Field Observations and Evaluations 

of Streambed Scour at Bridges

PUBLICATION NO. FHWA-RD-03-052 MAY 2005

Research, Development, and Technology

Turner-Fairbank Highway Research Center

6300 Georgetown Pike

McLean, VA 22101-2296
FOREWORD 

This report describes the most comprehensive set of real-time field measurements of bridge 

scour ever assembled. It represents more than 6 years of dedicated effort by the U.S. Geological 

Survey researchers to collect scour data during flood events wherever they occurred in the 

United States. The report will be of interest to bridge engineers and hydraulic engineers 

involved in bridge scour evaluations and to researchers involved in developing improved bridge 

scour evaluation procedures. Sufficient copies will be printed to provide at least two copies to 

each Federal Highway Administration (FHWA) Division Office. 

 T. Paul Teng, P.E. 

 Director, Office of Infrastructure 

 Research and Development 

Notice 

This document is disseminated under the sponsorship of the U.S. Department of Transportation 

in the interest of information exchange. The U.S. Government assumes no liability for the use of 

the information contained in this document. This report does not constitute a standard, 

specification, or regulation. 

The U.S. Government does not endorse products or manufacturers. Trademarks or 

manufacturers’ names appear in this report only because they are considered essential to the 

objective of the document. 

Quality Assurance Statement 

FHWA provides high-quality information to serve Government, industry, and the public in a 

manner that promotes public understanding. Standards and policies are used to ensure and 

maximize the quality, objectivity, utility, and integrity of its information. FHWA periodically 

reviews quality issues and adjusts its programs and processes to ensure continuous quality 

improvement.
1. Report No. 

FHWA–RD–03–052 

2. Government Accession No. 3. Recipient’s Catalog No. 

5. Report Date 

May 2005 

4. Title and Subtitle 

FIELD OBSERVATIONS AND EVALUATIONS OF 

STREAMBED SCOUR AT BRIDGES 6. Performing Organization Code 

7. Author (s) 

David S. Mueller and Chad R. Wagner 

8. Performing Organization Report No. 

9. Performing Organization Name and Address 10. Work Unit No. (TRAIS) 

U.S. Geological Survey 

Water Resources Division 

9818 Bluegrass Parkway 

Louisville, KY 40299 

11. Contract or Grant No. 

DTFH61–93–Y–00050 

13. Type of Report and Period Covered 

Final Report: 1993–1999 

12. Sponsoring Agency Name and Address 

Office of Engineering Research and Development 

Federal Highway Administration 

6300 Georgetown Pike 

McLean, VA 22101-2296 

14. Sponsoring Agency Code 

15. Supplementary Notes 

Contracting Officer’s Technical Representative (COTR): J. Sterling Jones, HRDI–07 

16. Abstract 

The variability and complexity of site conditions make it difficult to develop methodology for predicting scour at 

bridges. Laboratory investigations often oversimplify or ignore many complexities common in the field. The U.S. 

Geological Survey, in cooperation with the Federal Highway Administration and many State highway agencies, has 

collected and compiled field data on scour at bridges at 79 sites located in 17 States. These data have been analyzed to 

isolate pier scour, contraction scour, and abutment scour. The national data base contains 493 local pier scour 

measurements, 18 contraction scour measurements, and 12 abutment scour measurements. The pier scour 

measurements were used to evaluate 26 published pier scour equations. The Froehlich Design, HEC-18, HEC-18-K4, 

HEC-18-K4Mu, HEC-18-K4Mo (>2 millimeter), and Mississippi equations proved to be better than the other 

equations for predicting pier scour for design purposes. However, comparison of the scour predicted from these 

equations with the observed scour clearly shows that variability in the field data is not correctly accounted for in the 

equations. Relations between dimensionless variables developed from laboratory experiments did not compare well 

with the field data. Analysis of the pier scour data indicated the importance of bed-material characteristics as a variable 

in the predictive equations. A new K4 term for the HEC-18 pier-scour equation was developed based on the relative 

bed-material size (b/D50) where b = pier width and D50 is the median bed material. A review of published literature 

found 29 references to abutment and contraction scour data; however, only a few provided complete data sets. 

Published comparisons of observed versus computed scour were inconclusive. A detailed comparison of computed 

contraction and abutment scour with field observations for two sites in Minnesota was also inconclusive. The current 

methodology for computing scour depth provides reasonable estimates of the maximum total scour, but the individual 

estimates of contraction and abutment scour did not compare well with the observed data. The accuracy of the 

contraction and abutment scour equations may depend on the degree of contraction, the flow distribution in and 

configuration of the approach, and how well the hydraulic model represents the true flow distribution. 

17. Key Words 

Bridge scour, field data, contraction scour, abutment 

scour, pier scour, local scour, debris 

18. Distribution Statement 

19. Security Classif. (of this report) 

Unclassified 

20. Security Classif. (of this report) 

Unclassified 

21. No. of Pages 

134 

22. Price 

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

 

SI* (MODERN METRIC) CONVERSION FACTORS 

APPROXIMATE CONVERSIONS TO SI UNITS

Symbol When You Know Multiply By To Find Symbol 

LENGTH 

in inches 25.4 millimeters mm 

ft feet 0.305 meters m 

yd yards 0.914 meters m 

mi miles 1.61 kilometers km 

AREA 

in2 square inches 645.2 square millimeters mm2

ft2 square feet 0.093 square meters m2

yd2 square yard 0.836 square meters m2

ac acres 0.405 hectares ha

mi2 square miles 2.59 square kilometers km2

VOLUME 

fl oz fluid ounces 29.57 milliliters mL 

gal gallons 3.785 liters L 

ft3 cubic feet 0.028 cubic meters m3 

yd3 cubic yards 0.765 cubic meters m3 

NOTE: volumes greater than 1000 L shall be shown in m3

MASS 

oz ounces 28.35 grams g

lb pounds 0.454 kilograms kg

T short tons (2000 lb) 0.907 megagrams (or "metric ton") Mg (or "t") 

TEMPERATURE (exact degrees) o

F Fahrenheit 5 (F-32)/9 Celsius oC 

or (F-32)/1.8 

ILLUMINATION 

fc foot-candles 10.76 lux lx 

fl foot-Lamberts 3.426 candela/m2 cd/m2

FORCE and PRESSURE or STRESS 

lbf poundforce 4.45 newtons N 

lbf/in2 poundforce per square inch 6.89 kilopascals kPa 

APPROXIMATE CONVERSIONS FROM SI UNITS 

Symbol When You Know Multiply By To Find Symbol 

LENGTH

mm millimeters 0.039 inches in 

m meters 3.28 feet ft 

m meters 1.09 yards yd 

km kilometers 0.621 miles mi 

AREA mm2

 square millimeters 0.0016 square inches in2 

m2 square meters 10.764 square feet ft2 

m2 square meters 1.195 square yards yd2 

ha hectares 2.47 acres ac

km2 square kilometers 0.386 square miles mi2 

VOLUME 

mL milliliters 0.034 fluid ounces fl oz 

L liters 0.264 gallons gal 

m3 cubic meters 35.314 cubic feet ft3 

m3 cubic meters 1.307 cubic yards yd3 

MASS 

g grams 0.035 ounces oz

kg kilograms 2.202 pounds lb

Mg (or "t") megagrams (or "metric ton") 1.103 short tons (2000 lb) T 

TEMPERATURE (exact degrees) o

C Celsius 1.8C+32 Fahrenheit oF 

ILLUMINATION 

lx lux 0.0929 foot-candles fc 

cd/m2 candela/m2 0.2919 foot-Lamberts fl

FORCE and PRESSURE or STRESS 

N newtons 0.225 poundforce lbf 

kPa kilopascals 0.145 poundforce per square inch lbf/in2

*SI is the symbol for th International System of Units. Appr e opriate rounding should be made to comply with Section 4 of ASTM E380. 

(Revised March 2003)
iii 

TABLE OF CONTENTS 

CHAPTER 1: INTRODUCTION..........................................................................................1 

GENERAL....................................................................................................................1 

PURPOSE AND SCOPE.............................................................................................2 

CHAPTER 2: COMPONENTS OF SCOUR AT BRIDGES...............................................3 

GENERAL....................................................................................................................3 

AGGRADATION AND DEGRADATION................................................................4 

SHORT-TERM SCOUR AND FILL .........................................................................4 

GENERAL SCOUR.....................................................................................................5 

CONTRACTION SCOUR ..........................................................................................6 

LOCAL SCOUR ..........................................................................................................8 

CHAPTER 3: DESCRIPTION OF FIELD METHODS...................................................11 

GENERAL..................................................................................................................11 

LIMITED-DETAIL DATA.......................................................................................11 

DETAILED DATA ....................................................................................................12 

CHAPTER 4: ENHANCEMENTS TO THE BRIDGE SCOUR 

DATA MANAGEMENT SYSTEM..................................................15 

CHAPTER 5: LOCAL SCOUR AT PIERS .......................................................................17 

GENERAL..................................................................................................................17 

SUMMARY OF FIELD DATA ................................................................................18 

EVALUATION OF PUBLISHED EQUATIONS...................................................22 

Discussion of Equations.................................................................................22

Assumptions ...................................................................................................27

Comparison of Predictions with Field Data ................................................27 

EVALUATION OF LABORATORY RESEARCH...............................................41 

General ...........................................................................................................41

Pier Geometry ...............................................................................................41

Relative Velocity ............................................................................................44

Bed Material Parameters .............................................................................45

Depth of Approach Flow ..............................................................................48 

DEVELOPMENT OF SCOUR PREDICTION METHODOLOGY....................49 

Assessment of Basic Variables......................................................................49

Assessment of Current Methodology ...........................................................52

Development of New Methodology...............................................................56 

Importance of Sampling Bed Material ........................................................61 

CHAPTER 6: SCOUR CAUSED BY FLOW CONTRACTION AT BRIDGES ...........63 

GENERAL..................................................................................................................63 

DISCUSSION OF PREDICTIVE EQUATIONS ...................................................63 
iv 

TABLE OF CONTENTS (continued) 

General............................................................................................................63

Contraction Scour..........................................................................................63 

Live Bed Contraction Scour Equations................................................64 

Clear Water Contraction Scour Equations..........................................67 

Abutment Scour .............................................................................................68

Field Conditions...................................................................................68 

Discussion of Equations.......................................................................68 

EVALUATION OF PUBLISHED FIELD OBSERVATIONS..............................70 

General............................................................................................................70 

Summary of Selected References..................................................................70 

ANALYSIS OF REAL-TIME DATA ......................................................................78

General............................................................................................................78

U.S. Route 12 over the Pomme de Terre River ...........................................79 

Site Description....................................................................................79 

Discussion of Field Data .....................................................................79 

Model Calibration................................................................................82 

Assessment of Scour Computations .....................................................85 

Swift County Route 22 over the Pomme de Terre River............................86 

Site Description....................................................................................86 

Discussion of Field Data .....................................................................88 

