U.S. Department Publication No. FHWA-RD-03-083

of Transportation June 2003

Federal Highway 

Administration 

BRIDGE SCOUR IN 

NONUNIFORM SEDIMENT MIXTURES 

AND IN COHESIVE MATERIALS: 

SYNTHESIS REPORT 

Federal Highway Administration 

Office of Infrastructure Research and Development 

Turner-Fairbank Highway Research Center 

6300 Georgetown Pike 

McLean, Virginia 22101 
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. 

5. Report Date 

 June 2003

6. Performing Organization Code 

4. Title and Subtitle 

 BRIDGE SCOUR IN NONUNIFORM SEDIMENT MIXTURES 

 AND IN COHESIVE MATERIALS: SYNTHESIS REPORT

7. Author(s) Albert Molinas

8. Performing Organization Report No.

10. Work Unit No. (TRAIS) 

 NCP# 3D3C1-582 

11. Contract or Grant No. 

 DTFH61-91-C-00004

9. Performance Organization Name and Address 

 Colorado State University 

 Engineering Research Center 

 Fort Collins, CO

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

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*SI is the symbol for th International System of Units. Appropriate rounding should be made to comply with Section 4 of ASTM E e 380. 

(Revised March 2003)
iii

BRIDGE SCOUR IN NONUNIFORM SEDIMENT MIXTURES 

AND COHESIVE MATERIALS: SYNTHESIS REPORT 

TABLE OF CONTENTS

1. INTRODUCTION........................................................................................................... 1

2. EFFECTS OF GRADATION AND COARSE MATERIAL 

 FRACTION ON PIER SCOUR..................................................................................... 5

 2.1 General...................................................................................................... 5 

 2.2 Experimental Setup and Measurements.................................................... 5 

 2.3 Experimental Results ................................................................................ 14 

 2.4 Analysis..................................................................................................... 24 

 2.5 Adjustment to FHWA’s CSU Equation.................................................... 29 

 2.6 Summary and Conclusions ....................................................................... 31 

3. EFFECTS OF GRADATION AND COARSE MATERIAL 

 FRACTION ON ABUTMENT SCOUR ....................................................................... 47

 3.1 General...................................................................................................... 47 

 3.2 Experimental Setup and Measurements.................................................... 48 

 3.3 Experimental Results ................................................................................ 54 

 3.4 Analysis..................................................................................................... 75 

 3.5 Conclusions............................................................................................... 80 

4. BRIDGE SCOUR IN CLAYEY SANDS ...................................................................... 81

 4.1 General...................................................................................................... 81 

 4.2 Experimental Setup and Measurements.................................................... 82 

 4.3 Experimental Results ................................................................................ 83 

 4.4 Analysis..................................................................................................... 86 

 4.5 Conclusions............................................................................................... 88 

5. PIER SCOUR IN MONTMORILLONITE CLAY SOILS ........................................ 89

 5.1 General...................................................................................................... 89 

 5.2 Experimental Setup and Measurements.................................................... 90 

 

 5.3 Experimental Results ................................................................................ 91 

 5.4 Analysis..................................................................................................... 96 

 5.5 Conclusions............................................................................................... 99 

6. EFFECTS OF COHESION ON ABUTMENT SCOUR ........................................... 101

 6.1 General.................................................................................................... 101 

 6.2 Experimental Setup and Measurements...................................................102 
iv

 6.3 Experimental Results ...............................................................................103 

 6.4 Analysis....................................................................................................108 

 6.5 Conclusions..............................................................................................110 

REFERENCES...........................................................................................................................111
v

LIST OF FIGURES 

Figure Page 

1 Variation of scour depth with velocity for sand mixtures used 

in sets 1 through 3 ......................................................................................................... 33

2 Comparison of FHWA’s CSU equation with measured scour 

from sets 1 through 3...................................................................................................... 33

3 Velocity versus discrepancy ratio for set 1 through 3 experiments. .............................. 34

4 Flow velocity versus dimensionless scour for set 4 through 7 experiments .................. 34

5 Dimensionless excess velocity factor, ψ, versus depth of scour 

for sets 1 through 3......................................................................................................... 35

6 Variation of dimensionless scour with excess velocity factor 

for various mixtures ....................................................................................................... 35

7 Variation of dimensionless scour with excess velocity factor, ψ,

for different coarse fraction sizes used in sets 4 through 7 ............................................ 36

8 Variation of dimensionless scour with excess velocity factor, ψ, 

for different coarse fraction sizes used in sets 8 and 9................................................... 36

9 Variation of scour depth with excess velocity factor, ψ,

for different coarse fraction size ratios used in sets 4 through 7 experiments ............... 37

10 Variation of measured coarse fraction reduction factor K4 with excess 

velocity factor, ψ, for set 1 through 3 experiments........................................................ 37

11 Variation of scour depth with pier size for the set 8 experiments.................................. 38

12 Relationship describing variation of pier scour with pier diameter ............................... 38

13 Computed versus measured pier scour depths for set 8 experiments............................. 39

14 Variation of scour depth with excess velocity factor, ψ, for uniform sand and gravel.. 39

15 Computed versus measured pier scour depths using the uniform-mixture equation ..... 40

16 Measured and computed scour for uniform sediment using FHWA’s CSU equation ... 40

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

equation 14 .....................................................................................................................

41

18 Measured and computed scour for nonuniform-sand experiments in sets 1 through 3.. 41

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

equation)......................................................................................................................... 42
vi

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

equation 14 ..................................................................................................................... 42

21 Computed scour using FHWA’s CSU equation for uniform and nonuniform 

mixtures.......................................................................................................................... 43

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. ........................................................................................................... 43

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) .......................... 44

24 FHWA’s CSU equation adjusted with Ki and K4 and with the HEC 18 correction for 

K4 ................................................................................................................................... 44

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 ........................................................................................... 45

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.............................................................................................................................. 47

27 Variation of dimensionless abutment scour with deflected flow 

excess velocity................................................................................................................ 75

28 Adjustment factors for gradation and coarse material fraction: (a) Gradation 

reduction factor, Ks; (b) Coarse fraction adjustment, K15 .............................................. 78

29 Measured and computed abutment scour for hydrodynamics flume experiments: 

(a) For uniform mixtures; (b) All mixtures .................................................................... 79

30 Effect of clay content on abutment scour....................................................................... 81

31 Pier scour reduction factor for Montmorillonite clay mixtures...................................... 86

32 Abutment scour reduction factor for Montmorillonite clay mixtures ............................ 87

33 Abutment scour reduction factor for Kaolinite mixtures ............................................... 88

34 Computed and measured dimensionless pier scour depth 

for unsaturated Montmorillonite clay............................................................................. 98

35 Computed and measured dimensionless pier scour depth 

for saturated Montmorillonite clay................................................................................. 98

36 Computed and measured relative abutment scour for Montmorillonite clay................. 109

37 Computed and measured relative abutment scour for Kaolinite clay ............................ 110
vii

LIST OF TABLES 

Table Page 

1 Properties of sediment mixtures used in pier scour experiments ................................... 16

2 Summary of sand-scour experiments in the sedimentation flume for set 1 ................... 17

3 Summary of sand-scour experiments in sedimentation flume for set 2 ......................... 18

4 Summary of sand-scour experiments in sedimentation flume for set 3 ......................... 19

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

through 7 ........................................................................................................................ 20

6 Summary of river mechanics flume experiments to study pier width 

effects for set 8 ............................................................................................................... 22

7 Summary of gravel-scour experiments in sedimentation flume 

for sets 9 and 10 ............................................................................................................. 23

8 Experimental conditions for set A runs for abutment scour........................................... 58

9 Experimental conditions for set B runs for abutment scour........................................... 60

10 Experimental conditions for set C runs for abutment scour........................................... 62

11 Experimental conditions for set G runs for abutment scour........................................... 63

12 Experimental conditions for set H runs for abutment scour........................................... 64

13 Experimental conditions for set I runs for abutment scour ............................................ 65

14 Experimental conditions for set J runs for abutment scour............................................ 66

15 Experimental conditions for set K runs for abutment scour........................................... 67

16 Experimental conditions for set L runs for abutment scour........................................... 68

17 Experimental conditions for set M runs for abutment scour.......................................... 69

18 Experimental conditions for set N runs for abutment scour........................................... 70

19 Experimental conditions for set D runs for abutment scour........................................... 71

20 Experimental conditions for set E runs for abutment scour........................................... 72

21 Experimental conditions for set F runs for abutment scour ........................................... 73

22 Properties of sediment mixture types used in abutment scour experiments .................. 74

23 Summary of pier scour experiments in clayey sands ..................................................... 84

24 Summary of abutment scour experiments in clayey sands ............................................ 85
viii

25 Results of set 1 experiments to study effects of clay content......................................... 92

26 Summary of experimental conditions and results for set 2 

(effect of compaction on pier scour in cohesive soils)................................................... 93

27 Summary of experimental conditions and results for set 3 

(effect of IWC on pier scour in cohesive soils).............................................................. 94

28 Summary of experimental conditions and results for set 3 

(effects of initial water content on pier scour for saturated clay)................................... 95

29 Results of Montmorillonite clay experiments conducted in the 2.44-m wide 

sedimentation flume using 0.22 m abutment protrusion length ..................................... 104

30 Results of Montmorillonite clay experiments conducted in the 1.22-m wide flume 

using 0.11 m abutment protrusion length....................................................................... 105

31 Results of Montmorillonite clay experiments conducted in the 2.44-m wide flume 

using 0.11 m abutment protrusion length....................................................................... 106

32 Results of Kaolonite clay experiments in the 2.44-m wide flume 

with 0.22 m abutment width........................................................................................... 107
1

1. INTRODUCTION

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 (CSU) from 1991 through 1996. This study encompassed 

experiments conducted under four major categories: 

1. Effects of gradation and coarse material fraction on pier scour experiments. 

2. Effects of gradation and coarse material fraction on abutment scour experiments. 

3. Effects of cohesion on pier scour experiments. 

4. Effects of cohesion on abutment scour experiments. 

During this 5-year study, 5 different experimental flumes of varying sizes with 20 

noncohesive and 10 cohesive sediment mixtures were utilized. In pier scour experiments, nine 

different pier sizes were tested; in abutment scour experiments, seven different abutment 

protrusion lengths were utilized. Depending on the purpose and scale of the experiments, 

varying degrees of data collection programs were implemented. The measurements utilized 

various levels of sophistication and collected detailed flow and sediment data in the vicinity of 

pier and abutment models. These experimental results were then utilized to quantify effects of 

coarse material fraction and cohesion on bridge scour. The details of the study were presented in 

a separate, six-volume set. These reports are: 

Vol. 1. Effect of Sediment Gradation and Coarse Material Fraction on Clear Water 

Scour Around Bridge Piers(1)

Vol. 2. Experimental Study of Sediment Gradation and Flow Hydrograph Effects on 

Clear Water Scour Around Circular Piers (2)

Vol. 3. Abutment Scour for Nonuniform Mixtures (3)

Vol. 4. Experimental Study of Scour Around Circular Piers in Cohesive Soils (4)

Vol. 5. Effect of Cohesion on Bridge Abutment Scour (5)

Vol. 6. Abutment Scour in Uniform and Stratified Sand Mixtures (6)
2

In this synthesis report, the experimental facilities, experimental equipment and 

measurements, results, and analysis and conclusions for each of the components of the 

experimental study are presented individually in chapters 2 through 6. Each chapter 

tabulates the properties of the materials used, the approaching flow conditions, and the 

resulting scour measurements. The results from the various components of the study are 

also presented in terms of unified relationships and equations. 

In chapter 2, results of experiments to study the effects of gradation and coarse 

bed material fraction on pier scour are presented. In the first phase of the study, effects 

of gradation and coarse bed material fraction were investigated experimentally in the 

2.44-meter (m) wide by 60-m long tilting flume at the Engineering Research Center, 

CSU. Six different sand mixtures with the same median diameter, D50, of 0.75 millimeter 

(mm), but with gradation coefficients ranging from 1.38 to 3.4 were used in the 

experiments. To study the coarse material fraction effects, mixtures with the same 

median diameter, D50, and gradation coefficient, σg, but with varying D90 and D95 sizes 

were subjected to the same scour conditions. Experiments revealed that the coarse 

material fraction, rather than the gradation coefficient, is the controlling factor in pier 

scour. This set of experiments was limited to clear water scour around a circular pier 

with a diameter of 0.178 m. Flow intensities starting with incipient conditions were 

increased in the experiments until live-bed conditions were encountered. Extensive bed 

material samples from the scour hole and the approach were obtained and analyzed. 

Also, in chapter 2, the results of a smaller scale study for quantifying the effects of 

gradation on pier scour are presented using six sand mixtures with the same median sizes 

of 1.8 mm and 0.78 mm, but with different gradation coefficients. These smaller scale 

experiments, conducted in a 0.61-m wide by 18-m long experimental flume using circular 

piers with a diameter of 0.076 m, examined the effects of model scaling. These 

experiments, along with additional experiments conducted in the 5.1-m wide river 

mechanics flume, were used in defining the dependence of scour on pier diameter. It was 

found that local scour, Ds, varied with pier diameter, b, according to a simple power 

relationship: Ds ~ b0.6. Finally, the study was extended to larger sediment sizes by 

conducting experiments using uniform and coarse fraction enriched gravel mixtures. As 

a result of this effort, 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. 

In chapter 3, effects of size fraction on clear water abutment scour was studied 

using 16 different sediment mixtures ranging from very fine sand to gravel sizes. 

Experiments were conducted in 0.61-m, 2.44-m, and 5.18-m wide flumes to cover a range 

of geometrically similar flow conditions to analyze scale effects in physical modeling of 

abutment scour. In the experiments, for mixtures with median (D50) sediment sizes of 0.1 

mm, 0.55 mm, 0.78 mm, and 1.8 mm, the coarse size fractions present in mixtures 

corresponding to D90 and D95 sizes were varied while keeping the median size and 
3

sediment gradation constant. Results of abutment scour experiments were normalized 

using corresponding abutment scour values for uniform mixtures with the same median 

D50 sediment sizes. Similar to pier scour analysis, a coarse fraction reduction factor for 

graded mixtures was derived for different gradation coefficients. It was found that, for 

very low flow intensities and for intensities approaching live-bed conditions, the coarse 

material fractions have little effect. However, for a wide range of intermediate 

dimensionless flow intensities, abutment scour is very much dependent on the coarse 

material fraction and can be as low as 15 percent of scour in uniform material with the 

same D50 size. As a result of abutment scour experiments, two new equations were 

developed. The first equation was derived from a 0.1 mm uniform sand mixture and 

defines an envelope 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 represent the experimental data very 

accurately but have not been tested with field data. 

In chapter 4, effects of clay content on bridge scour are presented. For both pier 

and abutment scour experiments, clayey sand mixtures with varying amounts of clay 

were subjected to different approach flow conditions. The resulting scour values from 

these experiments were normalized with the corresponding scour experienced in pure 

sand used as the base material. In pier scour experiments, for Montmorillonitic clay 

mixtures, results showed that for sandy clays, increasing the clay content to up to 30 

percent may reduce scour by up to 40 percent. Beyond a certain clay content (30 to 40 

percent for the present mixture) parameters such as compaction, initial water content, 

degree of saturation, shear strength, etc., dominated the pier scour. Similarly, in 

abutment scour experiments, for Montmorillonitic clay mixtures, experimental results 

showed that for clayey sands, increasing the clay content up to 30 percent may reduce 

scour by up to 40 percent. For Kaolinitic clay mixtures the results are more dramatic; for 

the clear water scour range, scour reduction can be up to 80 percent of that observed in 

pure sands. Beyond a certain percentage of clay content (30 to 40 percent for the present 

mixture) parameters such as compaction, initial water content, degree of saturation, shear 

strength, etc., dominate the cohesive material scour. These experiments were conducted 

for the range of flows that represented clear-water scour conditions for the base material. 

However, it is expected that for velocities much in excess of those that could mobilize the 

base material, ultimate scour would be reached. 

In chapter 5, effects of cohesion on pier scour are investigated experimentally 

using 1.22-m, 2.44-m, and 5.18-m wide test flumes at the Engineering Research Center, 

CSU. In these experiments, flows with corresponding Froude numbers ranging from 0.2 

to 1.4 were used to investigate scour taking place in saturated and unsaturated 

Montmorillonitic sandy clay mixtures. Regression equations relating flow and selected 

cohesive soil parameters to pier scour were derived for a fixed circular pier geometry and 

a constant flow depth. These experiments showed that pier scour in cohesive material 

can be expressed in terms of cohesive soil parameters that can be obtained in the field. 
4

The equations derived from this study do not attempt to relate effects due to pier 

geometry, size, flow depth, gradation of sand in the mixtures, etc., and therefore are not 

intended for general application. These relationships are derived to explain the 

variability of pier scour with cohesive properties. 

In chapter 6, effects of cohesion on abutment scour were investigated 

experimentally using 1.22-m and 2.44-m wide test flumes. In abutment scour 

experiments, Montmorillonitic and Kaolinitic sandy clay mixtures were used to examine 

clay mineralogy effects as well as effects due to compaction, initial water content, and 

soil shear strength. Froude numbers in these experiments varied between 0.2 and 0.6 to 

cover a wider range of hydraulic conditions. Regression equations relating flow and clay 

properties to abutment scour express scour in terms of sands scour and were therefore 

developed for the range of flows that represented clear water scour conditions for the 

base material. 

The data resulting from the experimental study are presented in chapters 2 

through 6 in tabulated, consistent units. The majority of these data were derived from the 

six-volume report mentioned above. The results of pier scour experiments in gravel and 

the results of pier scour experiments to study effects of pier width were conducted 

beyond the initial set of experiments and are included in this report in chapter 2. 
5

2. EFFECTS OF GRADATION AND COARSE 

MATERIAL FRACTION ON PIER SCOUR 

2.1 GENERAL 

Pier scour has been extensively studied in the past for uniform and graded 

sediment mixtures. In general, pier scour equations account for the variation in sediment 

properties by either including a correction factor for sediment gradation or by using 

median size and the gradation coefficient in developing the experimental regression 

equations. In this study, a new governing sediment parameter was brought to attention. 

This parameter, which describes the characteristics of the coarse fraction available in 

mixtures, accounts for the wide variation in scour depth for mixtures with the same 

median size and size gradation factor. Along with a dimensionless flow intensity 

parameter, a new pier scour equation was developed to account for the sediment 

properties in clear-water scour range. This equation was shown to be applicable to a 

wide range of sediment sizes ranging all the way from 0.1 mm to 40 mm. Pier scour 

experiments conducted in noncohesive materials in this study were aimed at identifying 

the effects of sediment properties on the resulting scour. For this purpose 10 sets of 

experiments were designed to vary sediment size, gradation, and other size distribution 

properties. Pier scour experiments for each sediment mixture were conducted by varying 

the approach flow conditions. Since the primary goal of the study was to define effects 

of gradation and coarse material fraction on pier scour, other flow variables such as 

depth, flow angle of attack, pier shape, etc., were kept constant. 

2.2 EXPERIMENTAL SETUP AND MEASUREMENTS

This section presents experimental flumes, sediment mixtures used as bed 

materials, piers, experimental procedures, as well as the individual measurements 

employed in quantifying effects of gradation and coarse material fraction on pier scour. 

 

LABORATORY FLUMES

Three laboratory flumes, designated as the hydrodynamics flume, sedimentation 

flume, and the river mechanics flume, were simultaneously utilized for conducting the 

pier scour experiments in noncohesive sediment mixtures. The first two flumes are 

sediment recirculating facilities, while the latter does not recirculate sediment. All 

flumes are housed at the Hydraulics Laboratory of the Engineering Research Center at 

CSU. The water supply to these flumes is from the nearby Horsetooth Reservoir. The 

temperature of the water in the laboratory is controlled through a heating pipe system. 
6

Hydrodynamics Flume 

The hydrodynamics flume is a tilting, water and sediment recirculating laboratory 

facility. The flume is 0.6-m wide, 0.75-m deep, and 18-m long and is made of a steel 

bottom and Plexiglas side walls to facilitate visual observations. The facility is rigidly 

supported on U-shaped steel frames located every 1.2 m and is equipped with angled 

upper and lower flange stiffeners. The bottom flanges are supported on two I-beams 

spanning the full length of the flume and ground supported at the far upstream, the 

middle, and far downstream. Two carefully leveled guide rails are mounted on the top 

flanges to provide an escorting track for the measuring carriage. The flume can be tilted 

around its middle lateral axis through the synchronized operation of two mechanical jacks 

located at the upstream and downstream ends. Flow is supplied to the flume from a 

ground sump via a 0.3-m diameter steel pipe line, equipped with a 0.15-m diameter 

bypass for fine tuning of the flow, and a 20-horsepower (HP) centrifugal pump. The flow 

is first introduced to an upstream head box equipped with a multi-layer screen containing 

gravel at its outlet to serve as a flow guide to provide uniform velocities and turbulence 

characteristics at the entrance of the flume. A wave suppressor is then introduced to 

ensure accomplishment of the previous concerns. The flow depth is controlled by a 

downstream rotating gate hinged across the bottom of the flume, spanning the full width, 

and operated by a system of pulleys. Because of the tail gate control and the nature of the 

flume, a back water effect is sometimes noticed, causing the water depth to increase as 

the gate is approached. A uniform sediment layer 23 centimeters (cm) thick is prepared 

from the tested mixture and is spread along the full length of the flume with provisions 

made for a downstream 1.8-m long sediment trap and an upstream 1.8-m long transition 

zone. The upstream transition zone is composed of coarser sediments, with a sloping 

profile carefully designed to provide excess friction to ensure the existence of fully 

developed turbulent flow, with a boundary layer hitting the free surface, far upstream of 

the study reaches for all flow conditions. 

Sedimentation Flume

The sedimentation flume is an elevated sediment transport testing facility that 

provides for both longitudinal tilting and sediment recirculation. The flume is 60-m long, 

2.4-m wide, 1.2-m deep, and allows for slope adjustments up to 3 percent through a 

system of hydraulic jacks. The flume is constructed from steel plates at the bottom and 

sides, with provisions for Plexiglas windows along specific locations at its side. The 

structure's skeleton is composed of U-shaped lateral steel frames supported on boxsectioned longitudinal girders. A walkway is cantilevered from the lateral frames on each 

side of the structure. The upper flanges house guide rails for an electrically motorized 

measuring carriage that can virtually move to any point in the flume. Three different 

pumps (125, 150, and 250 HP), with a maximum combined capacity of 3 cubic meters 

per second (m3

/s), can be simultaneously operated to supply water to the flume from a 

reservoir sump via three separate steel pipe lines. The flow is first introduced to the 

upstream head box, which contains several guide vanes and flow straighteners followed 
7

by a honeycomb mesh. It then passes through a gravel-filled screen succeeded by a wave 

suppressor. Rapid development of the fully turbulent boundary layer is achieved through 

an upstream concrete ramp and/or artificial roughening of the entrance zone. The flow 

depth is regulated through a manually operated downstream adjustable tail gate. The 

sediment bed is built to a thickness of about 0.4 m, with provisions made for a 

downstream sediment trap that extends for 6 m. To facilitate drainage of the flume after 

the experiments, a perforated 10-cm diameter poly vinyl chloride (PVC) pipe was 

embedded in the bed material and spanned the full length of the study reach. A 

motorized instrument carriage runs longitudinally on rails mounted on the side walls of 

the flume. 

River Mechanics Flume 

The river mechanics flume is a fixed-slope facility. The flume is 6-m wide, 0.9-m 

deep, and around 30-m long. The test section, however, was 24-m long, 5.1-m wide, and 

0.9-m deep, providing for two Plexiglass viewing sections along one side of the flume 

and a large upstream reservoir to create uniform entrance conditions. I-beam rails are 

mounted on the side walls to provide a track for the measuring carriage. A 75-HP axial 

pump of maximum capacity around 0.6 m3

/s supplies water to the flume through a 0.6-m 

diameter pipeline. The upstream main ends in a similar size diffuser located orthogonal 

to the main flow direction to distribute the flow uniformly across the flume width. The 

flow then passes through a gravel-filled screen followed by an artificially coarsened 

concrete ramp that joins the main sediment bed. The setup also provides for a 

downstream sediment trap and a downstream sill for depth regulation. 

PIERS

For the pier scour experiments conducted in the sedimentation flume, three 

identical 1.22-m high clear Plexiglas cylindrical piers with an outside diameter of 0.18 m 

were utilized. Circular piers were used because of their symmetry and the abundance of 

data available for comparative purposes. All three piers were placed at the center line of 

the flume for each run. In the longitudinal direction, the leading pier was 13.7 m from 

the head box of the flume, the second one was 24.4 m from the head of the flume, and the 

third one was 36.6 m from the head of the flume. To keep side-wall effects insignificant, 

the maximum pier size (for use in the 2.4-m wide flume) was kept at 0.18 m resulting in a 

flume width-to-pier ratio of 13.7. The depth measurements for pier scour with time were 

achieved utilizing visual techniques. For this purpose, the piers were constructed of 

transparent Plexiglas material, and three measuring scales were glued to the front, side, 

and back of each pier, in addition to a mirror with handle angled at 45o

 placed at the base 

of the pier. The base of the scour hole and the deepest point of the scour hole at any time 

could be easily identified and recorded by sliding the mirror within the Plexiglas pier and 

reading the corresponding measurement on the scale. In this way, scour depth with 

elapsed time could be obtained up to an accuracy of ±0.0015 m. A bright light located 

above the water surface was used to improve the visibility of the scour region under clear 
8

water-scour conditions. For the hydrodynamics flume experiments, three Plexiglas piers 

with outside diameters of 0.051 m, 0.051 m, and 0.07 m were installed in the flume, 

equidistant from the walls. Pier scour experiments in the river mechanics flume 

investigated the effects of pier width on the resulting scour. For this purpose six 

additional pier diameters including 0.019 m, 0.032 m, 0.057 m, 0.089 m, 0.165 m, and 

0.216 m were tested. 

SEDIMENT MIXTURES

Previous researchers have indicated that there is a very strong tendency for 

alluvial sediments to follow the log-normal size distribution. Such size distribution can 

be represented by a straight line on plots using logarithmic-normal probability scales. In 

this case the median sediment size D50 is also the geometric mean diameter, Dg of the 

sediment mixture, where D50 is the sediment diameter for which 50 percent of the 

sediment material is finer by weight. The geometric standard deviation σg is given by: 

16

50

50

84

D

D

D

D

σ g = = (1) 

or 

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ = +

16

50

50

84

2

1

D

D

D

D

σ g (2) 

 

or 

16

84

D

D

σ g = (3) 

where: D16 , D50 , and D84 are the sediment diameters for which 16, 50, and 84 percent of 

the sediment material is finer by weight, respectively. The log-normal distribution 

function is a two-parameter distribution and is completely defined by D50 and σg. 

However, most natural sediments show an approximate log-normal distribution only 

through the mid part of the distribution, say D50 ± σg, but they usually have long tails in 

both the coarse and fine fractions. Thus, equations 1 through 3 are for gradation 

coefficients that measure the spread of the distribution only between D84 and D16 in most 

natural sediments. The presence of coarse material in sediment mixtures is better defined 

by sizes of different quantities, such as D98, D95, D90, etc. For the work here, D50 and σg

were held constant and sizes of D90, D95, D98, etc., were changed since armoring in the 

scour hole involves mostly the coarser fractions of the mixture. 

There is a specific requirement that needs to be met in determining the gradation of 

the initial grain size distribution for the sediment mixtures. This requirement is to keep the 

median size diameter constant throughout the study. In the sedimentation flume sand-scour 

experiments, the median diameter was chosen to be 0.75 mm, with gradation coefficients 
9

varying between 1.3 and 4.0. In the hydrodynamics flume sand-scour experiments, the 

median diameter was chosen to be 1.8 mm, with gradation coefficients varying between 1.1 

and 4. Additional runs using 0.75-mm diameter uniform sand were conducted to study 

scaling effects. In the river mechanics flume sand-scour experiments, 0.45-mm diameter 

sand with a gradation coefficient of 2.3 was used. Finally, in the gravel-scour experiments 

conducted in the sedimentation flume, the median diameter was chosen to be 18 mm, with 

gradation coefficients varying between 1.4 and 2.3. 

 The properties of the sediment mixtures used in the pier-scour experiments are 

given in table 1.

MEASUREMENTS

 A series of measurements is needed to define the relationship between local pier 

scour and the various hydraulic, geometric, and sediment parameters. These 

measurements are presented below. 

Flow Discharge 

The water discharge in all three test flumes was measured through a system of 

orifice-meter and a differential manometer. For the hydrodynamics flume, two orifice 

plates were available: one mounted on the 0.3-m diameter main, and the other attached to 

the 0.15-m diameter bypass line. Both orifice plates are connected to a dual watermercury manometer for detecting the pressure drop across the ends of the plate. The flow 

discharge was then computed from the calibration curves for the orifices. The pressure 

tapping across the orifice plate is connected to the manometer through hard vinyl tubing 

provided with bleeding valves for drainage and for ensuring an air-free environment. The 

sedimentation flume is equipped with three similar setups for measuring the discharge, 

each attached to a different pump. Extreme care was taken to ensure the release of air 

bubbles entrapped in all manometer lines. Flow discharges were also estimated by 

integrating the vertical velocity profiles over the entire cross section of the flume at 

several locations. The error in measuring the discharge in the hydrodynamics flume is 

around 3 percent, in the sedimentation flume around 4 percent, and in the river mechanics 

flume around 5 percent. These error estimates are due to the calibration errors of orifice 

plates, unsteadiness in the pump discharge, and fluctuations in manometer readings. 

Flow Velocity 

In the hydrodynamics flume, velocities were measured utilizing a twodimensional electromagnetic Marsh McBirney, Model 523 velocity meter attached to a 

point gauge to measure velocity components in two orthogonal directions in a plane 

parallel to the bottom of the flume. The meter consists of a spherical electromagnetic 

probe with cable and signal processor powered by 6-volts direct current (V DC) 

externally charged with 110-volts alternating current (V AC). The probe has a diameter 

of 12 mm and is mounted on a 6-mm diameter vertical standing rod. The analog signals 

corresponding to the two orthogonal velocities sensed by the probe are intercepted by a 
10

multichannel data acquisition board connected to a personal computer. The sampling 

duration was 30 s, with a frequency of 50 Hertz (Hz). Overall accuracy of the latter setup 

is around 3.5 percent. 

 In the river mechanics and sedimentation flumes, velocities were measured with a 

one-dimensional Marsh McBirney, Model 2000 electromagnetic flow meter with a 2.54-

cm elliptic probe and a digital display conversion voltmeter. The accuracy of the flow 

meter is reported to be ±2 percent by the manufacturer, and its operating range is from -

0.015 m/s to +6.1 m/s within temperature extremes of 0 o

C to 71 oC. Overall accuracy of 

the velocity measuring setup is estimated to be 5 percent. 

Flow Depth and Hydraulic Grade Line 

 In the case of nonuniform material, bed irregularities affect the accuracy of bed 

elevation measurements. Theoretically, during experiments the bed surface is not known 

and the bed elevation measurement at any section depends on the position of the tip of the 

point gauge relative to the larger grains on the bed. To reduce such errors, during the 

sedimentation flume experiments three different point gauge readings utilizing a flat tip 

were taken across each of the test sections. The bed elevation was accepted as the 

average of these three values. Using a pointed point-gauge tip, the corresponding water 

surface elevations were also measured. At specified test sections the depth of flow was 

then calculated as the average difference between the water surface and the bed surface 

elevations. For all experimental runs a uniform flow depth over the entire flume length 

was maintained at 0.3 m ±0.03 m by regulating the tail gate at the flume exit. However, 

the local flow depth varied along the flume length during runs No. MA-12, MA-19, and 

MA-27, in response to the presence of bed features. 

 Slope in laboratory flumes is one of the most difficult quantities to measure. 

Special attention was devoted to reduce the error in slope measurements as much as 

possible. For this reason bed levels were measured using the point gauge, then corrected 

through conversion factors obtained from a careful leveling of the carriage along the 

entire flume using a surveyor's level and rod. The bed slope was then computed as the 

slope of the line of best fit based on least square-criteria. The water surface slope was 

calculated in a similar manner.

