Clear-Water Abutment and Contraction Scour 

in the Coastal Plain and Piedmont Provinces of 

South Carolina, 1996-99

By Stephen T. Benedict

U.S. Geological Survey

Water-Resources Investigations Report 03-4064

Prepared in cooperation with the

SOUTH CAROLINA DEPARTMENT OF TRANSPORTATION

Columbia, South Carolina

2003
U.S. DEPARTMENT OF THE INTERIOR

GALE A. NORTON, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director

Use of trade, product, or firm names in this publication is for descriptive purposes only 

and does not imply endorsement by the U.S. Geological Survey

For additional information 

write to: 

District Chief 

U.S. Geological Survey

Suite 129

720 Gracern Road

Columbia, SC 29210-7651

Additional information about water resources in South Carolina is available on the 

internet at http://sc.water.usgs.gov

Copies of this report can be 

purchased from:

U.S. Geological Survey

Branch of Information Services

Box 25286

Denver, CO 80225-0286

888-ASK-USGS
Contents III

CONTENTS

Abstract...................................................................................................................................................................................... 1

Introduction................................................................................................................................................................................ 1

Purpose and Scope ........................................................................................................................................................... 2

Acknowledgments............................................................................................................................................................ 2

Previous Investigations..................................................................................................................................................... 2

Description of Study Area................................................................................................................................................ 4

Data Assumptions ...................................................................................................................................................................... 4

Justification for the Assumption of Large Flood Flows................................................................................................... 7

Justification for the Assumption of Equilibrium-Scour Conditions .............................................................................. 11

Basin Equilibrium Scour................................................................................................................................................ 11

Site selection ............................................................................................................................................................................ 12

SCDOT List of Bridges with Observed Scour ............................................................................................................... 12

Selection of Previously Studied Level 2 Bridge-Scour Sites......................................................................................... 13

Selection of Piedmont Sites Influenced by the August 1995 Flood............................................................................... 14

Selection of Piedmont Bridges with Wide, Flat Floodplains......................................................................................... 14

Selection of Coastal Plain Sites Influenced by the September 1999 Flood ................................................................... 14

Techniques for the Collection and Interpretation of Field Data .............................................................................................. 15

Data Collection in the Coastal Plain .............................................................................................................................. 15

Data Collection in the Piedmont .................................................................................................................................... 18

Flow Model Data............................................................................................................................................................ 19

Sediment Samples .......................................................................................................................................................... 19

Contraction Scour in the Abutment Area....................................................................................................................... 20

Pier Scour in the Abutment Area ................................................................................................................................... 21

Estimating Hydraulic Data....................................................................................................................................................... 22

Development of WSPRO Models .................................................................................................................................. 24

Estimates of Hypothetical Flows ................................................................................................................................... 25

Historical Flows ............................................................................................................................................................. 27

Flood of October 1992 ......................................................................................................................................... 27

Flood of August 1995........................................................................................................................................... 27

Flood of September 1999 ..................................................................................................................................... 29

Development of Theoretical Bridge-Scour Database .............................................................................................................. 31

Theoretical Pier Scour.................................................................................................................................................... 31

Theoretical Live-Bed Contraction Scour ....................................................................................................................... 36

Theoretical Clear-Water Contraction Scour................................................................................................................... 38

Theoretical Abutment Scour .......................................................................................................................................... 40

Variables Influencing Abutment Scour.................................................................................................................................... 43

Time and Flow Duration ................................................................................................................................................ 46

Flow Velocity ................................................................................................................................................................. 47

Flow Depth..................................................................................................................................................................... 52

Sediment Size................................................................................................................................................................. 54

Sediment Gradation........................................................................................................................................................ 57

Embankment Length ...................................................................................................................................................... 57

Abutment Shape ............................................................................................................................................................. 61

Embankment Skew......................................................................................................................................................... 62

Channel Geometry ......................................................................................................................................................... 65

Geometric-Contraction Ratio......................................................................................................................................... 65

Conclusions.................................................................................................................................................................... 66
IV Contents

Abutment Scour in the Piedmont .............................................................................................................................................69

Comparison of Observed and Theoretical Abutment-Scour Depths ..............................................................................70

Piedmont Floodplain Geometry as an Indicator of Potential Scour ...............................................................................71

Embankment Length as an Indicator of Potential Scour ................................................................................................71

Geometric-Contraction Ratio as an Indicator of Potential Scour...................................................................................75

Floodplain Land Cover as an Indicator of Potential Scour.............................................................................................77

Abutment Scour in the Coastal Plain........................................................................................................................................79

Abutment-Scour Hole Patterns.......................................................................................................................................79

Comparison of Observed and Theoretical Abutment-Scour Depths ..............................................................................81

Embankment Length as an Indicator of Potential Scour ................................................................................................84

Geometric-Contraction Ratio as an Indicator of Potential Scour...................................................................................84

Bridges 240 Feet or Less in Length................................................................................................................................88

Floodplain Land Cover as an Indicator of Potential Scour.............................................................................................90

Effect of Upstream Channel Alignment on Abutment Scour...................................................................................................90

Guidance for Assessing Abutment-Scour Depth Using the Envelope Curves .........................................................................91

Selecting a Reference Surface for Abutment-Scour Depth ............................................................................................93

Estimating the Embankment Length and the Geometric-Contraction Ratio..................................................................93

Selecting the Appropriate Abutment-Scour Depth Envelope.........................................................................................93

Single-Bridge Openings .......................................................................................................................................94

Multiple-Bridge Openings....................................................................................................................................94

Contraction and Pier Scour within the Abutment-Scour Area .......................................................................................97

Limitations of the Abutment-Scour Depth Envelope Curves.........................................................................................99

Guidance for Assessing Abutment-Scour Hole Location, Width, and Shape ..........................................................................99

Lateral Reference for Scour Hole.................................................................................................................................100

Scour-Hole Top Width..................................................................................................................................................100

Scour-Hole Longitudinal Location...............................................................................................................................100

Scour-Hole Shape .........................................................................................................................................................104

Clear-Water Contraction Scour in the Piedmont ....................................................................................................................105

Comparison of Observed and Theoretical Contraction-Scour Depths .........................................................................110

Geometric-Contraction Ratio as an Indicator of Potential Scour.................................................................................110

Selecting a Reference Surface for Clear-Water Contraction Scour..............................................................................112

Pier Scour Within Clear-Water Contraction-Scour Areas ............................................................................................115

Estimate of Clear-Water Contraction-Scour Hole Location.........................................................................................115

Permissible Velocities for Clear-Water Contraction Scour ..........................................................................................117

The South Carolina Bridge-Scour Database ..........................................................................................................................119

Summary ................................................................................................................................................................................120

Selected References................................................................................................................................................................121

Appendix A: Explanation of Variables in the South Carolina Bridge-Scour Database .........................................................123

Abutment_Scour Table .................................................................................................................................................125

Bridge Info Table..........................................................................................................................................................126

Clay Information Table.................................................................................................................................................127

Clearwater_Scour Table................................................................................................................................................127

Field_Observations Table .............................................................................................................................................128

Livebed_Scour Table ....................................................................................................................................................129

Pier_Scour Table...........................................................................................................................................................130

WSPRO_Scour Table ...................................................................................................................................................131

Selected References......................................................................................................................................................132

Appendix B: South Carolina Bridge-Scour Study Sites and Reference Numbers for Figures 1, 5, and 6.............................133
Contents V

FIGURES

1. Map showing location of physiographic provinces and bridge-scour study sites in South Carolina ......................... 3

2-4. Graphs showing:

2. Distribution of streambed slopes for bridge-scour study sites in the Coastal Plain and Piedmont of 

South Carolina .................................................................................................................................................... 5

3. Distribution of drainage areas for bridge-scour study sites in the Coastal Plain and Piedmont of 

South Carolina .................................................................................................................................................... 5

4. Distribution of bridge age for bridge-scour study sites in the Coastal Plain and Piedmont of South Carolina ........ 8

5. Map showing location of study bridges 40 years or older and streamflow gaging stations experiencing at least 

one flow equaling or exceeding 70 percent of the 100-year flow during 1956-96..................................................... 9

6. Map showing location of bridge-scour study sites affected by selected historic floods in South Carolina ............. 10

7. Graph showing distribution of bridge lengths for sites discarded from the South Carolina Department 

of Transportation list of bridges with observed scour problems .............................................................................. 13

8. Photograph of inflatable boat used to collect field data at swampy sites in the Coastal Plain of South Carolina.... 15

9. Photograph of paper-chart fathometer mounted on inflatable boat used to investigate limits of scour holes 

in the Coastal Plain of South Carolina...................................................................................................................... 16

10. Map showing example of scour-hole contour plot developed from topographic-survey data at Structure 

211009511400 on Interstate 95, crossing the Pee Dee River floodplain in Florence County, South Carolina, 

August 19, 1996........................................................................................................................................................ 17

11-13. Photographs of:

11. Drive-tube coring device used to obtain sediment samples from the bottom of submerged scour holes ......... 18

12. Example of sediment core collected from the bottom of a submerged scour hole using the drive-tube 

coring device..................................................................................................................................................... 18

13. Example of minimal scour in the clayey soils of the Piedmont at Structure 47026300100 on Road S-263, 

crossing the Rocky River in Anderson County, South Carolina....................................................................... 19

14. Graph showing comparison of median grain size between the original and second sediment samples................... 20

15. Graph showing percentile plot for the maximum pier or pile width for bridge-scour study sites in the Piedmont 

and Coastal Plain of South Carolina......................................................................................................................... 22

16-18. Sketches showing:

16. Example of pile bent located at the low point of a scour hole at Structure 367008100200 on Road S-81, 

crossing the Enoree River in Newberry County, South Carolina, April 7, 1997 .............................................. 23

17. Example of pile bent located on the side of a scour hole at Structure 152002100300 on U.S. Route 21, 

crossing Sandy Run Creek in Colleton County, South Carolina, December 18, 1996..................................... 24

18. Profile of pile bent............................................................................................................................................. 31

19-21. Photographs showing:

19. Timber pile bent at Structure 194023000500 on S.C. Route 230, crossing Horne Creek in Edgefield 

County, South Carolina..................................................................................................................................... 32

20. Steel H-pile bent at Structure 467072100100 on Road S-721, crossing Taylors Creek in York County, 

South Carolina .................................................................................................................................................. 32

21. Concrete pile bent at Structure 182007800200 on U.S. Route 78, crossing Polk Swamp in Dorchester 

County, South Carolina..................................................................................................................................... 33

22. Sketch showing profile of pier on spread footing and pile group............................................................................. 33

23. Photograph showing pier at Structure 262050103100 on U.S. Route 501, crossing the Waccamaw River in 

Horry County, South Carolina .................................................................................................................................. 34

24. Sketch showing profile of composite bent................................................................................................................ 34

25. Photograph showing composite bent at Structure 262050103200 on U.S. Route 501, crossing the Waccamaw 

River in Horry County, South Carolina .................................................................................................................... 35

26-30. Sketches showing:

26. Bridge profile with well-defined low-flow channel, showing subsections and pier identifications for 

theoretical pier-scour database.......................................................................................................................... 37

27. Bridge with swampy channel, showing subsection and pier identifications for theoretical 

pier-scour database............................................................................................................................................ 37

28. Typical bridge with well-defined low-flow channel, showing areas of clear-water scour................................ 39
VI Contents

29. Typical bridge cross section for a swampy channel or floodplain relief bridge, showing area of 

clear-water scour ...............................................................................................................................................39

30. Plan view and profile of spill-through abutment...............................................................................................41

31. Photograph of spill-through abutment at Structure 307011200100 on Road S-112, crossing the Enoree River 

in Laurens County, South Carolina ...........................................................................................................................42

32-34. Sketches showing:

32. Plan view and profile of vertical abutment........................................................................................................43

33. Embankment length and obstructed flow area determined by projection of the bridge cross section onto 

the approach cross section.................................................................................................................................44

34. Simplified streamline patterns at a bridge contraction......................................................................................44

35-51. Graphs showing:

35. Comparison of the simulated 100-year-flow hydrographs for 200-square mile basins in the Piedmont and 

lower Coastal Plain of South Carolina ..............................................................................................................46

36. Comparison of hydrograph durations at 95 percent of the peak flow estimated from simulated 100-year 

flow hydrographs for various basin sizes in the Piedmont and lower Coastal Plain of South Carolina ...........47

37. Relation of observed clear-water abutment-scour depth and the estimated peak-flow duration for the 

100-year flow at single-bridge openings in the Coastal Plain and Piedmont of South Carolina............................48

38. General relation of flow intensity and abutment-scour depth normalized by embankment length based on 

laboratory data...................................................................................................................................................49

39. Relation of abutment-scour depth, normalized by embankment length, and flow intensity for laboratory 

data and observed data in the Piedmont and Coastal Plain of South Carolina..................................................50

40. Relation of observed clear-water abutment-scour depth and the average 100-year-flow velocity, blocked 

by the embankment, at single-bridge openings in the Coastal Plain and Piedmont of South Carolina ............51

41. Comparison of envelopes for the relation of observed clear-water abutment-scour depth and the average

100-year-flow velocity, blocked by the embankment, for embankment lengths of 400 feet or less in 

the Coastal Plain and Piedmont, and embankment lengths of 600 feet and 700 feet or less in the 

Coastal Plain and Piedmont of South Carolina, respectively............................................................................53

42. Relation of clear-water abutment-scour depth and flow depth, normalized by embankment length, for 

selected laboratory data at spill-through abutments and field data collected in the Piedmont and Coastal 

Plain of South Carolina .....................................................................................................................................54

43. Relation of observed clear-water abutment-scour depth and the 100-year-flow depth near the abutment 

toe for single-bridge openings in the Coastal Plain and Piedmont of South Carolina ......................................55

44. Relation of observed clear-water abutment-scour depth and the median grain size of the original sediment 

sample for surface soils at single-bridge openings in the Coastal Plain and Piedmont of South Carolina.......56

45. Relation of abutment-scour depth and embankment length, normalized by flow depth, for laboratory 

data collected at flow intensities of 1 ................................................................................................................58

46. Relation of observed clear-water abutment-scour depth and the 100-year-flow embankment length, 

normalized by the 100-year-flow depth near the abutment toe, for the Piedmont and Coastal Plain of 

South Carolina with a complete horizontal axis, and a truncated horizontal axis ............................................59

47. Relation of observed clear-water abutment-scour depth and the 100-year-flow embankment length for the 

Coastal Plain and Piedmont of South Carolina .................................................................................................60

48. Comparisons of envelopes for observed clear-water abutment-scour depth and the 100-year-flow 

embankment length for the Piedmont and Coastal Plain of South Carolina.....................................................61

49. Relation of abutment-scour depth correction factor and embankment skew based on data obtained from 

laboratory investigations where experimental tests ran for 300 minutes or less ...............................................62

50. Relation of observed clear-water abutment-scour depth and embankment skew for selected sites with 

left and (or) right abutment scour in the Piedmont and Coastal Plain of South Carolina .................................63

51. Relation of observed clear-water abutment-scour depth, for positive and negative skews, at sites with

left and right abutment scour in the Piedmont and Coastal Plain of South Carolina ........................................64

52. Sketch showing profile of abutment projecting into main channel...........................................................................66

53. Sketch showing profile of setback abutment commonly found in the Piedmont of South Carolina.........................66
Contents VII

54-58. Graphs showing:

54. Example of Coastal Plain natural cross section at Structure 212037801000 on U.S. Route 378, crossing 

Big Swamp in Florence County, South Carolina.............................................................................................. 67

55. Example of Piedmont natural cross section at Structure 444005600100 on S.C. Route 56, crossing the 

Enoree River in Union County, South Carolina................................................................................................ 67

56. Relation of abutment-scour depth and geometric-contraction ratio for laboratory data................................... 68

57. Relation of observed clear-water abutment-scour depth and the 100-year-flow geometric-contraction 

ratio in the Coastal Plain and Piedmont of South Carolina .............................................................................. 68

58. Relation of observed clear-water abutment-scour depth and theoretical abutment-scour depth for 

the 100-year flow, and the August 1995 flood at selected sites in the Piedmont of South Carolina ................ 70

59. Map showing example of wide, flat floodplain at Structure 124000901100 on S.C. Route 9, crossing 

Turkey Creek in Chester County, South Carolina, and Structure 362017600400 on U.S. Route 176, crossing 

Indian Creek in Newberry County, South Carolina.................................................................................................. 72

60-68. Graphs showing:

60. Relation of observed clear-water abutment-scour depth and the 100-year-flow embankment length 

in the Piedmont of South Carolina.................................................................................................................... 73

61. Relation of observed clear-water abutment-scour depth and the 100-year-flow embankment length with 

identification of the 1995 flood sites in the Piedmont of South Carolina......................................................... 74

62. Relation of theoretical 100-year-flow abutment-scour depth and embankment length compared with 

the envelope of observed abutment scour for selected sites in the Piedmont of South Carolina...................... 75

63. Relation of observed clear-water abutment-scour depth and the 100-year-flow geometric-contraction 

ratio in the Piedmont of South Carolina ........................................................................................................... 76

64. Relation of observed clear-water abutment-scour depth and the 100-year-flow geometric-contraction 

ratio with identification of the 1995 flood sites in the Piedmont of South Carolina ........................................ 76

65. Relation of theoretical 100-year-flow abutment-scour depth and the 100-year-flow geometric-contraction 

ratio compared with the envelope of observed data in the Piedmont of South Carolina.................................. 77

66. Relation of observed clear-water abutment-scour depth and the 100-year-flow embankment length 

with the identification of sites with lowered floodplain flow resistance for the Piedmont of South Carolina........ 78

67. Percentile of observed clear-water abutment-scour depth in the Piedmont and Coastal Plain of 

South Carolina .................................................................................................................................................. 79

68. Percentile of the 100-year-flow floodplain width for study sites in the Piedmont and Coastal Plain 

of South Carolina .............................................................................................................................................. 80

69. Sketch showing example of single scour hole at shorter bridges in the Coastal Plain, as shown at Structure 

212030100100 on U.S. Route 301, crossing Douglas Swamp in Florence County, South Carolina, July 31, 1996...... 81

70. Sketch showing example of separate left and right abutment-scour holes at longer bridges in the Coastal Plain, 

as shown at Structure 277008700100 on Road S-87, crossing the Coosawhatchie River in Jasper County, 

South Carolina, November 12, 1997 ........................................................................................................................ 82

71-78. Graphs showing:

71. Relation of observed clear-water abutment-scour depth with theoretical abutment-scour depth for the

100-year flow, and the maximum historic flow at selected sites in the Coastal Plain of South Carolina......... 83

72. Relation of observed clear-water abutment-scour depth and the 100-year-flow embankment length for 

the Coastal Plain of South Carolina with a complete horizontal axis and a truncated horizontal axis............. 85

73. Relation of observed clear-water abutment-scour depth and the 100-year-flow embankment length 

identifying the 1992 and 1999 flood sites in the Coastal Plain of South Carolina ........................................... 86

74. Relation of theoretical 100-year-flow abutment-scour depth and the 100-year-flow embankment 

length compared with the envelope of observed abutment scour for selected sites in the Coastal Plain of 

South Carolina .................................................................................................................................................. 86

75. Relation of observed clear-water abutment-scour depth and the 100-year-flow geometric-contraction 

ratio for the Coastal Plain of South Carolina.................................................................................................... 87

76. Relation of observed clear-water abutment-scour depth and the 100-year-flow geometric-contraction 

ratio identifying sites with known maximum historic flows in the Coastal Plain of South Carolina............... 87

77. Relation of theoretical 100-year-flow abutment-scour depth and the 100-year-flow geometric-contraction 

ratio compared with the envelope of observed abutment scour in the Coastal Plain of South Carolina .......... 88
VIII Contents

78. Relation of observed clear-water abutment-scour depth and the 100-year-flow embankment length 

identifying sites with bridge lengths of 240 feet or less and single scour holes in the Coastal Plain of 

South Carolina with a complete horizontal axis and a truncated horizontal axis .............................................89

79. Sketch showing plan view of channel bends and resulting scour hole at Structure 304007200500 

on S.C. Route 72, crossing the Little River in Laurens County, South Carolina ......................................................91

80. Aerial photograph of severe meander on the Waccamaw River at Structure 264000920500 on S.C. Route 9 

in Horry County, South Carolina ..............................................................................................................................92

81. Photograph of scour hole at left abutment viewed from downstream at Structure 264000920500 

on S.C. Route 9, crossing the Waccamaw River in Horry County, South Carolina..................................................92

82-86. Graphs showing:

82. Relation of observed clear-water abutment-scour depth and the 100-year-flow embankment length 

with the identification of multiple-bridge openings for the Piedmont of South Carolina.................................95

83. Relation of observed clear-water abutment-scour depth and 100-year-flow geometric-contraction 

ratio with the identification of multiple-bridge openings for the Piedmont of South Carolina ........................95

84. Relation of observed clear-water abutment-scour depth and the 100-year-flow embankment length 

with the identification of multiple-bridge openings in the Coastal Plain of South Carolina ............................96

85. Relation of observed clear-water abutment-scour depth and the 100-year-flow geometric-contraction 

ratio with the identification of multiple-bridge openings in the Coastal Plain of South Carolina ....................97

86. Relation of observed clear-water abutment-scour depth and the 100-year-flow embankment length 

identifying sites with a pier or pile bent located at the low point of the scour hole for the Coastal Plain, 

and Piedmont of South Carolina .......................................................................................................................98

87. Sketch showing plan view of reference for estimating the width and lateral location of an abutment-scour hole .......101

88-90. Graphs showing:

88. Lateral location of scour-hole bank in reference to the abutment toe for observed abutment scour in the 

Piedmont and Coastal Plain of South Carolina ...............................................................................................101

89. Relation of abutment-scour-hole top width and abutment-scour depth at bridges greater than 240 feet 

in length, and swampy and floodplain relief bridges, 240 feet or less in length in the Piedmont and 

Coastal Plain of South Carolina ......................................................................................................................102

90. Relation of longitudinal location for the low point of the abutment-scour hole and the 100-year-flow 

top width at the bridge for shallow and deep scour holes in the Piedmont of South Carolina, and sites 

in the Coastal Plain of South Carolina ............................................................................................................103

91. Sketch showing example of scour-hole low point located upstream at Structure 367029900100 on 

Road S-299, crossing Cannons Creek in Newberry County, South Carolina, November 24, 1997 .......................104

92. Sketch showing example of scour-hole low point located downstream at Structure 344004100800 

on S.C. Route 41, crossing Maiden Down Swamp in Marion County, South Carolina, December 3, 1996 ..........105

93. Graph showing ground line of existing scour hole compared with the shape of a trapezoid and rectangle .......... 106

94. Sketch showing profile of overbank area for bridges with setback abutments .......................................................106

95. Sketch showing plan view of areas of clear-water abutment and contraction scour...............................................107

96. Map showing example of abutment- and clear-water contraction-scour areas at Structure 367008100200 

on Road S-81, crossing the Enoree River in Newberry County, South Carolina....................................................108

97-100. Photographs showing:

97. Right abutment-scour hole as viewed from downstream at Structure 367008100200 on Road S-81, crossing 

the Enoree River in Newberry County, South Carolina ..................................................................................109

98. Clear-water contraction scour on the right overbank as viewed from the left at Structure 367008100200

on Road S-81, crossing the Enoree River in Newberry County, South Carolina ............................................109

99. Example of shallow clear-water contraction scour at Structure 464032200300 on S.C. Route 322, 

crossing Fishing Creek in York County, South Carolina.................................................................................111

100. Example of shallow clear-water contraction scour at Structure 114000500200 on S.C. Route 5, 

crossing Buffalo Creek in Cherokee County, South Carolina.........................................................................111

101-104. Graphs showing relation of:

101. Observed and theoretical clear-water contraction-scour depth for the 100-year flow, and the August 1995 

flood at selected sites in the Piedmont of South Carolina...............................................................................113

102. Observed clear-water contraction-scour depth and the 100-year-flow geometric-contraction ratio in the

Piedmont of South Carolina ............................................................................................................................114
Contents IX

103. Observed clear-water contraction-scour depth and the 100-year-flow geometric-contraction ratio 

identifying sites for the 1995 flood in the Piedmont of South Carolina ......................................................... 114

104. Theoretical 100-year-flow clear-water contraction-scour depth and the 100-year-flow 

geometric-contraction ratio compared with the envelope of observed data in the Piedmont of 

South Carolina ................................................................................................................................................ 115

105. Photograph of the erosion of bed material from around piles caused by bank widening from the 

August 1995 flood at Structure 307026300100 on Road S-263, crossing the Enoree River in 

Laurens County, South Carolina............................................................................................................................. 116

106. Graph showing relation of longitudinal location for the low point of the clear-water contraction-scour hole 

and observed contraction-scour depth in the Piedmont of South Carolina ............................................................ 117

107. Graph showing relation of observed clear-water contraction scour depth and the average overbank velocity 

for the 100-year flow in the Piedmont of South Carolina ...................................................................................... 118

Tables

1. Percent risk for the occurrence of the 25-year recurrence-interval flow for selected bridge ages...................................... 7

2. Bridge-scour study sites influenced by the high-flow region in the Piedmont of South Carolina.................................... 26

3. Bridge-scour study sites with impervious area exceeding 10 percent of the drainage-basin area.................................... 27

4. Estimated peak flows from the October 1992 flood at bridge-scour study sites in the lower Coastal Plain of 

South Carolina................................................................................................................................................................... 28

5. Estimated peak flows at selected U.S. Geological Survey gaging stations and ungaged sites for the August 1995 

flood along the Reedy, South Tyger, and Enoree Rivers in South Carolina...................................................................... 28

6. Estimated peak flows at bridge-scour study sites for the August 1995 flood in the Piedmont of South Carolina ........... 29

7. Estimated peak flows at selected U.S. Geological Survey gaging stations for the September 1999 flood along the 

Waccamaw River in North Carolina and South Carolina.................................................................................................. 30

8. Estimated maximum historic flows for bridge-scour study sites along the Waccamaw, Pee Dee, and Little 

Pee Dee Rivers in South Carolina ..................................................................................................................................... 30

9. Estimated maximum historic flows at selected U.S. Geological Survey gaging stations along the Pee Dee and 

Little Pee Dee Rivers in South Carolina............................................................................................................................ 30

10. Range of selected parameters for 100 observations of clear-water abutment scour collected at 65 bridges in the 

Piedmont of South Carolina .............................................................................................................................................. 69

11. Field observations excluded from the development of the embankment-length envelope curve (figure 60) for the 

Piedmont of South Carolina .............................................................................................................................................. 74

12. The effect of changing floodplain flow resistance exemplified by model data at Structure 367029900100 on 

Road S-299, crossing Cannons Creek in Newberry County, South Carolina.................................................................... 78

13. Range of selected parameters for 109 observations of clear-water abutment scour collected at 81 bridges in the 

Coastal Plain of South Carolina ........................................................................................................................................ 80

14. Range of selected parameters for 76 observations of clear-water contraction scour collected at 53 bridges in the 

Piedmont of South Carolina ............................................................................................................................................ 112

15. Pre- and post-flood channel top widths for the 1995 flood at selected sites along the Enoree River in the Piedmont 

of South Carolina............................................................................................................................................................. 116
X Contents

CONVERSION FACTORS, TEMPERATURE, VERTICAL DATUM, ABBREVIATIONS, AND ACRONYMS

Equations for temperature conversion between degrees Celsius (°C) and degrees Fahrenheit (°F):

°C = 5/9 (°F - 32)

°F = (1.8 x °C) + 32

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 (NAVD 88); 

horizontal coordinate information is referenced to the North American Datum of 1927 (NAD 27).

ACRONYMS AND ABBREVIATIONS

Multiply By To obtain

Length

inch (in.) 25.4 millimeter

foot (ft) 0.3048 meter

mile (mi) 1.609 kilometer

Area

square mile (mi2

) 2.590 square kilometer

Volume

cubic foot (ft3

) 0.02832 cubic meter

Flow Rate

cubic foot per second (ft3

/s) 0.02832 cubic meter per second

foot per second (ft/s) 0.3048 meter per second

FHWA Federal Highway Administration

HEC Hydraulic Engineering Circular

HIRE Highways in the River Environment

PVC polyvinyl chloride

SCBSD South Carolina Bridge Scour Database

SCDOT South Carolina Department of Transportation

USGS U.S. Geological Survey

WSPRO water-surface profile

mm millimeter

< less than

≤ less than or equal to

> greater than

≥ greater than or equal to
Abstract 1

Clear-Water Abutment and Contraction Scour 

in the Coastal Plain and Piedmont Provinces 

of South Carolina, 1996-99

By Stephen T. Benedict

ABSTRACT

The U.S. Geological Survey, in cooperation 

with the South Carolina Department of Transportation, collected observations of clear-water abutment and contraction scour at 146 bridges in the 

Coastal Plain and Piedmont of South Carolina. 

Scour depths ranged from 0 to 23.6 feet. Theoretical 

scour depths were computed at each bridge and 

compared with observed scour. This comparison 

showed that theoretical scour depths, in general, 

exceeded the observed scour depths and often were 

excessive. A comparison of field data with dimensionless relations for laboratory data showed that 

the range of dimensionless variables used in laboratory investigations was outside of the range for 

field data in South Carolina, suggesting laboratory 

relations may not be applicable to field conditions 

in South Carolina. Variables determined to be 

important in developing scour within laboratory 

studies were investigated to understand their influence within the South Carolina field data, and 

many of these variables appeared to be insignificant under field conditions found in South Carolina. The strongest explanatory variables were 

embankment length, geometric-contraction ratio, 

approach velocity, and soil cohesion. Envelope 

curves developed with the field data are useful tools 

for assessing reasonable ranges of scour depth in 

South Carolina. These tools are simple to apply and 

are an improvement over the current methods for 

predicting theoretical scour. 

Data from this study have been compiled into 

a database that includes photographs, figures, 

observed scour depths, theoretical scour depths, 

limited basin characteristics, limited soil data, and 

theoretical hydraulic data. The database can be 

used to compare studied sites with unstudied sites 

to assess the potential for scour at the unstudied 

sites. In addition, the database can be used to assess 

the performance of various theoretical methods for 

predicting clear-water abutment and contraction 

scour.

INTRODUCTION

In the late 1980’s, bridge scour caused the 

catastrophic failures of the Schoharie Creek Bridge in 

New York State and the Hatchie River Bridge in 

Tennessee, resulting in the loss of life. These tragic 

events brought the issue of bridge scour to the forefront 

of the highway engineering community. To address the 

issue, the Federal Highway Administration (FHWA) 

published Hydraulic Engineering Circulars (HEC) 18 

and 20 (Richardson and others, 1991; Lagasse and 

others, 1991) to provide state-of-the-knowledge guidance 

on evaluating theoretical scour at bridges. In addition to 

these publications, the FHWA initiated a national 

program to evaluate the susceptibility of existing 

bridges to scour. This program encompassed all bridges 

under state jurisdiction and delegated the responsibility 

of the evaluations to each state.
2 Clear-Water Abutment and Contraction Scour in the Coastal Plain and Piedmont Provinces of South Carolina, 1996-99

To comply with this program, the South Carolina 

Department of Transportation (SCDOT), conducted 

level 1 assessments at 3,506 bridges and made detailed 

level 2 bridge-scour studies at approximately 700 bridges, 

using the methods presented in HEC-18 (Richardson 

and others, 1991; Richardson and others, 1993). The 

level 2 studies included estimates of theoretical scour 

caused by the 100- and 500-year flows using methods 

presented in HEC-18. These estimates indicated that 

the theoretical scour depths were often much greater 

than the scour depths observed in the field. These 

results, in conjunction with the lack of field verification 

of the HEC-18 laboratory-derived equations, prompted 

the SCDOT to question the applicability of the HEC-18 

methods to South Carolina streams.

To address the excessive estimates of clear-water 

contraction and abutment scour in the cohesive soils of 

the Piedmont, the U.S. Geological Survey (USGS) in 

cooperation with the SCDOT, initiated a study in May 

1996 to investigate clear-water contraction and abutment scour at bridges in South Carolina. The general 

objectives of the study were to (1) collect field observations of scour, (2) use the data to assess the HEC-18 

methods for predicting theoretical scour, and (3) if 

possible, improve the theoretical scour-prediction 

methods for streams in South Carolina. The scope of 

the investigation was limited to clear-water contraction 

and abutment scour in the sandy soils of Coastal Plain 

swamps and in the cohesive overbank soils of Piedmont 

streams. These regions (fig. 1) are characterized by 

thick floodplain vegetation that promotes conditions 

for clear-water scour. In addition, large depths of scour 

are known to frequently occur in the sandy soils of the 

Coastal Plain in contrast to infrequent occurrences in 

the cohesive soils of the Piedmont. This contrast of soil 

types and occurrences of scour between the regions 

should provide valuable insights into the scour 

processes for South Carolina streams.

Purpose and Scope

The purpose of this report is to describe 

(1) techniques used to collect clear-water contraction 

and abutment-scour data at 146 bridges in the Coastal 

Plain and Piedmont Provinces of South Carolina, 

(2) a comparison of theoretical clear-water contraction 

and abutment-scour depths with observed scour 

depths, (3) selected relations within the field data, 

and (4) envelope curves that may be used to estimate 

ranges of anticipated clear-water contraction and 

abutment scour at bridges in the Coastal Plain and 

Piedmont Provinces of South Carolina. In addition, a 

compilation of the data developed for each bridge site 

is provided on a compact disc (CD). This compilation 

includes, photographs, figures, observed scour depths, 

theoretical scour depths, limited basin characteristics, 

limited soil data, and theoretical hydraulic data, which 

can be viewed using Microsoft Access. 

Acknowledgments

The author gratefully acknowledges the 

assistance and support of William H. Hulbert, SCDOT 

(retired), and C. Lamar Sanders, Jr., (USGS). The 

expertise of both men in the area of stream and bridge 

hydraulics helped form the concept for this study and 

provided valuable direction throughout the investigation. The author also gratefully acknowledges the 

dedicated work of Andy W. Caldwell (USGS), in 

assisting with the collection and analysis of the field 

data used in this report.

Previous Investigations

The USGS, in cooperation with the SCDOT, 

investigated scour in South Carolina in two previous 

studies. The first investigation, level 1 bridge-scour 

assessment, was conducted during 1990-92 when 

limited structural, hydraulic, geomorphic and vegetative data at 3,506 bridges and culverts were collected 

(Hurley, 1996). This information was largely qualitative in nature and was used to develop observed- and 

potential-scour indexes at each site. The observed scour 

index provides a relative indicator of the amount of 

scour observed at the site during the site visit. The 

potential-scour index provides a relative indicator of 

the potential for scour at the site, based on site-specific 

data. These indexes, along with other variables, were 

used by the SCDOT to select sites that required additional investigation. Approximately 700 sites were 

selected for more detailed level 2 studies, in which 

theoretical scour was estimated using methods 

presented in HEC-18 (Richardson and others, 1991). 

The second cooperative investigation, conducted 

during 1992-95, included level 2 studies at 293 bridges 

selected from the first study. A detailed study was 

conducted at each bridge site including (1) estimates of 

the 100- and 500-year recurrence-interval watersurface profiles using the Water-Surface Profile 

(WSPRO) model (Shearman, 1990), (2) computation of 
Introduction 3

theoretical scour for the 100- and 500-year flows using 

methods in HEC-18 (Richardson and others, 1991, 

1993), and (3) comparison of theoretical scour depths 

to bridge foundations to provide an indicator of the 

vulnerability of the bridge to failure. This information 

is used by the SCDOT to assist in determining if additional studies and (or) remedial action are required to 

protect a bridge from the threat of scour.

The level 1 and level 2 bridge-scour studies gave 

a qualitative overview of scour, which helped form 

general concepts of the type, magnitude, and frequency 

of scour throughout South Carolina. In addition, the 

level 2 bridge-scour study provided evidence of the 

apparent discrepancy between the theoretical and 

observed scour in the cohesive soils of the Piedmont. 

This information was helpful in developing the 

approach for this investigation, to study clear-water 

scour in the Piedmont, where scour generally is insignificant, and in the Coastal Plain, where larger magnitudes of scour frequently occur. This study used 105 

of the level 2 bridge-scour sites, from the previous 

investigations. These sites, with previously developed 

WSPRO models (Shearman, 1990), reduced the time 

and cost associated with developing such models.

LAKE

0 20 40 60 80 MILES

0 20 40 60 80 KILOMETERS

BLUE RIDGE

EXPLANATION

PIEDMONT

UPPER COASTAL PLAIN

LOWER COASTAL PLAIN

STATE BOUNDARY

STREAM OR SHORELINE

COUNTY BOUNDARY

Atlantic Ocean

83°

83°

82°

82° 81°

81°

80°

80°

79°

79°

34°

33°

35° 35°

34°

33°

North Carolina

Georgia

Columbia

Greenville

Charleston

Br

oad

River Congare e

Ri ver

Salu da

River

Lynches

R

Pee

River

Wateree

Santee River

South F ork Edisto River

North Fork Edist o River

Edisto Salk ehatchie Rive r

Savannah

River

80 BRIDGE AND BRIDGE NUMBER (see Table 1)

Enoree

River

Reedy Riv

er S. T yger

Coosawhatchie River

Wa ccamaw

Litt el Pee Dee

PIEDMONT HIGH-FLOW REGION

Oconee

Pickens

Anderson

Greenville

Spartanburg

Cherokee

Union

Laurens

Abbeville

Greenwood

McCormick

Edgefield

Saluda

Newberry

Chester

York

Fairfield

Lexington

Aiken

Richland

Lancaster

Kershaw

Barnwell

Orangeburg

Calhoun

Lee

Chesterfield

Darlington

Marlboro

Florence

Williamsburg

Marion

Dillon

Horry

Georgetown

Berkeley

Clarendon

Sumter

Dorchester

Colleton

Charleston

Bamberg

Allendale

Jasper Beaufort

Hampton

1

2

3

4

5

6 7

8

9

10

11

13 12

14

15 16

17

18

20 19

21

22

23 24

25

26

27

28

29

30 31 32 33

34 35

36

37 38

39

40

41

42 43

44

45

46

47

48

49

50

51

52

53 54

55

56

57

58

59

60

61

62

63

64

65

66

67

68 69

70 71

72

73

74

75

76 77

78

79

80 81

82

83

84 86 85

87

88

89

90

91 92

93 94

95

96

97

98

99

100 101

102

103

104 105

106

107

108

109

110 111

112

113

114

115 116

117

118

119 120

121

122

123

124

125

126

127 128

129

130

131

132 133

134

135

136137138

139

140

141

142

143

144

Base modified from U.S. Geological Survey

digital data, 1:2,000,000 scale, 1972

Figure 1. Location of physiographic provinces and bridge-scour study sites in South Carolina. (Refer to appendix B at back 

of report to identify bridge with corresponding number.)
4 Clear-Water Abutment and Contraction Scour in the Coastal Plain and Piedmont Provinces of South Carolina, 1996-99

Description of Study Area

South Carolina has an area of about 31,100 mi2

and is divided into three physiographic provinces: 

the Blue Ridge, Piedmont, and Coastal Plain, with the 

Coastal Plain being divided into an upper and lower 

region (fig. 1). The study area includes most of South 

Carolina, but generally excludes the Blue Ridge Province 

and the tidally influenced area of the lower Coastal 

Plain.

The Piedmont physiographic province covers 

approximately 35 percent of South Carolina and lies 

between the Blue Ridge and Coastal Plain Provinces 

(fig. 1). Land-surface elevations range from about 400 ft 

near the Fall Line (Coastal Plain boundary) to roughly 

1,000 ft at the Blue Ridge boundary. The general 

topography includes rolling hills, elongated ridges, 

and moderately deep to shallow valleys. The drainage 

patterns are well developed with well-defined channels 

and densely vegetated floodplains. Stream slopes in 

the Piedmont range from approximately 0.00015 to 

0.0100 ft/ft. The geology of the Piedmont consists of 

fractured crystalline rock overlain by moderately to 

poorly permeable silty clay loams. Alluvial deposits 

along the valley floors consist of clay, silt, and sand, and 

form varying degrees of cohesive soils (Guimaraes and 

Bohman, 1992). The cohesive soils, typically found on 

Piedmont floodplains, provide some resistance to scour 

and are believed to be an important factor in minimizing 

scour depths in this region. The thick floodplain vegetation significantly impedes sediment transport, promoting 

clear-water scour conditions on the floodplain.

Limited data indicate that peak flows are higher 

in the northeastern region of the Piedmont than in the 

western region (Guimaraes and Bohman, 1992). This 

area is designated as the Piedmont high-flow region 

(fig. 1). In this study, 65 bridge sites in the Piedmont 

physiographic province (21 within or influenced by 

the high-flow region) were surveyed for clear-water 

contraction and abutment scour. Stream slopes and 

drainage areas for these sites range from 0.00015 to 

0.00290 ft/ft (fig. 2) and 11.0 to 1,620 mi2 (fig. 3), 

respectively. (Two sites within the Blue Ridge physiographic province (fig. 1) have characteristics similar to 

Piedmont streams and, therefore, were classified as 

Piedmont sites.) 

The upper Coastal Plain is bounded by the 

Piedmont and lower Coastal Plain, and covers approximately 20 percent of the State (fig. 1). The general 

topography in the upper Coastal Plain consists of 

rounded hills with gradual slopes, and land-surface 

elevations that range from less than 200 ft to more than 

700 ft. The geology consists primarily of sedimentary 

rocks composed of layers of sand, silt, clay, and gravel 

underlain by igneous rocks (Zalants, 1990). A shallow 

surface layer of permeable sandy soils is common. 

Low-flow channels bounded by densely vegetated 

floodplains characterize upper Coastal Plain streams. 

Stream slopes are moderate, ranging from approximately 0.0005 to 0.0040 ft/ft (Guimaraes and Bohman, 

1992). In this study, four bridge sites in the upper 

Coastal Plain were surveyed for clear-water contraction 

and abutment scour.

The lower Coastal Plain covers about 43 percent 

of the State (fig. 1). The topographic relief in the lower 

Coastal Plain is less pronounced than that of the upper 

Coastal Plain, and land-surface elevations range from 

sea level at the coast to nearly 200 ft at the boundary 

with the upper Coastal Plain. The geology of the lower 

Coastal Plain consists of loosely consolidated sedimentary rocks of sand, silt, clay, and gravel overlain by 

permeable sandy soils (Zalants, 1991). Stream slopes 

range from approximately 0.0001 to 0.0040 ft/ft (fig. 2), 

and streamflow patterns are tidally influenced near the 

coast (Guimaraes and Bohman, 1992). 

Although large rivers flow within well-defined 

channels in the lower Coastal Plain, the area is noted 

for its numerous swamps, which have wide, densely 

vegetated floodplains that are drained by a network of 

shallow, poorly defined channels. Because of the thick 

vegetation, the shallow channels have large root masses 

at or just below the ground surface. These root masses 

significantly impede the transport of bed sediments, 

thereby promoting clear-water scour conditions at 

bridge contractions. In this study, 77 bridge sites in the 

lower Coastal Plain were surveyed for clear-water 

contraction and abutment scour; 68 of these sites are in 

swamps. Stream slopes and drainage areas for the 81 

sites in the upper and lower Coastal Plain range from 

0.00007 to 0.002400 ft/ft (fig. 2) and 6.0 to 8,830 mi2

(fig. 3), respectively.

DATA ASSUMPTIONS

Live-bed scour occurs at a bridge when bed 

sediments are transported into the area of scour. Under 

these conditions, sediments will be deposited into the 

scoured area as flood flows recede, and the scoured 

area will be totally or partially obscured by the infill. 

In contrast, clear-water scour occurs at a bridge when 
Data Assumptions 5

0.000

0.001

0.002

0.003

0 20 40 60 80 100

PERCENTILE

STREAMBED SLOPE, IN FOOT PER FOOT

 Piedmont

 Coastal Plain

0

400

800

1,200

1,600

0 20 40 60 80 100

PERCENTILE

DRAINAGE AREA, IN SQUARE MILES

 Piedmont

 Coastal Plain

Figure 2. Distribution of streambed slopes for bridge-scour study sites in the Coastal Plain 

and Piedmont of South Carolina.