Model Calibration................................................................................90 

Assessment of Scour Computations .....................................................92 

CHAPTER 7: SUMMARY AND CONCLUSIONS ..........................................................93 

APPENDIX A: PIER SCOUR FIELD DATA.....................................................................95 

REFERENCES.....................................................................................................................113 
v 

LIST OF FIGURES 

1. Illustration of a specific gage plot showing stream degradation....................................4 

2. Example of short-term scour and fill with no long-term changes .................................5 

3. Illustration of reference surface for contraction scour...................................................5 

4. Illustration of nonuniformly distributed contraction scour............................................6 

5. Illustration of a reference surface for clear water contraction scour with material 

deposited immediately downstream of the scour hole...................................................7 

6. Illustration of reference surface sketched on a cross section plot..................................8 

7. Scatterplots of computed versus observed scour, in meters (m), for 

selected pier scour equations .......................................................................................28 

8. Evaluation of residuals for the Froehlich equation......................................................35 

9. Evaluation of residuals for the Froehlich Design equation..........................................36 

10. Evaluation of residuals for the HEC-18-K4 equation..................................................37 

11. Evaluation of residuals for the HEC-18-K4Mu equation ............................................38 

12. Evaluation of residuals for the Mississippi equation ...................................................39 

13. Evaluation of residuals for the HEC-18-K4Mo equation ............................................40 

14. Box plot illustrating the effect of pier shape on relative depth of scour......................43 

15. Box plot illustrating the effect of pier shape on the depth of scour with the 

effects of pier width, velocity, depth, and bed material removed by 

linear regression...........................................................................................................43 

16. Comparison of field observations with the curves developed by Chiew showing 

the effect of sediment size and relative velocity on relative depth of scour(44)............44 

17. Effect of gradation and relative velocity on relative depth of scour for field 

data, with hand-drawn envelope curves for selected gradation classes.......................45 

18. Effect of relative sediment size on relative depth of scour for field data ....................46 
vi 

LIST OF FIGURES (continued)

19. Effect of the coefficient of gradation on relative depth of scour for field data with 

hand-drawn envelope curves of ripple- and nonripple-forming sediments .................47 

20. Effect of relative flow depth on relative depth of scour with field data compared 

to the relation presented by Melville and Sutherland(2) ...............................................48 

21. Effect of relative flow depth on relative depth of scour for field conditions near 

incipient motion (0.8<Vo/Vc<1.2) compared to the relation presented by Melville and 

Sutherland(2) .................................................................................................................49 

22. Scatterplot matrix and frequency distribution of basic variables and depth 

of scour, log-transformed.............................................................................................50 

23. Example of difference between unweighted regression and weighted regression in 

developing a design curve............................................................................................51 

24. Relation between the ratio of the observed depth of pier scour to the depth of pier 

scour computed by the HEC-18 equation (idealized K4) and the K4 proposed by 

Mueller(5) as adopted by the fourth edition of HEC-18(77) ...........................................55 

25. Box plot of the variation in the ratio of the observed depth of pier scour to 

the depth of pier scour computed by the HEC-18 equation (idealized K4) for 

clear water and live bed conditions..............................................................................57 

26. Box plot of the variation in the ratio of the observed depth of pier scour to 

the depth of pier scour computed by the HEC-18 equation (idealized K4) 

 for low and high armor potential conditions...............................................................57 

27. Box plot of the variation in the ratio of the observed depth of pier scour to 

the depth of pier scour computed by the HEC-18 equation (idealized K4) for 

sediment size classes....................................................................................................58 

28. Relation between the ratio of the observed depth of pier scour to the 

depth of pier scour computed by the HEC-18 equation (idealized K4) 

and selected variables ..................................................................................................59 

29. Relation between relative errors in computed scour using the HEC-18 

equation and relative bed material size........................................................................60 

30. Illustration of flow contracted by an embankment constructed in a floodplain ..........70 
vii 

LIST OF FIGURES (continued)

31. Comparison of measured and computed contraction scour at State Road (S.R.) 16 

over the Pearl River near Edinburg, MS, at the left (east) relief bridge 

(1 ft = 0.305 m). ...........................................................................................................72 

32. Comparison of measured and computed contraction scour at S.R. 15 over the Pearl 

River near Burnside, MS, at the left (south) relief bridge (1 ft = 0.305 m).................72 

33. Illustration of U.S. Route 12 over Pomme de Terre River, Minnesota, showing spot 

elevations and surface current patterns on April 9, 1997. (Elevations are in meters 

referenced to NGVD of 1929.) ....................................................................................80 

34. Measured cross sections at U.S. Route 12 over the Pomme de Terre River in 

Minnesota.....................................................................................................................83 

35. Comparison of observed and model velocity distributions at U.S. Route 12 

over the Pomme de Terre River, Minnesota, for April 5, 1997...................................84 

36. Comparison of observed and model velocity distributions at U.S. Route 12 

over the Pomme de Terre River, Minnesota, for April 9, 1997...................................84 

37. Plan view of Swift County Route 22 over the Pomme de Terre River, Minnesota, 

(no scale) .....................................................................................................................87 

38. Sketch of flow conditions at Swift County Route 22 over the Pomme de Terre 

River, Minnesota (not to scale)........................................................................87 

39. Cross sections collected along the upstream edge of Swift County Route 22 

over the Pomme de Terre River, Minnesota ................................................................89 

40. Comparison of observed and model velocity distributions for April 5, 1997, 

at Swift County Route 22 over the Pomme de Terre River, Minnesota ......................91 

41. Comparison of observed and model velocity distributions for April 9, 1997, 

at Swift County Route 22 over Pomme de Terre River, Minnesota ............................91 
viii 

LIST OF TABLES 

Table 1. Number of bridges damaged and destroyed by scour, 1985–95....................................... 1 

Table 2. Summary statistics for selected pier scour measurements.............................................. 21 

Table 3. Summary of selected local pier scour equations.*.......................................................... 23 

Table 4. Summary of exponents for variables used in selected equations.................................... 26 

Table 5. Summary of the performance of the selected pier scour equations. ............................... 33 

Table 7. Summary of weighted and unweighted regression results using basic variables. .......... 54 

Table 8. Summary of variability in bed material data from sites in Ohio. .................................. 62 

Table 9. Summary of live bed contraction scour equation exponents. ......................................... 66 

Table 10. Summary of published field data for contraction and abutment scour......................... 71 

Table 11. Comparison of measured mean depth to calculated mean depth at Alaskan bridges 

where contraction was present during flood flows (modified from Norman(16)). ........ 73

Table 12. Comparison of computed and measured scour at U.S. 87 on Razor Creek, MT, June 

1991.............................................................................................................................. 74

Table 13. Measured and predicted mean depth of flow at bridges 331 and 1187 on the Copper 

River Highway, Alaska, in May 1992.......................................................................... 75

Table 14. Comparison of measured mean depth to calculated mean depth at bridges where 

contraction was present during flood flows ................................................................. 76

Table 15. Contraction scour data published by Jackson.(42).......................................................... 77 

Table 16. Summary of contraction scour measurements at U.S. Route 12 over the Pomme de 

Terre River in Minnesota ............................................................................................. 81 

Table 17. Summary of abutment scour data for U.S. Route 12 over the Pomme de Terre River in 

Minnesota ..................................................................................................................... 82

Table 18. Comparison of observed to computed contraction scour at U.S. Route 12 over the 

Pomme de Terre River in Minnesota. .......................................................................... 85 

Table 19. Comparison of observed to computed abutment and total scour at U.S. Route 12 over 

the Pomme de Terre River in Minnesota...................................................................... 86 
ix 

Table 20. Summary of hydraulic data collected at Swift County Route 22 over the Pomme de 

Terre River in Minnesota ............................................................................................. 88

Table 21. Summary of abutment scour field data for Swift County Route 22 over the Pomme de 

Terre River in Minnesota ............................................................................................. 90

Table 22. Comparison of observed to computed abutment scour at Swift County Route 22 over 

the Pomme de Terre River in Minnesota...................................................................... 92

Table 23. List of sites.................................................................................................................... 95 

Table 24. Pier scour observations. ................................................................................................ 97 
x 

LIST OF SYMBOLS 

b is the pier width. 

be is the effective pier width defined as bcosα + Lsinα . 

b1 is the bottom width in the uncontracted section. 

b2 is the bottom width in the contracted section. 

ca is the pier location code in the Arkansas pier scour equation, ca = 0 for main channel 

piers and ca = 1 for piers on the banks of the main channel or on the floodplain. 

D10 is the grain size of bed material for which 10 percent is finer. 

D16 is the grain size of bed material for which 16 percent is finer. 

D35 is the grain size of bed material for which 35 percent is finer. 

D50 is the grain size of bed material for which 50 percent is finer; the median grain size. 

D84 is the grain size of bed material for which 84 percent is finer. 

D90 is the grain size of bed material for which 90 percent is finer. 

D95 is the grain size of bed material for which 95 percent is finer. 

D99 Is the grain size of bed material for which 99 percent is finer. 

Di 

or Dx

is the grain size of bed material for which i or x percent is finer. 

Dm is the mean grain size of the bed material. 

DA is the drainage area. 

DCFM is an average of the coarse grain sizes used by Molinas; see table 3.(1)

Eb is the exponent on the ratio of bottom widths for live bed contraction scour equation. 

En is the exponent on the ratio of roughness coefficients or live bed contraction scour 

equation. 

EQ is the exponent on the ratio of discharges for live bed contraction scour equation. 

f ( ) is an undefined function of parameters enclosed in parentheses. 

F & 

Fo

is the flow Froude number defined as Vo/(gyo)

0.5. 

Fp is the pier Froude number defined as Vo/(gb)

0.5. 

G is the acceleration of gravity. 
xi 

LIST OF SYMBOLS (continued) 

ki is the standard normal deviate of i. 

K is a multiplying factor that varies from 1.3 to 2.3 

Kd is a coefficient to correct for sediment size by Melville and Sutherland.(2)

Ki is a coefficient to correct the HEC-18 equation for sediment size by Molinas; see 

table 3.(1)

KI is a coefficient to correct for flow intensity defined by Melville and Sutherland.(2)

Ks is a coefficient to correct for pier shape defined by Melville and Sutherland.(2)

Ksc is a coefficient for pier shape in the Simplified Chinese equation, defined by Gao et 

al. to be 1 for cylinders, 0.8 for round-nosed piers, and 0.66 for sharp nosed-piers.(3)

KS2 is a coefficient for pier shape used by Larras and is 1.0 for cylindrical piers and 1.4 

for rectangular piers.(4)

Ku is 1.0 for SI units and 1.81 for customary English units in the critical velocity 

equation. 

Ky is a coefficient to correct for flow depth defined by Melville and Sutherland.(2)

K1 is a coefficient based on the shape of the pier nose, defined as 1.1 for square-nose 

piers, 1.0 for circular- or round-nosed piers, 0.9 of sharp-nosed piers, and 1.0 for a 

group of cylinders. 