In the hydrodynamics flume, the water surface elevations were measured using 

point gauges with a resolution of ±0.3 mm. Water surface elevation measurements were 

conducted at a minimum of three approach cross sections per pier and at a minimum of 

four locations across the flume width at each cross section. At every location in the cross 

section, the water level was considered to be the average of detected values, to account 

for any residual fluctuations in the supply discharge and any surface waves induced by 

the setup. This tedious procedure assumes an accuracy of 2 percent in the computed 

water depth. The hydraulic grade line is identified through regressing the measured water 

surface values after being adjusted with the level correction factors. The regression 

results in high correlation coefficients (R2

 = 0.95). The velocity head was then added to 

the hydraulic grade line to define the total energy line. 
11

Free Stream Bed and Scour Hole Topographies

 The bed topographies for the scour holes and the free stream approaches are measured 

using point gauges. In all flumes, the standard topography measurement procedure started with 

the leveling of instrumentation carriages at each measurement location along and across the 

flume to account for the potential unevenness of tracks. Choosing an arbitrary fixed level, every 

location in the flume, as identified by its Cartesian coordinates, was assigned a correction factor 

reflecting its elevation relative to the fixed level. In the hydrodynamics flume, point gauges with 

flat/pointed tips were utilized for measuring purposes depending on the location and accuracy 

desired. The bed topographies at four different approach sections were measured for each pier 

model to define the upstream bed elevation. At each cross section, the bed level was considered 

to be the average of 10 measurements evenly distributed across the flume's width. To define bed 

topography in the vicinity of local scour requires more intensive measurements. An intensive 

measuring grid was adopted to describe the scour hole region for each pier. A similar procedure 

was followed for the other two flumes, with provisions made for measuring the initial as well as 

the final levels. The raw measurements are adjusted with the leveling correction factors for each 

location and then regressed together to yield the value of the bed slope. Approximately 300 

topography measurements were performed for each pier model per experiment. Due to the large 

sampling size, the error in bed elevation measurements is considered to be equal to 0.25 of the 

D90 grain size. The maximum scour depth value for each run was calculated as the difference 

between the mean initial bed elevation and the lowest measured point of scour around each pier. 

Scour versus Time Measurements 

 The measurement of the scour hole development for piers utilized visual techniques. 

As mentioned earlier, piers were made out of transparent Plexiglas. Measuring grid tapes were 

pasted on the interior wall at the front, side, and back of each pier. Using a simple periscope 

manufactured by an inclined-mirror, the development of scour with time was recorded without 

interference with the flow. A series of lights was used to facilitate the observation of the scour 

hole development. The depth of scour was recorded at regular intervals as the scour hole 

formed. The frequency of scour depth measurements decreased as the rate of scouring 

decreased. The experiments were stopped when no change occurred to the maximum depth of 

the scour hole over a minimum period of 4 hours. 

Sampling of the Armor Layer 

 At the end of each experiment, the particle size distribution of the armor layers 

formed around each pier in the scour hole, approach bed to each pier, and downstream of 

the pier were measured from samples obtained by the flour paste technique described by 

Abdou(7). Sieve analysis was then performed on the samples using U.S. standard sieves 

and the available shaker in the Sediment Laboratory of the Engineering Research Center, 

CSU. 

 To determine the grain size distribution of the armor layer, it was required to 

collect all grains in the top layer only. The most common method used by previous 
12

researchers for such a purpose is the wax method. Gessler(8) used molten resin at 200 o

C, 

Little and Mayer(9) used purified bee's wax at 65 oC to 68 oC, Davies(10) used molten 

petroleum wax at 76 o

C to 78 oC, and Proffitt(11) used paraffin wax at 55 oC to 57 oC. In 

previous work, the measured grain size distribution of the armor layers was found to be 

highly affected by the temperature at which the wax was poured onto the bed. If the 

temperature is outside the narrow ranges specified above, the wax either permeates down 

before solidifying or solidifies before all grains in the top layer adhere to it. 

 Day(12) used the paint method to identify grains in the top layer but still used the 

wax method to lift it up. This method predicted a coarser grain size distribution of the 

armor layer than the wax method. Day explained this to be caused by the penetration of 

the wax below the armor layer. 

 In the present study, the flour paste technique developed by Abdou(7) was used. 

The procedure proved to be much easier than the wax method in terms of preparation, 

use, time elapsed, and separation of the grains adhering to the paste. The paste was 

sticky enough for all grain sizes to adhere easily and thickly enough so that it did not 

penetrate further than the surface layer. 

 After the bed was allowed to dry, the paste was placed on the surface of the bed. 

A gentle uniform pressure was applied downward on the paste to pick up all the grain 

sizes on the surface layer. The paste was then lifted up bringing with it the grains that 

had been the surface layer. Washing the surface of the paste with warm water and then 

gently brushing by hand achieved the separation of grains from the paste. A visual 

observation of the paste surface clearly indicated that all grains, even the finest, were 

separated from the paste. The material was then dried, weighed, sieved, and the grain 

size distribution of the armor layer at a specific area for a given flow condition was 

obtained. 

Grain Size Distribution 

 Mechanical (or sieve) analysis was used to determine the particle sizes and their 

relative distribution for particles greater than 0.074 mm. The smallest sieve size used in 

this analysis was the U.S. No. 200. The sieve number corresponds to the number of 

openings per linear inch; for example, the U.S. Bureau of Standards No. 8 sieve has eight 

openings per inch. 

 To accomplish the mechanical analysis, sieves were stacked one on top of the 

other in the shape of a nest of sieves, in which the largest screen opening (smallest sieve 

number) was on top, progressing to the sieve with the smallest screen openings (largest 

sieve number) on the bottom of the nest. A lid was placed on top of the nest and a pan 

was placed below the bottom sieve to catch any sediment that passed through the smallest 

opening. A 10-minute shaking period was used in this procedure. A larger sample 

required a longer shaking period. Similarly, a sample composed primarily of fine-grained 

material requires a longer shaking period than a coarse-grained sample of equal weight. 
13

EXPERIMENTAL PROCEDURE

Preparation for the scour test was initiated by leveling the bed. Prior to each run, 

the sediment bed was leveled with the aid of a flat plate that was of the same width as the 

flume and was connected vertically to the instrument carriage by clamps. By employing 

the point gauge mounted on the carriage, initial bed elevations were taken to check the 

leveling of the flume and calculate the average initial bed elevation around each pier. 

 The gate was kept closed until the flume was filled with water. Then the gate was 

adjusted to get the desired depth, and the valves of the pump were adjusted to get the 

desired discharge, which was determined with an orifice inserted in the recirculating 

pipeline. Flow depth in the sedimentation flume experiments was maintained around 0.3 

m; water surface and bed slopes were almost parallel. For the hydrodynamics flume 

experiments flow depth was kept around 0.08 m. 

 Once the requested flow conditions were verified, the carriage and the point 

gauge were moved along the flume in such a way that any point in the study area could 

be reached with the measuring devices. Water surface profile was measured along the 

length of the flume to calculate the water surface slope. Vertical velocity profiles and 

development of scour with time were recorded during each experiment. In the 

sedimentation flume experiments, the duration of runs was selected to be 16 hours (h) to 

allow maximum scour to be reached and the final scour hole geometry to be established. 

This period was long enough to maintain the maximum scour depth constant for at least 3 

to 4 h. For the hydrodynamics flume experiments longer experiment durations were 

tested. Test runs up to 56 h showed that, for the ranges of sediment sizes and gradation 

used in the experiments, the longer experiment durations did not alter maximum scour. 

For a given discharge, once the surface armor layer was formed, bed profiles remained 

virtually constant. At the end of each run, the tail gate was slowly closed and the pump 

stopped to drain the flume without any disturbance. Then the flume was slowly drained 

with the aid of an efficient drainage system on the floor of the flume with its end open 

toward the tail gate. 

 The bed was then allowed to dry over a 24-h period, photos of scour holes around 

each pier were taken, and measurements of the final bed elevations were recorded to 

determine the maximum scour depth around each pier and the final bed slope. The bed 

was allowed to dry another 24 h, and then the armored layers around each pier and 

different areas in the approach and downstream of the piers were sampled using the flour 

paste technique described by Abdou(7). Sieve analysis was then performed on the 

samples using U.S. standard sieves (the sieving of the sediment samples was completed 

by using a series of sieves at intervals of 2 times sieve diameter). 

 This procedure was repeated for each run. In the sedimentation flume 

experiments, the area around each pier within 6.1 m had to be refilled with the proper 

mixtures, leveled, and saturated with water. Flow conditions were verified, and velocity 

was measured at the approach of each pier in addition to the water surface profile 

measurements and scour depth with time. After the scour depth became constant with 

time for at least 3.0 to 4.0 h, the flow was stopped to let the bed dry, then the final bed 
14

elevations were taken. Finally, the surface layer around each pier as well as the approach 

and downstream of each pier was sampled to determine the size distribution of the 

armored layers. 

2.3 EXPERIMENTAL RESULTS 

 A comprehensive experimental program was designed to investigate the different 

aspects of gradation and coarse material fraction effects on local pier scour. These 

experiments are categorized into 10 different sets of runs labeled 1 through 10. The 

experimental program was carried out concurrently in three different laboratory facilities. 

Sets 1 through 3, 9, and 10 were conducted in the sedimentation flume; sets 4 through 7 

were performed in the hydrodynamics flume; and set 8 experiments were carried out in 

the mechanics flume. Thirteen different sediment mixtures and 10 different pier models 

were subjected to a range of flow conditions, resulting in a total of 188 different pier 

scour case studies. 

 Set 1 experiments were conducted by subjecting three identical piers to specified 

flow conditions. The purpose of this set of runs was to check the repeatability of results 

for scour depth at the three piers subjected to the same flow conditions. This first set 

(runs MA-1 through MA-12) was performed using a graded sand mixture with a 

geometric standard deviation, σg of 2.43, and D50 of 0.75 mm. Set 2 experiments (runs 

MA-13 through MA-19) were conducted using the same sand with σg of 2.43 and D50 of 

0.75 mm as the bed material around pier 1. Around piers 2 and 3 the size of coarse 

material fraction in the original sediment mixture corresponding to 10 percent (around 

pier 2) and to 5 percent (around pier 3) was increased. The gradation coefficient, σg, and 

D50 were kept constant at 2.43 and 0.75 mm, respectively. The purpose of this second set 

of runs was to examine the behavior of the scour depth with increasing the sizes of 

sediments for the fraction above D90 and D95 in the original sediment mixture without 

changing the gradation coefficient. Set 3 experiments (runs MA-20 through MA-27) 

used a sediment mixture with σg of 3.4 and D50 of 0.75 mm as the bed material around 

pier 2 and increasing the coarse fraction above D90 in the same sediment mixture as the 

bed material around pier 1. For pier 3, a uniform sand with σg of 1.38 and D50 of 0.75 

mm was used. The purpose of set 2 and 3 experiments was to investigate the effect of 

increasing coarse material fraction and gradation of bed materials on local pier scour 

depth. 

 Sets 4 through 7 were conducted in a smaller flume with scaled down (1:4) flume 

width, flow depth, and pier width. In sets 4 through 6, a coarse sand mixture with the 

same median diameter, D50, of 1.8 mm, but different gradation coefficients were 

subjected to a range of approach flow conditions. In set 4, a uniform mixture with σg of 

1.15 was used; whereas in sets 5 and 6, σg was 2.43 and 3.4, respectively. Finally, in set 

7 the same sediment used in set 1 (with D50 of 0.75 mm and σg of 2.43) was used to study 

scaling effects. 

 Set 8 experiments were conducted in the 5.18-m wide river mechanics flume to 

examine and verify pier size effects. For this purpose, a series of circular piers with 
15

varying diameters was subjected to the same oncoming flows. The sediment used for 

these experiments was medium sand with a median diameter of 0.55 mm and gradation 

coefficient of 2.43. The results of these experiments were used to establish scaling 

parameters for pier widths. In general terms, it is found that pier scour is a function of 

b2/3. 

 Sets 9 and 10 were conducted in the sedimentation flume and examined the 

effects of coarse fraction on gravel scour. The two sediment mixtures used in these 

experiments had both a median diameter of 18 mm and gradation coefficient of 1.45. 

However, the gravel mixture used in set 9 experiments contained larger coarse fractions. 

The D90 for the mixtures used in sets 9 and 10 was 40 mm and 22 mm, respectively. The 

purpose of these experiments was to investigate the range of applicability of the theory 

developed from the study. 

 A summary table of the sediment characteristics associated with the different 

mixtures utilized in the study is given in table 1. Tables 2 through 7 present these cases. 

In the pier scour experiments presented in this section, the effects of the following 

parameters were investigated: 

• Effect of mean sediment size, D50 (all sets). 

• Effect of sediment gradation, σg (all sets). 

• Effect of the coarsest 10th percentile of the sediment size gradation (sets 1-3; 9-10). 

• Effect of flow depth, Y (all sets). 

• Effect of pier diameter, b (set 8). 

 The following sections present results of experiments. References to related 

summary tables are given whenever applicable. 
16

Table 1. Properties of sediment mixtures used in pier scour experiments. 

Mixture Mixture σg Dm D16 D35 D50 D65 D84.6 D85 D90 D95 D99 Dcfm Dcfm/D50

No. ID (mm) (mm) (mm) (mm) (mm) (mm) (mm) (mm) (mm) (mm) (mm)

(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) 

 

1 MA-1A 2.43 0.75 0.31 0.50 0.75 1.11 1.83 1.83 2.10 2.36 4.80 2.59 3.46 

2 MA-1B 2.43 0.75 0.31 0.50 0.75 1.11 1.83 1.83 2.80 5.00 8.00 4.24 5.65 

3 MA-1C 2.43 0.75 0.31 0.50 0.75 1.11 1.83 1.83 2.00 2.36 8.00 3.09 4.12 

4 MA-2E 3.40 0.75 0.23 0.45 0.75 1.31 2.65 2.65 4.76 6.40 8.00 5.50 7.33 

5 MA-2D 3.40 0.75 0.23 0.45 0.75 1.31 2.65 2.65 3.20 4.20 8.00 4.24 5.66 

6 MA-3 1.38 0.75 0.55 0.65 0.75 0.87 1.05 1.05 1.18 1.22 1.30 1.19 1.59 

7 HN-1 3.70 1.87 0.40 1.02 1.87 3.00 5.47 5.47 6.30 8.04 10.00 7.36 3.93 

8 HN-2 1.15 1.87 1.56 1.74 1.87 2.01 2.21 2.21 2.26 2.32 2.38 2.29 1.23 

9 HN-3 2.17 1.80 1.10 1.29 1.80 2.53 3.89 3.89 4.39 4.93 5.60 4.69 2.61 

10 HN-4 1.28 0.76 0.63 0.66 0.76 0.87 1.03 1.03 1.08 1.14 1.19 1.11 1.46 

11 MH-1 2.24 0.55 0.22 0.40 0.55 0.75 1.10 1.10 1.30 1.60 2.30 1.53 3.40 

12 MH-2 1.28 16.90 11.50 16.10 16.90 17.84 19.10 38.10 40.00 42.40 45.00 41.30 2.44 

13 MH-3 1.30 16.70 13.00 15.20 16.70 18.16 20.20 20.20 20.90 21.70 22.40 21.30 1.28 
17

Table 2. Summary of sand-scour experiments in sedimentation flume for set 1 

 (runs 1 through 12). 

Median Gradation Flow Approach Approach Energy Froude Scour Flow

Diameter Coefficient Discharge Depth Velocity Slope Number Depth Duration

D50 σg Q Y V Se Fr Ds t

Run

ID

Mixture

ID

(mm) (l/s) (m) (m/s) (m) (h)

MA-1-1 MA-1 0.75 2.43 206.43 0.384 0.213 0.00020 0.11 0.037 8 

MA-1-2 MA-1 0.75 2.43 206.43 0.396 0.204 0.00020 0.10 0.037 8 

MA-1-3 MA-1 0.75 2.43 206.43 0.399 0.201 0.00020 0.10 0.034 8 

MA-2-1 MA-1 0.75 2.43 246.07 0.287 0.347 0.00040 0.21 0.076 8 

MA-2-2 MA-1 0.75 2.43 246.07 0.293 0.341 0.00040 0.20 0.067 8 

MA-2-3 MA-1 0.75 2.43 246.07 0.293 0.338 0.00040 0.20 0.067 8 

MA-3-1 MA-1 0.75 2.43 300.16 0.287 0.421 0.00060 0.25 0.168 19 

MA-3-2 MA-1 0.75 2.43 300.16 0.290 0.405 0.00060 0.24 0.143 19 

MA-3-3 MA-1 0.75 2.43 300.16 0.293 0.405 0.00060 0.24 0.143 19 

MA-4-1 MA-1 0.75 2.43 300.16 0.381 0.302 0.00040 0.16 0.049 12 

MA-4-2 MA-1 0.75 2.43 300.16 0.375 0.302 0.00040 0.16 0.049 12 

MA-4-3 MA-1 0.75 2.43 300.16 0.378 0.290 0.00040 0.15 0.046 12 

MA-5-1 MA-1 0.75 2.43 263.63 0.354 0.296 0.00042 0.16 0.046 8 

MA-5-2 MA-1 0.75 2.43 263.63 0.344 0.293 0.00042 0.16 0.046 8 

MA-5-3 MA-1 0.75 2.43 263.63 0.341 0.296 0.00042 0.16 0.040 8 

MA-6-1 MA-1 0.75 2.43 280.34 0.335 0.332 0.00045 0.18 0.088 12 

MA-6-2 MA-1 0.75 2.43 280.34 0.335 0.332 0.00045 0.18 0.082 12 

MA-6-3 MA-1 0.75 2.43 280.34 0.335 0.329 0.00045 0.18 0.079 12 

MA-7-1 MA-1 0.75 2.43 323.10 0.323 0.396 0.00060 0.22 0.146 16 

MA-7-2 MA-1 0.75 2.43 323.10 0.326 0.390 0.00060 0.22 0.134 16 

MA-7-3 MA-1 0.75 2.43 323.10 0.326 0.387 0.00060 0.22 0.134 16 

MA-8-1 MA-1 0.75 2.43 360.76 0.320 0.442 0.00065 0.25 0.186 12 

MA-8-2 MA-1 0.75 2.43 360.76 0.326 0.411 0.00065 0.23 0.183 12 

MA-8-3 MA-1 0.75 2.43 360.76 0.323 0.421 0.00065 0.24 0.183 12 

MA-9-1 MA-1 0.75 2.43 267.03 0.320 0.335 0.00043 0.19 0.091 16 

MA-9-2 MA-1 0.75 2.43 267.03 0.311 0.335 0.00043 0.19 0.079 16 

MA-9-3 MA-1 0.75 2.43 267.03 0.305 0.341 0.00043 0.20 0.079 16 

MA-10-1 MA-1 0.75 2.43 390.49 0.332 0.469 0.00070 0.26 0.195 10 

MA-10-2 MA-1 0.75 2.43 390.49 0.326 0.457 0.00070 0.26 0.186 10 

MA-10-3 MA-1 0.75 2.43 390.49 0.317 0.460 0.00070 0.26 0.183 10 

MA-11-1 MA-1 0.75 2.43 429.00 0.335 0.491 0.00073 0.27 0.207 14 

MA-11-2 MA-1 0.75 2.43 429.00 0.329 0.479 0.00073 0.27 0.198 14 

MA-11-3 MA-1 0.75 2.43 429.00 0.320 0.494 0.00073 0.28 0.207 14 

MA-12-1 MA-1 0.75 2.43 473.74 0.363 0.503 0.00085 0.27 0.198 16 

MA-12-2 MA-1 0.75 2.43 473.74 0.363 0.524 0.00085 0.28 0.195 16 

MA-12-3 MA-1 0.75 2.43 473.74 0.384 0.482 0.00085 0.25 0.201 16 
18

Table 3. Summary of sand-scour experiments in sedimentation flume for set 2 (runs 13 

 through 19). 

Median Gradation Flow Approach Approach Energy Froude Scour Flow

Diameter Coefficient Discharge Depth Velocity Slope Number Depth Duration

D50 σg Q Y V Se Fr Ds t

Run 

ID

Mixture

ID

(mm) (l/s) (m) (m/s) (m) (h)

MA- 13-1 MA-1A 0.75 2.43 314.32 0.323 0.390 0.00055 0.22 0.155 16 

MA- 13-2 MA-1B 0.75 2.43 314.32 0.323 0.366 0.00055 0.19 0.049 16 

MA- 13-3 MA-1C 0.75 2.43 314.32 0.323 0.372 0.00055 0.21 0.067 16 

MA- 14-1 MA-1A 0.75 2.43 206.43 0.311 0.256 0.00029 0.15 0.049 16 

MA- 14-2 MA-1B 0.75 2.43 206.43 0.314 0.250 0.00029 0.14 0.012 16 

MA- 14-3 MA-1C 0.75 2.43 206.43 0.314 0.250 0.00029 0.14 0.027 16 

MA- 15-1 MA-1A 0.75 2.43 146.11 0.305 0.250 0.00022 0.11 0.009 16 

MA- 15-2 MA-1B 0.75 2.43 146.11 0.308 0.183 0.00022 0.11 0.003 16 

MA- 15-3 MA-1C 0.75 2.43 146.11 0.305 0.186 0.00022 0.11 0.006 16 

MA- 16-1 MA-1A 0.75 2.43 236.73 0.329 0.280 0.00045 0.16 0.082 16 

MA- 16-2 MA-1B 0.75 2.43 236.73 0.335 0.265 0.00045 0.15 0.027 16 

MA- 16-3 MA-1C 0.75 2.43 236.73 0.338 0.259 0.00045 0.14 0.046 16 

MA- 17-1 MA-1A 0.75 2.43 259.38 0.329 0.302 0.00050 0.17 0.091 16 

MA- 17-2 MA-1B 0.75 2.43 259.38 0.332 0.293 0.00050 0.16 0.030 16 

MA- 17-3 MA-1C 0.75 2.43 259.38 0.332 0.290 0.00050 0.16 0.049 16 

MA- 18-1 MA-1A 0.75 2.43 380.30 0.329 0.451 0.00062 0.25 0.213 16 

MA- 18-2 MA-1B 0.75 2.43 380.30 0.329 0.427 0.00062 0.24 0.085 16 

MA- 18-3 MA-1C 0.75 2.43 380.30 0.335 0.433 0.00062 0.24 0.128 16 

MA- 19-1 MA-1A 0.75 2.43 477.14 0.335 0.549 0.00098 0.30 0.226 16 

MA- 19-2 MA-1B 0.75 2.43 477.14 0.335 0.558 0.00098 0.31 0.180 16 

MA- 19-3 MA-1C 0.75 2.43 477.14 0.305 0.646 0.00098 0.37 0.201 16 
19

Table 4. Summary of sand-scour experiments in sedimentation flume for set 3 (runs 20 

 through 27). 

Median Gradation Flow Approach Approach Energy Froude Scour Flow

Diameter Coefficient Discharge Depth Velocity Slope Number Depth Duration

D50 σg Q Y V Se Fr Ds t

Run 

ID

Mixture

ID

(mm) (l/s) (m) (m/s) (m) (h)

MA-20-1 MA-2E 0.75 3.40 147.25 0.305 0.195 0.00047 0.11 0.003 16

MA-20-2 MA-2D 0.75 3.40 147.25 0.308 0.189 0.00047 0.11 0.003 16

MA-20-3 MA-3 0.75 1.38 147.25 0.308 0.186 0.00047 0.11 0.009 16

MA-21-1 MA-2E 0.75 3.40 184.06 0.341 0.207 0.00050 0.11 0.009 16

MA-21-2 MA-2D 0.75 3.40 184.06 0.338 0.210 0.00050 0.12 0.012 16

MA-21-3 MA-3 0.75 1.38 184.06 0.338 0.213 0.00050 0.12 0.040 16

MA-22-1 MA-2E 0.75 3.40 206.71 0.323 0.250 0.00054 0.14 0.012 16

MA-22-2 MA-2D 0.75 3.40 206.71 0.329 0.241 0.00054 0.13 0.015 16

MA-22-3 MA-3 0.75 1.38 206.71 0.332 0.238 0.00054 0.13 0.064 16

MA-23-1 MA-2E 0.75 3.40 259.38 0.335 0.305 0.00062 0.17 0.018 16

MA-23-2 MA-2D 0.75 3.40 259.38 0.335 0.299 0.00062 0.16 0.021 16

MA-23-3 MA-3 0.75 1.38 259.38 0.338 0.293 0.00062 0.16 0.128 16

MA-24-1 MA-2E 0.75 3.40 314.32 0.326 0.387 0.00070 0.22 0.030 16

MA-24-2 MA-2D 0.75 3.40 314.32 0.329 0.378 0.00070 0.21 0.052 16

MA-24-3 MA-3 0.75 1.38 314.32 0.326 0.381 0.00070 0.21 0.213 16

MA-25-1 MA-2E 0.75 3.40 379.45 0.332 0.451 0.00090 0.25 0.070 16

MA-25-2 MA-2D 0.75 3.40 379.45 0.332 0.436 0.00090 0.24 0.085 16

MA-25-3 MA-3 0.75 1.38 379.45 0.335 0.430 0.00090 0.24 0.250 16

MA-26-1 MA-2E 0.75 3.40 478.55 0.317 0.591 0.00150 0.34 0.189 16

MA-26-2 MA-2D 0.75 3.40 478.55 0.317 0.582 0.00150 0.33 0.189 16

MA-27-1 MA-2E 0.75 3.40 518.20 0.299 0.674 0.00200 0.39 0.219 16

MA-27-2 MA-2D 0.75 3.40 518.20 0.299 0.652 0.00200 0.38 0.201 16
20

Table 5. Summary of sand-scour experiments in hydrodynamics flume for sets 4 through 7. 

Run 

ID

Mixture

ID

Median

Diameter

D50

(mm)

Gradation

Coefficient

σg

Pier

Diameter

b

(m)

Flow

Discharge

Q

(l/s)

Approach

Depth

Y

(m)

Approach

Velocity

V

(m/s)

Froude

Number

Fr

Bed

Slope

So

Scour

Depth

Ds

(m)

Flow

Duration

 t

(h)

HN-01-1 HN-1 1.87 3.70 0.051 6.91 0.040 0.207 0.33 0.00418 0.010 8 

HN-01-2 HN-1 1.87 3.70 0.051 6.91 0.050 0.165 0.23 0.00418 0.003 8 

HN-01-3 HN-1 1.87 3.70 0.070 6.91 0.056 0.143 0.19 0.00418 0.004 8 

HN-02-1 HN-1 1.87 3.70 0.051 13.79 0.065 0.436 0.54 0.00418 0.025 8 

HN-02-2 HN-1 1.87 3.70 0.051 13.79 0.073 0.351 0.41 0.00418 0.009 8 

HN-02-3 HN-1 1.87 3.70 0.070 13.79 0.080 0.314 0.35 0.00418 0.013 8 

HN-03-1 HN-1 1.87 3.70 0.051 10.90 0.061 0.354 0.46 0.00363 0.015 8 

HN-03-2 HN-1 1.87 3.70 0.051 10.90 0.071 0.271 0.32 0.00363 0.008 8 

HN-03-3 HN-1 1.87 3.70 0.070 10.90 0.074 0.235 0.28 0.00363 0.008 8 

HN-04-1 HN-1 1.87 3.70 0.051 17.58 0.075 0.488 0.57 0.00336 0.028 10 

HN-04-2 HN-1 1.87 3.70 0.051 17.58 0.079 0.399 0.45 0.00336 0.012 10 

HN-04-3 HN-1 1.87 3.70 0.070 17.58 0.081 0.354 0.40 0.00336 0.014 10 

HN-05-1 HN-1 1.87 3.70 0.051 20.67 0.075 0.521 0.61 0.00368 0.032 10 

HN-05-2 HN-1 1.87 3.70 0.051 20.67 0.078 0.451 0.52 0.00368 0.027 10 

HN-05-3 HN-1 1.87 3.70 0.070 20.67 0.078 0.421 0.48 0.00368 0.036 10 

HN-10-1 HN-2 1.87 1.15 0.051 6.91 0.075 0.162 0.19 0.00375 0.002 11 

HN-10-2 HN-2 1.87 1.15 0.051 6.91 0.088 0.128 0.14 0.00375 0.000 11 

HN-10-3 HN-2 1.87 1.15 0.070 6.91 0.101 0.110 0.11 0.00375 0.000 11 

HN-11-1 HN-2 1.87 1.15 0.051 9.00 0.077 0.216 0.25 0.00375 0.007 9 

HN-11-2 HN-2 1.87 1.15 0.051 9.00 0.087 0.158 0.17 0.00375 0.000 9 

HN-11-3 HN-2 1.87 1.15 0.070 9.00 0.101 0.140 0.14 0.00375 0.000 9 

HN-12-1 HN-2 1.87 1.15 0.051 10.90 0.075 0.256 0.30 0.00391 0.048 14 

HN-12-2 HN-2 1.87 1.15 0.051 10.90 0.085 0.189 0.21 0.00391 0.009 14 

HN-12-3 HN-2 1.87 1.15 0.070 10.90 0.098 0.158 0.16 0.00391 0.014 14 

HN-13-1 HN-2 1.87 1.15 0.051 12.35 0.078 0.274 0.31 0.00418 0.058 21 

HN-13-2 HN-2 1.87 1.15 0.051 12.35 0.088 0.204 0.22 0.00418 0.012 21 

HN-13-3 HN-2 1.87 1.15 0.070 12.35 0.101 0.180 0.18 0.00418 0.017 21 

HN-14-1 HN-2 1.87 1.15 0.051 13.79 0.077 0.387 0.44 0.00417 0.077 19 

HN-14-2 HN-2 1.87 1.15 0.051 13.79 0.088 0.296 0.32 0.00417 0.042 19 

HN-14-3 HN-2 1.87 1.15 0.070 13.79 0.100 0.247 0.25 0.00417 0.018 19 

HN-20-1 HN-2 1.87 1.15 0.051 16.88 0.073 0.445 0.53 0.00417 0.078 30 

HN-20-2 HN-2 1.87 1.15 0.051 16.88 0.089 0.344 0.37 0.00417 0.066 30 

HN-20-3 HN-2 1.87 1.15 0.070 16.88 0.097 0.290 0.30 0.00417 0.061 30 

HN-21-1 HN-3 1.8 2.17 0.051 10.90 0.085 0.212 0.23 0.00341 0.012 17 

HN-21-2 HN-3 1.8 2.17 0.051 10.90 0.091 0.189 0.20 0.00341 0.003 17 

HN-21-3 HN-3 1.8 2.17 0.070 10.90 0.097 0.149 0.15 0.00341 0.007 17 

HN-22-1 HN-3 1.8 2.17 0.051 13.79 0.087 0.273 0.30 0.00341 0.014 22 

HN-22-2 HN-3 1.8 2.17 0.051 13.79 0.094 0.244 0.25 0.00341 0.012 22 

HN-22-3 HN-3 1.8 2.17 0.070 13.79 0.097 0.213 0.22 0.00341 0.012 22 

HN-23-1 HN-3 1.8 2.17 0.051 16.88 0.088 0.342 0.37 0.00341 0.031 25 
21

Table 5. Summary of sand-scour experiments in hydrodynamics flume (continued). 

Median Gradation Pier Flow Approach Approach Froude Bed Scour Flow

Diameter Coefficient Diameter Discharge Depth Velocity Number Slope Depth Duration

D50 σg b Q Y V Fr So Ds t

Run 

ID

Mixture

ID

(mm) (m) (l/s) (m) (m/s) (m) (h)

HN-23-3 HN-3 1.80 2.17 0.070 16.88 0.095 0.258 0.27 0.00341 0.020 25 

HN-24-1 HN-3 1.80 2.17 0.051 20.70 0.088 0.410 0.44 0.00323 0.048 24 

HN-24-2 HN-3 1.80 2.17 0.051 20.70 0.087 0.377 0.41 0.00323 0.042 24 

HN-29-3 HN-4 0.76 1.28 0.070 7.56 0.098 0.122 0.12 0.00000 0.002 8 

HN-30-1 HN-4 0.76 1.28 0.051 9.74 0.076 0.232 0.27 0.00000 0.062 20 

HN-30-2 HN-4 0.76 1.28 0.051 9.74 0.086 0.170 0.19 0.00000 0.024 20 

HN-30-3 HN-4 0.76 1.28 0.070 9.74 0.096 0.152 0.16 0.00000 0.017 20 

HN-31-1 HN-4 0.76 1.28 0.051 11.95 0.079 0.261 0.30 0.00000 0.081 21 

HN-31-2 HN-4 0.76 1.28 0.051 11.95 0.091 0.191 0.20 0.00000 0.058 21 

HN-31-3 HN-4 0.76 1.28 0.070 11.95 0.101 0.174 0.18 0.00000 0.047 21 

HN-32-1 HN-4 0.76 1.28 0.051 13.79 0.079 0.298 0.34 0.00000 0.085 23 

HN-32-2 HN-4 0.76 1.28 0.051 13.79 0.091 0.255 0.27 0.00000 0.066 23 

HN-32-3 HN-4 0.76 1.28 0.070 13.79 0.101 0.205 0.21 0.00000 0.083 23 
22

Table 6. Summary of river mechanics flume experiments to study pier width effects for 

 set 8. 

