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Publication Number: FHWA-RD-03-083
Date: June 2003

Bridge Scour in Nonuniform Sediment Mixtures and in Cohesive Materials: Synthesis Report

FOREWORD

This report is a summary of a six-volume series describing detailed laboratory experiments conducted at Colorado State University for the Federal Highway Administration as part of a study entitled "Effects of Sediment Gradation and Cohesion on Bridge Scour." This report will be of interest to hydraulic engineers and bridge engineers involved in bridge scour evaluations. It will be of special interest to other researchers conducting studies of the very complex problem of estimating scour in cohesive bed materials and to those involved in preparing guidelines for bridge scour evaluations. The six-volume series has been distributed to National Technical Information Service (NTIS) and to the National Transportation Library, but it will not be printed by FHWA. This summary report, which describes the key results from the six-volume series, will be published by FHWA.

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

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 use thereof. This report does not constitute a standard, specification, or regulation.

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

Technical Report Documentation Page

1. Report No.

FHWA-RD-03-083

2. Government Accession No.

3. Recipient's Catalog No.

4. Title and Subtitle

BRIDGE SCOUR IN NONUNIFORM SEDIMENT MIXTURES AND IN COHESIVE MATERIALS: SYNTHESIS REPORT

5. Report Date June 2003

6. Performing Organization Code

7. Author(s) Albert Molinas

8. Performing Organization Report No.

9. Performance Organization Name and Address

Colorado State University
Engineering Research Center
Fort Collins, CO

10. Work Unit No. (TRAIS)

NCP# 3D3C1-582

n

11. Contract or Grant No.

DTFH61-91-C-00004

13. Type of Report and Period Covered

Laboratory Final Report
Jan 1991 - Jan 1996

12. Sponsoring Agency and Address

Office of Infrastructure Research and Development
Federal Highway Administration
6300 Georgetown Pike
McLean, VA 22101

14. Sponsoring Agency Code

15. Supplementary Notes
Contracting Officer's Technical Representative: J. Sterling Jones HRDI-07

16. Abstract

This report presents the summary and synthesis of the various components of the experimental study entitled "Effects of Gradation and Cohesion on Bridge Scour" conducted at Colorado State University between the dates 1991 through 1996.

As a result of this effort, in excess of 250 new pier scour data was collected and a new equation was developed expressing pier scour in terms of the dimensionless excess velocity factor, flow depth, pier diameter and a correction factor for the coarse fractions present in mixtures was derived. The new method was tested with available data from previous research. This equation shows that gradation effects are not constant through the entire range of flow conditions but vary with flow intensity. Additionally, a new method to adjust FHWA's Colorado State University pier scour equation for initiation of motion and sediment size was developed.

Abutment scour experiments resulted in over 384 new points and 2 new abutment scour equations. The first equation was derived from a 0.1 mm uniform sand mixture and defines an envelop relationship. The second equation applies to mixtures with coarse fractions. A coarse size fraction compensation factor Wg is presented to account for the presence of varying amounts of coarse material in sediment mixtures under different dimensionless flow intensities. These new equations represented the experimental data accurately but have not been tested with field data.

Effects of cohesion on pier and abutment scour was studied systematically, and in excess of 200 new data points were collected covering a range of flow and cohesive parameter values. Relationships were developed to explain the variability of bridge scour in cohesive materials for various cohesive material properties.

This report summarizes results from the following six-volume series, which was not printed but was distributed to the National Technical Information Service and the National Transportation Library:

  • FHWA-RD-99-183 Volume 1. Effect of Sediment Gradation and Coarse Material Fraction on Clear-Water Scour Around Bridge Piers.
  • FHWA-RD-99-184 Volume 2. Experimental Study of Sediment Gradation and Flow Hydrograph Effects on Clear Water Scour Around Circular Piers.
  • FHWA-RD-99-185 Volume 3. Abutment Scour for Nonuniform Mixtures.
  • FHWA-RD-99-186 Volume 4. Experimental Study of Scour Around Circular Piers in Cohesive Soils.
  • FHWA-RD-99-187 Volume 5. Effect of Cohesion on Bridge Abutment Scour.
  • FHWA-RD-99-188 Volume 6. Abutment Scour in Uniform and Stratified Sand Mixtures.

17. Key Words:

Bridge scour, Pier scour, Abutment scour, Local scour, Sand Mixtures, Gravel scour, Gradation, Cohesion, Clay, Montmorillonite, Kaolinite, Clear-water, Experimental study

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

121

22. Price

Form DOT F 1700.7 (8-72) Reproduction of completed page authorized


Metric Conversion Chart


TABLE OF CONTENTS

  1. INTRODUCTION
  2. EFFECTS OF GRADATION AND COARSE MATERIAL FRACTION ON PIER SCOUR
  3. EFFECTS OF GRADATION AND COARSE MATERIAL FRACTION ON ABUTMENT SCOUR
  4. BRIDGE SCOUR IN CLAYEY SANDS
  5. PIER SCOUR IN MONTMORILLONITE CLAY SOILS
  6. EFFECTS OF COHESION ON ABUTMENT SCOUR

    REFERENCES


LIST OF FIGURES

1

Variation of scour depth with velocity for sand mixtures used in sets 1 through 3

2

Comparison of FHWA's CSU equation with measured scour from sets 1 through 3

3

Velocity versus discrepancy ratio for set 1 through 3 experiments

4

Flow velocity versus dimensionless scour for set 4 through 7 experiments

5

Dimensionless excess velocity factor, , versus depth of scour for sets 1 through 3

6

Variation of dimensionless scour with excess velocity factor for various mixtures