K2 is a coefficient to correct for the skew of the pier to the approach flow, defined as 

(cos α + (L/b)sin α)

0.65. 

K3 is a coefficient to correct for the channel bed condition, defined as 1.1 except when 

medium to large dunes are present, and then it can range from 1.2 to 1.3. 

K4 is a coefficient to correct for bed material size and gradation; see table 3. 

K4Mu K4 coefficient derived by Mueller.(5) 

K4Mo K4 coefficient derived by Molinas.(1)

KαL is a coefficient to correct for flow alignment defined by Melville and Sutherland 

(1988).(2)

L is the length of the pier. 

n1 is the Manning’s n in the uncontracted section. 

n2 is the Manning’s n in the contracted section. 

Q1 is the discharge in the uncontracted section. 
xii 

LIST OF SYMBOLS (continued) 

Q2 is the discharge in the contracted section. 

S is the slope of channel in the vicinity of the bridge. 

Vo is the approach velocity for pier scour. 

Vc is the critical (incipient-transport) velocity for the D50 size particle. 

Vcx is the critical (incipient-transport) velocity for the Dx size particle. 

VR is a velocity intensity term used by Richardson and Davis; see table 3.(6)

Vc

′ is the approach velocity corresponding to critical velocity and incipient scour of the 

D50 in the accelerated flow region at the pier. 

Vcx

′ is the approach velocity corresponding to critical velocity and incipient scour of the 

Dx in the accelerated flow region at the pier. 

Vi is the approach velocity corresponding to critical velocity and incipient scour in the 

accelerated flow region at the pier defined by Molinas; see table 3.(1)

Vcm is the critical (incipient-transport) velocity for the coarse size fraction defined by 

Molinas; see table 3.(1)

VLP is the live bed peak velocity defined by Sheppard.(7)

V2 is the velocity in the contracted section. 

yo is the approach depth of flow for pier scour. 

ys is the depth of scour. 

Y1 is the depth in the uncontracted section. 

Y2 is the depth in the contracted section. 

α is the skew of the pier to approach flow. 

ф is a pier shape factor in Froelich’s equations 

σ is the coefficient of gradation. 

θ is the Shield’s parameter. 

τ represents one or more shear stress variables. 

ν is the kinematic viscosity in Shen’s equation (ft2

/sec). 
1 

CHAPTER 1: INTRODUCTION 

GENERAL 

Bridge piers and highway embankments leading to a bridge often obstruct the flow of 

floodwaters, causing an increase in velocity and the development of vortices. The increased 

velocity and vortices often cause scour near the bridge foundations. The damage to and failure of 

bridges caused by scour are problems of national concern, as illustrated by the number of bridges 

damaged or destroyed by floods in the United States during 1985–95 (table 1). 

Table 1. Number of bridges damaged and destroyed by scour, 1985–95. 

Location and Year 

Number of Bridges 

Damaged or 

Destroyed 

Pennsylvania, West Virginia, Virginia, 1985 73 

New York and New England, 1987 17 

Midwestern United States, 1993 >2,500 

Georgia, 1994 >1,000 

Virginia, 1995 74 

California, 1995 45 

The U.S. Federal Highway Administration (FHWA) has issued several guidance documents 

addressing scour. Hydraulic Engineering Circular (HEC)–18 presents methods for predicting 

local and contraction scour at planned and existing bridges.(6) Although the methods presented in 

HEC–18 represent the state-of-the-knowledge at the time of publication, some of the potential 

limitations of these methods were identified. 

The current equations and methods for estimating scour at bridges are 

based primarily on laboratory research. Very little field data have been 

collected to verify the applicability and accuracy of the various design 

procedures for the range of soil conditions, streamflow conditions, and 

bridge designs encountered throughout the United States. (p. 3)(6)

The lack of and need for reliable and complete field data on scour at bridges has been a recurring 

conclusion of many researchers (See references 8, 9, 10, 11, and 12.) Other researchers have 

compiled field measurements on local pier scour.(3,13,14) These historical data sets contain 

valuable information, but most do not contain information on all major factors known to affect 

scour. Froehlich was unable to include the effect of sediment gradation in his analysis because 

many data sets did not include this information.(13) Johnson, in a comparison of seven published 

pier scour equations with field data, assumed uniform sediment size because sediment gradation 

information was not available for most of the data.(15) 

Field measurements of contraction and abutment scour are significantly fewer than of local pier 
2 

scour. In a recent review of published field data on contraction and abutment scour, 29 

references were found. Of these only Norman(16) presented detailed data collected during floods 

and only two other references included data on abutment scour.(17,18) Seven of the 29 papers 

presented data on contraction scour, and another 14 discussed sites that could yield contraction 

scour data, but only if additional data were available. 

Despite the recognized need for the collection of field data, few data were collected until the late 

1980s, a deficiency that is primarily a reflection of the difficulty in collecting the necessary data. 

Accurate and complete field measurements of scour are difficult to obtain because of complex 

hydraulic conditions at bridges during floods, inability to get skilled personnel to bridge sites 

during floods, and problems associated with existing measuring equipment.(19,20) Cooperative 

research among FHWA, State highway departments, and the U.S. Geological Survey (USGS) 

has allowed the collection of scour data at bridges during floods. Landers and Mueller published 

394 local pier scour measurements made by the USGS during the first national bridge scour 

study. (21) Most of these data collected by USGS contain bed material data and provide 

supporting channel cross sections and site-characterization data. 

PURPOSE AND SCOPE 

This report describes the results of the second USGS national field-data collection and analysis 

study on scour at bridges, funded by FHWA. The database originally developed during the first 

national study has been enhanced and many scour measurements added, including measurements 

of abutment and contraction scour.(22) Sufficient local pier scour data are now available to permit 

a detailed analysis of local pier scour. Scour depths computed from published pier scour 

equations are compared to the field measurements. Many commonly cited dimensionless 

variables believed to control the depth of scour are evaluated and compared with equations 

developed from laboratory data. The effect of the size and gradation of the bed material on the 

depth of scour is investigated, and a correction factor for the HEC–18 pier scour equation is 

proposed. Available data are insufficient to permit a detailed investigation of contraction and 

abutment scour; however, some basic comparisons and qualitative observations are presented on 

the basis of a review of the literature. The results of scour analyses for two contracted bridges are 

compared with real-time field data. 
3 

CHAPTER 2: COMPONENTS OF SCOUR AT BRIDGES 

GENERAL 

Total scour at a bridge is generally divided into three or more components. According to 

Richardson and Davis, bridge scour consists of the following three components:(6)

1. Aggradation or degradation—long-term changes in streambed elevation due to 

natural or human-induced causes, which can affect the reach of the river near the 

bridge. 

2. Contraction scour—removal of material from the bed and banks across all or most of 

the channel width, resulting from the contraction of the flow area. 

3. Local scour—removal of bed material from around piers, abutments, spurs, and 

embankments. Local scour is caused by the acceleration of flow and by vortices 

resulting from flow around an obstruction. 

Documents from Canada and Australia, along with field experience, indicate the need for a 

fourth component of scour, termed short-term scour, which refers to natural tendencies of the 

stream to scour and fill during relatively short-term streamflow runoff cycles.(23,24) Neill calls this 

component natural scour: “Natural scour in alluvial and tidal channels (is) associated with 

variations in flow conditions and associated channel processes including bed material transport, 

bed-form migration, and channel shifting.” (p. 77)(23)

Austroads defined this component as follows: “Scour due to river morphology—which would 

occur naturally in the stream and is a function of flow conditions and associated channel 

characteristics. It includes general bed movement and scour at channel contractions and bends.” 

(p. 55)(24)

The discussion of contraction scour in Richardson and Davis implies that this natural short-term 

scour is included in the contraction scour component.(6) This short-term scour does not depend 

on the contraction, but on the sediment transport processes in the stream. Short-term scour must 

be accounted for in the analysis of field data, or the reported contraction scour may include 

short-term scour. 

Data to quantify local (abutment and pier) and contraction scour must have a spatial and 

temporal distribution that encompasses the complex processes causing these types of scour. 

Scour caused by piers, abutments, and contractions cannot be measured directly, but must be 

interpreted from channel bathymetry data.(21,25) The spatial and temporal distribution of the data 

collected must include the data needed to identify the reference surfaces from which the various 

components of scour are measured. The components of scour cannot be isolated with any degree 

of certainty without adequate data to identify the reference surfaces. 
4 

Figure 1. Illustration of a specific gage plot 

showing stream degradation. 

AGGRADATION AND DEGRADATION 

Aggradation and degradation are the result of change in a geomorphic control within the 

watershed, which in turn causes a long-term change in the streambed elevation. The reference 

surface for aggradation and degradation must be established at some discrete time in the river’s 

history. Determining the long-term changes at a bridge site is very difficult if historical 

streamflow data are not available. In such situations, a paleogeomorphologist can evaluate 

sediment stratigraphy and vegetation to estimate the stability and magnitude of past and future 

geomorphic changes for a particular river. If streamflow data are available, then the development 

of specific gage plots, analysis of rating shifts, and analysis of historical changes in streambed 

elevation are the best means of estimating the stability and magnitude of geomorphic changes. 

Specific gage plots are developed by selecting one or more discharges to analyze using only 

measured stage and discharge at each discharge and then plotting a graph of stage-versus-time 

for each of the selected discharges. The general slope of the line will indicate the long-term 

change in the bed elevation, and the irregularity of the line indicates changes in roughness or 

short-term changes in bed elevation (figure 1). Comparing pre-flood and flood cross sections will 

not yield the magnitude of long-term streambed elevation change; it does yield a combination of 

long-term and short-term streambed elevation change, but then only if uncontracted sections 

(such as the approach section) are used. If the 

bridge section is used, the difference between 

pre-flood and flood cross sections will also 

include contraction scour. 

SHORT-TERM SCOUR AND FILL 

Short-term scour and fill is the change in bed 

elevation in a reach of river; it is caused by the 

temporary storage and transport of sediments 

by the river at different stages of the stream’s 

hydrograph. Depending on the shape of the 

rising and falling limbs of a hydrograph, the 

short-term changes in bed elevation may not 

return to the long-term equilibrium condition at 

the end of a single event; returning to this 

equilibrium condition may take more time, perhaps several events. Some streams may never 

achieve a single equilibrium elevation; however, over a long period of time the specific gage plot 

should have a horizontal trend indicating no long-term changes (figure 2). Because prediction of 

short-term scour is difficult, if not impossible, and because the contraction and local scour may 

override the short-term scour at a structure, bridge scour design guidance has not included this in 

computations for depth of scour, except as the movement of bed forms.(6)
5 

PLAN VIEW

PROFILE VIEW

FLOW

CONTRACTION SCOUR

REFERENCE

SURFACE

PLAN VIEW

PROFILE VIEW

FLOW

CONTRACTION SCOUR

REFERENCE

SURFACE

Figure 2. Example of short-term scour and 

fill with no long-term changes. 