Median Gradation Pier Approach Approach Bed Froude Scour Flow

Mixture Diameter Coeficient Diameter Depth Velocity Slope Number Depth Duration

ID D50 σg b Y V So Fr Ds t

Run

ID

(mm) (m) (m) (m/s) (m/m) (m) (h)

MH 11-1 MH-1 0.55 2.24 0.216 0.238 0.244 0.001 0.160 0.045 16 

MH 10-1 MH-1 0.55 2.24 0.216 0.157 0.448 0.001 0.361 0.196 16 

MH 9-1 MH-1 0.55 2.24 0.216 0.198 0.371 0.001 0.266 0.153 16 

MH 8-1 MH-1 0.55 2.24 0.216 0.212 0.255 0.001 0.177 0.060 16 

MH 7-1 MH-1 0.55 2.24 0.216 0.255 0.272 0.001 0.172 0.079 16 

MH 6-1 MH-1 0.55 2.24 0.216 0.239 0.257 0.001 0.168 0.072 16 

MH 5-1 MH-1 0.55 2.24 0.216 0.246 0.290 0.001 0.187 0.120 16 

MH 12-1 MH-1 0.55 2.24 0.152 0.237 0.280 0.001 0.184 0.088 16 

MH 13-1 MH-1 0.55 2.24 0.152 0.210 0.253 0.001 0.176 0.069 16 

MH 14-1 MH-1 0.55 2.24 0.152 0.224 0.274 0.001 0.185 0.089 16 

MH 15-1 MH-1 0.55 2.24 0.152 0.244 0.316 0.001 0.205 0.116 16 

MH 16-1 MH-1 0.55 2.24 0.152 0.214 0.282 0.001 0.195 0.081 16 

MH 17-1 MH-1 0.55 2.24 0.152 0.290 0.517 0.001 0.307 0.248 16 

MH 18-1 MH-1 0.55 2.24 0.152 0.247 0.361 0.001 0.232 0.191 16 

MH 19-1 MH-1 0.55 2.24 0.152 0.224 0.307 0.001 0.207 0.111 16 

MH 11-3 MH-1 0.55 2.24 0.165 0.219 0.246 0.001 0.168 0.049 16 

MH 10-3 MH-1 0.55 2.24 0.165 0.138 0.465 0.001 0.400 0.143 16 

MH 9-3 MH-1 0.55 2.24 0.165 0.182 0.408 0.001 0.305 0.158 16 

MH 8-3 MH-1 0.55 2.24 0.165 0.194 0.265 0.001 0.192 0.065 16 

MH 7-3 MH-1 0.55 2.24 0.165 0.237 0.307 0.001 0.201 0.088 16 

MH 11-2 MH-1 0.55 2.24 0.089 0.238 0.240 0.001 0.157 0.037 16 

MH 10-2 MH-1 0.55 2.24 0.089 0.157 0.479 0.001 0.385 0.117 16 

MH 9-2 MH-1 0.55 2.24 0.089 0.198 0.349 0.001 0.250 0.111 16 

MH 8-2 MH-1 0.55 2.24 0.089 0.212 0.238 0.001 0.165 0.066 16 

MH 7-2 MH-1 0.55 2.24 0.089 0.255 0.270 0.001 0.171 0.077 16 

MH 6-2 MH-1 0.55 2.24 0.089 0.239 0.276 0.001 0.180 0.073 16 

MH 11-4 MH-1 0.55 2.24 0.057 0.219 0.235 0.001 0.160 0.035 16 

MH 10-4 MH-1 0.55 2.24 0.057 0.138 0.436 0.001 0.374 0.057 16 

MH 9-4 MH-1 0.55 2.24 0.057 0.182 0.378 0.001 0.283 0.068 16 

MH 8-4 MH-1 0.55 2.24 0.057 0.194 0.250 0.001 0.181 0.036 16 

MH 7-4 MH-1 0.55 2.24 0.057 0.237 0.276 0.001 0.181 0.032 16 

MH 10-5 MH-1 0.55 2.24 0.032 0.138 0.463 0.001 0.398 0.029 16 

MH 9-5 MH-1 0.55 2.24 0.032 0.182 0.413 0.001 0.309 0.037 16 

MH 8-5 MH-1 0.55 2.24 0.032 0.194 0.266 0.001 0.193 0.022 16 

MH 7-5 MH-1 0.55 2.24 0.032 0.237 0.305 0.001 0.200 0.038 16 

MH 10-6 MH-1 0.55 2.24 0.019 0.157 0.437 0.001 0.352 0.014 16 

MH 9-6 MH-1 0.55 2.24 0.019 0.198 0.339 0.001 0.243 0.034 16 

MH 8-6 MH-1 0.55 2.24 0.019 0.212 0.219 0.001 0.152 0.018 16 
23

Table 7. Summary of gravel-scour experiments in sedimentation flume for sets 9 and 10. 

Median Gradation Pier Approach Approach Bed Froude Scour Flow

Mixture Diameter Coefficient Diameter Depth Velocity Slope Number Depth Duration

ID D50 σg b Y V So Fr Ds t

Run

ID

(mm) (m) (m) (m/s) (m/m) (m) (h)

MHG1-1 MH-2 16.90 1.28 0.178 0.296 0.850 0.0007 0.499 0.049 16 

MHG2-1 MH-2 16.90 1.28 0.178 0.320 1.073 0.0007 0.605 0.073 16 

MHG3-1 MH-2 16.90 1.28 0.178 0.338 1.192 0.0007 0.654 0.110 16 

MHG4-1 MH-2 16.90 1.28 0.178 0.354 1.228 0.0007 0.659 0.113 16 

MHG5-1 MH-2 16.90 1.28 0.178 0.372 1.384 0.0007 0.724 0.113 16 

MHG6-1 MH-2 16.90 1.28 0.178 0.290 1.859 0.0007 1.103 0.110 16 

MHG7-1 MH-2 16.90 1.28 0.178 0.238 2.286 0.0007 1.497 0.271 16 

MHG1-2 MH-3 16.70 1.30 0.178 0.335 0.771 0.0007 0.425 0.073 16 

MHG2-2 MH-3 16.70 1.30 0.178 0.357 0.969 0.0007 0.518 0.085 16 

MHG3-2 MH-3 16.70 1.30 0.178 0.381 1.079 0.0007 0.558 0.119 16 

MHG4-2 MH-3 16.70 1.30 0.178 0.357 1.186 0.0007 0.634 0.152 16 

MHG5-2 MH-3 16.70 1.30 0.178 0.375 1.320 0.0007 0.688 0.183 16 

MHG6-2 MH-3 16.70 1.30 0.178 0.250 2.018 0.0007 1.288 0.235 16 

MHG7-2 MH-3 16.70 1.30 0.178 0.219 2.478 0.0007 1.688 0.305 16 
24

2.4 ANALYSIS 

 This section presents the parameters affecting the pier scour in nonuniform 

mixtures and derives relationships to quantify their effects on the resulting scour depths. 

The equations derived from this analysis are then tested with the data from the 

experimental study and with data from earlier studies. 

GOVERNING PARAMETERS

 Experiments conducted for sets 1 through 3 varied the size gradation and coarse 

material fraction of six sand mixtures while keeping their median diameter constant. In 

these experiments, the flow depth was kept relatively constant, and the pier diameter 

remained 0.18 m while the discharge into the flume was incremented. Since the channel 

width and flow depth remained constant, this discharge variation in the experiments 

corresponded to varying velocity while keeping all other flow parameters constant. 

Figure 1 shows the variation of dimensionless scour depth in sets 1 through 3 

experiments with approach velocity. By keeping all other variables constant, these 

experiments isolate the effects of gradation and coarse material fraction on pier scour. As 

shown in figure 1, the initiation of pier scour takes place independent of the size of coarse 

material fractions for approach velocities of about 0.18 m/s. This velocity is termed as 

the scour initiation velocity, Vi, and marks the threshold condition for clear-water scour. 

For approach velocities greater than Vi , the largest scour depth in figure 1 takes place in 

uniform sediment mixtures (gradation coefficient , σg = 1.38). As the size gradation 

coefficient increases from 1.38 to 2.43 to 3.4, the depth of scour decreases. This finding 

is in agreement with previous research. However, the reduction of scour is not a constant 

factor as suggested by earlier studies, but is a function of flow intensity. While the 

largest scour reduction takes place for an intermediate velocity value, for velocities 

slightly greater than 0.18 m/s and for velocities greater than 0.6 m/s, the scour reduction 

remains small. Figure 1 also shows two mixtures with the same median sediment sizes 

and gradation coefficients but with enlarged coarse fractions. In mixtures identified as 

2.38A and 2.38B, while D50 and σg were kept at 0.75 mm and 2.43, respectively, the 

coarsest 5-percent and 10-percent fractions were enlarged by replacing these size groups 

with coarser sediments. As a result, as shown in figure 1, the scour depths corresponding 

to these mixtures are smaller. In fact, the scour observed for the mixture with enlarged 

coarsest 10-percent fraction (mixture 2.43 A) is the same as the scour observed in 

mixture 3.4A with a gradation coefficient of 3.4. Similarly, introducing larger coarse 

fractions to mixture 3.4A results in further reduction in scour depth. 

 Figure 2 compares the results of sets 1 through 3 experiments with the computed 

scour values from the CSU equation given in Federal Highway Administration’s 

(FHWA) HEC-18(13). In figure 2 several observations can be made: 1) As the intensity of 

flow increases (indicated by larger scour depth) the computed scour depths approach the 

measured values; 2) For larger gradation factors and for mixtures with larger coarse 

fractions, the convergence of computed and measured values takes place at higher flow 

intensities; and 3) At low flow intensities the computed values are in the order of 8 to 10 
25

times the measured values. In figure 3, the ratio of measured to computed scour depth is 

plotted against the flow velocity for sets 1 through 3 experiments. For these data, this 

ratio approaches 1 (perfect agreement) as the flow velocity (or intensity) increases. For a 

given flow velocity, the ratio is closer to 1 for uniform mixtures (illustrated in the figure 

by the 1.38 gradation coefficient above the plot points) than for mixtures with large size 

variations (illustrated by the 3.40 gradation coefficient above the plot points). A general 

conclusion from figures 2 and 3 is that the discrepancy between measured and computed 

scour depths, using the current CSU equation, becomes worse as the gradation coefficient 

increases and as the velocity (or flow intensity) decreases. 

 Figure 4 shows results from set 4 through 7 experiments conducted using coarser 

sediment mixtures and compares these results with the finer uniform sand mixture used in 

sets 1 through 3. In these experiments smaller depths and pier diameters were used; 

without applying proper modeling scale ratios for flow depth and pier diameter the results 

cannot be superimposed on the previous results. However, the pattern of scour depth 

variation with flow intensity remains identical. Both in figures 1 and 4 the relationship 

between velocity and Ds /b shows that for uniform material, the variation of scour with 

velocity is almost linear. For graded material, and material with larger coarse fractions, 

this relationship assumes the characteristics of 2nd or 3rd degree polynomial (concave). At 

high flow velocities, both figures show that scour values tend to converge to an 

“ultimate” value. The velocity at which maximum clear-water scour takes place is a 

function of the size of coarsest size fractions present in mixtures. This velocity is 

identified as the critical velocity, Vc, at which the entire bed is mobilized (live-bed 

conditions). 

 It is possible to define a dimensionless excess velocity, ψ, which is a relative 

velocity with respect to the critical velocity that fully mobilizes the bed given by: 

0 ≤ ≤1 −

− ψ = ; ψ V V

V V

c i

i (4) 

 

The value of ψ varies between 0 and 1; the 0 corresponding to initiation of scour and 1 

corresponding to the condition of fully mobilized bed. The values of Vi and Vc can be 

determined by relating these velocities to critical flow conditions corresponding to 

initiation of motion. Using Shields’ relationship for critical shear 

 

r

' τ c = Kγ sD (5) 

 

or 

r

' γ R S = Kγ sD (6) 

 
26

where τc = critical shear; K = experimental constant ( ≈ 0.047 ); ,

s γ = submerged specific 

weight of sediment ( ≈1.65γ ); R = hydraulic radius; S = slope of the energy grade line; 

and Dr = characteristic sediment size. For critical conditions, using Manning-Strickler 

equation to express the slope of the energy line in terms of approach velocity ( S = Vc

2

 n

2

/ R2/3 , where Vc and R are in metric units) and using a relationship expressing the 

roughness coefficient, n, in terms of the characteristic sediment size (n = Dr

1/6 /26.1, 

where Dr is in meters) it is possible to obtain: 

r

,

s

c r . D . R

V D

γ 0 047 γ 26 12 1 3

2 1 3

= (7) 

 

or 

1 3 1 6

*

1 3 1 6

, 0.047 V 26.1 Dr R K Dr R s

c = = γ

γ (8) 

where Vc , R, and Dr are in SI units. Replacing ,

s γ /γ with 1.65 and after simplifications, 

equation 8 reduces to 1 3 1 6 V (m/s) 7.27D (m) Y (m) c ≈ r . In English units, the critical 

velocity expression becomes Vr (ft/sec) ≈ 13.16 Dr (ft)1/3 Y (ft)1/6. For the purposes of this 

study, however, the constant K* in equation 8 is left to be an experimentally determined 

value. Using results of pier scour experiments, K* was found to be 6.625 for SI units 

(using Dr in meters) and 12 for English units (using Dr in feet). To reflect the 

characteristics of the coarse material fractions, the representative sediment size, Dr, is 

defined by the median size of the coarse material fraction, Dcfm, given by: 

6

D85 2D90 2D95 D99 Dcfm+ + + = (9) 

The parameter Dcfm is a representative size (in meters for SI units and in feet for English 

units) for the coarse fractions present in sediment mixtures. Experimental evidence in 

figures 1 and 4 indicate that fully mobilized bed cannot be achieved without mobilizing 

coarser sizes. In the absence of extensive size information, or in cases where there are no 

discontinuities in the size gradation curves, it is possible to utilize D90 to represent coarse 

fractions. 

 Velocities in the vicinity of piers are amplified. From potential flow theory, this 

amplification is in the order of 1.7 times the approach velocity. The scour initiation takes 

place when the accelerated flows past the pier are capable of removing the bed material 

from the pier region. Experimental evidence indicates that these velocities are dependent 

on the finer size fractions that are significantly available in the bed. For this study, the 

representative size for initiation of motion was determined to be D35. This size was used 
27

in the sedimentation literature by Einstein and Chien(14), Ackers and White(15) to account 

for the gradation effects in the transport of bed material. The expression for the initiation 

of pier scour can be derived from the critical velocity relationship and can be expressed 

as: 

1 3 1 6 Vi = K′′D35 Y (10) 

where K” is an experimental coefficient. From the pier scour experiments, the value of

K” was found to be 2.65 for SI units using D35 in meters (4.8 for English units using D35

in feet). 

 Using the dimensionless velocity factor ψ , the data presented earlier in figure 1 

are expressed in figure 5. Figure 5 shows that while maximum scour depth is reached at 

ψ = 0.6 for the uniform mixture, for graded sediments higher flow intensities may be 

needed. This figure also shows that for mixtures with coarse fractions, low relative flow 

intensities produce significantly smaller scour depths. For these mixtures, ultimate scour 

is produced sharply beyond a threshold intensity. Figure 5 indicates that even though the 

ultimate scour might be the same, for intermediate flows, different mixtures exhibit 

different scour patterns. The information in figure 5 is reproduced in figure 6 in 

nondimensional form. 

 Figures 7 and 8 present dimensionless velocity versus depth of scour for sets 4 

through 7, 8, and 9. In these figures the parameter Dcfm is used to differentiate between 

mixtures with the same median size and gradation coefficient. As shown, the 

representative coarse fraction size, Dcfm, can reliably identify mixtures and therefore can 

be used in relationships to quantify the associated scour depths. In general, for the same 

dimensionless velocity factor, smaller Dcfm values are associated with larger scour depths. 

However, a more reliable factor in differentiating sediment properties of mixtures is the 

Dcfm /D50 ratio used in figure 9. This dimensionless parameter can be used to normalize 

different sediment sizes for their expected scour potential. In figure 9, Dcfm /D50 values of 

1.23 and 1.46 represent two uniform mixtures with median sizes of 1.80 mm and 0.75 

mm, respectively. For a given dimensionless velocity factor, the mixture with the larger 

sediment size but with smaller Dcfm /D50 ratio produces larger scour holes. This 

experimental observation can be used to formulate an expression by relating scour to 

flow intensity (as represented by dimensionless flow velocity factor, ψ ) and the relative 

coarse fraction size, Dcfm /D50. 

 To achieve this goal, scour taking place in uniform material must first be 

evaluated. Then, ratios of scour values observed in mixtures with varying amounts of 

coarse material to scour in uniform material must be evaluated. This ratio, which is 

termed as the “Coarse Fraction Reduction Factor,” and denoted by K4 , must then be 

related to flow intensity and Dcfm /D50. Figure 10 shows the results of this procedure for 

set 1 through 3 experiments. Several conclusions can be drawn from figure 10: 

(1) Scour reduction due to presence of coarse material cannot be expressed with a 

single value. 
28

(2) Scour reduction is a function of the coarse sediment fraction ratio Dcfm /D50. The 

higher the ratio, the lower the minimum value of K4. 

(3) For low flow intensities, and therefore ψ values near zero, the K4 value must be 

unity since at low flow intensities there could be no effects due to coarse fractions 

or gradation. 

(4) For high flow intensities, and therefore ψ values near or greater than unity, the K4

value must also approach unity. At high flow intensities with fully mobilized bed, 

effects due to the presence of coarse fractions must be minimal. 

(5) There exists a certain flow intensity ψ at which the scour reduction is minimum for 

a given sediment mixture. The location and magnitude of the minimum depend on 

the distribution and modality of sediment mixtures. 

DERIVATION OF K4 RELATIONSHIP

 Two steps are needed in order to derive a functional relationship for K4: 1) 

develop an expression for pier scour in uniform mixtures; and 2) separate the effects due 

to coarse fractions and develop an appropriate function. 

 Set 8 experiments given in table 6 were used to define the variation of local scour 

with pier diameter. Figures 11 and 12 show the variation of dimensionless scour depth 

with approach velocity for the six pier diameters used in the study. Since the flow depth 

was kept relatively constant for these experiments, and since the investigation of these 

effects was beyond the scope of the experimental study, the commonly accepted depth 

dependency of Y 1/6 was assumed for this study in normalizing the results. The best-fit 

line for describing the variation of scour with pier width in the pier width effect 

experiments utilizing 0.55-mm graded sand and the corresponding correlation coefficient 

are given by: 

0 97 0 90 0 72 2

0 66 0 17 . ; R . b Y

D .

. .

s = ψ = (11) 

where Ds, b, and Y are in meters and R2

 is the correlation coefficient. This relationship 

demonstrates that scour is related to pier diameter according to 0.66 D . s ~ b The goodness 

of fit of this relationship is shown in figure 13. 

 Next, utilizing Ds /(b0.66Y0.17) and ψ as variables, an expression for pier scour in 

uniform mixtures was developed. For this purpose, the present experiments with median 

sediment sizes ranging from 0.75 mm to 1.80 mm and to 17 mm were utilized. The 

resulting expression in International System (SI) units is: 

0.99 ; 0 1 (12) 0.55

0.66 0.17 = ψ ≤ψ ≤

b Y

Ds

in which Ds, b, and Y are in meters. For preferred English units, the coefficient 0.99 

becomes 1.21; Ds, b, and Y are in feet. 
29

In using equation 12, a limiting value of 1 must be imposed on ψ to reflect 

maximum clear-water scour conditions. Figures 14 and 15 show the goodness of fit of 

the data to this equation. In figure 15 additional data (126 points) from Chabert and 

Engeldinger(16) and Shen, Schneider, Karaki(17, 18) that were used in the development of 

FHWA’s CSU equation are included. This demonstrates the agreement of the new 

equation with other data sources. For comparison purposes, figure 16 presents the same 

uniform material data with the CSU equation. As expected, for coarse material and 

gravel, the CSU equation does not perform well. 

 The last stage in the development was the derivation of an expression to separate 

the effects due to coarse material fractions. This expression was derived through an 

extensive search for a function that could describe the physical phenomenon explained 

earlier in figure 10. These conditions are: 

• At low flow intensities ( 0 ψ ≈ ), effects due to coarse fraction are negligible since at 

these flows only finer fractions are scoured; 

• At high flow intensities ( 8 ψ ≥ 0. ), effects due to coarse fraction are also negligible 

since at these flows the entire mixture is mobilized; 

• At intermediate flow intensities scour reduction is a function of both ψ and Dcfm /D50 

; and 

• Maximum scour reduction takes place for ψ values varying from 0.1 to 0.3. 

The resulting expression is: 

; 0 1 (13) 0.55

0.66 0.17 = K K1 K2 K3 K4 ψ ≤ψ ≤

b Y

D

U

s

where KU = 0.99 for SI units, in which Ds, b, and Y are in meters (=1.21 for preferred 

English units, in which Ds, b, and Y are in feet); K1, K2, and K3 are as defined in HEC18(13); and K4 is the Coarse Fraction Factor defined by: 

1 25 3 0 5 0 1, 0 1 (14) 4

0 60

50

4 = + ψ ln(ψ + . ); ≤ K ≤ ≤ψ ≤

D

D

K . cfm .

where ψ is the dimensionless excess velocity from equation 4. 

 Figure 17 shows the data from set 1 through 3 experiments along with predictions 

from FHWA’s CSU equation(13) and the new K4 relationship. The goodness of fit is 

demonstrated in figure 18. In these figures, at low flow intensities and in the presence of 

coarse material, the performance of the CSU equation was poor. However, at high flow 

intensities, the CSU predictions converged with the new method and the measurements. 

 Figures 19 and 20 show the new equations with all available data from this study 

and with the data from earlier studies that were used in the development of the earlier 

CSU equation (a total of 310 data values). The performance of the CSU equation with 

the same data set is illustrated in figure 21. 

2.5 ADJUSTMENTS TO FHWA’S CSU EQUATION

 Figure 22 compares the present HEC-18(13) correction for coarse material size 

with the new approach. As shown, the modifications proposed in HEC-18 cannot fully 
30

accommodate size corrections since this factor does not involve any sizes and provides 

maximum correction at ψ = 0 (no scour condition). 

 From the analysis of all data, it is concluded that two adjustments are needed for 

FHWA’s CSU equation: 1) implementation of initiation of scour for uniform mixtures 

with larger sediment diameters than those used in the derivation of the model; and 

2) implementation of gradation and coarse fraction size correction for nonuniform 

sediment mixtures. 

 Since the equation was originally developed for fine sands, the initiation of 

motion took place at very low velocities, and therefore the need for such correction was 

not obvious. For coarser sediments at low flow intensities, the present analysis amplifies 

this deficiency. The initiation of scour may be implemented into the CSU equation by 

the inclusion of a scour initiation factor, Ki. This factor was found to be: 

1 (15)

0 45

i

.

i

i ; V V

V

V K ⎟ > ⎠

⎞ ⎜

⎝

⎛ = −

For values of V ≤Vi , the value of the initiation of scour factor, Ki , is 0. 

 Figure 23 compares the adjusted scour computations with the presently used K4

adjustment. The results are almost identical. The reason for this is due to the fact that the 

current K4 is merely a correction for the initiation of motion since the expression used for 

K4 in HEC-18 is independent of relative sizes. 

 The second adjustment to the CSU equation to implement gradation and coarse 

fraction size correction for nonuniform sediment mixtures may be accomplished through 

the K4 factor defined earlier: 

1 25 3 0 5 0 1, 0 1 (14) 4

0 60

50

4 = + ψ ln(ψ + . ); ≤ K ≤ ≤ψ ≤

D

D

K . cfm .

The final form of the CSU equation is: 

2 (16)

0 43 0 65

1 2 3 4

. .

i

s

gY

V

Y

b K K K K K

Y

D

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ ⎟

⎠

⎞ ⎜

⎝

⎛ =

where the definition of terms K1, K2, and K3 are as defined in HEC-18. Figures 24 and 25 

show the results of pier scour depth computations using both Ki and K4 and compares the 

results with the results from this study. As seen, major improvement takes place in the 

predictions. The final results are comparable to those obtained from the study with slight 

overestimations.
31

2.6 SUMMARY AND CONCLUSIONS

 A new pier scour equation describing effects of gradation and coarse material 

fraction on pier scour was developed. This equation is given as: 

; 0 1 (13) 0.55

0.66 0.17 = K K1 K2 K3 K4 ψ ≤ψ ≤

b Y

D

U

s

where KU = 0.99 for SI units, in which Ds, b, and Y are in meters (= 1.21 for preferred 

English units, in which Ds, b, and Y are in feet); K1, K2, and K3 are as defined in HEC-18; 

and ψ = the dimensionless excess velocity factor given by 

0 ≤ ≤1 −

− ψ = ; ψ V VV V

c i

i (4) 

The definitions of the critical and scour initiating velocities, Vc and Vi , respectively are: 

1 3 1 6 Vc = Kc Dcfm Y 

(8) 

 1 3 1 6 Vi = Ki D35 Y 

(10) 

where Kc = 6.625 for SI units (= 12.0 for preferred English units); K i = 2.65 for SI units 

(= 4.8 for preferred English units); and Dcfm is the median size of the coarse material 

fractions (in meters for SI units and in feet for English units) computed from 

6

D85 2D90 2D95 D99 Dcfm+ + + = (9) 

The coarse fraction reduction factor K4 is given by: 

1 25 3 0 5 0 1, 0 1 (14) 4

0 60

50

4 = + ψ ln(ψ + . ); ≤ K ≤ ≤ψ ≤

D

D

K . cfm .

By definition, both K4 and ψ cannot be greater than 1. The new equation has the 

following characteristics: 

(1) For uniform mixtures, it accommodates the initiation of motion at low flow 

intensities; for velocities smaller than the scour initiating velocities, no scour is 

computed. 

(2) The computed results are bounded by the imposition of a limiting condition for ψ of 

1 (live-bed conditions). 
32

(3) It is physically based. No scour reduction at initiation of motion or at high flow 

intensities. 

(4) Scour reduction is expressed as a function of relative coarse fraction size and the 

intensity of flow. 

(5) It was verified with past experimental data (170 points), including data used in the 

development of FHWA’s CSU equation. It also showed excellent agreement with 

Jain and Fisher’s data for supercritical flows. 

(6) It was shown to be applicable to size ranges from 0.24-mm fine sand to 17-mm 

gravel mixtures. 

(7) It was tested successfully with 370 data sets with close agreement. 

 The work on the existing FHWA CSU equation also provided very promising 

results. The adjustments needed for the FHWA equation were: 1) implementation of 

initiation of scour for uniform mixtures with larger sediment diameters than those used in 

the derivation of the model; and 2) implementation of gradation and coarse fraction size 

correction for nonuniform sediment mixtures. 

 The initiation of scour is implemented into the CSU equation by the inclusion of 

Ki factor. This factor was found to be: 

1 (15)

0 45

i

.

i

i ; V V

V

V K ⎟ > ⎠

⎞ ⎜

⎝

⎛ = −

For values of V ≤Vi , the value of the initiation of scour factor, Ki , is 0. 

 Gradation and coarse fraction size correction for nonuniform sediment mixtures is 

implemented into the CSU equation through the K4 factor defined earlier as: 

1 25 3 0 5 0 1, 0 1 (14) 4

0 60

50

4 = + ψ ln(ψ + . ); ≤ K ≤ ≤ψ ≤

D

D

K . cfm .

The final form of the CSU equation is: 

2 (16)

0 43 0 65

1 2 3 4

. .

i

s

gY

V

Y

b K K K K K

Y

D

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ ⎟

⎠

⎞ ⎜

⎝

⎛ =

where the definition of terms K1, K2, and K3 are as defined in HEC-18. 
33

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

Figure 2. Comparison of FHWA’s CSU equation with measured scour from sets 1 through 3. 
34

Figure 3. Velocity versus discrepancy ratio for sets 1 through 3 experiments. 

Figure 4. Flow velocity versus dimensionless scour for sets 4 through 7 experiments. 
35

Figure 5. Dimensionless excess velocity factor, ψ, versus depth of scour for sets 1 through 3. 

Figure 6. Variation of dimensionless scour with excess velocity factor for various mixtures. 
36

Figure 7. Variation of dimensionless scour with excess velocity factor, ψ, for different 

coarse fraction sizes used in sets 4 through 7 experiments. 

Figure 8. Variation of dimensionless scour with excess velocity factor, ψ, for different 

coarse fraction sizes used in sets 8 and 9. 
37

Figure 9. Variation of scour depth with excess velocity factor, ψ, for different coarse 

fraction size ratios used in sets 4 through 7 experiments. 

Figure 10. Variation of measured coarse fraction reduction factor, K4, with excess 

velocity factor, ψ, for sets 1 through 3 experiments. 
38

Figure 11. Variation of scour depth with pier size for the set 8 experiments. 

Figure 12. Relationship describing variation of pier scour with diameter. 
39

Figure 13. Computed and measured scour depths for the set 8 experiments. 

Figure 14. Variation of scour depth with excess velocity factor, ψ, for uniform sand and gravel. 
40

Figure 15. Measured and computed depth of scour for uniform sands and gravel using equation 12. 

Figure 16. Measured and computed scour for uniform sediment using FHWA’s CSU equation. 
41

Figure 17. Computed scour for sets 1 through 3 experiments using equation 12 with K4

from equation 14. 

Figure 18. Measured and computed scour for nonuniform-sand experiments in sets 1 through 3. 
42

Figure 19. Computed and measured scour for all data using equation 12 (uniform-mixture equation). 

Figure 20. Computed and measured scour for all data using equation 12 with K4 

correction from equation 14. 
43

Figure 21. Computed scour using FHWA’s CSU equation for uniform and nonuniform mixtures. 

Figure 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. 
44

Figure 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). 

Figure 24. FHWA’s CSU equation adjusted with Ki and K4 and with the HEC 18 correction for K4. 
45

Figure 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. 
46
47

3. EFFECTS OF GRADATION AND COARSE 

MATERIAL FRACTION ON ABUTMENT 

SCOUR

 

3.1 GENERAL 

The majority of currently available abutment scour predictors, including some of 

the methodologies recommended by FHWA, relate abutment scour to a characteristic 

length (such as flow depth, Y, abutment protrusion length, a, etc.) and the approach flow 

Froude number. Some of these regression equations include sediment size and gradation 

as independent variables. However, since these relationships were developed from 

limited laboratory and field data, they cannot reflect effects due to sediment size 

properties; therefore they often result in unrealistic scour estimations. As shown in 

figures 26(a) and 26(b), they provide no guidance when applied to graded sediment 

mixtures such as those used in the present experimental study. 

Figure 26. Variation of dimensionless abutment scour with Froude number: (a) abutment 

protrusion length, a, as characteristic length; (b) Lc = aY as characteristic length. 
48

 This chapter presents results of the experiments to analyze effects of gradation 

and coarse material fraction on abutment scour. As a result of this study, 384 new 

abutment scour data points covering a range of selected sediment mixtures have been 

contributed to the literature. Parameters controlling abutment scour in mixtures have 

been identified, and two new relationships for estimating abutment scour have been 

developed. The close agreement of these new relationships with laboratory experiments 

is encouraging. 