Figure 3. Distribution of drainage areas for bridge-scour study sites in the Coastal Plain and 

Piedmont of South Carolina. (Note: Vertical scale has been truncated for graph clarity at 

small drainage areas.)
6 Clear-Water Abutment and Contraction Scour in the Coastal Plain and Piedmont Provinces of South Carolina, 1996-99

upstream approach flows do not transport bed 

sediments into the area of scour. Under these conditions, sediments do not refill the scoured area and a 

nonobscured record of the maximum scour is preserved 

at that location. This record can be readily measured 

during postflood investigations, and the measured 

scour represents the maximum clear-water scour 

that has occurred during the life of the bridge. (This 

assumes that the scoured area has not been disturbed 

by repair of previous scour; efforts should be made to 

validate this assumption through investigation of 

bridge-maintenance records.) For older bridges, this 

record of clear-water scour provides some indication of 

the maximum anticipated scour that may occur during 

the life of a bridge. Because of the relative ease of 

measuring clear-water scour, in contrast to live-bed 

scour, this study focused on the collection of clearwater scour data.

Many bridge-scour data-collection studies have 

focused on collecting data during floods in order to 

associate the measured scour with the hydraulic conditions that produced it. The laboratory-derived equations 

in HEC-18 (Richardson and Davis, 1995) use hydraulic 

variables, such as velocity and depth, to predict scour. 

Therefore, an obvious advantage of collecting concurrent scour- and hydraulic-field data is the ability to 

directly verify the laboratory-derived equations and 

(or) develop new equations from measured hydraulic 

properties. An important limitation of such studies is 

the dependency on the occurrence of floods. If floods 

of substantial magnitude do not occur during the study 

duration, no data are collected. The probability of 

collecting scour data during large-magnitude floods 

increases as the geographic area and duration of the 

study increases. However, with the geographic area 

limited to the boundaries of South Carolina and with a 

data-collection period of about 2 years, the probability 

of collecting a significant quantity of field data during 

large floods was considered low. In fact, no significant 

floods occurred in South Carolina during the primary 

data-collection phase (1996-97) of this study.

To overcome this limitation, the scope of this 

study focused on the collection of maximum historic 

scour depths at numerous bridge sites, rather than 

collecting scour depths associated with a particular 

flood. The maximum historic scour depth is defined as 

the maximum contraction or abutment-scour depth 

that has occurred at a given bridge since construction. 

Although these data do not allow the direct verification 

of the HEC-18 scour equations, they do provide ranges 

of maximum historic scour depths that should be anticipated for site-specific conditions. These ranges can be 

used to assess the reasonableness of the theoretical 

values obtained from HEC-18 and provide a basis for 

modifying the theoretical value, if necessary. For example, if the theoretical contraction scour on the clayey 

overbank at a Piedmont bridge is estimated to be 20 ft 

for the 100-year flow, yet the range of observed maximum scour depths on the overbanks at bridges with 

similar site conditions is 0.0 to 4.5 ft, one could 

conclude that the theoretical scour is excessive. Using 

the field data, along with the comparison of sitespecific conditions, the theoretical scour depth could be 

adjusted to conform to observed data. In this case, the 

theoretical-contraction scour depth might be adjusted 

to 4.5 ft to reflect the upper limit of the maximum 

observed scour depths.

When using observed scour data in such a 

manner, it must be assumed that (1) the collected field 

data reflect scour resulting from floods, such as those 

approaching the 100-year flow magnitude that have 

occurred since bridge construction and that (2) the 

scour depths are near equilibrium conditions for such 

flows. [The concept of equilibrium-scour conditions 

has been derived from laboratory investigations, and 

refers to the state where scour has reached an ultimate 

depth for a constant flow. To obtain clear-water scour 

equilibrium conditions, laboratory experiments are 

often conducted for several days (Dongol, 1993).] 

If the collected field data reflect scour that has resulted 

only from minor floods, then the data cannot be used to 

assess scour resulting from large floods, such as those 

near the 100-year flow magnitude. Likewise, if the 

observed scour depths are not near equilibrium conditions for large flows, then the collected field data will 

tend to underestimate ultimate scour depths for large 

floods. In reality, assumptions 1 and 2 are interdependent, in that a site cannot achieve equilibrium-scour 

conditions for large flows unless large flows have 

occurred at the site. The assumption will be made that, 

if a large flow has occurred at a given site, then the 

observed clear-water scour is at equilibrium or is at 

least representative of scour that could be expected for 

other bridges with similar characteristics.
Data Assumptions 7

Justification for the Assumption of Large 

Flood Flows

Support for assumption 1 can be obtained in 

several ways. From simple statistics, it is understood 

that as a bridge increases in age, the probability that 

the bridge has experienced a large flood also increases. 

Therefore, if clear-water scour data are collected at 

older bridges, then it is likely that the scour resulted 

from a large flood. If it can be shown that scour data in 

this study were collected at older bridges and that such 

bridges have a high probability of having experienced 

large flows, then assumption 1 can be substantiated. 

For purposes of this study, a large flood will be defined 

as any flow that equals or exceeds 70 percent of the 

100-year flow magnitude. In South Carolina, the rural 

regression equations for predicting flow magnitudes 

(Guimaraes and Bohman, 1992) show that the 25-year 

flow is equal to approximately 70 percent of the 100-

year flow magnitude. Therefore, a large flood, as 

defined in this study, is any flood that equals or exceeds 

the magnitude of the 25-year flow. Using this recurrence interval with a binomial distribution, a risk analysis can be made (Bedient and Huber, 1988) to 

determine the probability that the 25-year flow will 

occur at least once at a bridge of given age. The equation 

is defined as follows:

Risk = 1– (1–1/T)n, (1)

where

For selected bridge ages, table 1 lists the risk for 

the occurrence of a 25-year flow. As can be seen, 

bridges that are 30 years or older have a fairly high 

probability of having experienced a flood exceeding 

the 25-year recurrence interval.

Risk is the probability that the T-year event will occur 

at least once in n years;

T is the recurrence interval, in years; and

n is the period for assessing risk, in years.

A percentile plot of bridge age (fig. 4), shows 

that about 80 percent of the bridges in this study are 

30 years or older and, therefore, have a 71-percent 

probability of having experienced a flow magnitude 

that equals or exceeds the 25-year flow. For bridges 

that are 30 years or older, this simple risk analysis 

provides support for the validity of the assumption that 

the collected scour data resulted from large floods. 

A review of historic flood records for the 27 bridges 

that are less than 30 years of age shows that 11 of these 

sites are known to have experienced floods that have 

exceeded the 25-year flow, giving further support for 

assumption 1. 

Assumption 1 can be further substantiated from 

streamflow gaging station records. Referring to figure 4, 

about 70 percent of the bridges (101 out of 146) in this 

study are 40 years or older. A review of 277 USGS 

streamflow gaging station records (for stations that were 

operated for all or part of the 40-year period, 1956-96) 

shows that flows equaling or exceeding 70 percent of the 

100-year flow magnitude occurred at 50 gages in South 

Carolina. (Only 32 of the 277 streamflow gaging stations 

were operational for the full 40-year period from 

1956-96.) A plot of the 50 gages on a map of South 

Carolina (fig. 5) shows a wide spatial distribution, indicating that much of South Carolina has experienced at 

least one large flood during the 40-year period of interest. When 40-year-old bridges, surveyed in this study, 

are superimposed on the same map (fig. 5), there is 

significant overlap between the gaging stations and 

bridge sites, again indicating that large floods probably 

occurred at many of the bridges 40 years or older. A 

review of historic flood records for the 45 bridges that 

are less than 40 years of age shows that 16 of these sites 

are known to have experienced floods that exceeded the 

25-year flow, giving further support for assumption 1. 

[An additional gaging station (02110704) influenced by 

the 1999 flood along the Waccamaw River is included 

on figure 5. This gaging station did not experience a flow 

exceeding 70 percent of the 100-year flow prior to 1999; 

however, the 1999 flood exceeded the 100-year flow and 

postflood data were collected at selected bridges crossing the Waccamaw River.]

Finally, assumption 1 can be substantiated by 

reviewing four historic floods that have occurred since 

1990. South Carolina is in a hurricane-susceptible region 

and frequently experiences heavy rainfalls associated 

Table 1. Percent risk for the occurrence of the 25-year recurrence-interval flow for 

selected bridge ages 

Bridge age, in years 10 20 30 40 50 60 70

Risk, in percent 33 56 71 80 87 91 94
8 Clear-Water Abutment and Contraction Scour in the Coastal Plain and Piedmont Provinces of South Carolina, 1996-99

0

20

40

60

80

100

0 20 40 60 80 100

PERCENTILE

BRIDGE AGE, IN YEARS

with tropical storms. In addition, thunderstorms occur 

frequently, creating heavy rainfall for more localized 

areas. Since 1990, South Carolina has experienced four 

storms that have caused extensive flooding in various 

parts of the State. In October 1990, Tropical Storms 

Klaus and Marcos stalled over South Carolina dropping 

8-10 inches (in.) of rainfall in the Coastal Plain over a 

24-hour period (R.N. Cherry, U.S. Geological Survey, 

written commun., January 1991). Flow magnitudes were 

high, exceeding the 100-year recurrence interval at various locations and causing the failure of 80 bridges 

(Hurley, 1996). Twenty-five bridges in this study exist in 

this region (fig. 6). Although documentation of flows 

from these storms is not available at these bridge sites, 

it is probable that they experienced flows equaling or 

exceeding the 25-year flow magnitude. 

In October 1992, approximately 9 in. of rain fell 

in 24 hours over Allendale, Bamberg, Colleton, and 

Hampton Counties. This storm produced extensive 

flooding with flow magnitudes exceeding the 100-year 

flow (T.H. Lanier, U.S. Geological Survey, written 

commun., December 1993). Eighteen bridges in the 

current study exist within the region of this storm 

(fig. 6) and flow magnitudes for 8 of these sites have 

been estimated from high-water marks, indirect computations, and gaging station data (fig. 5). Recurrence 

intervals for these sites ranged from 70 years to greater 

than 100 years with six sites exceeding the 100-year 

flow magnitude. Although documentation of flow is 

not available at the remaining 10 bridges, it is probable 

that they experienced flows that equaled or exceeded 

the 25-year flow magnitude. 

In 1995, extensive flooding occurred in the 

Piedmont when Tropical Storm Jerry produced 8-20 in. 

of rainfall over a 4-day period. Flows with recurrence 

intervals ranging from 10 years to greater than 100 years 

were recorded in this region and flow magnitudes along 

the Enoree River ranged from 1.4 to 5.2 times the 100-

year flow (A.W. Caldwell, U.S. Geological Survey, 

written commun., January 1996). Twenty-seven bridges 

in the current study exist within the region affected by 

this storm (fig. 6), and flow magnitudes for 12 of these 

sites have been estimated from high-water marks, indirect computations, and gaging station data (fig. 5). Eight 

of the sites with estimated flows have magnitudes that 

exceed the 100-year flow magnitude. Although 15 of the 

27 sites affected by the 1995 flood do not have flow 

documentation, it is probable that 8 of these sites have 

experienced flows that equaled or exceeded the 25-year 

flow magnitude. Therefore, about 20 sites in this study 

experienced flows equaling or exceeding the 25-year 

flow during the 1995 flood.

Figure 4. Distribution of bridge age for bridge-scour study sites in the Coastal Plain and 

Piedmont of South Carolina. 
Data Assumptions 9

In September 1999, Hurricane Floyd produced 

extensive flooding along the Waccamaw River Basin in 

Horry County, South Carolina, where flow magnitudes 

exceeded the 100-year flow. Seven sites with estimated 

flows along the Waccamaw River (fig. 6) were 

included in this study. In addition, eight bridges along 

the Pee Dee and Little Pee Dee Rivers (fig. 6) also were 

influenced by this storm and were included in this 

study. Although flows on the Little Pee Dee and Pee 

Dee Rivers did not exceed the 25-year magnitude 

during the 1999 flood, long-term gaging station data at 

these bridges indicate maximum historic flows near the 

25-year flow magnitude have occurred on both rivers 

since the construction of the bridges.

LAKE 0 20 40 60 80 MILES

0 20 40 60 80 KILOMETERS

BLUE RIDGE

EXPLANATION

PIEDMONT

UPPER COASTAL PLAIN

LOWER COASTAL PLAIN

STATE BOUNDARY

STREAM OR SHORELINE

BRIDGE AND BRIDGE NUMBER (SEE APPENDIX B)

Atlantic Ocean

83°

83°

82°

82° 81°

81°

80°

80°

79°

79°

34°

33°

35° 35°

34°

33°

North Carolina

Georgia

Columbia

Greenville

Charleston

Broad

River Congare e

Ri ver

Saluda

River

Lynches

R

Pee

River

Wateree

Santee River

South

F ork Edisto River

North F ork Edist o River

Edisto Sal kehatchie Rive r

Savanna h

River

80

Litt el Pee Dee

02162500

02131500

02184500

02169550

02157000

02156500

02174000

02187900

02173000

02163500

02185200

02153500

02175000

02185000

02162010

02174250

02196000

02165200 02160105

02136000

02177000

02130900

02131110

02131309

02131990

02132100

02135300

02147500

02147600

02154500

02160000

02169630

02172500

02192500

02135500

Wa ccamaw

1

3 4

5

6 7

9

11

12

15 16

17

18

19 20

21

22

23 24

25

26

27

28

29

30 31 32 33

34 35

38

39

40

42 43

44

47

48

49

52

53 54

55

56

57

58

61

63

67

68 69

70

71

72

73

74

78

79

80 81

82

83

84 85

87

88

89

90

96

97

98

99

100 101

102

103

104

106

107

108

109

114

116

117

118

119 120

121

122

123

124

125

131

132 133

134135

136137138

139

142

143

02174000 USGS STREAMFLOW GAGE AND GAGE NUMBER

Enoree

River

Reedy

River S. T yger

Coosawhatchie River

PIEDMONT HIGH-FLOW REGION

02175450

02175500

02176500

02164000

02164110

02165000

02158500

02159000

02160326

02160390

02160500

02160700

02110500

02110704

02135000

02131000

USGS STREAMFLOW GAGE AFFECTED BY 1992 FLOOD AND GAGE NUMBER

USGS STREAMFLOW GAGE AFFECTED BY 1995 FLOOD AND GAGE NUMBER

USGS STREAMFLOW GAGE AFFECTED BY 1999 FLOOD AND GAGE NUMBER

02176500

02110704

02165000

Base modified from U.S. Geological Survey

digital data, 1:2,000,000 scale, 1972

Figure 5. Location of study bridges 40 years or older and streamflow gaging stations experiencing at least one flow equaling 

or exceeding 70 percent of the 100-year flow during 1956-96. (Refer to appendix B at back of report to identify bridge with 

corresponding number.)
10 Clear-Water Abutment and Contraction Scour in the Coastal Plain and Piedmont Provinces of South Carolina, 1996-99

LAKE

0 20 40 60 80 MILES

0 20 40 60 80 KILOMETERS

BLUE RIDGE

EXPLANATION

PIEDMONT

UPPER COASTAL PLAIN

LOWER COASTAL PLAIN

STATE BOUNDARY

STREAM OR SHORELINE

BRIDGE AFFECTED BY 1990 FLOOD

Atlantic Ocean

83°

83°

82°

82° 81°

81°

80°

80°

79°

79°

34°

33°

35° 35°

34°

33°

North Carolina

Georgia

Columbia

Greenville

Charleston

Broad

River Congare e

Ri ver

Saluda

River

Lynches

R

Pee

River

Wateree

Santee River

South F ork Edisto River

North F ork Edist o River

Edisto Sal kehatchie Rive r

S av anna h River

60

30 BRIDGE AFFECTED BY 1992 FLOOD

90 BRIDGE AFFECTED BY 1995 FLOOD

70 BRIDGE AFFECTED BY 1999 FLOOD

Enoree

River

Reedy River S. T yger

Coosawhatchie River

Wa ccamaw

Litt el Pee Dee

PIEDMONT HIGH-FLOW REGION

Oconee

Pickens

Anderson

Greenville

Spartanburg

Cherokee

Union

Laurens

Abbeville

Greenwood

McCormick

Edgefield

Saluda

Newberry

Chester

York

Fairfield

Lexington

Aiken

Richland

Lancaster

Kershaw

Barnwell

Orangeburg

Calhoun

Lee

Chesterfield

Darlington

Marlboro

Florence

Williamsburg

Marion

Dillon

Horry

Georgetown

Berkeley

Clarendon

Sumter

Dorchester

Colleton

Charleston

Bamberg

Allendale

Jasper Beaufort

Hampton

SCDOT DISTRICT BOUNDARY

DISTRICT 1

DISTRICT 2

DISTRICT 3

DISTRICT 4

DISTRICT 7

DISTRICT 6

DISTRICT 5

COUNTY BOUNDARY

3 4

5

6 7

8 30 31

32

3433

36 38 37

67

68

69

83

11

23 24

25

26

27

28

42 43

50

53

55

56

59

60

61

85 84

117

118

119 120

121

132 133

54

70 71

72 73

74

75

76 77

97

99

100 101

102 103

1

62

63

64

65

88

89

90

91 92

93 94

106

107

109

110 111

112

113

125

127 128

129

130

131

134

135

Base modified from U.S. Geological Survey

digital data, 1:2,000,000 scale, 1972

The four reviewed floods occurred over a 9-year 

period (1990-99) and affected approximately 78 

bridges in the current study. It is probable that these 

bridges experienced flows equaling or exceeding the 

25-year flow magnitude. Thirty-five of the 78 bridges 

have some estimate of peak flood flows and 21 of these 

had flows that equaled or exceeded the 100-year flow 

magnitude. It also should be kept in mind that other 

floods occurred prior to 1990, but were not reviewed 

for this study because documentation was not readily 

available. An investigation of floods prior to 1990 

would certainly increase the number of bridges known 

to have actually experienced large flows. The evidence 

from the four recent floods provides support of the 

assumption that the scour data collected in this study 

represent scour that has resulted from large flows.

Figure 6. Location of bridge-scour study sites affected by selected historic floods in South Carolina. (Refer to appendix B 

at the back of report to identify bridge with corresponding number.)
Data Assumptions 11

Although not all bridges in this study can be 

guaranteed to have experienced at least one large flood, 

from a review of statistical inference, gaging station 

records, and historic flood data, strong evidence has 

been provided to show that 78 of the 146 bridges have 

likely been subjected to floods equaling or exceeding 

the magnitude of the 25-year flow. This supports the 

assumption that the scour data collected in this study 

reflect scour resulting from large floods. Therefore, the 

data will likely provide some estimate of anticipated 

ranges of scour for high-flow conditions at bridges in 

South Carolina.

Justification for the Assumption of 

Equilibrium-Scour Conditions

The assumption that the observed scour depths 

collected in this study reflect equilibrium depths is 

more difficult to support. As mentioned previously, the 

inference that a large number of the bridges in this 

investigation have experienced large flows, gives 

support for the assumption that collected scour data are 

at or near equilibrium conditions resulting from such 

flows. 

 Local scour develops when bed-shear stresses 

around a pier or abutment exceed the critical shear 

stress for a given bed material. As the scour hole deepens, the bed shear stresses are reduced. Under steadyflow conditions, the progression of clear-water scour 

will reach a point where shear stresses are no longer 

sufficient to remove significant amounts of bed material. This condition is typically defined as the equilibrium-scour depth and represents the maximum scour 

that will occur for a given constant flow (Richardson 

and Davis, 1995). Because peak flood flows of 

prolonged duration are rarely observed in the field, it is 

questionable if the concept of equilibrium-scour depth 

for a constant peak flood flow is applicable to field 

conditions.

Raudkivi and Sutherland (1981) investigated the 

effects of unsteady flow on clear-water pier scour and 

compared the results with steady-flow equilibrium 

scour. Their research shows that the passage of a single 

flood hydrograph produces a scour depth less than the 

equilibrium-scour depth associated with the peak flow, 

under steady-state conditions. The duration of the 

hydrograph also was found to affect the scour depth, 

with unsteady-flow scour depths approaching steadyflow equilibrium depths as the hydrograph duration 

increased. In addition, the research showed that successive flood waves produced a progression of scour that 

eventually reached an equilibrium state after eight 

flood waves. However, the equilibrium-scour depth 

caused by the series of unsteady-flow events was 

always less than the steady state equilibrium-scour 

depth. Although this research was conducted for clearwater pier scour, it is reasonable to assume that the 

progression for clear-water contraction and abutment 

scour is similar. Based on this research, it is questionable if the concept of equilibrium-scour depth for a 

constant flood flow is applicable to field conditions.

The scour data collected in this study may not be 

at equilibrium-scour conditions as defined in laboratory 

studies. It is important to keep in mind that the data 

include observations at a large number of older bridges 

and at a large number of bridges that have likely experienced large floods. Regardless of the mechanism that 

produced the scour (sustained flows or a series of 

floods) the observations should give some indication of 

anticipated scour depths during the life of a bridge. As 

such, the data can be used in a limited manner to assess 

theoretical scour and to understand potential scour at 

similar sites.

Basin Equilibrium Scour

Although the concept of steady-flow equilibrium 

scour may not be applicable to field conditions, it is 

reasonable to assume that scour depths will approach 

some type of equilibrium condition for a given bridge 

and drainage-basin system. Some support for the 

concept of a basin equilibrium-scour depth can be 

developed by utilizing the principles of fluvial geomorphology. Numerous factors comprise and influence a 

drainage-basin system. Some of the major components 

include topography, geology, land cover, and hydrology 

(Simons and Associates, 1982). Given sufficient time, 

these components will develop a stable drainage-basin 

system that is considered to be in a state of equilibrium. 

When modifications, such as land-use changes, 

are superimposed upon a stable basin, the system 

becomes unstable and adjustments within the system 

will occur to reestablish equilibrium conditions. In 

general, the stream networks of South Carolina can be 

considered relatively stable systems. If the principles of 

fluvial geomorphology are assumed applicable to 
12 Clear-Water Abutment and Contraction Scour in the Coastal Plain and Piedmont Provinces of South Carolina, 1996-99

bridges, a bridge can be viewed as a modification to a 

stable system that will force adjustments within that 

system. Flood flows that previously were in equilibrium with the floodplain geometry have now been 

forced to contract and pass through a bridge. This new 

flow path will force system adjustments, one of which 

will be bridge scour. With time, a bridge will experience the general flood patterns of the drainage basin, 

bridge scour will occur, and these scour depths will 

approach equilibrium conditions for the given drainage-basin system. If this model is correct, then sites 

with similar bridge geometry and basin characteristics 

will show similar trends in basin equilibrium-scour 

depths. Therefore, basins with common regional characteristics will likely show regional trends in bridgescour depths.

The principles of fluvial geomorphology offer 

some defense for the concept of a basin equilibriumscour depth, but leave some question about the time 

required to attain that state. Although a definitive 

answer to this question may not be available, some 

practical insights may be drawn. Because large flows 

are the driving force that will push a site towards basin 

equilibrium-scour conditions, sites that have experienced large flows are more likely to be at or near this 

condition. Likewise, by statistical inference, older 

bridges are more likely to have experienced several 

large flows and, therefore, are more likely to be at or 

near equilibrium conditions. As discussed previously, 

the sites for this study are fairly old (119 of 146 bridges 

were 30 years or older in 1996) and there is strong 

evidence to suggest that many of the sites have experienced large flows. Therefore, it is likely that much of 

the collected scour data for this study are at or near 

basin equilibrium-scour conditions.

SITE SELECTION

Five sources were used to find candidate sites 

for measuring clear-water contraction and abutment 

scour. These included (1) an SCDOT list of bridges 

with observed scour problems, (2) sites previously 

studied by the USGS in the level 2 bridge-scour study, 

(3) Piedmont sites influenced by the August 1995 

flood, (4) Piedmont bridges with wide, flat floodplains 

as identified on topographic maps, and (5) Coastal 

Plain sites influenced by the September 1999 flood. 

SCDOT List of Bridges with 

Observed Scour

The SCDOT makes routine inspections of all 

bridges within its jurisdiction. Part of the inspection 

includes observations of scour-related problems. 

To develop a list of potential study sites, the seven 

SCDOT Districts (fig. 6) were queried by letter regarding bridges with known scour problems. Scour problems were limited to three categories: (1) contraction 

scour at swampy sites, (2) pier scour, and (3) abutment 

scour. Scour at swampy sites was requested because 

swamps commonly have clear-water contraction and 

abutment-scour holes. Swamps were a primary focus 

of this study and such sites would provide good sources 

of data. Scour at piers was requested because such 

scour would give some indication that the site had 

experienced large flows providing potential for 

contraction or abutment scour. Sites with abutment 

scour were requested because of their relevance to this 

study. Additionally, abutment scour would indicate a 

potential site for clear-water contraction scour.

The list provided by the SCDOT contained 

149 bridges for consideration. Seven of the sites were 

discarded because they were culverts or tidally influenced; 19 bridges were in category one, 104 in category two, and 19 in category three. Although site 

visits were made to 123 bridges to determine if they 

would be candidates for collecting field observations of 

scour, only 9 of these sites were selected as potential 

candidates for data collection. The primary reason for 

discarding most of the bridges was that the predominant type of scour occurring at these sites was caused 

by bank failure. Most of these bridges were short in 

length and crossed streams having a defined channel 

with narrow overbank areas under the bridge. The abutments were commonly vertical timber walls with the 

abutment toes near the channel banks. Contractionscour processes were widening the channel at the 

bridge by bank failure and bank failure presented 

potential problems for the timber wall abutments or 

nearby pile bents. Because the area of scour predominately encompassed the live-bed channel, these sites 

were considered outside the scope of investigating 

clear-water scour. In addition, scour at these sites was 

not primarily bed degradation, but channel widening, 

which again placed the sites outside the scope of this 

investigation. Figure 7 shows the distribution of bridge 

lengths for the 114 discarded sites. Approximately 83 

percent of these bridges were less than 100 ft long and 

94 percent were less than 200 ft long. 
Site Selection 13

0

0 20 40 60 80 100

100

200

300

400

PERCENTILE

BRIDGE LENGTH, IN FEET

The nine sites selected for possible field 

measurements had a range of bridge lengths from 75 

to 420 ft with abutment scour being the predominant 

observed scour. Five of these sites were selected for 

this study, one of which had been studied in the level 2 

bridge-scour study.

Although the list of bridges received from the 

SCDOT did not result in many sites for this study, the 

list did provide insights about the scour problems that 

occur at small bridges in South Carolina. These smaller 

bridges create considerable contraction of flow as 

evidenced by channel widening at the sites. Additionally, these bridges often experience abutment failure 

during floods. The nature of the abutment failure is 

typically not degradation but washout of the road fill at 

the abutments, leaving the bridge structure intact, but 

the road approaches destroyed. This is the most 

common type of bridge failure in South Carolina. For 

example, floods caused by rainfall from Tropical 

Storms Klaus and Marco in October 1990, caused 80 

bridges to fail in South Carolina, 79 of them by the 

washout of the road fill at the abutment (Hurley, 1996).

Selection of Previously Studied 

Level 2 Bridge-Scour Sites

A primary objective of this investigation was to 

analyze field measurements of scour with selected 

explanatory variables to determine if relations exist that 

help explain scour at bridges in South Carolina. 

Hydraulic variables were considered an important part 

of this analysis. Because concurrent flow data were not 

collected with the scour measurements, hydraulic 

variables at each site were estimated using the WSPRO 

model (Shearman, 1990). Developing WSPRO models is 

a time consuming and expensive process. To minimize 

these costs, level 2 bridge-scour sites, with previously 

developed WSPRO models, were used for this study. 

In the level 2 bridge-scour study, the USGS 

analyzed 293 bridges in South Carolina with 133 

located in the lower Coastal Plain, 37 in the upper 

Coastal Plain, and 123 in the Piedmont. An initial 

subset of bridge sites was taken from these studies 

using selection criteria that indicated high potential for 

clear-water contraction and abutment scour. Selection 

criteria included an assessment of hydraulic conditions 

and theoretical clear-water scour depths for the 

Figure 7. Distribution of bridge lengths for sites discarded from the South Carolina Department 

of Transportation list of bridges with observed scour problems.
14 Clear-Water Abutment and Contraction Scour in the Coastal Plain and Piedmont Provinces of South Carolina, 1996-99

100-year flow as determined in the level 2 studies. 

Large values of contraction ratio, bridge-flow velocity, 

bridge backwater, and theoretical clear-water scour 

were considered good indicators for high scour potential. An attempt was made to select sites that would 

provide some variation in drainage-area size, bridge 

length, and spatial distribution. From this subset of 

bridge sites, 105 sites were included in the present 

study with 60 located in the lower Coastal Plain, 4 in 

the upper Coastal Plain, and 41 in the Piedmont.

Selection of Piedmont Sites Influenced 

by the August 1995 Flood

To select additional Piedmont sites, a reconnaissance trip was made to bridges that had experienced the 

August 25-29, 1995, flood caused by rainfall from 

Tropical Storm Jerry. Flood flows for this event were 

documented by the USGS using high-water marks, 

indirect computations, and gage data. Records show 

that flows during this event often exceeded the 100-

year flow magnitude. Bridges that experienced this 

flood should provide good examples of anticipated 

scour depths during high flows. Forty-eight sites along 

the Enoree, Reedy, and South Tyger Rivers were visited 

and most sites showed only minor signs of abutment 

and contraction scour. Fourteen representative sites 

were included in the present study, one of which was 

also on the SCDOT list of problem sites.

Three of the surveyed flood sites had large abutment-scour holes that ranged from 5.2 to 13.7 ft deep. 

These depths approached the scour depths seen more 

commonly in the sandy Coastal Plain swamps, and 

significantly exceeded depths typically observed in the 

clayey soils of the Piedmont. The soils at these sites 

appeared to be more susceptible to scour than the cohesive soils typically associated with the Piedmont and 

played some role in the development of the larger scour 

depths. But an additional factor that could be associated 

with these large abutment-scour depths was the floodplain geometry consisting of wide, flat floodplains. 

Because streams of the Piedmont tend to have narrow 

river valleys, the wide, flat floodplains are prominent 

features on the USGS 7.5-minute series topographic 

maps, making the topographic map a useful tool for 

identifying such sites having high scour potential.

Selection of Piedmont Bridges with 

Wide, Flat Floodplains

Based on the observation that high scour potential 

in the Piedmont could be associated with wide, flat 

floodplains, all (220) topographic maps covering the 

Piedmont Province were reviewed to identify such 

sites. The review showed that most bridge crossings in 

the Piedmont are at the more narrow river valleys. 

However, 58 sites were noted to have wide, flat floodplains and reconnaissance trips were made to these 

sites to determine if they might be good candidates for 

measuring clear-water scour. Of these 58 bridge sites, 

11 had short bridges with little or no overbanks; 6 had 

only minor scour; 1 had scour-remedial action present 

(riprap on overbanks); 8 had bridges that were being 

replaced and, therefore, scour could not be assessed; 

and 32 had moderate to large scour depths ranging 

from approximately 3 to 18 ft. Using the pool of 32 

bridges with observed scour, 18 sites were selected for 

inclusion in this study, 11 of which had been previously 

selected from the level 2 studies, the 1995 flood sites, 

or the SCDOT list of problem bridges. The remaining 

7 were surveyed for clear-water contraction and 

abutment scour. 

Selection of Coastal Plain Sites Influenced 

by the September 1999 Flood

In September 1999, rainfall from Hurricane Floyd 

produced extensive flooding along the Waccamaw River 

Basin in Horry County. Flow magnitudes on the Waccamaw River exceeded the 100-year flow. This event also 

produced flooding along the Pee Dee and Little Pee Dee 

Rivers. The Waccamaw, Pee Dee, and Little Pee Dee 

Rivers have wide floodplains ranging from approximately 8,000 to 16,000 ft, and bridge crossings often 

create large contractions of flow providing a high potential for bridge scour. Because sites along these rivers had 

experienced a significant flood, a survey was conducted 

to select a number of sites to be included in this study. 

Eight bridges with estimated flows were selected along 

the Waccamaw River, as well as eight along the Pee Dee, 

and Little Pee Dee Rivers. (One of the Waccamaw River 

bridges was later dropped because the site had been 

disturbed by construction equipment.) Although flows 

on the Little Pee Dee and Pee Dee Rivers did not exceed 

the 25-year magnitude during the 1999 flood, long-term 

gages at the selected bridges indicated that maximum 

historic flows near the 25-year flow had occurred since 

construction of the bridges.
Tehniques for the Collection and Interpretation of Field Data 15

TECHNIQUES FOR THE COLLECTION 

AND INTERPRETATION OF FIELD DATA

Basic field data collected at each clear-water 

scour site included (1) measurements of scour depths, 

(2) collection of bed-material samples from the 

unscoured and scoured areas, (3) estimates of infill 

depths, and (4) description of the site by photographs, 

sketches, and written records. Because the magnitude 

of the scour-hole depths varies between the Coastal 

Plain and Piedmont, the approach to collecting the field 

data in these regions was slightly different. 

Data Collection in the Coastal Plain

In the Coastal Plain swamps, standing water 

typically covers the floodplain throughout the year, 

making scour holes difficult to locate visually. To 

determine the location and extent of scour at such sites, 

it is necessary to rely upon sonar equipment deployed 

by a small manned craft. A two-man inflatable boat and 

paper-chart fathometer (figs. 8 and 9, respectively) 

were used to investigate the lateral and longitudinal 

limits of scour. After locating the scour-hole limits, 

more detailed fathometer traces were obtained, including longitudinal and cross-sectional bed profiles. Based 

on the fathometer traces, appropriate cross-section 

locations were determined for the collection of detailed 

topographic-survey data. These data were collected 

with a total-station survey instrument and were used to 

develop contour plots of the scour holes (fig. 10). Scour 

holes in the Coastal Plain were commonly large enough 

to encompass piers or pile bents, but the effect of pier 

scour could not be isolated. Therefore, measured scour, 

in general, represents total scour and not just the 

component of abutment scour. 

In the Coastal Plain, it is possible to have shallow 

low-flow channels that pass through a large scour hole. 

In this study, such conditions occurred most commonly 

at bridges under 240 ft in length where a large, single 

Figure 8. Inflatable boat used to collect field data at swampy sites in the Coastal Plain of South Carolina. (Photograph by the 

South Carolina District, U.S. Geological Survey, 1996.)
16 Clear-Water Abutment and Contraction Scour in the Coastal Plain and Piedmont Provinces of South Carolina, 1996-99

scour hole developed at the bridge rather than separate 

left and right abutment-scour holes. Out of the 109 

observations of abutment scour in the Coastal Plain, 

42 observations had this shallow channel and all but one 

were bridges 240 ft or less in length. The depth of these 

shallow channels ranged from 1.0 to 4.6 ft with a median 

depth of 2.5 ft. At such sites, it was possible to use the 

shallow channel as a reference surface. However, the 

average, undisturbed floodplain elevation in the area of 

the observed scour was considered a more stable reference, as well as a reference that could be more consistently assessed. Therefore, scour depths at these sites, as 

well as all other Coastal Plain sites, were referenced to 

the average, undisturbed floodplain elevation in the area 

of the scour hole. In general, the upstream and downstream floodplain elevations, outside the area affected 

by scour and excluding the influence of any shallow 

channels, were used to determine this reference surface. 

In addition, available cross-section and SCDOT road 

plans data were reviewed to confirm the estimated 

reference surface. In certain cases, the survey of the 

upstream and downstream reference surface was not 

taken at a sufficient distance from the scour hole and the 

measured surface did not represent the undisturbed 

floodplain. In these cases, surveyed cross sections and 

SCDOT plans were relied upon to determine the average, undisturbed floodplain elevation. Although abutment-scour depths referenced to the shallow channels 

were not used in this report, the data were included for 

information in the bridge-scour database.

After collecting the survey data, bed-material 

samples were obtained by driving a 2-in.-diameter 

polyvinyl chloride (PVC) pipe (figs. 8, 11, and 12) into 

the sediment, then removing the sediment from the 

tube. The drive-tube sampler allowed the collection of 

submerged bed material that was useful for estimating 

sediment infill. In general, bed samples were collected 

from the low point of the scour hole and from the 

Figure 9. Paper-chart fathometer mounted on inflatable boat used to investigate limits of scour holes in the Coastal Plain of 

South Carolina. (Photograph by the South Carolina District, U.S. Geological Survey, 1996.)
Tehniques for the Collection and Interpretation of Field Data 17

upstream natural bed. Two cores were collected from 

each location. One core was preserved for future reference and a grain-size distribution analysis was made on 

the other sample. The material at the bottom of the 

scour hole was often different from that of the 

upstream bed, because the scour had cut through the 

surface alluvium and into a geologic formation. 

Because sediment transport is limited under 

clear-water scour conditions, infill in clear-water scour 

holes is typically shallow or even nonexistent. In this 

study, for the 109 observations of abutment scour in the 

Coastal Plain, infill ranged from 0.0 to 4.6 ft with a 

mean of 0.7 ft and median of 0.3 ft. Two types of infill 

were observed for Coastal Plain sites and are defined in 

this study as pluff infill and sediment infill. Pluff infill 

is a very soft layer of material that rests on the surface 

of the scour hole and is comprised of organics and fine 

sediments. This layer can be easily penetrated and 

measured by probing with a range pole. Sediment infill 

is a layer of sandy material below the pluff that is 

somewhat harder to penetrate. The depth of sediment 

infill was determined from cores obtained from the low 

point of the scour hole. A distinct layer commonly 

observed in these cores clearly distinguished the sediment infill from the unscoured material and was used to 

estimate the sediment-infill depth. The combination of 

pluff and sediment infill gives the total infill, which 

was added to the measured scour-hole depth to determine the maximum scour depth. Pictures, sketches, and 

general descriptions of sites were made as needed. 

78

80

56

78

72

70

56

68

58

60

66

62

64

80

66

68

70

72

74

74

76

76

78

78

80

78

0 10 20 30 40 50 FEET

0 5 10 15 20 METERS

EXPLANATION

80 TOPOGRAPHIC CONTOUR--Interval 1.0 foot.

Arbitrary datum

SURVEYED DATA POINT

BRIDGE DECK

BENT LOCATION

AVERAGE UNSCOURED REFERENCESURFACE ELEVATION

78.1 FEET

FLOW

Elevation at bridge

deck corner = 97.0 feet

Figure 10. Example of scour-hole contour plot developed from topographic-survey data at Structure 

211009511400 on Interstate 95, crossing the Pee Dee River floodplain in Florence County, South Carolina, 

August 19, 1996.
18 Clear-Water Abutment and Contraction Scour in the Coastal Plain and Piedmont Provinces of South Carolina, 1996-99

Data Collection in the Piedmont

In contrast to the Coastal Plain, the clear-water 

scour holes of the Piedmont are located on the overbank area and tend to have shallower scour depths 

(fig. 13). These scour holes are typically dry and easily 

observed visually. Because the scour holes are relatively small, less detailed survey data were collected at 

these sites. Data included elevation measurements of 

the upstream- and downstream-unscoured surface and 

the low point of the scour hole. The average unscoured 

surface was used as a reference to determine the scour 

depth. This reference surface typically represented the 

average, undisturbed floodplain elevation in the vicinity of the scour hole. Available cross-section and 

SCDOT road plans data also were reviewed to confirm 

the estimated reference surface. In the case of abutment 

scour, the measured scour depth represented total 

scour, excluding the effect of pier scour. At some of the 

deeper abutment-scour holes, however, the measured 

abutment-scour depth did include scour created from 

piers or pile bents, which could not be separated from 

the abutment scour. In the case of clear-water contraction scour in the overbank area of Piedmont bridges, 

the measured scour depth, in general, represented scour 

created from the contraction alone and did not include 

local pier scour.

After collecting survey data, bed-material 

samples were taken at the low point of the scour hole 

and from the upstream unscoured area, and a grainsize distribution analysis was made on these samples. 

Core

hammmer

Core

sample

tube

Handle

Capped PVC

core plunger

PVC

Cutting Shoe

Steel

5 FEET

Figure 11. Drive-tube coring device used to obtain 

sediment samples from the bottom of submerged scour 

holes. (Photograph by the South Carolina District, U.S. 

Geological Survey, 2001.)

Figure 12. Example of sediment core collected from the 

bottom of a submerged scour hole using the drive-tube 

coring device. (Photograph by the South Carolina District, 

U.S. Geological Survey, 1996.)
Tehniques for the Collection and Interpretation of Field Data 19

Post-hole diggers or the PVC drive-tube sampler were 

used to collect the sediment samples. In the clayey 

overbanks of the Piedmont, infill of scour holes by 

washed deposits is minimal and typically can be considered nonexistent. The amount of infill was determined by visual inspection of the scour hole and the 

material obtained from the bed samples. Pictures, 

sketches, and general descriptions of sites were made 

as needed.

Flow Model Data

In this study, 105 bridges had previously developed WSPRO (Shearman, 1990) models from the level 

2 bridge-scour study. However, 41 bridges had not previously been studied, so appropriate field data were 

collected to develop the WSPRO model at these sites. 

Data included cross-section surveys along the bridge 

faces, road grade, and at appropriate locations 

upstream and (or) downstream from the bridge; 

collection of bridge geometry; and estimates of channel 

and floodplain roughness coefficients.

Sediment Samples

During the data-collection phase of the study, the 

analysis of grain-size distributions for sediment 

samples was limited to grain sizes greater than or equal 

to 0.062 mm. This is the break point between the sand 

and silt/clay soils. For most Coastal Plain sites, this 

analysis was adequate for defining the median grain 

size (D50). However, this size was occasionally inadequate for clayey Piedmont soils where the D50 can be 

smaller than 0.062 mm. At such sites, the D50 was 

assumed equal to 0.062 mm and clear-water contraction scour was computed for this grain size. In an 

attempt to better understand and define the clay content 

of Piedmont soils, revisits were made to all Piedmont 

sites and a second grab sample was obtained and 

analyzed for the full range of grain size. (In this report, 

the first sample is referred to as the initial or original 

sediment sample, and the revisit sample is called the 

second sample.)

Figure 13. Example of minimal scour in the clayey soils of the Piedmont at Structure 047026300100 on Road S-263, 

crossing the Rocky River in Anderson County, South Carolina. (Photograph by the South Carolina District, U.S. Geological 

Survey, 1997.)
20 Clear-Water Abutment and Contraction Scour in the Coastal Plain and Piedmont Provinces of South Carolina, 1996-99

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

MEDIAN GRAIN SIZE OF ORIGINAL SAMPLE, IN MILLIMETERS

MEDIAN GRAIN SIZE OF SECOND SAMPLE,

IN MILLIMETERS

Line of agreement

 In figure 14, the D50 of the original sample is 

compared with that of the second sample. Although the 

second set of samples was obtained in close proximity 

to the original sample, there was often poor agreement 

in sediment size, and differences sometimes varied by 

an order of magnitude. This discrepancy in grain size, 

between multiple samples at the same site, highlights 

the nonhomogeneity of floodplain soils and indicates 

that grain-size analyses should be viewed with caution. 

(Because of this discrepancy, results obtained from 

scour-predicting equations that use D50 as an explanatory variable, also should be viewed with caution.) 

Although the results of the grain-size analyses in this 

study may be useful in obtaining a general understanding of soil conditions, it is questionable whether soil 

variables, such as the D50 obtained from grab samples 

are accurate representations of sediment characteristics 

that will allow for strong distinction of soils between 

various sites. 

Contraction Scour in the Abutment Area

When assessing total theoretical scour at abutments, HEC-18 (Richardson and Davis, 1995) considers the various components that create scour to be 

independent and additive. Therefore, to determine the 

total theoretical scour, the individual components of 

long-term streambed change, pier scour, contraction 

scour, and abutment scour, within the abutment area 

must be estimated and then summed. This approach, 

in part, stems from the difficulty in isolating the impact 

of the various scour components when they develop 

scour concurrently. Therefore, laboratory investigations 

typically have focused on understanding each scour 

component in isolation, necessitating the above 

approach for estimating total scour. Field observations, 

in conjunction with the theory of flow patterns at short 

contractions, indicate that this view of scour in the 

abutment area is inappropriate. 

Figure 14. Comparison of median grain size between the original and second sediment samples.
Tehniques for the Collection and Interpretation of Field Data 21

Consideration of contraction and local abutment 

scour as independent and separate processes in the 

abutment area is a particular concern. The assessment 

of clear-water contraction scour is often based on the 

simplifying assumption of uniform flow distributions 

within a long contraction. Therefore, flow patterns 

within a contraction are assumed to be rectilinear and 

equations for predicting scour can be derived using the 

concept of critical bed-shear stress for rectilinear flow. 

(This was the procedure used to derive the Laursen 

(1963) clear-water contraction-scour equation.) With 

lateral distance from the abutment, flow patterns are 

approximately rectilinear and the assumptions used to 

develop Laursen’s 1963 equation are more appropriate. 