Figure 3. Illustration of reference surface for 

contraction scour. 

If the river is not undergoing long-term changes in streambed elevation, the difference between 

uncontracted cross-sectional geometry measured before and during the flood provides an 

estimate of the short-term scour at that location; however, if long-term changes in streambed 

elevation are occurring on the river, this same measurement would include these long-term 

changes. These long-term changes cannot be easily removed from the data because long-term 

geomorphic changes often occur catastrophically at thresholds and not gradually at a uniform 

rate. A typical value of short-term scour could be estimated from the scatter of the specific gage 

plot around the long-term trend line (figures 1 and 2). 

 

GENERAL SCOUR 

General scour is a combination of short- and long-term scour. General scour at a specific period 

in time can be measured by determining the difference in bed elevation between pre-flood and 

flood measurements of uncontracted cross sections; however, measurements of uncontracted 

cross sections during floods are rarely available. Contracted sections should not be used because 

the scour measurements based on these sections will include contraction scour, in addition to the 

short- and long-term scour components. 
6 

CONTRACTION SCOUR 

Contraction scour results from the contraction of 

the normal flow by natural contractions or 

manmade contractions (such as highway 

embankments and bridge piers); it is the difference 

in bed elevation between contracted and 

uncontracted cross sections (figure 3). Contraction 

scour does not necessarily occur evenly over the 

entire cross section; however, it usually is defined 

as the average difference between the contracted 

and uncontracted sections (figure 4). Contractions 

can be both lateral and vertical. A vertical 

contraction would occur when a bridge enters a 

pressure flow condition. In this report, contraction 

scour will refer to only lateral contractions, and the 

term pressure scour will be used to refer to the depth of scour caused by a vertical contraction. 

 

Blodgett reported one of the few field investigations in which contraction scour was quantified 

separately from aggradation and degradation and short-term scour and fill.(26) Blodgett’s 

contraction scour reference surface was represented by a straight line projected over the 

contracted section from the thalweg (minimum elevation) profile upstream and downstream from 

the contracted area. The contraction scour was reported as the difference between this reference 

surface and the thalweg of the contracted sections; Blodgett stated that using the thalweg to 

measure contraction scour represented a worst-case condition. Natural pool and riffle sequences 

must be accounted for when using the thalweg as a reference surface or the results could be 

misleading. Differences between thalweg elevations may not be consistent with published 

contraction scour equations, which are based on average changes in the contracted section 

required to achieve equilibrium sediment transport. Consequently, these equations cannot be 

used to compute the lateral distribution of contraction scour.(21) 

The reference surface should characterize the mean bed elevation of an uncontracted section near 

the location of the contraction scour measurement. The reference surface is established by 

passing a surface through the average elevation of uncontracted cross sections located upstream 

and downstream from the contracted section.(25) For live bed conditions, the contracted and 

uncontracted cross sections should ideally be measured concurrently to eliminate effects from 

aggradation, degradation, and short-term scour. The effects of local scour on the average 

elevation in the contracted section are removed by excluding the locally scoured areas from the 

average. The cross section should be subdivided into subareas, based on the mode of transport 

(live bed transport or clear water conditions) and the scour in each subarea computed using the 

proper equation for the mode of transport. 

Landers and Mueller acknowledged several potential problems with this ideal reference surface 

for live bed conditions:(25)

RANGE

ELEVATION

UNCONTRACTED SECTION

CONTRACTED SECTION

Figure 4. Illustration of nonuniformly 

distributed contraction scour. 
7 

LONGITUDINAL DISTANCE

ELEVATION

FLOW

REFERENCE SURFACE

DEPOSITION CONTRACTION 

SCOUR

LONGITUDINAL DISTANCE

ELEVATION

FLOW

REFERENCE SURFACE

DEPOSITION CONTRACTION 

SCOUR

Figure 5. Illustration of a reference surface for clear 

water contraction scour with material deposited 

immediately downstream of the scour hole. 

1. Identification of the bottom width over which active bed-load transport occurs is 

often difficult because of irregular cross section geometry. 

2. Upstream and downstream cross sections may be in natural contractions or 

expansions because of channel bends or other factors, so they do not represent an 

uncontracted section at the bridge. Alternating pool and riffle sequences in some 

high-gradient streams can also present problems in establishing an uncontracted 

reference surface at the bridge. 

3. Large dune bed forms can produce misleading results when dune crests or troughs 

predominate in one of the measured sections. 

4. Slow downstream migration of large sand and gravel bars can make a measurement 

nonrepresentative of equilibrium conditions. 

5. Measured contraction scour may not represent equilibrium scour if the scour develops 

over many years because of the infrequency of channel-formative flows and the 

resistance of the bed to scour. 

6. Most flood-flow scour measurements are made only from the bridge deck along the 

upstream and downstream sides of the bridge because boats are usually unavailable to 

obtain concurrent uncontracted channel geometry. 

Pre-flood and (or) post-flood 

measurements of uncontracted 

sections often are used to establish a 

contraction scour reference surface 

because of the difficulty in obtaining 

measurements of the uncontracted 

sections during a flood. This 

reference surface is valid for clear 

water conditions but may not isolate 

contraction scour from short-term 

scour and degradation or aggradation 

for live bed conditions. Pre- and postflood measurements may be useful 

for live bed contraction scour 

measurements if data are sufficient to 

support the assumption of stable 

approach and exit sections; however, 

the stability of the uncontracted sections and the accuracy of the measurement should be assessed 

by comparing pre- and post-flood measurements throughout the study reach or for multiple 

floods at a specific location. 

For clear water contraction scour, collection of real-time bathymetric data are not necessary. The 

bed upstream of the bridge and the scour hole will maintain the same geometry after the passage 
8 

of the flood because no sediment is being transported into the scour hole. Therefore, post-flood 

surveys can be used to measure clear water contraction scour. The post-flood survey must extend 

far enough downstream to end beyond the effect of the material that was scoured and deposited 

downstream (figure 5). 

LOCAL SCOUR 

Local scour near an obstruction is caused by the vortexes and flow acceleration resulting from 

flow striking the obstruction and moving past it. The depth of local scour is the difference 

between where the bed would be if the obstruction were not present and where the bed is with 

the obstruction in place. The concurrent ambient bed surface is the preferred reference for local 

scour determinations (figure 6). The concurrent ambient bed surface is determined from data 

collected at the time the scour hole was measured (concurrent with the scour data). This 

reference surface is typically taken as the average of several points measured in the vicinity of 

the obstruction but beyond the limits of the local scour hole. 

Figure 6. Illustration of reference surface sketched 

on a cross section plot. 
9 

The concept and description of the concurrent ambient bed are simple; however, a representative 

concurrent ambient bed surface is not always apparent given the range of channel geometries and 

data limitations. Establishing local scour reference surfaces can be difficult and requires 

considerable judgment for complex cross section geometries, such as a site where the thalweg 

coincides with the local scour hole. Additional factors that must be evaluated in measuring local 

scour include remnant scour holes, debris, scour countermeasures, time-rate of scour, and dune 

bed forms. Determining a representative and repeatable reference surface for local scour can be 

difficult.(25) A good method for determining the concurrent ambient bed surface is to use a 

section of the river that has the same general shape as the bridge section (such as the approach, 

exit, or preconstruction sections), and move that section vertically until it best fits the bridge 

section, ignoring the scour holes. The local scour is the difference between the concurrent 

ambient bed surface and the lowest elevation in the scour hole. Another method is to sketch in 

the concurrent ambient bed surface (figure 6). Both methods are subjective and may yield 

different answers when applied by different people to the same cross section. 

The concurrent ambient- or mean-equilibrium (for dunes) bed surface has been used as the 

reference surface in most flume studies of local scour. (See references 9, 27, 28, 29, and 30.) 

Other reference surfaces used in flume studies include the initial-condition bed surface and the 

water surface. Field measurements of local scour have generally used the concurrent ambient bed 

surface as a reference (see references 16, 31, 32, and 33); however, Inglis(34) used the water 

surface as the reference surface, and Jarrett and Boyle(35) used the highest observed bed elevation 

at the point where local scour was being measured. Other commonly used references that are not 

valid for isolating the local scour component include: 

• The average bed elevation that includes the local scour hole(s). 

• The pre- or post-flood cross sections because the piers and corresponding local scour 

holes may be present in these cross sections. 

• The preconstruction cross section on the bridge plans because this cross section does 

not reflect the long-term, short-term, and contraction scour that may have occurred. 

Local scour measurements based on reference surfaces other than concurrent ambient bed 

surface may include components of contraction scour, short-term scour, or aggradation or 

degradation. Total scour measurements recorded with these types of reference surfaces cannot be 

effectively analyzed in relation to separate local, contraction, and sediment-supply processes; 

therefore, the concurrent ambient bed surface should be used to determine local scour depths 

from the channel geometry.(25)

11 

CHAPTER 3: DESCRIPTION OF FIELD METHODS 

GENERAL 

The type of data collected at a bridge is determined by the scour processes being studied and the 

equipment’s capability to collect the desired data. The type of scour being studied (pier, 

abutment, or contraction) determines which variables need to be measured and the spatial 

distribution of these data. For local pier scour, measurement of the approach velocity, bed 

material properties, and cross-sectional data to define the reference surface and maximum depth 

of scour may be sufficient. For abutment and contraction scour, hydraulic conditions in the 

approach section are important. Unfortunately, the site configuration may limit the data that can 

be collected accurately and safely. Floodplains commonly are covered by trees and other woody 

vegetation, which makes measurement of the velocity and flow distribution virtually impossible. 

Debris and ice accumulations may prevent measuring streambed elevations in key areas. On 

small streams, the duration of the flood may not allow sufficient time to collect all of the desired 

data; all desired data are rarely collected at a site. 

The USGS has defined two classes of field data: limited-detailed and detailed.(21) In general, 

limited-detailed data can be collected from the bridge deck, and the spatial extent of the data is 

restricted to the upstream and downstream edges of the bridge. Detailed data require the 

deployment of equipment from a boat to collect data over an area extending beyond the 

hydraulic effect of the bridge. Most available data are from limited-detail data collection efforts 

sponsored by State highway departments. The FHWA-sponsored national bridge scour studies 

are responsible for most of the detailed data sets that have been collected. 

A brief summary of these types of data is presented here. For a more detailed discussion of 

limited-detail data collection see Landers and Mueller,(21) Jarrett and Boyle,(35) and Mueller and 

Landers.(36) A comprehensive presentation of detailed data collection and the necessary 

equipment can be found in Landers and Mueller,(21) Mueller and Landers,(36) and Mueller.(5)

LIMITED-DETAIL DATA 

Limited-detail data primarily are collected to evaluate published equations, investigate the 

relations between local scour and explanatory variables, and develop envelope curves for the 

maximum observed scour. Limited-detail data sets should include the following data:(21) 

• Water discharge. 

• Water-surface elevations at the bridge. 

• Cross section data along the upstream and downstream edges of the bridge. 