3.2 EXPERIMENTAL SETUP AND MEASUREMENTS

 This section presents details of the experimental setup, testing procedure, 

laboratory facilities, and measuring equipment that were utilized in the abutment scour 

experiments. First, the three laboratory test flumes used to study the effects of gradation 

on abutment scour are described. Next, individual measurements along with associated 

instrumentation and their accuracy are discussed. These measurements include initial and 

final bed topographies and slopes, hydraulic grade line, flow discharge, flow velocity, 

scour hole topography and its evolution with time, flow visualization, grain size 

distribution, and sampling of the armor layer. Finally, the experimental procedure 

followed for the various runs is discussed along with measures taken to ensure the 

accuracy and reliability of results. 

LABORATORY FLUMES

Three laboratory flumes, designated as the hydrodynamics flume, sedimentation 

flume, and the river mechanics flume, were simultaneously utilized for conducting the 

abutment scour experiments in noncohesive sediment mixtures. The former two flumes 

are sediment recirculating facilities, while the latter does not recirculate sediment. All 

flumes are housed at the Hydraulics Laboratory of the Engineering Research Center at 

CSU. The water supply to these flumes is from the nearby Horsetooth Reservoir. The 

temperature of the water in the laboratory is controlled through a heating pipe system. 

Hydrodynamics Flume 

 The hydrodynamics flume is a tilting, water and sediment recirculating laboratory 

facility. The flume is 0.6 m wide, 0.75 m deep, and 18 m long and is made of a steel 

bottom and Plexiglas side walls to facilitate visual observations. The facility is rigidly 

supported on U-shaped steel frames located every 1.2 m and is equipped with angled 

upper and lower flange stiffeners. The bottom flanges are supported on two I-beams 

spanning the full length of the flume and ground supported at the far upstream, middle, 

and far downstream. Two carefully leveled guide rails are mounted on the top flanges to 

provide an escorting track for the measuring carriage. The flume can be tilted around its 

middle lateral axis through the synchronized operation of two mechanical jacks located at 

the upstream and downstream ends. Flow is supplied to the flume from a ground sump 

via a 0.3-m diameter steel pipe line, equipped with a 0.15-m diameter bypass for fine 
49

tuning of the flow, and a 20-HP centrifugal pump. The flow is first introduced to an 

upstream head box equipped with a multilayer screen containing gravel at its outlet to 

serve as a flow guide to provide uniform velocities and turbulence characteristics at the 

entrance of the flume. A wave suppressor is then introduced to ensure the 

accomplishment of the previous concerns. The flow depth is controlled by a downstream 

rotating gate hinged across the bottom of the flume, spanning the full width, and operated 

by a system of pulleys. Due to the tail gate control and the nature of the flume, a back 

water effect is sometimes noticed, causing the water depth to increase as the gate is 

approached. A 23-cm thick uniform sediment layer prepared from the tested mixture is 

spread along the full length of the flume, with provisions made for a downstream 1.8-m 

long sediment trap and an upstream 1.8-m long transition zone. The upstream transition 

zone is composed of coarser sediments, with a sloping profile carefully designed to 

provide excess friction to ensure the existence of fully developed turbulent flow and with 

a boundary layer hitting the free surface far upstream of the study reaches for all flow 

conditions. 

Sedimentation Flume

 The sedimentation flume is an elevated sediment transport testing facility that 

provides for both longitudinal tilting and sediment recirculation. The flume is 60 m long, 

2.4 m wide, and 1.2 m deep and allows for slope adjustments up to 3 percent through a 

system of hydraulic jacks. The flume is constructed from steel plates at the bottom and 

sides with provisions for Plexiglas windows along specific locations at its side. The 

structure's skeleton is composed of U-shaped lateral steel frames with cantilevers for 

sidewalks and supported on box-sectioned longitudinal girders. The upper flanges house 

guide rails for an electrically motorized measuring carriage that can virtually move to any 

point in the flume. Three different pumps (125, 150, and 250 HP), with a maximum 

combined capacity of 3 m3

/s, can be simultaneously operated to supply water to the flume 

from a reservoir sump via three separate steel pipe lines. The flow is first introduced to 

the upstream head box, which contains several guide vanes and flow straighteners 

followed by a honeycomb mesh. The flow then passes through a gravel-filled screen 

succeeded by a wave suppressor. Rapid development of the fully turbulent boundary 

layer is achieved through an upstream concrete ramp and/or artificial roughening of the 

entrance zone. The flow depth is regulated through a manually operated downstream 

adjustable tail gate. The sediment bed is built to a thickness of about 0.4 m, with 

provisions made for a downstream sediment trap that extends for 6 m. To facilitate 

drainage of the flume after the experiments, a perforated 10-cm diameter PVC pipe was 

embedded in the bed material and spanned the full length of the study reach. 

River Mechanics Flume 

 The river mechanics flume is a fixed-slope facility. The flume is 6 m wide, 0.9 m 

deep, and around 30 m long. The test section, however, was 24 m long, 5.1 m wide, and 

0.9 m deep, providing for two Plexiglas viewing sections along one side of the flume and 
50

a large upstream reservoir to create uniform entrance conditions. I-beam rails are 

mounted on the side walls to provide a track for the huge measuring carriage. A 75-HP 

axial pump of maximum capacity around 0.6 m3

/s supplies water to the flume through a 

0.6-m diameter pipeline. The upstream main ends in a similar size diffuser located 

orthogonal to the main flow direction to distribute the flow uniformly across the flume 

width. The flow then passes through a gravel-filled screen followed by an artificially 

coarsened concrete ramp that joins the main sediment bed. The setup also provides for a 

downstream sediment trap and a downstream sill for depth regulation. 

MEASUREMENTS

 A series of measurements are needed in order to define the relationship between 

local scour at abutments and the various hydraulic, geometric, and sediment parameters, 

both quantitatively and qualitatively. These measurements are described below. 

Flow Discharge 

The water discharge in all three test flumes was measured through a system of 

orifice-meter and a differential manometer. For the hydrodynamics flume, two orifice 

plates were available: one mounted on the 0.3-m diameter main and the other attached to 

the 0.15-m diameter bypass line. Both orifice-plates are connected to a dual watermercury manometer for detecting the pressure drop across the ends of the plate. The flow 

discharge was then computed from the calibration curves for the orifices. The pressure 

tapping across the orifice plate is connected to the manometer through hard vinyl tubing 

provided with bleeding valves for drainage and for ensuring an air-free environment. The 

sedimentation flume is equipped with three similar setups for measuring the discharge, 

each attached to a different pump. Extreme care was taken to ensure the release of air 

bubbles entrapped in all manometer lines. The error in measuring the discharge in the 

hydrodynamics flume is around 3 percent, in the sedimentation flume around 4 percent, 

and in the river mechanics flume around 5 percent. These error estimates are due to the 

calibration errors of orifice plates, unsteadiness in the pump discharge, and fluctuations in 

manometer readings. 

Flow Velocity 

In the hydrodynamics flume, velocities were measured utilizing two different size 

pitot tubes and a two-dimensional magnetic flow meter depending on the desired 

accuracy. A 3-mm diameter minute pitot tube, tapered to a 1.5-mm diameter total head 

sensor that is connected to a membrane-type transducer, was first used to detect the flow 

velocity with an accuracy of ±1.5 percent. However, the problems encountered with air 

entrainment and blockage of the sensing and conveying tubing led to the replacement of 

this pitot tube with a larger one. The second pitot tube had a 3-mm diameter sensing 

stem with a pressure take-off diameter of 3 mm and a probe length of 0.3 m. The 

pressure transducer that was connected to the pitot tubes contained a 0.7 kilo pascal (kPa) 

steel membrane and is commercially available as P7D±5 PSID SER. 11534. The output 
51

signals from the transducer are amplified, filtered, and analyzed by a data acquisition 

board and a personal computer. The velocities sensed by this device are in the order of 

±2.5 percent of accuracy. The second method for velocity measurements in the 

hydrodynamics flume utilized the 2-D electromagnetic Marsh McBirney, Model 523 

flowmeter. The flowmeter consists of a spherical electromagnetic probe with cable and 

signal processor powered by 6-V DC externally charged with 110-V AC. The probe has 

a diameter of 12 mm and is mounted on a 6-mm diameter vertical standing rod. The 

analog signals corresponding to the two orthogonal velocities sensed by the probe are 

intercepted by a multichannel data acquisition board connected to a personal computer. 

The sampling frequency was 50 Hz. Overall accuracy of the latter setup is around 3.5 

percent. 

 In the river mechanics and sedimentation flumes, velocities were measured with a 

one-dimensional Marsh McBirney, Model 2000 electromagnetic flowmeter with a 2.54-

cm elliptic probe and a digital display conversion voltmeter. The accuracy of the 

flowmeter is reported to be ±2 percent by the manufacturer, and its operating range is 

from -0.015 m/s to +6.1 m/s within temperature extremes of 0 o

C to 71 oC. Overall 

accuracy of the velocity measuring setup is estimated to be 5 percent. 

Flow Visualization

 Examination of flow field in the vicinity of the abutment was achieved through 

flow visualization techniques, including: 

• Surface and subsurface floats made of hollow glass spheres partially filled with a 

fluid to maintain neutral buoyancy. These floats were 1 cm in diameter and were 

used in determining separation, reattachment, and reversed flow zones. 

• Lightweight plastic pellets sprayed on the water surface gave a good picture of the 

surface stream lines, especially at low flows. 

• Dye injection of potassium permanganate solution with the same density as water was 

very indicative of the nature of the horse vortex and other subsurface features. Other 

chemicals were tried but did not provide further improvement. 

• Potassium permanganate crystals sprayed on the sediment bed surface, inside the 

scour hole or around it, provided visualization of boundary currents. 

• Video and still picture cameras for documenting various phenomena.

Free Stream Bed and Scour Hole Topographies

 The bed topographies for the scour holes and the free stream approaches are 

measured using point gauges. In all flumes, the standard topography measurement 

procedure started with the leveling of instrumentation carriages at each measurement 

location along and across the flume to account for the potential unevenness of tracks. 

Choosing an arbitrary fixed level, every location in the flume, as identified by its 

Cartesian coordinates, was assigned a correction factor reflecting its elevation relative to 

the fixed level. 
52

 In the hydrodynamics flume, three different point gauges were utilized for 

measuring purposes depending on the location and accuracy desired. The bed 

topographies at four different approach sections were measured for each abutment model 

to define the upstream bed elevation. At each cross section, the bed level was considered 

to be the average of 10 measurements evenly distributed across the flume's width. To 

define bed topography in the vicinity of local scour requires more intensive 

measurements. A measuring grid of an average of 17 cross sections, with a minimum of 

21 points per section, was adopted to describe the scour hole region for each abutment. 

 A similar procedure was followed for the other two flumes, with provisions made 

for measuring the initial as well as the final levels. The raw measurements are adjusted 

with the leveling correction factors for each location and then regressed together to yield 

the value of the bed slope. Approximately 400 topography measurements were 

performed for each abutment model per experiment. Due to the large sampling size, the 

error in bed measurements is considered to be equal to 0.25 of the D90 grain size. 

Hydraulic Grade Line 

The water surface elevations were measured using point gauges with a resolution 

of ±0.3 mm. Water surface elevation measurements were conducted at a minimum of 

three approach cross sections per abutment and at a minimum of five locations across the 

flume width at each cross section. At every location in the cross section, the water level 

was considered to be the average of two minimum and maximum detected values, to 

account for any residual fluctuations in the supply discharge and any surface waves 

induced by the setup. This tedious procedure assumes an accuracy of 2 percent in the 

computed water depth. Again the hydraulic grade line is identified through regressing the 

measured water surface values after being adjusted with the level correction factors. The 

regression results in high correlation coefficients (R2

 = 0.95). The velocity head was then 

added to the hydraulic grade line to define the total energy line. 

Sampling of the Armor Layer 

 Surface samples representing the armor coat formed at the approach to the 

abutment and inside the scour hole were carefully collected after the experiments were 

over and the bed was dried. Various techniques, including molten paraffin wax, sticky 

polymers, modeling clay, scraping the surface, and pebble counting from still 

photographs, were tried. Among these, the modeling clay method proved to be the most 

practical and the most accurate. 

 Soft modeling clay, sometimes treated with castor oil to make it softer, was used 

to prepare a thin paste around 5 mm thick. The paste was placed at the desired sampling 

location and gently stamped down under a uniformly distributed applied pressure. The 

paste was then carefully lifted up, thus collecting a patch of adhered sediments 

representing the armor coat. After waiting for a short time to allow the paste to become 

drier, the collected sediments were removed by scraping the surface. If the clay is left on 

for a long time (one day or more) it becomes very stiff and the removal process has to be 
53

done under warm water. The following step was to wash the sediments to make sure that 

no residual clays are present, then oven dry them and finally prepare them for sieve 

analysis.

Grain Size Distribution 

The relative size distributions for the various sediment mixtures and samples was 

determined using the sieve analysis method. The sieve sizes used were those defined by 

the U.S. Bureau of Standards, which represents the sieve number by the number of 

openings it has per square inch. An average shaking time of 10 minutes was applied for 

all tested samples. 

EXPERIMENTAL PROCEDURE

The experimental procedure for the abutment scour experiments is summarized as 

follows: 

(1) The sediment mixture forming the bed of the flume is naturally compacted in 

layers. A low flow is then introduced to the flume while raising the downstream 

gate (for the Hydrodynamics and river mechanics flumes), or lowering the 

downstream gate (for the sedimentation flume), and is allowed to prevail for long 

enough to saturate the bed and drive away any entrapped air. 

(2) The flow is stopped and the bed is leveled to the desired elevation. Initial bed 

levels are then recorded at the desired sections. The bed around the abutments 

where local scour is expected to occur is protected with specially prepared shields. 

(3) The flow is introduced to flumes at a very slow rate in order to avoid disturbing the 

bed topography, especially around the abutments. A depth of flow greater than the 

desired depth for the experiment is maintained within the flume. The shields 

around the abutments are then removed. 

(4) The required flow depth and discharge are then achieved through simultaneous 

adjustment of the discharge valve and the downstream gate (or sill) position. 

Recording of scour development with time is immediately started. 

(5) Water surface elevations and flow velocities are recorded following the procedure 

given above. 

(6) Flow visualization and video recording, if any, are then performed. 

(7) The shut down process includes raising the depth inside the flume through 

downstream gate adjustment while decreasing the flow rate in steps. At complete 

shut down, the flume has to be holding a sizable depth of water that is then allowed 

to drain very slowly. The current step is crucial for preserving the local scour 

features produced through the run. 

(8) After allowing the bed to be completely drained, the final bed topographies are 

recorded, still camera photography is performed, and sampling of the armor layer is 

conducted. 

(9) The local scour areas are then replenished with new sediments and the previous 

procedure is repeated for the next experiment. 
54

3.3 EXPERIMENTAL RESULTS 

 A comprehensive experimental program was designed to investigate the different 

aspects of gradation and coarse material fraction effects on local abutment scour. These 

experiments are categorized into 14 different sets of runs labeled set A through set N. 

The experimental program was carried out concurrently in three different laboratory 

facilities. Sets A, B, C, G, H, I, J, K, L, M, and N were performed in the hydrodynamics 

flume, while sets D and E were conducted in the sedimentation flume. The river 

mechanics flume was assigned to set F experiments. Sixteen different sediment mixtures 

and 10 different abutment models were subjected to a range of flow conditions, resulting 

in a total of 384 different abutment case studies. Tables 8 through 21 present these cases. 

A summary table of the sediment characteristics associated with the different mixtures 

utilized in the study is given in table 22. 

 In the abutment scour experiments presented in this section, the effects of the 

following parameters on local abutment scour were investigated: 

• Effect of mean sediment size, D50 (in sets I, G, and L). 

• Effect of sediment gradation, σg (in sets A, B, and C, D-M1, E-M1, and E-M3). 

• Effect of the coarsest tenth percentile of the sediment size gradation (sets I, K, and 

L, M, N, D-M1, D-M2, D-M3, E-M1, E-M2). 

• Effect of flow depth, Y (sets H, I, and J). 

• Effect of abutment protrusion length, a (all sets). 

 The following sections present details of each set of experiments and references to 

related summary tables whenever applicable. 

HYDRODYNAMICS FLUME EXPERIMENTS

 The majority of the experiments (304 different abutment case studies) were 

carried out in the hydrodynamics flume. The details of each set of experiments are given 

below. 

Set A Experiments

 Set A consisted of 18 different runs (A-1 through A-18). Four abutment models 

were tested in each of the runs A-1, A-2, and A-3. All the four abutment models had the 

same length of 11.4 cm in the direction of flow; however, their protrusion lengths (length 

orthogonal to flow) varied. The abutment with a protrusion length of 2.54 cm was 

identified as M1, that with a protrusion length of 5.08 cm was identified as M2, that with 

a protrusion length of 7.62 cm was identified as M3, and that with a protrusion length of 

10.16 cm was identified as M4. In runs A-4 through A-18, the 2.54-cm model (M1) was 

dismantled, since its small size could introduce undesirable scale effects with the selected 

sediment sizes and therefore could influence the validity of the results. The bed material 

tested (sediment Type IV) was coarse graded sand with a D50 of 1.8 mm and a σg of 2.1, 

with an approximate log-normal size fraction distribution. The flow depth to abutment 
55

protrusion length (Y/a) varied between 0.5 and 2.0, the average velocity was between 18 

cm/s and 40 cm/s, and the mean Froude number ranged from 0.2 to 0.5. Table 8 shows a 

summary of the experimental conditions for set A runs. It should be noted that flow 

depth (Y) and flow velocities were all measured at an approach section located at a 

distance of (10a) upstream from each abutment, where a is the abutment protrusion 

length. 

Set B Experiments

 Set B included 16 different runs (B-1 through B-16). Three abutment models 

(M2, M3, and M4) were tested in each of the runs except for runs B-12 and B-13 where 

only M2 and M4 were examined. Sediment of Type III, which is a coarse uniform sand 

with a D50 of 1.8 mm and a σg of 1.17, was used throughout this set. The flow depth to 

abutment protrusion length (Y/a) varied between 0.3 and 2.3, the average velocity ranged 

between 0.21 m/s and 0.45 m/s, and the mean Froude number was between 0.25 and 0.6. 

Table 9 presents a summary of the experimental conditions for set B runs. 

Set C Experiments

 As shown in table 10, 21 case studies were included in set C (C-2 through C-8). 

Throughout this series and in the remaining sets that were conducted in the 0.61-m-wide 

flume, only abutment models M2, M3, and M4 were used. The bed material selected for 

this set was a coarse graded sand with a D50 of 1.8 mm, a σg of 3.9, and an approximate 

log-normal size fraction distribution (Type V sediment). The flow depth to abutment 

protrusion length (Y/a) varied between 0.38 and 1.7, the average velocity (Vu) was 

between 0.21 m/s and 0.44 m/s, and the mean Froude number ranged from 0.3 to 0.5. A 

summary of the experimental conditions for set C runs is presented in table 10. 

Set G Experiments

 This set was designed to test a second uniform medium sand mixture with a D50

of 0.78 mm, and a σg of 1.3 identified as Type II sediment. The set consisted of seven 

runs, which provided 21 data points as indicated in table 11. The experiments were 

conducted using a constant flow depth of 0.075 m for M2, 0.08 m for M3, and 0.09 m for 

M4. Mean velocity in set G experiments varied between 0.13 m/s and 0.3 m/s, and mean 

Froude number varied between 0.14 and 0.35. 

 

Sets H, I, and J Experiments 

 Sets H, I, and J were all conducted with fine silica sand with a D50 of 0.1 mm and 

σg of 1.4 (Type I sediment). Set H consisted of 24 cases, all of which were run for a 

constant depth of 0.10 m. Set I included 18 study cases, all being tested at a constant 

depth of 0.075 m, and similarly, set J consisted of 18 tests conducted at a constant depth 

of 0.05 m. Tables 12 through 14 demonstrate the experimental conditions associated 

with each set. In each of these sets the most severe flow condition resulted in live bed 

scour, whereas the rest of the experiments were all conducted under clear-water 
56

conditions. For set H, velocities and Froude numbers ranged between 0.01 m/s and 0.33 

m/s and between 0.09 and 0.3, respectively; for set I, velocities and Froude numbers 

ranged between 0.075 m/s and 0.40 m/s and between 0.13 and 0.46, respectively; and for 

set J velocities and Froude numbers ranged between 0.11 m/s and 0.31 m/s and between 

0.15 and 0.44, respectively. 

Set K Experiments

 Table 15 lists test conditions for the seven runs in the set K experiments using 

Type VII sediment. The Type VII sediment resembles the Type I sediment in all of its 

features except that the coarsest 15 percent fraction matches that of sediment Type III. 

All experiments were conducted at an almost constant flow depth of 0.075 m, while the 

flow velocity varied from conditions which initiated local scour at the abutments to 

conditions that resulted in the initiation of live-bed scour (clear-water scour limit). 

Set L Experiments

 This set of experiments is similar to set B in which Type III sediment with a D50

of 1.8 mm and a σg of 1.17 was used. However, in this set the flow depth was maintained 

at a constant value of about 0.08 m. Flow conditions for the 24 cases considered in set L 

are given in table 16. 

Set M Experiments

 As shown in table 17, 10 different experiments were performed in set M using a 

constant depth of 0.075 m. The bed material tested was Type VI sediment, which 

basically had the same D50 and σg as uniform coarse sand of Type III (1.8 mm and 1.17, 

respectively), while having the D90 (i.e. the same coarsest fraction) of the graded 

sediment mixture of Type IV. Flow conditions in this set of experiments varied from 

conditions that initiated scour to conditions that initiated bed forms. 

Set N Experiments

 This set of experiments is similar to set A, which used Type IV graded coarse 

sand. However, in these experiments the flow depth was kept at a constant value of 

0.075 m. Flow conditions pertaining to set N experiments (27 cases) are given in table 

18. 

SEDIMENTATION FLUME EXPERIMENTS

 Sixty-eight different abutment case studies were tested in the sedimentation 

flume; these studies constituted about 17 percent of the total number of abutment scour 

cases involved in this research. All the tests performed in the 2.44-m wide flume were 

conducted at a constant depth of 0.3 m. A total of eight different sediment mixtures 

were tested. Five different sand types, all having the same D50 of 0.78 mm but with 

varying gradation coefficients and ninety-percentile diameters, were selected to study 
57

sensitivity to various sediment size characteristics. The experiments are divided into two 

major sets: D and E. 

Set D Experiments 

 Set D consisted of eight different runs. As shown in table 19, four abutment 

models were tested in each of these runs. All the abutment models were 0.46 m long (in 

direction of flow) and were identified as M1, M2, M3, and M4. Abutments M1, M2, and 

M3 had a protrusion length of 0.22 m, while the protrusion length of M4 was 0.18 m. 

Abutment models M1 and M4 were used for testing an approximate log-normally graded 

medium sand mixture of D50 equal to 0.78 mm and σg equal to 2.43 (Type VIII 

sediment). Abutment M2 was designated for testing a mixture similar to that at M1, but 

with a coarser upper ten-percentile fraction (Type XI sediment). Similarly, a third 

mixture (Type X sediment), that was basically Type VIII sediment but with a coarser 

upper five-percentile fraction, was introduced at abutment M3. 

Set E Experiments

 Table 20 summarizes the 36 test conditions for the set E experiments. Four 

abutment models, which were also identified as M1, M2, M3, and M4, with protrusion 

lengths of 0.22 m and 0.45 m, were used in these experiments. In runs E-1 through E-8, 

abutment M1 was used for testing a graded medium sand mixture of D50 equal to 0.78 

mm and σg equal to 3.4 (sediment Type IX). Abutment M2 was for testing a mixture 

similar to that at M1, but with a coarser upper ten-percentile fraction (sediment Type 

XII). A uniform medium sand with a D50 of 0.78 mm and σg of 1.3 (Type II ) was 

utilized at M3. For comparison purposes, a mixture of sand and clay (70 percent sand 

and 30 percent clay) was introduced at M4. Runs E-9 and E-10 were conducted using a 

gravel mixture to examine potential variations due to median sediment size. 

RIVER MECHANICS FLUME EXPERIMENTS

 In the set F experiments, which were conducted in the 5.2-m wide river 

mechanics flume, a sediment mixture with mean diameter of 0.55 mm, a gradation 

coefficient of 2.1, and a D90 of 1.3 mm (sediment Type XV) was used. Table 21 

summarizes the experimental conditions for this series. Set F experiments were 

conducted for the qualitative description of the scour hole and related geometric features. 
58

Table 8. Experimental conditions for set A runs for abutment scour. 

MODEL FLOW APPROACH AVERAGE ABUTMENT DIVERTED SCOUR SEDIMENT

SIZE DISCHARGE DEPTH VELOCITY VELOCITY VELOCITY DEPTH TYPE

RUN 

ID

(cm x cm) Q (l/s) Y (m) Vu (m/s) Va (m/s) Vj (m/s) Ds (m)

A1 - M1 2.5x11.4 12.621 0.061 0.34 0.33 0.31 0.016 IV

A1 - M2 5.1x11.4 12.621 0.211 1.06 0.98 0.93 0.072 IV

A1 - M3 7.6x11.4 12.621 0.220 1.01 0.85 0.83 0.089 IV

A1 - M4 10.2x11.4 12.621 0.238 0.94 0.78 0.72 0.098 IV

A2 - M1 2.5x11.4 16.170 0.068 0.39 0.38 0.36 0.023 IV

A2 - M2 5.1x11.4 16.170 0.211 1.24 1.17 1.13 0.072 IV

A2 - M3 7.6x11.4 16.170 0.220 1.17 0.99 0.96 0.089 IV

A2 - M4 10.2x11.4 16.170 0.238 1.11 0.96 0.89 0.098 IV

A3 - M1 2.5x11.4 18.502 0.087 0.35 0.33 0.32 0.025 IV

A3 - M2 5.1x11.4 18.502 0.211 1.14 1.06 1.02 0.072 IV

A3 - M3 7.6x11.4 18.502 0.220 1.04 0.88 0.85 0.089 IV

A3 - M4 10.2x11.4 18.502 0.238 1.01 0.85 0.79 0.098 IV

A4 - M2 5.1x11.4 19.502 0.075 0.42 0.40 0.40 0.073 IV

A4 - M3 7.6x11.4 19.502 0.082 0.39 0.33 0.32 0.054 IV

A4 - M4 10.2x11.4 19.502 0.084 0.38 0.34 0.32 0.083 IV

A5 - M2 5.1x11.4 19.884 0.080 0.41 0.39 0.38 0.064 IV

A5 - M3 7.6x11.4 19.884 0.087 0.38 0.32 0.31 0.054 IV

A5 - M4 10.2x11.4 19.884 0.091 0.36 0.31 0.29 0.052 IV

A6 - M2 5.1x11.4 15.874 0.066 0.40 0.38 0.36 0.048 IV

A6 - M3 7.6x11.4 15.874 0.071 0.37 0.31 0.30 0.040 IV

A6 - M4 10.2x11.4 15.874 0.075 0.35 0.30 0.28 0.042 IV

A7 - M2 5.1x11.4 20.445 0.083 0.40 0.38 0.37 0.062 IV

A7 - M3 7.6x11.4 20.445 0.088 0.38 0.32 0.32 0.053 IV

A7 - M4 10.2x11.4 20.445 0.094 0.35 0.31 0.29 0.066 IV

A8 - M2 5.1x11.4 17.306 0.071 0.40 0.38 0.37 0.059 IV

A8 - M3 7.6x11.4 17.306 0.076 0.38 0.32 0.31 0.055 IV

A8 - M4 10.2x11.4 17.306 0.080 0.36 0.31 0.29 0.060 IV

A9 - M2 5.1x11.4 14.130 0.065 0.36 0.34 0.32 0.047 IV

A9 - M3 7.6x11.4 14.130 0.069 0.33 0.28 0.27 0.043 IV

A9 - M4 10.2x11.4 14.130 0.074 0.31 0.27 0.25 0.044 IV

A10 - M2 5.1x11.4 10.227 0.049 0.34 0.32 0.31 0.026 IV

A10 - M3 7.6x11.4 10.227 0.055 0.30 0.25 0.25 0.030 IV

A10 - M4 10.2x11.4 10.227 0.062 0.27 0.22 0.20 0.029 IV

A11a - M2 5.1x11.4 4.573 0.029 0.26 0.23 0.22 0.007 IV

A11a - M3 7.6x11.4 4.573 0.030 0.25 0.21 0.20 0.010 IV

A11a - M4 10.2x11.4 4.573 0.039 0.19 0.15 0.13 0.006 IV
59

Table 8. Experimental conditions for set A runs for abutment scour (continued). 

MODEL FLOW APPROACH AVERAGE ABUTMENT DIVERTED SCOUR SEDIMENT

SIZE DISCHARGE DEPTH VELOCITY VELOCITY VELOCITY DEPTH TYPE

RUN 

ID

(cm x cm) Q (l/s) Y (m) Vu (m/s) Va (m/s) Vj (m/s) Ds (m)

A11b - M2 5.1x11.4 7.232 0.039 0.30 0.28 0.26 0.015 IV

A11b - M3 7.6x11.4 7.232 0.044 0.27 0.22 0.22 0.016 IV

A11b - M4 10.2x11.4 7.232 0.050 0.24 0.19 0.17 0.010 IV

A11c - M2 5.1x11.4 20.454 0.082 0.41 0.39 0.38 0.068 IV

A11c - M3 7.6x11.4 20.454 0.086 0.39 0.33 0.32 0.062 IV

A11c - M4 10.2x11.4 20.454 0.093 0.36 0.32 0.30 0.067 IV

A15a - M2 5.1x11.4 13.790 0.102 0.22 0.20 0.18 0.010 IV

A15a - M3 7.6x11.4 13.790 0.113 0.20 0.17 0.16 0.005 IV

A15a - M4 10.2x11.4 13.790 0.121 0.19 0.14 0.13 0.005 IV

A15b - M2 5.1x11.4 13.790 0.077 0.29 0.27 0.25 0.023 IV

A15b - M3 7.6x11.4 13.790 0.087 0.26 0.22 0.21 0.008 IV

A15b - M4 10.2x11.4 13.790 0.096 0.24 0.19 0.17 0.010 IV

A16 - M2 5.1x11.4 13.529 0.057 0.39 0.37 0.36 0.049 IV

A16 - M3 7.6x11.4 13.529 0.059 0.38 0.32 0.31 0.061 IV

A16 - M4 10.2x11.4 13.529 0.062 0.36 0.32 0.30 0.080 IV

A18a - M2 5.1x11.4 15.263 0.065 0.39 0.36 0.35 0.057 IV

A18a - M3 7.6x11.4 15.263 0.067 0.37 0.31 0.31 0.047 IV

A18a - M4 10.2x11.4 15.263 0.073 0.34 0.30 0.28 0.075 IV

A18b - M2 5.1x11.4 15.574 0.068 0.37 0.35 0.34 0.043 IV

A18b - M3 7.6x11.4 15.574 0.073 0.35 0.30 0.29 0.043 IV

A18b - M4 10.2x11.4 15.574 0.080 0.32 0.27 0.26 0.049 IV

Notes:

1. Average Velocity: Mean velocity prevailing at the approach cross section. 

2. Abutment Velocity: Velocity measured along the longitudinal passing through the abutment nose at the approach 

section. 

3. Diverted Velocity: Integrated mean velocity prevailing across the deflected flow portion 

4. All experiments are performed in a 0.61-m wide flume measured at the approach section. 

5. Bed slope is set to a value of 0.0037. 
60

Table 9. Experimental conditions for set B runs for abutment scour. 