In the vicinity of the abutment, however, flow patterns 

typically have severe flow curvature. This curvature 

promotes vortices, which are the primary mechanism 

for the development of scour in the abutment area 

(Dongol, 1993). Because rectilinear flow patterns in 

the abutment area are absent, it is reasonable to assume 

that scour produced by this flow pattern is absent as 

well. Therefore, total scour in an abutment area should 

consist of long-term streambed change, local abutment 

scour generated from the severely curved flow field, 

and local pier scour generated from piers within this 

same flow field. Contraction scour produced by rectilinear flow should not be considered a component of 

total scour within this area.

With respect to field observations of abutment 

scour in this study, contraction scour should not be 

viewed as a contributing component of scour. This can 

possibly be a point of confusion when comparing field 

data with various abutment-scour equations derived 

from laboratory data. Many laboratory investigations 

have sought to separate contraction scour from abutment 

scour by subtracting observed contraction scour that has 

occurred beyond the abutment area from the total 

observed scour depth at the modeled abutment. For 

example, laboratory investigations by Dongol (1993) 

measured contraction scour at the flume wall opposite 

from the abutment, and subtracted it from the total scour 

at the abutment in an attempt to isolate the scour created 

by the abutment alone. Flow patterns near the flume wall 

will largely be rectilinear in contrast to the curved flow 

field at the modeled abutment. Therefore, it is unreasonable to expect contraction scour generated by rectilinear 

flow near the flume wall to be duplicated near the abutment were flow patterns are distinctly different. The 

scour at the modeled abutment and the flume wall are 

created by distinct flow patterns that produce different 

scouring mechanisms, and the scour generated at the 

abutment should not be considered a combination of 

scour generated by these two mechanisms. This convention has been used in many laboratory investigations, 

however, and abutment-scour equations, such as the 

Froehlich (1989) equation, have been derived using data 

developed in this manner. This should be kept in mind 

when comparing various laboratory relations with the 

field data in this report. When assessing scour in the 

abutment area using techniques presented in this report, 

no adjustment for contraction scour in the abutment area 

is required.

Pier Scour in the Abutment Area

When collecting abutment-scour field data, it 

was often difficult to isolate pier scour from abutment 

scour. This was particularly true for deeper scour holes 

approximately 5 ft or greater. (In general, pier scour 

was isolated and not included in the measurement of 

scour in the shallow abutment-scour holes.) In this 

study, there are 85 abutment-scour holes deeper than 

5 ft with scour hole widths ranging from 19 to 130 ft 

(perpendicular to flow) and lengths ranging from 24 to 

395 ft (parallel to flow). Piers or pile bents located 

within these large scour holes generally showed no 

significant signs of local pier-scour holes distinct from 

the abutment-scour hole. Although it is reasonable to 

assume that the presence of the piers or pile bents may 

have contributed to the scour within the abutment-scour 

hole, there was no reference surface that would allow 

the distinction to be made between the local pier and 

abutment scour. Therefore, abutment-scour depths for 

the deeper scour holes in this study should be viewed as 

including scour effects generated from piers or pile 

bents within the scour hole.

Perhaps part of the reason why local pier scour 

holes were, in general, not detected in the deeper abutment-scour holes is that multiple-column bents with 

widths ranging from 0.8 to 2.3 ft were the most common type of bridge support found in this study (fig. 15).

These small column widths tend to cause relatively 

minor amounts of scour that may go undetected within 

the larger abutment-scour holes. In this study, 85 percent 

of the bridge sites have multiple-column bents with 

column widths ≤2.3 ft (fig. 15). Although the multiplecolumn bents may only have a minor contribution to 

the total scour at a given abutment, the field measurements were determined at the lowest point of the scour 

hole and, therefore, should include their effect.
22 Clear-Water Abutment and Contraction Scour in the Coastal Plain and Piedmont Provinces of South Carolina, 1996-99

The location of a pier or multiple-column 

bent within a given abutment-scour hole will in some 

measure indicate the impact the pier has had on the 

total scour within the abutment area. Because the 

impact of local pier scour is typically confined to 

the close proximity of the pier, it seems reasonable to 

assume that a pier or multiple-column bent located 

at the low point of an existing abutment-scour hole 

creates the greatest impact on the total scour. Figure 16 

gives an example of a pile bent located at the low point 

of the scour hole. In contrast, a pier or multiple-column 

bent located on the slope of an existing scour hole 

(fig. 17) is likely to have less impact on the total scour. 

The bridge-scour database included with this report 

indicates whether a pier or pile bent is located at the 

low point of the abutment-scour hole. In addition, 80 

of the 146 bridge sites have scour-hole contour plots 

that indicate where piers or pile bents are located in 

relation to the measured scour. This information can be 

reviewed to gain understanding of the impact of pier 

scour on total abutment scour at a given site. 

ESTIMATING HYDRAULIC DATA 

As noted previously, data collected for this study 

reflect maximum clear-water scour depths for the life 

of a bridge rather than scour produced by a unique flow 

event. The limitation of such data is that observed scour 

cannot be associated with the hydraulic conditions that 

produced the scour. Because many of the theoretical 

scour equations are driven by hydraulic properties, 

such as flow depth and flow velocity, direct verification 

of these equations was limited in this study. In an 

attempt to minimize this limitation, the one-dimensional step-backwater model, WSPRO (Shearman, 

1990) was used for each bridge to gain insights about 

hydraulic conditions during large flows. Because the 

magnitude of large floods at most of these sites is 

unknown, the 100-year flow was modeled as 

a common flood condition. In addition, known 

maximum historic flows at 35 sites were modeled. 

Hydraulic data generated from WSPRO were used 

in the theoretical scour equations to make a limited 

comparison of theoretical and observed scour. 

PERCENTILE

PIER WIDTH, IN FEET

0 20 40 60 80 100

0

2

4

6

8

Widths 1.7 feet or less are pile bents.

Widths greater than 1.7 feet are piers set

on spread footings or pile groups. 

Figure 15. Percentile plot for the maximum pier or pile width for bridge-scour study sites in the 

Piedmont and Coastal Plain of South Carolina.
Estimating Hydraulic Data 23

0 10 20 30 40 50 FEET

0 5 10 15 20 METERS

EXPLANATION

88 TOPOGRAPHIC CONTOUR--Interval 1.0 foot.

Arbitrary datum

SURVEYED DATA POINT

BRIDGE DECK

BENT LOCATION

AVERAGE UNSCOURED REFERENCESURFACE ELEVATION

86.1 FEET

0 20 40 FEET

0 10 20 METERS

86

84

86

82

74

7

8

8

6

76 78

88

80

80

82

86

84 82

84

82

84

84

86

FLOW Approximate location

of channel banks

Elevation at bridge 

deck corner = 100.0 feet

Figure 16. Example of pile bent located at the low point of a scour hole at Structure 367008100200 on 

Road S-81, crossing the Enoree River in Newberry County, South Carolina, April 7, 1997.
24 Clear-Water Abutment and Contraction Scour in the Coastal Plain and Piedmont Provinces of South Carolina, 1996-99

35

24

34

33

32

31

30

21

29 28

32

33

34

22

23 24

25

26

27

28

29

30

31

32

34

33

0 10 20 30 40 50 FEET

0 5 10 15 20 METERS

FLOW

27

EXPLANATION

32 TOPOGRAPHIC CONTOUR--Interval 0.5 foot.

Arbitrary datum

SURVEYED DATA POINT

BRIDGE DECK

BENT LOCATION

AVERAGE UNSCOURED REFERENCESURFACE ELEVATION

33.5 FEET

Elevation at bridge 

deck corner = 46.4 feet

Hydraulic properties from WSPRO, along with theoreticalscour properties, were entered in a database and these 

data, along with field observations, were used to investigate relations that may help explain scour in South 

Carolina.

Development of WSPRO Models

WSPRO models (Shearman, 1990) for 105 bridge 

sites had been developed previously in the level 2 

bridge-scour study and were used in this investigation. 

An additional 41 WSPRO models were developed for 

sites not included in the level 2 study. These models 

typically used only the minimal cross sections (exit, 

full valley, bridge, and approach) required by WSPRO 

to analyze flow through a bridge. If the floodplain was 

relatively uniform in width, one surveyed cross section 

was used as a template to represent the exit, full valley, 

and approach cross sections. If sharp changes in floodplain widths or channel slopes occurred near a bridge, 

an attempt usually was made to incorporate these 

features into the WSPRO model by synthesizing cross 

sections from topographic maps and surveyed data. 

Surveyed data at the upstream or downstream bridge 

face were used to represent the bridge cross section in 

the WSPRO model. Manning’s roughness coefficients 

Figure 17. Example of pile bent located on the side of a scour hole at Structure 152002100300 on 

U.S. Route 21, crossing Sandy Run Creek in Colleton County, South Carolina, December 18, 1996.
Estimating Hydraulic Data 25

for conditions at the time of the site visit were 

estimated from field observations and incorporated into 

the models. For most sites, no gage or historic flood 

data were available. Therefore, the starting watersurface elevation for the models was determined by 

slope conveyance. Where flow data were available, 

high-water marks or gage data were used to estimate 

starting water-surface elevations. When necessary, 

cross sections were synthesized at downstream controls 

as evident from topographic maps.

The hydraulic models developed for this study 

used limited field data and simplifying assumptions. 

Results from the models will likely have some error 

associated with them and, therefore, the solutions 

should be viewed as approximate rather than precise. 

Also, observed field conditions at the time of model 

development might not reflect the conditions that 

produced the scour. Of particular concern are forested 

floodplains that may have been clear-cut in the past and 

pastures that may have been forested or overgrown. 

Such changes in land cover can significantly affect 

Manning’s roughness coefficients which, in turn, can 

affect the estimate of hydraulic variables. These issues 

should be kept in mind when reviewing hydraulic properties (such as embankment length, flow velocity, and 

flow depth) generated from the WSPRO models. 

 When designing a new bridge to withstand 

scour, the theoretical scour at that bridge typically is 

estimated using pre-scour hydraulic conditions. To 

follow this pattern, the theoretical scour computed for 

each site was based on pre-scour hydraulics. In many 

cases, the existing scour hole at a bridge significantly 

changed the hydraulics from the pre-scour conditions. 

To estimate pre-scour hydraulics at these sites, the 

WSPRO model (Shearman, 1990) was adjusted to 

reflect conditions prior to scour. The typical modification required changing the ground elevations at the 

bridge cross section to represent pre-scour conditions. 

Bridge plans, field data, and judgment were used to 

make these adjustments. Where scour depths were 

small (approximately 3 ft or less) and the areal extent 

of scour was limited, no adjustments were made to the 

WSPRO models. Because scour depths were small at 

most Piedmont sites, no model adjustments were usually 

required. Models of Coastal Plain sites, however, were 

usually modified because of the larger scour depths.

To obtain an understanding of the impact of large 

scour holes on bridge hydraulics, WSPRO models 

(Shearman, 1990) reflecting the existing scour holes 

also were developed. To determine bridge hydraulics, 

the WSPRO model requires a cross section at the 

downstream bridge face. In many cases, the deepest 

part of a scour hole did not coincide with this location. 

Therefore, to estimate the change in hydraulics induced 

by a scour hole, the scour-hole geometry was superimposed on the bridge cross section. This was typically 

accomplished by hand projecting the survey data at the 

deepest part of the scour hole onto the downstream 

bridge face. Because scour-hole geometry is commonly 

complex and the primary axis of the hole may be 

skewed to the bridge face, judgment was often required 

to accomplish this task. Because scour depths were 

small at most Piedmont sites, post-scour hydraulics 

were assumed to be the same as pre-scour hydraulics. 

Exceptions to this were the Piedmont sites with large 

abutment-scour holes. Post-scour WSPRO models 

were developed at most Coastal Plain sites because 

large scour holes typically exist at these bridges.

Estimates of Hypothetical Flows

The hypothetical flows that were used in the 

WSPRO model were the 100-year flow and an index 

flow having a recurrence interval equal to the age of the 

bridge. The 100-year flow is recommended in HEC-18 

(Richardson and Davis, 1995) as a standard condition 

for estimating theoretical scour and was used as a 

common flow at all sites. Because older bridges have a 

higher probability of having experienced large, scourproducing flows, bridge age might have a significant 

influence on scour depth. To incorporate the influence 

of bridge age, an index flow having a recurrence interval equal to the bridge age was determined for each 

site. For example, a 20-year old bridge was assigned an 

index flow equal to the 20-year flow. Theoretical scour 

based on the hypothetical flows was compared with the 

observed scour to assess the reasonableness of the 

HEC-18 methods. In addition, the hydraulic- and theoretical-scour data generated with the hypothetical flows 

were used to investigate relations that might help 

explain scour in South Carolina. Initial comparison 

indicated large discrepancies between the index flows 

based on bridge age and known maximum historic 

flows at selected sites. The index flows were often 

much lower than the maximum historic flows. Therefore, investigations of relations within the database 

focused primarily on data produced with the 100-year 

flow and the maximum historic flows.

The 100-year flow and index flow for rural basins 

were computed using the flood-frequency equations and 
26 Clear-Water Abutment and Contraction Scour in the Coastal Plain and Piedmont Provinces of South Carolina, 1996-99

methods presented in Guimaraes and Bohman (1992). 

Where the recurrence interval of the index flow did not 

directly correspond with the recurrence interval of the 

flood-frequency equations (2-, 5-, 10-, 25-, 50-, or 100-

year flow), estimates of the index flows were obtained 

by interpolating between the flood-frequency equations. 

Rural flows in the high-flow region of the Piedmont 

(Guimaraes and Bohman, 1992) (fig. 1) were estimated 

using the North Carolina Piedmont rural floodfrequency equations (Gunter and others, 1987) as 

recommended in an unpublished USGS study (C.L. 

Sanders, U.S. Geological Survey, written commun., 

November 1993). This study showed that the North 

Carolina equations tend to better represent the limited 

stream-gage data in the high-flow region. The North 

Carolina Piedmont equations give larger peak-flow 

magnitudes than the South Carolina equations. 

For example, the 100-year peak-flow equation for the 

Piedmont of South Carolina (Guimaraes and Bohman, 

1992) is:

474A0.63, (2)

where A is the drainage area in square miles.

The 100-year peak-flow equation for the North Carolina 

Piedmont (Gunter and others, 1987) is:

719A0.643. (3)

The difference between these equations is significant. 

Because the recommendation to use the North Carolina 

equations is based on limited field data, the estimated 

peak flows for the high-flow region may contain some 

error. This should be kept in mind when reviewing sites 

in this region. Twenty-one bridge sites in the current 

study were influenced by the high-flow region (table 2). 

Flows for urban drainage basins with an impervious 

area greater than 10 percent of the basin were computed 

Table 2. Bridge-scour study sites influenced by the high-flow region in 

the Piedmont of South Carolina

[SCDOT, South Carolina Department of Transportation; S.C., South Carolina Route; 

S, Secondary Road; I, Interstate Highway; U.S., United States Route] 

County Road Stream

SCDOT 

structure 

number

Reference 

number 

(fig. 1 

and 

app. B)

Cherokee S.C. 5 Buffalo Creek 114000500200 12

Cherokee S-348 Buffalo Creek 117034800100 13

Chester I-77 Fishing Creek 121007710700 14

Chester S.C. 9 Turkey Creek 124000900200 15

Chester S.C. 9 Fishing Creek 124000901100 16

Chester S.C. 72 Sandy River 124007200200 17

Chester S.C. 97 Turkey Creek 124009700100 18

Chester S.C. 97 Rocky Creek 124009700800 19

Chester S.C. 215 Sandy River 124021500200 20

Chester S.C. 223 Fishing Creek 124022300100 21

Chester S.C. 901 Rocky Creek 124090100200 22

Fairfield I-77 Little Wateree Creek 201007710600 a 45

Fairfield I-77 Big Wateree Creek 201007710700 46

Fairfield U.S. 21 Dutchmans Creek 202002100200 a 47

Fairfield U.S. 21 Big Wateree Creek 202002100400 48

Fairfield S.C. 200 Wateree Creek 204020000500 49

York S.C. 97 Bullocks Branch 464009700300 140

York S.C. 322 Fishing Creek 464032200300 141

York S.C. 322 Tools Fork Creek 464032200500 142

York S.C. 557 Crowders Creek 464055700200 143

York S-721 Taylors Creek 467072100100 144

a

 Site is not in the high-flow region, but flows at the site are thought to be similar to 

or influenced by the high-flow region.
Estimating Hydraulic Data 27

using the urban-runoff equations presented in Bohman 

(1992). Two sites were found to have impervious areas 

exceeding 10 percent (table 3). 

Historical Flows

Although most bridge sites in this study had no 

record of maximum historic flows, these data were 

available at or near 35 sites and were used to estimate 

maximum flows during the life of the bridge. The 

maximum historic flows were estimated using data 

obtained from USGS gaging stations or from floods 

documented by the USGS using indirect methods for 

flow computation. Particular focus was given to several 

floods documented in the Coastal Plain and the Piedmont. These maximum historic flows were used in the 

WSPRO model (Shearman, 1990) to estimate hydraulic 

conditions during those floods. The hydraulic variables 

were then used to estimate theoretical scour for the 

maximum flows and compared with the observed 

scour. In addition, the hydraulic- and theoretical-scour 

data generated with the maximum historic flows were 

used to investigate relations that might help explain 

scour in South Carolina.

Flood of October 1992

In October 1992, approximately 9 in. of rain fell 

in 24 hours over Allendale, Bamberg, Colleton, and 

Hampton Counties (fig. 6). This storm produced extensive flooding with flow magnitudes exceeding the 

100-year flow (T.H. Lanier, U.S. Geological Survey, 

written commun., December 1993). The USGS estimated peak flows at 11 sites within this region, 8 being 

determined by indirect flow computations and 3 from 

USGS gaging station data (gaging station numbers 

02175450, 02175500, and 02176500, fig. 5.) The range 

of recurrence intervals for the estimated flows was 

from 60 to greater than 100 years, with eight sites 

exceeding the 100-year flow magnitude. The three 

gaging stations had periods of record in 1992 ranging 

from 29 to 41 years, and the flows for this event were 

the peak flows for the period of record. (Stations 

02175500 and 02176500 are currently active (2002) 

and the flows recorded in October 1992 remain the 

peak flow of record.) Based on these gaging station 

records, it was assumed that the flows computed by 

indirect measurements at the ungaged sites would most 

likely represent the maximum flow during the life of 

the bridge. Seven of the 11 sites were used in this study 

and the maximum historic flow at an additional site 

was estimated by interpolating these data (table 4). 

Flood of August 1995

In August 1995, extensive flooding occurred in 

the Piedmont of South Carolina when Tropical Storm 

Jerry produced 8 to 20 in. of rainfall over a 4-day 

period (fig. 6). The USGS estimated peak flows at 26 

sites within this region, and the recurrence intervals for 

the estimated flows ranged from 10 to greater than 100 

years (A.W. Caldwell, U.S. Geological Survey, written 

commun., January 1996). The heaviest flooding 

occurred on the Reedy, South Tyger, and Enoree Rivers 

(fig. 6), and focus was given to these rivers for collecting field data. The USGS estimated peak flows at three 

locations along the Reedy River, at three locations 

along the South Tyger River and at five locations along 

the Enoree River (table 5). 

Table 3. Bridge-scour study sites with impervious area exceeding 10 percent of the drainage-basin area

 [SCDOT, South Carolina Department of Transportation; mi2, square miles; S.C., South Carolina Route] 

County Road Stream SCDOT structure 

number

Drainage 

area 

(mi2)

Impervious 

area of 

drainage 

basin, 

in percent

Reference 

number 

(fig. 1 

and 

app. B)

Spartanburg S.C. 146 Enoree River 424014600100 127 13 127

Spartanburg S.C. 296 Enoree River 424029600100 119 14 128
28 Clear-Water Abutment and Contraction Scour in the Coastal Plain and Piedmont Provinces of South Carolina, 1996-99

Table 4. Estimated peak flows from the October 1992 flood at bridge-scour study sites in the lower Coastal Plain 

of South Carolina

[mi2, square miles; ft3/s, cubic feet per second; S-, Secondary Road; >, greater than; S.C., South Carolina Route; 

U.S., United States Route] 

County Road Stream

Drainage 

area 

(mi2)

Method for 

estimating 

peak flow

Discharge

(ft3/s)

Recurrence 

interval

 (years)

Reference 

number 

(fig. 6; 

app. B)

Allendale S-107 Gaul Branch 8.5 Indirect measurement 2,240 >100 8

Allendale S.C. 3 King Creek 17.2 Indirect measurement 1,560 70 5

Allendale S.C. 3 Gaul Creek 17.9 Indirect measurement 4,320 >100 6

Hampton S-13 Whippy Swamp 134 Indirect measurement 10,100 >100 69

Allendale S-21 Coosawhatchie River 48.1 Indirect measurement 11,900 >100 7

Hampton S.C. 363 Coosawhatchie River 124 Interpolation 5,380 100 68

Hampton U.S. 601 Coosawhatchie River 203 Stage-flow rating 8,800* >100 67

Jasper S-87 Coosawhatchie River 382 Indirect measurement 14,100 >100 83

*A discharge of 8.200 ft3/s was inadvertently used in this study to represent the peak flow for the October 1992 flood at the U.S. Route 601 

crossing of the Coosawatchie River. This is a 7 percent reduction in the correct peak flow of 8,800 ft3/s and should not produce significant errors in 

the WSPRO (Shearman, 1990) model and theoretical scour computations of this site.

Table 5. Estimated peak flows at selected U.S. Geological Survey gaging stations and ungaged sites for the 

August 1995 flood along the Reedy, South Tyger, and Enoree Rivers in South Carolina

[USGS, U.S. Geological Survey; mi2, square miles; ft3/s, cubic feet per second; S.C., South Carolina; >, greater than] 

USGS gaging 

station number 

(fig. 5)

Station name 

Period of 

record at 

time of 

flood

Drainage 

area 

(mi2)

Method for 

determining 

peak flow

Discharge

(ft3/s)

Recurrence 

interval 

(years)

02164000 Reedy River near Greenville, S.C. 1942-1995 48.6 Stage-flow rating a 5,400 b Urban

02164110 Reedy River near Fork shoals, S.C. 1939-1995 104 Stage-flow rating 8,200 b Urban

02165000 Reedy River near Ware Shoals, S.C. 1939-1995 236 Stage-flow rating a 9,870 20

Ungaged South Tyger River near Duncan, S.C. No record 91.8 Indirect 

measurement

13,900 >100

02158500 

(discontinued) South Tyger River near Reidville, S.C. 1935-1967 106 Stage-flow rating c9,650 >100

02159000 

(discontinued) South Tyger River near Woodruff, S.C. 1934-1971 174 Stage-flow rating c16,500 >100

02160326 Enoree River at Pelham, S.C. 1993-1995 84.2 Indirect 

measurement

a 11,300 >100

Ungaged Enoree River near Woodruff, S.C. No record 177 Indirect 

measurement

65,000 >100

02160390 Enoree River near Woodruff, S.C. 1993-1995 249 Indirect 

measurement

a 50,400 >100

02160500 

(discontinued) Enoree River at Enoree, S.C. 1930-1993 307 Stage-flow rating c 43,800 >100

02160700 Enoree River at Whitmire, S.C. 1974-1995 444 Stage-flow rating a31,200 >100

a

 Peak flow for period of record.

b Upper part of basin is urbanized, but flows exceeded the magnitude for the 100-year rural flows.

c

 Flow exceeded peak flow for period of record.
Estimating Hydraulic Data 29

Based on the gaging station records for the 

Reedy, South Tyger, and Enoree Rivers, it was assumed 

that the flows during the 1995 flood would most likely 

represent the maximum historic flow for most bridges 

presently located on these rivers. Exceptions were in 

the lower part of the Reedy River where recurrence 

intervals dropped to 20 years. After post-flood reconnaissance, two bridges on the Reedy River, three 

bridges on the South Tyger River and seven on the 

Enoree River were selected for inclusion in this study 

(table 6). Sites with unknown peak flows for the 1995 

flood were estimated by prorating known peak flows 

by drainage area. [Peak flows at S.C. Route 146 and 

S.C. Route 296 crossing the Enoree River (reference 

numbers 127 and 128, respectively, appendix B) were 

not estimated.] Most of these estimates appear reasonable; however, Road S-81 crossing the Enoree River is 

10 mi downstream from the last flow measurement on 

the river (USGS gaging station 02160700, Enoree 

River at Whitmire, S.C., fig. 5), and the extrapolated 

flood flow might be too low. All flow measurements 

along the Enoree River from the 1995 flood exceeded 

the 100-year flow, but flows began to decrease at a 

drainage area of 177 mi2 in the downstream direction 

(table 5). This decreasing trend was used to extrapolate 

flows downstream from gaging station 02160700 (fig. 5), 

causing flows to eventually fall below the 100-year 

flow magnitude. Based on the fact that measured flows 

at five sites along the Enoree River exceeded the 100-

year flow and that the extrapolation is significantly 

beyond the measured data, flows at Road S-81 might 

have been higher than the estimated 23,400 ft3

/s. 

Flood of September 1999

In September 1999, rainfall from Hurricane 

Floyd produced extensive flooding along the Waccamaw River in Horry County, South Carolina (fig. 6), 

where flow magnitudes exceeded the 100-year flow. 

Peak flows were estimated at three USGS gaging 

stations along the Waccamaw River (table 7 and fig. 5), 

and flows at each gage were the peak for the period of 

record. Based on the gaging station records, it was 

assumed that flows along the Waccamaw River during 

the 1999 flood would most likely represent the maximum historic flow for existing structures. After postflood reconnaissance, three multiple-bridge crossings 

on the Waccamaw River, encompassing eight bridges, 

were selected for inclusion in the current bridge-scour 

study (table 8). (One bridge on the Waccamaw River 

was subsequently removed from the study because of 

disturbance to the floodplain from construction equipment.) Peak flows along the Waccamaw River were 

estimated by prorating peak gage flows by drainage 

area. 

This flood also influenced the Pee Dee and Little 

Pee Dee Rivers. Although flows did not exceed the 

25-year magnitude during the 1999 flood, long-term 

gages indicate that flows near the 25-year flow 

magnitude have occurred on both rivers (table 9). 

One multiple-bridge crossing with six bridges on the 

Table 6. Estimated peak flows at bridge-scour study sites for the August 1995 flood in the Piedmont of South Carolina

[mi2, square miles; ft3/s, cubic feet per second; S-, Secondary Road; U.S., United States Route; >, greater than; S.C., South Carolina Route] 

County Road Stream

Drainage 

area

(mi2)

Method for 

estimating 

peak flow

Discharge

(ft3/s)

Recurrence 

interval

(years)

Reference 

number 

(fig. 6; 

app. B)

Greenville S-68 Reedy River 173 Extrapolation 9,530 35 64

Laurens S-36 Reedy River 236 Stage-flow rating 9,870 20 91

Spartanburg U.S. 29 South Tyger River 76.0 Extrapolation 6,500 50 125

Spartanburg S-62 South Tyger River 91.8 Indirect measurement 13,900 >100 129

Spartanburg S-242 South Tyger River 94.4 Extrapolation 8,400 >100 131

Spartanburg S-118 Enoree River 186 Extrapolation 64,200 >100 130

Laurens S-263 Enoree River 249 Indirect measurement 50,400 >100 94

Laurens S-112 Enoree River 256 Extrapolation 50,000 >100 93

Laurens S.C. 49 Enoree River 307 Stage-flow rating 43,800 >100 88

Union S.C. 56 Enoree River 371 Extrapolation 37,500 >100 134

Union S-22 Enoree River 395 Extrapolation 35,700 >100 135

Newberry S-81 Enoree River 677 Extrapolation a 23,400 40 112

a

 Extrapolated discharge is significantly beyond limits of measured flow data and may be too low.
30 Clear-Water Abutment and Contraction Scour in the Coastal Plain and Piedmont Provinces of South Carolina, 1996-99

Table 7. Estimated peak flows at selected U.S. Geological Survey gaging stations for the September 

1999 flood along the Waccamaw River in North Carolina and South Carolina

[USGS, U.S. Geological Survey; mi2, square miles; ft3/s, cubic feet per second; N.C., North Carolina; >, greater than; 

S.C., South Carolina] 

USGS gaging 

station 

number 

(fig. 5)

Station name Period of record

Drainage 

Area 

(mi 2)

Discharge

(ft 3/s)

Recurrence 

interval

(years)

a 02109500 Waccamaw River at Freeland, N.C. 1940-1999 680 b 31,200 >100

 02110500 Waccamaw River at Longs, S.C. 1950-1999 1,110 b 28,200 >100

 02110704 Waccamaw River at Conway 

Marina at Conway, S.C.

1991-1999 c 1,420 b 24,800 undetermined

a Site is located in North Carolina and is not shown on figure 5. b Peak flow for period of record.

c Approximate drainage area; site tidally influenced at low stages.

Table 8. Estimated maximum historic flows for bridge-scour study sites along the Waccamaw, Pee Dee, and Little 

Pee Dee Rivers in South Carolina

[mi2, square miles; ft3/s, cubic feet per second; U.S., United States Route; S.C., South Carolina Route] 

County Road Stream

Number of 

bridges at 

crossing

Drainage 

area

(mi 2)

Date of 

maximum 

historic flow

Discharge

(ft3/s)

Recurrence 

interval

(years)

Florence U.S. 76 Pee Dee River 2 8,830 March 1979 103,000 a Regulated

Horry S.C. 22 Waccamaw River 4 b1,200 September 1999 27,000 c Tidally influenced 

Horry U.S. 501 Waccamaw River 3 b1,420 September 1999 24,000 c Tidally influenced

Horry U.S. 501 By Pass Waccamaw River 1 b1,420 September 1999 24,000 c Tidally influenced

Horry U.S. 501 Little Pee Dee River 6 2,790 October 1965 27,600 40

a The Pee Dee River is regulated making it difficult to assign a flood frequency at this location. However, if it is assumed that no regulation 

exists, the recurrence interval for this flow based on the gaging station record is 25 years. b Approximate drainage area only.

c These sites are tidally influenced and therefore are not assigned a flood frequency. The historic flows at these bridges are believed to be the 

maximum historic flow for the life of the bridges. For purposes of this study, the maximum historic flows at these bridges also were assumed to be 

equal to the magnitude of the 100-year flow.

Table 9. Estimated maximum historic flows at selected U.S. Geological Survey gaging stations along the Pee Dee 

and Little Pee Dee Rivers in South Carolina 

[USGS, U.S. Geological Survey; mi2, square miles; ft3/s, cubic feet per second; S.C., South Carolina] 

USGS gaging 

station number 

(fig. 5)

Station name Period of 

record

Drainage 

area 

(mi 2)

Date of 

maximum 

historic flow

Discharge

(ft 3/s)

Recurrence 

interval 

(years)

02135000 Little Pee Dee River at Galivants

Ferry, S.C.

1938-1999 2,790 October 1965 27,600 40

02131000 Pee Dee River at Pee Dee, S.C. 1942-1999 8,830 March 1979 103,000 a Regulated

a The Pee Dee River is regulated making it difficult to assign a flood frequency at this location. However, if it is assumed that no 

regulation exists, then the recurrence interval for this flow based on the gage record is 25 years.
Estimating Hydraulic Data 31

Little Pee Dee River and one multiple-bridge crossing 

with two bridges on the Pee Dee River also were selected 

(table 8). The long-term gaging stations in table 9 are 

located at these crossings and provide data to estimate 

the maximum historic flow. 

DEVELOPMENT OF THEORETICAL 

BRIDGE-SCOUR DATABASE

Theoretical scour was computed at each bridge 

for the 100-year flow, index flow, and, where available, 

maximum historic flow. Methods and equations 

described in HEC-18 (Richardson and Davis, 1995) 

were used to calculate theoretical pier, abutment, and 

contraction scour. The hydraulic variables required for 

these equations were obtained from the WSPRO model 

(Shearman, 1990). Computer programs were written to 

automate the extraction of hydraulic data from the 

WSPRO output files and to calculate theoretical scour. 

Theoretical scour depths and variables required to 

compute these depths are available in a database 

contained on the CD in back of this report.

Theoretical Pier Scour

Pile bents are a common foundation for bridges 

in South Carolina and were the primary foundation at 

approximately 70 percent of the bridges studied in this 

investigation (fig. 15). Pile bents consist of a row of 

piles driven into the ground and interconnected by a 

bent cap at the top of the piles (fig. 18) that provides 

support for the bridge deck. The three types of piles 

observed in this study were round timber, steel H, and 

square concrete (figs. 19, 20, and 21, respectively). 

The widths for these piles varied from 0.8 to 1.7 ft. 

Another type of bridge foundation was a pier 

supported on spread footings or pile groups (figs. 22 

and 23) that existed at about 30 percent of the bridges 

in this study (fig. 15). The piers were generally larger 

than piles and ranged in width from 1.8 to 6.1 ft. On 

bridges that had been widened to accommodate additional traffic lanes, it was common to find a combination of piers and piles forming a composite bent to 

support the bridge. Composite bents typically have 

piers supporting the original structure with piles added 

upstream and downstream from the old piers to support 

the newly added lanes (figs. 24 and 25).

Although pier scour was not a primary focus for 

this study, the theoretical pier scour was computed at 

each bridge to provide some indication of the scour 

potential of piers when they fall within areas of abutment and contraction scour. HEC-18 (Richardson and 

Davis, 1995) recommends the following equation for 

computing theoretical scour at bridges:

, (4)

where

ys is the theoretical pier-scour depth, in feet;

a is the pier width, in feet;

K1 is the dimensionless correction factor for 

pier-nose shape;

K2 is the dimensionless correction factor for angle

of attack;

K3 is the dimensionless correction factor for bed

conditions;

K4 is the dimensionless correction factor for bed

armoring;

y1 is the approach flow depth, in feet; and

Fr1 is the approach flow Froude number defined as

Fr1 = V1/(gy1)

0.5

where

V1 is the mean approach velocity, in feet per second; and

g is the acceleration of gravity, in feet per squared 

second.

ys

a

---- 2.0K1K2K3K4

y1

a

---- 0.35

Fr1

0.43 =

Figure 18. Profile of pile bent.
32 Clear-Water Abutment and Contraction Scour in the Coastal Plain and Piedmont Provinces of South Carolina, 1996-99

Figure 20. Steel H-pile bent at Structure 467072100100 on Road S-721, crossing Taylors Creek in York County, 

South Carolina. (Photograph by the South Carolina District, U.S. Geological Survey, January 29, 1997.)

Figure 19. Timber pile bent at Structure 194023000500 on S.C. Route 230, crossing Horne Creek in Edgefield County, 

South Carolina. (Photograph by the South Carolina District, U.S. Geological Survey, February 6, 1997.)
Estimating Hydraulic Data 33

Figure 22. Profile of pier on spread footing and pile group.

Figure 21. Concrete pile bent at Structure 182007800200 on U.S. Route 78, crossing Polk Swamp in Dorchester County,

South Carolina. (Photograph by the South Carolina District, U.S. Geological Survey, November 26,1996.)
34 Clear-Water Abutment and Contraction Scour in the Coastal Plain and Piedmont Provinces of South Carolina, 1996-99

Figure 24. Profile of composite bent.

Figure 23. Pier at Structure 262050103100 on U.S. Route 501, crossing the Waccamaw River in Horry County, South 

Carolina. (Photograph by the South Carolina District, U.S. Geological Survey, June 13, 2000.)
Estimating Hydraulic Data 35

When applying this equation to compute local 

scour around piers and pile bents, the following 

assumptions and methods were used. In general, the 

width of the pier or pile was determined by using the 

pier or pile dimension parallel with the bridge face 

opening and perpendicular to the direction of flow. 

For composite bents with columns of varying widths 

(figs. 24 and 25), the widest column within each bent 

was used to represent the pier width in the HEC-18 

equation. Most bridges in this study had piers or piles 

that were constant in width along the vertical axis. 

However, several bridges had piers where width diminished as elevation increased. In such cases, the pier 

width at the ground line was used in the HEC-18 equation. Although the pier or pile bent length is not used 

directly in the HEC-18 equation, it is required to determine the correction factor for angle of attack. For pile 

and composite bents, the pier length was determined by 

summing the length of each pile or pier parallel with 

the direction of flow. For solid piers, the pier dimension parallel with the flow was used to represent the 

pier length. 

The correction factor for pier-nose shape, K1, 

was obtained from HEC-18 (Richardson and Davis, 

1995). Pile bents with square piles were assumed to 

have the shape of a square-nosed pier, whereas pile 

bents with circular piles were treated as a group of 

cylinders. The correction factor for angle of attack, K2, 

also was obtained from HEC-18. To determine this 

factor, there must be an estimate of the high-flow angle 

of attack. This angle is typically based on visualizing 

the flow patterns during high-flow conditions and, as 

such, has a measure of subjectivity. Field observations 

and USGS topographic maps were used to estimate the 

high-flow angle of attack with weight typically given to 

the topographic map. In general, a single angle of 

attack was determined for each bridge crossing and 

applied to all piers at that bridge. For Coastal Plain 

swamps, backwater upstream from the bridge 

commonly causes flood flows to pass relatively straight 

through the bridge opening irrespective of the bridge 

orientation to the floodplain. In such cases, the angle of 

attack was considered to be zero. 

Figure 25. Composite bent at Structure 262050103200 on U.S. Route 501, crossing the Waccamaw River in Horry

County, South Carolina. (Photograph by the South Carolina District, U.S. Geological Survey, June 13, 2000.)
36 Clear-Water Abutment and Contraction Scour in the Coastal Plain and Piedmont Provinces of South Carolina, 1996-99

Because this study primarily focused on the 

occurrence of clear-water scour, the bed conditions at 

piers and pile bents were generally assumed to be clear 

water. In the case where pier scour was computed in the 

main channel (live-bed conditions) the plane-bed form 

was assumed to exist. For both cases, the correction 

factor for bed conditions, K3, is 1.1 and this was the 

assigned value for all pier-scour computations. The 

minimum median grain size (D50) required for applying the armoring correction factor, K4, is 60 mm. The 

largest D50 for all bridge sites in the study was 0.99 

mm. Therefore, the effects of bed armoring on pier 

scour were considered negligible, and the correction 

coefficient, K4, was set to 1.0 for all computations of 

pier scour. 

To calculate the Froude number at a given pier, 

the stream-tube algorithm within the WSPRO (Shearman, 

1990) model was applied to the bridge cross section to 

obtain estimates of the flow velocity and depth. This 

algorithm divides the bridge cross section into 20 

stream tubes of equal conveyance and computes the 

flow area and the average velocity within each tube. 

The stream tube that corresponds with the location of 

a given pier or pile bent was selected, and the velocity 

and depth associated with that tube were used to 

compute the Froude number for the pier or pile bent of 

interest. 

When computing theoretical scour at piers with 

footings, HEC-18 (Richardson and Davis, 1995) 

recommends adjusting the bed elevation at a pier to 

account for the theoretical contraction scour. If a footing is exposed, based on this adjusted bed elevation, 

then special considerations must be made for computing scour at this pier. In general, field observations 

showed that piers in this study rarely had exposed footings due to construction constraints or contraction 

scour. Based on this observation, the special considerations for exposed footings were assumed unnecessary 

when computing theoretical pier scour.

Because pier scour was not a primary focus of 

this study, only limited theoretical data were stored in 

the pier-scour database. At a site with a well-defined 

low-flow channel, the cross section at the bridge was 

divided into three subsections defined as the left overbank, right overbank, and main channel (fig. 26). The 

piers or pile bents within a given subsection were typically identical in geometry and had similar theoretical 

scour depths. For a given subsection, the pier with the 

largest theoretical pier-scour depth was selected to 

represent pier scour for that subsection. The theoretical 

pier-scour depth and the hydraulic variables used to 

compute the scour were stored in the theoretical pierscour database. The selected pier-scour depth was identified with the flow of interest (100-year flow, index 

flow, or maximum historic flow) and the subsection 

location. The left overbank subsection was identified as 

‘lfp’ for left floodplain. The right overbank subsection 

was identified as ‘rfp’ for right floodplain. The mainchannel subsection was identified as ‘mcl,’ ‘mcm,’ or 

‘mcr’ for main-channel left, main-channel middle, or 

main-channel right, respectively (figs. 26 and 27). Piers 

or pile bents located on the overbanks near the main 

channel were generally excluded from selection as 

representative overbank piers. The theoretical scour at 

these piers was typically influenced by the high velocities within the main channel and did not represent theoretical scour associated with just overbank flows. At 

swampy sites where a low-flow channel was not well 

defined, the entire bridge cross section was defined as 

the main channel and only one pier or pile bent was 

chosen to represent theoretical pier scour at that bridge 

(fig. 27). For further details on the variables stored 

within the theoretical pier-scour database, see 

appendix A.

Theoretical Live-Bed Contraction Scour

As with pier scour, live-bed contraction scour 

was not a primary focus of this study, but was 

computed, where applicable, to provide some indication of the theoretical contraction scour in live-bed 

channels. Live-bed contraction scour occurs when bed 

sediments upstream from a contraction are transported 

into the contraction scour hole. The low-flow velocities 

and thick vegetation on the floodplains of most South 

Carolina streams, limit the transport of bed materials, 

creating clear water rather than live-bed scour conditions on the floodplain. In contrast, sediments in welldefined low flow channels are available for transport 

and may experience live-bed contraction scour within 

the limits of the defined channel. Theoretical live-bed 

contraction scour was computed at sites containing this 

live-bed feature, which excluded most Coastal Plain 

sites and included all Piedmont sites. For computing 

live-bed contraction scour, HEC-18 (Richardson and 

Davis, 1995) recommends the use of a modified 

version of the 1960 Laursen equation for live-bed scour 

at long contractions, which is defined as: 
Estimating Hydraulic Data 37

Figure 27. Bridge with swampy channel, showing subsection and pier identifications for theoretical pier-scour database. 

[mcl, main-channel left; mcm, main-channel middle; mcr, main-channel right]

Figure 26. Bridge profile with well-defined low-flow channel, showing subsections and pier identifications for theoretical 

pier-scour database. [lfp, left flood plain; mcm, main-channel middle; rfp, right flood plain]
38 Clear-Water Abutment and Contraction Scour in the Coastal Plain and Piedmont Provinces of South Carolina, 1996-99

, and (5)

ys = y2 – y1 , (6)

where

The live-bed contraction-scour equation defines 

W1 and W2 as the bottom widths at the upstream and 

contracted channel, respectively. In natural channels, 

the bottom width is often difficult to define. In such 

cases, HEC-18 (Richardson and Davis, 1995) recommends the use of the channel top widths for defining 

W1 and W2. This convention was used in all computations of live-bed contraction scour, and the channel top 

widths were determined as the distance between the 

channel banks. The flows in the upstream and 

contracted channels were defined as the flow bounded 

by the channel banks. This flow was determined by 

prorating the total flow by the ratio of conveyance 

within the channel to that of the entire cross section. 

The average flow depth in the upstream channel was 

determined by dividing the channel flow area by the 

channel top width. The energy grade line of the 

y2 is the average flow depth in the main channel at 

the contracted section, in feet;

y1 is the average flow depth in the upstream main 

channel, in feet;

Q1 is the flow in the upstream main channel transporting sediment, in cubic feet per second;

Q2 is the flow in the main channel at the contracted 

section, in cubic feet per second;

W1 is the bottom width of the upstream main channel, 

in feet;

W2 is the bottom width of the main channel at the 

contracted section adjusted by subtracting the 

pier width(s) within the channel, in feet;

k1 is an exponent determined from V*, ω, and the 

tables in HEC-18; 

ys is the average scour depth, in feet;

ω is the fall velocity of the median bed material D50, 

in feet per second; and

V* is the shear velocity in the upstream main channel, 

in feet per second, which is defined as 

V* = (gy1S1)

1/2,

where

g is the acceleration of gravity, in feet per squared 

second for g; and 

S1 is the energy grade line of the main channel, 

in feet per foot.

y2

y1

---- Q2

Q1

------

6

7

-- W1

W2

------

k1

=

upstream channel was determined from the WSPRO 

(Shearman, 1990) model. The D50 was determined 

from a grain-size analysis of a sediment grab sample 

obtained from the main channel. The fall velocity for 

the D50 was determined from an algorithm developed 

by the USGS (Richard J. Huizinga, U.S. Geological 

Survey, written commun., 1997). The algorithm uses a 

best-fit equation of the fall-velocity curve presented in 

HEC-18 (Richardson and Davis, 1995). 

Theoretical scour depths and the variables used 

to compute those depths were stored in the theoretical 

live-bed contraction-scour database and associated 

with the flow of interest (100-year flow, index flow, or 

maximum historic flow). For further details on the 

stored variables, see appendix A.