• Cross sections, approximately one bridge width upstream and downstream of the 

bridge (it is desirable to measure this during the flood, but low-water approach and 

exit sections are usually acceptable). 

• Approach flow velocity for each pier location. 
12 

• Bed material samples (it is desirable to collect these during a flood, but low-water 

samples are usually acceptable). 

• Notes on debris accumulations, surface currents, roughness, and vegetation. 

• Photographs of the bridge and stream reaches upstream and downstream. 

• Water temperature. 

• Bridge and pier geometry. 

• Soil boring logs for the bridge crossing. 

Rantz et al.(37) and Landers and Mueller(21) describe equipment and procedures for discharge and 

approach velocity measurements. Bed material samples can be collected using the equipment and 

procedures described in Landers and Mueller, Mueller and Landers, Edwards and Glysson, 

Ashmore et al., Yuzyk, and the International Organization for Standardization (1992). (See 

references 21, 36, 38, 39, 40, and 41.) 

DETAILED DATA 

Detailed data sets are similar to limited-detail data except the data are denser and their spatial 

extent broader. Ideally, detailed data sets include real-time measurements of hydraulic and 

channel-geometry data at several times during the flood hydrograph. Data are collected both 

upstream and downstream in an area that extends just beyond the hydraulic effect of the bridge. 

The extent of the hydraulic effect of the bridge both upstream and downstream depends on the 

degree of contraction, the size and configuration of the floodplains and approach embankments, 

and the slope of the stream. The goal is to collect data at least 10 bridge widths upstream and 

downstream, but this may be adjusted in the field as site conditions dictate. Detailed data sets 

allow distinction between local, contraction, and general scour occurring at the highway 

crossing, and are needed to advance the understanding of complex bridge scour processes. 

Detailed data sets should include the following: 

• Water-discharge hydrograph. 

• Water-surface elevation hydrograph. 

• Water-surface slope. 

• Detailed channel-geometry data at and near the bridge. 

• Channel geometry in the river reach upstream and downstream of the bridge. 

• Flow velocities (magnitude and direction) in the entire study reach. 

• Bed material samples. 

• Suspended-load and bed-load measurements (if possible). 

• Notes on surface currents, channel roughness, and vegetation. 

• Approximate measurements of debris piles present. 
13 

• Photographs of the bridge and stream reaches upstream and downstream. 

• Water temperature. 

• Bridge and pier geometry. 

• Soil boring logs for the bridge crossing. 

The procedures and equipment for making discharge measurements and collecting suspendedload and bed material samples are the same as those used for the limited-detail measurements. 

The spatial extent of the bathymetric and velocity data is much greater for detailed data sets than 

for limited-detail data sets; therefore, instruments must be deployed from the water surface rather 

than from the bridge deck. It is also highly desirable to measure the direction of flow in addition 

to the velocity magnitude. 

15 

CHAPTER 4: ENHANCEMENTS TO THE BRIDGE SCOUR DATA 

MANAGEMENT SYSTEM 

The Bridge Scour Data Management System (BSDMS) was developed to support the 

preparation, compilation, and analysis of bridge scour data.(22) Bridge scour data are stored in the 

BSDMS as data sets that are defined for each bridge scour site in the database. Each data set 

contains more than 200 site and measurement attributes of the channel geometry, flow 

hydraulics, hydrology, sediment, geomorphic setting, location, and bridge specifications. The 

BSDMS provides interactive storage, retrieval, selection, editing, and display of bridge scour 

data sets; it was originally programmed in FORTRAN and was available in both DOS and Unix®

versions. 

Changes to the structure of the database were difficult, which restricted adapting the database to 

different types of data and adding new parameters as research evolved on scour at contracted 

bridge openings. The capabilities of the BSDMS for extracting data from the database and 

putting the data into tables for subsequent analysis were also limited. It was determined that the 

BSDMS would be more efficient and useful if it were ported to a commonly used database 

engine, such as Microsoft® Access, rather than to try to adapt the FORTRAN code to meet the 

changing requirements of the BSDMS. Microsoft Access provides a much more flexible 

database structure with user-friendly methods for querying and extracting data. The use of 

Microsoft Access will also allow future versions of the BSDMS to be served on the World Wide 

Web. 

The enhancement of the BSDMS and the serving of the data on the Internet are an ongoing effort 

that has received subsequent funding from the National Cooperative Highway Research Program 

(NCHRP) Project 24–14. As of 2000, all data from the original BSDMS had been ported to 

Microsoft Access and the transferred data checked for any errors in conversion. Most variables 

of the original BSDMS have been retained and some new variables added to better characterize 

bed material and contraction and abutment scour. Some features, such as entry of hydrographs 

and cross sections and all graphics capabilities, currently are not supported directly by the 

database. Hydrographs and cross sections can be stored as individual files and referenced in the 

files section of the database. This simplifies the database, and the files section provides the 

capability of including many different types of data, such as Acoustic Doppler Current Profiler 

(ADCP) data, xyz topography data, digital photographs, scanned bridge plans, maps, and satellite 

imagery. Data are easily extracted into tables so that the user may construct ASCII data tables 

and import those tables into the graphics or statistical software of their choice. 

Corrections and enhancements to existing data were made during the data transfer and validation 

process. The data from Alaska, reported in Norman, provided only the D90 and D50 grain sizes.(16)

The data set in the original BSDMS included D90 as D95 and noted this in the comments; this has 

been changed in the new BSDMS. The D95, D84, and D16 are computed from the data provided. 

The D84 was interpolated from the D90 and D50 by use of a log-probability interpolation. The 

gradation parameter (σ ) was computed as D84/D50 (although this parameter could also be 

computed as D50/D16 or (D84/D16)

0.5), and the D95 and D16 were computed from the following 

equation: 
16 

 (1) 

where 

Di is the grain size of which i percent is finer; 

D50 is the median grain size of the bed material; and 

ki is the standard normal deviate of i, defined as follows: 

Di Ki

 D10 -1.282 

 D16 -0.994 

 D35 -0.385 

 D50 0 

 D65 +0.385 

 D84 +0.994 

 D90 +1.282 

 D95 +1.645 

 D99.99 +3.72 

The limit on the number of bed material samples that could be entered for a given site in the old 

BSDMS did not allow full presentation of the data for sites in Ohio. The new BSDMS does not 

have this restriction, and all the bed material data are included from a very extensive sampling 

program used in the Ohio bridge scour study.(42) The database now contains 557 bed material 

samples collected at bridge scour study sites. 

New sites and additional data at existing sites were also added to the database. Landers et al. 

reported 384 local pier scour measurements at 56 bridges located in 14 States.(22) This has been 

expanded to 493 local pier scour measurements, 18 contraction scour measurements, and 12 

abutment scour measurements. The database now contains data from 79 sites located in 17 

States. Some of the contraction and abutment scour data sets are not complete, but they represent 

the best available information and should prove useful to other researchers working on abutment 

and contraction scour. 
17 

CHAPTER 5: LOCAL SCOUR AT PIERS 

GENERAL 

Laboratory research has dominated the field of local scour at bridge piers. Such research is 

limited by the range of hydraulic conditions typically tested and is conducted primarily under 

steady-flow conditions with uniform bed material. Relations and predictive equations developed 

from laboratory research have not been adequately verified by use of field data. This report 

compares the results of predictive equations to field measurements of local pier scour. Relations 

developed in the laboratory are evaluated for application to conditions typical of natural streams. 

The effects of nonuniform bed material and other explanatory variables on field measurements of 

scour are investigated by use of a partial residual analysis. 

A complete evaluation of all equations for the prediction of local scour around bridge piers is 

beyond the scope of this investigation; however, 26 commonly cited equations are compared 

with field measurements of scour to evaluate their potential for use as design equations. A design 

equation should accurately predict scour; however, predicting sediment transport and scour 

accurately is very difficult. If a design equation predicts too little scour, the bridge could be 

under-designed and the traveling public put at risk. A good design equation should be as 

accurate as possible, but when in error the equation should overpredict scour to ensure that the 

design is always safe. 

Results from laboratory investigations of the effects of velocity, approach flow depth, pier width, 

and bed material properties on the depth of local pier scour are compared with similar plots of 

field data. Laboratory research has shown a relation between relative depth of scour (ys/b) and 

flow intensity (Vo/Vc) for ripple- and nonripple-forming sediments and for uniform and 

nonuniform sediments. (See references 29, 43, 44, and 45.) Melville and Sutherland investigated 

the effect of relative flow depth (yo/b) on relative depth of scour (ys/b).(2) The effect of relative 

sediment size (b/D50) on relative scour depth for relative sediment sizes ranging from 2 to 1,000 

has also been studied in the laboratory.(43,46,47) Nonuniform sediments consistently have resulted 

in less relative scour than similar uniform sediments used in laboratory investigations. (See 

references 45, 46, 47, and 48.) The applicability of these relations and the range of the laboratory 

data are evaluated by plotting field data with the same combination of variables as the laboratory 

data. 

Analysis of bridge scour field data is more complicated than analysis of laboratory data because 

in the field all explanatory variables have the potential to vary at the same time. In the 

laboratory, all explanatory variables can be held constant and a specific variable systematically 

changed to study its effect. These controlled laboratory investigations, however, may not 

adequately describe the variability and interaction of variables present in natural conditions. In 

the field, all variables can change and interact; the effect of individual variables cannot be easily 

isolated. Although variables and dimensionless parameters from field data can be compared with 

laboratory data, the effect of all variables is present in the field data. For example, in a 

comparison of the effects of velocity, the field data will also include the effects of flow depth, 

bed material properties, pier shape, and pier size. Most of the laboratory relations account for 

pier size by plotting normalized scour depth (ys/b); however, if the pier size does not affect scour 
18 

linearly, then the relation has been distorted by the selection of the parameters. 

Bed material size may affect local scour through its effect on bed forms and on the energy 

required to transport grains from the scour hole. Initially, the energy available to erode bed 

material is higher at the pier than in the approach section. As erosion occurs at the pier, the scour 

hole forms and deepens. The energy available to transport sediment decreases as the scour hole 

deepens until equilibrium with upstream transport is achieved. Insufficient energy to transport a 

larger particle should be achieved at a shallower depth of scour than the depth needed for a 

smaller particle; thus, sediment size affects the depth of scour. Laboratory research is typically 

conducted at a constant Vo/Vc ratio: as the bed material size increases, so does the approach 

velocity. These laboratory conditions are not representative of most field conditions. The effect 

of bed material properties on the depth of scour is evaluated by comparing subsets of the field 

data that represent conditions similar to those modeled in the laboratory with the results of 

laboratory investigations. In addition, an analysis of the residuals generated by comparing the 

computed scour from the HEC–18 equation with field data is used to evaluate the importance 

and relation of bed material properties on the maximum depth of scour. Furthermore, because 

laboratory research has shown a significant effect of bed forms on the depth of scour in rippleforming sediments, the difference in the depth of scour in ripple- and nonripple-forming 

sediments is evaluated using field data. 