MODEL FLOW APPROACH AVERAGE ABUTMENT DIVERTED SCOUR SEDIMENT

RUN SIZE DISCHARGE DEPTH VELOCITY VELOCITY VELOCITY DEPTH TYPE

ID (cm x cm) Q (l/s) Y (m) Vu (m/s) Va (m/s) Vj (m/s) Ds (m)

B1 - M2 5.1x11.4 14.130 0.064 0.365 0.341 0.329 0.1173 III

B1 - M3 7.6x11.4 14.130 0.064 0.362 0.296 0.293 0.1295 III

B1 - M4 10.2x11.4 14.130 0.066 0.349 0.304 0.285 0.1494 III

B2 - M2 5.1x11.4 11.118 0.048 0.384 0.361 0.349 0.0832 III

B2 - M3 7.6x11.4 11.118 0.046 0.395 0.322 0.321 0.1256 III

B2 - M4 10.2x11.4 11.118 0.052 0.348 0.303 0.284 0.1347 III

B3 - M2 5.1x11.4 8.011 0.037 0.352 0.328 0.315 0.0811 III

B3 - M3 7.6x11.4 8.011 0.038 0.348 0.283 0.281 0.1140 III

B3 - M4 10.2x11.4 8.011 0.040 0.329 0.283 0.264 0.1222 III

B4 - M2 5.1x11.4 4.876 0.027 0.298 0.274 0.258 0.0472 III

B4 - M3 7.6x11.4 4.876 0.026 0.303 0.247 0.243 0.0622 III

B4 - M4 10.2x11.4 4.876 0.028 0.282 0.234 0.215 0.0634 III

B5 - M2 5.1x11.4 6.895 0.035 0.326 0.301 0.287 0.0680 III

B5 - M3 7.6x11.4 6.895 0.039 0.292 0.238 0.234 0.0716 III

B5 - M4 10.2x11.4 6.895 0.045 0.252 0.204 0.187 0.0457 III

B6 - M2 5.1x11.4 8.011 0.036 0.369 0.345 0.333 0.0872 III

B6 - M3 7.6x11.4 8.011 0.042 0.312 0.254 0.251 0.0719 III

B6 - M4 10.2x11.4 8.011 0.048 0.272 0.223 0.205 0.0515 III

B7 - M2 5.1x11.4 11.117 0.046 0.395 0.373 0.362 0.1000 III

B7 - M3 7.6x11.4 11.117 0.048 0.382 0.311 0.310 0.1207 III

B7 - M4 10.2x11.4 11.117 0.052 0.350 0.305 0.286 0.1122 III

B8 - M2 5.1x11.4 9.121 0.040 0.372 0.349 0.336 0.0899 III

B8 - M3 7.6x11.4 9.121 0.044 0.343 0.280 0.277 0.0838 III

B8 - M4 10.2x11.4 9.121 0.052 0.290 0.242 0.224 0.0796 III

B9 - M2 5.1x11.4 9.121 0.048 0.311 0.286 0.271 0.0393 VI

B9 - M3 7.6x11.4 9.121 0.051 0.294 0.240 0.236 0.0402 VI

B9 - M4 10.2x11.4 9.121 0.055 0.274 0.226 0.208 0.0244 VI

B10 - M2 5.1x11.4 11.118 0.048 0.378 0.355 0.342 0.0241 VI

B10 - M3 7.6x11.4 11.118 0.050 0.366 0.298 0.296 0.0323 VI

B10 - M4 10.2x11.4 11.118 0.052 0.352 0.308 0.288 0.0372 VI

B12 - M2 5.1x11.4 8.011 0.036 0.362 0.338 0.325 0.0896 III

B12 - M4 10.2x11.4 8.011 0.050 0.264 0.215 0.197 0.0664 III

B13 - M2 5.1x11.4 33.980 0.120 0.463 0.445 0.440 0.1167 III

B13 - M4 10.2x11.4 33.980 0.152 0.366 0.323 0.304 0.1149 III
61

Table 9. Experimental conditions for set B runs for abutment scour (continued). 

MODEL FLOW APPROACH AVERAGE ABUTMENT DIVERTED SCOUR SEDIMENT

RUN SIZE DISCHARGE DEPTH VELOCITY VELOCITY VELOCITY DEPTH TYPE

ID (cm x cm) Q (l/s) Y (m) Vu (m/s) Va (m/s) Vj (m/s) Ds (m)

B14 - M2 5.1x11.4 9.504 0.042 0.368 0.345 0.332 0.078 III

B14 - M3 7.6x11.4 9.504 0.052 0.297 0.242 0.238 0.068 III

B14 - M4 10.2x11.4 9.504 0.058 0.269 0.221 0.203 0.076 III

B15 - M2 5.1x11.4 7.862 0.048 0.271 0.247 0.231 0.027 III

B15 - M3 7.6x11.4 7.862 0.055 0.235 0.192 0.187 0.023 III

B15 - M4 10.2x11.4 7.862 0.062 0.206 0.160 0.145 0.020 III

B16 - M2 5.1x11.4 11.742 0.054 0.359 0.335 0.322 0.091 III

B16 - M3 7.6x11.4 11.742 0.056 0.345 0.282 0.279 0.117 III

B16 - M4 10.2x11.4 11.742 0.063 0.304 0.256 0.237 0.126 III

Notes:

1. Average Velocity: Mean velocity prevailing at the approach cross section. 

2. Abutment Velocity: Velocity measured along the longitudinal passing through the abutment nose at the approach 

section. 

3. Diverted Velocity: Integrated mean velocity prevailing across the deflected flow portion. 

4. All experiments are performed in a 0.61-m wide flume measured at the approach section. 

5. Bed slope is set to a value of 0.0037. 
62

Table 10. Experimental conditions for set C runs for abutment scour.

MODEL FLOW APPROACH AVERAGE ABUTMENT DIVERTED SCOUR SEDIMENT

RUN ID SIZE DISCHARGE DEPTH VELOCITY VELOCITY VELOCITY DEPTH TYPE

(cm x cm) Q (l/s) Y (m) Vu (m/s) Va (m/s) Vj (m/s) Ds (m)

 C2 - M2 5.1x11.4 7.553 0.039 0.318 0.293 0.278 0.003 V

C2 - M3 7.6x11.4 7.553 0.042 0.297 0.242 0.238 0.004 V

C2 - M4 10.2x11.4 7.553 0.046 0.270 0.221 0.204 0.005 V

C3 - M2 5.1x11.4 17.306 0.071 0.402 0.380 0.370 0.024 V

C3 - M3 7.6x11.4 17.306 0.075 0.381 0.311 0.309 0.038 V

C3 - M4 10.2x11.4 17.306 0.078 0.362 0.319 0.299 0.038 V

C4 - M2 5.1x11.4 4.876 0.033 0.243 0.220 0.204 0.003 V

C4 - M3 7.6x11.4 4.876 0.035 0.228 0.186 0.181 0.003 V

C4 - M4 10.2x11.4 4.876 0.038 0.210 0.164 0.148 0.005 V

C5 - M2 5.1x11.4 20.454 0.084 0.401 0.379 0.369 0.026 V

C5 - M3 7.6x11.4 20.454 0.086 0.392 0.321 0.319 0.033 V

C5 - M4 10.2x11.4 20.454 0.090 0.374 0.332 0.313 0.045 V

C6 - M2 5.1x11.4 14.298 0.062 0.375 0.352 0.340 0.010 V

C6 - M3 7.6x11.4 14.298 0.070 0.335 0.273 0.270 0.011 V

C6 - M4 10.2x11.4 14.298 0.076 0.310 0.263 0.244 0.022 V

C7 - M2 5.1x11.4 21.912 0.081 0.443 0.424 0.417 0.039 V

C7 - M3 7.6x11.4 21.912 0.087 0.414 0.339 0.338 0.032 V

C7 - M4 10.2x11.4 21.912 0.093 0.388 0.348 0.329 0.039 V

C8 - M2 5.1x11.4 19.502 0.075 0.428 0.408 0.400 0.032 V

C8 - M3 7.6x11.4 19.502 0.079 0.405 0.331 0.330 0.035 V

C8 - M4 10.2x11.4 19.502 0.082 0.390 0.351 0.332 0.036 V

Notes:

1. Average Velocity: Mean velocity prevailing at the approach cross section. 

2. Abutment Velocity: Approach velocity measured along the longitudinal passing through the abutment nose. 

3. Diverted Velocity: Integrated mean velocity prevailing across the deflected flow portion measured at the approach 

 section. 

4. All experiments are performed in a 0.61-m wide flume.
63

Table 11. Experimental conditions for set G runs for abutment scour.

MODEL FLOW APPROACH AVERAGE ABUTMENT DIVERTED SCOUR SEDIMENT

RUN ID SIZE DISCHARGE DEPTH VELOCITY VELOCITY VELOCITY DEPTH TYPE

(cm x cm) Q (l/s) Y (m) Vu (m/s) Va (m/s) Vj (m/s) Ds (m)

 G1 - M2 5.1x11.4 7.552 0.076 0.163 0.144 0.130 0.006 II

G1 - M3 7.6x11.4 7.553 0.086 0.144 0.118 0.113 0.005 II

G1 - M4 10.2x11.4 7.553 0.092 0.135 0.099 0.087 0.001 II

G2 - M2 5.1x11.4 8.990 0.083 0.178 0.158 0.143 0.020 II

G2 - M3 7.6x11.4 8.990 0.094 0.158 0.129 0.124 0.022 II

G2 - M4 10.2x11.4 8.990 0.098 0.151 0.112 0.099 0.012 II

G3 - M2 5.1x11.4 9.751 0.075 0.214 0.192 0.176 0.047 II

G3 - M3 7.6x11.4 9.751 0.082 0.195 0.160 0.154 0.042 II

G3 - M4 10.2x11.4 9.751 0.092 0.173 0.131 0.117 0.030 II

G4 - M2 5.1x11.4 11.010 0.073 0.246 0.222 0.207 0.079 II

G4 - M3 7.6x11.4 11.010 0.086 0.210 0.172 0.167 0.068 II

G4 - M4 10.2x11.4 11.010 0.092 0.197 0.152 0.137 0.046 II

G5 - M2 5.1x11.4 11.941 0.076 0.258 0.234 0.218 0.084 II

G5 - M3 7.6x11.4 11.943 0.089 0.220 0.180 0.175 0.077 II

G5 - M4 10.2x11.4 11.943 0.095 0.207 0.161 0.145 0.049 II

G6 - M2 5.1x11.4 12.884 0.080 0.265 0.241 0.225 0.098 II

G6 - M3 7.6x11.4 12.884 0.093 0.227 0.186 0.181 0.093 II

G6 - M4 10.2x11.4 12.884 0.096 0.221 0.174 0.158 0.066 II

G7 - M2 5.1x11.4 14.130 0.076 0.304 0.280 0.265 0.113 II

G7 - M3 7.6x11.4 14.130 0.089 0.260 0.213 0.207 0.117 II

G7 - M4 10.2x11.4 14.130 0.091 0.253 0.205 0.188 0.119 II

Notes:

1. Average Velocity: Mean velocity prevailing at the approach cross section. 

2. Abutment Velocity: Approach velocity measured along the longitudinal passing through the abutment nose. 

3. Diverted Velocity: Integrated mean velocity prevailing across the deflected flow portion measured at the approach 

 section. 

4. All experiments are performed in a 0.61-m wide flume.
64

Table 12. Experimental conditions for set H runs for abutment scour. 

MODEL FLOW APPROACH AVERAGE ABUTMENT DIVERTED SCOUR SEDIMENT

RUN # SIZE DISCHARGE DEPTH VELOCITY VELOCITY VELOCITY DEPTH TYPE

(cm x cm) Q (l/s) Y (m) Vu (m/s) Va (m/s) Vj (m/s) Ds (m)

H1 - M2 5.1x11.4 8.155 0.119 0.112 0.098 0.087 0.008 I

H1 - M3 7.6x11.4 8.155 0.123 0.108 0.089 0.085 0.002 I

H1 - M4 10.2x11.4 8.155 0.129 0.104 0.074 0.064 0.003 I

H2 - M2 5.1x11.4 10.228 0.116 0.144 0.127 0.114 0.026 I

H2 - M3 7.6x11.4 10.228 0.121 0.139 0.114 0.109 0.035 I

H2 - M4 10.2x11.4 10.228 0.125 0.134 0.098 0.086 0.036 I

H3 - M2 5.1x11.4 11.941 0.119 0.164 0.145 0.131 0.040 I

H3 - M3 7.6x11.4 11.941 0.123 0.159 0.130 0.125 0.047 I

H3 - M4 10.2x11.4 11.941 0.128 0.153 0.114 0.101 0.048 I

H4 - M2 5.1x11.4 14.464 0.122 0.195 0.173 0.158 0.060 I

H4 - M3 7.6x11.4 14.464 0.126 0.188 0.155 0.149 0.064 I

H4 - M4 10.2x11.4 14.464 0.129 0.184 0.140 0.125 0.084 I

H5 - M2 5.1x11.4 16.605 0.122 0.224 0.201 0.186 0.087 I

H5 - M3 7.6x11.4 16.605 0.124 0.220 0.181 0.175 0.105 I

H5 - M4 10.2x11.4 16.605 0.128 0.213 0.167 0.151 0.125 I

H6 - M2 5.1x11.4 17.913 0.122 0.241 0.218 0.202 0.105 I

H6 - M3 7.6x11.4 17.913 0.124 0.237 0.195 0.189 0.125 I

H6 - M4 10.2x11.4 17.913 0.127 0.232 0.184 0.167 0.132 I

H7 - M2 5.1x11.4 19.502 0.122 0.262 0.238 0.222 0.119 I

H7 - M3 7.6x11.4 19.502 0.123 0.259 0.213 0.208 0.129 I

H7 - M4 10.2x11.4 19.502 0.126 0.254 0.205 0.188 0.139 I

H8 - M2 5.1x11.4 25.683 0.125 0.336 0.312 0.298 0.115 I

H8 - M3 7.6x11.4 25.683 0.126 0.333 0.274 0.270 0.129 I

H8 - M4 10.2x11.4 25.683 0.129 0.326 0.279 0.260 0.140 I

Notes: 

1. Average Velocity: Mean velocity prevailing at the approach cross section. 

2. Abutment Velocity: Approach velocity measured along the longitudinal passing through the abutment nose. 

3. Diverted Velocity: Integrated mean velocity prevailing across the deflected flow portion measured at the approach 

 section. 

4. All experiments are performed in a 0.61-m wide flume.
65

Table 13. Experimental conditions for set I runs for abutment scour. 

MODEL FLOW APPROACH AVERAGE ABUTMENT DIVERTED SCOUR SEDIMENT

RUN ID SIZE DISCHARGE DEPTH VELOCITY VELOCITY VELOCITY DEPTH TYPE

(cm x cm) Q (l/s) Y (m) Vu (m/s) Va (m/s) Vj (m/s) Ds (m)

I1 - M2 5.1 x 11.4 6.881 0.071 0.158 0.140 0.126 0.0393 I 

I1 - M3 7.6 x 11.4 6.881 0.076 0.148 0.121 0.116 0.0299 I 

I1 - M4 10.2 x 11.4 6.881 0.080 0.140 0.103 0.091 0.0335 I 

I2 - M2 5.1 x 11.4 8.438 0.073 0.190 0.169 0.154 0.0594 I 

I2 - M3 7.6 x 11.4 8.438 0.076 0.182 0.149 0.143 0.0524 I 

I2 - M4 10.2 x 11.4 8.438 0.080 0.173 0.130 0.116 0.0640 I 

I3 - M2 5.1 x 11.4 9.990 0.077 0.213 0.190 0.175 0.0914 I 

I3 - M3 7.6 x 11.4 9.990 0.079 0.208 0.170 0.165 0.1052 I 

I3 - M4 10.2 x 11.4 9.990 0.080 0.204 0.158 0.143 0.1219 I 

I4 - M2 5.1 x 11.4 11.330 0.075 0.249 0.225 0.209 0.1082 I 

I4 - M3 7.6 x 11.4 11.330 0.075 0.249 0.203 0.198 0.1210 I 

I4 - M4 10.2 x 11.4 11.330 0.075 0.247 0.199 0.181 0.1362 I 

I5 - M2 5.1 x 11.4 18.501 0.076 0.398 0.376 0.365 0.1167 I 

I5 - M3 7.6 x 11.4 18.501 0.077 0.395 0.323 0.322 0.1356 I 

I5 - M4 10.2 x 11.4 18.501 0.077 0.392 0.353 0.334 0.1402 I 

I6 - M2 5.1 x 11.4 5.768 0.075 0.127 0.111 0.099 0.0046 I 

I6 - M3 7.6 x 11.4 5.768 0.079 0.119 0.098 0.093 0.0027 I 

I6 - M4 10.2 x 11.4 5.768 0.081 0.117 0.084 0.074 0.0009 I 

Notes:

1. Average Velocity: Mean velocity prevailing at the approach cross section. 

2. Abutment Velocity: Approach velocity measured along the longitudinal passing through the abutment nose. 

3. Diverted Velocity: Integrated mean velocity across the deflected flow portion measured at the approach section. 

4. All experiments are performed in a 0.61-m wide flume. 

5. Bed slope is set to a value of 0.0017. 
66

Table 14. Experimental conditions for set J runs for abutment scour. 

MODEL FLOW APPROACH AVERAGE ABUTMENT DIVERTED SCOUR SEDIMENT

RUN ID SIZE DISCHARGE DEPTH VELOCITY VELOCITY VELOCITY DEPTH TYPE

 (cm x cm) Q (l/s) Y (m) Vu (m/s) Va (m/s) Vj (m/s) Ds (m)

 J1 - M2 5.1 x 11.4 3.786 0.045 0.137 0.120 0.107 0.0162 I

J1 - M3 7.6 x 11.4 3.786 0.048 0.128 0.104 0.100 0.0216 I

J1 - M4 10.2 x 11.4 3.786 0.053 0.118 0.085 0.074 0.0104 I

J2 - M2 5.1 x 11.4 5.295 0.054 0.160 0.141 0.128 0.0460 I

J2 - M3 7.6 x 11.4 5.295 0.054 0.162 0.132 0.127 0.0747 I

J2 - M4 10.2 x 11.4 5.295 0.055 0.158 0.118 0.105 0.0829 I

J3 - M2 5.1 x 11.4 6.711 0.052 0.210 0.188 0.173 0.0823 I

J3 - M3 7.6 x 11.4 6.711 0.050 0.222 0.181 0.176 0.0948 I

J3 - M4 10.2 x 11.4 6.711 0.049 0.226 0.178 0.162 0.1076 I

J4 - M2 5.1 x 11.4 7.561 0.051 0.245 0.222 0.206 0.0969 I

J4 - M3 7.6 x 11.4 7.561 0.047 0.266 0.217 0.212 0.1024 I

J4 - M4 10.2 x 11.4 7.561 0.045 0.275 0.226 0.208 0.1158 I

J5 - M2 5.1 x 11.4 10.449 0.059 0.290 0.266 0.250 0.1052 I

J5 - M3 7.6 x 11.4 10.449 0.055 0.311 0.253 0.250 0.1036 I

J5 - M4 10.2 x 11.4 10.449 0.054 0.320 0.272 0.254 0.1234 I

J6 - M2 5.1 x 11.4 3.455 0.045 0.126 0.110 0.098 0.0021 I

J6 - M3 7.6 x 11.4 3.455 0.049 0.116 0.095 0.091 0.0009 I

J6 - M4 10.2 x 11.4 3.455 0.049 0.116 0.083 0.073 0.0003 I

Notes:

1. Average Velocity: Mean velocity prevailing at the approach cross section. 

2. Abutment Velocity: Velocity measured along the longitudinal passing through the abutment nose at the approach 

section. 

3. Diverted Velocity: Integrated mean velocity prevailing across the deflected flow portion. 

4. All experiments are performed in a 0.61-m wide flume measured at the approach section. 

5. Bed slope is set to a value of 0.0017. 
67

Table 15. Experimental conditions for set K runs for abutment scour. 

MODEL FLOW APPROACH AVERAGE ABUTMENT DIVERTED SCOUR SEDIMENT

RUN SIZE DISCHARGE DEPTH VELOCITY VELOCITY VELOCITY DEPTH TYPE

ID (cm x cm) Q (l/s) Y (m) Vu (m/s) Va (m/s) Vj (m/s) Ds (m)

K1 - M2 5.1 x 11.4 6.895 0.073 0.155 0.136 0.123 0.0021 VII

K1 - M3 7.6 x 11.4 6.895 0.076 0.148 0.121 0.117 0.0009 VII

K1 - M4 10.2 x 11.4 6.895 0.081 0.140 0.102 0.090 0.0006 VII

K2 - M2 5.1 x 11.4 8.444 0.072 0.193 0.172 0.157 0.0128 VII

K2 - M3 7.6 x 11.4 8.444 0.075 0.185 0.151 0.146 0.0043 VII

K2 - M4 10.2 x 11.4 8.444 0.080 0.174 0.132 0.118 0.0034 VII

K3 - M2 5.1 x 11.4 9.992 0.079 0.208 0.186 0.170 0.0287 VII

K3 - M3 7.6 x 11.4 9.992 0.078 0.209 0.171 0.166 0.0250 VII

K3 - M4 10.2 x 11.4 9.992 0.080 0.204 0.158 0.143 0.0277 VII

K4 - M2 5.1 x 11.4 11.640 0.074 0.257 0.233 0.217 0.0491 VII

K4 - M3 7.6 x 11.4 11.640 0.073 0.261 0.213 0.208 0.0631 VII

K4 - M4 10.2 x 11.4 11.640 0.073 0.262 0.214 0.196 0.0728 VII

K5 - M2 5.1 x 11.4 12.898 0.073 0.290 0.266 0.251 0.0762 VII

K5 - M3 7.6 x 11.4 12.898 0.072 0.293 0.239 0.235 0.0741 VII

K5 - M4 10.2 x 11.4 12.898 0.073 0.292 0.243 0.225 0.0860 VII

K6 - M2 5.1 x 11.4 14.626 0.076 0.316 0.292 0.277 0.0835 VII

K6 - M3 7.6 x 11.4 14.626 0.074 0.323 0.264 0.260 0.0933 VII

K6 - M4 10.2 x 11.4 14.626 0.074 0.325 0.279 0.260 0.1149 VII

K7 - M2 5.1 x 11.4 16.889 0.078 0.356 0.333 0.319 0.0884 VII

K7 - M3 7.6 x 11.4 16.889 0.075 0.370 0.302 0.300 0.1012 VII

K7 - M4 10.2 x 11.4 16.889 0.075 0.368 0.326 0.306 0.1244 VII

Notes:

1. Average Velocity: Mean velocity prevailing at the approach cross section. 

2. Abutment Velocity: Velocity measured along the longitudinal passing through the abutment nose at the approach 

section. 

3. Diverted Velocity: Integrated mean velocity prevailing across the deflected flow portion. 

4. All experiments are performed in a 0.61-m wide flume measured at the approach section. 

5. Bed slope is set to a value of 0.0017. 
68

Table 16. Experimental conditions for set L runs for abutment scour. 

MODEL FLOW APPROACH AVERAGE ABUTMENT DIVERTED SCOUR SEDIMENT

RUN SIZE DISCHARGE DEPTH VELOCITY VELOCITY VELOCITY DEPTH TYPE

ID (cm x cm) Q (l/s) Y (m) Vu (m/s) Va (m/s) Vj (m/s) Ds (m)

L1 - M2 5.1 x 11.4 11.327 0.075 0.249 0.225 0.209 0.0290 III

L1 - M3 7.6 x 11.4 11.327 0.077 0.241 0.197 0.192 0.0283 III

L1 - M4 10.2 x 11.4 11.327 0.080 0.234 0.186 0.169 0.0302 III

L2 - M2 5.1 x 11.4 15.106 0.077 0.323 0.298 0.284 0.0991 III

L2 - M3 7.6 x 11.4 15.106 0.076 0.325 0.266 0.262 0.1015 III

L2 - M4 10.2 x 11.4 15.106 0.077 0.324 0.277 0.258 0.1082 III

L3 - M2 5.1 x 11.4 16.888 0.079 0.350 0.326 0.312 0.1082 III

L3 - M3 7.6 x 11.4 16.888 0.079 0.352 0.288 0.285 0.1109 III

L3 - M4 10.2 x 11.4 16.888 0.077 0.359 0.316 0.297 0.1222 III

L4 - M2 5.1 x 11.4 18.882 0.081 0.381 0.358 0.346 0.1155 III

L4 - M3 7.6 x 11.4 18.883 0.077 0.400 0.327 0.326 0.1183 III

L4 - M4 10.2 x 11.4 18.882 0.078 0.399 0.361 0.342 0.1433 III

L5 - M2 5.1 x 11.4 20.453 0.084 0.402 0.380 0.369 0.1201 III

L5 - M3 7.6 x 11.4 20.453 0.080 0.422 0.345 0.345 0.1253 III

L5 - M4 10.2 x 11.4 20.453 0.078 0.432 0.400 0.382 0.1420 III

L6 - M2 5.1 x 11.4 9.992 0.073 0.226 0.203 0.188 0.0027 III

L6 - M3 7.6 x 11.4 9.992 0.075 0.219 0.179 0.174 0.0018 III

L6 - M4 10.2 x 11.4 9.992 0.078 0.211 0.165 0.149 0.0015 III

L7 - M2 5.1 x 11.4 9.251 0.073 0.207 0.186 0.170 0.0003 III

L7 - M3 7.6 x 11.4 9.251 0.076 0.201 0.164 0.159 0.0003 III

L7 - M4 10.2 x 11.4 9.251 0.078 0.194 0.149 0.134 0.0003 III

L8 - M2 5.1 x 11.4 13.790 0.078 0.291 0.267 0.251 0.0692 III

L8 - M3 7.6 x 11.4 13.790 0.078 0.289 0.236 0.232 0.0732 III

L8 - M4 10.2 x 11.4 13.790 0.079 0.285 0.237 0.219 0.0741 III

Notes:

1. Average Velocity: Mean velocity prevailing at the approach cross section. 

2. Abutment Velocity: Velocity measured along the longitudinal passing through the abutment nose at the approach 

section. 

3. Diverted Velocity: Integrated mean velocity prevailing across the deflected flow portion. 

4. All experiments are performed in a 0.61-m wide flume measured at the approach section. 

5. Bed slope is set to a value of 0.0017. 
69

Table 17. Experimental conditions for set M runs for abutment scour. 

MODEL FLOW APPROACH AVERAGE ABUTMENT DIVERTED SCOUR SEDIMENT

RUN SIZE DISCHARGE DEPTH VELOCITY VELOCITY VELOCITY DEPTH TYPE

ID (cm x cm) Q (l/s) Y (m) Vu (m/s) Va (m/s) Vj (m/s) Ds (m)

M1 - M2 5.1 x 11.4 13.441 0.075 0.293 0.269 0.253 0.0232 VI

M1 - M3 7.6 x 11.4 13.441 0.077 0.287 0.235 0.230 0.0238 VI

M1 - M4 10.2 x 11.4 13.441 0.079 0.279 0.231 0.213 0.0344 VI

M2 - M2 5.1 x 11.4 14.627 0.075 0.320 0.296 0.281 0.0283 VI

M2 - M3 7.6 x 11.4 14.627 0.075 0.320 0.262 0.258 0.0280 VI

M2 - M4 10.2 x 11.4 14.627 0.076 0.317 0.270 0.251 0.0421 VI

M3 - M2 5.1 x 11.4 15.571 0.075 0.341 0.317 0.303 0.0293 VI

M3 - M3 7.6 x 11.4 15.571 0.076 0.338 0.276 0.273 0.0344 VI

M3 - M4 10.2 x 11.4 15.571 0.077 0.333 0.286 0.267 0.0546 VI

M4 - M2 5.1 x 11.4 17.169 0.076 0.373 0.349 0.337 0.0479 VI

M4 - M3 7.6 x 11.4 17.169 0.075 0.377 0.308 0.306 0.0515 VI

M4 - M4 10.2 x 11.4 17.169 0.076 0.371 0.329 0.310 0.0762 VI

M5 - M2 5.1 x 11.4 17.714 0.077 0.378 0.355 0.343 0.0527 VI

M5 - M3 7.6 x 11.4 17.714 0.076 0.384 0.314 0.312 0.0594 VI

M5 - M4 10.2 x 11.4 17.714 0.076 0.384 0.344 0.325 0.0866 VI

M6 - M2 5.1 x 11.4 18.372 0.077 0.389 0.367 0.355 0.0588 VI

M6 - M3 7.6 x 11.4 18.372 0.076 0.396 0.323 0.322 0.0671 VI

M6 - M4 10.2 x 11.4 18.372 0.076 0.399 0.361 0.342 0.0887 VI

M7 - M2 5.1 x 11.4 19.502 0.079 0.407 0.385 0.375 0.0728 VI

M7 - M3 7.6 x 11.4 19.502 0.077 0.413 0.338 0.337 0.0744 VI

M7 - M4 10.2 x 11.4 19.502 0.077 0.415 0.380 0.361 0.1113 VI

M8 - M2 5.1 x 11.4 20.685 0.079 0.430 0.410 0.401 0.0789 VI

M8 - M3 7.6 x 11.4 20.685 0.077 0.438 0.358 0.359 0.0796 VI

M8 - M4 10.2 x 11.4 20.685 0.074 0.456 0.431 0.412 0.1177 VI

M9 - M2 5.1 x 11.4 22.128 0.084 0.433 0.413 0.405 0.0853 VI

M9 - M3 7.6 x 11.4 22.128 0.082 0.444 0.364 0.364 0.0856 VI

M9 - M4 10.2 x 11.4 22.128 0.081 0.449 0.422 0.404 0.1228 VI

M10 - M2 5.1 x 11.4 11.327 0.076 0.245 0.221 0.206 0.0037 VI

M10 - M3 7.6 x 11.4 11.327 0.077 0.242 0.198 0.193 0.0030 VI

M10 - M4 10.2 x 11.4 11.327 0.079 0.235 0.188 0.171 0.0055 VI

Notes:

1. Average Velocity: Mean velocity prevailing at the approach cross section. 

2. Abutment Velocity: Velocity measured along the longitudinal passing through the abutment nose at the approach 

section. 

3. Diverted Velocity: Integrated mean velocity prevailing across the deflected flow portion. 

4. All experiments are performed in a 0.61-m wide flume measured at the approach section. 

5. Bed slope is set to a value of 0.0017. 
70

Table 18. Experimental conditions for set N runs for abutment scour. 

MODEL FLOW APPROACH AVERAGE ABUTMENT DIVERTED SCOUR SEDIMENT

RUN ID SIZE DISCHARGE DEPTH VELOCITY VELOCITY VELOCITY DEPTH TYPE

(cm x cm) Q (l/s) Y (m) Vu (m/s) Va (m/s) Vj (m/s) Ds (m)

N1 - M2 5.1 x 11.4 13.441 0.075 0.294 0.270 0.254 0.0098 IV

N1 - M3 7.6 x 11.4 13.441 0.076 0.289 0.237 0.232 0.0079 IV

N1 - M4 10.2 x 11.4 13.441 0.080 0.277 0.229 0.210 0.0158 IV

N2 - M2 5.1 x 11.4 14.463 0.075 0.315 0.291 0.276 0.0207 IV

N2 - M3 7.6 x 11.4 14.463 0.076 0.311 0.255 0.250 0.0174 IV

N2 - M4 10.2 x 11.4 14.463 0.078 0.303 0.255 0.236 0.0213 IV

N3 - M2 5.1 x 11.4 15.416 0.077 0.331 0.306 0.292 0.0235 IV

N3 - M3 7.6 x 11.4 15.416 0.076 0.332 0.271 0.268 0.0283 IV

N3 - M4 10.2 x 11.4 15.416 0.077 0.327 0.280 0.261 0.0360 IV

N4 - M2 5.1 x 11.4 17.029 0.078 0.358 0.334 0.321 0.0338 IV

N4 - M3 7.6 x 11.4 17.029 0.077 0.362 0.296 0.294 0.0347 IV

N4 - M4 10.2 x 11.4 17.029 0.077 0.364 0.321 0.302 0.0610 IV

N5 - M2 5.1 x 11.4 18.372 0.078 0.386 0.364 0.352 0.0488 IV

N5 - M3 7.6 x 11.4 18.372 0.076 0.397 0.325 0.323 0.0619 IV

N5 - M4 10.2 x 11.4 18.372 0.074 0.407 0.371 0.352 0.0914 IV

N6 - M2 5.1 x 11.4 19.502 0.079 0.407 0.385 0.375 0.0527 IV

N6 - M3 7.6 x 11.4 19.502 0.077 0.418 0.342 0.342 0.0689 IV

N6 - M4 10.2 x 11.4 19.502 0.077 0.416 0.382 0.363 0.0963 IV

N7 - M2 5.1 x 11.4 20.685 0.080 0.423 0.403 0.394 0.0619 IV

N7 - M3 7.6 x 11.4 20.685 0.078 0.435 0.356 0.356 0.0728 IV

N7 - M4 10.2 x 11.4 20.685 0.078 0.437 0.406 0.388 0.1045 IV

N8 - M2 5.1 x 11.4 22.128 0.085 0.427 0.406 0.398 0.0689 IV

N8 - M3 7.6 x 11.4 22.128 0.082 0.443 0.362 0.363 0.0789 IV

N8 - M4 10.2 x 11.4 22.128 0.080 0.456 0.431 0.413 0.1128 IV

N9 - M2 5.1 x 11.4 11.740 0.075 0.257 0.233 0.217 0.0040 IV

N9 - M3 7.6 x 11.4 11.740 0.076 0.253 0.207 0.202 0.0027 IV

N9 - M4 10.2 x 11.4 11.740 0.079 0.243 0.195 0.178 0.0021 IV

Notes:

1. Average Velocity: Mean velocity prevailing at the approach cross section. 

2. Abutment Velocity: Approach velocity measured along the longitudinal passing through the abutment nose. 

3. Diverted Velocity: Integrated mean velocity prevailing across the deflected flow portion. 

4. All experiments are performed in a 0.61-m wide flume measured at the approach section. 

5. Bed slope is set to a value of 0.0017. 
71

Table 19. Experimental conditions for set D runs for abutment scour. 