Theoretical Clear-Water 

Contraction Scour

Clear-water contraction scour occurs where 

upstream bed sediments are not transported through a 

contracted section. This condition may occur when 

velocities upstream from a contraction are insufficient 

to transport bed materials from the upstream reach into 

the contraction. Clear-water scour conditions may 

further be enhanced by dense vegetation that limits 

sediment transport, regardless of upstream flow velocities. Both conditions typically prevail on the floodplains and swamps of South Carolina making them 

good candidates for clear-water scour. On streams with 

well-defined channels, low velocities and dense vegetation on the floodplain create clear-water scour conditions on the bridge overbanks (fig. 28). Likewise, low 

velocities and thick vegetation in Coastal Plain swamps 

produce clear-water scour conditions across the entire 

bridge opening (fig. 29). These types of clear-water 

contraction scour were the primary focus of this study.

For computing theoretical clear-water contraction 

scour, HEC-18 (Richardson and Davis, 1995) recommends the use of the Laursen’s 1963 equation for clearwater scour at long contractions, and is defined as:

, and (7)

ys = y2 – y1 , (8)

y2

Q2

120Dm

2

3

--

W2

---------------------------

3

7

--

=
Estimating Hydraulic Data 39

Figure 28. Typical bridge with well-defined low-flow channel, showing areas of clear-water scour.

Figure 29. Typical bridge cross section for a swampy channel or floodplain relief bridge, showing area of 

clear-water scour.
40 Clear-Water Abutment and Contraction Scour in the Coastal Plain and Piedmont Provinces of South Carolina, 1996-99

where

For sites with well-defined low flow channels, 

the left and right overbanks were the part of the bridge 

opening experiencing clear-water contraction scour. 

Theoretical scour was computed at each overbank. The 

contracted width for a given overbank was defined as 

the distance from the abutment toe to the channel bank 

(fig. 28). The flow across the overbank was determined 

by prorating the total flow through the bridge by the 

ratio of conveyance within the overbank to that of the 

entire bridge cross section. The average depth of flow 

prior to the occurrence of contraction scour was 

obtained by dividing the flow area at the overbank by 

the overbank width. For Coastal Plain sites with 

swampy channels, the entire channel at the bridge 

opening experiences clear-water contraction scour 

(fig. 29). In this case, the contracted width was defined 

as the distance from the abutment toe to abutment toe 

and procedures defined above were used to determine 

the other variables. The D50 was determined from a 

grain-size analysis of a sediment grab sample. To 

obtain a representation of the pre-scour sediments, the 

sample was taken upstream from the contraction, 

outside the limits of any scour.

Theoretical scour depths and the variables used 

to compute those depths were stored in the theoretical 

clear-water contraction-scour database and associated 

with the flow of interest (100-year flow, index flow, or 

maximum historic flow). For further details on the 

stored variables, see appendix A.

y2 is the average depth of flow in the contracted 

section after the occurrence of contraction 

scour, in feet;

Q is the flow associated with the contraction width 

W, in cubic feet per second;

Dm is the diameter of the smallest nontransportable 

particle in the bed material at the contracted 

section, in feet, and is defined as Dm = 1.25D50; 

W is the width of the contracted section adjusted by 

subtracting the pier width(s) within the section, 

in feet;

ys is the average scour depth in the contracted 

section, in feet;

y1 is the average depth of flow in the contracted 

section prior to contraction scour, in feet; and 

D50 is the median grain size of the bed material, 

in feet.

Theoretical Abutment Scour

Abutment scour was a primary focus of this 

study, and therefore, theoretical abutment-scour 

depths were computed at all bridge sites. The current 

study had spill-through abutments at 143 bridges with 

the remaining 3 bridges having vertical abutments. 

Spill-through abutments have a sloping, earthen 

embankment under the bridge deck (figs. 30 and 31) 

and are often protected by riprap. Vertical abutments 

have vertical concrete walls that function as a retaining 

wall for the road embankment fill (fig. 32). Hydraulic 

Engineering Circular Number 18 (Richardson and Davis, 

1995) recommends the use of the Froehlich (1989) 

live-bed abutment-scour equation or the “Highways 

in the River Environment” (HIRE) (Richardson and 

others, 1990) equation for predicting theoretical abutment scour. These equations were used in this study 

and are defined below.

The Froehlich (1989) live-bed abutment-scour 

equation is defined as:

, (9)

where

ys is the abutment-scour depth, in feet;

ya is the average depth of flow on the floodplain 

upstream from the abutment, in feet;

K1 is the dimensionless correction factor for abutment shape determined from table in HEC-18;

K2 is the dimensionless correction factor for the angle 

of embankment to flow;

L is the length of the embankment projected normal 

to flow, in feet; (Many laboratory investigations 

define the road embankment that blocks 

approaching flows as the abutment length. In 

this report, the term embankment length is 

used.); and

Fr is the Froude number of the flow upstream from 

the embankment, which is defined as: 

Fr = Ve /(gya ) 0.5,

where

Ve is the average flow velocity upstream from the 

embankment, in feet per second, and is defined 

as: Ve = Qe /Ae ,

where

Qe is the flow obstructed by the embankment, in 

cubic feet per second; and

Ae is the flow area obstructed by the embankment, in 

square feet; and

g is previously defined.

ys

ya

---- 2.27K1K2

L

ya

---- 0.43

Fr0.61 = + 1
Estimating Hydraulic Data 41

The value of 1 at the end of the equation is the 

safety factor that forces the equation to encompass 

98 percent of the laboratory data (Froehlich, 1989).

The HIRE (Richardson and others, 1990) equation defined below is applicable only when the ratio of 

embankment length and flow depth at the abutment is 

greater than 25:

, (10) ys

y1

---- 4

K1

0.55

---------- Fr0.33 =

where

ys is the abutment-scour depth, in feet;

y1 is the depth of flow upstream and adjacent to the 

abutment, in feet;

K1 is the dimensionless correction factor for abutment shape determined from a table in HEC-18;

Fr is the Froude number of the flow upstream and 

adjacent to the abutment, and is defined as:

Fr = Ve /(gy1 )

0.5,

where

Ve is the flow velocity upstream and adjacent to the 

abutment, in feet per second; and

g is previously defined.

Figure 30. Plan view and profile of spill-through abutment.
42 Clear-Water Abutment and Contraction Scour in the Coastal Plain and Piedmont Provinces of South Carolina, 1996-99

The Froehlich (1989) equation was used to 

compute theoretical scour at all bridges in the study. 

Although the HIRE (Richardson and others, 1990) 

equation is limited to sites with embankment length to 

flow depth ratio greater than 25, computations were 

made at all sites regardless of the ratio. The variables 

used in the Froehlich (1989) equation were determined 

in the following manner. The length of the embankment was determined by projecting the bridge cross 

section onto the approach cross section and using the 

distance from edge of water to the projected abutment 

toe as the embankment length (fig. 33).

The obstructed flow area was determined by 

using the approach cross-section geometry and watersurface elevation to calculate the flow area across the 

embankment length (fig. 33). The obstructed flow was 

determined by prorating the total flow by the ratio of 

conveyance across the embankment length to that of 

the entire approach cross section. The average flow 

depth on the approach floodplain was determined by 

dividing the obstructed area by the embankment length. 

Abutment-scour computations with the Froehlich 

(1989) equation were made with and without the safety 

factor.

For the HIRE (Richardson and others, 1990) 

equation, the velocity adjacent to the abutment was 

determined by using the WSPRO stream-tube algorithm at the bridge cross section. The average velocity 

in the tube at the abutment was chosen to represent Ve. 

The depth of flow at the abutment was determined by 

subtracting the ground elevation at the abutment toe 

from the water-surface elevation at the bridge.

Theoretical abutment-scour depths and the 

variables used to compute those depths were stored in 

the theoretical abutment-scour database and were associated with the flow of interest (100-year flow, index 

flow, or maximum historic flow). For further details on 

the stored variables, see appendix A.

Figure 31. Spill-through abutment at structure 307011200100 on Road S-112, crossing the Enoree River in Laurens County, 

South Carolina. (Photograph by the South Carolina District, U.S. Geological Survey, April 2,1997.)
Variables Influencing Abutment Scour 43

VARIABLES INFLUENCING ABUTMENT 

SCOUR

Local bridge scour is the erosion of bed material 

from around flow obstructions such as piers and abutments. The mechanism that causes the erosion is the 

combined effect of flow acceleration and the resulting 

vortices that are induced by the obstructions (Richardson 

and Davis, 1995). In the case of a bridge abutment, 

flood flow is diverted from its natural flow path by a 

road embankment and contracted into a bridge opening 

(fig. 34). Under ideal conditions, contraction causes the 

acceleration of flow along the path of a streamline, and 

the largest velocity magnitudes exist near the edge of 

the contracted section (Rouse, 1946). At a bridge, 

the edge of the contracted section corresponds to the 

tip of a road embankment and is defined as a bridge 

abutment. In addition to the accelerated flows near 

the abutment, the severe flow curvature in this area 

promotes the development of vortices that have been 

identified in laboratory studies as the primary mechanism for abutment scour (Dongol, 1993).

The accelerated flows caused by a bridge 

contraction can be shown schematically by the use 

of streamlines and stream tubes. A streamline represents the path that a particle of water will follow. 

A stream tube represents a hypothetical flow tube 

formed by adjacent streamlines. The volumetric flow 

within a stream tube is assumed to remain constant. 

Figure 32. Plan view and profile of vertical abutment.
44 Clear-Water Abutment and Contraction Scour in the Coastal Plain and Piedmont Provinces of South Carolina, 1996-99

Figure 34. Simplified streamline patterns at a bridge contraction.

Figure 33. Embankment length and obstructed flow area determined by projection of the bridge cross section onto the 

approach cross section.
Variables Influencing Abutment Scour 45

Therefore, as a stream tube is constricted, the flow 

velocity at the constriction must increase to conserve the 

constant flow. Figure 34 shows a simplified schematic of 

steady-flow streamline and stream-tube patterns at a 

bridge contraction, illustrating the severe stream-tube 

constriction and curvature of streamlines in the abutment 

area. The combination of increased velocities and flow 

curvature, as illustrated by the stream tubes, provides an 

ideal environment for producing vortices that in turn will 

promote abutment scour. With lateral distance from the 

abutment, the stream-tube constriction and streamline 

curvature significantly diminish, indicating a reduced 

potential for scour. Field observations indicate that abutment scour in South Carolina commonly occurs where 

the severe constriction and curvature of the stream tubes 

would be anticipated. Likewise, where the severity of 

these flow patterns would be expected to diminish, with 

distance from the abutment, much smaller depths of 

scour are typically observed. 

Numerous laboratory studies have investigated 

the variables that influence abutment scour. Dongol 

(1993), reviewing previous studies of abutment scour, 

compiled a list of influencing variables and described 

the effect of each. A detailed discussion of these variables is beyond the scope of this report, and for more 

details, the reader is referred to Dongol (1993). Dongol 

classified the variables influencing abutment scour into 

the seven categories listed below:

(1) Variables describing the channel

• channel width • channel slope

• channel geometry

(2) Variables describing the abutment

• embankment length • abutment shape 

• skew

(3) Variables describing the flow

• flow depth • mean approach velocity

• energy slope • gravitational acceleration

(4) Variables describing the bed material

• median size • specific gravity

• gradation • fall velocity

• particle shape factor • angle of repose

• cohesiveness • particle Reynolds number

• dimensionless critical-shear stress

(5) Variables describing the fluid

• dynamic viscosity • density

(6) Temperature (7) Time

This list includes 23 variables that have been observed 

to influence abutment scour in laboratory studies, demonstrating that scour processes can be complex. 

To simplify the study of scour processes, laboratory 

investigations of abutment scour often use noncohesive, granular bed materials of constant specific gravity 

and water flows of constant temperature. Under these 

conditions, cohesion, particle shape factor, angle of 

repose, fall velocity, sediment specific gravity, water 

density, dynamic viscosity, and temperature can be 

assumed constant, and the relation of abutment scour to 

influencing variables can be simplified (Dongol, 1993). 

Using this simplified relation, along with other simplifying assumptions, Dongol presents a shortened list of 

the primary influencing variables and describes their 

effect on abutment scour as observed in various laboratory investigations. This list of influencing variables 

includes time, flow velocity, flow depth, sediment size, 

sediment gradation, embankment length, abutment 

shape, embankment skew, and channel geometry. 

The simplifying assumptions presented in Dongol 

(1993) may not be valid for field conditions. For example, a constant specific gravity of the bed material may 

be a reasonable assumption; however, to assume a 

noncohesive bed material is probably unreasonable. 

Even the sandy soils of the Coastal Plain of South 

Carolina commonly have some silts, clays, and organic 

materials mixed with the sands, providing some degree 

of cohesion. In addition, water temperatures can significantly vary as the seasons change. For example, in water 

year 1998 (from October 1997-September 1998) the 

USGS gaging station 02163001 on the Saluda River near 

Williamston, S.C., recorded a minimum water temperature of 39 °F on January 3, 1998, and a maximum 

temperature of 86 °F on July 1, 1998. A temperature 

change from 86 to 39 °F changes the water density by 

only 0.4 percent. The same temperature change, 

however, causes a 95-percent increase in the dynamic 

viscosity (Gerhart and others, 1992). Because fluid shear 

stresses are directly related to viscosity, an increase of 

this magnitude could influence the scour process. 

Because conditions in the field can be substantially different from the simplified conditions of the 

laboratory, direct application of laboratory results to 

the field may not be justified. However, it is reasonable 

to assume that trends within the laboratory also should 

be observed in the field and, therefore, laboratory 

investigations should provide valuable insights for 

understanding abutment scour under field conditions. 

With this assumption in mind, a brief description of 
46 Clear-Water Abutment and Contraction Scour in the Coastal Plain and Piedmont Provinces of South Carolina, 1996-99

0

2,000

4,000

6,000

8,000

10,000

12,000

14,000

16,000

0 20 40 60 80 100 120 140 160

TIME, IN HOURS

FLOW, IN CUBIC FEET PER SECOND

Simulated hydrograph for the Piedmont

Simulated hydrograph for the lower Coastal Plain

Hydrograph duration at 95 percent of peak flow

some laboratory findings and how they may relate to 

abutment scour in South Carolina is presented. Discussion, in general, will be limited to the influencing 

variables for the simplified abutment-scour relation 

(Dongol, 1993). In this study, hydraulic variables associated with field observations were estimated using the 

one-dimensional model, WSPRO (Shearman, 1990). 

Because most sites do not have historical flow data, the 

theoretical 100-year flow was used as a common condition to gain insights about the hydraulics that may 

occur during a large flood. Therefore, when hydraulic 

variables, such as embankment length, flow velocity, 

flow depth, and geometric-contraction ratio, are 

mentioned in this report, they should be viewed as estimated rather than measured data. Because clear-water 

scour is the focus of the current study, laboratory findings related to clear-water scour is the primary focus in 

the following review. 

Time And Flow Duration

To reach clear-water equilibrium-scour depths 

under laboratory conditions, experiments must typically run at constant-flow conditions for several days 

(Dongol, 1993). Because of scaling effects, events in 

small laboratory models (such as flumes) in general, 

should have shorter durations than a similar event 

under field conditions. Therefore, to reach clear-water equilibrium-scour conditions in the 

field, peak-flow durations may 

need to be longer than several 

days. Because natural hydrographs may peak and recede 

within hours rather than days, 

the concept of equilibrium 

scour, as defined in the laboratory, may not be applicable to 

the field. In South Carolina, it 

is unlikely that peak flows for 

events such as the 100-year 

flow will be sustained for 

several days. This can be 

demonstrated using the regionalized dimensionless hydrographs developed for South 

Carolina (Bohman, 1990) 

where regional dimensionless 

hydrographs were derived from 

observed data—three dimensionless hydrographs for the Coastal Plain and one for 

the Piedmont.

To provide some perspective on the differing 

shapes of the hydrographs between the Coastal Plain 

and Piedmont, a plot of the 100-year hydrograph for 

two hypothetical 200-mi2

 basins are shown in figure 35.

As demonstrated in figure 35, Piedmont hydrographs 

are much shorter in duration than Coastal Plain hydrographs. Therefore, scour depths in the Coastal Plain 

will more likely approach equilibrium-scour depths for 

peak flows than will those in the Piedmont. 

Assuming that the hydrograph duration for 95 

percent of the 100-year flow (fig. 35) represents the 

duration of the peak flow, a plot of the relation of the 

100-year peak-flow duration and drainage area is 

shown in figure 36. This figure demonstrates that South 

Carolina drainage basins under 2,000 mi2 are unlikely 

to have sustained flow durations of 2 days for flows 

approaching the 100-year magnitude. Drainage areas 

for the bridge sites used in this study range from 6 to 

8,830 mi2 with 94 percent of the drainage areas less 

than 2,000 mi2

 (fig. 3). Because of shorter duration 

flows associated with drainage basins under 2,000 mi2, 

field measurements of scour depths in this study will 

likely be less than corresponding equilibrium-scour 

depths associated with scaled laboratory studies. 

Figure 35. Comparison of the simulated 100-year-flow hydrographs for 200-square 

mile basins in the Piedmont and lower Coastal Plain of South Carolina.
Variables Influencing Abutment Scour 47

0

5

10

15

20

25

30

35

40

45

0 500 1,000 1,500 2,000

DRAINAGE AREA, IN SQUARE MILES

ESTIMATED 95-PERCENT PEAK-FLOW

DURATION FOR THE 100-YEAR FLOW, IN HOURS

Piedmont

Lower Coastal Plain

To provide some perspective on the relation of 

abutment-scour depth and peak-flow duration for field 

data, figure 37A shows the relation of abutment-scour 

depth and the estimated peak-flow duration for the 

100-year flow at single-bridge openings in the Coastal 

Plain. The peak-flow duration for each site was estimated by using methods presented in Bohman (1990), 

assuming that the hydrograph duration at 95 percent 

of the 100-year flow represented the duration for the 

100-year peak flow. Multiple-opening bridges were 

excluded from this figure because two-dimensional 

flow patterns at such sites can vary from the typical 

single-bridge opening. Data were grouped into three 

categories: (1) scour holes with embankment lengths 

less than or equal to 400 ft, (2) scour holes with 

embankment lengths less than or equal to 600 ft, but 

greater than 400 ft, and (3) scour holes with embankment lengths less than or equal to 5,300 ft, but greater 

than 600 ft. As can be seen in figure 37A, there is a 

large scatter of data within these three categories; 

however, the envelope curves, drawn to encompass the 

data in each category, are relatively flat implying that 

the effect of peak-flow duration on abutment-scour 

depths may only have a minor effect for the range of 

peak-flow durations in this study. It is noteworthy that 

all peak-flow durations for the simulated hydrographs 

are less than 26 hours, which 

may be insufficient to produce 

scour depths comparable to 

laboratory investigations that 

run for several days or longer.

The relation of abutmentscour depth and the estimated 

peak-flow duration for the 

100-year flow (Bohman, 1990) 

for bridges in the Piedmont is 

shown in figure 37B. [Two outlying data points were excluded 

from this figure (reference 

number 89 and 90, appendix B, 

fig. 1).] Data were grouped into 

three categories based on 

embankment length. The envelope curves of these categories 

are relatively flat implying that 

peak-flow duration in the Piedmont may have only minor 

effects on abutment-scour 

depths. Comparing the Coastal 

Plain and Piedmont plots highlights the difference in 

peak-flow duration between these regions. A large part 

of the Piedmont sites have peak-flow durations less 

than 5 hours in contrast to those in the Coastal Plain 

that exceed 5 hours. As with the Coastal Plain, figure 

37B shows that Piedmont peak-flow durations may be 

insufficient to produce scour comparable to laboratory 

investigations that run for several days or longer.

Flow Velocity

Laboratory investigations indicate that clear-water 

abutment-scour depths increase with increasing 

approach velocity (Dongol, 1993). Figure 38 (Dongol, 

1993) presents a typical laboratory relation of flow 

velocity and equilibrium-scour depth for uniform sediments. The shape of this curve is slightly different for 

nonuniform sediments because of the effect of armoring, but the trends are similar. The vertical axis in 

figure 38 represents the equilibrium-scour depth 

normalized by the embankment length. The horizontal 

axis represents flow intensity, which is defined as the 

ratio of the average approach flow velocity (U) to the 

critical average velocity (Uc) required to initiate motion 

of the given sediment. For clear-water scour conditions, 

the flow intensity will be less than 1, with the transition 

Figure 36. Comparison of hydrograph durations at 95 percent of the peak flow 

estimated from simulated 100-year-flow hydrographs for various basin sizes in the 

Piedmont and lower Coastal Plain of South Carolina.
48 Clear-Water Abutment and Contraction Scour in the Coastal Plain and Piedmont Provinces of South Carolina, 1996-99

0

5

10

15

20

25

0 5 10 15 20 25 30

ESTIMATE OF PEAK-FLOW DURATION FOR THE 100-YEAR FLOW, IN HOURS

OBSERVED ABUTMENT-SCOUR DEPTH, IN FEET

0

5

10

15

20

25

0 5 10 15 20

OBSERVED ABUTMENT-SCOUR DEPTH, IN FEET

ESTIMATE OF PEAK-FLOW DURATION FOR THE 100-YEAR FLOW, IN HOURS

Envelope for embankment lengths 400 feet or less

Envelope for embankment lengths 600 feet or less

Embankment lengths ≤ 400 feet

Embankment lengths ≤ 600 feet and > 400 feet

Embankment lengths ≤ 5,300 feet and > 600 feet

Envelope for embankment lengths > 600 feet

A.

B.

Envelope for embankment lengths 100 feet or less

Envelope for embankment lengths 400 feet or less

Envelope for embankment lengths 700 feet or less

Embankment lengths ≤ 100 feet

Embankment lengths ≤ 400 feet and > 100 feet

Embankment lengths ≤ 700 feet and > 400 feet

Embankment lengths ≤ 1,670 feet and > 700 feet

Figure 37. Relation of observed clear-water abutment-scour depth and the estimated peak-flow 

duration for the 100-year flow at single-bridge openings in the (A) Coastal Plain and (B) Piedmont of 

South Carolina.
Variables Influencing Abutment Scour 49

FLOW INTENSITY

ABUTMENT-SCOUR DEPTH A

B

U

T

M

E

N

T-S

C

O

U

R D

E

P

T

H

DIVIDED BY EMBANKMENT LENGTH DIVID

E

D B

Y E

M

B

A

N

K

M

E

N

T L

E

N

G

T

H

Clear-Water

Scour Zone

Live-Bed

Scour Zone

0.0 1.0

Not to Scale

from clear-water to live-bed scour occurring when the 

flow intensity equals 1. The curve in the area of clearwater scour has a relatively steep slope, indicating that 

small changes in approach velocity can produce relatively large changes in scour depth when other variables remain constant. 

Figure 39 shows selected laboratory data for 

spill-through abutments (Dongol, 1993) grouped by the 

dimensionless variable of flow depth divided by 

embankment length. The patterns are similar to that 

depicted in figure 38. Figure 39 also shows the relation 

of flow intensity (based on the 100-year obstructed 

approach velocity) and normalized scour depth for field 

observations in the current study. [The critical velocity, 

Uc, was determined from the relation published by 

Vanoni (1977) using the D50 for the unscoured surface 

soils at each site. The critical velocities were not 

adjusted for flow depth.] The field data show no strong 

relation, and actually indicate that the effect of velocity 

on abutment-scour depth may be minor. This relation in 

the field data should be viewed with some caution 

because of potential errors associated with the D50 and 

the estimated flow velocities. Soils in this study were 

heterogeneous, and grab samples used to determine the 

D50 might not properly represent the average soil characteristics at a given site. Error associated with the grain 

size will, in turn, introduce error in the estimate of the 

critical velocity. In addition, the flows that produced the 

measured scour are unknown, and the estimated flow 

velocities obtained from the WSPRO (Shearman, 1990) 

model may have some error. Cohesive soils of the Piedmont might also introduce some discrepancy with the 

laboratory relations that were developed for loosegrained soils. 

Despite these potential errors within the field data, 

figure 39 provides a good indication for the range of 

dimensionless variables that should be anticipated for 

field conditions in South Carolina. As can be seen, the 

range for the normalized abutment-scour depth for the 

laboratory data does not encompass the range for the 

field data, making it difficult to apply the laboratory relations to field conditions. Although the laboratory data 

shown in figure 39 are only a subset of all the laboratory 

data, the subset implies that many laboratory investigations of abutment scour do not encompass the range 

of dimensionless variables typically found in the field. 

Figure 38. General relation of flow intensity and abutment-scour depth normalized by embankment 

length based on laboratory data. (From Dongol, 1993.)
50 Clear-Water Abutment and Contraction Scour in the Coastal Plain and Piedmont Provinces of South Carolina, 1996-99

Therefore, abutment-scour relations developed 

from laboratory data may not translate well to field 

conditions.

To provide some perspective on the relation of 

abutment-scour depth and approach velocity for nonnormalized field data, figure 40A shows the relation of 

abutment-scour depth and the average 100-year flow 

velocity obstructed by the embankment for singlebridge openings in the Coastal Plain. Data were 

grouped into three categories similar to the peak-flow 

duration relation in figure 37A. Envelope curves were 

developed for these categories assuming that an 

approach-flow velocity of 0.0 ft/s would produce 0.0 ft 

of scour. As can be seen in figure 40A, there is a large 

scatter of data within these three categories; however, 

the envelope curves drawn to encompass the data of 

these categories display a relation that indicates an 

increase in abutment-scour depth with increasing 

approach velocity, which is consistent with laboratory 

investigations of clear-water abutment scour (figs. 38 

and 39). The slopes of the three envelope curves 

are relatively flat for the range of the 100-year-flow 

velocities between 0.3 to 0.7 ft/s. This indicates that 

abutment-scour depth in the Coastal Plain may be 

relatively insensitive for this range of approach velocity. If the extrapolation of the envelope curves to zero is 

correct, then abutment-scour depth will be more sensitive for velocities less than 0.3 ft/s, and small changes 

in approach velocity in this area will produce relatively 

large changes in abutment-scour depth. 

In general, the slopes of the envelope curves 

increase with increasing embankment length. Because 

larger scour depths are associated with longer embankment lengths, this trend appears reasonable. This trend 

is also consistent with the theory of hydrodynamics. 

For a constant-approach flow velocity, an increase in 

embankment length will produce an increase in velocity at the tip of the abutment (fig. 34), which then 

produces an increase in potential for scour. As a result, 

for a constant-approach flow velocity, larger scour 

depths at longer embankments are expected.

 Figure 40B shows the relation of abutmentscour depth and the average 100-year-flow velocity 

obstructed by the abutment for bridges in the Piedmont. 

(Two outlying data points were excluded from this 

figure.) Data were grouped into three categories 

similar to the peak-flow duration relation in figure 37B.

0.0

0.5

1.0

1.5

0 12 3 456

FLOW INTENSITY

ABUTMENT-SCOUR DEPTH

DIVIDED BY EMBANKMENT LENGTH 

Observed field data

Laboratory data for flow depth divided by embankment length = 0.43

Laboratory data for flow depth divided by embankment length = 0.77

Laboratory data for flow depth divided by embankment length = 1.08

Laboratory data for flow depth divided by embankment length = 1.69

Figure 39. Relation of abutment-scour depth, normalized by embankment length, and flow 

intensity for laboratory data (Dongol, 1993) and observed data in the Piedmont and Coastal 

Plain of South Carolina.
Variables Influencing Abutment Scour 51

0

5

10

15

20

25

01 2 34

AVERAGE 100-YEAR FLOW VELOCITY 

BLOCKED BY THE EMBANKMENT, IN FEET PER SECOND

OBSERVED ABUTMENT-SCOUR DEPTH, IN FEET

0

5

10

15

20

25

0.0 0.5 1.0

OBSERVED ABUTMENT-SCOUR DEPTH, IN FEET

AVERAGE 100-YEAR FLOW VELOCITY 

BLOCKED BY THE EMBANKMENT, IN FEET PER SECOND

Envelope for embankment lengths 400 feet or less

Envelope for embankment lengths 600 feet or less

Embankment lengths ≤ 400 feet

Embankment lengths ≤ 600 feet and > 400 feet

Embankment lengths ≤ 5,300 feet and > 600 feet

Envelope for embankment lengths > 600 feet

A.

B.

Envelope for embankment lengths 100 feet or less

Envelope for embankment lengths 400 feet or less

Envelope for embankment lengths 700 feet or less

Embankment lengths ≤ 100 feet

Embankment lengths ≤ 400 feet and > 100 feet

Embankment lengths ≤ 700 feet and > 400 feet

Embankment lengths ≤ 1,670 feet and > 700 feet

Figure 40. Relation of observed clear-water abutment-scour depth and the average 100-year-flow 

velocity, blocked by the embankment, at single-bridge openings in the (A) Coastal Plain and 

(B) Piedmont of South Carolina.
52 Clear-Water Abutment and Contraction Scour in the Coastal Plain and Piedmont Provinces of South Carolina, 1996-99

The trends are similar to those in the Coastal Plain 

(fig. 40A); however, there are some notable differences. 

A comparison between figures 40A and 40B shows that 

velocities in the Piedmont will, in general, exceed those 

in the Coastal Plain. This can be attributed to steeper 

stream slopes associated with Piedmont streams (fig. 2). 

In addition, envelope curves for comparable embankment lengths indicate that a given approach velocity will 

produce larger scour depths in the Coastal Plain than the 

Piedmont. This is particularly obvious for embankment 

lengths of 400 ft or less (fig. 41A). The difference 

between the envelope curves can most likely be attributed to the cohesive nature of the Piedmont soils compared to the sandy soils of the Coastal Plain. Figure 41A 

shows that the cohesive Piedmont soils can sustain 

higher velocities and will produce smaller scour depths 

in contrast to the sandy soils of the Coastal Plain. This 

provides some perspective of the effect of soil cohesion 

on abutment scour. Comparing the Piedmont envelope 

for embankment lengths of 700 ft or less with the 

Coastal Plain envelope for embankment lengths of 600 

ft or less (fig. 41B) shows that the Coastal Plain still has 

larger scour depths at embankment lengths exceeding 

400 ft. The difference between these envelope curves, 

however, is less than the envelope curves for embankment lengths of 400 ft or less. This indicates that the 

scour patterns in the Piedmont approach those of the 

Coastal Plain as embankment length increases. 

Flow Depth

Laboratory investigations indicate that clearwater abutment-scour depths increase at a diminishing 

rate with increasing flow depth (Dongol, 1993). This 

trend is shown in figure 42 (Dongol, 1993) where a 

best-fit line through selected laboratory data for spillthrough abutments with uniform sediments is plotted.

The scour and flow depth variables have been normalized by the embankment length. Although figure 42 

represents a subset of laboratory data from previous 

investigations, it provides insight regarding the hydraulic conditions under which these investigations were 

typically made. The ranges for the normalized flow 

depth and the normalized scour depth for the selected 

laboratory data are 0.1 to 1.7 and 0.3 to 1.0, respectively. In contrast, the ranges of these dimensionless 

variables for 207 of the 209 field observations of 

abutment scour (204 are spill-through abutments) are 

0.0016 to 0.54 and 0.0 to 0.13, respectively. As can be 

seen from figure 42, all of the field data fall outside the 

limits of the selected laboratory data, indicating that 

laboratory experiments do not cover the range of field 

variables typically found in South Carolina. Because 

the normalizing variable in figure 42 is embankment 

length, it is likely that the range discrepancy between 

laboratory and field data can be attributed to the use of 

relatively short embankments in laboratory studies in 

contrast to relatively long embankments found in the 

field. Because the South Carolina field data fall outside 

the range of laboratory data, it is difficult to draw 

conclusions on what effect flow depth will have on 

scour depths under field conditions. However, the 

concentration of field data in the lower left-hand corner 

of figure 42 may indicate that flow depth has only a 

minor effect on abutment-scour depths in South 

Carolina.

To provide some perspective on the relation of 

abutment-scour depth and flow depth for non-normalized field data, figure 43A shows the relation of abutment-scour depth and the 100-year flow-depth near the 

abutment toe, for single-bridge openings in the Coastal 

Plain. Data were grouped into three categories similar 

to the peak-flow duration relation presented in figure 

37A. As can be seen, there is a large scatter of data 

within these three categories and the range of the 100-

year-flow depth for the majority of the data is relatively 

small (4 to 8 ft). Envelope curves for embankment 

lengths of 400 ft or less and 600 ft or less show a small 

increase in scour depth with increase in flow depth. 

The increase is relatively small implying that abutment-scour depth in the Coastal Plain is relatively 

insensitive for this range of 100-year-flow depths.

The relation of abutment-scour depth and the 

100-year-flow depth near the abutment toe for bridges 

in the Piedmont is shown in figure 43B. Data were 

grouped into categories similar to the peak-flow duration relation presented in figure 37B. The envelope 

curves in this figure generally show a small increase 

in abutment-scour depth with increasing flow depth. 

The exception to this is for embankment lengths of 100 ft 

or less where observed scour depths drop slightly with 

increasing flow depth. Regardless, all three envelope 

curves indicate that abutment-scour depth in the 

Piedmont is relatively insensitive to flow depth. 
Variables Influencing Abutment Scour 53

0

5

10

15

0.0 0.5 1.0 1.5 2.0

AVERAGE 100-YEAR-FLOW VELOCITY 

BLOCKED BY THE EMBANKMENT, IN FEET PER SECOND

OBSERVED ABUTMENT-SCOUR DEPTH, IN FEET

Envelope of observed scour for embankment lengths

 of 400 feet or less in the Piedmont

Envelope of observed scour for embankment lengths 

of 400 feet or less in the Coastal Plain

0

5

10

15

20

0.0 0.5 1.0 1.5

OBSERVED ABUTMENT-SCOUR DEPTH, IN FEET

AVERAGE 100-YEAR-FLOW VELOCITY 

BLOCKED BY THE EMBANKMENT, IN FEET PER SECOND

Envelope of observed scour for embankment lengths

 of 700 feet or less in the Piedmont

Envelope of observed scour for embankment lengths 

of 600 feet or less in the Coastal Plain

A.

B.

Figure 41. Comparison of envelopes for the relation of observed clear-water abutment-scour depth 

and the average 100-year-flow velocity, blocked by the embankment, for (A) embankment lengths 

of 400 feet or less in the Coastal Plain and Piedmont, and (B) embankment lengths of 600 feet and 

700 feet or less in the Coastal Plain and Piedmont of South Carolina, respectively.
54 Clear-Water Abutment and Contraction Scour in the Coastal Plain and Piedmont Provinces of South Carolina, 1996-99

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0.0 0.5 1.0 1.5 2.0

FLOW DEPTH DIVIDED BY EMBANKMENT LENGTH 

SCOUR DEPTH DIVIDED BY EMBANKMENT LENGTH

Observed abutment scour from field investigation

Trend line through laboratory data

Laboratory abutment-scour data (Dongol, 1993)

Sediment Size

Although numerous properties influence the 

process of clear-water abutment-scour, laboratory data 

suggest that, as the embankment length increases, the 

influence of some properties diminishes and may 

become negligible (Melville, 1992). Sediment size is 

one property that functions in such a manner. For 

uniform noncohesive sediments, laboratory studies 

show that the influence of sediment size on scour-hole 

depths diminishes with increasing embankment length. 

When the dimensionless ratio of embankment length 

(L) to median grain size (D50) is greater than or equal 

to 50, the effect of sediment size on equilibrium-scour 

depths becomes negligible (Dongol, 1993). Although 

soils in South Carolina are not uniform and commonly 

have some degree of cohesion, a computation of this 

dimensionless variable provides some insights into the 

effect of sediment size on abutment scour in South 

Carolina. For the 209 observations of abutment scour in 

this study, D50 ranges from 0.007 to 0.990 mm, 

L ranges from 18 to 7,440 ft, and the dimensionless 

variable L/D50 ranges from 4.4x104

 to 7.3x107. The 

range of L/D50 greatly exceeds 50, indicating that the 

sediment sizes typically found in South Carolina may 

have negligible influence on equilibrium-scour depths 

because embankment lengths are relatively long. 

To provide some perspective on the relation of 

abutment-scour depth and median grain size for nonnormalized field data, figure 44A shows the relation of 

abutment-scour depth and the median grain size for 

surface soils at single-bridge openings in the Coastal 

Plain. Data were grouped into three categories similar 

to the peak-flow duration relation presented in figure 

37A, and the scatter within these categories is fairly 

large. In general, the envelope curves indicate an 

increase in scour depth with increasing grain size, 

which seems counterintuitive. Perhaps one explanation 

for this trend is that the smaller grain sizes are associated with more cohesive soils, which reduce the potential for scour. An example of this can be seen at 

Structure 342050100300 on U.S. Route 501, crossing 

Smith Swamp in Marion County, which is identified on 

figure 44A. The site has a clayey soil that may reduce 

the amount of scour. If this site had noncohesive soils, 

scour depths would be more in line with sites having 

larger median grain sizes. Excluding Smith Swamp, 

the general increase in scour depth with increasing 

Figure 42. Relation of clear-water abutment-scour depth and flow depth, normalized by 

embankment length, for selected laboratory data at spill-through abutments (Dongol, 1993) 

and field data collected in the Piedmont and Coastal Plain of South Carolina.
Variables Influencing Abutment Scour 55

0

5

10

15

20

0 5 10 15

100-YEAR-FLOW DEPTH NEAR THE ABUTMENT TOE, IN FEET

OBSERVED ABUTMENT-SCOUR DEPTH, IN FEET 

Envelope for embankment lengths 400 feet or less

Envelope for embankment lengths 600 feet or less

Embankment lengths ≤ 400 feet

Embankment lengths ≤ 600 feet and > 400 feet

Embankment lengths ≤ 5,300 feet and > 600 feet

Envelope for embankment lengths 100 feet or less

Envelope for embankment lengths 400 feet or less

Envelope for embankment lengths 700 feet or less

Embankment lengths ≤ 100 feet

Embankment lengths ≤ 400 feet and > 100 feet

Embankment lengths ≤ 700 feet and > 400 feet

Embankment lengths ≤ 1,670 feet and > 700 feet

0

5

10

15

20

25

0 5 10 15 20 25 30

100-YEAR-FLOW DEPTH NEAR THE ABUTMENT TOE, IN FEET

OBSERVED ABUTMENT-SCOUR DEPTH, IN FEET

A.

B.

Figure 43. Relation of observed clear-water abutment-scour depth and the 100-year-flow depth 

near the abutment toe for single-bridge openings (A) in the Coastal Plain and (B) Piedmont of 

South Carolina.
56 Clear-Water Abutment and Contraction Scour in the Coastal Plain and Piedmont Provinces of South Carolina, 1996-99

0

5

10

15

20

25

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

MEDIAN GRAIN SIZE, IN MILLIMETERS

OBSERVED ABUTMENT-SCOUR DEPTH, IN FEET

Envelope for embankment lengths 100 feet or less

Envelope for embankment lengths 400 feet or less

Envelope for embankment lengths 700 feet or less

Embankment lengths ≤ 100 feet

Embankment lengths ≤ 400 feet and > 100 feet

Embankment lengths ≤ 700 feet and > 400 feet

Embankment lengths ≤ 1,670 feet and > 700 feet

A.

B.

0

5

10

15

20

25

0.0 0.2 0.4 0.6 0.8 1.0

MEDIAN GRAIN SIZE, IN MILLIMETERS

OBSERVED ABUTMENT-SCOUR DEPTH, IN FEET

Envelope for embankment lengths 400 feet or less

Envelope for embankment lengths 600 feet or less

Embankment lengths ≤ 400 feet

Embankment lengths ≤ 600 feet and > 400 feet

Embankment lengths ≤ 5,300 feet and > 600 feet

Envelope for embankment lengths > 600 feet

Structure 342050100300 on Route U.S. 501 

crossing Smith Swamp

Figure 44. Relation of observed clear-water abutment-scour depth and the median grain size of 

the original sediment sample for surface soils at single-bridge openings in the (A) Coastal Plain and 

(B) Piedmont of South Carolina.
Variables Influencing Abutment Scour 57

sediment size is relatively small, and it is likely that the 

effect of sediment size is minimal for the range of D50

observed in the Coastal Plain.

The relation of abutment-scour depth and the 

median grain size for the original sample of the surface 

soils at bridges in the Piedmont is shown in figure 44B. 

Data were grouped into categories similar to the peakflow duration relation presented in figure 37B. (S.C. 

Route 97 crossing Turkey Creek in Chester County had 

a thin layer of clayey surface soils overlying sand. 

Because the sandy soil was the dominant soil type, the 

D50 for the sandy soil was used in figure 44B instead of 

the D50 for the clayey surface soil.) The envelope 

curves in figure 44B generally show little change in 

abutment-scour depth with increasing median grain 

size. For embankment lengths of 100 or 400 ft or less, 

the soils are clayey and fairly cohesive. Therefore, the 

flat nature of these envelope curves may likely be 

attributed to cohesion. Longer embankment lengths 

associated with wider floodplains also have clayey 

soils, but are commonly more sandy or have a relatively high moisture content reducing the degree of 

cohesion. Because of the large scatter within the data 

for embankment lengths greater than 400 ft, it is difficult to draw strong conclusions about the effect of 

grain size on scour depths. If the envelope for embankment lengths of 700 ft or less is correct, then the effect 

is minimal. 

Sediment Gradation

The above observation is for uniform sediments 

and, therefore, the effects of armoring that are often 

encountered with graded soils may influence scour 

depths at a given site. However, if the largest grain size 

of a graded sediment is too small to armor the streambed, then the effect of gradation is insignificant. 

If the dimensionless ratio L/Dmax equals or exceeds 50, 

then it would seem reasonable to assume that grain 

sizes ≤Dmax (and thus gradation) will have negligible 

influence on equilibrium-scour depths. Because the 

sediment size associated with the 84th percentile by 

weight (D84) was available in the bridge-scour database, the D84 was assumed to represent the largest 

grain size for a given soil. Although the D84 will be 

smaller than the Dmax, it is near the upper limits of 

the grain size range and should give insights into the 

effects of gradation. For the 209 observations of 

abutment scour in South Carolina, the D84 ranges 

from 0.040 to 3.80 mm and the dimensionless ratio, 

L/D84, ranges from 1.6x104

 to 1.2 x107. Because the 

dimensionless ratio L/D84 greatly exceeds 50, it is 

perhaps reasonable to assume that the effect of sediment gradation on equilibrium abutment-scour depths 

for South Carolina sites is negligible.

Embankment Length

Dongol (1993) states that, “Abutment length is 

one of the most important parameters influencing the 

process of local abutment scour.” (Many laboratory 

investigations, including Dongol, define the road 

embankment that blocks approaching flows as the abutment length; however, in this report, the term embankment length is used.) Laboratory data show that scour 

depth increases logarithmically with embankment 

length and will approach a value beyond which 

embankment length has minimal influence on scour 

depth (Dongol, 1993). Figure 45 shows the relation of 

embankment length and scour depth, normalized by 

flow depth, for laboratory data collected at a flow 

intensity (U/Uc) of 1 (Melville, 1992). The plot shows 

an increase of scour depth with increasing embankment 

length with an asymptotic limit of about 10. Melville 

notes that the limit of 10 for the scour depth to flow 

depth ratio is likely too high for field conditions, and 

states that a maximum ratio of 4 was observed for 

limited field data along the Mississippi River (Richardson 

and others, 1990). 

Figure 46A shows the South Carolina field data 

plotted in a similar manner to Melville (1992). The 

observed abutment-scour depth and the 100-year-flow 

embankment length have been normalized by the 

100-year-flow depth near the abutment toe. To provide 

a better comparison of the smaller normalized embankment lengths, figure 46B shows the same data with a 

truncated horizontal axis. The field data show a similar 

trend to the laboratory relation, but there is more scatter 

within the field data and the limit of 3.4 is significantly 

lower. The scatter within the field data may, in part, be 

attributed to flow intensities not equaling 1, and to 

varying degrees of soil cohesion. Note that the maximum ratio of scour depth to flow depth for the South 

Carolina field data (3.4) is relatively close to the maximum ratio of 4 observed along the Mississippi River 

(Richardson and others, 1990). These observed scour 

depth to flow depth ratios in South Carolina and along 

the Mississippi River need additional verification; 

however, they provide some perspective on the range of 

maximum abutment-scour depths observed in the field 

in contrast to laboratory data.
58 Clear-Water Abutment and Contraction Scour in the Coastal Plain and Piedmont Provinces of South Carolina, 1996-99

0

5

10

15

0 20 40 60 80 100

EMBANKMENT LENGTH DIVIDED BY FLOW DEPTH

 ABUTMENT-SCOUR DEPTH DIVIDED BY FLOW DEPTH

Envelope curve based on points identified by circles

Laboratory data from various investigators (Melville, 1992)

Data points arbitrarily chosen to develop envelope curve

ys/y = 1.5ln(L/y)+3.4

L/y Embankment length divided by flow depth

ln Natural logarithm

ys/y Abutment-scour depth divided by flow depth

 Using the laboratory data presented in figure 45, 

Melville (1992) defined three classes of abutments 

based on the dimensionless ratio of embankment length 

(L) to flow depth (y): (1) abutments with short embankments (L/y < 1), (2) abutments with intermediate 

embankments (1 ≤ L/y ≤ 25), and (3) abutments with 

long embankments (L/y > 25). For long embankments, 

the data suggest that the equilibrium-scour depth is less 

dependent of embankment length, and more dependent 

on the approach flow velocity and depth. This does not 

mean a long embankment has no effect on scour depth. 

