design 100-year hydrograph for Burdell Creek
  50. Cross sections for Highway 22 over the Pomme de Terre River for the flood of April 9, 1997
  51. Cross sections for Highway 12 over the Pomme de Terre River for the flood of April 9, 1997

LIST OF TABLES

  1. Experimental parameters
  2. Raw experimental results for fixed-bed hydraulic variables and equilibrium scour depth
  3. Range of dimensionless variables in experimental relationships
  4. WSPRO input data file for Burdell Creek, Q100
  5. WSPRO water-surface profile output for Burdell Creek, Q100
  6. Velocity distribution from HP record for unconstricted flow at approach section, Burdell Creek, Q100
  7. Velocity distribution from HP record for constricted flow at approach section, Burdell Creek, Q100
  8. WSPRO input data file for Highway 22 bridge
  9. WSPRO water-surface profile output for Highway 22 bridge
  10. Velocity distribution from HP record for unconstricted flow at approach section, Highway 22 bridge
  11. Velocity distribution from HP record for constricted flow at approach section, Highway 22 bridge
  12. WSPRO input data file for Highway 12 bridge
  13. WSPRO water-surface profile output for Highway 12 bridge
  14. Velocity distribution from HP record for unconstricted flow at approach section, Highway 12 bridge
  15. Velocity distribution from HP record for constricted flow at approach section, Highway 12 bridge

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