MODEL FLOW APPROACH AVERAGE ABUTMENT DIVERTED SCOUR SEDIMENT

RUN SIZE DISCHARGE DEPTH VELOCITY VELOCITY VELOCITY DEPTH TYPE

ID (cm x cm) Q (l/s) Y (m) Vu (m/s) Va (m/s) Vj (m/s) Ds (m)

D1 - M1 21.6 x 45.7 184.060 0.323 0.233 0.236 0.235 0.0579 VIII

D1 - M2 21.6 x 45.7 184.060 0.326 0.231 0.206 0.195 0.0320 XI

D1 - M3 21.6 x 45.7 184.060 0.326 0.231 0.195 0.192 0.0323 X

D1 - M4 17.8 x 45.7 184.060 0.326 0.231 0.175 0.161 0.0253 VIII

D2 - M1 21.6 x 45.7 176.980 0.324 0.224 0.222 0.216 0.0472 VIII

D2 - M2 21.6 x 45.7 176.980 0.326 0.223 0.191 0.189 0.0122 XI

D2 - M3 21.6 x 45.7 176.980 0.326 0.223 0.186 0.180 0.0137 X

D2 - M4 17.8 x 45.7 176.980 0.317 0.229 0.172 0.155 0.0198 VIII

D3 - M1 21.6 x 45.7 198.218 0.308 0.264 0.247 0.244 0.0625 VIII

D3 - M2 21.6 x 45.7 198.218 0.309 0.263 0.219 0.204 0.0140 XI

D3 - M3 21.6 x 45.7 198.218 0.314 0.259 0.201 0.189 0.1234 X

D3 - M4 17.8 x 45.7 198.218 0.316 0.257 0.183 0.172 0.0256 VIII

D4 - M1 21.6 x 45.7 229.366 0.317 0.296 0.267 0.265 0.0686 VIII

D4 - M2 21.6 x 45.7 229.366 0.321 0.293 0.234 0.219 0.0204 XI

D4 - M3 21.6 x 45.7 229.366 0.323 0.291 0.219 0.213 0.0344 X

D4 - M4 17.8 x 45.7 229.366 0.326 0.288 0.202 0.192 0.0445 VIII

D5 - M1 21.6 x 45.7 257.117 0.324 0.325 0.300 0.299 0.1103 VIII

D5 - M2 21.6 x 45.7 257.117 0.331 0.319 0.265 0.250 0.0299 XI

D5 - M3 21.6 x 45.7 257.117 0.334 0.316 0.255 0.244 0.0485 X

D5 - M4 17.8 x 45.7 257.117 0.325 0.325 0.249 0.216 0.0564 VIII

D6 - M1 21.6 x 45.7 295.628 0.327 0.371 0.331 0.329 0.1692 VIII

D6 - M2 21.6 x 45.7 295.628 0.331 0.367 0.299 0.277 0.0451 XI

D6 - M3 21.6 x 45.7 295.628 0.329 0.369 0.289 0.274 0.0671 X

D6 - M4 17.8 x 45.7 295.628 0.327 0.371 0.265 0.245 0.0817 VIII

D7 - M1 21.6 x 45.7 242.392 0.327 0.304 0.272 0.271 0.0866 VIII

D7 - M2 21.6 x 45.7 242.392 0.333 0.299 0.241 0.229 0.0320 XI

D7 - M3 21.6 x 45.7 242.392 0.336 0.296 0.231 0.223 0.0421 X

D7 - M4 17.8 x 45.7 242.392 0.336 0.296 0.219 0.202 0.0415 VIII

D8 - M1 21.6 x 45.7 144.416 0.311 0.191 0.189 0.183 0.0204 VIII

D8 - M2 21.6 x 45.7 144.416 0.311 0.190 0.165 0.152 0.0024 XI

D8 - M3 21.6 x 45.7 144.416 0.312 0.190 0.158 0.149 0.0076 X

D8 - M4 17.8 x 45.7 144.416 0.312 0.190 0.146 0.131 0.0058 VIII

Notes:

1. Average Velocity: Mean velocity prevailing at the approach cross section. 

2. Abutment Velocity: Velocity measured along the longitudinal passing through the abutment nose at the approach 

section. 

3. Diverted Velocity: Integrated mean velocity prevailing across the deflected flow portion. 

4. All experiments are performed in a 2.44-m wide flume measured at the approach section. 

5. Bed slope is set to a value of 0.0007. 
72

Table 20. Experimental conditions for set E runs for abutment scour. 

MODEL FLOW APPROACH AVERAGE ABUTMENT DIVERTED SCOUR SEDIMENT

RUN ID SIZE DISCHARGE DEPTH VELOCITY VELOCITY VELOCITY DEPTH TYPE

(cm x cm) Q (l/s) Y (m) Vu (m) Va (m) Vj (m/s) Ds (m)

E1 - M1 21.6 x 45.7 147.25 0.279 0.216 0.206 0.204 0.0189 IX

E1 - M2 21.6 x 45.7 147.25 0.284 0.213 0.182 0.173 0.0168 XII

E1 - M3 21.6 x 45.7 147.25 0.292 0.207 0.166 0.155 0.0320 II

E1 - M4 21.6 x 45.7 147.25 0.283 0.213 0.152 0.136 0.0213 XIII

E2 - M1 21.6 x 45.7 172.73 0.305 0.232 0.236 0.234 0.0317 IX

E2 - M2 21.6 x 45.7 172.73 0.297 0.238 0.202 0.188 0.0213 XII

E2 - M3 21.6 x 45.7 172.73 0.293 0.242 0.183 0.178 0.0774 II

E2 - M4 21.6 x 45.7 172.73 0.299 0.237 0.155 0.155 0.0579 XIII

E3 - M1 21.6 x 45.7 202.47 0.293 0.283 0.268 0.265 0.0152 IX

E3 - M2 21.6 x 45.7 202.47 0.299 0.278 0.230 0.223 0.0229 XII

E3 - M3 21.6 x 45.7 202.47 0.305 0.272 0.212 0.204 0.0844 II

E3 - M4 21.6 x 45.7 202.47 0.301 0.276 0.193 0.168 0.0457 XIII

E4 - M1 21.6 x 45.7 232.20 0.294 0.324 0.300 0.297 0.0360 IX

E4 - M2 21.6 x 45.7 232.20 0.300 0.317 0.260 0.248 0.0256 XII

E4 - M3 21.6 x 45.7 232.20 0.305 0.312 0.243 0.233 0.0957 II

E4 - M4 21.6 x 45.7 232.20 0.299 0.319 0.213 0.173 0.0579 XIII

E5 - M1 21.6 x 45.7 263.35 0.299 0.362 0.332 0.329 0.0500 IX

E5 - M2 21.6 x 45.7 263.35 0.303 0.357 0.298 0.286 0.0399 XII

E5 - M3 21.6 x 45.7 263.35 0.313 0.345 0.272 0.267 0.1481 II

E5 - M4 21.6 x 45.7 263.35 0.301 0.359 0.251 0.223 0.0671 XIII

E6 - M1 21.6 x 45.7 311.49 0.293 0.437 0.416 0.417 0.1094 IX

E6 - M2 21.6 x 45.7 311.49 0.298 0.429 0.356 0.337 0.0884 XII

E6 - M3 21.6 x 45.7 311.49 0.307 0.417 0.320 0.292 0.2060 II

E6 - M4 21.6 x 45.7 311.49 0.287 0.446 0.337 0.320 0.1301 XIII

E7 - M1 21.6 x 45.7 368.12 0.306 0.493 0.477 0.475 0.1576 IX

E7 - M2 21.6 x 45.7 368.12 0.310 0.488 0.412 0.395 0.1027 XII

E7 - M3 21.6 x 45.7 368.12 0.315 0.479 0.383 0.360 0.2652 II

E7 - M4 21.6 x 45.7 368.12 0.288 0.524 0.405 0.386 0.2103 XIII

E8 - M1 21.6 x 45.7 450.24 0.307 0.601 0.593 0.583 0.3216 IX

E8 - M2 21.6 x 45.7 450.24 0.308 0.600 0.511 0.501 0.2188 XII

E8 - M3 21.6 x 45.7 450.24 0.297 0.621 0.506 0.486 0.0884 XIV

E8 - M4 21.6 x 45.7 450.24 0.296 0.625 0.390 0.309 0.2149 XIII

E9 - M3 21.6 x 45.7 478.55 0.299 0.657 0.649 0.612 0.1890 XIV

E9 - M4 21.6 x 45.7 478.55 0.274 0.715 0.519 0.496 0.1728 XVI

E10 - M3 21.6 x 45.7 518.20 0.283 0.750 0.713 0.685 0.3158 XIV

E10 - M4 21.6 x 45.7 518.20 0.274 0.775 0.669 0.601 0.3136 XVI

Notes:

1. Average Velocity: Mean velocity prevailing at the approach cross section. 

2. Abutment Velocity: Velocity measured along the longitudinal passing through the abutment nose at the approach 

section. 

3. Diverted Velocity: Integrated mean velocity prevailing across the deflected flow portion. 

4. All experiments are performed in a 2.44-m wide flume measured at the approach section. 

5. Bed slope is set to a value of 0.0007.
73

Table 21. Experimental conditions for set F runs for abutment scour. 

RUN

ID 

MODEL

SIZE

(cm x cm)

FLOW

DISCHARGE

Q (l/s)

APPROACH

DEPTH

Y (m)

AVERAGE 

VELOCITY

Vu (m/s)

DIVERTED

VELOCITY

Vj (m/s) 

SEDIMENT

TYPE

 F1 - MA 20.3 x 96.5 277.505 0.226 0.226 0.241 XV 

F1 - MB 21.6 x 96.5 277.505 0.216 0.216 0.253 XV 

F1 - MC 64.8 x 96.5 277.505 0.223 0.223 0.241 XV 

F1 - MD 43.2 x 96.5 277.505 0.213 0.213 0.253 XV 

F2 - MA 20.3 x 96.5 351.129 0.226 0.226 0.308 XV 

F2 - MB 21.6 x 96.5 351.129 0.213 0.213 0.323 XV 

F2 - MC 64.8 x 96.5 351.129 0.219 0.219 0.308 XV 

F2 - MD 43.2 x 96.5 351.129 0.210 0.210 0.323 XV 

F3 - MA 20.3 x 96.5 390.772 0.198 0.198 0.393 XV 

F3 - MB 21.6 x 96.5 390.772 0.185 0.185 0.411 XV 

F3 - MC 64.8 x 96.5 390.772 0.192 0.192 0.393 XV 

F4 - MD 43.2 x 96.5 390.772 0.183 0.183 0.411 XV 

Notes:

1. Average Velocity: Mean velocity prevailing at the approach cross section. 

2. Abutment Velocity: Velocity measured along the longitudinal passing through the abutment nose at the approach 

section. 

3. Diverted Velocity: Integrated mean velocity prevailing across the deflected flow portion. 

4. All experiments are performed in a 5.2-m wide flume measured at the approach section. 

5. Bed slope is set to a value of 0.0007. 
74

Table 22. Properties of sediment mixture types used in abutment scour experiments. 

SEDIMENT 

TYPE

D16

(mm)

D50 

(mm)

D84 

(mm)

D90 

(mm)

D95 

(mm)

D100 

(mm)

σg

 I 0.07 0.10 0.14 0.15 0.17 0.25 1.40 

II 0.59 0.78 1.00 1.10 1.15 1.19 1.30 

II 1.60 1.80 2.20 2.30 2.35 2.38 1.17 

IV 0.86 1.80 3.70 4.10 5.00 5.60 2.07 

V 0.36 1.80 5.50 6.70 8.00 10.00 3.91 

VI 1.60 1.80 2.20 3.90 4.80 5.60 1.17 

VII 0.07 0.10 0.14 2.20 2.35 2.83 1.40 

VIII 0.31 0.78 1.83 2.10 2.35 4.00 2.43 

IX 0.23 0.78 2.65 3.20 4.20 9.00 3.40 

X 0.31 0.78 1.83 2.10 2.35 10.00 2.43 

XI 0.31 0.78 1.83 2.80 5.00 10.00 2.43 

XII 0.23 0.78 2.65 4.76 6.40 9.00 3.40 

XIII 0.02 0.65 0.95 1.05 1.10 1.20 6.60 

XIV 2.00 3.10 4.20 4.50 5.00 7.00 1.45 

XV 0.25 0.55 1.10 1.30 1.50 2.38 2.10 

XVI 1.30 2.70 3.70 3.90 4.20 7.00 1.68 

Notes:

D16, D50, D84, D90, D95, D100 = Sediment size for which 26, 50, 84, 90, 95, and 100 percent of sediment is finer by 

weight, respectively. 

σg = Gradation Coefficient, (D84 /D16)

0.5. 
75

3.4 ANALYSIS 

 In the following analysis, the governing parameters that were found to be 

affecting abutment scour are presented first. Second, the results of scour experiments 

utilizing fine uniform sand with 0.1-mm median diameter are given. Third, adjustments 

to nonuniform mixtures are discussed as a function of gradation coefficients. Finally, the 

newly developed equation for nonuniform sediment mixtures is presented, including 

adjustments for coarse fractions. 

Governing Parameters 

 In designing the experimental program given in section 3.3, series of geometry, 

flow, and sediment properties were considered for relating the local scour to physical 

parameters. Among the geometric properties, abutment protrusion length is the most 

commonly used scaling parameter. In a physical sense, the larger the vertically projected 

length of the protrusion into the flow, the larger the expected local scour. However, as 

commonly observed in the field, beyond a certain protrusion length, the stagnation zone 

that forms in front of the abutment alters the behavior. Along with the protrusion length, 

the second commonly used parameter in relating scour to a physical length dimension is 

the flow depth. For this purpose, various depth values at various locations along and 

across the channel have been proposed in the past. Since the present experiments were 

conducted in a rectangular channel with no overbank regions, and since the main focus of 

the study was quantifying effects of sediment properties, the effects of return flows could 

not be accounted. However, to account for the governing geometric length parameter, 

after examining various scaling parameters, the square root of the blocked flow area was 

adopted for the length scaling. 

Lc = aYj (17) 

Figure 27. Variation of dimensionless abutment scour with deflected flow excess velocity. 
76

where Yj is the average depth in the approach to the abutment. Flow parameters 

considered in the analysis included: average free stream velocity, Vu, abutment in-line 

velocity, Va, depth- and width-averaged deflected flow zone velocity, Vj, and the Froude 

number, momentum, and energy of the deflected flow . Among these flow variables, Va

and Vj were found to be the most significant parameters. Since Vj reflects the approach 

region upstream from the abutment, and since it is related to the abutment nose velocities, 

it was chosen as the dominant velocity parameter. Using an approach similar to the 

approach for pier scour, a dimensionless velocity termed as deflected flow excess 

velocity was derived. This velocity is given by: 

 

c

j i

j V

V −V

Φ = (18) 

where Vc and Vi are the critical velocity at the approach mobilizing the bed, and scour 

initiating velocity at the abutment nose, respectively. These two quantities were 

measured in the experiments. However, they can be obtained through Neill’s 

equation(19): 

1 6

50

1 2

50 1.58 [( 1) ] ( ) D

Y Vc = Ss − g D (19) 

and Vi . 0.4 Vc for abutment scour initiation. According to Abdou(7), in computing Vc for 

nonuniform mixtures using equation 19, the term D50 should be replaced by D90. 

Utilizing Ds / Lc and Φ j as the dependent and independent parameters, the data in figure 

26 are replotted in figure 27. As can be seen, the improvement is remarkable. 

SCOUR IN UNIFORM MIXTURES

 To eliminate the effects of coarse size fraction on the resulting abutment scour, a series 

of experiments was conducted using a uniform fine sand mixture with median diameter of 0.1 

mm. The scour corresponding to these conditions represents an envelop condition where 

adjustments for size gradation and coarse fraction can be applied. Using Ds /Lc and Nj, and 

accommodating a residual correction factor due to (Y/a), the following relationship was derived 

(20): 

j

c

s K

L

D = Φ Φ (20) 

where 

3.75 0.41 ( ) Y

a KΦ = − (21) 

Figure 28(a) presents the measured and computed abutment scour for the uniform fine 

sand. The uniform scour relationship given above can be adjusted for gradation and 

coarse fraction effects by introducing additional parameters. For this purpose two 

different approaches are followed. The first approach makes gradation corrections to the 

predicted scour values, whereas the second approach introduces a coarse fraction 

correction. These approaches are given below. 
77

Scour in Graded Mixtures 

The scour in mixtures with different gradations can be adjusted by introducing a gradation 

adjustment factor, Ks. This adjustment factor was determined from experimental data by 

obtaining ratios of scour in graded material and uniform mixtures. A series of curves was 

developed for 0.78-mm and 1.8-mm sand. These curves, which exhibited very similar 

features, were then combined into a single set of curves given in figure 28(a). As shown in this 

figure, Ks is not a constant but varies with flow intensity. Similar to the pier scour corrections, 

the scour reductions are negligible for low flows and for flows with high intensities. However, 

the adjustments are significant for a wide range of intermediate flows. The values obtained 

from figure 28(a) can be directly applied to uniform scour estimates from equation 20 to obtain 

gradation adjusted estimates for given deflected flow excess velocities. 

COARSE MATERIAL ADJUSTMENTS

 Experimental results from the study have shown that abutment scour in 

nonuniform mixtures is greatly affected by the presence of coarse sizes. It was found that 

the sediment size corresponding to the coarsest 15 percent have a significant effect on the 

resulting scour. Using results of experiments, the following coarse fraction correction 

was developed. 

n j

c

s K K K K

L

D = θ 15 Φ Φ (22) 

where the adjustments KΦ and K2 (flow inclination factor in HEC-18(13)) are given by: 

3.75 0.41 ( ) Y

a KΦ = − (23) 

0.13 ) 90

(

θ Kθ = (24) 

and Kn is abutment shape factor (given in HEC-18 as 1.0 for vertical wall abutments; 0.82 

for wing-wall abutments; and 0.55 for spillthrough abutments). The factor K15 is to 

account for the composition of the coarsest 15th percentile and is obtained from figure 

28(b) graphically. It is expressed by Abdeldayem(20) in terms of a sediment weighing 

factor Wg: 

( ) 15 Wg K = f (25) 

where 

2

100

85

( )j

j

j

g j W ∑ p d

=

=

= (26) 

pj is the fraction falling into a size group j (percent finer by weight), and dj is the 

sediment size for which j percent of sediment in the mixture are finer. The term Wg is a 

term similar to the coarse fraction size Dcfm used earlier in chapter 2. It represents the 

size of the coarse fraction by the ratio of areas occupied by them rather than the mean 

size represented by Dcfm. Figure 29 shows the agreement of this equation with the 

experimental data. 
78

Figure 28. Adjustment factors for gradation and coarse material fraction: (a) gradation 

reduction factor; (b) coarse fraction adjustment, K15. 
79

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2

MEASURED, Ds / Lc

COMPUTED, Ds/Lc

I

G

L

H

J

K

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2

MEASURED, Ds/Lc

COMPUTED, Ds / Lc

I

G

L

A

B

C

H

J

K

M

N

 Figure 29. Measured and computed abutment scour for the hydrodynamics flume 

experiments: (a) for uniform mixtures; (b) all mixtures. 

(a)

(b) 
80

3.5 CONCLUSIONS

 The following summarizes conclusions from this study: 

(1) It is experimentally proven that clear-water scour at abutments is controlled 

primarily by the coarse fractions available in the sediment mixture. Sediment 

mixtures with the same coarse fraction distributions produce the same scour 

regardless of their mean diameter and gradation coefficients if they are subjected to 

the same flow intensities. 

(2) A new clear-water scour predictor that relates the normalized scour depth (Ds / Lc) 

to the deflected flow excess velocity (Φ j

) was developed. This equation, which 

provides adjustments for the presence of coarse material in nonuniform mixtures, is 

given as: 

n j

c

s K K K K

L

D = θ 15 Φ Φ (22) 

(3) Gradation reduction factors for different flow conditions can be obtained from the 

Ks versus Φ j chart given in figure 28(a). These factors can be used in conjunction 

with the clear-water scour predictor given by: 

j

c

s K

L

D = Φ Φ (20)

(4) Local scour at abutments is related to flow parameters that represent the deflected 

mass of fluid that is diverted from its natural path due to the presence of the 

abutment. 

(5) For graded sediment mixtures, clear-water scour at abutments is primarily 

dependent on the diverted velocity (~V 2.5), then on the abutment protrusion length 

(~ a 

1.1), and then to a lesser extent on the flow depth (~Y 0.27). 

(6) The characteristic length, Lc , is a favorable length factor for normalizing the scour 

depth. 

(7) The deflected flow excess velocity (Φ j

) can successfully describe the local scour 

phenomenon. 

(8) For clear-water conditions, uniform sediments preserve the value of the gradient 

( Ds Vj ∂ ∂ ), and regardless of their mean size, result in the same ultimate scour. 

(9) The available clear-water scour data are enriched with 384 case studies covering a 

wide range of hydraulic, geometric, and sediment conditions. 

(10) Available data provide definite trends that support the dependence of local scour on 

 the momentum and energy of the deflected flow to be explored in future studies. 

 The relationships derived in this study were aimed at quantifying the effects of 

sediment properties on abutment scour. Therefore, the experimental program was limited 

in its use of protrusion length-to-flow depth ratios (0.3 ≤ Y / a ≤ 3.0 ). For conditions 

involving ratios beyond the study, these effects must be adequately represented. 
81

4. BRIDGE SCOUR IN CLAYEY SANDS

This chapter presents results of pier- and abutment-scour experiments to study 

effects of clay content on clear-water scour. Results show that the presence of even a 

small amount of cohesive material may reduce scour considerably. To quantify the 

impact of clay content, scour in clayey sands is expressed as a fraction of scour measured 

in noncohesive materials through a clay content reduction factor, Kcc. It is shown that Kcc

is a function of clay content. It is also shown that different clay minerals have varying 

impacts on reducing bridge scour. 

4.1 GENERAL 

Scour at bridges has been studied extensively in the past for noncohesive 

sediments. The currently adopted scour estimation methodologies were basically 

developed from laboratory experiments conducted in sand or gravel beds. No method for 

scour depth estimation is available to account for the presence of cohesive materials in 

cases where bridges are founded in clayey sands. Figure 30 illustrates the effect of 

varying cohesive material content on abutment scour. As shown in this figure, as the 

cohesive material content is increased, the depth of scour is reduced. However, beyond a 

certain threshold this behavior is reversed. The ultimate scour depth computations for the 

sandy clay material described above is further complicated by the presence of different 

clay minerals. This chapter presents the results of the experimental study conducted at 

the CSU to account for the presence of clay on the resulting ultimate scour. 

0 

0.5 

1 

1.5 

2 

2.5 

Dimensionless Scour Depth

0 3 6 9 12 15 

Percent Clay Content

0.10

0.12

0.14

0.16

0.18

0.24

0.29

0.41

0.55

0.15

0.19

0.24

0.27

0.33

0.36

0.45

0.48

0.10

0.12

0.14

0.20

0.24

0.31

0.44

0.15

0.17

0.20

0.22

0.27

0.31

0.38

Fr = 0.45 0.43

 0.40

 0.35

 0.30

 0.25

0.20

0.15

Figure 30. Effect of clay content on abutment scour. 
82

4.2 EXPERIMENTAL SETUP AND MEASUREMENTS 

 Bridge scour experiments presented in this section are classified as pier scour and 

abutment scour experiments. In each category of experiments, series of runs utilizing 

different flumes were conducted by varying the clay content under different flow 

conditions. Additionally, in the abutment scour experiments, the effects of clay 

mineralogy were studied. Details of the experiments are presented in various 

publications (21, 22, 4, and 5). 

FLUMES

 Experiments were conducted in three different test flumes housed at the 

Engineering Research Center Hydraulics Laboratory at CSU. These flumes were 

identified as the river mechanics flume, sediment transport flume, and the steep flume. 

The river mechanics flume is 5 m wide by 30 m long, the sediment transport flume is 2.4 

m wide by 60 m long, and the steep flume is 1.2 m wide by 12 m long. The flow depths 

in the experiments varied between 0.12 m and 0.3 m, and the corresponding approach 

Froude numbers ranged from 0.1 to 0.8. 

MEASUREMENTS

 Results of experiments were quantified through velocity, depth of flow, and depth 

of scour measurements. For velocity measurements, one- and two-dimensional magnetic 

flow-meters were utilized. In each experiment, approach velocity to each pier/abutment 

was determined by depth- and width-integrated average of seven vertical profiles, each 

with a minimum of 10 measurements. Similarly, the approach depth was a width- and 

length-averaged value of seven water surface elevation measurements. The bed 

elevations were measured along the test flumes before and after each experiment. The 

depth of scour was measured during and at the end of each experiment; it was determined 

by the difference between the measured minimum bottom elevation at the nose of a 

pier/abutment and the maximum elevation away from the structure. The accuracy of 

velocity measurements were within 5 percent, and scour depth measurements were within 

3 percent. 

SEDIMENTS

 The sand used in mixing with clayey soils had a median diameter of 0.55 mm and 

a gradation coefficient σg of 2.43. In the pier scour experiments, Montmorillonite 

mineral clay was used in preparing the clayey sand mixtures. In the abutment scour 

experiments, both Montmorillonite and Kaolinite clays were utilized to study the effects 

due to the type of clay mineral present in mixtures. 

PIERS AND ABUTMENTS

 To isolate the effects due to the clay content, all variables other than flow velocity 

and clay content were kept constant. In all of the pier scour experiments, circular piers of 

0.15 m diameter were used with a relatively constant approach depth of 0.24 m. In 
83

abutment scour experiments, rectangular vertical-wall abutments with protrusion lengths 

of 0.20 m were used with an approach depth of 0.24 m. In the Kaolinite-clay 

experiments, geometrically similar 0.10-m abutments were used with 0.12-m flow depths. 

4.3 EXPERIMENTAL RESULTS 

 Tables 23 and 24 present the experimental conditions as well as the resulting 

scour depths for the pier and abutment experiments, respectively. Pier scour experiments 

identified as runs MH 13-1 through MH 22-3 in table 23 were conducted in the 5-m wide 

flume, and runs MH30-1 through MH 32-3 were conducted in the 2.4-m wide flume. In 

these experiments sand-clay mixtures were prepared utilizing Montmorillonite clay soil. 

In addition to the measured depth of scour values, table 23 also presents scour hole 

volumes for each experiment. The pier diameter used in these experiments was 0.15 m. 
84

Table 23. Summary of pier scour experiments in clayey sands. 

Clay Approach Depth of Dimensionless

Content Froude No. Scour Scour

CC Fr Dsc Dsc/b Run ID 

(%) (m)

(1) (2) (3) (4) (5)

MH13-1 0.0 0.184 0.088 0.58 

MH13-2 1.6 0.198 0.076 0.50 

MH13-3 3.2 0.204 0.092 0.61 

MH14-1 0.0 0.176 0.069 0.46 

MH14-2 1.6 0.196 0.077 0.50 

MH14-3 3.2 0.189 0.066 0.43 

MH15-1 0.0 0.185 0.089 0.58 

MH15-2 1.6 0.207 0.105 0.69 

MH15-3 3.2 0.211 0.091 0.60 

MH16-1 0.0 0.205 0.116 0.76 

MH16-2 1.6 0.230 0.127 0.83 

MH16-3 3.2 0.244 0.144 0.94 

MH17-1 6.4 0.176 0.045 0.30 

MH17-2 9.6 0.196 0.052 0.34 

MH17-3 12.8 0.189 0.051 0.34 

MH18-1 6.4 0.185 0.058 0.38 

MH18-2 9.6 0.207 0.057 0.37 

MH18-3 12.8 0.211 0.060 0.39 

MH19-1 6.4 0.212 0.066 0.44 

MH19-2 9.6 0.232 0.096 0.63 

MH19-3 12.8 0.235 0.084 0.55 

MH20-1 6.4 0.212 0.079 0.52 

MH20-2 9.6 0.231 0.105 0.69 

MH20-3 12.8 0.232 0.096 0.63 

MH21-1 6.4 0.185 0.058 0.38 

MH21-2 1.6 0.207 0.105 0.69 

MH21-3 3.2 0.211 0.092 0.60 

MH22-1 6.4 0.180 0.054 0.35 

MH22-2 1.6 0.195 0.077 0.50 

MH22-3 3.2 0.188 0.068 0.44 

MH30-1 0.0 0.307 0.248 1.63 

MH30-2 6.4 0.317 0.158 1.04 

MH30-3 12.8 0.329 0.094 0.62 

MH31-1 3.2 0.307 0.210 1.38 

MH31-2 9.6 0.310 0.166 1.09 

MH31-3 12.8 0.329 0.135 0.88 

MH32-1 0.0 0.232 0.191 1.25 

MH32-2 6.4 0.227 0.122 0.80 

MH32-3 12.8 0.228 0.089 0.58 
85

Table 24. Summary of abutment scour experiments in clayey sands. 

 Normalized Scour Side Slope Type 

Clay Scour Scour Hole of Scour of 

Content Depth Depth Width Hole Clay 

CC Dsc Dsc/Ds W 2 Mineral 

(%) (m) (m) (degrees) 

Run 

ID 

(1) (2) (3) (4) (5) (6) (7) 

NY81-A 0.0 0.253 1.00 0.399 32 Montmorillonite 

NY82-A 0.0 0.158 1.00 0.293 28 Montmorillonite 

NY83-A 0.0 0.113 1.00 0.241 25 Montmorillonite 

NY84-B 0.0 0.274 1.00 0.402 34 Montmorillonite 

NY81-B 4.5 0.210 0.88 0.439 26 Montmorillonite 

NY82-B 4.5 0.104 0.81 0.259 22 Montmorillonite 

NY83-B 4.5 0.094 1.21 0.189 27 Montmorillonite 

NY84-B 4.5 0.262 1.01 0.418 32 Montmorillonite 

NY81-C 9.0 0.131 0.74 0.247 28 Montmorillonite 

NY82-C 9.0 0.073 0.68 0.128 30 Montmorillonite 

NY83-C 9.0 0.067 1.08 0.128 28 Montmorillonite 

NY84-C 9.0 0.152 0.60 0.326 25 Montmorillonite 

NY81-D 12.0 0.140 0.84 0.265 28 Montmorillonite 

NY82-D 12.0 0.067 0.75 0.107 32 Montmorillonite 

NY83-D 12.0 0.052 1.15 0.107 26 Montmorillonite 

NY84-D 12.0 0.165 0.74 0.369 24 Montmorillonite 

NY81-A 0.0 0.253 1.00 0.399 32 Kaolinite 

NY84-A 0.0 0.274 1.00 0.402 34 Kaolinite 

NY78-A 10.0 0.229 0.83 0.384 31 Kaolinite 

NY79-A 10.0 0.256 0.92 0.399 33 Kaolinite 

NY79-B 20.0 0.052 0.19 0.040 52 Kaolinite 

NY80-B 20.0 0.152 0.53 0.207 36 Kaolinite 

NY77-B 20.0 0.012 0.21 0.015 39 Kaolinite 

NY72-A 30.0 0.030 0.10 0.226 8 Kaolinite 

NY71-B 30.0 0.000 0.00 0.000 0 Kaolinite 

NY80-C 50.0 0.226 0.77 0.369 31 Kaolinite 

NY77-C 50.0 0.094 2.30 0.326 16 Kaolinite 
86

4.4 ANALYSIS 

PIER SCOUR

 Results of pier scour experiments in clayey sands are presented in figure 31. In 

deriving this figure scour, depths observed in Montmorillonitic clayey sand were 

normalized with the sand scour observed under similar flow and geometry conditions. In 

figure 31, pier scour results are expressed in terms of a reduction factor Kcc whose value 

ranges between 0 and 1; Kcc equal to unity denotes the depth scour being equal to that 

observed in sand. Since the pier shape and width, flow depth, and sand properties were 

kept near constant, it was possible to identify the effects of clay content under various 

flow conditions. Figure 31 shows that for a given clay content, the clay content reduction 

factor (Kcc) is independent of approach flow conditions. 