 

The depth of scour in nonuniform sediments is often less than the depth of scour in uniform 

sediments. In uniform sediments, the energy is sufficient to transport the material (live bed) or it 

is insufficient to transport the material (clear water). In nonuniform sediments, the energy of the 

flow may only be sufficient to transport some material, allowing the coarser material to armor 

the bed. Armoring can occur in both the approach section (reducing the sediment transport to the 

scour hole) and in the scour hole (limiting the depth of the hole). The combination of armoring 

that occurs depends on the energy available for transport in the approach, the energy available 

for transport at the pier, and the gradation of the bed material. If the armoring occurs in the 

approach and no armoring occurs in the scour hole, the scour can be deeper than for identical 

conditions in uniform sediments because the armoring of the approach has reduced the sediment 

supply to the scour hole. If armoring of the scour hole occurs, the depth of scour is likely to be 

less than that for uniform sediments regardless of the transport condition at the approach (live 

bed or clear water). The traditional classification of live bed and clear water conditions is 

insufficient to describe the conditions that may occur in nonuniform bed material. 

SUMMARY OF FIELD DATA 

The 493 local pier scour measurements currently available in the BSDMS were filtered to ensure 

that the data used for this report met basic criteria necessary to complete the objectives of this 

analysis (for a complete listing of the data, see appendix A). The data collection techniques 

typically limited the data to cross sections along the upstream and downstream edges of the 

bridge. For a pier aligned with the flow, maximum scour typically occurs at the nose of the pier; 

therefore, the data collection method previously described measures maximum scour. 

Laboratory research indicates that for a pier skewed to the flow, maximum scour can occur along 

the sides of the pier rather than at the nose.(8) Because data were seldom collected along the sides 
19 

of the piers, all measurements where the flow was not aligned with the pier were removed from 

the data set. Where there are measurements along the upstream and downstream edges of the 

bridge, only the maximum depth of scour is used. Debris accumulations on the piers have an 

unknown effect on local scour and often make measurement of maximum scour impossible. The 

effect of debris on the depth of scour (unknown, insignificant, moderate, substantial) was noted 

for each measurement. All measurements where the effect of debris on the depth of scour was 

rated “substantial” were removed from the data set. The time required for scour to reach its 

maximum depth in cohesive material is considerably longer than in noncohesive material.(6)

Therefore, observations with scour in cohesive material also were removed from this analysis. 

The hydraulic parameters measured should be the conditions that caused the measured depth of 

scour. It is difficult to exactly associate hydraulics with a depth of scour because of the temporal 

development of the scour hole. Except at a few sites, the temporal development of the scour 

holes reported in BSDMS is not available. It was rationalized that if the scour hole can be 

reasonably associated with the reported hydraulic conditions, the velocity at the pier must be 

competent to erode the bed material. Gao et al. published the following equation to compute the 

critical approach velocity that results in transport of the bed material at the pier based on the 

critical velocity for incipient transport of the bed material:(3) 

(2) 

 where 

V′c is the approach velocity corresponding to critical velocity and incipient scour in the 

accelerated flow region at the pier; 

D50 is the mean grain size of the bed material; 

b is the pier width; and 

Vc is the critical (incipient-transport) velocity for the D50-size particle. 

 

Equation 2 was used with Neill’s formulation of the critical velocity equation: (23) 

 (3) 

where 

 θ is the Shield’s parameter; 

Ku is 1.0 for SI units and 1.81 for customary English units; 

 yo is the depth of flow; and 

D50 is the median grain size. 

to compute the critical approach velocity (V′c) for transport of the D50 grain size at the pier. All 

measurements having an approach velocity (Vo) less than the critical approach velocity for 

transport at the pier (V′c) were removed from the data set. 

The appropriate value for the Shield’s parameter, θ, has been a topic of considerable research 

and discussion, with no conclusive answer. Miller et al.(49) and Buffington and Montgomery(50)
20 

compiled and analyzed all available data on incipient sediment transport. Both investigations 

found scatter in the data caused by inconsistencies in the definition of incipient motion, the 

experimental method utilized, the experimental facility used, and the type of bed material used. 

According to these studies, the Shield’s parameter may vary from 0.02 to 0.086 with a common 

average value for gravel of about 0.046.(50) Miller et al.(49) presented a method based on Inman(51)

that relates grain size to shear velocity; this method is only valid for water at a temperature of 

20 °C and for bed material with a specific gravity of 2.65. The method was presented 

graphically, but has been reduced to equations for the Shield’s parameter by Mueller.(5) 

 

 for D50 < 0.0009 (meter) (4) 

 for 0.0009 (meter) < D50 < 0.020 (meter) (5) 

 for D50 > 0.020 (meter) (6) 

The method is easily applied, provides for variation in the Shield’s parameter for smaller grain 

sizes, and is within the range of variation defined by previous research; therefore, this method 

was used to evaluate the Shield’s parameter needed to estimate the critical velocity for incipient 

sediment transport. 

Of the 493 pier scour measurements in the BSDMS, 266 were selected for this analysis. The 

criterion that flow be aligned with the pier was satisfied by 322 measurements; 476 

measurements satisfied the criterion that debris not be a substantial effect; 488 measurements 

were in noncohesive material; and 437 measurements satisfied the criterion that the velocity at 

the pier has to be competent to erode the median particle size of the bed material. Where there 

were measurements both at the upstream and downstream edges of the bridge, only a single 

measurement representing the maximum depth of scour was included; thus, 41 measurements 

were excluded. All criteria were satisfied by 266 measurements, which represent 106 different 

piers at 53 bridges located in 15 States. 

A summary of the selected data and commonly used dimensionless variables is provided in table 

2. The maximum and minimum values of the data and of the dimensionless variables represent a 

range equal to or greater than most laboratory investigations. Unlike laboratory investigations, 

the distribution of the data cannot be precisely controlled in the field, and the data tend to be 

grouped near the low end of most of the primary variables.
21 

Table 2. Summary statistics for selected pier scour measurements. 

Variable Units 

Number 

of 

Points Minimum

25th

Quartile Median 

75th

Quartile Maximum Mean 

Standa

Deviati

Depth of scour (ys) m 266 0.00 0.43 0.61 1.19 7.65 1.06 Pier width (b) m 266 0.38 0.88 1.13 1.52 5.52 1.53 

Approach velocity (Vo) m 266 0.18 0.66 1.14 1.89 4.48 1.36 

Approach depth (yo) m 266 0.12 2.45 4.39 6.45 20.03 4.86 D16 mm 262 0.03 0.20 0.35 3.78 68.00 5.48 1

D50 mm 266 0.15 0.48 0.74 8.00 108.00 10.61 2D84 mm 262 0.26 1.30 2.35 29.00 233.00 22.36 3D95 mm 262 0.28 2.08 7.45 44.00 350.00 34.87 5Gradation coefficient (σg ) – 262 1.20 2.03 2.30 3.65 21.80 3.21 

Drainage area (DA) km2 192 166 1197 3680 9402 1805222 32706 135

Slope (S) m/m 219 0.00010 0.00016 0.00050 0.00105 0.00500 0.00090 0.00yo / b – 266 0.12 1.87 3.05 5.31 13.84 3.91 

b / D50 – 266 8.47 129.54 1024.54 1828.80 14224.00 2219.03 340

ys / b – 266 0.00 0.40 0.58 0.88 2.09 0.68 

Vo / Vc – 266 0.43 0.75 1.14 1.53 4.92 1.32 

Vo / (gyo)0.5 – 266 0.04 0.11 0.17 0.33 0.83 0.23 
22 

EVALUATION OF PUBLISHED EQUATIONS 

Discussion of Equations 

Local pier scour has been a popular topic of study for many laboratory researchers. A literature 

review by McIntosh found that more than 35 equations had been proposed for predicting the 

depth of scour at a bridge pier.(52) Most local scour equations are based on research in laboratory 

flumes with noncohesive, uniform bed material and limited verification of results with field 

data.(52) In evaluating and applying scour-prediction equations, it is valuable to know the 

limitations of the equations, the conditions for which they were developed, how the underlying 

data were interpreted, and the methods used to develop the equations. Such information about 

each equation has been previously published in Landers and Mueller,(21) Mueller,(5) and 

Pritsivelis.(53) The equation, published reference, and the equation name for the 26 equations 

used in this report are presented in table 3. 

The equations presented here were developed to predict the maximum depth of scour; there are 

three approaches to developing such an equation. The first is to predict the maximum depth of 

scour that could occur at the bridge pier under any conditions. The second is to predict, as 

accurately as possible, the maximum depth of scour for a given set of hydraulic and bed material 

conditions. These equations often are developed by multiple regression analysis and, by 

definition, underpredict the depth of scour for about one-half of the observations used in the 

equation development. The third approach is to develop a design equation. A good design 

equation should accurately predict the depth of scour for a given set of site and flood conditions, 

but when in error should always err by predicting too much scour. 

Analysis of how each equation addresses pier width, approach velocity, approach depth, and bed 

material properties provides an indication of the effect of these variables on the depth of scour. 

The selected equations are formulated into two patterns. The regime equations are written in the 

form: 

 (7) 

and compute the total depth of flow including local scour. Nonregime equations are written in 

the form: 

 (8) 

and compute the depth of local scour only. The pier width is included in more than 75 percent of 

the equations (table 4). The regime equations have an exponent on pier width between 0.2 and 

0.25. The exponent on pier width ranges from 0.6 to 0.75 in more than one-half of the nonregime 

equations when the pier width could be isolated. The smaller exponents on pier width for the 

regime equations are justified because pier width should have less effect on the total depth than 

on the depth of local pier scour. The exponents on approach velocity range from 0.2 to 0.68 and 

on approach depth from 0.135 to 0.8. This variability indicates that there is a lack of agreement 

among the equations on the effect of approach depth and velocity on the depth of scour. The 
23 

median grain size is only included in 11 equations; it can only be isolated in 4 equations, where it 

has a small negative exponent. 

Table 3. Summary of selected local pier scour equations.* 

Name and 

Reference 

Number 

Equation Equation 

Number 

Ahmad(54) (9) 

Arkansas(55)** (10) 

Blench-Inglis 

I

(56)** (11) 

Blench-Inglis 

II(56)*** (12) 

Breusers(57) (13) 

BreusersHancu(53) (14) 

Chitale(58) (15) 

Froehlich(13) (16) 

Froehlich 

Design(13) (17) 

HEC-18(59) (18) 

HEC-18-

K4(6) (19) 

HEC-18-

K4Mo(1) (20) 
24 

Table 3. Summary of selected local pier scour equations, continued. 

Name and 

Reference 

Number 

Equation Equation 

Number 

HEC-18-

K4Mu(5) (21)

Inglis-Poona 

I

(34)** (22)

Inglis-Poona 

II(34) (23)

Larras(4)** (24)

Laursen I(60) (25)

Laursen II(61) (26)

LaursenCallander(62) (27)

Melville and 

Sutherland(2) (28)

Mississippi(63) (29)

Molinas(1) (30)

Shen(9)** (31)

Shen-Maza(9) (32)

Sheppard(64)

as reported 

by 

Alkhalidi(65) 

(33)
25 

Table 3. Summary of selected local pier scour equations, continued. 