7

Variation of dimensionless scour with excess velocity factor, , for different coarse fraction sizes used in sets 4 through 7

8

Variation of dimensionless scour with excess velocity factor, , for different coarse fraction sizes used in sets 8 and 9

9

Variation of scour depth with excess velocity factor, , for different coarse fraction size ratios used in sets 4 through 7 experiments

10

Variation of measured coarse fraction reduction factor K4 with excess velocity factor, , for set 1 through 3 experiments

11

Variation of scour depth with pier size for the set 8 experiments

12

Relationship describing variation of pier scour with pier diameter

13

Computed versus measured pier scour depths for set 8 experiments

14

Variation of scour depth with excess velocity factor, , for uniform sand and gravel

15

Computed versus measured pier scour depths using the uniform-mixture equation

16

Measured and computed scour for uniform sediment using FHWA's CSU equation

17

Computed scour for sets 1 through 3 experiments using equation 12 with K4 from equation 14

18

Measured and computed scour for nonuniform-sand experiments in sets 1 through 3

19

Computed and measured scour for all data using equation 12 (uniform-mixture equation)

20

Computed and measured scour for all data using equation 12 with K4 correction from equation 14

21

Computed scour using FHWA's CSU equation for uniform and nonuniform mixtures

22

Computed scour by using FHWA's CSU equation with and without K4 correction from HEC 18, and by using the newly developed equation 12 with K4 correction from equation 14

23

Comparison of FHWA's CSU equation with K4 correction (according to HEC 18) and the initiation of motion correction, Ki (according to equation 15)

24

FHWA's CSU equation adjusted with Ki and K4 and with the HEC 18 correction for K.

25

Comparison between computed and measured scour using Ki and K4 corrections to the FHWA's CSU equation (equation 16) and by using equation 12 with K4 correction from equation 14

26

Variation of dimensionless abutment scour with Froude number: (a) abutment protrusion length, a, as characteristic length; (b) Lc= (a Y)0.5 as characteristic length

27

Variation of dimensionless abutment scour with deflected flow excess velocity

28

Adjustment factors for gradation and coarse material fraction: (a) Gradation reduction factor, Ks; (b) Coarse fraction adjustment, K15

29

Measured and computed abutment scour for hydrodynamics flume experiments: (a) For uniform mixtures; (b) All mixtures

30

Effect of clay content on abutment scour

31

Pier scour reduction factor for Montmorillonite clay mixtures

32

Abutment scour reduction factor for Montmorillonite clay mixtures

33

Abutment scour reduction factor for Kaolinite mixtures

34

Computed and measured dimensionless pier scour depth for unsaturated Montmorillonite clay

35

Computed and measured dimensionless pier scour depth for saturated Montmorillonite clay

36

Computed and measured relative abutment scour for Montmorillonite clay

37

Computed and measured relative abutment scour for Kaolinite clay

LIST OF TABLES

1

Properties of sediment mixtures used in pier scour experiments

2

Summary of sand-scour experiments in the sedimentation flume for set 1

3

Summary of sand-scour experiments in sedimentation flume for set 2

4

Summary of sand-scour experiments in sedimentation flume for set 3

5

Summary of sand-scour experiments in hydrodynamics flume for sets 4 through 7

6

Summary of river mechanics flume experiments to study pier width effects for set 8

7

Summary of gravel-scour experiments in sedimentation flume for sets 9 and 10

8

Experimental conditions for set A runs for abutment scour

9

Experimental conditions for set B runs for abutment scour

10

Experimental conditions for set C runs for abutment scour

11

Experimental conditions for set G runs for abutment scour

12

Experimental conditions for set H runs for abutment scour

13

Experimental conditions for set I runs for abutment scour

14

Experimental conditions for set J runs for abutment scour

15

Experimental conditions for set K runs for abutment scour

16

Experimental conditions for set L runs for abutment scour

17

Experimental conditions for set M runs for abutment scour

18

Experimental conditions for set N runs for abutment scour

19

Experimental conditions for set D runs for abutment scour

20

Experimental conditions for set E runs for abutment scour

21

Experimental conditions for set F runs for abutment scour

22

Properties of sediment mixture types used in abutment scour experiments

23

Summary of pier scour experiments in clayey sands

24

Summary of abutment scour experiments in clayey sands

25

Results of set 1 experiments to study effects of clay content

26

Summary of experimental conditions and results for set 2 (effect of compaction on pier scour in cohesive soils)

27

Summary of experimental conditions and results for set 3 (effect of IWC on pier scour in cohesive soils)

28

Summary of experimental conditions and results for set 3 (effects of initial water content on pier scour for saturated clay)

29

Results of Montmorillonite clay experiments conducted in the 2.44-m wide sedimentation flume using 0.22 m abutment protrusion length

30

Results of Montmorillonite clay experiments conducted in the 1.22-m wide flume using 0.11 m abutment protrusion length

31

Results of Montmorillonite clay experiments conducted in the 2.44-m wide flume using 0.11 m abutment protrusion length

32

Results of Kaolonite clay experiments in the 2.44-m wide flume with 0.22 m abutment width

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The Federal Highway Administration (FHWA) is a part of the U.S. Department of Transportation and is headquartered in Washington, D.C., with field offices across the United States. is a major agency of the U.S. Department of Transportation (DOT).
The Federal Highway Administration (FHWA) is a part of the U.S. Department of Transportation and is headquartered in Washington, D.C., with field offices across the United States. is a major agency of the U.S. Department of Transportation (DOT). The hydraulics and hydrology research program at the TFHRC Federal Highway Administration's (FHWA) R&T Web site portal, which provides access to or information about the Agency’s R&T program, projects, partnerships, publications, and results.
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