Field data and laboratory data clearly show that deeper 

abutment-scour holes are associated with longer 

embankments. This only implies that at long embankments, the rate of change in abutment-scour depth as 

embankment length increases is diminished. For the 

209 observations of abutment scour in South Carolina, 

model data show that L ranges from 18 to 7,440 ft and 

y ranges from 1.3 to 20.5 ft. Applying Melville’s classification to these sites, 2 abutments have short embankments, 47 have intermediate embankments, and 160 

have long embankments. The Coastal Plain has 109 

observations of abutment scour, of which 103 are classified as long embankments. In contrast, there are 100 

observations of abutment scour in the Piedmont of 

which 57 are classified as long embankments. If the 

observed data are representative of the bridge abutment 

population in South Carolina, then approximately 77 

percent of all abutments would be classified as having 

long embankments and the rate of change in abutmentscour depth with increasing embankment length would 

be less pronounced. Figure 46 shows most field observations falling under the horizontal part of the envelope 

where the effects of embankment length on scour depth 

are diminished, giving support to this conclusion.

To provide some perspective on the relation of 

scour depth and embankment length for non-normalized field data, figure 47 shows plots of field data 

collected in the Coastal Plain and Piedmont of South 

Carolina. The envelope curves of these data show an 

increase of clear-water abutment-scour depth with 

increasing embankment length. In addition, the rate of 

change in abutment-scour depth diminishes as embankment length increases. Both of these trends are similar 

to laboratory data. Figure 48 compares the envelope 

Figure 45. Relation of abutment-scour depth and embankment length, normalized by flow 

depth, for laboratory data collected at flow intensities of 1. (Modified from Melville, 1992.)
Variables Influencing Abutment Scour 59

0

5

10

15

0 100 200 300 400 500 600 700

EMBANKMENT LENGTH DIVIDED BY FLOW DEPTH

 ABUTMENT-SCOUR DEPTH DIVIDED BY FLOW DEPTH

Envelope of field data

Envelope of laboratory data (Melville, 1992)

Field data

A.

B.

0

5

10

15

0 20 40 60 80 100

EMBANKMENT LENGTH DIVIDED BY FLOW DEPTH

 ABUTMENT-SCOUR DEPTH DIVIDED BY FLOW DEPTH

Envelope of field data

Envelope of laboratory data (Melville, 1992)

Field data

Figure 46. Relation of observed clear-water abutment-scour depth and the 100-year-flow embankment 

length, normalized by the 100-year-flow depth near the abutment toe, for the Piedmont and Coastal Plain 

of South Carolina with a (A) complete horizontal axis, and a (B) truncated horizontal axis.
60 Clear-Water Abutment and Contraction Scour in the Coastal Plain and Piedmont Provinces of South Carolina, 1996-99

0

5

10

15

20

25

0 1,000 2,000 3,000 4,000 5,000 6,000 7,000 8,000

EMBANKMENT LENGTH, IN FEET

OBSERVED ABUTMENT-SCOUR DEPTH, IN FEET

Envelope curve based on a trend line 

through the points identified by circles

Observed abutment scour from field 

investigation

Data points arbitrarily chosen to develop 

envelope curve and equation

A.

B.

0

5

10

15

20

0 200 400 600 800 1,000

EMBANKMENT LENGTH, IN FEET

OBSERVED ABUTMENT-SCOUR DEPTH, IN FEET

Envelope curve based on a trend line 

through the points identified by circles

Observed abutment scour from field 

investigation

Data points arbitrarily chosen to develop 

envelope curve and equation

Figure 47. Relation of observed clear-water abutment-scour depth and the 100-year-flow 

embankment length for the (A) Coastal Plain and (B) Piedmont of South Carolina.
Variables Influencing Abutment Scour 61

0

5

10

15

20

0 200 400 600 800 1,000

EMBANKMENT LENGTH, IN FEET

OBSERVED ABUTMENT-SCOUR DEPTH, IN FEET

 Envelope of observed abutment scour in the Piedmont

 Envelope of observed abutment scour in the Coastal Plain

curves between the Piedmont and Coastal Plain. Interestingly, the envelope curves are similar, with the Piedmont envelope being lower for embankment lengths 

less than 700 ft. This trend is consistent for the differing soils between the Piedmont and Coastal Plain. 

Because the envelope curves were developed using 

different data sets, the similarity between the curves 

gives some support that the envelope curves are showing anticipated trends. Such envelope curves can serve 

as tools for assessing clear-water abutment-scour 

depth; various envelope curves are discussed later in 

the report under “Abutment Scour in the Piedmont” 

and “Abutment Scour in the Coastal Plain.”

Abutment Shape

Laboratory studies indicate that vertical abutments (fig. 32) create larger scour depths than wingwall and spill-through abutments (fig. 30). To account 

for this phenomenon, empirical abutment-scour equations often include a correction coefficient for shape, 

with vertical-wall abutments having a correction coefficient of 1 and wing-wall and spill-through abutments 

having values less than 1. Melville (1992) suggests that 

as the embankment length increases, the benefits 

derived from wing-wall and spill-through abutments is 

diminished and will become negligible for long 

embankments. Melville’s suggestion seems reasonable, 

and, if correct, would indicate that abutment shape has 

little influence on the equilibrium-scour depths for long 

embankments. Based on Melville’s embankment length 

model, 77 percent of the bridges sampled in this study 

are classified as long embankments, indicating that 

abutment shape may have negligible influence on abutment-scour depths at many sites in South Carolina. 

This phenomenon will be more pronounced in the 

Coastal Plain where the percentage of long embankments is higher. With respect to the current study, all 

but 5 of 209 field observations of abutment scour were 

for spill-through abutments. Therefore, conclusions 

drawn from this study must, in general, be limited to 

bridges with spill-through abutments, and the influence 

of abutment shape cannot be verified with this data set.

Figure 48. Comparisons of envelopes for observed clear-water abutment-scour depth and the 

100-year-flow embankment length for the Piedmont and Coastal Plain of South Carolina.
62 Clear-Water Abutment and Contraction Scour in the Coastal Plain and Piedmont Provinces of South Carolina, 1996-99

0.6

0.7

0.8

0.9

1.0

1.1

1.2

-80 -60 -40 -20 0 20 40 60 80

SKEW ANGLE, IN DEGREES

CORRECTION FACTOR FOR ABUTMENT-SCOUR DEPTH

Flow

Zero degree reference

perpendicular to flow

Embankments

Embankment Skew

Laboratory investigations of the effects of 

embankment skew on scour depth have yielded contradictory results (Dongol, 1993). Early investigations 

indicated that embankments skewed upstream toward 

the approaching flow (defined as positive angles in this 

study) produced larger scour depths than its perpendicular counterpart. In contrast, embankments skewed 

downstream, away from the approaching flow (defined 

as negative angles in this study) produced smaller scour 

depths. Two later investigations (Kwan, 1984; 

Kandasamy, 1985) contradicted these results and 

showed that embankments skewed upstream or downstream produced smaller scour depths than their 

perpendicular counterparts. Dongol (1993) suggests 

that a major reason for this contradiction is that the 

earlier studies were run for 300 minutes or less, providing insufficient time for the experiments to reach equilibrium-scour conditions. In contrast, the later studies 

were run for periods greater than 5,000 minutes providing adequate time to reach equilibrium conditions and 

in turn providing more reliable results. If these later 

observations are correct, then embankments with no 

skew are worst-case conditions for equilibrium-scour 

depth and skews on embankments would be considered 

beneficial.

Even if results from the older studies are considered reliable for assessing the effects of skew on abutment-scour depths, from a practical view, the effects 

are often minor. Using an envelope of the older laboratory data, Froehlich (1989) developed an equation to 

estimate an adjustment factor for the effects of skew. 

A modified form of the equation is [(90+θ)/ 90]0.13, 

where θ is the embankment skew in degrees measured 

from a perpendicular reference to the flow (fig. 49), 

with upstream skews being positive and downstream 

skews being negative. Figure 49 shows a plot of this 

equation and as can be seen, even for severe cases of 

skew (60 degrees from perpendicular) the maximum 

increase in scour depth is about 7 percent. Of the 209 

observations of abutment scour in the current study, 

70 observations have absolute skew angles greater than 

0 degrees, with 11 observations exceeding 20 degrees 

Figure 49. Relation of abutment-scour depth correction factor and embankment skew based 

on data obtained from laboratory investigations where experimental tests ran for 300 minutes 

or less. (Modified from Dongol, 1993.)
Variables Influencing Abutment Scour 63

Flow

Zero degree reference

perpendicular to flow

Embankments

0

5

10

15

20

25

-50 -40 -30 -20 -10 0 10 20 30 40 50

EMBANKMENT SKEW, IN DEGREES

OBSERVED ABUTMENT-SCOUR DEPTH, IN FEET

Observed abutment scour 

from perpendicular and the maximum skew being 

45 degrees. With a skew of ±45 degrees, the correction 

to abutment-scour depth ranges from –9 to 6 percent 

depending on orientation to flow. For skews of ±20 

degrees the correction factor ranges from –3 to 3 

percent. If these corrections are applied to the largest 

abutment-scour depth observed in this study (23.6 ft), 

then the theoretical variation in scour depth for skews 

of ±45 degrees is –2.1 to 1.4 ft and for ±20 degrees 

is –0.7 to 0.7 ft. For scour depths less than 23.6 ft, the 

magnitude of the corrections will be smaller. Because 

of the large scatter associated with field data and the 

complexities of field conditions in contrast to laboratory conditions, adjustments of this small magnitude 

cannot be identified within field data. 

Initial reviews of the field data in this study do 

not indicate any major differences in scour depths 

between skewed and unskewed embankments. Investigating the effects of skew on abutment-scour depths is 

difficult to accomplish using field data because skewed 

and unskewed conditions at a given site cannot be 

compared. In addition, left and right embankment 

lengths at a skewed bridge are commonly different, 

making it difficult to draw any conclusions about the 

effect of negative and positive skews at that site. 

However, some broad observations can be made that 

may provide qualitative insights about the effect of 

skew. Figure 50 shows the relation of observed abutment-scour depth and embankment skew for the 209 

observations of abutment scour in this study. Because 

influencing variables, such as embankment length and 

flow velocity, are not incorporated in this figure, the 

effect of skew cannot be readily assessed. The figure 

indicates that the range for abutment-scour depth 

(approximately 0 to 14 ft) at embankments with negative and positive skews does not exceed the range for 

unskewed (0 degrees) abutment-scour depths (0 to 

23.6 ft). This observed trend in figure 50 does not 

define the effect of skew at a given abutment. It does, 

however, indicate that the upper limit for abutmentscour depths at skewed embankments will likely be 

within the range for unskewed embankments for sites 

similar to those investigated in this study. This suggests 

that skews similar to those found in this study will have 

minimal impact on scour depths. 

Figure 50. Relation of observed clear-water abutment-scour depth and embankment skew 

for selected sites with left and (or) right abutment scour in the Piedmont and Coastal Plain of 

South Carolina.
64 Clear-Water Abutment and Contraction Scour in the Coastal Plain and Piedmont Provinces of South Carolina, 1996-99

0

5

10

15

20

25

0 5 10 15

OBSERVED ABUTMENT-SCOUR DEPTH FOR POSITIVE SKEW, IN FEET

OBSERVED ABUTMENT-SCOUR DEPTH FOR NEGATIVE SKEW, IN FEET

Flow

Zero degree reference

perpendicular to flow

Embankments

Absolute value of skew is ≤ 10 degrees

Absolute value of skew is > 10 but ≤ 20 degrees

Absolute value of skew is > 20 but ≤ 32 degrees

1.5 feet offset

1.5 feet offset

Line of agreement

The relation of abutment-scour depths for 

bridges having both negative and positive skews with 

observations of left and right abutment scour (18 sites) 

is shown in figure 51. For a given skewed bridge, the 

left and right embankments will have the same absolute 

value of skew, with one of the embankment-skew 

values being positive and the other negative, depending 

on the orientation of the embankment to flow. Comparison of the left and right abutment-scour depths at a 

given site (based on the positive and negative skew) 

may provide some insight of the effect of skew on 

scour depth. If the trend in Froehlich’s (1989) equation 

is correct, then negative values of skew should produce 

smaller scour depths in comparison with the complimentary abutment with the positive skew. 

A weakness in the relation in figure 51 is that 

most sites have different left and right embankment 

lengths. For example, the left abutment at S.C. Route 

201 crossing the Little River in Abbeville County 

(reference number 1, app. B) has an observed scour 

depth of 4.3 ft associated with a 100-year-flow 

embankment length of 620 ft and an embankment skew 

of 5 degrees. In contrast, the right abutment at this site 

has an observed scour depth of 3.5 ft associated with an 

embankment length of 479 ft and an embankment skew 

of –5 degrees. 

Because embankment length is an important 

explanatory variable for scour depth, the difference in 

left and right abutment-scour depths at this site may be 

more appropriately explained by embankment length 

rather than skew. To isolate the effect of skew, it would 

be best to have skewed bridges with equal left and right 

embankment lengths. However, sites with these conditions were observed infrequently in this study. For the 

sites shown in figure 51, the difference between the left 

and right embankment lengths at a given site ranges 

from 17 to 1,549 ft with a median of 262 ft. This large 

range indicates that the relation in figure 51 likely does 

not isolate the effect of skew. Eleven of the sites in 

figure 51 have left and right abutment-scour depths 

that differ by 1.6 ft or less. From a practical view, 

this difference is minimal. The seven remaining 

bridges, with differences greater than 1.6 ft, have 

site-specific conditions other than skew, that provide 

Figure 51. Relation of observed clear-water abutment-scour depth, for positive and negative 

skews, at sites with left and right abutment scour in the Piedmont and Coastal Plain of South 

Carolina.
Variables Influencing Abutment Scour 65

good explanation for the difference between left and 

right abutment-scour depths. These conditions include 

longer embankment lengths or significantly lower 

Manning’s roughness coefficient at the deeper of the 

left and right scour holes, and site anomalies such as 

channel bends or multiple-bridge openings. The relation in figure 51 does not define the effect of skew on 

abutment-scour depths; however, it does appear to indicate that the effect of skew is probably minor.

Channel Geometry 

Until recent years, much of the laboratory 

research on abutment scour has been made in simple 

rectangular flumes with minimal attention given to the 

effects of irregular shaped channels that are typically 

found in the field. Natural rivers commonly have 

compound channels that consist of a main channel and 

floodplain with variations in Manning’s roughness 

coefficients. The cross-section geometry and roughness 

changes associated with compound channels create 

flow distributions that can be significantly different 

from the uniform flow patterns of simple rectangular 

channels. In general, compound channels will tend to 

reduce abutment-scour depths in comparison to rectangular channels. Richardson and Richardson (1992, as 

cited in Richardson and Davis, 1995) have argued that 

this is a major cause for the overprediction of scour 

depths by laboratory-derived equations. Laboratory 

data suggest that reduction in scour depths will be most 

noticeable at embankments projecting into the main 

channel (fig. 52) and abutments set at the channel bank. 

Laboratory investigations of this type of abutment indicate that scour depths can be reduced by as much as 40 

percent from scour depths in simple rectangular channels (Dongol, 1993). However, when abutments are 

removed from the channel and set back some distance 

from the channel bank (fig. 53), laboratory data suggest 

that the effect of channel geometry is diminished and 

likely becomes negligible (Melville and Coleman, 

2000). Under such conditions it is reasonable to isolate 

the floodplain as a separate channel and assume the 

main channel has no influence on abutment-scour 

depth. This is the general procedure recommended in 

HEC-18 (Richardson and Davis, 1995). 

With regard to the influence of channel geometry 

on abutment scour in South Carolina, several observations can be made. In general, the swampy low-flow 

channels of the Coastal Plain are narrow and shallow and 

convey only a small percentage of the total flood flow. 

In many cases, these channels are almost indistinguishable from the floodplain. In addition, the 

Manning’s roughness coefficient is typically uniform 

across the floodplain creating a relatively uniform flow 

distribution. An example of a Coastal Plain cross 

section is shown on figure 54. The shape and flow 

distributions associated with a swampy cross section 

approximate flow conditions in a simple rectangular 

flume. At swampy Coastal Plain sites, the effect of 

irregular channel geometry is expected to be minor. 

In contrast, the channel geometry of the Piedmont 

streams includes a low-flow channel that often carries a 

large part of the total flood flow (fig. 55). All study 

sites in the Piedmont had setback abutments where 

embankments block only floodplain flows. Piedmont 

floodplains are generally rectangular in shape with 

uniform boundary roughness. Laboratory investigations of setback abutments in compound channels 

having rectangular floodplains with uniform floodplain 

roughness indicate that the effect of channel geometry 

on scour depths is minimal. Therefore, it is likely that 

the effect of channel geometry on setback abutments in 

the Piedmont is minimal as well. 

Geometric-Contraction Ratio

 Although the geometric-contraction ratio (m) is 

not directly mentioned in Dongol’s (1993) list of influencing variables, the dimensionless ratio is directly 

related to embankment length and channel width, and 

is defined as m = 1 – b/B, where B is the approach flow 

width in feet and b is the bridge-opening width in feet. 

The geometric-contraction ratio is an indicator of the 

severity of flow contraction created by a bridge and 

with an increase in this ratio, it would be reasonable to 

anticipate larger scour depths. Das (1973) identified the 

geometric-contraction ratio as an important explanatory variable for abutment scour and showed that abutment-scour depth increases with increasing geometriccontraction ratio (fig. 56). A similar trend is evident in 

the envelope of the South Carolina field data (fig. 57). 

Even though the vertical scales in the figures have not 

been made dimensionless, the shapes of the envelope 

curves are similar, indicating that the field data in 

South Carolina display similar trends to laboratory 

data. Envelope curves like that shown in figure 57 can 

serve as tools for assessing clear-water abutment scour. 

Several envelope curves are discussed later in this 

report under “Abutment Scour in the Piedmont” and 

“Abutment Scour in the Coastal Plain.” 
66 Clear-Water Abutment and Contraction Scour in the Coastal Plain and Piedmont Provinces of South Carolina, 1996-99

Conclusions

 The review of variables that influence abutment 

scour in laboratory investigations indicates that many of 

these variables likely have minimal influence on abutment-scour depths for the prevailing field conditions 

in South Carolina. The variables that appear to have 

minimal influence include flow duration, flow depth, 

sediment size, sediment gradation, abutment shape, abutment skew, and channel geometry. The variables that 

have the most significant effect on abutment-scour depth 

appear to be embankment length, geometric-contraction 

ratio, flow velocity, and soil cohesion. Many of the fielddata relations reviewed in this report were developed 

from modeled rather than measured hydraulics and all of 

the relations clearly show a large scatter. With these limitations in mind, conclusions drawn from the field-data 

relations should be viewed with caution. It is important 

to realize that variables that appear to have minimal 

effect within the South Carolina data may be important 

explanatory variables for abutment scour in regions that 

have field conditions that deviate from those in South 

Carolina.

Figure 52. Profile of abutment projecting into main channel.

Figure 53. Profile of setback abutment commonly found in the Piedmont of South Carolina.
Variables Influencing Abutment Scour 67

20

30

40

50

60

70

0 1,000 1,500 2,000 2,500 3,000 500

DISTANCE FROM LEFT END OF CROSS SECTION, IN FEET

ELEVATION ABOVE MEAN SEA LEVEL, IN FEET

Ground line

Projection of bridge length onto cross section

60

65

70

75

80

85

90

95

100

105

0 200 400 600 800 1,000 1,200

DISTANCE FROM LEFT END OF CROSS SECTION, IN FEET

ELEVATION ABOVE ARBITRARY DATUM, IN FEET

Projection of bridge length onto cross section

Ground line

Figure 55. Example of Piedmont natural cross section at Structure 444005600100 on S.C. Route 56, 

crossing the Enoree River in Union County, South Carolina. (Data from field survey by the U.S. 

Geological Survey, 1998.)

Figure 54. Example of Coastal Plain natural cross section at Structure 212037801000 on U.S. Route 

378, crossing Big Swamp in Florence County, South Carolina. (Data from the South Carolina Department 

of Transportation road plans, File Number 21.253, dated 1947.)
68 Clear-Water Abutment and Contraction Scour in the Coastal Plain and Piedmont Provinces of South Carolina, 1996-99

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0.0 0.2 0.4 0.6 0.8 1.0

GEOMETRIC-CONTRACTION RATIO

OBSERVED ABUTMENT-SCOUR DEPTH, IN FEET

Envelope curve

Envelope curve based on a trend line 

through the points identified by circles

Observed abutment scour for laboratory 

investigation

Data points arbitrarily chosen to develop 

envelope curve

0

5

10

15

20

25

0.0 0.2 0.4 0.6 0.8 1.0

100-YEAR FLOW GEOMETRIC-CONTRACTION RATIO

OBSERVED ABUTMENT-SCOUR DEPTH, IN FEET

Envelope curve

Envelope curve based on a trend line 

through the points identified by circles

Observed abutment scour from field 

investigation

Data points arbitrarily chosen to develop 

envelope curve

Figure 56. Relation of abutment-scour depth and geometric-contraction ratio for 

laboratory data. (Das, 1973.)

Figure 57. Relation of observed clear-water abutment-scour depth and the 100-year-flow 

geometric-contraction ratio in the Coastal Plain and Piedmont of South Carolina.
Abutment Scour in the Piedmont 69

The envelope relations of approach velocity and 

abutment-scour depth (fig. 40) are helpful in understanding the effect of approach velocity on abutmentscour depths. However, the envelope curves are limited 

to broad categories of embankment lengths, and the 

modeled velocities for the 100-year flow likely do not 

represent historic flow velocities that created the scour. 

Because of these limitations, the approach velocity and 

scour depth envelope relation in these figures are not 

recommended as primary tools for assessing scour in 

South Carolina. 

When comparing the Piedmont and Coastal Plain 

envelope curves for the relation of approach velocity 

and scour depth (fig. 41), the scour depths in the Piedmont are significantly lower. This difference highlights 

the scour resistant nature of the cohesive Piedmont 

soils in contrast to the sandier soils of the Coastal 

Plain. Data collected in this study are insufficient to 

quantify the varying degrees of cohesion that exist 

within the soils of South Carolina. Only the broad 

categories of abutment scour in the Piedmont and the 

Coastal Plain can be used to, in some measure, incorporate the effect of varying soil cohesion. 

Embankment length appears to be the strongest 

explanatory variable for abutment-scour depth in South 

Carolina. Although embankment lengths in this study 

were estimated with the WSPRO model using the 100-

year flow, these lengths (at a given site) are approximately constant for a large range of flood elevations and 

the estimates likely represent 

historic embankment lengths that 

created the scour. Embankment 

length can be estimated from topographic maps, road plans, or 

hydraulic models. The strong relation of embankment length within 

the field data, as well as the ease of 

estimating this parameter, makes 

embankment length a good explanatory variable for assessing abutment-scour depth in South Carolina. 

Tools for assessing abutment scour 

in South Carolina, as discussed in 

the following report sections, focus 

on envelope curves for the Piedmont and Coastal Plain regions 

developed with embankment 

length. Envelope curves developed 

with the geometric-contraction ratio 

also are discussed. In addition to 

envelope curves for assessing potential abutment-scour 

depth, tools for assessing the location and width of 

abutment-scour holes are described.

A regression analysis of observed abutmentscour depth and selected explanatory variables indicated a weak correlation within the data and a large 

scatter of the observed data around the regression. 

From these results, it was concluded that envelope 

curves of the field data with selected explanatory 

variables would provide more reliable estimates of 

maximum observed scour depths than regression equations. Therefore, focus was given to developing these 

envelope curves for assessing scour in South Carolina.

ABUTMENT SCOUR IN THE PIEDMONT

One hundred observations of clear-water 

abutment scour were collected at 65 bridge sites in the 

Piedmont Province, including 21 bridges (table 2) influenced by the high-flow region (fig. 1) (Guimaraes and 

Bohman, 1992). The abutment configuration at all sites 

is a setback abutment where the toe of the abutment is 

located on the floodplain at some distance away from 

the main channel (fig. 53). With only two exceptions, all 

abutments were spill through (fig. 30). Table 10 lists the 

range for selected parameters associated with the 100 

abutment-scour observations. Values for hydraulic data 

were estimated with the WSPRO model (Shearman, 

1990) using the 100-year flow. 

Table 10. Range of selected parameters for 100 observations of clear-water 

abutment scour collected at 65 bridges in the Piedmont of South Carolina

[mi2, square mile; ft/ft, feet per foot; ft/s, feet per second; ft, feet; mm, millimeter; <, less than] 

Parameter Minimum 

value

Median 

value

Maximum 

value

Drainage area (mi2) 11.0 74.9 a 1,620

Channel slope determined from topographic 

map (ft/ft)

0.00015 0.0012 0.0029

100-year average approach velocity obstructed by 

embankment (ft/s) b

0.1 0.85 3.2

100-year average approach depth obstructed by 

embankment (ft) b

1.0 5.4 14.6

Embankment length based on 100-year flow (ft) b 18.0 276 c 953

Median grain size (mm) < 0.062 0.073 0.990

Observed abutment-scour depth (ft) 0 1.3 18

a

 Approximately 97 percent of the study sites in the Piedmont have drainage areas less than 400 

mi2 (fig. 3). b

 Parameter was estimated from the one-dimensional water-surface profile model, WSPRO 

(Shearman, 1990). c

 Three observations had embankment lengths exceeding 950 ft and were significantly outside 

the range for the majority of the Piedmont data. These sites were excluded from development of the 

embankment-length envelope curve. Therefore, the range of Piedmont embankment lengths used in 

this study should be limited to 950 ft.
70 Clear-Water Abutment and Contraction Scour in the Coastal Plain and Piedmont Provinces of South Carolina, 1996-99

Comparison of Observed and Theoretical 

Abutment-Scour Depths

To estimate theoretical abutment-scour depth for 

clear-water scour conditions, Hydraulic Engineering 

Circular Number 18 (HEC-18) (Richardson and Davis, 

1995) recommends the use of Froehlich’s (1989) 

equation (eq. 9) that was derived from laboratory data 

for noncohesive sediments. Because Piedmont soils typically have some degree of cohesion, the equation often 

gives excessive scour depths for this region. Figure 58A 

compares the theoretical abutment-scour depth 

computed for the 100-year flow with the observed 

abutment-scour depth and clearly shows the excessive 

0

5

10

15

20

25

30

35

0 5 10 15 20

OBSERVED ABUTMENT-SCOUR DEPTH, IN FEET

THEORETICAL ABUTMENT-SCOUR DEPTH, IN FEET

Line of agreement

0

5

10

15

20

25

30

35

0 5 10 15 20

OBSERVED ABUTMENT-SCOUR DEPTH, IN FEET

THEORETICAL ABUTMENT SCOUR-DEPTH, IN FEET

Line of agreement

A.

B.

Figure 58. Relation of observed clear-water abutment-scour depth and theoretical 

abutment-scour depth for the (A) 100-year flow, and the (B) August 1995 flood at 

selected sites in the Piedmont of South Carolina. (Theoretical abutment scour 

calculated with the Froehlich (1989) equation.)
Abutment Scour in the Piedmont 71

scour depths given by Froehlich’s equation for Piedmont 

soils. Only three observations of abutment scour are 

under predicted by the Froehlich equation with the 

largest underprediction being 3 ft; each of these sites 

has conditions that may explain this underprediction. 

One site is a multiple bridge opening where complex 

flow patterns may be improperly estimated by the onedimensional WSPRO (Shearman, 1990) model. At the 

other two sites, it is possible that the floodplain roughness coefficients in the WSPRO model were set too 

high for the conditions that may have created the scour. 

These sites were modeled for existing conditions at the 

time of the site visit. However, evidence indicates that 

floodplains at these sites were clear in years past, which 

would significantly lower the roughness coefficients and 

increase the theoretical scour depth computed by the 

Froehlich equation. 

It should be kept in mind that the flows that 

created the observed abutment-scour depths are 

unknown for most sites plotted in figure 58A. The 

discrepancy between the observed and theoretical 

scour depths could be attributed to historic flows at 

these sites being less than the 100-year flow magnitude. Maximum historic flows have been documented 

at 12 sites along the Reedy, South Tyger and Enoree 

Rivers (table 6, fig. 6). Flows at these 12 sites occurred 

during the August 1995 flood, and 8 sites experienced 

flows exceeding the 100-year flow magnitude. A comparison of the theoretical abutment-scour depths, computed using the August 1995 flow magnitudes, with the 

observed scour still indicates excessive scour depths 

given by Froehlich’s equation (fig. 58B). The results 

of figure 58A are based on the hypothetical 100-year 

flow, whereas figure 58B represents observed and 

theoretical scour for known maximum historic flows. 

Therefore, the results shown in figure 58B likely provide a better indicator of how well the Froehlich equation predicts theoretical scour. Regardless, both figures 

show a large discrepancy between observed and theoretical scour depths, suggesting that the Froehlich 

equation is a poor predictor for abutment scour at 

Piedmont sites and that another method is needed to 

estimate abutment scour in this region.

Piedmont Floodplain Geometry as an 

Indicator of Potential Scour

Because of the common occurrence of the scourresistant clayey soils in the Piedmont, deep abutmentscour holes were not anticipated to be found in this 

region. During initial reconnaissance in the Piedmont, 

however, several bridges with deep abutment-scour 

holes were discovered. The soils at these sites appeared 

to have more sand content or were more saturated than 

the clayey soils typically associated with the Piedmont. 

These conditions apparently made the soils more 

susceptible to scour and played a role in the development of the larger scour depths. An additional factor, 

however, could be the floodplain geometry consisting 

of wide, flat floodplains. Because of the rolling 

topography of the Piedmont, the stream network is 

fairly dense with numerous smaller streams combining 

to form larger streams. The smaller streams typically 

have narrow river valleys, and bridge crossings tend to 

make less severe contractions of flow. In general, large 

abutment-scour depths are not observed at such sites. 

In contrast, some of the larger streams have alternating 

reaches of narrow and wide river valleys (fig. 59).

When a bridge crossing occurs at the wider section of 

the river valley, the potential for abutment scour can be 

significantly increased because the bridge has more 

potential to create a large contraction of flow. Strictly 

speaking, the wide river valley is not the cause of deep 

abutment scour, but serves as an indicator for the 

potential of a large contraction of flow. If the wide river 

valley is associated with a bridge that creates a large 

contraction of flow and with scour-susceptible soils, 

then large abutment-scour depths can be anticipated. 

As shown on figure 59, the wide river valleys of the 

Piedmont are prominent features that can be easily 

identified on USGS 7.5-minute series topographic 

maps. Therefore, topographic maps can be useful tools 

in identifying bridges crossing wide river valleys and 

having a high potential for abutment scour. 

Embankment Length as an Indicator of 

Potential Scour

In this study, the embankment length is defined 

for a given flow, by projecting the bridge section onto 

the approach cross section and determining the top 

width of the obstructed-flow area for the embankment 

of interest (fig. 33). Laboratory studies indicate that 

scour depth increases with increasing embankment 

length and logarithmically approaches a limit for 

longer embankments (fig. 45). The relation of the 

observed abutment-scour depths in the Piedmont and 

the embankment length for the 100-year flow shows 

similar trends to laboratory data (fig. 60). The envelope 

curve for observed data in the Piedmont clearly indicates 

that larger abutment-scour depths are associated with 
72 Clear-Water Abutment and Contraction Scour in the Coastal Plain and Piedmont Provinces of South Carolina, 1996-99

Figure 59. Example of wide, flat floodplain at (A) Structure 124000901100 on S.C. Route 9, 

crossing Turkey Creek in Chester County, South Carolina, and (B) Structure 362017600400 on 

U.S. Route 176, crossing Indian Creek in Newberry County, South Carolina.
Abutment Scour in the Piedmont 73

0

5

10

15

20

0 200 400 600 800 1,000

EMBANKMENT LENGTH, IN FEET

OBSERVED ABUTMENT-SCOUR DEPTH, IN FEET

Envelope curve based on a trend line 

through the points identified by circles

Observed abutment scour from field 

investigation

Data points arbitrarily chosen to develop 

envelope curve and equation

Equation for envelope curve 

ys = -0.000009L2

 + 0.0276L

 for L < 950

ys Estimate of abutment scour depth

 along envelope curve, in feet

 L 100-year-flow embankment length, in feet 

 (not to exceed 950 feet)

longer embankment lengths. As with the laboratory 

data, the influence of embankment length diminishes 

with increasing length and appears to be approaching a 

limit. An equation for this envelope is:

ys = -0.000009L2 + 0.0276L , (11)

where

Six observations of abutment scour (table 11) 

were excluded from figure 60 because they were 

outside the range for most of the data or because anomalies at the site made the measured data or modeled 

hydraulic variables questionable. Figure 61 shows the 

same information as figure 60, but highlights the 

observed data from the 1995 flood. Several of the data 

ys is the upper limit for the range of anticipated 

abutment-scour depth, in feet; and

L is the 100-year-flow embankment length, in feet; 

and should not exceed 950 ft. 

points fall along or near the envelope, indicating that 

the curve provides some representation of the upper 

limit for abutment-scour depths that might occur at 

sites experiencing large flood flows.

The envelope in figure 60 encompasses the 

majority of field data, as well as all sites influenced by 

the 1995 flood, and it is reasonable to assume that it 

represents an upper range of potential scour for a given 

embankment length. With this assumption, the envelope can be used as a tool for assessing potential abutment-scour depths in the Piedmont. Because the 

envelope represents an upper limit of observed scour 

depth, it will likely overpredict scour depth for various 

site conditions; however, the envelope will not exceed 

the upper bound of observed field data. In contrast, 

theoretical scour computed with the Froehlich (1989) 

equation frequently exceeds the upper bound of 

observed data (fig. 62), indicating the embankmentlength envelope is a better method for assessing 

potential abutment-scour depths in the Piedmont.

Figure 60. Relation of observed clear-water abutment-scour depth and the 100-year-flow 

embankment length in the Piedmont of South Carolina.
74 Clear-Water Abutment and Contraction Scour in the Coastal Plain and Piedmont Provinces of South Carolina, 1996-99

0

5

10

15

20

0 200 400 600 800 1,000

EMBANKMENT LENGTH, IN FEET

OBSERVED ABUTMENT-SCOUR DEPTH, IN FEET

Envelope of field data

Observed abutment scour for all Piedmont sites

Envelope curve

Observed abutment scour from 1995 flood

Table 11. Field observations excluded from the development of the embankment-length 

envelope curve (fig. 60) for the Piedmont of South Carolina

[U.S., United States Route; S.C., South Carolina Route; I, Interstate Highway] 

County Road Stream

Reference 

number 

in figure 1 

and 

appendix B

Embankment 

lengtha

(feet)

Abutment

scour 

depth

(feet)

Chester b I-77 Fishing Creek 14 1,593 0.0

Laurens c S.C. 72 Little River 89 52 13.3

Laurens d S.C. 72 Duncan Creek 90 473 17.0

Newberry b,e U.S. 176 Kings Creek 107 1,205 1.6

Newberry b,f S.C. 121 Saluda River 109 1,669 2.3

Oconee e U.S. 76 Coneross Creek 114 711 0.0

a

 Embankment length estimated from the WSPRO model (Shearman, 1990) using the theoretical 100-year flow. b

 Embankment length is beyond the range of the majority of observed data. c

 Abutment scour at this site is not created by classical abutment-scour processes. The channel at this site has a 

severe bend (approximately 90 degrees) upstream from the bridge. During high-flow conditions, flow jumps the 

main channel and enters the abutment region, creating the large abutment-scour depth. For more information on this 

site see the report section, “Effect of Upstream Channel Alignment.” d

 This is a multiple-bridge opening crossing that may have distributions of flow significantly different from a 

single-bridge opening. These irregular flow patterns make it difficult to estimate an embankment length and 

therefore this site was excluded from figure 60. For more information on multiple-bridge openings see the 

“Multiple-Bridge Openings” section under “Guidance for Assessing Abutment-Scour Depth with the Envelope 

Curves.” e

 A small relief culvert in the road embankment makes the measured scour data questionable. f

 A large road ditch on the upstream road embankment makes the measured scour data questionable.

Figure 61. Relation of observed clear-water abutment-scour depth and the 100-year-flow 

embankment length with identification of the 1995 flood sites in the Piedmont of South Carolina.
Abutment Scour in the Piedmont 75

0

5

10

15

20

25

30

35

0 200 400 600 800 1,000

EMBANKMENT LENGTH, IN FEET

THEORETICAL ABUTMENT-SCOUR DEPTH, IN FEET

Envelope of field data

Theoretical abutment scour (Froehlich, 1989)

Envelope curve

Geometric-Contraction Ratio as an 

Indicator of Potential Scour

The amount of flow contraction caused by a bridge 

can be assessed by using the geometric-contraction ratio, 

m, defined as m = 1 – b/B, where the variables are 

previously defined. In general, as the geometriccontraction ratio increases, the velocity and flow curvature at a bridge will rise, increasing the potential for 

scour. Therefore, deeper scour depths associated with 

larger geometric-contraction ratios were expected, as 

was the trend of the laboratory data shown in figure 56. 

This general trend is also apparent in the relation of 

observed scour depth and the 100-year-flow geometriccontraction ratio for field data in the Piedmont 

(fig. 63). The envelope of these data clearly indicates 

that larger abutment-scour depths are associated with 

larger geometric-contraction ratios. An equation for 

this envelope is:

ys = 19.96m2 + 6.163m , (12)

where

Because the largest observation of abutment 

scour in the Piedmont (18.0 ft) has a geometriccontraction ratio of 0.82 and an embankment length of 

950 ft, the envelope curve should be limited to sites 

with geometric-contraction ratios and embankment 

lengths less than or equal to these values. Figure 64 

shows the same information as figure 63, but highlights 

the observed data from the 1995 flood. Several of the 

data points fall along or near the envelope, indicating 

that the curve provides some representation of the 

upper limit for abutment-scour depths that might occur 

at sites experiencing large flood flows. 

ys is the upper limit of the range for anticipated 

abutment-scour depth in feet; and

m is the 100-year-flow geometric-contraction ratio;

where 

m = 1 – b/B with the variables previously defined.

Figure 62. Relation of theoretical 100-year-flow abutment-scour depth and embankment length 

compared with the envelope of observed abutment scour for selected sites in the Piedmont of 

South Carolina. (Theoretical abutment scour calculated with the Froehlich (1989) equation.)
76 Clear-Water Abutment and Contraction Scour in the Coastal Plain and Piedmont Provinces of South Carolina, 1996-99

0

5

10

15

20

0.0 0.2 0.4 0.6 0.8 1.0

GEOMETRIC-CONTRACTION RATIO

OBSERVED ABUTMENT-SCOUR DEPTH, IN FEET

Envelope curve based on a trend line 

through the points identified by circles

Observed abutment scour from field 

investigation

Data points arbitrarily chosen to develop 

envelope curve and equation

ys Estimate of abutment-scour depth

 along envelope curve, in feet

 m 100-year-flow geometric-contraction ratio 

 (not to exceed 0.82)

Equation for envelope curve 

ys = 19.96m2 + 6.163m 

 for m ≤ 0.82

0

5

10

15

20

0.0 0.2 0.4 0.6 0.8 1.0

GEOMETRIC-CONTRACTION RATIO

OBSERVED ABUTMENT-SCOUR DEPTH, IN FEET

Envelope curve

Envelope curve 

Observed abutment scour for all Piedmont sites

Observed abutment scour from 1995 flood

Figure 63. Relation of observed clear-water abutment-scour depth and the 100-year-flow 

geometric-contraction ratio in the Piedmont of South Carolina.

Figure 64. Relation of observed clear-water abutment-scour depth and the 100-year-flow 

geometric-contraction ratio with identification of the 1995 flood sites in the Piedmont of 

South Carolina.
Abutment Scour in the Piedmont 77

0

5

10

15

20

25

30

35

0.0 0.2 0.4 0.6 0.8 1.0

GEOMETRIC-CONTRACTION RATIO

THEORETICAL ABUTMENT-SCOUR DEPTH, IN FEET

Envelope curve for observed abutment scour

Theoretical abutment scour (Froehlich, 1989)

Envelope curve

The envelope in figure 63 encompasses all field 

data, as well as all sites influenced by the 1995 flood, 

and it is reasonable to assume that it represents an 

upper range of potential scour for a given geometriccontraction ratio. With this assumption, the envelope 

can be used as a tool for assessing potential abutmentscour depths in the Piedmont. Because the envelope 

represents an upper limit of observed scour depth, it 

will likely overpredict scour depth for various site 

conditions; however, the envelope will not exceed 

the upper bound of observed field data. In contrast, 

theoretical scour computed with the Froehlich (1989) 

equation frequently exceeds this upper bound (fig. 65).

For geometric-contraction ratios of 0.75 or less, 45 of 

the 55 theoretical values exceed the envelope, indicating that this part of the envelope will provide better 

estimates of abutment scour. For ratios greater than 

0.75, 38 of the 45 theoretical values fall below the 

envelope, indicating that this part of the envelope 

may overpredict scour depths. This overprediction can 

be attributed to the dimensionless character of the 

geometric-contraction ratio that obscures the effect 

of embankment length. This will be discussed in more 

detail in the report section, “Selecting the Appropriate 

Abutment-Scour Depth Envelope.”

Floodplain Land Cover as an Indicator 

of Potential Scour

Perhaps the most commonly observed land cover 

for floodplains in the Piedmont of South Carolina is 

forest. Forested floodplains have relatively high resistance to flow compared to the main channel. The highflow resistance reduces flow velocities and the percentage of total flow carried by the floodplain. When floodplain flow resistance is significantly reduced, flow 

velocities and the percentage of total flow carried by 

the floodplain will increase. This, in turn, will increase 

the potential for abutment scour. Reduction in floodplain flow resistance can occur when forested floodplains are clearcut for timber harvest or are converted 

to pastures. To provide some perspective of the impact 

that changes in flow resistance can have on floodplain 

flows, the right floodplain of Cannons Creek at Road 

S-299 in Newberry County was modeled for forested 

and pastured conditions. Table 12 lists the right floodplain average velocity and flow, showing a significant 

difference between these conditions. 

Figure 66 shows the relation of observed abutment-scour depth and the 100-year-flow embankment 

length, identifying sites with reduced flow resistance 

Figure 65. Relation of theoretical 100-year-flow abutment-scour depth and the 100-year-flow 

geometric-contraction ratio compared with the envelope of observed data in the Piedmont of 

South Carolina. (Theoretical abutment scour calculated with the Froehlich (1989) equation.)
78 Clear-Water Abutment and Contraction Scour in the Coastal Plain and Piedmont Provinces of South Carolina, 1996-99

caused by pastures or timber harvest. Three categories 

of sites are identified: (1) sites that had pastures at the 

time of the site visit; (2) sites that had woods at the 

time of the site visit, but had strong evidence of 

pastures or timber harvest in the past; and (3) sites that 

had sparse woods at the time of the site visit, and weak 

evidence of existing pastures or timber harvest in the 

past. A noteworthy observation is that these sites 

include all but one of the observed abutment-scour 

depths greater than 6.5 ft. These sites also form the 

boundary of the envelope for scour depths 9.7 ft or 

greater. This suggests that sites with lowered flow 

resistance have a high abutment-scour potential. The 

lowered floodplain flow resistance is not sufficient, in 

itself, to produce large scour depths, and also must be 

accompanied by a significant contraction of flow as 

well as soils that will scour for the given conditions.