Name and 

Reference 

Number 

Equation Equation 

Number 

c2 is a coefficient defined by Sheppard as 

where: tanh is the hyperbolic tangent function 

(34)

c3 is a coefficient defined by Sheppard as 

 

(35)

k is a coefficient defined by Sheppard as 

(36)

Simplified 

Chinese(3) (37)

*

See List of Symbols in the front of the report for definition of symbols not defined in the table. **Units are English units in feet. 

***Units are English units in feet, except D50, which is in millimeters. 
26 

Table 4. Summary of exponents for variables used in selected equations. 

Equation 

Pier 

Width

Approach 

Velocity 

Approach 

Depth D50

Other 

Bed 

Material 

Ahmad – 0.667 0.667 – – 

Arkansas – 0.684 – -0.117 – 

Blench-Inglis I* 0.25 – 0.75 – – 

Blench-Inglis II* 0.25 0.5 0.5 -0.125 – 

Breusers 1.0 – – – – 

Breusers-Hancu X – X – – 

Chitale – X X – – 

Froehlich 0.62 0.2 0.36 -0.08 – 

Froehlich Design X 0.2 0.36 -0.08 – 

HEC-18 0.65 0.43 0.135 – – 

HEC-18-K4 X X X X X 

HEC-18-K4Mo X X X X X

HEC-18-K4Mu X X X X X

Inglis-Poona I* 0.22 0.52 X – – 

Inglis-Poona II* 0.22 – X – – 

Larras 0.75 – – – – 

Laursen I 0.7 – 0.3 – – 

Laursen II X – X – – 

Laursen-Callander 0.5 – 0.5 – – 

Melville and Sutherland X X X X X 

Mississippi 0.6 – 0.4 – – 

Molinas 0.66 X X X X 

Shen 0.62 0.62 – – – 

Shen-Maza Fp<0.2 – 2.0 – – – 

Shen-Maza Fp>0.2 0.67 0.67 – – – 

Sheppard X X X X – 

Simplified Chinese X X X X – *

Regime equation that in its original form computed total depth including pier scour and approach depth (see equation 7). 

X—Equation uses this variable, but the equation is complex, and this variable cannot be algebraically isolated. 
27 

Assumptions 

Laboratory experiments are designed to isolate specific scour processes; thus, the resulting 

equations may not account for complex and dynamic field conditions. Some field conditions that 

effect scour are undefined in the selected equations and assumptions are required to apply the 

equations. The flow in the field is assumed to be steady state and uniform to allow the 

application of laboratory-based equations to predict scour at bridges. All equations presented 

here are used to estimate scour for both live bed and clear water conditions. 

Many equations do not include corrections for pier shape, or they include corrections for only a 

few pier shapes. Pile groups are classified as round-nose or circular piers for equations that do 

not specify a shape correction for pile groups.(59) However, pile groups are treated as cylinders 

for the Simplified Chinese equation. Blench-Inglis I and II, Inglis-Poona I and II, Chitale, 

Ahmad, Shen, Arkansas, Breusers, and Mississippi equations do not include procedures to 

correct for pier shape, and corrections were not applied in this evaluation. Because the Larras 

equation specifies only square-nose and circular pier shapes, sharp-nose piers and pile groups are 

classified as circular piers in this evaluation of the Larras equation. 

When it was possible, each equation was used to compute a depth pier scour for each field 

measurement. Several sites did not have adequate bed material data. The D90 grain size was 

interpolated from the D84 and D95 grain sizes using log-probability interpolation. The D99 was 

estimated by extrapolating the D84 and D95 values in log-probability space. If neither D84 nor D95

were reported, equations requiring D90 and D99 were not applied. A water temperature of 20 °C 

was assumed if no temperature was reported. 

Comparison of Predictions with Field Data 

This evaluation of the selected equations focuses primarily on the ability of the equations to be 

used as design equations for different site and flood conditions. The objective is to find an 

equation that accurately predicts the depth of scour for the specified conditions, but that 

overestimates the depth of scour when in error. The potential causes for significant inaccuracies 

or under-predictions are also investigated. 

The ability of the equations to accurately predict the depth of scour for the variety of field 

conditions represented in this data set varies greatly (figure 7). Some equations (Ahmad, 

Breusers-Hancu, Chitale, Inglis-Poona I, Melville and Sutherland, and Shen-Maza) show trends 

away from the line of equality, indicating that those equations do not properly represent the 

processes responsible for local pier scour that occur in the field. Several equations (Arkansas, 

Blench-Inglis I, Blench-Inglis II, Froehlich, Shen, and Simplified Chinese) underpredict the 

depth of scour for a significant number of the observations and are not good candidates for 

design equations. The other equations have some trend along the line of equality with few 

underpredictions, but they display a broad scatter of data and often do not accurately predict the 

observed scour. 
28 

(A) AHMAD EQUATION (B) ARKANSAS EQUATION 

(C) BLENCH-INGLIS I EQUATION (D) BLENCH-INGLIS II EQUATION 

(E) BREUSERS EQUATION (F) BREUSERS-HANC EQUATION 

Figure 7. Scatterplots of computed versus observed scour, in meters (m), 

 for selected pier scour equations.
29 

(G) CHITALE EQUATION (H) FROEHLICH EQUATION 

(I) FROEHLICH DESIGN EQUATION (J) HEC-18 EQUATION 

(K) HEC-18-K4-EQUATION (L) HEC-18-K4Mo EQUATION 

Figure 7. Scatterplots of computed versus observed scour, in meters (m), 

for selected pier scour equations, continued. 
30 

(M) HEC-18-K4Mo (>2MM) EQUATION (N) HEC-18-K4Mu EQUATION 

(O) INGLIS-POONA I EQUATION (P) INGLIS-POONA II EQUATION 

(Q) LARRAS EQUATION (R) LAURSEN I EQUATION 

Figure 7. Satterplots of computed versus observed scour, in meters (m), 

for selected pier scour equations, continued.
31 

(S) LAURSEN II EQUATION (T) MELVILLE AND SUTHERLAND EQUATION

(U) MISSISSIPPI EQUATION (V) MOLINAS EQUATION 

(W) SHEN EQUATION (X) SHEN-MAZA EQUATION 

Figure 7. Scatterplots of computed versus observed scour, in meters (m), 

for selected pier scour equations, continued. 
32 

(Y) SHEPPARD EQUATION (Z) SIMPLIFIED CHINESE EQUATION 

Figure 7. Scatterplots of computed versus observed scour, in meters (m), 

for selected pier scour equations, continued. 

Ranking the performance of scour-prediction equations is difficult because of the tradeoff 

between accuracy and underpredictions. If only accuracy is considered, the sum of squared errors 

can be used to evaluate the equations’ performance (table 5). This statistic shows the Froehlich 

equation to be the most accurate; however, the Froehlich is a regression expression and 

underpredicted the depth of scour for 129 of 266 field observations. If the smallest number of 

underpredictions is used to evaluate the equations, the Froehlich Design equation is the best 

equation because it underestimated only four observations. The Froehlich Design equation, 

however, ranked 19th based on the sum of squared errors criteria. The magnitude of the 

underpredictions is just as important, if not more so, than the number of underpredictions; thus, 

the sum of squared errors for those observations that were underpredicted is another factor that 

should be considered. The Melville and Sutherland equation had the lowest sum of squared 

errors for the underpredicted observations, but this equation ranked 26th in overall sum of 

squared errors. The Melville and Sutherland equation did not underestimate scour by much, but 

grossly overestimated scour for many cases (figure 7T). The Froehlich Design, HEC-18-K4, 

HEC-18, HEC-18-K4Mu, and HEC-18-K4Mo (>2 mm) equations all had low sum of squared 

errors for the underpredicted observations. If the all the ranks are totaled, the Froehlich Design 

equation appears to be the top equation, followed by the HEC-18-K4Mu, HEC-18-K4, HEC-18, 

Mississippi, and HEC-18-K4Mo (>2 mm) equations; however, the Froehlich Design equation 

had the largest sum of squared errors for this group. If only the ranks based on the two sum of 

squared error categories are used, the HEC-18-K4Mu equation is favored and the Froehlich 

Design equation drops to a rank of 8.5. No single equation is conclusively better than the rest, 

but the top six equations generally appear to be the Froehlich Design, HEC-18-K4, 

HEC-18-K4Mu, HEC-18-K4Mo (>2 mm), Mississippi, and HEC-18 equations. 
33 

Table 5. Summary of the performance of the selected pier scour equations. 

Number of Underpredictions Summat

Number of SSE Count SSE All Ranks 

Equation Observations Magnitude Rank Number Rank Magnitude Rank Total Ran

Ahmad 266 7536.86 27 61 14 159.48 22 63 23 Arkansas 266 239.52 4 74 20.5 165.61 23 47.5 20 Blench-Inglis I 266 265.83 5 74 20.5 52.14 17 42.5 18 Blench-Inglis II 266 954.55 17 174 27 824.60 27 71 25 Breusers 266 670.40 13 18 9.5 7.14 9 31.5 7.5 Breusers-Hancu 266 1205.60 21 77 22 201.18 25 68 24 Chitale 266 2299.40 25 90 23 169.37 24 72 26 Froehlich 266 160.67 1 129 26 98.24 21 48 21 

Froehlich Design 266 1067.77 19 4 1 1.51 2 22 1 HEC-18 266 822.38 15 13 7 2.16 4 26 4.5 HEC-18-K4 262 791.54 14 15 8 1.93 3 25 3 HEC-18-KMO (All) 266 495.18 11 65 16 17.01 13 40 15.5 HEC-18-KMO (> 2 mm) 266 608.79 12 21 11 2.47 6 29 6 HEC-18-K4Mu 266 448.53 9 18 9.5 2.23 5 23.5 2 Inglis-Poona I 266 1758.81 24 119 25 597.74 26 75 27 Inglis-Poona II 266 229.68 3 72 19 45.67 16 38 12 Larras 266 311.13 7 48 13 72.09 20 40 15.5 Laursen I 266 1277.71 23 6 2 5.20 8 33 10 Laursen II 266 930.57 16 9 3.5 10.95 12 31.5 7.5 Laursen-Callander 266 960.55 18 9 3.5 10.39 11 32.5 9 Melville and Sutherland 262 3092.08 26 28 12 1.45 1 39 13.5 Mississippi 266 465.05 10 12 6 7.90 10 26 4.5 Molinas 262 199.79 2 103 24 55.96 18 44 19 Shen 266 300.77 6 69 18 37.00 15 39 13.5 

Shen-Maza 266 1133.23 20 67 17 36.90 14 51 22 Sheppard 262 1276.04 22 11 5 3.89 7 34 11 Simplified Chinese 254 344.46 8 62 15 56.21 19 42 17 SSE—sum of squared errors
34 

Sheppard’s equations have been revised several times as reported by Alkhalidi, Sheppard et al., 

and Sheppard.(65,66,67) The 2001 version of the equation as listed in table 5 was used for 

comparisons in their analysis because it was the version provided by Sheppard to the author at 

the time the analysis was conducted. That version of the Sheppard equations ranked 19th based 

on the two sum of squared error categories. 