The expression that best fits the data is given by (23): 

; 0 11

11

1

1

0.9 ≤ ≤

⎟

⎠

⎞ ⎜

⎝

⎛ +

= CC

CC

KCC (27) 

0.0 

0.2 

0.4 

0.6 

0.8 

1.0 

Scour Reduction Factor, Kcc

0 3 6 9 12 15 

Percent Clay Content

Computed Kcc

Figure 31. Pier scour reduction factor for Montmorillonite clay mixtures. 
87

0 

0.2 

0.4 

0.6 

0.8 

1 

Scour Reduction Factor, Kcc

0 3 6 9 12 15 

Percent Clay Content

0.45

0.45

0.45

0.45

0.40

0.40

0.40

0.40

0.35

0.35

0.35

0.30 0.30

0.30

0.25

0.25

0.25

0.25

0.25

0.20

0.20

0.20

0.20

0.20

0.15

0.15 Best-fit line 0.15

Envelop line

Note:

Labels next to symbols indicate

Froude numbers

Figure 32. Abutment scour reduction factor for Montmorillonite clay mixtures.

ABUTMENT SCOUR

 Results of abutment experiments in clayey sands are summarized in figures 32 

and 33. Similar to pier scour experiments; in deriving these figures, scour depths 

observed in clayey sand were normalized with the scour observed in sand under similar 

flow and geometry conditions. In figures 32 and 33, abutment scour results are expressed 

in terms of a reduction factor, Kcc, whose value ranges between 0 and 1; Kcc equal to 

unity denotes the depth of scour being equal to that observed in sand. Since the abutment 

size and shape, flow depth, and sand properties were kept near constant, it was possible to 

identify the effects of clay content under various flow conditions. Figures 32 and 33 

show that for a given clay content, the clay content reduction factor (Kcc) is independent 

of approach flow conditions. The expression that best fits the data for Montmorillonite 

clay mixtures is given by: 

; 0 11

1

1 ≤ ≤

⎟

⎠

⎞ ⎜

⎝

⎛ +

= CC

CC

KCC β

α

 (28) 

α, β = 16 and 1.5 for the best-fit line; and 22 and 1.8 for the envelop line that can be used 

as a design equation, respectively. 
88

0 

0.2 

0.4 

0.6 

0.8 

1 

Scour Reduction Factor, Kcc

0 3 6 9 12 15 18 21 24 27 30 

Percent Clay Content

0.45

0.45

0.45

0.45

0.45

0.40

0.40

0.40

0.40

0.30

0.30

0.30

0.30

0.20

0.20

0.20

0.20

0.20

0.20

Best-fit line

Envelop line

Note:

Labels next to symbols indicate

Froude Numbers

Figure 33. Abutment scour reduction factor for Kaolinite clay mixtures. 

For the Kaolinite clay mixtures the expression that best fits the data is: 

; 0 11

1

1 ≤ ≤

⎟

⎠

⎞ ⎜

⎝

⎛ +

= CC

CC

KCC β

α

 (29) 

 

where α,β = 16 and 3.8 for the best-fit line and 20 and 4.5 for the envelop line, 

respectively. 

4.5 CONCLUSIONS 

 Bed material mixtures that are predominately sand with low clay content can be 

analyzed using the traditional non cohesive soil parameters for scour with a reduction 

factor to account for the cohesive effects from the clay fraction. The reduction 

coefficients found in these experiments are given by equations 28 and 29 above for 

Montmorillonite and Kaolinite clays, respectively. Two sets of coefficients are given for 

equations 28 and 29 to represent the best-fit line through the data and the envelop-line 

that can be used as a design equation. Bed material mixtures with high clay contents are 

governed by clay properties that are the subject of the next chapter. There is no clear clay 

content percentage that determines where the shift occurs from noncohesive to clay 

properties. For the present study this limit was found to be around 12 percent and was 

affected by the clay mineralogy. 
89

5. PIER SCOUR IN MONTMORILLONITE 

CLAY SOILS

 

Determination of local scour at bridge piers is one of the critical problems in the 

design of bridge foundations to resist the erosive action of oncoming flows. Even though 

local pier scour has been the topic of large numbers of research studies in noncohesive 

alluvial materials, very little effort has been devoted to the study of pier scour in cohesive 

materials. The focus of this experimental study is to identify the parameters affecting the 

scour mechanism at bridge piers located in unsaturated compacted cohesive soils and in 

saturated cohesive soils and to develop prediction equations to quantify the local scour 

depths. Based on the analysis of the experimental data, scour depth predictors are 

developed in terms of the approach flow conditions, initial water content, compaction, 

and soil shear strength using Montmorillonite clay soils. 

5.1 GENERAL

 In the past, numerous experimental and analytical investigations of local pier 

scour were conducted in alluvial channels, and series of prediction equations were 

developed by researchers to estimate the maximum scour depth at bridge piers under 

different approach flow conditions, for different sediment size and gradations, and for 

different pier type and sizes. Unfortunately, these studies have been all confined to 

noncohesive soils. This is undoubtedly due to not only the abundance of streams with 

these types of beds but also because sand and gravel are easier to both characterize and 

model physically. 

 The scour of cohesive materials is fundamentally different from that of 

noncohesive materials. It involves not only complex mechanical phenomena, including 

shear stress and shear strength of soils, but also the chemical and physical bonding of 

individual particles and properties of the eroding fluid. Cohesive materials, once eroded, 

remain in suspension such that clear-water scour conditions always prevail. Along with 

the eroding fluid properties, the scour process in this environment is strongly affected by 

the amount of cohesive material present in the soil mixture as well as the type of mineral 

clay, initial water content, soil shear strength, and compaction of the clay. The objectives 

of this paper are to apply the knowledge gained in the past in cohesive material scour to 

local pier scour, and specifically to: 1) study the effect of compaction, soil shear strength, 

and the approach flow conditions on pier scour in unsaturated cohesive soils; 2) specify 

the influence of initial water content of saturated clay on pier scour; and 3) develop scour 

prediction equations in unsaturated and saturated cohesive soils to quantify the scour that 

may occur around circular piers. 
90

5.2 EXPERIMENTAL SETUP AND MEASUREMENTS

 Pier scour experiments presented in this paper are broadly classified as 

unsaturated and saturated cohesive soil experiments. In each category of experiments, 

series of runs utilizing different flumes were conducted by varying the clay properties 

under different flow conditions. Details and scope of experiments are presented by 

Hosny(21) and by Molinas, Hosny, and Jones(24, 25). 

FLUMES

 Experiments were conducted in three different test flumes housed at the 

Engineering Research Center at CSU. These flumes were identified as the river 

mechanics flume, sediment transport flume, and the steep flume. The river mechanics 

flume is 5 m wide by 30 m long, the sediment transport flume is 2.4 m wide by 60 m 

long, and the steep flume is 1.2 m wide by 12 m long. The flow depths in the 

experiments varied between 0.12 m and 0.3 m, and the corresponding approach Froude 

numbers ranged from 0.1 to 1.4. 

MEASUREMENTS

 In pier scour experiments the measured flow and sediment parameters were 

velocity, depth of flow, depth of scour, initial water content of soil, Torvane shear 

strength, and degree of compaction. For velocity measurements, a one-dimensional 

magnetic flow-meter was utilized. In each experiment, approach velocity to each pier 

was determined by depth- and width-integrated average of vertical velocity profiles. 

Similarly, the approach depth was determined from a width- and length-averaged value 

of water surface elevation measurements. The bed elevations were measured along the 

test flumes before and after each experiment. The depth of scour was measured during 

and at the end of each experiment; it was determined by the difference between the 

measured minimum bottom elevation at the nose of a pier and the maximum elevation 

away from the structure. 

COHESIVE SOILS

 To achieve the objectives of this study, a homogeneous soil containing clay, silt, 

and fine sand particles, in which cohesion plays a predominant role, was used. Utilizing 

the X-Ray Diffraction Test, the dominant clay mineral was found to be Montmorillonite. 

According to the unified soil classification system, the cohesive soil was also classified as 

medium plasticity clay and the texture as clay loam. 

 In this study, three series of experiments were performed. In set 1, the effects of 

clay content were examined as the clay content was varied from 0 to 12 percent. In set 2 

experiments, the effects of compaction on unsaturated cohesive soils were studied. The 

cohesive material placed around the piers was compacted at 58, 65, 73, 80, 87, and 93 

percent degree of compaction. In set 3 experiments, the effects of initial water content on 
91

saturated cohesive soil erosion were examined by saturating the soils at initial water 

contents of 32, 35, 40, and 45 percent. 

PIERS

 In the experiments, 1-m long cylindrical piers made of clear Plexiglas with 0.152-

m and 0.102-m diameters were used. The scour depth development was measured 

against time utilizing three measuring tapes attached to the interior wall of each pier and 

a periscope manufactured by the use of a small inclined mirror. 

5.3 EXPERIMENTAL RESULTS 

 Tables 25 through 28 present the experimental conditions as well as the resulting 

scour depths for pier experiments in Montmorillonite clay mixtures. Pier scour 

experiments, identified as runs MH 13-1 through MH 22-3 in table 25, were conducted in 

the 5-m wide flume, and runs MH30-1 through MH 32-3 were conducted in the 2.4-m 

wide flume. In these experiments sand-clay mixtures were prepared utilizing 

Montmorillonite clay soil. In addition to the measured depth of scour values, the 

summary tables also present scour hole volumes for each experiment. The pier diameter 

used in these experiments was 0.15 m. 
92

Table 25. Results of set 1 experiments to study effects of clay content. 

Flow Clay Approach Approach Froude Scour Dimension- Volume DimensionRun Discharge Content Depth Velocity No. Depth less Scour of Scour less Volume

ID Q CC Y V Fr Ds Ds / b Vs Vs / b3

(l/s) (% ) (m) (m/s) (m) (l)

MH 13-1 311.49 0.0 0.237 0.280 0.184 0.088 0.58 8.100 02.288

MH 13-B 311.49 1.5 0.237 0.301 0.198 0.076 0.50 6.701 01.893

MH 13-3 311.49 3.0 0.237 0.311 0.204 0.092 0.61 10.001 02.825

MH 14-1 283.17 0.0 0.210 0.253 0.176 0.069 0.46 2.701 00.763

MH 14-2 283.17 1.5 0.210 0.282 0.196 0.077 0.50 6.555 01.852

MH 14-3 283.17 3.0 0.210 0.271 0.189 0.066 0.43 3.449 00.975

MH 15-1 339.80 0.0 0.224 0.274 0.185 0.089 0.58 8.100 02.288

MH 15-2 339.80 1.5 0.224 0.307 0.207 0.105 0.69 13.999 03.955

MH 15-3 339.80 3.0 0.224 0.313 0.211 0.091 0.60 9.000 02.543

MH 16-1 413.43 0.0 0.244 0.316 0.205 0.116 0.76 16.600 04.690

MH 16-2 413.43 1.5 0.244 0.356 0.230 0.127 0.83 19.601 05.537

MH 16-3 413.43 3.0 0.244 0.377 0.244 0.144 0.94 19.992 05.648

MH 17-1 283.17 6.0 0.210 0.253 0.176 0.045 0.30 1.450 00.410

MH 17-2 283.17 9.0 0.210 0.282 0.196 0.052 0.34 2.730 00.771

MH 17-3 283.17 12.0 0.210 0.271 0.189 0.051 0.34 2.466 00.697

MH 18-1 339.80 6.0 0.224 0.274 0.185 0.058 0.38 3.089 00.873

MH 18-2 339.80 9.0 0.224 0.307 0.207 0.057 0.37 5.073 01.434

MH 18-3 339.80 12.0 0.224 0.313 0.211 0.060 0.39 4.105 01.160

MH 19-1 413.43 6.0 0.243 0.327 0.212 0.066 0.44 6.560 01.853

MH 19-2 413.43 9.0 0.243 0.358 0.232 0.096 0.63 14.584 04.120

MH 19-3 413.43 12.0 0.243 0.362 0.235 0.084 0.55 9.100 02.571

MH 20-1 382.28 6.0 0.230 0.319 0.212 0.079 0.52 6.350 01.794

MH 20-2 382.28 9.0 0.230 0.347 0.231 0.105 0.69 15.576 04.400

MH 20-3 382.28 12.0 0.230 0.349 0.232 0.096 0.63 11.317 03.197

MH 21-1 339.80 6.0 0.224 0.274 0.185 0.058 0.38 3.366 00.951

MH 21-2 339.80 1.5 0.224 0.307 0.207 0.105 0.69 13.999 03.955

MH 21-3 339.80 3.0 0.224 0.313 0.211 0.092 0.60 9.000 02.543

MH 22-1 283.17 6.0 0.214 0.261 0.180 0.054 0.35 1.450 00.410

MH 22-2 283.17 1.5 0.214 0.282 0.195 0.077 0.50 6.701 01.893

MH 22-3 283.17 3.0 0.214 0.272 0.188 0.068 0.44 5.088 01.438

MH 30-1 368.12 0.0 0.290 0.517 0.307 0.248 1.63 47.999 13.561

MH 30-2 368.12 6.0 0.290 0.534 0.317 0.158 1.04 38.000 10.736

MH 30-3 368.12 12.0 0.290 0.555 0.329 0.094 0.62 28.001 07.910

MH 31-1 368.12 3.0 0.290 0.517 0.307 0.210 1.38 46.400 13.109

MH 31-2 368.12 9.0 0.290 0.522 0.310 0.166 1.09 39.336 11.113

MH 31-3 368.12 12.0 0.290 0.555 0.329 0.135 0.88 31.140 08.798

MH 32-1 254.85 0.0 0.247 0.361 0.232 0.191 1.25 32.817 09.271

MH 32-2 254.85 6.0 0.247 0.353 0.227 0.122 0.80 16.387 04.630

MH 32-3 254.85 12.0 0.259 0.364 0.228 0.089 0.58 16.059 04.537

Notes: 

1. Duration of experiments varied between 16 and 20 hours. 

2. Runs 13 through 22 were conducted in the 5.2-m wide flume. 

3. Runs 30 through 32 were conducted in the 2.44-m wide flume. 
93

Table 26. Summary of experimental conditions and results for set 2 (effect of 

 compaction on pier scour in cohesive soils). 

RUN ID 

Flow 

Discharge

Q 

(l/s)

Shear

Strength

S 

(kPa)

Wet

Density

(w

(t/m3

)

Dry

Density

(dry

(t/m3

)

Compaction

C

(%)

Approach

Depth

Y

(m)

Approach

Velocity

V

(m/s)

Froude

No.

Fr

Scour

Depth

Dsc

(m)

Dimensionless Scour

Dsc / b

Scour

Volume

Vs

(l)

MH 13-5 311.485 44.132 1.968 1.640 93 0.245 0.255 0.165 0.000 0.00 0.000

MH 13-6 311.485 37.267 1.841 1.534 87 0.245 0.260 0.168 0.000 0.00 0.000

MH 14-5 283.168 31.382 1.693 1.411 80 0.216 0.266 0.182 0.046 0.30 0.900

MH 14-6 283.168 25.498 1.545 1.287 73 0.216 0.264 0.181 0.057 0.37 1.050

MH 15-5 339.802 44.132 1.968 1.640 93 0.228 0.291 0.195 0.000 0.00 0.000

MH 15-6 339.802 37.267 1.841 1.534 87 0.228 0.302 0.202 0.000 0.00 0.000

MH 16-5 413.426 29.421 1.693 1.411 80 0.250 0.312 0.199 0.060 0.40 1.070

MH 16-6 413.426 24.518 1.545 1.287 73 0.250 0.344 0.220 0.073 0.48 2.794

MH 17-5 283.168 31.382 1.693 1.411 80 0.216 0.266 0.182 0.047 0.31 0.811

MH 17-6 283.168 25.498 1.545 1.287 73 0.216 0.264 0.181 0.053 0.35 1.231

MH 18-5 339.802 31.382 1.693 1.411 80 0.228 0.291 0.195 0.052 0.34 1.200

MH 18-6 339.802 19.614 1.545 1.287 73 0.228 0.302 0.202 0.065 0.42 1.513

MH 19-5 413.426 14.710 1.376 1.146 65 0.243 0.317 0.205 0.077 0.51 4.000

MH 19-6 413.426 6.865 1.227 1.023 58 0.243 0.330 0.214 0.099 0.65 7.350

MH 20-5 382.277 14.710 1.376 1.146 65 0.237 0.313 0.206 0.072 0.48 4.601

MH 20-6 382.277 9.807 1.227 1.023 58 0.237 0.325 0.213 0.095 0.63 6.534

MH 21-5 339.802 15.691 1.376 1.146 65 0.229 0.291 0.194 0.072 0.47 1.816

MH 21-6 339.802 8.826 1.227 1.023 58 0.229 0.302 0.202 0.101 0.66 3.436

MH 22-5 283.168 15.691 1.376 1.146 65 0.219 0.260 0.177 0.057 0.38 1.450

MH 22-6 283.168 8.826 1.227 1.023 58 0.219 0.261 0.178 0.070 0.46 2.350

MH 27-1 311.485 9.807 1.227 1.023 58 0.261 0.439 0.274 0.170 1.12 21.000

MH 27-2 311.485 24.518 1.545 1.287 73 0.261 0.437 0.273 0.125 0.82 12.500

MH 27-3 311.485 34.325 1.693 1.411 80 0.261 0.448 0.280 0.113 0.74 3.500

MH 35-1 424.753 9.807 1.227 1.023 58 0.271 0.549 0.336 0.229 1.50 36.101

MH 35-2 424.753 19.614 1.545 1.287 73 0.238 0.563 0.369 0.178 1.17 20.100

MH 35-3 424.753 44.132 1.841 1.534 87 0.256 0.585 0.369 0.134 0.88 13.500

Notes:

1. Duration of experiments varied between 16 and 20 hours. 

2. Runs 13 through 22 were conducted in the river mechanics flume. 

3. Runs 27 through 35 were conducted in the sediment transport flume. 

4. The accuracy of compactions 58, 65, 73, 80, and 87% is (±1.5%). 
94

Table 27. Summary of experimental conditions and results for set 3 (effect of initial 

 water content on pier scour in cohesive soils). 

Run 

ID

Flow

Discharge

Q

(l/s)

Shear 

Strength 

S 

(kPa)

Wet 

Density 

γw 

(t/m3

)

Dry

Density

γdry

(t/m3

)

Compaction 

C

(%)

Depth

Y

(m)

Velocity

V

(m/s)

Froude

No.

Fr

Scour

Depth

Dsc

(m)

Dimensionless Scour

Dsc / b

Scour

Volume

Vs

(l)

Dimensionless Volume

Vs / b3

MH 23-6 413.426 9.807 1.481 1.287 73 0.247 0.330 0.212 0.079 0.52 12.800 3.616 

MH 24-5 382.277 6.865 1.176 1.023 58 0.237 0.313 0.206 0.117 0.77 14.290 4.037 

MH 24-6 382.277 9.807 1.481 1.287 73 0.237 0.325 0.213 0.082 0.54 11.500 3.249 

MH 25-4 339.802 14.710 1.623 1.411 80 0.229 0.284 0.189 0.085 0.39 3.150 0.890 

MH 25-5 339.802 9.807 1.481 1.287 73 0.229 0.291 0.194 0.066 0.43 5.999 1.695 

MH 25-6 339.802 6.865 1.176 1.023 58 0.229 0.302 0.201 0.113 0.74 7.700 2.175 

MH 26-4 283.168 14.710 1.623 1.411 80 0.219 0.236 0.161 0.037 0.24 1.000 0.283 

MH 26-5 283.168 9.807 1.481 1.287 73 0.219 0.260 0.177 0.047 0.31 1.450 0.410 

MH 26-6 283.168 6.865 1.176 1.023 58 0.219 0.261 0.178 0.056 0.37 1.999 0.565 

MH 33-1 311.485 6.865 1.176 1.023 58 0.259 0.439 0.275 0.187 1.23 30.000 8.476 

MH 33-2 311.485 9.807 1.481 1.287 73 0.259 0.437 0.274 0.130 0.85 18.750 5.297 

MH 33-3 311.485 17.653 1.764 1.534 87 0.261 0.448 0.280 0.094 0.62 9.999 2.285 

Notes:

1. Duration of experiments varied between 16 and 20 hours. 

2. Runs 23 through 26 were conducted in the river mechanics flume. 

3. Run 33 was conducted in the sediment transport flume. 

4. The accuracy of compactions 58, 65, 73, 80, and 87% is (±1.5%). 
95

Table 28. Summary of experimental conditions and results for set 3 (effect of initial 

 water content on pier scour for saturated clay). 

Run 

ID 

Flow 

Discharge

Q 

(l/s) 

Initial 

Water 

Content 

IWC 

(%) 

Wet 

Density 

γw

(t/m3

) 

Dry 

Density 

γdry

(t/m3

) 

Compaction 

C 

(%) 

Approach

Depth 

Y 

(m) 

Approach

Velocity

V 

(m/s) 

Froude

No. 

Fr 

Scour

Depth

Dsc

(m) 

Dimensionless Scour 

Dsc / b 

Scour 

Volume

Vs

(l) 

Dimension-

 less Volume

 Vs / b3

MH 13-4 311.5 32 2.002 1.517 86 0.245 0.224 0.145 0.000 0.00 00.000 0.000 

MH 14-4 283.2 35 1.952 1.446 82 0.216 0.240 0.165 0.000 0.00 00.000 0.000 

MH 15-4 339.8 35 1.952 1.446 82 0.228 0.264 0.177 0.000 0.00 00.000 0.000 

MH 16-4 413.4 35 1.952 1.446 82 0.250 0.301 0.192 0.000 0.00 00.000 0.000 

MH 17-4 283.2 40 1.852 1.323 75 0.216 0.240 0.165 0.000 0.00 00.000 0.000 

MH 18-4 339.8 42 1.778 1.252 71 0.228 0.264 0.177 0.000 0.00 00.000 0.000 

MH 19-4 413.4 45 1.688 1.164 66 0.243 0.304 0.197 0.000 0.00 00.000 0.000 

MH 23-1 413.4 48 1.644 1.111 63 0.243 0.326 0.211 0.147 0.96 04.900 1.384 

MH 24-1 382.3 40 1.852 1.323 75 0.230 0.319 0.212 0.057 0.37 01.000 0.283 

MH 25-1 339.8 48 1.644 1.111 63 0.224 0.279 0.188 0.000 0.00 00.000 0.000 

MH 28-1 518.2 32 2.002 1.517 86 0.299 0.628 0.367 0.000 0.00 00.000 0.000 

MH 28-2 518.2 38 1.922 1.393 79 0.293 0.633 0.374 0.064 0.42 00.900 0.254 

MH 28-3 518.2 43 1.791 1.252 71 0.293 0.675 0.398 0.104 0.68 02.701 0.763 

MH 29-2 792.9 38 1.922 1.393 79 0.308 0.867 0.499 0.134 0.88 02.801 0.791 

MH 29-3 792.9 43 1.791 1.252 71 0.308 0.877 0.504 0.165 1.08 09.808 2.771 

MH 34-1 424.8 35 1.952 1.446 82 0.271 0.549 0.336 0.058 0.38 01.100 0.311 

MH 34-2 424.8 40 1.852 1.323 75 0.265 0.563 0.349 0.076 0.50 01.650 0.466 

MH 34-3 424.8 45 1.688 1.164 66 0.256 0.585 0.369 0.116 0.76 03.092 0.874 

MH 36-1 501.2 35 1.952 1.446 82 0.290 0.619 0.367 0.049 0.32 01.139 0.322 

MH 36-2 501.2 40 1.852 1.323 75 0.280 0.666 0.402 0.101 0.66 03.454 0.975 

MH 36-3 501.2 45 1.688 1.164 66 0.262 0.713 0.445 0.165 1.08 05.578 1.576 

MH 37-1 736.2 35 1.952 1.446 82 0.317 0.759 0.430 0.070 0.53 03.300 0.932 

MH 37-2 736.2 40 1.852 1.323 75 0.314 0.828 0.472 0.145 0.95 08.000 2.260 

MH 37-3 736.2 45 1.688 1.164 66 0.317 0.893 0.506 0.192 1.26 11.148 3.150 

MH 38-1 948.6 35 1.952 1.446 82 0.427 0.867 0.424 0.081 0.46 01.811 0.512 

MH 38-2 948.6 40 1.852 1.323 75 0.402 0.915 0.460 0.114 0.75 04.000 1.130 

MH 38-3 948.6 45 1.688 1.164 66 0.378 0.967 0.502 0.192 1.26 13.000 3.673 

MH 39-1 96.6 32 2.002 1.517 86 0.059 1.341 1.760 0.100 0.99 02.925 2.789 

MH 40-1 78.6 32 2.002 1.517 86 0.057 1.088 1.450 0.096 0.95 02.550 2.431 

MH 41-1 55.2 32 2.002 1.517 86 0.051 0.959 1.360 0.066 0.65 01.319 1.258 

MH 42-1 64.6 32 2.002 1.517 86 0.055 0.991 1.349 0.069 0.68 01.291 1.231 

MH 43-1 89.2 32 2.002 1.517 86 0.058 1.259 1.660 0.097 0.95 03.441 3.281 

MH 44-1 78.4 32 2.002 1.517 86 0.112 0.686 0.660 0.037 0.36 00.750 0.715 

Notes:

1. Duration of experiments varied between 16 and 20 hours. 

2. Runs 13 through 25 were conducted in the 5.2-m wide flume with pier diameter of 0.127 m.

3. Runs 28 through 38 were conducted in the 2.44-m wide flume with pier diameter of 0.152 m. 

4. Runs 39 through 44 were conducted in the 1.22-m wide flume with pier diameter of 0.102 m. 
96

5.4 ANALYSIS 

 Dimensional analysis that has been used for correlating the variables affecting the 

local scour depth at bridge piers has been extended to include cohesive soil properties in 

order to account for the cohesive bed material. The variables used in the analysis are 

parameters defining the soil, the fluid, and the geometry of the modeled system. Depth of 

pier scour, Ds, which is the dependent variable in this analysis, can be expressed as a 

function of the following independent variables: 

D f (Y ,b,V ,D , ,t,g, , ,S,CC,Mn,C,IWC ) s s σ , ,ρ ρ ν = 50 g φ (30) 

in which Ds = depth of scour; Y = depth of approach flow; b = pier width; V = velocity of 

approach flow; D50 = mean sediment diameter; σg = standard deviation of sediment size; 

φ = pier shape factor; ρs=density of sediment particles; t = time; g = gravitational 

acceleration; ρ = fluid density; ν = fluid kinematic viscosity; S = soil shear strength; CC

= clay content; Mn = origin of clay minerals (e.g. Kaolinite, Illite, Montmorillonite); 

C = degree of compaction; and IWC = initial water content. 

 Applying the dimensional analysis using b, V, and ρ as repeating variables, and 

using appropriate simplifications, the following set of dimensionless parameters can be 

obtained: 

, )

ρ

( , , 2 C

V

S f F IWC

b

D r

s = (31) 

in which Fr is the approach Froude number (=V gY ). In deriving equation 31, the clay 

content (CC) was eliminated as a variable since, as shown in chapter 4, the effects of this 

parameter was found to be an independent factor only up to 12 percent clay content. In 

the cohesive pier scour experiments in Montmorillonite clays, the clay content was kept 

constant at 32 percent. In the experiments, the variation of scour with time was 

measured. This relationship was shown to be an asymptotic function with a sharp initial 

scour development followed by a gradual increase(26). The initial rate of scour hole 

development is generally controlled by the nature of the clay mineral and other cohesive 

material parameters such as compaction, initial water content, etc. Past experimental and 

theoretical studies have shown that the velocity at the nose region of a circular pier is 

amplified by a factor of 1.6 to 1.7 times the approach velocity, V. Accordingly, the 

bottom shear stresses that are related to V2

 are also amplified and cause local scour in the 

affected zone. If approach velocities are increased beyond a threshold value defined as 

critical velocity, the entire approach channel bottom becomes subject to general scour in 

addition to the local scour. Under these conditions, the scour hole development process 

continues until equilibrium slopes are attained for the entire reach and may last 

indefinitely. The experimental study presented in this report limited itself to conditions in 

which the oncoming flows do not scour the approach reach (clear water conditions). 

Under these conditions, as soon as the scour hole reaches a depth where the shear stress 

within the base becomes equal to the critical shear stress of the cohesive material, local 
97

scouring ceases. The duration of experiments in the study were long enough to maintain 

the equilibrium condition for at least 4 hours. The final scour depth values obtained 

under these conditions are independent of time, and therefore in deriving equation 31 the 

time parameter, t, is eliminated. 

 Pier scour analysis in this study was conducted under two major categories: 1) 

unsaturated Montmorillonite clay scour and 2) saturated Montmorillonite clay scour. The 

distinction is made since for saturated cohesive materials, parameters such as Torvane 

shear strength and compaction have no physical significance; whereas these parameters 

are important in unsaturated cohesive material scour. 

UNSATURATED CONDITIONS

 The measured values of (Ds /b) were regressed against the remaining 

dimensionless groups in equation 31. The best-fit regression equation resulting from the 

statistical analysis of experimental data is: 

0.36 1.92 1.62 24,715 ( )− − = IWC F C

b

D r

s (32) 

where the initial water content (IWC) and compaction (C) are in percent. In deriving this 

expression, the initial water content ranged from 15 to 50 percent, and compaction ranged 

from 50 to 100 percent. The development of equation 32 is based on laboratory tests in 

which Froude numbers ranged from 0.18 to 0.37, and soil shear strength values ranged 

from 0.1 to 0.45 kg/cm2

. The higher value of the correlation coefficient, R2 = 0.95, 

between the observed and predicted scour ratio indicates the strong correlation between 

measured scour depths and the parameters selected for defining flow and sediment 

properties. The plot of equation 32 with observed data is presented in figure 34. Pier 

scour corresponding to Froude numbers less than 0.2 and for compaction ratios higher 

than 85 percent is zero (scour threshold conditions). 

SATURATED CONDITIONS

 For saturated Montmorillonite clay soils, equation 31 can be simplified further by 

eliminating the dimensionless soil shear strength parameter, (S/ρV2

), since this term has 

no physical meaning for saturated clays at high initial water contents (it approaches to 0). 

Also, for saturated conditions the compaction of cohesive soils, C, is mainly related to the 

water content and can therefore be removed from the list of independent variables. 

Introducing the pier scour initiating Froude number, Fi, to define threshold conditions for 

pier scour and replacing Fr by the excess Froude number, (Fr-Fi), equation 31 becomes: 

f (F F , IWC) b

D r i

s = − (33) 
98

 Using the results of experimental study, Fi and Ds / b are determined as(25, 26): 

( )2

350

IWC

Fi = (34) 

and 

( ) r i r i

s IWC F F F F

b

D = 0.0288( ) − ; ≥ 1.14 0.6 (35) 

For approach Froude numbers less than the pier scour initiating Froude number (i.e., Fr < 

Fi ), depth of scour is zero. For supercritical approach conditions, the value of 

experimental coefficient 0.0288 was found to be 0.0131. The plot of equation 35 with 

observed data is presented in figure 35. 

0.0 

0.2 

0.4 

0.6 

0.8 

1.0 

1.2 

1.4 

1.6 

1.8 

2.0 

Computed Dimensionless Scour

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 

Observed Dimensionless Scour

Line of Perfect Agreement

Figure 34. Computed and measured dimensionless pier scour depth for unsaturated 

Montmorillonite clay. 