Because site histories are incomplete, the floodplain land cover at the time the observed scour holes 

were created is not known, and the scour-producing 

floods might have occurred at these sites when the 

floodplains were forested. If this is the case, then 

observed scour cannot be associated with a reduction in 

flow resistance. However, the data in figure 66 strongly 

suggests that the risk of greater scour depths increases 

when flow resistance is lowered by clear-cutting or 

conversion to pasture land. Because observed data at 

floodplains with lowered flow resistance fall near or on 

the boundary of both the embankment-length and the 

geometric-contraction-ratio envelope curves, it is 

reasonable to assume that the envelope curves will 

account for such conditions. When assessing abutment 

scour in the Piedmont, one should carefully consider 

existing and future conditions that could lower flow 

resistance and, in turn, increase scour potential.

Table 12. The effect of changing floodplain flow resistance 

exemplified by model data at Structure 367029900100 on 

Road S-299, crossing Cannons Creek in Newberry County, 

South Carolina

[ft 3/s, cubic feet per second; ft/s, feet per second] 

Right 

floodplain

land cover

Estimate of 

Manning’s 

roughness 

coefficient 

on right 

floodplain

Flow 

on right 

floodplain

(ft3/s)a

Average 

flow velocity 

on right 

floodplain

(ft/s)a

Forest 0.16 430 0.3

 Pasture 0.035 1,510 1.1

a

 Estimated from the WSPRO model (Shearman, 1990) using the 

theoretical 100-year flow.

0

5

10

15

20

0 200 400 600 800 1,000

EMBANKMENT LENGTH, IN FEET

OBSERVED ABUTMENT-SCOUR DEPTH, IN FEET

Envelope for observed data 

Observed abutment scour for all 

Piedmont sites

Observed abutment scour at 

sites with existing pasture

Observed abutment scour at sites with 

existing woods, but strong evidence of 

pasture or timber harvest in the past

Observed abutment scour at sites with 

existing sparse woods, and weak 

evidence of timber harvest in the past

Figure 66. Relation of observed clear-water abutment-scour depth and the 100-year-flow 

embankment length with the identification of sites with lowered floodplain flow resistance for the 

Piedmont of South Carolina. 
Abutment Scour in the Coastal Plain 79

0

5

10

15

20

25

0 20 40 60 80 100

PERCENTILE

OBSERVED ABUTMENT-SCOUR DEPTH, IN FEET

Coastal Plain

Piedmont

ABUTMENT SCOUR IN THE 

COASTAL PLAIN

A total of 109 observations of clear-water 

abutment scour were collected at 81 bridge sites in the 

Coastal Plain of South Carolina, consisting of 7 observations from the upper Coastal Plain and 102 from the 

lower Coastal Plain. Eleven of the 81 bridges have a 

significant main channel conveying water through the 

bridge. The remaining 70 bridges are located on 

swampy floodplains where shallow low-flow channels 

convey only a small percentage of the total flow. 

For the 11 bridges with main channels, the abutment 

configuration is a setback abutment where the toe of 

the abutment is located on the floodplain at some 

distance away from the main channel (fig. 53). For the 

swampy bridges, the abutment configuration is similar 

to the setback abutment, excluding the main channel, 

and the bridges resemble floodplain relief bridges 

(fig. 29). Three of the abutments are vertical wing walls 

with the remainder being spill-through abutments.

A plot of observed abutment scour in the 

Coastal Plain and Piedmont of South Carolina indicates that larger scour depths occur more frequently 

in the Coastal Plain than in the Piedmont (fig. 67).

These larger scour depths can be attributed to the 

longer embankment lengths and larger contraction 

ratios associated with the wider floodplains of the 

Coastal Plain. In addition, the scour-susceptible soils 

and longer flow durations of the Coastal Plain promote 

larger scour depths. Figure 68 is a plot of the observed 

floodplain widths for this study and gives some 

perspective on the more common occurrence of wide 

river valleys in the Coastal Plain compared with the 

Piedmont. Table 13 provides the range for selected 

parameters associated with the 109 abutment-scour 

observations. Values for hydraulic data were estimated 

with the WSPRO (Shearman, 1990) model using the 

100-year flow. 

Abutment-Scour Hole Patterns

In general, two scour-hole patterns were 

observed in the Coastal Plain. For smaller bridges 

(approximately 240 ft in length or less) it was common 

to find a large, single scour hole with a top width that 

often encompassed the entire bridge opening (fig. 69).

Figure 67. Percentile of observed clear-water abutment-scour depth in the Piedmont and 

Coastal Plain of South Carolina.
80 Clear-Water Abutment and Contraction Scour in the Coastal Plain and Piedmont Provinces of South Carolina, 1996-99

In contrast, bridges exceeding 240 ft in length tended to 

develop separate scour holes at each abutment (fig. 70).

This phenomenon at shorter bridges appears to be 

caused by the overlap of turbulent and curvilinear flow 

coming from the left and right abutments. If the bridge 

had been sufficiently long to prevent the overlap of 

these flow patterns, then separate left and right abutment-scour holes, typically found at longer bridges, 

would likely have developed. Because the large, single 

scour holes at shorter bridges appear to be created by 

turbulent flow patterns typically associated with abutment scour, it was assumed that these scour holes 

should be classified as abutment scour. To associate the 

single scour holes at short bridges with abutment variables, it was assumed that the longer of the left or right 

abutments provided the stronger influence in developing the scour hole and should be associated with the 

observed scour. For example, U.S. Route 301 crossing 

Douglas Swamp in Florence County has an 88-ft-long 

bridge with a single scour hole that is 10.6 ft deep 

(fig. 69). The left and right embankment lengths for 

the 100-year flow were 452 and 695 ft, respectively. 

Table 13. Range of selected parameters for 109 observations 

of clear-water abutment scour collected at 81 bridges in the 

Coastal Plain of South Carolina

[mi2, square mile; ft/ft., feet per foot; ft/s, feet per second; ft, feet; 

mm, millimeter; <, less than] 

Parameter

Minimum 

value

Median 

value

Maximum 

value

Drainage area (mi2) 6.0 120 a 8,830

Channel slope determined from topographic map (ft/ft)0.00007 0.0005 0.0024

100-year average approach velocity 

obstructed by embankment (ft/s) b

0.1 0.5 1.6

100-year average approach depth 

obstructed by embankment (ft) b

1.5 4.7 17.4

Embankment length based on 

100-year flow (ft) b

87 557 c 7,440

Median grain size (mm) < 0.062 0.18 0.780

Observed abutment-scour depth (ft) 0.0 7.0 23.6

a

 Approximately 80 percent of the study sites in the Coastal Plain 

have drainage areas less than 426 mi2

 (fig. 3). b Parameter was estimated from the one-dimensional water-surface 

profile model, WSPRO (Shearman, 1990). c

 Only seven observations have embankment lengths that 

exceed 2,000 ft.

0

500

1,000

1,500

2,000

2,500

3,000

0 40 60 80 100 20

PERCENTILE

FLOODPLAIN WIDTH, IN FEET

Coastal Plain

Piedmont

Figure 68. Percentile of the 100-year-flow floodplain width for study sites in the Piedmont and Coastal 

Plain of South Carolina. (NOTE: The vertical scale has been truncated in order to provide detail for the 

smaller floodplain widths.)
Abutment Scour in the Coastal Plain 81

Therefore, hydraulic variables for the longer embankments (695 ft) such as flow velocity, flow depth, and 

embankment length, were associated with the observed 

scour at this site. Hydraulic variables for all of the 

large, single scour holes were obtained in this manner. 

This assumption should be kept in mind when reviewing hydraulic abutment data from sites that display this 

pattern. These data, along with sites with distinct left 

and (or) right abutment-scour holes, were used to 

develop the embankment-length envelope curve 

presented later in the report.

Comparison of Observed and Theoretical 

Abutment-Scour Depths

To estimate theoretical abutment-scour depth for 

clear-water scour conditions, HEC-18 (Richardson and 

Davis, 1995) recommends the use of Froehlich’s 

(1989) equation (eq. 9), which was derived from laboratory data for noncohesive sediments. Because the 

sandy soils and nearly rectangular floodplains associated with the Coastal Plain are similar to laboratory 

conditions, Froehlich’s equation may perform better in 

91

83

84 85

92

92

86

91

87

88

89

86

87

88

90

92

91

91

0 10 20 30 40 50 FEET

0 5 10 15 20 METERS

FLOW

EXPLANATION

92 TOPOGRAPHIC CONTOUR--Interval 0.5 foot.

Arbitrary datum

SURVEYED DATA POINT

BRIDGE DECK

BENT LOCATION

AVERAGE UNSCOURED REFERENCESURFACE ELEVATION

91.9 FEET

Elevation at bridge

deck corner = 99.9 feet

Figure 69. Example of single scour hole at shorter bridges in the Coastal Plain, as shown at Structure 212030100100 on 

U.S. Route 301, crossing Douglas Swamp in Florence County, South Carolina, July 31, 1996.
82 Clear-Water Abutment and Contraction Scour in the Coastal Plain and Piedmont Provinces of South Carolina, 1996-99

this region than in the Piedmont. Figure 71A compares 

the theoretical abutment-scour depth computed for the 

100-year flow with the observed abutment-scour depth. 

Note that the Froehlich equation overpredicts scour for 

sites where less than 7 ft of scour was observed and 

begins to underpredict scour for sites with observed 

scour greater than 7 ft, indicating that the equation 

cannot be relied upon to always provide conservative 

design estimates of scour depth. Although the Froehlich 

equation performs better in the Coastal Plain than in 

the Piedmont, the results still have significant error. 

As with the Piedmont, it should be kept in mind 

that flows that created the observed abutment-scour 

depths in the Coastal Plain are unknown for most sites. 

It is possible that the discrepancy between the observed 

and theoretical scour depths may be attributed to historic 

flows at these sites differing from the 100-year flow 

magnitude; however, maximum historic flows have 

been documented at 23 bridges in the Coastal Plain 

(tables 4 and 8). Eight of the bridges experienced 

the October 1992 flood where flows equaled or 

exceeded the 100-year magnitude at seven of these sites. 

Seven bridges experienced the September 1999 flood 

along the Waccamaw River where flows exceeded the 

100-year magnitude. Eight bridges along the Pee Dee 

and Little Pee Dee River have maximum historic flows 

near the 25-year flow. Comparing the theoretical abutment-scour depth computed with the maximum historic 

flow with the observed scour indicates excessive scour 

depths given by Froehlich’s equation (fig. 71B). The 

results of figure 71A are based on the hypothetical 

100-year flow, whereas figure 71B represents observed 

and theoretical scour for known maximum historic 

flows. Therefore, the results of figure 71B likely 

provide a better indicator of how well the Froehlich 

equation predicts theoretical scour in the Coastal Plain. 

Both figures, however, show that large discrepancies 

between observed and theoretical scour depths can 

exist, suggesting that the Froehlich equation is a poor 

predictor for abutment scour at Coastal Plain sites 

and that another method should be used to estimate 

abutment scour in this region.

20

18

20

18

18

20

20

14 1

6

18

16

18

20

16

18

18

20

20 20

16

16

20

18

18

20

18

22

20

0 50 FEET

0 10 20 METERS

1

FLOW

Elevation at bridge 

deck corner = 30.6 feet

EXPLANATION

20 TOPOGRAPHIC CONTOUR--Interval 1.0 foot.

Arbitrary datum

SURVEYED DATA POINT

BRIDGE DECK

BENT LOCATION

AVERAGE UNSCOURED REFERENCESURFACE ELEVATION

19.8 FEET

Figure 70. Example of separate left and right abutment-scour holes at longer bridges in the Coastal Plain, as shown at Structure 

277008700100 on Road S-87, crossing the Coosawhatchie River in Jasper County, South Carolina, November 12, 1997.
Abutment Scour in the Coastal Plain 83

0

5

10

15

20

25

30

0 5 10 15 20 25

OBSERVED ABUTMENT-SCOUR DEPTH, IN FEET

OBSERVED ABUTMENT-SCOUR DEPTH, IN FEET

THEORETICAL ABUTMENT-SCOUR DEPTH, IN FEET

Line of agreement

A.

B.

0

5

10

15

20

25

0 5 10 15 20

THEORETICAL ABUTMENT-SCOUR DEPTH, IN FEET

Line of agreement

Figure 71. Relation of observed clear-water abutment-scour depth with theoretical abutmentscour depth for the (A) 100-year flow, and the (B) maximum historic flow at selected sites in 

the Coastal Plain of South Carolina. (Theoretical abutment scour calculated with the Froehlich 

(1989) equation.)
84 Clear-Water Abutment and Contraction Scour in the Coastal Plain and Piedmont Provinces of South Carolina, 1996-99

Embankment Length as an Indicator 

of Potential Scour

Similar to abutment scour in the Piedmont, 

embankment length can provide some indication of the 

anticipated range of scour depth for Coastal Plain sites. 

Comparing the observed scour depths in the Coastal 

Plain with the embankment length for the 100-year 

flow, larger scour depths are associated with longer 

embankment lengths (fig. 72A). (Figure 72B is similar 

to figure 72A with the horizontal scale truncated to 

2,000 ft to provide more detail for smaller embankment 

lengths.) The envelope curve for the observed data 

suggests that the influence of embankment length 

decreases with increasing embankment length, which is 

consistent with trends from laboratory investigations. A 

series of equations for this envelope is:

L ≤ 426, ys = 0.0338L, and (13)

426 < L ≤ 7,440, ys = 14.4 + 0.00131(L-426),

where

This envelope and associated equation are limited to 

embankment lengths of 7,440 ft, and because of sparse 

field observations, may not fully represent trends for 

embankment lengths exceeding 2,000 ft. Figure 73 

shows the same information as figure 72A, but 

highlights the observed data from the 1992 and 1999 

floods. Several of the data points fall along or near the 

envelope, indicating that the curve provides some representation of the upper limit for abutment-scour depths 

that might occur at sites experiencing large flood flows. 

The envelope in figure 72 encompasses the 

majority of field data, as well as all sites influenced by 

the 1992 and 1999 floods, and it is reasonable to 

assume that the envelope represents an upper range of 

potential scour depth for a given embankment length. 

With this assumption, the envelope can be used as a 

tool for assessing potential abutment-scour depths in 

the Coastal Plain. Figure 74 compares theoretical 

scour, computed with the Froehlich (1989) equation, 

with the embankment-length envelope. Most of the 

theoretical values are within the envelope, indicating 

that the Froehlich equation may often provide reasonable 

results in Coastal Plain of South Carolina. However, 

ys is the upper limit for the range of anticipated 

abutment-scour depth, in feet; and

L is the 100-year-flow embankment length, in feet. 

figure 71A indicates that the Froehlich equation can 

occasionally underpredict scour depth by as much as 

7.3 ft. Because the envelope represents the upper bound 

of observed data, underprediction is minimized, 

making the envelope a preferable method for assessing 

potential abutment-scour depths in the Coastal Plain.

Geometric-Contraction Ratio as an 

Indicator of Potential Scour

As with abutment scour in the Piedmont, the 

geometric-contraction ratio provides some indication 

of the potential for abutment scour at Coastal Plain 

bridges. Comparing the observed scour depths in the 

Coastal Plain with the geometric-contraction ratio 

for the 100-year flow, the data clearly show an increase 

in abutment-scour depth with increasing geometriccontraction ratio (fig. 75). An equation for this 

envelope is:

ys = 29.62m3 – 10.182m2 + 5.538m, (14)

where

This envelope and associated equation are 

limited to geometric-contraction ratios of 0.98 or less 

and embankment lengths of 7,440 ft, and because of 

sparse field observations, may not fully represent 

trends for embankment lengths exceeding 2,000 ft. 

Figure 76 shows the same information as figure 75, but 

highlights the observed data from the 1992 and 1999 

floods. Several of the data points fall near the envelope, 

indicating that the curve provides some representation 

of the upper limit for abutment-scour depths that might 

occur at sites experiencing large flood flows. 

The envelope in figure 75 encompasses all 

field data, as well as all sites influenced by the 1992 

and 1999 floods, and it is reasonable to assume that it 

represents an upper range of potential scour depth for 

a given geometric-contraction ratio. Based on this 

assumption, the envelope can be used as a tool for 

assessing potential abutment-scour depths in the 

Coastal Plain. Figure 77 compares theoretical scour, 

computed using the Froehlich (1989) equation, with the 

ys is the upper limit for the range of anticipated abutment-scour depth, in feet; and

m is the 100-year-flow geometric-contraction ratio;

where 

m = 1 – b/B with the variables previously defined.
Abutment Scour in the Coastal Plain 85

0

5

10

15

20

25

0 1,000 2,000 3,000 4,000 5,000 6,000 7,000 8,000

Envelope curve based on a trend line 

through the points identified by circles

Observed abutment scour from field 

investigation

Data points arbitrarily chosen to develop 

envelope curve and equation

ys Estimate of abutment scour depth

 along envelope curve, in feet

 L 100-year-flow embankment length, in feet 

 

Equation for lower segment of envelope curve 

ys

 = 0.0338L, 

where L < 426

Equation for upper segment of envelope curve 

ys

 = 14.4 + 0.00131(L- 426), 

where 426 < L < 7,440

A.

B.

0

5

10

15

20

25

0 500 1,000 1,500 2,000

EMBANKMENT LENGTH, IN FEET

EMBANKMENT LENGTH, IN FEET

OBSERVED ABUTMENT-SCOUR DEPTH, IN FEET OBSERVED ABUTMENT-SCOUR DEPTH, IN FEET

Envelope curve

Envelope curve based on points identified by circles

Observed abutment scour

Points arbitrarily chosen to develop envelope curve

Figure 72. Relation of observed clear-water abutment-scour depth and the 100-year-flow 

embankment length for the Coastal Plain of South Carolina with a (A) complete horizontal axis 

and a (B) truncated horizontal axis.
86 Clear-Water Abutment and Contraction Scour in the Coastal Plain and Piedmont Provinces of South Carolina, 1996-99

0

5

10

15

20

25

0 1,000 2,000 3,000 4,000 5,000 6,000 7,000 8,000

EMBANKMENT LENGTH, IN FEET

OBSERVED ABUTMENT-SCOUR DEPTH, IN FEET

Envelope curve

Envelope curve 

Observed abutment scour for all 

Coastal Plain sites

Observed abutment scour at sites 

with known maximum historic flow

Figure 73. Relation of observed clear-water abutment-scour depth and the 100-year-flow 

embankment length identifying the 1992 and 1999 flood sites in the Coastal Plain of South 

Carolina.

0

5

10

15

20

25

0 1,000 2,000 3,000 4,000 5,000 6,000 7,000 8,000

EMBANKMENT LENGTH, IN FEET

THEORETICAL ABUTMENT-SCOUR DEPTH, IN FEET

Envelope curve for observed abutment scour

Theoretical abutment scour (Froehlich, 1989)

Figure 74. Relation of theoretical 100-year-flow abutment-scour depth and the 100-year-flow 

embankment length compared with the envelope of observed abutment scour for selected sites 

in the Coastal Plain of South Carolina. (Theoretical abutment scour calculated with the Froehlich 

(1989) equation.)
Abutment Scour in the Coastal Plain 87

0

5

10

15

20

25

0.0 0.2 0.4 0.6 0.8 1.0

GEOMETRIC-CONTRACTION RATIO

OBSERVED ABUTMENT-SCOUR DEPTH, IN FEET

Envelope curve based on a trend line 

through the points identified by circles

Observed abutment scour from field 

investigation

Data points arbitrarily chosen to develop 

envelope curve and equation

ys Estimate of abutment-scour depth

 along envelope curve, in feet

 m 100-year-flow geometric-contraction ratio 

 (not to exceed 0.98)

Equation for envelope curve 

ys = 29.62m3-10.182m2 + 5.538m

 where m ≤ 0.98 

Figure 75. Relation of observed clear-water abutment-scour depth and the 100-year-flow 

geometric-contraction ratio for the Coastal Plain of South Carolina.

0

5

10

15

20

25

0.0 0.2 0.4 0.6 0.8 1.0

GEOMETRIC-CONTRACTION RATIO

OBSERVED ABUTMENT-SCOUR DEPTH, IN FEET

Envelope curve

Envelope curve 

Observed abutment scour for all 

Coastal Plain sites

Observed abutment scour at sites 

with known maximum historic flow

Figure 76. Relation of observed clear-water abutment-scour depth and the 100-year-flow 

geometric-contraction ratio identifying sites with known maximum historic flows in the 

Coastal Plain of South Carolina.
88 Clear-Water Abutment and Contraction Scour in the Coastal Plain and Piedmont Provinces of South Carolina, 1996-99

geometric-contraction ratio envelope. For geometriccontraction ratios of 0.75 or less, 15 of the 16 theoretical 

values exceed the envelope, indicating that this part of 

the envelope may provide better estimates of abutment 

scour than the Froehlich equation. For ratios greater 

than 0.75, 89 of the 93 theoretical values fall below the 

envelope, indicating that this part of the envelope may 

overpredict scour depths. As mentioned previously, this 

overprediction can be attributed to the dimensionless 

character of the geometric-contraction ratio that 

obscures the effect of embankment length and will 

be discussed in more detail in the report section, 

“Selecting the Appropriate Abutment-Scour Depth 

Envelope.”

As with the embankment-length envelope, 

most of the theoretical values are within the geometriccontraction ratio envelope, indicating that the Froehlich 

equation may often provide reasonable results in this 

region of South Carolina. As previously discussed, 

however, the Froehlich equation can occasionally 

underpredict observed scour (fig. 71A), making the 

envelope a preferable method for assessing potential 

abutment-scour depths in the Coastal Plain. 

Bridges 240 Feet or Less in Length

As described previously, bridges approximately 

240 ft or less in length, tend to form a large, single scour 

hole (fig. 69) rather than separate left and right abutment-scour holes (fig. 70). This phenomenon appears to 

be caused by the overlap of turbulent and curvilinear 

flow coming from the left and right abutments. Because 

turbulent and curvilinear flows are typically associated 

with abutment scour, scour holes at these sites were 

classified as abutment scour. To associate these large, 

single scour holes with a particular abutment, the longer 

of the left or right embankment was used in the development of the embankment-length envelope curve. Figure 

78A identifies the large, single scour holes at bridges 

240 ft or less in length, in relation to the embankmentlength envelope for the Coastal Plain. (Figure 78B is 

similar to figure 78A with the horizontal scale truncated 

to 2,000 ft to provide more detail for smaller embankment lengths.) For embankment lengths of 750 ft or 

less, these sites appear to dominate the upper bound of 

the envelope, indicating that the single scour holes are 

generally deeper than separate left and (or) right abutment-scour holes at longer bridges. Comparing the 

0

5

10

15

20

25

0.0 0.2 0.4 0.6 0.8 1.0

GEOMETRIC-CONTRACTION RATIO

THEORETICAL ABUTMENT-SCOUR DEPTH, IN FEET

Envelope curve for observed abutment scour

Theoretical abutment scour (Froehlich, 1989)

Envelope curve

Figure 77. Relation of theoretical 100-year-flow abutment-scour depth and the 100-yearflow geometric-contraction ratio compared with the envelope of observed abutment scour in 

the Coastal Plain of South Carolina. (Theoretical abutment scour calculated with the 

Froehlich (1989) equation.)
Abutment Scour in the Coastal Plain 89

0

5

10

15

20

25

0 1,000 2,000 3,000 4,000 5,000 6,000 7,000 8,000

EMBANKMENT LENGTH, IN FEET

OBSERVED ABUTMENT-SCOUR DEPTH, IN FEET

Envelope curve

Envelope curve of field data 

Bridges 240 feet or less in length

with single scour holes

Bridges with separate left and (or)

right abutment scour holes

Structure 274000300200

on S.C. Route 3 crossing 

Cypress Creek

0

5

10

15

20

25

0 500 1,000 1,500 2,000

EMBANKMENT LENGTH, IN FEET

OBSERVED ABUTMENT-SCOUR DEPTH, IN FEET

Envelope curve

Envelope curve for field data 

Bridges 240 feet or less in length

with single scour holes

Bridges with separate left and (or)

Structure 274000300200 right abutment scour holes

on S.C. Route 3 crossing 

Cypress Creek

A.

B.

Figure 78. Relation of observed clear-water abutment-scour depth and the 100-year-flow embankment length identifying sites with bridge lengths of 240 feet or less and single scour holes in the 

Coastal Plain of South Carolina with a (A) complete horizontal axis and a (B) truncated horizontal 

axis.
90 Clear-Water Abutment and Contraction Scour in the Coastal Plain and Piedmont Provinces of South Carolina, 1996-99

upper limit of scour depth at separate left and (or) right 

abutment-scour holes with the single scour holes indicates that the single scour holes can be about 3 to 4 ft 

deeper (fig. 78). The one exception is Structure 

274000300200 on S.C. Route 3 crossing Cypress Creek 

in Jasper County (reference number 81, app. B and fig. 

1), which is located at the apex of the envelope (fig. 78). 

This bridge is 210 ft in length and could be classified as 

a short bridge, but individual left and right abutmentscour holes were observed at this site. Excluding 

Cypress Creek, it may be possible to develop a separate 

envelope that encompasses separate left and right abutment-scour holes that are typically associated with 

longer bridges. Because of limited data, however, it was 

considered more prudent to develop one envelope based 

on all of the data. 

When assessing abutment scour at shorter bridges 

in the Coastal Plain, it should be kept in mind that these 

sites tend to have larger scour depths than comparable 

sites with longer bridges. Using a bridge length of 240 ft 

as the breakpoint between shorter bridges with single 

scour holes and longer bridges with separate left and 

right abutment-scour holes should be viewed as an 

approximate region of transition rather than an absolute 

value. When applying the abutment-scour envelope 

curves to shorter bridges, judgment should be used to 

ensure that the conditions at the site of interest are 

comparable to the sites included in this study.

Floodplain Land Cover as an Indicator 

of Potential Scour

As with the Piedmont, the most commonly 

observed land cover for floodplains in the Coastal Plain of 

South Carolina is forest. In the Piedmont, lowered flow 

resistance from deforestation was observed at a number of 

bridges and deeper scour holes commonly existed at these 

sites. In the Coastal Plain, only one site with a deep scour 

hole could be associated with deforestation: Interstate 95 

overflow bridge crossing the Pee Dee River floodplain in 

Florence County. This site had the deepest observed scour 

in this study, 23.6 ft. Investigations of historic aerial 

photographs by the SCDOT indicated that significant 

deforestation occurred on the upstream floodplain after 

construction of this bridge (William Hulbert, South Carolina Department of Transportation, oral commun., 1996). 

Therefore, it is likely that deforestation had a significant 

influence in the development of this scour hole. With only 

one observation in the Coastal Plain associated with 

deforestation, it is inappropriate to draw general 

conclusions about the effect of deforestation on scour in 

this region, but the one observation is consistent with the 

pattern observed in the Piedmont. 

EFFECT OF UPSTREAM CHANNEL 

ALIGNMENT ON ABUTMENT SCOUR

Channel bends can present a unique problem at 

bridges because approaching flow distributions at flood 

stage can be significantly altered when flows leave a 

channel and enter a floodplain at a channel bend. When 

channel bends occur upstream, near a bridge, concentrated channel flows can be directed to a section of a 

bridge opening that would not typically experience this 

magnitude of flow if the channel were straight. Under 

these conditions, large scour holes can develop even 

when embankment lengths and geometric-contraction 

ratios are relatively small. Two sites of this nature were 

noted during field investigations: Structure 

304007200500 on S.C. Route 72 crossing Little River in 

Laurens County and Structure 264000920500 on S.C. 

Route 9 crossing the Waccamaw River in Horry County. 

(Data from the S.C. Route 9 bridge were not included in 

the analysis for this report, but are presented here for 

information purposes.) 

The Little River in Laurens County has several 

sharp bends upstream from the bridge on S.C. Route 72 

(fig. 79). During flood conditions, concentrated flows 

leave the main channel and enter the right abutment area 

creating a deep scour hole (13.3 ft) that is associated 

with an embankment length of 52 ft. This is significantly 

outside the embankment-length envelope for the Piedmont (fig. 60). The USGS 7.5-minute series topographic 

map (U.S. Geological Survey, 1971) and aerial photographs of this site indicate that the Little River does 

meander; however, the severe bends near the bridge 

cannot be readily identified on the map or photograph. 

Field visits quickly identified these bends and the associated scour problem, underscoring the importance of site 

visits to help assess scour at bridges. 

The Waccamaw River in Horry County has a sharp 

meander bend just upstream from the bridge on S.C. 

Route 9 (fig. 80). During the 1999 flood, flow measurements at this bridge (USGS gaging station 02110500, 

Waccamaw River at Longs, S.C.) exceeded the 100-year 

flow magnitude. During flood conditions, concentrated 

flows apparently overtopped the main channel and entered 

the left abutment area creating a scour hole that was 

approximately 15 ft deep and 150 ft wide (fig. 81). The 
Guidance for Assessing Abutment-Scour Depth Using the Envelope Curves 91

embankment associated with this scour hole is approximately 800 ft long. The observed scour depth is slightly 

outside the embankment-length envelope for the Coastal 

Plain (fig. 72) and the scour-hole width is larger than 

those observed in this study. Because the Waccamaw 

River has a wide channel, the USGS 7.5-minute series 

topographic map (U.S. Geological Survey, 1990) and 

aerial photographs are useful tools for identifying this 

potential scour problem. Envelope curves developed in 

this report should not be used to assess scour at sites that 

have channel bends in the upstream reach near a bridge. 

Instead, one should review all available resources for the 

site of interest and use judgment to make a final decision 

on anticipated scour depths.

GUIDANCE FOR ASSESSING 

ABUTMENT-SCOUR DEPTH 

USING THE ENVELOPE CURVES

The envelope curves presented in figures 60, 63, 

72, and 75 can be useful tools for assessing abutmentscour depth at bridges in South Carolina. When using 

these envelope curves, one must select a reference 

surface, determine the scour depth using the appropriate abutment-scour envelope curve, and consider other 

scour components in the abutment region. Although 

these steps require judgment, some guidance for 

accomplishing these steps is presented in this section.

Figure 79. Plan view of channel bends and resulting scour hole at Structure 304007200500 on S.C. Route 72, 

crossing the Little River in Laurens County, South Carolina. (Note: Not to scale.)
92 Clear-Water Abutment and Contraction Scour in the Coastal Plain and Piedmont Provinces of South Carolina, 1996-99

Figure 81. Scour hole at left abutment viewed from downstream at Structure 264000920500 

on S.C. Route 9, crossing the Waccamaw River in Horry County, South Carolina. (Photograph 

by the South Carolina District, U.S. Geological Survey, June 2000.)

Figure 80. Severe meander on the Waccamaw River at Structure 264000920500 on S.C. 

Route 9 in Horry County, South Carolina. (Photograph from South Carolina DOQQ Archive, 

1994.)
Guidance for Assessing Abutment-Scour Depth Using the Envelope Curves 93

Selecting a Reference Surface for 

Abutment-Scour Depth

The envelope curves presented in this report can 

be used to assess potential scour depths at bridge abutments. The scour depth, in turn, must be subtracted 

from the appropriate reference surface to determine the 

bottom elevation of the scour hole and the effect that 

the scour will have on bridge foundations. In this study, 

the average, undisturbed floodplain elevation in the 

abutment-scour region was used to determine abutment-scour depth; this reference surface should be used 

when assessing abutment scour with the envelope 

curves. The reference surface can be determined by 

reviewing floodplain elevations from SCDOT road and 

bridge plans, surveyed cross sections, and (or) site visit 

observations. In many cases, the floodplain in the 

region of the abutment is relatively flat, so estimating 

an average floodplain elevation in the region of anticipated scour is not a difficult task. However, there can 

be cases where the floodplain significantly slopes in 

the lateral and (or) longitudinal direction making the 

determination of a reference surface more difficult. In 

such cases, judgment must be applied, bearing in mind 

that lower reference-surface elevations will produce 

lower scour-hole elevations.

Estimating the Embankment Length 

and the Geometric-Contraction Ratio

The explanatory variables used in the abutmentscour depth envelope curves are the embankment 

length and the geometric-contraction ratio. The envelope curves can be sensitive to these explanatory variables; therefore, it is critical that these variables be 

accurately determined. When developing the envelope 

curves, the embankment length and the geometriccontraction ratio were determined using a flow model 

for the 100-year flow condition. When applying the 

envelope curves at specific sites, these variables also 

should be estimated for the 100-year flow at the site of 

interest. It also is recommended that topographic maps 

and road plans be used to verify embankment lengths 

and the geometric-contraction ratio. When discrepancies exist between these sources, judgment should be 

used to determine the most reasonable estimate of the 

explanatory variables.

Selecting the Appropriate AbutmentScour Depth Envelope 

When assessing abutment-scour depths using 

envelope curves presented in this report, one must select 

envelope curves for the appropriate physiographic region 

(Piedmont or Coastal Plain), and then use the embankment-length and the geometric-contraction-ratio envelope, for the selected region, to assess potential 

abutment-scour depth. These two envelope curves will 

often provide different estimates for the upper range of 

abutment-scour depth, so one must select the most 

appropriate depth. Application of these envelope curves 

to selected sites in South Carolina has shown that the 

geometric-contraction-ratio envelope, for both physiographic regions, will typically provide higher estimates 

of abutment-scour depth than the embankment-length 

envelope curves. At times, the scour-depth estimate from 

the geometric-contraction-ratio envelope is excessive. 

Excessive estimates can be attributed to the dimensionless nature of the geometric-contraction ratio and its 

inability to account for changes in embankment length. 

For example, a 100-ft-long bridge crossing a 500-ft-wide 

Piedmont floodplain has a geometric-contraction ratio of 

0.8. Similarly, a 200-ft-long bridge crossing a 1,000-ftwide floodplain will have the same geometric-contraction ratio. If these bridges are symmetrically located on 

the floodplain, then the embankment lengths will be 200 

and 400 ft, respectively. Using the geometric-contraction-ratio envelope curve (fig. 63) to estimate abutmentscour depth, the scour depth will be identical for both 

bridges. It is reasonable, however, to assume (with all 

other variables the same) that the bridge with the shorter 

embankments will have smaller scour depths than the 

bridge with longer embankments. Another example of 

where excessive estimates of abutment scour depth may 

occur, is at sites having eccentric bridge crossings where 

one embankment is substantially shorter in length. 

In such cases, the geometric-contraction-ratio envelope 

will give identical scour depths at both abutments. 

However, with other variables remaining constant, it is 

reasonable to assume that scour depths will be less at the 

shorter embankment of an eccentric bridge crossing. 

To avoid overestimates of the range for abutment-scour 

depth, it may be reasonable to use the embankmentlength envelope curve as the primary tool for estimating 

abutment-scour depth in the Piedmont and Coastal Plain 

of South Carolina. If a conservative estimate of the range 

of abutment-scour depth is needed, then the larger scour 

depth obtained from the geometric-contraction ratio 

or embankment-length envelope curve may be more 

appropriate. Special consideration must be given to 
94 Clear-Water Abutment and Contraction Scour in the Coastal Plain and Piedmont Provinces of South Carolina, 1996-99

the selection of the appropriate envelope curve when 

dealing with multiple-bridge openings, and a discussion of assessing abutment-scour depth at single-and 

multiple-bridge openings follows.

Single-Bridge Openings

In the Piedmont, 63 of the 65 bridges surveyed 

in this study were single-bridge openings. All of these 

data, with the exception of Structure 304007200500 at 

S.C. Route 72 crossing Little River in Laurens County 

(reference number 89, appendix B and fig. 1), fall 

within the embankment-length and geometric-contraction-ratio envelope curves for the Piedmont region of 

South Carolina (figs. 60 and 63). Therefore, if a singlebridge opening in the Piedmont has site conditions 

similar to bridges included in this study, it is appropriate to use either envelope curve to estimate the range of 

anticipated abutment-scour depth. 

In the Coastal Plain, 50 of the 81 bridges 

surveyed in this study were single-bridge openings. 

All of these data fall within the embankment-length 

and geometric-contraction-ratio envelope curves for the 

Coastal Plain region of South Carolina (figs. 72 and 

75). As with the Piedmont, if a single-bridge opening 

in the Coastal Plain has site conditions similar to 

bridges included in this study, it is appropriate to use 

either envelope curve to estimate the range of anticipated abutment-scour depth. As previously discussed, 

it may be reasonable to use the embankment-length 

envelope curve as the primary tool for estimating 

abutment-scour depth at single-bridge openings in the 

Piedmont and Coastal Plain of South Carolina. If a 

conservative estimate of the range of abutment-scour 

depth is needed, then the larger scour depth obtained 

from the geometric-contraction-ratio or embankmentlength envelope curve may be more appropriate.

When assessing Coastal Plain abutment-scour 

depths at single-bridge openings crossing swamps with 

poorly defined low-flow channels, with bridge lengths 

approximately 240 ft or less, the longest of the left or 

right embankment lengths should be used in conjunction with the embankment-length envelope curve 

(fig. 72). The geometric-contraction-ratio envelope 

(fig. 75) also may be used to assess abutment-scour 

depth at these bridges, bearing in mind that this estimate may be high for shorter embankment lengths. 

The estimated scour depth will be assumed to represent 

the depth of the large, single scour hole that will likely 

extend from abutment toe to abutment toe.

Multiple-Bridge Openings

 Floodplain widths in the Piedmont are relatively 

narrow and, therefore, multiple-bridge openings are 

uncommon. Two of the 65 Piedmont bridges surveyed in 

this study were associated with multiple-bridge openings: Structure 237004000100 on Road S-40 crossing 

South Saluda River in Greenville County and Structure 

304007201200 on S.C. Route 72 crossing Duncan Creek 

in Laurens County (reference numbers 63 and 90, 

respectively, appendix B and fig. 6.) These crossings 

include main-channel bridges and floodplain relief structures. The two bridges of interest are floodplain relief 

structures with bridge lengths of 60 and 100 ft, respectively. Because of the short bridge lengths, large, single 

scour holes developed at these sites similar to scour 

holes at short bridges in the Coastal Plain. (For more 

details on this scour-hole pattern refer to the “Bridge 

Lengths of 240 Feet or Less” section under ”Abutment 

Scour in the Coastal Plain.”) Figure 82 shows the 

Piedmont embankment-length envelope, previously 

described, with identification of data from the two multiple-bridge openings. Observed scour at Duncan Creek 

falls outside of the envelope curve. This anomaly is possibly caused by the irregular distribution of flow that 

commonly occurs at a multiple-bridge site. In addition, 

stagnation points for flow between multiple-bridges, as 

estimated by the simplified routine in the WSPRO 

(Shearman, 1990) model, may be in error. Any error in 

this estimate will provide improper estimates of the 

embankment lengths. In contrast, all data (single and 

multiple bridges) fall near or within the Piedmont geometric-contraction-ratio envelope curves (fig. 83). 

Therefore, when estimating abutment scour at multiplebridge openings in the Piedmont, it appears reasonable 

to use the geometric-contraction-ratio envelope (fig. 63) 

rather than the embankment-length envelope (fig. 60).

In contrast to the Piedmont, floodplains in the 

Coastal Plain are typically flat and wide and often have 

multiple-bridge openings. Thirty-one of the 81 Coastal 

Plain bridges surveyed in this study were associated 

with multiple-bridge openings. When developing the 

Coastal Plain embankment-length envelope curve, 4 

of the 48 multiple-bridge observations of abutment 

scour fell outside of the envelope (fig. 84). As with the 

Piedmont, this anomaly is possibly caused by complex 

flow patterns and errors associated with the WSPRO 

(Shearman, 1990) model. The four multiple bridges in 

the Coastal Plain that fall outside of the embankmentlength envelope have embankment lengths of 193 ft or 

less. In contrast, all data (single and multiple bridges) 
Guidance for Assessing Abutment-Scour Depth Using the Envelope Curves 95

0

5

10

15

20

0 500 1,000 1,500 2,000

EMBANKMENT LENGTH, IN FEET

OBSERVED ABUTMENT-SCOUR DEPTH, IN FEET

Envelope curve

Envelope curve

Observed abutment scour at 

single-bridge openings

Observed abutment scour at 

multiple-bridge openings

0

5

10

15

20

0.0 0.2 0.4 0.6 0.8 1.0

GEOMETRIC-CONTRACTION RATIO

OBSERVED ABUTMENT-SCOUR DEPTH, IN FEET

Envelope curve

Envelope of field data 

Observed abutment scour at single-bridge openings

Observed abutment scour at multiple-bridge openings

Figure 83. Relation of observed clear-water abutment-scour depth and 100-year-flow 

geometric-contraction ratio with the identification of multiple-bridge openings for the 

Piedmont of South Carolina. 

Figure 82. Relation of observed clear-water abutment-scour depth and the 100-yearflow embankment length with the identification of multiple-bridge openings for the 

Piedmont of South Carolina.
96 Clear-Water Abutment and Contraction Scour in the Coastal Plain and Piedmont Provinces of South Carolina, 1996-99

0

5

10

15

20

25

0 500 1,000 1,500 2,000

EMBANKMENT LENGTH, IN FEET

OBSERVED ABUTMENT-SCOUR DEPTH, IN FEET

Envelope curve

Envelope curve 

Observed abutment scour at single-bridge openings

Observed abutment scour at multiple-bridge openings

fall near or within the Coastal Plain geometric-contraction-ratio envelope (fig. 85). Therefore, if a multiplebridge opening in the Coastal Plain has site conditions 

similar to bridges included in this study, it is appropriate to use the geometric-contraction-ratio envelope (fig. 

75) rather than the embankment-length envelope (fig. 

72) to estimate the range of anticipated abutment-scour 

depth. There are some cases, however, where the use of 

the embankment-length envelope for multiple-bridge 

openings in the Coastal Plain is appropriate and these 

cases are described in the following paragraphs.

As previously discussed, the geometric-contraction-ratio envelope tends to overpredict the range of 

scour depth. To minimize this effect when assessing 

multiple-bridge openings in the Coastal Plain, the 

following steps should be followed. Examination of the 

embankment-length envelope curve for the Coastal 

Plain (fig. 72) shows that the envelope has a sharp 

break in slope at an embankment length of 425 ft. 

Estimates of the upper limit for the range of abutmentscour depths will be sensitive for embankment lengths 

less than 425 ft, and relatively insensitive for embankment lengths greater than 425 ft. Because observed 

scour at all Coastal Plain multiple-bridge openings 

with embankment lengths greater than 425 ft fall 

within the embankment-length envelope (fig. 84), 

it is reasonable to assume that the embankment-length 

envelope would be applicable to Coastal Plain multiple-bridge openings with embankment lengths exceeding 425 ft. In the case of multiple-bridge openings with 

embankment lengths less than 425 ft, the geometriccontraction-ratio envelope (fig. 75) may be used.

As with single-bridge openings in the Coastal 

Plain, field data show that multiple-bridge openings 

crossing swamps with poorly defined low-flow channels, with bridge lengths of approximately 240 ft or 

less, tend to form one large scour hole rather than separate left and right abutment-scour holes. The width of 

the single scour hole commonly encompasses the entire 

bridge opening extending from one abutment toe to the 

other. To estimate scour at such bridges, the following 

approach may be reasonable. Select the longest of the 

left and right embankment lengths. If the longest 

embankment length is <425 ft, then use the geometriccontraction-ratio envelope to estimate the abutmentscour depth. If the length is ≥425 ft, then use the 

embankment-length envelope to estimate the range of 

abutment-scour depth. The estimated scour depth is 

assumed to represent the depth of the large, single 

scour hole that will likely extend from abutment toe to 

abutment toe.

Figure 84. Relation of observed clear-water abutment-scour depth and the 

100-year-flow embankment length with the identification of multiple-bridge 

openings in the Coastal Plain of South Carolina.
Guidance for Assessing Abutment-Scour Depth Using the Envelope Curves 97

0

5

10

15

20

25

0.0 0.2 0.4 0.6 0.8 1.0

GEOMETRIC-CONTRACTION RATIO

OBSERVED ABUTMENT-SCOUR DEPTH, IN FEET

Envelope curve

Envelope curve 

Observed abutment scour at single-bridge openings

Observed abutment scour at multiple-bridge openings

Contraction and Pier Scour within the 

Abutment-Scour Area

As previously discussed in the report section, 

“Techniques for the Collection and Interpretation of 

Field Data,” contraction scour should not be considered 

a contributing component to total scour in the abutment 

region. Therefore, when using the envelope curves to 

assess total scour depth at abutments, no adjustment for 

contraction scour in the abutment region is required.

In the case of pier scour, consideration must be 

given to the impact that piers in the abutment region 

may have on total scour. As previously discussed in the 

report section, “Techniques for the Collection and 

Interpretation of Field Data,” measured scour at deeper 

abutment-scour holes represents total scour, including 

the impact of pier scour, if a pier was located within the 

scour hole. The assumption also was made that piers or 

multiple-column bents located at the bottom of the 

scour hole produced the largest effect on total scour. 

Figure 86 shows the previously developed envelope 

curves for the relation of embankment length 

and abutment-scour depth for the Coastal Plain and 

Piedmont, identifying sites where the low point of the 

abutment-scour hole coincides with a pier or multiplecolumn bent. The figure shows many such sites falling 

on or near the envelope curves, indicating that the envelope curves, include the worst-case impact of local pier 

scour on total scour at abutments. 

When examining more closely the sites that have 

piers at the low point of the abutment-scour hole, the 

supporting substructures for all but one of the sites 

were found to be multiple-column bents that range in 

width from 1 to 2.3 ft with little or no skew. Based on 

this observation, it is reasonable to assume that the 

envelope curves include the impact of such bents on 

total scour in the abutment region and no adjustment 

for pier scour is required. When the column and pier 

width exceeds 2.3 ft, judgment should be used to determine if the results of the abutment-scour envelope 

curves should be adjusted to account for the effects 

of the wider piers. Judgment also should be used if 

the effects of debris and (or) severe skews must be 

considered. 

The above guidance must be slightly modified 

for abutment-scour holes of approximately 5 ft or less 

Figure 85. Relation of observed clear-water abutment-scour depth and the 100-yearflow geometric-contraction ratio with the identification of multiple-bridge openings in the 

Coastal Plain of South Carolina.
98 Clear-Water Abutment and Contraction Scour in the Coastal Plain and Piedmont Provinces of South Carolina, 1996-99

0

5

10

15

20

25

0 1,000 2,000 3,000 4,000 5,000 6,000 7,000 8,000

EMBANKMENT LENGTH, IN FEET

OBSERVED ABUTMENT-SCOUR DEPTH, IN FEET

Envelope curve

Observed abutment scour without pier

located at low point of scour hole

Observed abutment scour with pier

located at low point of scour hole 

Envelope curve

A.

B.

0

5

10

15

20

0 500 1,000 1,500 2,000

EMBANKMENT LENGTH, IN FEET

OBSERVED ABUTMENT-SCOUR DEPTH, IN FEET

Envelope curve

Observed abutment scour without pier

located at low point of scour hole

Observed abutment scour with pier

located at low point of scour hole 

Envelope curve

Figure 86. Relation of observed clear-water abutment-scour depth and the 100-yearflow embankment length identifying sites with a pier or pile bent located at the low point 

of the scour hole for the (A) Coastal Plain and (B) Piedmont of South Carolina.
Guidance for Assessing Abutment-Scour Hole Location, Width, and Shape 99

in the Piedmont. These scour holes were typically dry 

and shallow, making it possible to distinguish the 

region of pier scour from the region of abutment scour. 

When collecting data at these sites, scour around the 

pier generally was not included in the measurement. 

Therefore, when the range of abutment-scour depth in 

the Piedmont is estimated to be 5 ft or less with the 

envelope curve, judgment should be used to account 

for the effect of pier scour within the abutment region.

Limitations of the Abutment-Scour 

Depth Envelope Curves

When using the abutment-scour depth envelope 

curves for assessing clear-water abutment scour in 

South Carolina, the potential error and limitations of 

these curves should be kept in mind. The abutmentscour depth envelope curves were developed using 

modeled hydraulic data that only estimate the true 

hydraulic conditions that may have created the 

observed scour. It is probable that errors exist within 

the hydraulic estimates introducing error within the 

envelope curves. This error is partially mitigated by the 

large number of observations, but the actual error and 

the degree of mitigation is unknown. Although the 209 

observations of abutment scour in this study are 

considered a statistically large data sample, this sample 

may not encompass the upper range of potential scour 

depths in South Carolina. Therefore, abutment-scour 

depths in South Carolina could possibly exceed the 

envelope curves developed in this study. As a result, it 

may be prudent to include a safety factor when using 

the envelope curves to assess scour.

When assessing abutment scour at a bridge in 

South Carolina, it is important that site characteristics 

at that bridge are similar to the characteristics of the 

sites used to develop the envelope curves. Site conditions at a bridge of interest should be compared with 

the range of parameters included in this study (tables 

10 and 13, and figs. 2, 3, and 68) to help assess the 

appropriateness of using the envelope curves. Also, the 

bridge-scour database, included with this report, can be 

used to review studied sites to gain insights about 

potential scour at unstudied sites. Limits for embankment length and the geometric-contraction ratio have 

been defined for each envelope and care should be 

given to remain within those limits. When site characteristics are significantly different from the sites used 

in this study, other methods for assessing abutmentscour depth should be considered. 

The abutment-scour depth envelope curves were 

developed using hydraulic data estimated with the 

hypothetical 100-year flow. Although it is unlikely that 

all study sites experienced this flow magnitude, there is 

evidence to suggest that many sites experienced flows 

equaling or exceeding 70 percent of this flow magnitude. Therefore, estimates of abutment-scour depth 

obtained from the envelope curves should represent 

scour that may occur at such flow magnitudes. The 

envelope curves should not be used to assess scour for 

extreme events, such as the 500-year flow. 

An additional scour problem that frequently 

occurs in South Carolina is the washout of road 

embankments. Typically, washout occurs at smaller 

bridges that create a large contraction of flow. The 

magnitude of this problem is seen from bridge failure 

statistics from the October 1990 flood. This event 

caused 80 bridges to fail in South Carolina with 79 of 

these failures attributed to embankment washout 

(Hurley, 1996). Data collected in this study did not 

include failure from embankment washout and, therefore, envelope curves in this report cannot be used to 

assess this type of abutment scour.

Although the envelope curves presented in 

this report can serve as a valuable tool in assessing abutment-scour depths in South Carolina, the potential errors 

and limitations restrict their use. Therefore, the envelope 

curves should not be relied upon as the only tool for 

assessing abutment scour. To best assess anticipated 

scour, one should compile and study the available information for a given site and then bring sound engineering 

principals to bear on the final estimate of anticipated 

abutment-scour depth.

GUIDANCE FOR ASSESSING ABUTMENTSCOUR HOLE LOCATION, WIDTH, AND 

SHAPE

In addition to assessing abutment-scour depth at 

a given site, it is important to consider the scour-hole 

geometry and location within the bridge cross section. 

Estimating scour-hole characteristics is critical for 

assessing the impact of scour on the various bridge 

supports (abutments, piers, and pile bents). This section 

discusses and gives guidance for assessing the location, 

width, and shape of an abutment-scour hole.
100 Clear-Water Abutment and Contraction Scour in the Coastal Plain and Piedmont Provinces of South Carolina, 1996-99

Lateral Reference for Scour Hole

In general, the edge of an abutment-scour hole 

was observed to be most commonly located near the 

toe of the abutment (fig. 87). Figure 88 shows the relation of observed abutment-scour depth and distance 

from the top edge of the scour-hole bank to the toe of 

the abutment. In the case of left abutments, the distance 

is measured from the left abutment toe to the top left 

bank of the scour hole created by the left abutment. 

In the case of right abutments, the distance is measured 

from the right abutment toe to the top right bank of the 

scour hole created by the right abutment. 

The plot in figure 88 includes only 112 of the 

209 observations of abutment scour. A total of 97 

observations were excluded because: (1) there was no 

observed scour (33 sites); (2) the scour hole occurred at 

a short bridge where a large, single scour hole developed rather than two separate scour holes (43 sites); (3) 

the low point of the scour hole was located at some 

distance upstream or downstream from the bridge, 

making it difficult to reference to the abutment toe (13 

sites); and (4) the data were incomplete (8 sites). 

The scour-hole banks were contiguous with abutment toes for 68 percent of the observations. The banks 

were 1 to 15 ft from the toe for 22 percent of the observations. The remaining observations that were located 

more than 15 ft from the abutment toe can be associated with shallow scour depths (4.3 ft or less) that typically would not threaten bridge foundations. Although 

field data indicate variation in the lateral location of the 

abutment-scour hole, the abutment toe is the dominant 

location and appears to be a reasonable reference for 

locating the edge of the scour hole. Error in the prediction of the lateral location of deep scour holes could 

result in underestimates of scour at certain piers. This 

error can be minimized by using a conservative estimate of scour-hole top width as discussed in the 

following section. 

Scour-Hole Top Width

Using the abutment toe as a reference location 

for the bank of the abutment-scour hole, the lateral 

extent of the area affected by scour can be assessed by 

estimating the scour-hole top width. Figure 89 shows 

the relation of observed scour-hole top widths and 

observed scour depths for two site conditions. Figure 

89A represents the envelope for longer bridges in the 

Piedmont and Coastal Plain where two individual 

abutment-scour holes typically develop. Short bridges 

(approximately 240 ft or less) located on swamps or 

serving as floodplain relief structures, tend to develop 

single-scour holes and are represented in figure 89B. 

The scatter in the depth-to-width ratio of the 

observed scour is large, making estimates of scour-hole 

top width highly variable. Using the envelope curves in 

figure 89, a conservative estimate of top width can be 

obtained. These estimates could be unreasonable under 

some site conditions. At Piedmont sites where the overbank width (at setback abutments) is less than the estimated abutment-scour-hole top width, it may be 

reasonable to limit the estimated scour-hole top width 

to the width of the overbank. At relatively short 

bridges, where the left and right scour-hole top widths 

overlap or the estimated top width exceeds the toe-totoe width between the left and right abutments, it may 

be reasonable to select the deepest of the left and right 

abutment-scour-hole depths as an estimate of the abutment-scour depth at the bridge and then limit the estimated scour-hole top width to the toe-to-toe width. 

Scour-Hole Longitudinal Location

The upstream-to-downstream longitudinal location for the low point of the scour hole is highly variable. Figure 90 shows the relation of the 100-year-flow 

top width at the bridge and the longitudinal location of 

the scour-hole low point for the Piedmont and Coastal 

Plain. For the shallow scour depths (4.5 ft or less) in the 

clayey soils of the Piedmont, the low point of the scour 

hole is typically along the centerline of the roadway 

directly under the bridge (fig. 90A). However, the 

deeper scour holes of the Piedmont can have longitudinal locations that extend beyond the limits of the 

bridge. For the Coastal Plain sites, the longitudinal 

location of the scour hole can vary significantly for 

100-year-flow bridge top widths of 300 ft or less, 

(fig. 90B). However, beyond 300 ft, the longitudinal 

location falls close to the roadway centerline directly 

under the bridge. Although data for the deeper scour 

holes in the Piedmont are not as abundant as data in the 

Coastal Plain, a similar trend can be observed. 

Figure 90 shows that, for bridges with 100-yearflow top widths approximately 300 ft or less, the low 

point of the scour hole might be located outside of the 

bridge limits. In some cases, the low point of the scour 

hole was largely removed from the bridge so as to not 

significantly threaten bridge foundations (figs. 91 and 

92). Interaction of turbulent flow coming from the left 

and right abutments likely causes instabilities in the 
Guidance for Assessing Abutment-Scour Hole Location, Width, and Shape 101

0

5

10

15

20

0 5 10 15 20 25 30 35 40

DISTANCE FROM TOP EDGE OF SCOUR TO ABUTMENT TOE, IN FEET

OBSERVED ABUTMENT-SCOUR DEPTH, IN FEET

Figure 88. Lateral location of scour-hole bank in reference to the abutment toe for observed abutment 

scour in the Piedmont and Coastal Plain of South Carolina.

Figure 87. Plan view of reference for estimating the width and lateral location of an abutment-scour hole.
102 Clear-Water Abutment and Contraction Scour in the Coastal Plain and Piedmont Provinces of South Carolina, 1996-99

0

10

20

30

40

50

60

70

80

0 5 10 15 20

OBSERVED ABUTMENT-SCOUR DEPTH, IN FEET

OBSERVED ABUTMENT-SCOUR-HOLE TOP WIDTH, IN FEET

Envelope curve

Envelope curve based on points identified by circles

Observed abutment scour from field investigation

Points arbitrarily chosen to develop envelope curve

0

20

40

60

80

100

120

140

0 5 10 15 20 25

OBSERVED ABUTMENT-SCOUR DEPTH, IN FEET

OBSERVED ABUTMENT-SCOUR-HOLE TOP WIDTH, IN FEET

Envelope curve Envelope curve based on points identified by circles

Observed abutment scour from field investigation

Points arbitrarily chosen to develop envelope curve

A.

B.

Figure 89. Relation of abutment-scour-hole top width and abutment-scour depth at bridges 

(A) greater than 240 feet in length, and (B) swampy and floodplain relief bridges, 240 feet or less 

in length in the Piedmont and Coastal Plain of South Carolina.
Guidance for Assessing Abutment-Scour Hole Location, Width, and Shape 103

-120

-100

-80

-60

-40

-20

0

20

40

60

0 200 400 600 800

100-YEAR FLOW TOP WIDTH AT BRIDGE, IN FEET

LONGITUDINAL LOCATION OF 

 SCOUR-HOLE LOW POINT, IN FEET

Scour depths greater than 4.5 feet

Scour depths less than or equal to 4.5 feet

A.

B.

-120

-100

-80

-60

-40

-20

0

20

40

60

80

100

120

0 200 400 600 800 1,000 1,200

100-YEAR FLOW TOP WIDTH AT BRIDGE, IN FEET

LONGITUDINAL LOCATION OF 

 SCOUR-HOLE LOW POINT, IN FEET

FLOW

Positive

Negative

0

All observations of Coastal Plain abutment

scour with depths ranging from 0 to 23.6 feet

Bridge Center Line of Road FLOW

Positive

Negative

0 Bridge Center Line of Road

Figure 90. Relation of longitudinal location for the low point of the abutment-scour hole and the 

100-year-flow top width at the bridge for (A) shallow and deep scour holes in the Piedmont of 

South Carolina, and (B) sites in the Coastal Plain of South Carolina.
104 Clear-Water Abutment and Contraction Scour in the Coastal Plain and Piedmont Provinces of South Carolina, 1996-99

92

90

82

92

84

86

88

90

92

88

90

86

86

88

88

92 90

90

0 10 20 30 40 50 FEET

0 5 10 15 20 METERS

FLOW

90

EXPLANATION

92 TOPOGRAPHIC CONTOUR--Interval 1.0 foot.

Arbitrary datum

SURVEYED DATA POINT

BRIDGE DECK

BENT LOCATION

AVERAGE UNSCOURED REFERENCESURFACE ELEVATION

91.9 FEET

Elevation at bridge 

deck corner = 101.5 feet

flow patterns for shorter bridges, creating the scatter 

within the longitudinal pattern. As the bridge lengthens, the interaction of turbulent flow from left and right 

abutments is diminished and more stable flow patterns 

are established around each separate abutment. The 

stable flow patterns promote the creation of the classical abutment-scour hole pattern where the low point 

falls longitudinally near the bridge. Additional research 

is needed to confirm this speculation. Because the 

accurate prediction of longitudinal location of the scour 

hole is presently not possible, it must be assumed, for 

scour assessment purposes, that the scour-hole low 

point will be located directly at the bridge.

Scour-Hole Shape

Although cross-sectional shape at the low point 

of an abutment-scour hole varies, the general pattern is 

similar to a parabola. Data from the current study may 

be adequate to define a dimensionless shape for abutment-scour holes; however, this is outside the scope of 

this study. For assessing scour-hole shape at bridge 

foundations, it may be reasonable to use either a rectangle or trapezoid (fig. 93). The rectangle shape will 

certainly be conservative, but may overestimate scour 

at bridge foundations near the scour edges. Perhaps a 

more reasonable estimate of shape is the trapezoid, 

which somewhat mimics the side slopes of a parabola. 

Figure 91. Example of scour-hole low point located upstream at Structure 367029900100 on Road S-299, crossing 

Cannons Creek in Newberry County, South Carolina, November 24, 1997.
Clear-Water Contraction Scour in the Piedmont 105

90

90

90

88

76

88

78

80

82

84

88

86

86

90

90

88

0 10 20 30 40 50 FEET

0 5 10 15 20 METERS

FLOW

Elevation at bridge

deck corner = 100.3 feet

EXPLANATION

90 TOPOGRAPHIC CONTOUR--Interval 1.0 foot.

Arbitrary datum

SURVEYED DATA POINT

BRIDGE DECK

BENT LOCATION

AVERAGE UNSCOURED REFERENCESURFACE ELEVATION

89.4 FEET

Use of a trapezoid requires judgment to select a side 

slope that will reflect anticipated slopes in the soil of 

interest. It should be kept in mind that steeper side 

slopes will widen the scour-hole bottom width providing a more conservative estimate of the impact of the 

scour hole.

CLEAR-WATER CONTRACTION 

SCOUR IN THE PIEDMONT

Densely vegetated floodplains of Piedmont 

streams, in combination with lower floodplain flow 

velocities, promote a region of clear-water scour on 

bridge overbanks (fig. 94). In this study, the width of 

the bridge overbank is defined as the distance from the 

abutment toe to the top of the channel bank. In the case 

of a floodplain relief bridge with no defined channel, 

the region of clear-water scour will be the width 

between the abutment toes (fig. 29). Under ideal conditions, flood flows contracted through a bridge opening 

will have flow patterns similar to those shown in figure 

34. In the region near the abutment, contracted stream 

tubes produce increased velocities and the severe flow 

curvature produces vortices. Both conditions serve to 

promote abutment scour. Moving laterally away from 

the abutment, velocities and flow curvature diminish, 

reducing the potential for scour. If a bridge overbank is 

sufficiently wide, it will experience both the severely 

curved as well as the relatively straight flow patterns, 

and clear-water abutment scour and contraction scour 

may develop in these regions, respectively (fig. 95).

Figure 92. Example of scour-hole low point located downstream at Structure 344004100800 on S.C. Route 41, 

crossing Maiden Down Swamp in Marion County, South Carolina, December 3, 1996.
106 Clear-Water Abutment and Contraction Scour in the Coastal Plain and Piedmont Provinces of South Carolina, 1996-99

55

60

65

70

75

80

85

90

95

-10 0 10 20 30 40 50 60 70 80

DISTANCE FROM LEFT END OF SCOUR HOLE, IN FEET

ELEVATION ABOVE ARBITRARY DATUM, IN FEET

Ground line for existing scour hole

Estimate of scour hole shape using trapezoid

Estimate of scour hole shape using rectangle

Figure 94. Profile of overbank area for bridges with setback abutments.

Figure 93. Ground line of existing scour hole compared with the shape of a trapezoid 

and rectangle.
Clear-Water Contraction Scour in the Piedmont 107

In general, field observations of clear-water contraction 

scour in the Piedmont were collected in the clear-water 

contraction-scour region, as defined in figure 95, 

outside of the region of abutment scour.

In this study, field data were collected during 

low-flow conditions and the boundary between 

severely curved and straight flow patterns at a given 

site was not defined. This boundary can often be estimated from scour-hole patterns observed in the field. 

Deeper abutment-scour holes typically occurred near 

the abutment toe with a substantially shallower scour 

occurring beyond this region. An example of this 

pattern can be seen at Structure 367008100200 on 

Road S-81 crossing the Enoree River in Newberry 

County (fig. 96). The large abutment-scour hole 

(fig. 97) is 13.7 ft deep and is located in the abutmentscour region where severe flow curvature would be 

expected. Beyond the abutment-scour hole, the maximum clear-water contraction-scour depth is approximately 4 ft (fig. 98), and is in the overbank region 

where straighter flow patterns would likely occur. Piedmont sites with smaller abutment-scour depths, often 

display a similar pattern of distinct abutment- and 

clear-water contraction-scour regions. The idealized 

flow patterns associated with a contraction provide 

some explanation of the differing scour potentials 

between the abutment- and clear-water contractionscour regions. Sediment-transport theory associated 

with rectilinear flow may provide insight for scour 

processes in this overbank region where straighter flow 

patterns dominate.

A total of 76 observations of clear-water contraction scour were collected at 53 bridge sites in the Piedmont Province, including 16 bridges in the high-flow 

region (fig. 1) (Guimaraes and Bohman, 1992). Scour 

depths ranged from 0 to 4.5 ft with a median scour 

depth of 0.8 ft. Overbank widths ranged from 7 to 684 ft 

with a median width of 61 ft. The dense vegetation and 

low velocities associated with Piedmont floodplains 

typically create clear-water scour conditions in the 

bridge overbank; all observed scour was considered 

clear-water in nature. The soils of the Piedmont tend to 

be scour-resistant clayey soils, but sandier soils also are 

present at some sites. Observations of clear-water contraction scour in the Piedmont included soils with varying 

degrees of cohesion. The clear-water contraction-scour 

holes on the bridge overbank region were typically limited to beneath the bridge and commonly formed a 

Figure 95. Plan view of areas of clear-water abutment and contraction scour.
108 Clear-Water Abutment and Contraction Scour in the Coastal Plain and Piedmont Provinces of South Carolina, 1996-99

0 10 20 30 40 50 FEET

0 5 10 15 20 METERS

EXPLANATION

88 TOPOGRAPHIC CONTOUR--Interval 1.0 foot.

Arbitrary datum

SURVEYED DATA POINT

BRIDGE DECK

BENT LOCATION

AVERAGE UNSCOURED REFERENCESURFACE ELEVATION

86.1 FEET

0 20 40 FEET

0 10 20 METERS

86

84

86

82

74

7

8

8

6

76 78

88

80

80

82

86

84 82

84

82

84

84

86

FLOW Approximate location

of channel banks

Elevation at bridge 

deck corner = 100.0 feet

Abutment-scour

region

Clear-water-contraction

scour region

Figure 96. Example of abutment- and clear-water contraction-scour areas at Structure 367008100200 on 

Road S-81, crossing the Enoree River in Newberry County, South Carolina (April 7, 1997).
Clear-Water Contraction Scour in the Piedmont 109

Figure 98. Clear-water contraction scour on the right overbank as viewed from the left at Structure 

367008100200 on Road S-81, crossing the Enoree River in Newberry County, South Carolina. 

(Photograph by the South Carolina District, U.S. Geological Survey, April 7,1997.)

Figure 97. Right abutment-scour hole as viewed from downstream at Structure 367008100200 on 

Road S-81, crossing the Enoree River in Newberry County, South Carolina. The abutment-scour 

hole is inundated with ponded water. (Photograph by the South Carolina District, U.S. Geological 

Survey, April 7,1997.)
110 Clear-Water Abutment and Contraction Scour in the Coastal Plain and Piedmont Provinces of South Carolina, 1996-99

shallow, parabolic depression running perpendicular to 

the flow (figs. 98, 99, and 100). Scour-hole patterns 

were commonly sufficient to distinguish clear-water 

contraction scour from abutment scour. When the 

scour-hole patterns were insufficient to distinguish 

these regions, clear-water contraction-scour data were 

collected away from the abutment toe. For small overbanks, the regions were often indistinguishable and 

judgment was required in classifying the scour as abutment or contraction. Table 14 lists the range of selected 

parameters associated with the 76 clear-water contraction-scour observations. Values for hydraulic data were 

estimated with the WSPRO model (Shearman, 1990) 

using the 100-year flow. 

Comparison of Observed and 

Theoretical Contraction-Scour 

Depths

To estimate theoretical contraction-scour depth 

for clear-water scour conditions, HEC-18 (Richardson 

and Davis, 1995) recommends the use of Laursen’s 

(1963) equation, which was derived for noncohesive 

sediments. Because Piedmont soils typically have some 

degree of cohesion, the equation often yields excessive 

scour depths for this region. Figure 101A compares the 

theoretical clear-water contraction-scour depth 

computed for the 100-year flow with the observed 

clear-water contraction-scour depth, and shows the 

excessive scour depths given by Laursen’s equation for 

Piedmont soils. (To provide a meaningful scale in 

figure 101A, three theoretical scour depths of 30.3, 

82.6, and 88.0 ft were excluded.) It should be kept in 

mind that the flows creating the observed contractionscour depths are unknown for most sites. The discrepancy between the observed and theoretical scour depths 

may, in part, be attributed to historic flows at these sites 

being less than the 100-year flow magnitude. However, 

maximum historic flows have been documented at 12 

sites along the Reedy, South Tyger and Enoree Rivers 

(table 6). These flows occurred during the August 1995 

flood and eight sites experienced flows exceeding the 

100-year flow magnitude. A comparison of the theoretical 

contraction-scour depths, computed using the August 

1995 flow magnitudes, with the observed scour still 

indicates excessive scour depths given by Laursen’s 

equation (fig. 101B). The results of figure 101A are 

based on the hypothetical 100-year flow, whereas 

figure 101B represents observed and theoretical scour 

for known maximum historic flows. Therefore, the 

results of figure 101B likely provide a better indicator 

of how well the Laursen equation predicts theoretical 

scour in the Piedmont. Both figures, however, show 

that large discrepancies between observed and theoretical scour depths exist, suggesting that the Laursen 

equation is a poor predictor for clear-water contraction 

scour at Piedmont sites and that another method is 

needed to estimate clear-water contraction scour in this 

region.

Geometric-Contraction Ratio as an 

Indicator of Potential Scour

In general, as the geometric-contraction ratio 

increases, the flow velocity through a bridge opening 

will rise, increasing the potential for scour. Similar to 

abutment-scour in the Piedmont, it would seem reasonable to expect an increase in clear-water contractionscour depth with increasing geometric-contraction 

ratios. This general trend is apparent in the relation of 

clear-water contraction-scour depth and the 100-year 

flow geometric-contraction ratio for observed data in 

the Piedmont (fig. 102). Although the range of 

observed scour depth is small, the envelope for this 

data indicates that large clear-water contraction-scour 

depths are associated with large geometric-contraction 

ratios. An equation for this envelope is:

ys = -6.29m2 + 10.23m + 0.59, (15)

where

Because observations of clear-water contraction 

scour are sparse beyond a geometric-contraction ratio 

of 0.85, the envelope curve should be limited to sites 

with geometric-contraction ratios less than or equal to 

this value. Figure 103 shows the same information as 

presented in figure 102, but highlights the observed 

data from the 1995 flood. Several of the data points fall 

along or near the envelope, indicating that the curve 

provides some representation of the upper limit for 

clear-water contraction-scour depths that might occur 

at sites experiencing large flood flows.

ys is the upper limit of the range for anticipated 

clear-water contraction-scour depth, in feet; and

m is the 100-year-flow geometric-contraction ratio;

where

m = 1 – b/B with the variables previously defined.
Clear-Water Contraction Scour in the Piedmont 111

Figure 99. Example of shallow clear-water contraction scour at Structure 464032200300 on S.C. 

Route 322, crossing Fishing Creek in York County, South Carolina. (Photograph by the South 

Carolina District, U.S. Geological Survey, January 28,1997.)

Figure 100. Example of shallow clear-water contraction scour at Structure 114000500200 on S.C. 

Route 5, crossing Buffalo Creek in Cherokee County, South Carolina. (Photograph by the South 

Carolina District, U.S. Geological Survey, October 7,1996.)
112 Clear-Water Abutment and Contraction Scour in the Coastal Plain and Piedmont Provinces of South Carolina, 1996-99

The envelope curve in figure 102 encompasses all 

of the field data, as well as all sites influenced by the 

1995 flood, so it is reasonable to assume that the envelope represents an upper range of potential scour for a 

given geometric-contraction ratio. With this assumption, 

the envelope can be used as a tool for assessing potential 

clear-water contraction-scour depths in the overbanks of 

Piedmont streams. Because the envelope represents an 

upper limit of observed scour depth, it will likely overpredict scour depth for various site conditions; however, 

the envelope will not exceed the upper bound of 

observed field data. In contrast, theoretical scour 

computed with the Laursen (1963) equation frequently 

exceeds the upper bound of observed data (fig. 104), 

indicating the geometric-contraction-ratio envelope is a 

better method for assessing potential clear-water 

contraction-scour depths in the Piedmont. (To provide a 

meaningful scale in figure 104, three theoretical scour 

depths of 30.3, 82.6, and 88.0 ft were excluded.) 

The envelope curve shown on figure 102 and equation 15 can be used for sites in the Piedmont to estimate 

a range of anticipated clear-water contraction-scour 

depths for geometric-contraction ratios of 0.85 or less. 

From a practical view, it may be appropriate to use an 

envelope value of 4.5 ft for all geometric-contraction 

ratios less than or equal to 0.85. Only sites having 

similar characteristics to sites used in this study should 

be assessed using this envelope. Because the envelope 

was developed from a limited sample of bridges in the 

Piedmont, scour depths could exceed the envelope. 

Therefore, it may be prudent to apply some type of 

safety factor to the envelope. When using this envelope, 

it is critical to properly estimate the geometric-contraction ratio at a given site. To ensure that the ratio is properly assessed, various sources of data should be 

reviewed, including, but not limited to, topographic 

maps, hydraulic models, road plans, and field observations. The envelope was developed using field data 

encompassing sites experiencing flows approaching the 

100-year flow and, therefore, should not be used to 

assess clear-water contraction-scour depths for extreme 

events such as the 500-year flow.

Selecting a Reference Surface for 

Clear-Water Contraction Scour

In this study, the average, undisturbed floodplain 

elevation in the clear-water contraction-scour region was 

used to determine contraction-scour depth. This reference 

surface should be used when assessing clear-water 

contraction scour with the contraction-scour-depth envelope. This reference surface can be determined by 

reviewing floodplain elevations from SCDOT road and 

bridge plans, surveyed cross ections, and (or) site visit 

observations. In many cases, the floodplain in the region 

of clear-water contraction scour is relatively flat, and estimating an average floodplain elevation is not a difficult 

task. However, there can be cases where the floodplain 

slopes substantially in the lateral and (or) longitudinal 

direction making the determination of a reference surface 

more difficult. In such cases, judgment should be applied, 

bearing in mind that lower reference-surface elevations 

will produce lower scour-hole elevations.

Table 14. Range of selected parameters for 76 observations of clear-water 

contraction scour collected at 53 bridges in the Piedmont of South Carolina

[mi2, square mile; ft/ft, feet per foot; ft/s, feet per second; ft, feet; mm, millimeter; <, less than] 

Parameter Minimum 

value

Median 

value

Maximum 

value

Drainage area (mi2) 11 76 a 1,620

Channel slope determined from topographic

map (ft/ft)

0.00015 0.0012 0.0029

100-year flow average overbank velocity at

the bridge (ft/s) b

1.6 3.2 6.2

100-year flow average overbank depth at the

bridge (ft) b

1.3 7.4 20.5

Median grain size (mm) <0.062 0.073 0.989

Observed clear-water contraction-scour depth (ft) 0.0 0.8 4.5

a

 Approximately 97 percent of the study sites in the Piedmont have drainage areas less than 

400 mi2 (fig. 3). b Parameter was estimated from the one-dimensional water-surface profile model, WSPRO 

(Shearman, 1990).
Clear-Water Contraction Scour in the Piedmont 113

0

5

10

15

20

25

012345

OBSERVED CONTRACTION-SCOUR DEPTH, IN FEET

THEORETICAL CONTRACTION-SCOUR

DEPTH, IN FEET

Line of agreement

A.

B.

0

10

20

30

40

50

60

0 12345

OBSERVED CONTRACTION-SCOUR DEPTH, IN FEET

THEORETICAL CONTRACTION-SCOUR DEPTH, IN FEET

Line of agreement

Figure 101. Relation of observed and theoretical clear-water contraction-scour depth for the 

(A) 100-year flow, and the (B) August 1995 flood at selected sites in the Piedmont of South Carolina. 

(Theoretical contraction scour calculated with the Laursen (1963) equation.)
114 Clear-Water Abutment and Contraction Scour in the Coastal Plain and Piedmont Provinces of South Carolina, 1996-99

0

1

2

3

4

5

0.0 0.2 0.4 0.6 0.8 1.0

GEOMETRIC-CONTRACTION RATIO

OBSERVED CONTRACTION-SCOUR DEPTH, IN FEET

Envelope curve based on a trend line

Observed clear-water contraction scour

 ys Estimate of contraction-scour depth

 along envelope curve, in feet

 m 100-year-flow geometric-contraction ratio 

 (not to exceed 0.85)

ys = -6.29m2 + 10.23m + 0.59

 where m < 0.85

0

1

2

3

4

5

0.0 0.2 0.4 0.6 0.8 1.0

GEOMETRIC-CONTRACTION RATIO

OBSERVED CONTRACTION-SCOUR DEPTH, IN FEET

Envelope of observed data

Observed overbank contraction 

scour for all Piedmont sites

Observed overbank contraction 

scour from 1995 flood

Figure 103. Relation of observed clear-water contraction-scour depth and the 100-year-flow 

geometric-contraction ratio identifying sites for the 1995 flood in the Piedmont of South Carolina.

Figure 102. Relation of observed clear-water contraction-scour depth and the 100-year-flow 

geometric-contraction ratio in the Piedmont of South Carolina.
Clear-Water Contraction Scour in the Piedmont 115

0

5

10

15

20

25

30

0.0 0.2 0.4 0.6 0.8 1.0

GEOMETRIC-CONTRACTION RATIO

THEORETICAL CONTRACTION-SCOUR DEPTH, IN FEET

Envelope for observed clear-water contraction scour

Theoretical clear-water contraction scour (Laursen,1963)

Pier Scour within Clear-Water 

Contraction-Scour Areas

Because of the shallow nature of clear-water 

contraction-scour holes, it was generally possible to 

distinguish the region of pier scour from the region of 

clear-water contraction-scour. When collecting data at 

these sites, scour around piers generally was not 

included in the measurement of clear-water contraction 

scour. Therefore, the envelope in figure 102 represents 

contraction scour only and not the total scour. This 

envelope can be used to assess anticipated ranges of 

clear-water contraction scour in Piedmont overbanks, 

but judgment should be used to account for any additional scour created by piers or bents.

A potential threat to overbank piers that is not 

addressed in this study is channel widening. Channel 

widening can undermine overbank piers that are 

located near a channel bank. In 1995, the Enoree River 

in the Piedmont of South Carolina experienced a large 

flood that significantly exceeded the 100-year flow 

magnitude at various locations along the river (table 5). 

Post-flood field visits to bridges along the Enoree 

River, indicated that various reaches of the main 

channel had experienced channel widening from bank 

erosion. At some locations widening was sufficient to 

remove material from around piles set near the channel 

bank (fig. 105). Table 15 compares pre- and post-flood 

channel top widths at three sites along the Enoree River 

and demonstrates the effect that large floods can have 

on channel widening and the potential threat to piers 

near channel banks. One should be aware of this potential problem and use judgment when assessing scour at 

overbank piers or bents near channel banks.

Estimate of Clear-Water Contraction-Scour 

Hole Location

In general, the shape of clear-water contractionscour holes in the overbank region, observed in the 

Piedmont, consisted of shallow parabolic depressions 

running perpendicular to flow (fig. 100) and covering 

most of the overbank region unaffected by abutment 

scour (fig. 95). Figure 106 shows the relation of clearwater contraction-scour depth and the longitudinal 

location of the low point of the scour hole in reference 

to the centerline of the roadway. In general, the low 

Figure 104. Relation of theoretical 100-year-flow clear-water contraction-scour depth and the 

100-year-flow geometric-contraction ratio compared with the envelope of observed data in the 

Piedmont of South Carolina. (Theoretical clear-water contraction scour calculated with the 

Laursen (1963) equation.)
116 Clear-Water Abutment and Contraction Scour in the Coastal Plain and Piedmont Provinces of South Carolina, 1996-99

point is in close proximity to the roadway centerline 

and all observations are located beneath the bridge 

deck. The left and right lateral extent of the clear-water 

contraction scour typically began at the edge of the 

abutment-scour hole and ran toward the bank (fig. 95). 

Scour depths over the lateral extent of the scour hole 

varied. To account for this, observed scour was 

measured at the deepest area over the lateral extent.

Data are insufficient to predict the exact location 

of clear-water contraction-scour holes. The data 

suggest, however, that it is reasonable to assume the 

scour will occur under the bridge and will extend 

laterally from the edge of the abutment-scour hole to 

the channel bank. The range of anticipated contractionscour depths can be assessed using the clear-water 

contraction-scour envelope (fig. 102). The upper limit 

of this range can be applied across the entire overbank 

region from the edge of the abutment-scour hole to the 

channel bank. Because the edge of the abutment-scour 

hole will be a limiting boundary for the clear-water 

contraction scour, abutment scour at the bridge should 

be assessed first. 

Figure 105. Erosion of bed material from around piles caused by bank widening from the August 

1995 flood at Structure 307026300100 on Road S-263, crossing the Enoree River in Laurens County, 

South Carolina. (Photograph by the South Carolina District, U.S. Geological Survey, April 1, 1997.)

Table 15. Pre- and post-flood channel top widths for the 1995 flood at selected sites along 

the Enoree River in the Piedmont of South Carolina

[mi2, square miles; ft3/s, cubic feet per second; ft, feet; S-, Secondary Road] 

County Road

Drainage 

Area

(mi2)

Peak flow 

for 

1995 Flood 

(ft 3/s)

Pre-flood 

channel 

top width a

(ft)

Post-flood 

channel 

top width 

(ft)

Reference 

number 

(fig. 1 

and 

app. B)

Spartanburg S-118 186 64,200 95 121 130

Laurens S-263 249 50,400 113 140 94

Laurens S-112 256 50,000 113 148 93

a

 Widths determined from the South Carolina Department of Transportation bridge plan surveys.
Clear-Water Contraction Scour in the Piedmont 117

-15

-10

-5

0

5

10

15

012345

OBSERVED CONTRACTION-SCOUR DEPTH, IN FEET

LONGITUDINAL LOCATION OF 

SCOUR-HOLE LOW POINT, IN FEET

FLOW

Positive

Negative

0 Bridge Center Line of Road

Permissible Velocities for Clear-Water 

Contraction Scour

The concept of permissible velocities can 

provide some insights as to why clear-water contraction scour in the Piedmont is relatively small. Using 

field data from irrigation canals and the experience of 

field engineers, Fortier and Scobey (1926) defined 

ranges of permissible velocities that a given soil could 

sustain without significant erosion. Fortier and Scobey 

(1926) state that the permissible velocity should not be 

confused with the critical velocity that defines the point 

of incipient motion for a loose particle of given grain 

size. In general, permissible velocities for given field 

conditions will be higher than critical velocities. 

According to the study, cohesive clayey soils have 

permissible velocities in the range of 3.75 to 5.0 ft/s. In 

addition, firm loamy materials can sustain flows in the 

range of 2.5 to 3.5 ft/s. Richardson and others (1990) 

cite information from a U.S. Army Corps of Engineers 

publication (Keown and others, 1977) that gives 

suggested ranges for permissible velocities for cohesive 

and noncohesive soils somewhat similar to Fortier and 

Scobey (1926). (This publication describes the velocities as nonscour velocities instead of permissible velocities.) The cohesive soils are defined as sandy loams, 

which are a mixture of sands, silts, clays, and organics 

of varying degrees. In general, this description encompasses soils typically found in the Piedmont. The 

permissible velocities suggested by Keown and others 

(1977) for this type of soil ranges from 3.1 to 4.9 ft/s, 

with the permissible velocity increasing as flow depth 

increases.

 Permissible velocities, as described by Fortier 

and Scobey (1926) and Keown and others (1977) are 

for rectilinear flows. As discussed previously, flow 

patterns in the overbank region are approximately rectilinear and the use of permissible velocities to assess the 

potential for clear-water contraction scour at bridge 

overbanks may be reasonable. Figure 107 shows the 

relation of the observed clear-water contraction-scour 

depth and the average overbank velocity for the 100-

year flow. (The actual historic velocities at these sites 

Figure 106. Relation of longitudinal location for the low point of the clear-water contraction-scour 

hole and observed contraction-scour depth in the Piedmont of South Carolina.
118 Clear-Water Abutment and Contraction Scour in the Coastal Plain and Piedmont Provinces of South Carolina, 1996-99

0

1

2

3

4

5

01234567

AVERAGE OVERBANK VELOCITY FOR THE 100-YEAR FLOW, IN FEET PER SECOND

OBSERVED CONTRACTION-SCOUR DEPTH, IN FEET

Observed contraction scour

are unknown, but the 100-year flow velocity provides 

insight on the range of velocities that could occur 

during large floods.) This relation indicates that most 

sites have velocities that are less than 3.5 ft/s, which is 

near the lower limit of permissible velocity for clayey 

soils and near the upper limit for firm loams, according 

to Fortier and Scobey (1926). According to Keown and 

others (1977), 3.5 ft/s is near the lower end of permissible velocities for cohesive sandy loams. Of the 25 

observations that have velocities exceeding 3.5 ft/s, 20 

are associated with clayey soils that in general are more 

scour-resistant and could possibly sustain higher velocities without significant scour. Figure 107 illustrates 

that many sites investigated in this study have average 

overbank velocities that are within the range of permissible velocities. Therefore, the potential for scour is 

low and significant amounts of scour should not be 

anticipated. This provides some explanation for the 

relatively small amounts of contraction scour observed 

on Piedmont overbanks. This also indicates that 

permissible velocity may be a useful tool to qualitatively assess the potential for contraction scour in the 

overbank region. The works of Fortier and Scobey 

(1926), Keown and others (1977), and Richardson and 

others (1990) should be consulted to further understand 

the application and limitations of permissible velocities. 

If many of the overbanks in the current study 

have high-flow velocities that are within the range of 

permissible velocities, one could question why scour is 

observed at many of these sites. A definitive answer is 

not available, but some speculation follows. Although 

the overbank region has flow patterns that are relatively 

straight in comparison to the severe curvature near the 

abutment, the flow patterns still have some curvature. 

This may introduce additional bed stresses and vortices 

that may increase the potential for scour giving some 

explanation as to why small amounts of scour are 

observed. Scour in the overbank region could also, in 

part, be attributed to lateral flows under the bridge. At 

several sites, there was evidence that flows had run 

along the overbank region toward the main channel 

rather than perpendicular to the bridge opening. One 

source for such flow is stormwater runoff from the 

roadway and bridge deck, as well as seepage of stormwater through bridge deck joints. An additional source 

Figure 107. Relation of observed clear-water contraction-scour depth and the average overbank 

velocity for the 100-year flow in the Piedmont of South Carolina.
Clear-Water Contraction Scour in the Piedmont 119

is from shallow floodplain flows that occur during 

smaller flood events. Under these conditions, shallow 

flows might seek a path of least resistance along the 

bare soils found in the overbank regions of many 

bridges. It is suspected that observed scour in the overbank region is a combination of various factors including contraction scour, which will occur at higher flow 

conditions, as well as lateral flow from stormwater 

runoff and shallow flood flows. 

THE SOUTH CAROLINA BRIDGE-SCOUR 

DATABASE

Selected data generated from this study has been 

compiled into a database called the South Carolina 

Bridge Scour Database (SCBSD), and can be viewed 

using Microsoft Access. The SCBSD includes photographs, figures, selected field data, variables used to 

compute theoretical scour, computed scour depths, 

limited basin characteristics, limited soil data, and 

selected hydraulic data estimated with the WSPRO 

model (Shearman, 1990). These raw data are compiled 

in various data tables within the database, and automated reports have been developed to allow extraction 

of selected data for a chosen bridge. The SCDOT 

assigns a unique 12-digit number for each bridge under 

its jurisdiction and these numbers are used in the database as the primary search variable for extracting 

selected data from the raw data tables for display in the 

automated reports. Appendix A contains a description 

of the SCBSD automated reports, raw data tables, and 

variable definitions.

The SCBSD was developed using Microsoft 

Access 2000. The electronic file for the database 

requires approximately 638 megabytes of computer storage and is located on the compact disc (CD) in the pocket 

at the back of the report. To install the database, the file 

“scour.mdb” should be copied from the CD to a directory 

of the user’s choice. After copying this file to the computer, the properties of the file should be changed from 

“Read-only” by (1) right clicking on the file and selecting “Properties” on the popup menu, (2) unselecting the 

“Read-only” option in the Properties menu box, and 

then (3) clicking “OK” at the bottom of the menu box. 

To invoke the SCBSD, the file “scour.mdb” should be 

opened in Access. Upon opening this file, a menu box 

titled, “Bridge-Scour Data Reports,” will appear. This 

menu box lists the 10 automated reports described in 

appendix A and allows the user to generate the automated reports for a selected bridge. Near the top of this 

menu box is a drop-down menu designated by a menu 

button with a downward pointing arrowhead. Clicking 

on this button will produce a list of sites included in this 

study. From this list, the user should select a bridge of 

interest. Once a specific bridge has been selected, the 

user can view the various automated reports, for the 

bridge of interest, by selecting the “View Selected 

Record” menu button that is to the right of the report of 

interest. Selecting the ”View All Reports” menu button 

will generate, in numeric order, the corresponding report 

for all bridges. 