 
99

0.0 

0.3 

0.6 

0.9 

1.2 

1.5 

Computed Dimensionless Scour Depth

0.0 0.3 0.6 0.9 1.2 1.5 

Observed Dimensionless Scour Depth

Line of Perfect Agreement

Figure 35. Computed and measured dimensionless pier scour depth for saturated 

 Montmorillonite clay.

5.5 CONCLUSIONS

 There is a distinction between pier scour taking place in unsaturated compacted 

clayey soils and saturated clayey soils. This differentiation affects parameters controlling 

local pier scour. For unsaturated compacted Montmorillonite clay soils, a new scour 

depth predictor is proposed in terms of initial water content, Froude number, soil shear 

strength, and degree of compaction. The pier scour depth and volume decrease as the 

compaction of cohesive soils increases. For saturated cohesive soils, the scour depth can 

be expressed as a function of initial water content and excess Froude number, (Fr-Fi). 

Under saturated conditions, the scour depth is directly proportional to excess Froude 

number and is inversely proportional to initial water content. 
100
101

6. EFFECT OF COHESION ON ABUTMENT 

 SCOUR

 

 In this chapter, effects of cohesion on local abutment scour are investigated 

experimentally for Montmorillonitic and Kaolinitic clay mixtures using larger scale, 1.2-

m and 2.4-m wide test flumes at the Engineering Research Center, CSU. For cohesive 

soils with significant clay content (30 percent for the present mixtures), soil parameters 

such as compaction, initial water content, degree of saturation, shear strength, and type of 

clay mineral dominate the abutment scour. In this study these effects are quantified for 

flow conditions with Froude numbers ranging from 0.2 to 0.9. Equations relating flow 

and selected cohesive soil parameters to abutment scour were developed to explain the 

variability of abutment scour with cohesion properties. These equations express 

abutment scour in cohesive soils relative to clear-water scour in noncohesive material for 

the same flow and geometry conditions. Under the same geometric and flow conditions, 

measured scour in cohesive materials varies from 7 percent to 140 percent of that 

measured in medium sand. 

6.1 GENERAL

 Flow structure around abutments and the resulting local scour were studied in the 

past in numerous experimental, numerical, and analytical investigations for noncohesive 

materials. As a result, series of empirical and semiempirical prediction equations were 

developed to relate the scour depth at bridge abutments to different approach flow 

conditions, to sediment size and gradations, and to different abutment types and sizes. 

 The mechanism of cohesive-material scour is significantly different from scouring 

of alluvial noncohesive materials. The process involves not only the balancing of flowinduced shear stresses and the shear strength of soils to withstand scour, but also the 

chemical and physical bonding of individual particles and the properties of the eroding 

fluid. Cohesive materials, once eroded, remain in suspension. As a result, the 

phenomenon identified as clear-water local scour in noncohesive materials always 

prevails. Along with the eroding fluid properties, the scour process in cohesive soils is 

strongly affected by the amount of cohesive material present in the soil mixture as well as 

the type of mineral clay, initial water content, soil shear strength, and compaction of the 

clay. The objectives of this paper are to apply the knowledge gained in the past in 

cohesive material scour to study local scour around abutments and to analyze effects of 

compaction, initial water content, soil shear strength, and the approach flow conditions 

on abutment scour. For this purpose two different types of clay mixtures were used. The 

first cohesive mixture was a naturally occurring soil that contained 32 percent 

Montmorillonite mineral clay with almost equal amounts of fine sand and silt. The 

second cohesive mixture was prepared by blending 30 percent pure Kaolinite clay with 

medium sand. Experiments using the first soil were conducted under unsaturated and 

saturated soil conditions. The Kaolinite clay scour experiments were limited to 
102

unsaturated conditions where compaction and initial water content of the mixture were 

varied. As a result of this experimental study, empirical relationships were developed 

relating the scour in cohesive material to that observed in the noncohesive material that 

was used in preparing the mixtures under the same flow and geometric conditions. 

6.2 EXPERIMENTAL SETUP AND MEASUREMENTS

 The two categories of abutment scour experiments presented in this paper are 

classified as Montmorillonite and Kaolinite clay experiments. In each category of 

experiments, series of runs utilizing two experimental flumes were conducted by varying 

the clay properties under different flow conditions. A total of 126 experiments were 

conducted covering a wide range of flow and soil conditions. Details of the experiments 

are presented by Yakoub(22) and Molinas and Reiad (5). 

FLUMES

 Experiments were conducted in two different larger scale test flumes housed at 

the Engineering Research Center at CSU to achieve desired flow intensities. These 

flumes are identified as the sediment transport flume and the steep flume. The sediment 

transport flume is 2.4 m wide by 60 m long, and the steep flume is 1.2 m wide by 12 m 

long. The flow depths used in the experiments varied between 0.12 m and 0.3 m, and 

approach Froude numbers ranged from 0.1 to 0.9. 

MEASUREMENTS

 Flow and sediment parameters measured in experiments were velocity 

distribution (vertical, longitudinal, and lateral), depth of flow, depth of scour, initial water 

content of soil, Torvane shear strength, and degree of compaction. Velocity 

measurements were carried out by the use of a magnetic flow-meter. In the experiments, 

approach velocity to each abutment was determined by depth- and width-integrated 

average of vertical velocity profiles. Similarly, the approach depth was determined from 

a width- and length-averaged value of water surface and bed elevation measurements. 

The bed elevations were measured along the flumes across the flow channel before and 

after each experiment. The depth of abutment scour was measured during and at the end 

of each experiment; it was determined by the difference between the minimum bottom 

elevation at the nose region of abutment and the maximum elevation away from the 

structure. 

COHESIVE SOILS

 In this study, two cohesive soil mixtures were used. The first is a naturally 

occurring homogeneous soil containing 32 percent clay, 30 silt, and 38 percent fine sand 

particles. Utilizing the X-Ray Diffraction Test, the dominant clay mineral was found to 

be Montmorillonite. According to the unified soil classification system, the cohesive soil 

is classified as medium plasticity clay and the texture as clay loam. The second cohesive 

soil mixture was prepared by blending commercially obtained pure Kaolinite clay with 
103

medium sand. This mixture was composed of 30 percent clay and 70 percent sand. 

Montmorillonite clay experiments studied the effects of compaction, initial water content, 

and shear strength for unsaturated and saturated soil conditions. This was achieved by 

compacting the cohesive material placed around the abutments at various degrees of 

compaction and by using soils of different initial water content, whereas Kaolinite clay 

experiments investigated the effects of initial water content on unsaturated clay erosion 

by preparing mixtures with varying initial water contents. In these experiments 

compaction was also varied over a narrow range of conditions. 

ABUTMENTS

 In the experiments, rectangular vertical-wall abutments constructed of clear 

Plexiglas with protrusion lengths of 0.22 m and 0.11 m were used. Lengths of these 

abutments in the direction of flow were 0.44 m and 0.22 m, respectively. The scour 

depth development was measured against time utilizing three measuring tapes attached to 

the interior wall of each abutment and by the use of a small inclined mirror. 

6.3 EXPERIMENTAL RESULTS

 Results of abutment scour experiments in cohesive materials are presented in 

tables 29 through 32. These tables present the flow conditions and volumetric scour 

measurements along with cohesive soil properties and measured maximum local scour 

values. The clay mineral present in the cohesive soil mixtures used in set 1 and set 2 

experiments (given in tables 29 and 30) was Montmorillonite. Tables 31 and 32 present 

results of experiments utilizing Kaolinite clay mixtures. The analysis that is presented in 

this study is aimed at quantifying cohesive material effects in terms of easily obtainable 

soil parameters. For this purpose, the parameters selected to represent the scour 

resistance of cohesive materials were limited to those given in tables 29 through 32. 

They were: clay content, compaction, initial water content, shear strength, and clay 

mineral. The effects of clay content on bridge scour are treated in a separate chapter in 

this report. In the following analysis, effects due to cohesive material properties other 

than clay content are investigated. In the past, erodibility of cohesive soils was also 

related to surrounding fluid, disturbed versus undisturbed soils, sodium adsorption ratio, 

plasticity, etc. Due to the complexities involved in quantifying these effects (a much 

larger variety of clays), these parameters were excluded from the present analysis.
104

Table 29. Results of Montmorillonite clay experiments conducted in the 2.44-m wide 

 sedimentation flume using a 0.22-m abutment protrusion length. 

Torvane

Shear

Strength

S RUN ID 

Flow

Discharge

Q

(l/s)

Percent

Clay

Content

CC

(%)

Initial

Water

Content

IWC

(%)

Dry

Density

γdry

(t/m3)

Compaction

C

(%) (kPa)

Flow

Depth

Y

(m)

Approach

Velocity

V

(m/s)

Scour

Depth

Dsc

(m)

Scour

Volume

Vsc

(l)

Flow

Temp.

T 

(

o

C)

8-12-MA 144.42 0 4.0 1.758 91.4 3.655 0.311 0.171 0.020 1.812 17.2 

8-13-MA 184.06 0 4.0 1.758 91.4 3.655 0.312 0.223 0.058 4.106 17.2 

8-14-MA 176.98 0 4.0 1.758 91.4 3.655 0.320 0.207 0.047 3.313 17.2 

8-15-MA 198.22 0 4.0 1.758 91.4 3.655 0.309 0.235 0.060 4.191 17.2 

8-16-MA 229.37 0 4.0 1.758 91.4 3.655 0.319 0.250 0.069 4.899 17.2 

8-17-MA 257.12 0 4.0 1.758 91.4 3.655 0.322 0.287 0.110 9.090 17.2 

8-18-MA 295.63 0 4.0 1.758 91.4 3.655 0.326 0.317 0.171 14.102 17.2 

8-19-MA 242.39 0 4.0 1.758 91.4 3.655 0.327 0.253 0.087 3.398 17.2 

8-20-MA 172.73 10 5.5 1.510 74.2 9.204 0.275 0.171 0.088 5.607 17.2 

8-21-MA 147.25 10 5.5 1.510 74.2 9.204 0.282 0.162 0.027 2.039 17.2 

8-22-MA 202.47 10 5.5 1.510 74.2 9.204 0.296 0.210 0.049 3.511 17.2 

8-23-MA 232.20 10 5.5 1.510 74.2 9.204 0.289 0.229 0.061 4.304 17.2 

8-24-MA 263.35 10 5.5 1.510 74.2 9.204 0.307 0.271 0.066 4.587 16.7 

8-25-MA 311.49 10 5.5 1.510 74.2 9.204 0.298 0.341 0.142 10.987 16.7 

8-26-MA 368.12 10 5.5 1.510 74.2 9.204 0.307 0.424 0.204 25.287 16.7 

8-27-MA 311.49 32 29.7 1.483 85.8 9.786 0.245 0.448 0.000 0.000 13.3 

8-27-MB 311.49 32 13.1 1.266 73.2 44.037 0.258 0.378 0.110 9.005 13.3 

8-28-MA 518.20 32 27.5 1.483 85.8 14.679 0.248 0.704 0.000 0.000 12.2 

8-28-MB 518.20 32 38.1 1.363 78.8 2.936 0.242 0.646 0.107 2.492 12.2 

8-29-MA 792.87 32 27.5 1.091 63.1 14.679 0.244 0.869 0.000 0.000 12.2 

8-29-MB 792.87 32 38.1 1.141 66.0 2.936 0.258 0.674 0.162 8.495 12.2 

8-30-MA 368.12 32 16.7 1.271 73.5 46.973 0.252 0.549 0.249 15.489 12.8 

8-30-MB 368.12 32 20.8 1.295 74.9 44.037 0.260 0.479 0.122 3.993 12.8 

8-31-MA 368.12 32 17.7 1.000 57.9 17.615 0.257 0.451 0.354 54.000 12.2 

8-31-MB 368.12 32 13.4 1.110 64.3 10.765 0.273 0.415 0.271 47.997 12.2 

8-32-MA 254.85 13 8.7 1.581 80.0 11.743 0.252 0.351 0.140 10.506 13.3 

8-32-MB 254.85 5 6.1 1.691 86.5 5.872 0.264 0.314 0.107 9.005 13.3 

8-33-MA 311.49 13 6.4 1.352 68.4 11.743 0.253 0.436 0.277 28.005 12.8 

8-33-MB 311.49 5 4.0 1.432 73.3 5.872 0.265 0.390 0.195 24.013 12.8 

8-34-MA 424.75 13 2.4 1.420 71.9 9.786 0.233 0.646 0.445 95.003 13.3 

8-34-MB 424.75 5 3.1 1.501 76.8 7.829 0.238 0.558 0.366 78.013 13.3 

8-35-MA 424.75 32 17.1 1.306 75.5 48.930 0.245 0.631 0.198 25.995 13.3 

8-35-MB 424.75 32 16.4 1.260 72.9 34.251 0.244 0.594 0.357 44.995 13.3 

8-36-MA 501.21 32 34.4 1.380 79.8 2.936 0.246 0.838 0.155 16.509 15.0 

8-36-MB 501.21 32 44.6 1.240 71.7 0.979 0.230 0.799 0.238 25.995 15.0 

8-37-MA 736.24 32 35.7 1.363 78.8 2.936 0.306 0.911 0.082 2.407 15.0 

8-37-MB 736.24 32 45.3 1.329 76.9 0.979 0.292 0.951 0.271 44.004 15.0 

8-38-MA 948.61 32 36.8 1.340 77.5 2.936 0.351 0.997 0.091 4.502 15.6 

8-38-MB 948.61 32 44.4 1.237 71.6 0.979 0.320 1.039 0.338 95.994 15.6 

Note: 

Duration of experiments varied between 12 to 16 hours.
105

Table 30. Results of Montmorillonite clay experiments conducted in the 1.22-m wide 

 flume using a 0.11-m abutment protrusion length. 

Percent Initial Torvane

Flow Clay Water Dry Shear Flow Approach Scour Scour Flow

Run Discharge Content Content Density Compaction Strength Depth Velocity Depth Volume Temp.

ID Q CC IWC γdry C S Y V Dsc Vsc T

(l/s) (%) (%) (t/m3) (%) (kPa) (m) (m/s) (m) (l) (

o

C)

4-39 36.81 32 20.2 1.363 78.8 34.251 0.140 0.149 0.025 0.113 13.3 

4-40 59.47 32 19.7 1.369 79.2 31.316 0.143 0.271 0.064 1.161 13.3 

4-41 48.14 32 19.8 1.300 75.2 34.251 0.146 0.198 0.041 0.510 13.3 

4-42 70.79 32 19.6 1.317 76.2 40.123 0.144 0.360 0.109 4.191 13.3 

4-43 39.64 32 20.0 1.088 63.0 13.113 0.135 0.165 0.029 0.255 13.3 

4-44 55.78 32 19.6 1.139 65.9 13.700 0.140 0.256 0.063 1.104 13.9 

4-45 67.96 32 20.2 1.134 65.6 14.092 0.140 0.351 0.137 8.495 13.9 

4-46 90.61 32 19.5 1.409 81.5 66.056 0.129 0.503 0.168 9.486 13.9 

4-47 49.55 32 12.3 1.214 70.2 37.187 0.131 0.238 0.081 3.398 13.9 

4-48 36.81 32 12.3 1.168 67.6 36.209 0.129 0.192 0.049 1.246 13.9 

4-49 55.22 32 11.1 1.139 65.9 27.401 0.132 0.311 0.145 9.713 13.9 

4-50 66.54 32 10.8 1.141 66.0 23.487 0.135 0.372 0.193 17.500 13.9 

4-51 45.31 5 6.2 1.472 75.3 5.480 0.130 0.223 0.031 0.425 13.9 

4-52 70.79 5 6.2 1.472 75.3 5.480 0.131 0.378 0.130 11.497 13.9 

4-53 53.80 5 6.2 1.472 75.3 5.480 0.130 0.305 0.071 1.897 13.9 

4-54 35.40 5 6.2 1.472 75.3 5.480 0.126 0.168 0.030 0.113 13.9 

4-55 87.78 5 6.2 1.472 75.3 5.480 0.130 0.506 0.209 25.513 13.9 

4-56 38.23 13 11.2 1.478 74.8 27.401 0.132 0.198 0.030 0.113 13.3 

4-57 48.14 13 11.2 1.478 74.8 27.401 0.129 0.226 0.055 0.850 13.3 

4-58 56.63 13 11.2 1.478 74.8 27.401 0.135 0.308 0.096 3.200 13.9 

4-59 65.13 13 11.2 1.478 74.8 27.401 0.130 0.357 0.130 5.493 13.3 

4-60 82.12 13 11.2 1.478 74.8 27.401 0.129 0.491 0.137 7.108 12.8 

4-61 80.70 32 35.6 1.371 79.3 4.698 0.129 0.482 0.081 0.595 13.3 

4-62 101.94 32 30.4 1.413 81.7 4.893 0.131 0.616 0.066 1.897 13.3 

4-63 120.35 32 29.0 1.420 82.1 5.480 0.134 0.683 0.034 2.209 13.3 

4-64 106.19 32 35.3 1.380 79.8 3.327 0.134 0.637 0.085 3.511 13.3 

4-65 121.76 32 34.8 1.415 81.9 4.306 0.133 0.686 0.107 3.908 14.4 

4-66 52.39 32 43.3 1.323 76.5 1.566 0.129 0.287 0.074 1.388 14.4 

4-67 65.13 32 40.3 1.363 78.8 1.175 0.130 0.384 0.093 2.209 14.4 

4-68 82.12 32 44.5 1.323 76.5 1.370 0.128 0.482 0.119 5.409 13.9 

4-69 48.14 32 45.8 1.351 78.1 1.370 0.131 0.250 0.044 0.311 13.3 

4-70 104.77 32 45.8 1.351 78.1 1.370 0.141 0.619 0.172 5.805 13.9 

Note: 

Duration of experiments varied between 12 to 16 hours. 
106

Table 31. Results of Montmorillonite clay experiments conducted in the 2.44-m wide 

 flume using a 0.1-m abutment protrusion length. 

RUN 

ID 

Flow 

Discharge 

Q 

(l/s) 

Percent 

Clay 

Content 

CC 

(%) 

Initial 

Water 

Content 

IWC 

(%) 

Dry 

Density 

γdry

(t/m3

) 

Compaction

C 

(%) 

Torvane

Shear 

Strength

S 

(kPa) 

Flow 

Depth

Y 

(m) 

Approach 

Velocity 

U 

(m/s) 

Scour 

Depth 

Dsc

(m) 

Scour 

Volume

Vsc

(l) 

Flow 

Temp.

T 

(

o

C) 

8-80-MA 356.79 32 46.7 1.226 70.9 0.979 0.177 0.664 0.113 4.899 20.0 

8-81-MA 240.69 0 4.0 1.758 91.4 3.655 0.155 0.503 0.253 21.011 18.3 

8-81-MB 240.69 5 6.9 1.363 69.7 1.957 0.148 0.433 0.210 5.097 18.3 

8-81-MC 240.69 10 11.4 1.463 71.9 7.829 0.155 0.384 0.131 0.963 18.3 

8-81-MD 240.69 13 11.0 1.254 63.5 7.829 0.146 0.366 0.140 1.161 18.3 

8-82-MA 172.73 0 4.0 1.758 91.4 3.655 0.152 0.360 0.158 16.990 18.9 

8-82-MB 172.73 5 4.3 1.389 71.1 1.957 0.148 0.308 0.104 3.596 18.9 

8-82-MC 172.73 10 10.1 1.449 71.2 7.829 0.158 0.277 0.073 1.104 18.9 

8-82-MD 172.73 13 5.6 1.329 67.3 7.829 0.155 0.250 0.067 1.246 18.9 

8-83-MA 133.09 0 4.0 1.758 91.4 3.655 0.143 0.280 0.113 6.513 19.4 

8-83-MB 133.09 5 4.2 1.300 66.5 1.957 0.145 0.229 0.094 1.897 19.4 

8-83-MC 133.09 10 8.5 1.320 64.9 7.829 0.148 0.204 0.067 0.850 19.4 

8-83-MD 133.09 13 9.9 1.335 67.6 7.829 0.146 0.177 0.052 0.453 19.4 

8-84-MA 300.16 0 4.0 1.758 91.4 3.655 0.155 0.680 0.274 63.996 20.0 

8-84-MB 300.16 5 6.2 1.289 66.0 1.957 0.151 0.591 0.262 32.989 20.0 

8-84-MC 300.16 10 11.3 1.193 58.7 7.829 0.151 0.533 0.152 16.990 20.0 

8-84-MD 300.16 13 9.8 1.286 65.1 7.829 0.149 0.454 0.165 19.001 20.0 

Note: 

Duration of experiments varied between 12 to 16 hours. 
107

Table 32. Results of Kaolinite clay experiments in the 2.44-m wide flume, with 0.22-m 

 abutment width. 

Run

ID

Flow

Discharge

Q

(l/s)

Percent

Clay

Content

CC

(%)

Initial

Water

Content

IWC

(%)

Dry

Density

(dry

(t/m3)

Compaction

C

(%)

Torvane

Shear

Strength

S

(kPa)

Flow

Depth

Y

(m)

Approach

Velocity

V

(m/s)

Scour

Depth

Dsc

(m)

Scour

Volume

Vsc

(l)

Flow

Temp.

T

(

o

C)

8-71-MA 157.16 30 14.8 1.661 84.1 30.337 0.177 0.308 0.015 0.453 19.4 

8-71-MB 157.16 30 13.4 1.672 84.7 41.101 0.173 0.262 0.000 0.000 19.4 

8-71-MC 157.16 30 13.3 1.684 85.3 42.080 0.168 0.238 0.055 1.303 19.4 

8-72-MA 421.92 30 15.4 1.707 86.4 23.487 0.191 0.811 0.030 0.793 19.4 

8-72-MB 421.92 30 14.7 1.758 89.0 41.101 0.183 0.820 0.037 0.963 19.4 

8-72-MC 421.92 30 15.4 1.707 86.4 16.636 0.175 0.960 0.070 1.897 19.4 

8-73-MA 286.00 30 20.1 1.741 88.2 7.829 0.159 0.637 0.021 0.651 18.9 

8-73-MB 286.00 30 26.5 1.621 82.1 3.914 0.157 0.527 0.040 1.048 18.9 

8-73-MC 286.00 30 29.7 1.523 77.1 1.957 0.165 0.457 0.037 0.991 18.9 

8-74-MA 351.13 30 18.8 1.732 87.7 10.765 0.165 0.719 0.030 0.850 20.0 

8-74-MB 351.13 30 23.4 1.641 83.1 4.893 0.155 0.686 0.046 1.189 20.0 

8-74-MC 351.13 30 27.5 1.541 78.0 2.936 0.139 0.777 0.058 1.388 20.0 

8-75-MA 237.86 30 21.8 1.695 85.8 4.893 0.158 0.512 0.000 0.000 20.0 

8-75-MB 237.86 30 25.1 1.641 83.1 1.957 0.161 0.460 0.000 0.000 20.0 

8-75-MC 237.86 30 29.7 1.532 77.6 0.979 0.170 0.439 0.000 0.000 20.0 

8-76-MA 489.88 30 21.8 1.695 85.8 4.893 0.193 0.817 0.037 0.963 20.0 

8-76-MB 489.88 30 25.1 1.641 83.1 1.957 0.177 0.808 0.067 1.812 20.0 

8-76-MC 489.88 30 29.7 1.532 77.5 0.979 0.167 1.201 0.158 9.486 20.0 

8-77-MA 170.75 10 15.5 1.695 85.8 2.936 0.152 0.030 0.146 8.014 19.4 

8-77-MB 170.75 20 17.1 1.741 84.5 3.914 0.158 0.223 0.012 0.396 19.4 

8-77-MC 170.75 50 16.6 1.552 85.2 45.016 0.173 0.195 0.094 3.511 19.4 

8-77-MD 170.75 50 27.1 1.478 81.1 2.936 0.174 0.171 0.000 0.000 19.4 

8-78-MA 235.03 10 13.5 1.692 85.6 2.936 0.155 0.512 0.229 13.989 19.4 

8-78-MB 235.03 20 14.7 1.518 73.6 3.914 0.155 0.460 0.046 0.481 19.4 

8-78-MC 235.03 50 15.7 1.335 73.3 45.016 0.168 0.439 0.192 6.513 19.4 

8-78-MD 235.03 50 25.8 1.586 87.0 2.936 0.171 0.372 0.000 0.000 19.4 

8-79-MA 288.83 10 16.2 1.841 93.2 2.936 0.162 0.582 0.256 21.011 20.0 

8-79-MB 288.83 20 16.8 1.775 86.1 3.914 0.161 0.479 0.052 1.359 20.0 

8-79-MC 288.83 50 15.7 1.335 73.3 45.016 0.174 0.424 0.177 12.006 20.0 

8-79-MD 288.83 50 30.9 1.455 79.9 2.936 0.174 0.357 0.000 0.000 20.0 

8-80-MB 356.79 20 16.8 1.775 86.1 3.914 0.170 0.607 0.152 10.987 20.0 

8-80-MC 356.79 50 15.7 1.335 73.3 45.016 0.176 0.539 0.226 18.010 20.0 

8-80-MD 356.79 50 30.9 1.455 79.9 2.936 0.168 0.472 0.000 0.000 20.0 

Note: 

Duration of experiments varied between 12 to 16 hours. 
108

6.4 ANALYSIS

 The functional relationship between the maximum depth of abutment scour and 

the parameters defining the soil, fluid, and the geometry of the abutment is derived 

through dimensional analysis. The depth of abutment scour in cohesive material (Dsc) is 

expressed as a function of the following independent variables: 

D f ( D ,Y ,a,L,V ,IWC,C,CC,S,Mn,T ,t,g, , ) sc = s α φ, ρ,υ (36) 

in which Ds = depth of abutment scour in noncohesive material for conditions 

corresponding to Dsc; Y = depth of approach flow; a = abutment protrusion length; L = 

length of abutment in the direction of flow; V = velocity of approach flow; IWC = initial 

water content; C = compaction related to the optimum compaction; CC = clay content; S

= Torvane shear stress, Mn = type of clay (e.g., Kaolinite, Illite, Montmorillonite); T = 

water temperature; t = duration of experiment; g = gravitational acceleration, " = angle of 

attack; N = abutment shape factor; D = fluid density; and L = kinematic viscosity of fluid. 

 Applying the dimensional analysis using Ds, V, and D as repeating variables, and 

using appropriate simplifications, the following set of dimensionless parameters can be 

obtained: 

( , , , ) 2 C Mn

V

S f IWC

D

D

s

sc

ρ = (37) 

In the derivation of equation 37, the CC was eliminated as a variable since, as shown in 

Chapter 4, the effects of this parameter was found to be an independent factor up to 12 

percent clay content. In the cohesive abutment scour experiments, the clay content was 

kept above 30 percent. In abutment scour experiments the variation of scour with time 

was measured. This relationship was shown by Molinas and Reiad to be an asymptotic 

function with a sharp initial scour development followed by a gradual increase(5). The 

initial rate of scour hole development is generally controlled by the nature of the clay 

mineral and other cohesive material parameters such as compaction, initial water content, 

etc. Experimental and theoretical studies have shown that the velocity at the nose region 

of a vertical wall abutment is amplified by a factor of 1.2 to 1.8 times the approach V. 

According to Molinas, Khereldin, and Wu, bottom shear stresses that are related to V2

 are 

also amplified (by up to 11 times) and cause local scour in the affected zone(26). If 

approach velocities are increased beyond a threshold value defined as critical velocity, 

the entire approach channel bottom becomes subject to general scour in addition to the 

local scour. Under these conditions, the scour hole development process continues until 

equilibrium slopes are attained for the entire reach and may last indefinitely. The 

experimental study presented in this report limited itself to conditions in which the 

oncoming flows do not scour the approach reach (clear water conditions). Under these 

conditions, as soon as the scour hole reaches a depth where the shear stress within the 

base becomes equal to the critical shear stress of the cohesive material, local scouring 

ceases. The duration of experiments in the study were long enough to maintain the 

equilibrium condition for at least 4 hours. The final scour depth values obtained under 
109

these conditions are independent of time, and therefore in deriving equation 37 the time 

parameter, t, is eliminated. Additionally, the dimensionless S can be related to C and 

IWC, further reducing the number of parameters. 

The local abutment scour analysis in this study was conducted separately for the 

two clay types. This distinction is made since the type of clay mineral (Mn) was found to 

have a dominant effect on the scourability of bed material. 

 

MONTMORILLONITE SCOUR

 The measured values of (Dsc /Ds) were regressed against the remaining 

dimensionless groups in equation 37 using nonlinear multiple regression analysis. The 

best-fit regression equations resulting from the statistical analysis of experimental data 

were reported earlier by Molinas, Reiad, and Jones(27) and are given below. 

For unsaturated clays with initial water content less than 25 percent: 

(2.186 0.05342 ) (15.407 0.522 0.006087 0.0000235 ) 2 3 IWC C C C

D

D

s

sc = − − + − (38) 

where the IWC and C are in percent. 

For saturated clays with initial water content in the range of 28 to 45 percent: 

(4.76 0.451 0.01361 0.000126 ) (0.339 0.01744 ) 2 3 IWC IWC IWC C

D

D

s

sc = − + − + (39) 

In deriving equations 38 and 39, IWC ranged from 12 to 45 percent, and C ranged 

from 58 to 89 percent. Equations 38 and 39 were developed based on laboratory tests in 

which Froude numbers ranged from 0.1 to 0.6 and soil shear strength values ranged from 

0.1 to 0.63 kg/cm2

. The plot of equations 38 and 39 with observed data is presented in 

figure 36. 

0 

0.2 

0.4 

0.6 

0.8 

1 

1.2 

1.4 

Computed Relative Scour, Dsc/Ds

0 0.2 0.4 0.6 0.8 1 1.2 1.4 

Measured Relative Scour, Dsc/Ds

Figure 36. Computed and measured relative abutment scour for Montmorillonite clay. 
110

0 

0.05 

0.1 

0.15 

0.2 

0.25 

Computed Relative Scour, Dsc/Ds

0 0.05 0.1 0.15 0.2 0.25 

Measured Relative Scour, Dsc/Ds

Figure 37. Computed and measured relative abutment scour for Kaolinite clay.

KAOLINITE SCOUR

 For unsaturated Kaolinite clay soils, the basic form of equation 37 was retained. 

For the cohesive soil consisting of 30 percent Kaolinite clay and 70 percent medium sand 

with a median diameter of 0.81 mm, the best-fit regression equation from the statistical 

analysis of experimental data is: 

(0.12 0.014 0.00205 0.000057 ) (0.4 0.017 ) 2 3 IWC IWC IWC C

D

D

s

sc = + − − + (40) 

Scour initiating velocities for the Kaolinite clay mixture were experimentally determined 

to be 0.6 m/s. For velocities smaller than this value, the scour ratio is zero. The plot of 

equation 40 with observed data is presented in figure 37. 

 

6.5 CONCLUSIONS

 Abutment scour in cohesive soils shows a wide range of variability depending on 

the properties of soils. In the experiments, under the same geometric and flow conditions, 

measured scour in cohesive materials varied anywhere from 7 percent to 140 percent of 

that measured in medium sand. This is due to initial water content, soil shear strength, 

degree of compaction, and type of clay mineral present in the soil. For Montmorillonitic 

soils, the relative scour depth is expressed in terms of initial water content and degree of 

compaction. For unsaturated Montmorillonitic mixtures, abutment scour depth decreases 

as the compaction and initial water content increases. For saturated cohesive soils, 

however, the scour depth is mainly a function of initial water content and is inversely 

proportional to initial water content. For unsaturated Kaolinitic mixtures the initiation of 

scour takes place under higher flow intensities, and under same flow conditions, the total 

scour may be up to 80 percent smaller than the corresponding scour encountered in 

noncohesive material. 
111

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Scour, Vol. 1, Effect of Sediment Gradation and Coarse Material Fraction on Clear 

Water Scour Around Bridge Piers, Federal Highway Administration, FHWA-RD99-183.

2. Molinas, A., and Noshi, M. H. (1998). Effects of Gradation and Cohesion on 

Bridge Scour, Vol. 2, Experimental Study of Sediment Gradation and Flow 

Hydrograph Effects on Clear Water Scour Around Circular Piers, Federal Highway 

Administration, FHWA-RD-99-184.

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Administration, FHWA-RD-99-185. 

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112

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113

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