The SCBSD is a valuable tool for use in investigating clear-water abutment scour and contraction 

scour. For one who is assessing scour at bridges in 

South Carolina, the SCBSD provides a tool for making 

site comparisons. Sites under investigation, but not 

included in the current study, can be compared with 

sites in the SCBSD to gain insights about the range of 

anticipated scour depths. The SCBSD also provides a 

source of data to assess various theoretical methods for 

predicting clear-water abutment and contraction scour. 

Most equations for predicting scour are driven by 

hydraulic variables, such as flow depth and velocity. 

These variables can be extracted from the SCBSD and 

used in various theoretical equations to compute theoretical scour depths. The theoretical scour depths can 

then be compared with observed scour, as well as the 

field-data envelope curves, to assess the chosen equation’s performance. (Hydraulic data in the SCBSD may 

need to be manipulated to obtain specific variables 

required for a given theoretical equation.) 

One should keep in mind that the hydraulic data in 

the SCBSD were generated from a model and, therefore, 

do not necessarily represent the flow conditions that 

created the observed scour. As a result, some error is 

likely to be introduced into the comparison of theoretical 

scour with observed scour because of inaccuracies in the 

hydraulic data. However, the abundance of data points in 

the SCBSD will allow such comparisons to show the 

general trends of a theoretical equation and will provide 

some indication of the equation’s performance.

The SCBSD provides only limited information at 

each study site and, therefore, cannot be relied upon to 

provide a complete understanding of the sites. If more 

detailed information is required to understand conditions at a given site, then other data sources, such as 

topographic maps and bridge plans, should be consulted. Under certain circumstances, site visits may be 

required to gain a full appreciation of the observed 

scour and the conditions that created it.
120 Clear-Water Abutment and Contraction Scour in the Coastal Plain and Piedmont Provinces of South Carolina, 1996-99

SUMMARY

Observations of clear-water abutment scour and 

clear-water contraction scour at 146 bridges in the 

Piedmont and Coastal Plain of South Carolina were 

collected by the U.S. Geological Survey, in cooperation 

with the South Carolina Department of Transportation. 

The 209 observations of clear-water abutment-scour 

depth ranged from 0 to 23.6 feet, and the 76 observations of clear-water contraction-scour depth in the 

Piedmont ranged from 0 to 4.5 feet. The collected 

scour data represent the maximum clear-water abutment- and (or) clear-water contraction-scour depth that 

has occurred at a given bridge since construction. 

Although flow conditions creating the observed scour 

are not known for most sites, evidence suggests that 

approximately 80 percent of the bridges may have 

experienced at least one event equaling or exceeding 

the 25-year flow. Thirty-five of these sites have documented maximum historic flows exceeding the 20-year 

flow, with 24 sites having flows exceeding the 100-year 

flow magnitude. Because the collected data encompasses a number of sites that have experienced relatively large flows, the data should provide a reasonable 

range for anticipated scour depths at bridges with similar site characteristics.

To gain insights into hydraulic conditions that 

may have created the observed scour, hydraulic models 

were developed for each site using the one-dimensional 

step-backwater model, WSPRO. Because the magnitude of large floods was unknown at most sites, the 

100-year flow was modeled as a common flood. In 

addition, known maximum historic flows were 

modeled at 35 sites. Hydraulic data generated from the 

WSPRO model were used to compute theoretical-scour 

with methods presented in HEC-18. A comparison of 

theoretical and observed scour showed that theoretical 

abutment- and contraction-scour depths generally 

exceeded the observed scour depths and often times 

were excessive.

Modeled hydraulic data, theoretical scour data, 

and field data were compiled into a database and were 

used to investigate relations that may help explain scour 

in South Carolina. Abutment-scour field data were 

compared with dimensionless relations for laboratory 

data. This comparison showed that the range of dimensionless variables used in laboratory investigations was 

often outside the range of the dimensionless variables for 

field data in South Carolina. This implies that abutmentscour relations derived from laboratory data may not be 

applicable to field conditions in South Carolina. 

Variables determined to be influential in creating 

abutment scour within laboratory studies were investigated to understand their influence within the South 

Carolina field data. Many of these variables appeared 

to be insignificant under field conditions found in 

South Carolina. The strongest explanatory variables for 

abutment scour appeared to be embankment length, 

geometric-contraction ratio, approach velocity, and soil 

cohesion. 

The investigation identified several envelope 

curves within the field data that could be used for 

assessing reasonable ranges of abutment-scour depth 

in South Carolina. These envelope curves include 

embankment length and the geometric-contraction 

ratio as the primary explanatory variable. The envelope 

curves show that abutment-scour depth increases as 

these explanatory variables increase. The envelope 

curves are simple to apply and are an improvement 

over the current methods for predicting theoretical 

abutment scour in South Carolina.

Observed contraction-scour depths on the 

overbanks of the Piedmont were relatively small. 

The concept of permissible flow velocity was used to 

help understand and explain this phenomenon. Previous studies indicate that cohesive soils, similar to those 

of Piedmont overbanks, can sustain flow velocities in 

the range of 3 to 5 feet per second without experiencing 

significant erosion. Approximately 60 percent of the 

clear-water contraction-scour observations have 

modeled overbank-flow velocities less than 3.5 feet per 

second with only five observations (7 percent) having 

flow velocities exceeding 5 feet per second. Based on 

this observation, significant erosion on Piedmont overbanks should not be anticipated. This is consistent with 

the minimal scour depths observed in the field, and 

indicates that the concept of permissible velocity may 

be useful as a qualitative indicator for assessing the 

potential for contraction scour on Piedmont overbanks. 

The investigation also identified an envelope 

within the field data that could be used for assessing 

reasonable ranges of clear-water contraction-scour 

depths in the Piedmont of South Carolina. The envelope includes the geometric-contraction ratio as the 

primary explanatory variable. The envelope shows that 

clear-water contraction-scour depth increases as this 

ratio increases and eventually reaches a limit of about 

4.5 feet. The envelope is simple to apply and is an 

improvement over the current methods for predicting 

theoretical clear-water contraction scour on overbanks 

in the Piedmont of South Carolina. 
Selected References 121

Although the methods presented show considerable improvement over existing theoretical methods for 

predicting scour in South Carolina, the limitations of 

these empirical methods are important. The methods 

should not be used outside the range of data for which 

they were developed and special consideration is 

required for sites with bends directly upstream from the 

bridge or at sites with a high potential for channel 

widening. These methods also do not consider the 

washout of road embankments, which is a common 

problem in South Carolina.

Data for each bridge site have been compiled 

into a database that includes photographs, figures, 

observed scour depths, theoretical scour depths, limited 

basin characteristics, limited soil data, and theoretical 

hydraulic data. The database can be used to compare 

studied sites with unstudied sites in order to assess the 

potential for scour at the unstudied sites. In addition, 

the database provides a large source of field data that 

can be used to assess the performance of various theoretical methods for predicting clear-water abutment and 

contraction scour.

SELECTED REFERENCES

Bedient, P.B., and Huber, W.C., 1988, Hydrology and floodplain analysis: Reading, Mass., Addison-Wesley Publishing Co., 650 p.

Bohman, L.R., 1990, Determination of flood hydrographs 

for streams in South Carolina: Volume 1. Simulation 

of flood hydrographs for rural watersheds in South 

Carolina: U.S. Geological Survey Water-Resources 

Investigations Report 89-4087, 53 p.

——— 1992, Determination of flood hydrographs for 

streams in South Carolina: Volume 2. Estimation of 

peak-discharge frequency, runoff volumes, and flood 

hydrographs for urban watersheds: U.S. Geological 

Survey Water-Resources Investigations Report 92-4040, 

79 p.

Das, B.P., 1973, Bed scour at end-dump channel constrictions: 

Journal of the Hydraulics Division, American Society 

of Civil Engineers, v. 99, no. HY12, Paper no.10228, 

p. 2273-2291.

Dongol, D.M.S., 1993, Local scour at bridge abutments: 

University of Auckland, New Zealand, School of 

Engineering Report no. 544, 410 p.

Fortier, Samuel, and Scobey, F.C., 1926, Permissible canal 

velocities: Transactions of the American Society of 

Civil Engineers, v. 89, no. 1588, p. 940-984.

Froehlich, D.C., 1989, Local scour at bridge abutments: 

Hydraulic Engineering, Proceedings of the 1989 

National Conference on Hydraulic Engineering: New 

York, American Society of Civil Engineering, p. 13-18.

Gerhart, P.M., Gross, R.J., and Hochstein, J.I., 1992, Fundamentals of Fluid Mechanics (2d ed.): Reading, Mass., 

Addison-Wesley Publishing Co., 983 p.

Guimaraes, W.B, and Bohman, L.R., 1992, Techniques for 

estimating magnitude and frequency of floods in South 

Carolina, 1988: U.S. Geological Survey WaterResources Investigations Report 91-4157, 174 p.

Gunter, H.C., Mason, R.R., and Stamey, T.C., 1987, Magnitude 

and frequency of floods in rural and urban basins of 

North Carolina: U.S. Geological Survey WaterResources-Investigations Report 87-4096, 52 p.

Hurley Jr., N.M., 1996, Assessment of scour-critical data 

collected at selected bridges and culverts in South 

Carolina, 1990-92: U.S. Geological Survey Open-File 

Report 96-350, 119 p.

Kandasamy, J.K., 1985, Local scour at skewed abutments: 

University of Auckland, New Zealand, School of Engineering Report no. 375 as cited in Dongol, 1993.

Keown, M.P., Oswalt N.P., Perry, E.B., and Dordeau Jr., 

E.A., 1977, Literature survey and preliminary evaluation 

of streambank protection methods: U.S. Army Corps of 

Engineers, Waterways Experiment Station, Technical 

Report H-79-9. 

Kwan, T.F., 1984, Study of abutment scour: University of 

Auckland, New Zealand, School of Engineering Report 

no. 328 as cited in Dongol, 1993.

Lagasse, P.F., Schall, J.D., Johnson, F., Richardson, E.V., 

Richardson, J.R., and Chang, F., 1991, Stream stability 

at highway structures: Federal Highway Administration 

Hydraulic Engineering Circular No. 20, Publication 

FHWA-IP-90-014, 195 p.

Laursen, E.M., 1960, Scour at bridge crossings: Journal 

Hydraulic Division, American Society of Civil Engineering, v. 89, no. HY3.

——— 1963, An analysis of relief bridge scour: Journal 

Hydraulic Division, American Society of Civil Engineering, v. 92, no. HY3.

Melville, B.W., 1992, Local scour at bridge abutments: 

Journal of Hydraulic Engineering, American Society 

of Civil Engineering, v. 118, no. 4, p. 615-630.

Melville, B.W. and Coleman, S.E., 2000, Bridge Scour: 

Highlands, Colorado, Water Resources Publications, 

LLC, 550 p.

Melville, B.W., and Sutherland, A.J., 1988, Design method 

for local scour at bridge piers: Journal of Hydraulic 

Engineering, American Society of Civil Engineers, 

v. 114, no. 10, p. 1210-1226.

Raudkivi, A.J., and Sutherland, A.J., 1981, Scour at bridge 

crossings: Wellington, New Zealand, National Roads 

Board, Road Research Unit, Report No. 51.
122 Clear-Water Abutment and Contraction Scour in the Coastal Plain and Piedmont Provinces of South Carolina, 1996-99

Richardson, E.V., and Davis, S.R., 1995, Evaluating scour 

at bridges: Federal Highway Administration Hydraulic 

Engineering Circular No. 18, Publication FHWA-IP90-017, 204 p.

Richardson, E.V., Harrison, L.J, and Davis, S.R., 1991, 

Evaluating scour at bridges: Federal Highway Administration Hydraulic Engineering Circular No. 18, 

Publication FHWA-IP-90-017, 105 p.

Richardson, E.V., Harrison, L.J., Richardson, J.R., and 

Davis, S.R., 1993, Evaluating scour at bridges: Federal 

Highway Administration Hydraulic Engineering 

Circular No. 18, Publication FHWA-IP-90-017, 131 p.

Richardson, E.V., and Richardson, J. R., 1992, Discussion of 

Melville, B.W., 1992 Local Scour at Bridge Abutments, 

submitted to American Society of Civil Engineers, Journal of Hydraulics Division. 

Richardson, E.V., Simons, D.B., and Julien, P.Y., 1990, 

Highways in the river environment participant notebook: Federal Highway Administration, Publication 

FHWA-HI-90-016, 650 p.

Rouse, Hunter, 1946, Elementary mechanics of fluids: New 

York, Wiley and Sons, 376 p.

Shearman, J.O., 1990, User’s manual for WSPRO—A computer model for water-surface profile computations: 

Federal Highway Administration, Report no. FHWA-IP89-027, 175 p.

Simons, Li, and Associates, 1982, Engineering analysis of 

fluvial systems: p. 54.

South Carolina DOQQ Archive, 1994, Longs-SC-SW, digital 

orthophoto quarter quadrangle, web address: 

http://water3.dnr.state.sc.us/sidserver/bin/show_jpg.pl?

dir=W&client=sample&image=Longs-SCNCSE.sid&size=1&indexFg=2.

U.S. Geological Survey, 1969, Whitmire South, S.C., quadrangle: U.S. Geological Survey 7.5-minute series topographic map, scale 1:24,000. 

——— 1971, Clinton, S.C., quadrangle: U.S. Geological 

Survey 7.5-minute series topographic map, scale 

1:24,000.

——— 1983, Lockhart, S.C., quadrangle: U.S. Geological 

Survey 7.5-minute series topographic map, scale 

1:24,000.

——— 1990, Longs, S.C., quadrangle: U.S. Geological Survey 7.5-minute series topographic map, scale 1:24,000.

Vanoni, V.A., 1977, Sedimentation Engineering, American 

Society of Civil Engineers Task Committee for the Preparation of the Manual on Sedimentation of the Sedimentation Committee of the Hydraulics Division.

Yakoub, N.G.R., 1995, Effect of cohesion on bridge abutment scour: Colorado State University, Civil Engineering Department, dissertation, 231 p.

Yang, T.Y., 1996, Sediment transport theory and practice: 

New York, McGraw-Hill, 396 p.

Zalants, M.G., 1990, Low-flow characteristics of natural 

streams in the Blue Ridge, Piedmont, and upper Coastal 

Plain physiographic provinces of South Carolina: U.S. 

Geological Survey Water-Resources Investigations 

Report 90-4188, 92 p.

——— 1991, Low-flow frequency and flow duration of 

selected South Carolina streams through 1987: U.S. 

Geological Survey Water-Resources Investigations 

Report 91-4170, 87 p.
Appendix A 123

APPENDIX A

Explanation of Variables in the South Carolina Bridge-Scour Database
124 Clear-Water Abutment and Contraction Scour in the Coastal Plain and Piedmont Provinces of South Carolina, 1996-99

Data for this project have been compiled into a database, including photographs, figures, observed scour 

depths, theoretical scour depths, limited basin characteristics, limited soil data, and theoretical hydraulic data and 

can be viewed using Microsoft Access1. The South Carolina Bridge Scour Database (SCBSD) provides automated 

report formats that can be used to view data for a given site. The raw data also can be viewed in tabular format. 

Although most data for a given site can be viewed through the report formats, some data can only be viewed in the 

raw data tables. Blank data entries that appear in the reports or raw data tables indicate that data are not applicable 

or are missing. Following is a list and brief description of the automated report formats that are in the SCBSD. 

(1) Information Report

• includes site location information, bridge length, construction history, bridge age, drainage 

area, and channel slope.

(2) Abutment Scour Report

• includes theoretical abutment-scour depths computed with the Froehlich (1989) equation 

and the variables used to compute those depths; hydraulic variables in this report were 

estimated with the Water-Surface Profile (WSPRO) model (Shearman, 1990).

(3) Clay Information Report

• includes selected grain-size data for the second set of soil samples obtained at all Piedmont 

sites and at nine Coastal Plain sites; the second set of samples were collected to better 

define the percent of clays and silts at the selected sites.

(4) Clearwater Scour Report

• includes theoretical clear-water contraction scour depths computed with the Laursen 

(1963) equation and the variables used to compute those depths; hydraulic variables in this 

report were estimated with the WSPRO model (Shearman, 1990).

(5) Field Information Report

• includes selected scour-hole dimensions, observed infill depths, median grain size based 

on the initial soil samples, and general soil type at the site.

(6) Livebed Scour Report

• includes theoretical live-bed contraction-scour depths computed with the modified Laursen 

(1960) equation presented in HEC-18 (Richardson and Davis, 1995) and the variables used 

to compute those depths; live-bed scour was computed only at sites with significant lowflow channels; hydraulic variables in this report were estimated with the WSPRO model 

(Shearman, 1990).

(7) Pier Scour Report

• includes theoretical pier-scour depths computed with the HEC-18 (Richardson and Davis, 

1995) pier scour equation and the variables used to compute those depths; only limited 

theoretical pier-scour data are included in the SCBSD as described in the “Theoretical Pier 

1 Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement 

by the U.S. Government.
Appendix A 125

Scour” section of the report; hydraulic variables in this report were estimated with the 

WSPRO model (Shearman, 1990).

(8) Wspro Report

• includes selected hydraulic variables computed with the WSPRO model (Shearman, 

1990).

(9) Photos

• includes photographs and captions for most sites.

(10) Scour Figures

• includes scour contour plots for 80 sites (primarily located in the Coastal Plain).

There are eight raw data tables in the SCBSD; a brief description of each table and the associated variables 

follows. The headings for the following sections correspond with the table names in the database and are listed in 

alphabetical order. It should be kept in mind that hydraulic variables in the database are estimates obtained from 

the WSPRO (Shearman, 1990) model and errors could exist within these estimates.

Abutment_Scour Table

Theoretical abutment-scour depths and the variables used to compute those depths are stored in this table. 

Theoretical scour was computed for the 100-year flow, index flow, and the maximum historic flow when available. 

Scour depths were computed using the Froehlich (1989) and the Highways in the River Environment (HIRE) 

(Richardson and others, 1990) equations as presented in HEC-18 (Richardson and Davis, 1995). For more details 

about the computation of abutment scour, refer to the “Theoretical Abutment Scour” section of the report. The 

variables in the database table are briefly defined below:

bridgeno South Carolina Department of Transportation (SCDOT) bridge identification number

abut_qtype identifies the flow used in the theoretical computation as the 100-year flow (Q100), 

the index flow (QAGE), or the maximum historic flow (QHIS)

abut_location identifies abutment location as either the left or right abutment as determined by an 

observer looking downstream

abut_blocked cfs approach flow obstructed by the embankment, in cubic feet per second (ft3/s)

abut_blocked area approach flow area obstructed by embankment, in square feet (ft2)

abut_blocked length length of embankment blocking flow, in feet (ft)

abut_flowdepth flow depth directly at abutment toe, in ft

abut_blocked flowdepth average approach flow depth obstructed by the embankment, in ft

abut_skew skew of embankment to flow, in degrees;

positive skews indicate the embankment points upstream;

negative skews indicate the embankment points downstream

abut_tube flowvel flow velocity at abutment toe determined from the WSPRO stream tube located at the 

bridge abutment in the bridge cross section, in feet per second (ft/s)

abut_blocked flowvel average approach flow velocity obstructed by the embankment, in ft/s

abut_k1 coefficient for abutment type

abut_k2 coefficient for embankment skew

abut_froel froude Froude number for the approach flow obstructed by the embankment
126 Clear-Water Abutment and Contraction Scour in the Coastal Plain and Piedmont Provinces of South Carolina, 1996-99

abut_froel scourdepth theoretical abutment-scour depth computed using the Froehlich (1989) equation, 

including safety factor

abut_scourdepth wo theoretical abutment-scour depth computed using the Froehlich (1989) equation, but 

without safety factor

abut_hire froude Froude number for HIRE (Richardson and others, 1990) equation 

abut_hire scourdepth theoretical abutment-scour depth computed using the HIRE (Richardson and others, 

1990) equation

abut_type identification of abutment type (spill through or vertical)

Bridge Info Table

This table provides basic site information including bridge identification, location, limited basin 

characteristics data, construction dates, SCDOT bridge-plan file numbers, and bridge age. The variables are 

defined below:

bridgeno SCDOT bridge identification number

county county in which the bridge is located

long longitude of bridge, in degrees, minutes, seconds

lat latitude of bridge, in degrees, minutes, seconds

province physiographic province in which the bridge is located

road road type and number

stream name of stream

drainagearea drainage area at bridge, in square miles (mi2)

channel_slope channel slope at the bridge as determined from U.S. Geological Survey (USGS) 7.5-

minute series topographic map, in feet per foot (ft/ft)

bridgelength bridge length, in ft

bridgeconstdate calendar year in which bridge was originally constructed

bridgeplannumber SCDOT road plans file number from which construction date was estimated

widened indicates if bridge has been widened since original construction date

widendate calendar year when bridge was widened

widenplannumber SCDOT road plans file number from which widening date was estimated

bridgeage age of bridge in 1996; if bridge was widened, an attempt was made to assess if the 

construction at the time of widening disturbed the area of scour; if the assessment 

indicated that the area of scour was disturbed, the age was based on the widening date; 

otherwise the age was based on the original construction date

oldbridge indicates if an old bridge was in place (but removed) at the time of the original 

construction of the existing bridge

oldbridgedata calendar year in which the old structure was constructed
Appendix A 127

Clay Information Table

This table provides data for the second set of soil samples obtained to better define the percent of clays and 

silts at all Piedmont sites and at nine Coastal Plain sites that were noted to have clayey surface soils. The median 

grain size (D50) in the second set of samples often varies from the D50 of the original sample. This in part is 

attributed to the heterogeneous nature of the soils, and indicates that all soil data in this report should be viewed 

with caution. This table also includes original soil sample data from the 1999 flood sites along the Waccamaw, Pee 

Dee, and Little Pee Dee Rivers. The variables in the database table are briefly defined below:

bridgeno SCDOT bridge identification number

clay_50mm the D50 for the second sediment sample, in millimeters (mm)

percentfiner_0625 the percent finer than 0.0625 mm by weight in the second sediment sample

percentfiner_004 the percent finer than 0.004 mm by weight in the second sediment sample

Clearwater_Scour Table

Theoretical clear-water contraction-scour depths and the variables used to compute those depths are stored in 

this table. Theoretical scour was computed for the 100-year flow, index flow, and the maximum historic flow 

when available. Scour depths were computed using the Laursen (1963) clear-water contraction-scour equation as 

presented in HEC-18 (Richardson and Davis, 1995). For more details about the computation of clear-water 

contraction scour refer to the “Theoretical Clear-Water Contraction Scour” section of the report. The variables in 

the database table are briefly defined below:

bridgeno SCDOT bridge identification number

cw_qtype identifies the flow used in the theoretical computation as either the 100-year flow 

(Q100), the index flow (QAGE), or the maximum historic flow (QHIS)

cw_location identifies overbank location as either the left or right overbank as determined by an 

observer looking downstream or as a swampy channel with no well-defined low-flow 

channel

cw_scourdepth theoretical clear-water contraction-scour depth computed using the Laursen (1963) 

equation and not subtracting the cumulative pier width from the contracted width, in ft

cw_scourdepth_minuspiers theoretical clear-water contraction-scour depth computed using the Laursen (1963) 

equation and subtracting the cumulative pier width from the contracted width, in ft

cw_cfs flow in the contracted section, in ft3/s

cw_width width of contracted section, in ft

cw_flowdepth average flow depth at the contracted section, in ft

cw_d50mm the D50 based on the original soil sample at each site; for sites with a D50 less than 

0.062 mm, the D50, was set to 0.062 mm; for additional information on these soils, 

refer to the Clay Information Table

cw_cum_pierwidth the cumulative pier width within the contracted section, in ft
128 Clear-Water Abutment and Contraction Scour in the Coastal Plain and Piedmont Provinces of South Carolina, 1996-99

Field_Observations Table

This table provides field data collected at each site, including scour-hole geometry and soil data for the 

original soil sample. The variables are defined below:

bridgeno SCDOT bridge identification number

obs_location identifies the location where the scour hole was observed; the left and right overbank 

or abutment is determined by an observer looking downstream; a swampy channel, in 

general, refers to shorter bridges (240 ft or less) with a single large scour hole 

developing at the site rather than individual left and (or) right abutment scour hole

scourdepth_floodpn scour depth referenced to the average floodplain elevation in the region of the 

observed scour, in ft

scourdepth_channel scour depth referenced to the average channel bed elevation in the region of the 

observed scour, in ft 

This situation often occurs at swampy sites with shorter bridges, where a shallow 

channel runs through the scoured region; these data were not used in the analysis of 

this report but are provided here for information.

dataqual_of_scourdepth subjective indicator of the quality of the measured scour

infill the amount of infill at the low point of the scour hole, in ft

dataqual_of_infill subjective indicator of the quality of the measured infill

pierexistence indicator of existing pier at the low point of the scour hole

piershape shape of the pier at the low point of the scour hole

pierwidth width of the pier at the low point of the scour hole, in ft

dist_bridgecenterline_to_hole the upstream or downstream distance from the low point of the scour hole to the 

roadway centerline, in ft;

Negative numbers are downstream from the roadway centerline and positive numbers 

are upstream; refer to the “Scour Hole Longitudinal Location” section of this report 

for more details.

dist_to_leftedgeofhole distance from the left edge of the scour hole to the abutment toe as determined by an 

observer looking downstream, in ft

Refer to the “Lateral Reference for Scour Hole” section of this report for more details.

dist_to_scourlowpt distance from the low point of the scour hole to the abutment toe as determined by an 

observer looking downstream, in ft

dist_to_rightedgeofhole distance from the right edge of the scour hole to the abutment toe as determined by an 

observer looking downstream, in ft

Refer to the “Lateral Reference for Scour Hole” section of this report for more details.

scourwidth top width at the low point of the scour hole, in ft

Refer to the “Scour Hole Top Width” section of this report for more details.

scourlength the longitudinal length of the scour hole, in ft

soiltype_unscour a subjective indicator of the general surface soils in the unscoured region of the 

observed scour; this information is not necessarily an indicator of the measured grain 

size and should be viewed with caution; the information can be used to determine if 

there is a difference between the surface soils and the soils at the bottom of the scour 

hole; following is a description of each class:
Appendix A 129

• clay – a relatively cohesive soil

• sand – a sandy soil with relatively low cohesion

• layered – alternating layers of clay and sand

• mix – a mixture of sand and clay

<comment_d50mm_unscour indicator if the D50 is less than 0.0625 mm, but was assumed to be 0.0625 mm, 

because the grain-size analysis of the original soil samples did not go below 

0.0625 mm

d50mm_unscour the median grain size, D50,of the original sediment sample, in mm 

soiltype_scour a subjective indicator of the general soils at the low point of the scour hole; this 

information is not necessarily an indicator of the measured grain size and should be 

viewed with caution; the information can be used to determine if there is a difference 

between the surface soils and the soils at the bottom of the scour hole; following is a 

description of each class:

• clay – a relatively cohesive soil

• sand – a sandy soil with relatively low cohesion

• layered – alternating layers of clay and sand

• mix – a mixture of sand and clay

formation a subjective judgment that indicates if the soil at the bottom of the scour hole is a 

material from an older geologic formation in contrast to the newer surface alluviums; 

this is more common in the Coastal Plain where scour initially removes the sandy soils 

and then cuts into an older geologic formation; the soil characteristics of the formation 

are distinctly different from the surface alluviums and is often a clayey soil

<comment_d50mm_hole indicator if the D50 is less than 0.0625 mm, but was assumed to be 0.0625 mm, 

because the grain-size analysis of the original soil samples did not go below 0.0625 

mm

d50mm_hole the median grain size, D50, of the original sediment sample, in mm

wide_enough_scour indicator if abutment scour hole encompasses most of overbank area precluding the 

development of a separate clear-water contraction scour hole.

Livebed_Scour Table

Theoretical live-bed contraction-scour depths and the variables used to compute those depths are stored in 

this table. Theoretical scour was computed for the 100-year flow, index flow, and the maximum historic flow 

when available. Scour depths were computed using a modified version of the Laursen (1960) live-bed-scour 

equation as presented in HEC-18 (Richardson and Davis, 1995). For more details about the computation of livebed contraction scour, refer to the “Theoretical Live-Bed Contraction Scour” section of the report. The variables in 

the database table are briefly defined below:

bridgeno SCDOT bridge identification number

cw_qtype identifies the flow used in the theoretical computation as either the 100-year flow 

(Q100), the index flow (QAGE), or the maximum historic flow (QHIS)
130 Clear-Water Abutment and Contraction Scour in the Coastal Plain and Piedmont Provinces of South Carolina, 1996-99

lb_scourdepth theoretical live-bed contraction-scour depth computed using the modified Laursen 

(1960) equation presented in HEC-18 (Richardson and Davis, 1995), in ft

lb_flowdepth_approach average flow depth in the approach channel, in ft

lb_cfs_approach flow in the approach channel, in ft3/s

lb_width_approach bank-to-bank top width at approach channel, in ft

lb_cfs_bridge flow in the bridge channel, in ft3/s

lb_width_bridge bank-to-bank top width at bridge channel, in ft

lb_eslope slope of the energy grade line between the approach and bridge cross section, in ft/ft

lb_d50mm the D50 in the live-bed channel, in mm; in most cases these data were obtained from 

level 2 bridge-scour studies; when this information was not available, a grab sample 

from the channel was obtained and analyzed for grain-size distribution

lb d50mm fallvel the fall velocity for the D50, in ft/s

Pier_Scour Table

Theoretical pier-scour depths and the variables used to compute those depths are stored in this table. 

Theoretical scour was computed for the 100-year flow, index flow, and the maximum historic flow when available. 

Scour depths were computed using the HEC-18 pier-scour equation (Richardson and Davis, 1995). Only limited 

theoretical pier-scour data are included in the database. For more details about the computation of pier scour and 

the data included in the database, refer to the “Theoretical Pier Scour” section of the report. The variables in the 

database table are briefly defined below:

bridgeno SCDOT bridge identification number

pier_qtype identifies the flow used in the theoretical computation as either the 100-year flow 

(Q100), the index flow (QAGE), or the maximum historic flow (QHIS)

pier_location identifies the general location of the pier within the bridge opening

pier_scourdepth theoretical pier-scour depth computed using the HEC-18 pier-scour equation 

(Richardson and Davis, 1995), in ft

pier_flowdepth average approach flow depth at the pier, in ft

pier_flowvel approach flow velocity at the pier, in ft/s

pier_width width of the pier, in ft

pier_length length of the pier, in ft

pier_skew the skew of the pier to the approaching flow, in degrees

pier_k1 the dimensionless correction factor for pier nose shape

pier_k2 the dimensionless correction factor for angle of attack

pier_k3 the dimensionless correction factor for bed conditions

pier_k4 the dimensionless correction factor for bed armoring

pier_froude the approach flow Froude number
Appendix A 131

WSPRO_Scour Table

This table provides hydraulic data estimated with the WSPRO (Shearman, 1990) model for various flow 

conditions, including the 100-year flow, the index flow, and the maximum historic flow when available. Hydraulic 

data for pre- and post-scour conditions are included for bridge sites with relatively deep scour holes (approximately 

4 ft or greater). The names of the hydraulic variables in this table are, in most cases, identical to the variables in the 

WSPRO manual. For more details about the variables refer to the WSPRO manual (Shearman, 1990). For more 

details on the approach for developing the models in this study refer to the “Estimating Hydraulic Data“ section of 

this report. The variables in the database table are briefly defined below:

bridgeno SCDOT bridge identification number

wspro_condition identifies the model data as pre-scour or post scour condition

wspro_qtype identifies the flow used in the model as either the 100-year flow (Q100), the index 

flow (QAGE), or the maximum historic flow (QHIS)

wspro_crosssection identifies the cross section for the given hydraulic data

lew station for left edge of water, in ft

area cross-section flow area, in ft2

vhd cross-section velocity head, in ft

hf friction loss, in ft

egl energy grade line elevation, in ft

crws critical water-surface elevation, in ft

q flow, in ft3/s

wsel computed or assumed water-surface elevation, in ft

flen effective flow length from approach to bridge, in ft

rew station for right edge of water, in ft

k cross-section conveyance

alph velocity head correction factor for uniform velocity distribution

ho other losses, in ft

fr Froude number

vel flow velocity, in ft/s

type type of bridge opening

ppcd code to distinguish between piers and piles

flow indicates flow class for bridge

c coefficient of discharge for bridge opening

pa ratio of pier (pile) area to gross area in the bridge opening

lsel test value for low-chord elevation in a bridge used to test for possible pressure flow

mg geometric-contraction ratio

mk conveyance-contraction ratio

kq conveyance of Kq segment of the approach cross section

xlkq left station of Kq section

xrkq right station of Kq section
132 Clear-Water Abutment and Contraction Scour in the Coastal Plain and Piedmont Provinces of South Carolina, 1996-99

Selected References

Froehlich, D.C., 1989, Local scour at bridge abutments: Hydraulic Engineering, in Proceedings of the 1989 

National Conference on Hydraulic Engineering: New York, American Society of Civil Engineering, p. 13-18.

Laursen, E.M., 1960, Scour at bridge crossings: Journal Hydraulic Division, American Society of Civil 

Engineering, v. 89, no. HY3.

_____ 1963, An analysis of relief bridge scour: Journal Hydraulic Division, American Society of Civil 

Engineering, v. 92, no. HY3.

Richardson, E.V., and Davis, S.R., 1995, Evaluating scour at bridges: Federal Highway Administration Hydraulic 

Engineering Circular No. 18, Publication FHWA-IP-90-017, 204 p.

Richardson, E.V., Simons, D.B., and Julien, P.Y., 1990, Highways in the river environment participant notebook: 

Federal Highway Administration, Publication FHWA-HI-90-016, 650 p.

Shearman, J.O., 1990, User’s manual for WSPRO—A computer model for water-surface profile computations: 

Federal Highway Administration, Report no. FHWA-IP-89-027, 175 p.
Appendix B 133

APPENDIX B

South Carolina Bidge-Scour Study Sites and Reference Numbers for Figures 1, 5, and 6

[At twin bridge crossings only the structure number for the North or East bound bridge is provided. 

S.C., South Carolina Route; I-, Interstate Highway; U.S., United States Route; S-, Secondary Road]
134 Clear-Water Abutment and Contraction Scour in the Coastal Plain and Piedmont Provinces of South Carolina, 1996-99

Appendix B. South Carolina bridge-scour study sites and reference numbers for figures 1, 5, and 6

Reference 

number for 

figures 1,5, and 6

County Road Stream Structure number

1 Abbeville S.C. 201 Little River 014020100300

2 Aiken I-20 South Edisto River 021002021200

3 Allendale U.S. 301 Salkehatchie River 032030100600

4 Allendale U.S. 301 Salkehatchie River 032030100700

5 Allendale S.C. 3 King Creek 034000300100

6 Allendale S.C. 3 Gaul Creek 034000300200

7 Allendale S-21 Coosawhatchie River 037002100100

8 Allendale S-107 Gaul Branch 037010700100

9 Anderson I-85 Brushy Creek 041008511200

10 Anderson S-263 Rocky River 047026300100

11 Calhoun U.S. 176 Flea Bite Creek 092017600400

12 Cherokee S.C. 5 Buffalo Creek 114000500200

13 Cherokee S-348 Buffalo Creek 117034800100

14 Chester I-77 Fishing Creek 121007710700

15 Chester S.C. 9 Turkey Creek 124000900200

16 Chester S.C. 9 Fishing Creek 124000901100

17 Chester S.C. 72 Sandy River 124007200200

18 Chester S.C. 97 Turkey Creek 124009700100

19 Chester S.C. 97 Rocky Creek 124009700800

20 Chester S.C. 215 Sandy River 124021500200

21 Chester S.C. 223 Fishing Creek 124022300100

22 Chester S.C. 901 Rocky Creek 124090100200

23 Chesterfield S.C. 9 Thompson Creek 134000900400

24 Chesterfield S.C. 109 Thompson Creek 134010900100

25 Clarendon U.S. 15 Sammy Swamp 142001500400

26 Clarendon U.S. 378 Douglas Swamp 142037800300

27 Clarendon U.S. 521 Ox Swamp 142052100300

28 Clarendon S.C. 261 Sammy Swamp 144026100100

29 Colleton U.S. 21 Sandy Run Creek 152002100300

30 Colleton S.C. 63 Deep Creek 154006300300

31 Colleton S.C. 63 Little Salkehatchie River 154006300400

32 Colleton S.C. 63 Little Salkehatchie River 154006300500

33 Colleton S.C. 63 Little Salkehatchie River 154006300600

34 Colleton S.C. 63 Little Salkehatchie River 154006300700

35 Colleton S.C. 64 Jones Swamp 154006400700

36 Colleton S.C. 212 Buckhead Creek 154021200100

37 Colleton S.C. 212 Buckhead Creek 154021200200

38 Colleton S.C. 641 Willow Swamp 154064100200

39 Dillon S.C. 41 Buck Swamp 174004107100

40 Dillon S.C. 41 Buck Swamp 174004107200
Appendix B 135

41 Dorchester I-26 Four Hole Swamp 181002620300

42 Dorchester U.S. 78 Cattle Creek 182007800100

43 Dorchester U.S. 78 Polk Swamp 182007800200

44 Edgefield S.C. 230 Horne Creek 194023000500

45 Fairfield I-77 Little Wateree Creek 201007710600

46 Fairfield I-77 Big Wateree Creek 201007710700

47 Fairfield U.S. 21 Dutchmans Creek 202002100200

48 Fairfield U.S. 21 Big Wateree Creek 202002100400

49 Fairfield S.C. 200 Wateree Creek 204020000500

50 Florence I-95 Lake Swamp 211009510400

51 Florence I-95 Pee Dee River 211009511400

52 Florence U.S. 52 Lynches Lake 212005200100

53 Florence U.S. 52 Jefferies Creek 212005200700

54 Florence U.S. 76 Great Pee Dee River 212007621100

55 Florence U.S. 301 Douglas Swamp 212030100100

56 Florence U.S. 301 Sparrow Swamp 212030100400

57 Florence U.S. 378 Big Swamp 212037801000

58 Florence S.C. 51 Big Swamp 214005100200

59 Florence S.C. 51 Middle Swamp 214005100400

60 Florence S.C. 51 Jeffries Creek 214005100500

61 Florence S.C. 403 Deep Hole Swamp 214040300500

62 Greenville S.C. 417 Horse Pen Creek 234041700200

63 Greenville S-40 South Saluda River 237004000100

64 Greenville S-68 Reedy River 237006800100

65 Greenville S-125 Saluda River 237012500100

66 Greenwood S.C. 246 Wilson Creek 244024600200

67 Hampton U.S. 601 Coosawhatchie River 252060100300

68 Hampton S.C. 363 Coosawhatchie River 254036300100

69 Hampton S-13 Whippy Swamp 257001300500

70 Horry U.S. 501 Waccamaw River 262050103100

71 Horry U.S. 501 Waccamaw River 262050103200

72 Horry U.S. 501 Waccamaw River 262050103300

73 Horry U.S. 501 Waccamaw River 262050105200

74 Horry U.S. 501 Little Pee Dee River 262050110100

75 Horry S.C. 22 Waccamaw River 264002220200

76 Horry S.C. 22 Waccamaw River 264002220300

77 Horry S.C. 22 Waccamaw River 264002220400

78 Jasper U.S. 278 Cypress Creek 272027800100

79 Jasper U.S. 321 Cypress Swamp 272032100700

80 Jasper U.S. 321 Black Swamp Long Branch 272032100800

Appendix B. South Carolina bridge-scour study sites and reference numbers for figures 1, 5, and 6 (Continued)

Reference 

number for 

figures 1,5, and 6

County Road Stream Structure number
136 Clear-Water Abutment and Contraction Scour in the Coastal Plain and Piedmont Provinces of South Carolina, 1996-99

81 Jasper S.C. 3 Cypress Creek 274000300200

82 Jasper S.C. 336 Great Swamp 274033600100

83 Jasper S-87 Coosawhatchie River 277008700100

84 Kershaw U.S. 1 Little Lynches River 282000100500

85 Kershaw U.S. 521 Granneys Quarter Creek 282052100900

86 Kershaw S.C. 97 White Oak Creek 284009700300

87 Kershaw S.C. 97 Granneys Quarter Creek 284009700400

88 Laurens S.C. 49 Enoree River 304004900400

89 Laurens S.C. 72 Little River 304007200500

90 Laurens S.C. 72 Duncan Creek 304007201200

91 Laurens S-36 Reedy River 307003600200

92 Laurens S-102 Little River 307010200100

93 Laurens S-112 Enoree River 307011200100

94 Laurens S-263 Enoree River 307026300100

95 Lexington S.C. 113 North Edisto River 324011300100

96 McCormick S.C. 67 Cuffeytown Creek 334006700100

97 Marion U.S. 76 Great Pee Dee River 342007620100

98 Marion U.S. 501 Smith Swamp 342050100300

99 Marion U.S. 501 Little Pee Dee River 342050110700

100 Marion U.S. 501 Little Pee Dee River 342050110800

101 Marion U.S. 501 Little Pee Dee River 342050110900

102 Marion U.S. 501 Little Pee Dee River 342050111000

103 Marion U.S. 501 Little Pee Dee River 342050111100

104 Marion S.C. 41 Maiden Down Swamp 344004100800

105 Marion U.S. 576 Smith Swamp 344057620200

106 Newberry U.S. 176 Indian Creek 362017600400

107 Newberry U.S. 176 Kings Creek 362017600500

108 Newberry S.C. 34 Little River 364003400300

109 Newberry S.C. 121 Saluda River 364012100101

110 Newberry S-32 Indian Creek 367003200200

111 Newberry S-45 Enoree River 367004500100

112 Newberry S-81 Enoree River 367008100200

113 Newberry S-299 Cannons Creek 367029900100

114 Oconee U.S. 76 Coneross Creek 372007620500

115 Oconee S.C. 11 Colonels Fork Creek 374001100500

116 Oconee S.C. 183 Coneross Creek 374018300200

117 Orangeburg U.S. 176 Providence Swamp 382017600200

118 Orangeburg U.S. 176 Providence Swamp 382017600300

119 Orangeburg U.S. 301 Four Hole Swamp 382030110800

120 Orangeburg U.S. 301 Goodbys Swamp 382030110900

Appendix B. South Carolina bridge-scour study sites and reference numbers for figures 1, 5, and 6 (Continued)

Reference 

number for 

figures 1,5, and 6

County Road Stream Structure number
Appendix B 137

121 Orangeburg S.C. 453 Four Hole Swamp (Br 1) 384045300200

122 Richland U.S. 321 Crane Creek 402032100200

123 Saluda U.S. 378 Red Bank Creek 412037800100

124 Saluda U.S. 378 Little Saluda River 412037800200

125 Spartanburg U.S. 29 South Tyger River 422002900100

126 Spartanburg U.S. 176 Lawsons Fork Creek 422017620900

127 Spartanburg S.C. 146 Enoree River 424014600100

128 Spartanburg S.C. 296 Enoree River 424029600100

129 Spartanburg S-62 South Tyger River 427006200500

130 Spartanburg S-118 Enoree River 427011800001

131 Spartanburg S-242 South Tyger River 427024200200

132 Sumter U.S. 401 Rock Bluff Creek 432040100100

133 Sumter U.S. 401 Rock Bluff Creek 432040100200

134 Union S.C. 56 Enoree River 444005600100

135 Union S-22 Enoree River 447002200100

136 Williamsburg S.C. 261 Paisley Swamp 454026100300

137 Williamsburg S.C. 261 Boggy Swamp 454026100400

138 Williamsburg S.C. 261 Indiantown Swamp 454026100500

139 Williamsburg S-16 Johnson's Creek 457001600100

140 York S.C. 97 Bullocks Branch 464009700300

141 York S.C. 322 Fishing Creek 464032200300

142 York S.C. 322 Tools Fork Creek 464032200500

143 York S.C. 557 Crowders Creek 464055700200

144 York S-721 Taylors Creek 467072100100

NOTE: The following bridge sites have been excluded from the above list because the area of scour 

at the bridge was disturbed by construction equipment, bringing the field measurements of scour into 

question.

Allendale U..S. 301 Salkehatchie River 032030100800 

Horry S.C. 22 Waccamaw River 264002220100

Appendix B. South Carolina bridge-scour study sites and reference numbers for figures 1, 5, and 6 (Continued)

Reference 

number for 

figures 1,5, and 6

County Road Stream Structure number