Since no single equation was superior to the others and none of the equations accurately 

predicted the scour for all conditions, it is important to assess where the equations failed. 

Residuals of selected equations were plotted against Froude number (Vo/(gyo)

0.5), relative 

velocity (Vc/Vo), median grain size (D50), pier width (b), relative bed material size (b/D50), and 

relative depth (yo/b) to assess where the equations may fail to properly account for the scour 

processes. Figure 8 shows that the Froehlich equation, which is a regression equation, has no 

significant patterns; it fits the data reasonably well. However, to convert the Froehlich equation 

from a regression equation to a design equation, Froehlich added the pier width as a factor of 

safety. The factor of safety increases the scatter in the data significantly. The plot of residuals 

versus pier width shows that factor of safety becomes too large as the pier width increases (figure 

9). The HEC-18-K4 equation shows patterns of increasing overprediction as Froude number (0–

0.4), median grain size, and pier width increase (figure 10). The K4, proposed by Mueller, 

reduces the effect of the Froude number and median grain size, but patterns are still evident in 

the pier width (figure 11).(5) Only pier width displays a pattern in the residuals of the Mississippi 

equation (figure 12). The revised HEC-18 equation, HEC-18-K4Mo, also shows patterns in the 

residuals with Froude number and median grain size, and the most dominant pattern is the 

bottom envelope on the pier width (figure 13).(1) Most underpredictions seem to occur for grain 

sizes less than 2 mm. Table 5 shows that two thirds of the underpredictions by HEC-18-K4Mo 

occur at grain sizes less than 2 mm. Thus, limiting the Ki and K4 corrections to grain sizes greater 

than 2 mm improves the performance of the Molinas correction. 

Although many equations have been proposed for predicting the depth of local pier scour, no 

equation presented here accurately predicts the depth of scour for the wide variety of conditions 

represented in the field data set. Six equations are identified as being better than the others when 

assessed for their value as design equations; however, even these display patterns in their 

residuals that indicate they are not properly accounting for the scour processes. Additional 

research and analysis are needed to develop a better local pier scour prediction equation.
35 

(A) FROUDE NUMBER (B) RELATIVE VELOCITY 

(C) 50% FINER GRAIN SIZE, IN MM (D) PIER WIDTH, IN M 

(E) RELATIVE BED MATERIAL SIZE (F) RELATIVE DEPTH 

 

Figure 8. Evaluation of residuals for the Froehlich equation. 
36 

(A) FROUDE NUMBER (B) RELATIVE VELOCITY 

(C) 50% FINER GRAIN SIZE, IN MM (D) PIER WIDTH, IN M 

(E) RELATIVE BED MATERIAL SIZE 

(F) RELATIVE DEPTH 

 

Figure 9. Evaluation of residuals for the Froehlich design equation. 
37 

(A) FROUDE NUMBER (B) RELATIVE VELOCITY 

(C) 50% FINER GRAIN SIZE, IN MM (D) PIER WIDTH, IN M 

(E) RELATIVE BED MATERIAL SIZE 

(F) RELATIVE DEPTH 

Figure 10. Evaluation of residuals for the HEC-18-K4 equation. 
38 

(A) FROUDE NUMBER (B) RELATIVE VELOCITY 

(C) 50% FINER GRAIN SIZE, IN MM (D) PIER WIDTH, IN M 

(E) RELATIVE BED MATERIAL SIZE (F) RELATIVE DEPTH 

Figure 11. Evaluation of residuals for the HEC-18-K4Mu equation. 
39 

(A) FROUDE NUMBER (B) RELATIVE VELOCITY 

(C) 50% FINER GRAIN SIZE, IN MM (D) PIER WIDTH, IN M 

(E) RELATIVE BED MATERIAL SIZE (F) RELATIVE DEPTH 

Figure 12. Evaluation of residuals for the Mississippi equation. 
40 

(A) FROUDE NUMBER (B) RELATIVE VELOCITY 

(C) 50% FINER GRAIN SIZE, IN MM (D) PIER WIDTH, IN M 

(E) RELATIVE BED MATERIAL SIZE (F) RELATIVE DEPTH 

Figure 13. Evaluation of residuals for the HEC-18-K4Mo equation. 
41 

EVALUATION OF LABORATORY RESEARCH 

General 

Laboratory research has been the primary tool for defining the relations among variables 

affecting the depth of pier scour. The validity of these relations has not been proven in the field. 

Landers and Mueller evaluated many relations developed in the laboratory by use of transformed 

data (to obtain a more normal distribution) and smoothing techniques to assess general trends in 

the data.(21) They found only minimal agreement between the field data and laboratory-based 

relations. The assessment presented here investigates the relations in the field data for variable 

combinations commonly reported by laboratory investigations. Unlike the data set used by 

Landers and Mueller, all data at skewed piers were removed to prevent bias by these data, as 

previously discussed.(21) No transformations were applied unless necessary for consistency with 

published relations. Using the basic data without transformation results in a less uniform 

distribution of the data, but it provides for a more direct comparison with laboratory work. 

Pier Geometry 

For piers aligned with the approach flow, laboratory research indicates that streamlining the pier 

nose reduces the depth of scour. Many shapes have been tested in the laboratory (table 6) to 

determine what effect the pier shape has on the depth of scour, relative to a circular pier. The pier 

shapes for the field data are classified as unknown, cylinder, round-nose, sharp, or square. 

Landers and Mueller did not account for variables known to affect the depth of scour other than 

pier shape and found no correlation between pier shape and scour depth.(21) It is necessary to 

remove the effects of pier width, velocity, depth, and bed material before comparing the depth of 

scour among different pier shapes. Figures 14 and 15 show the effect of pier nose shape on 

scour. First, only the effect of pier width is accounted for by dividing the depth of scour (ys) by 

the pier width (b). Figure 14 shows that the median of the relative depth of scour decreases as the 

pier is more streamlined; however, there is overlap in the interquartile range, and the differences 

are not significant. Second, the effects of pier width, velocity, depth, and bed material are 

removed by regressing these variables with the depth of scour and by plotting the residuals of a 

regression against pier shape, which is not included in the regression. The residuals from the 

regression analysis (figure 15) show the same trend as observed in figure 14. The pier shape does 

not affect the depth of scour in the field as much as in the laboratory. In the field, flow directions 

are variable, pier shapes vary with depth, and the effect of submerged debris is not easily 

accounted for; these combine to reduce the effect of pier shape on the depth of scour. 
42 

Table 6. Coefficients for the effect of pier shape relative to the scour that would be expected 

at a circular pier. 

Shape 

LengthWidth 

Ratio Tison(68)

Laursen 

and 

Toch(8)

Chabert and 

Engeldinger(69)

Garde 

et al. (70) Venkatadri(71)

Neill 

(ed.) (23) Dietz(72)

Circular 1 1.0 1.0 1.0 — 1.0 1.0 1.0 

2 — 0.91 — 0.9 — — — 

3 — 0.76 — 0.8 — — — 

4 0.67 — 0.73 0.7 — — — 

Lenticular 

7 0.41 — — — — 0.8 — 

Triangular 

nose 60° — — — — — 0.75 — 0.65 

Triangular 

nose 90° — — — — — 1.25 — 0.76 

1 — — — — — — 1.22 

2 — 1.11 — — — — — 

3 — — — — — — 1.08 

4 1.4 — 1.11 — — — — 

5 — — — — — — — 

Rectangular 

6 — 1.11 — — — — 0.99 

Rectangular 

chamfered — — — — — — — 1.01 
43 

Figure 14. Box plot illustrating the effect of pier shape on relative depth of scour. 

Figure 15. Box plot illustrating the effect of pier shape on the depth of scour 

with the effects of pier width, velocity, depth, and bed material removed by 

linear regression. 
44 

Relative Velocity 

Through a series of laboratory experiments, Chiew found relative scour depths (ys/b) were less 

for ripple-forming sediments than for nonripple-forming sediments at relative velocities (Vo/Vc) 

ranging from 0.6 to 2.(44) He determined that this reduction in scour depth was caused by the 

roughness and sediment transport associated with the formation of ripples near incipient motion. 

Ripple-forming sediments are those with a D50 less than about 0.6 mm. Figure 16 shows that the 

upper envelope of the field data generally fits the curves that Chiew developed.(44) A few 

measurements with ripple-forming sediments exceed the envelope. The maximum depth of scour 

observed in the field does not appear to be strongly affected by whether the sediment is ripple 

forming or nonripple forming. The scatter of data below the envelope curves indicates that the 

relation between relative depth of scour and relative velocity developed in the laboratory does 

not adequately explain the scour processes in the field. Nonuniformity of the bed material and 

variable flow depth in the field probably cause some of the scatter. 

Baker also did laboratory investigations of the effect of bed material properties on the relation 

between relative scour depths and relative velocity, using nonuniform bed material characterized 

by the coefficient of gradation.(45) He found that as the coefficient of gradation increased, the 

relative depth of scour was reduced, and the maximum scour occurred at a relative velocity 

greater than one. The field data categorized by the coefficient of gradation are shown in figure 17 

with hand-drawn envelope curves for the four categories of gradation. The effect of gradation 

012345

RELATIVE VELOCITY (Vo / Vc)

0.0

0.5

1.0

1.5

2.0

2.5

RELATIVE DEPTH OF SCOUR (ys/b)

NONRIPPLE FORMING

RIPPLE FORMING

012345

RELATIVE VELOCITY (Vo / Vc)

0.0

0.5

1.0

1.5

2.0

2.5

RELATIVE DEPTH OF SCOUR (ys/b)

NONRIPPLE FORMING

RIPPLE FORMING

Figure 16. Comparison of field observations with the curves developed by Chiew showing the 

effect of sediment size andrelative velocity on relative depth of scour.(44)
45 

has no consistent pattern in the relation between normalized scour depth and relative velocity for 

the field observations. 

 

Baker changed the gradation while maintaining a constant D50 during his experiments.(45) To 

simulate a constant D50 in the field data, Mueller used partial residuals to remove the effect of 

D50 from the field data.(5) This approach did not improve the comparison between the field data 

and Baker’s laboratory observations.(45) 

Bed Material Parameters 

The scale of laboratory experiments prevents the effect of relative sediment size (b/D50) on 

relative scour depth from being directly compared with field conditions. The maximum relative 

sediment size obtained in the laboratory was about 800. In the laboratory, ripple-forming 

sediments had lower relative scour depths than nonripple-forming sediments for relative 

sediment sizes ranging from 100 to 800. The field data do not contain ripple-forming sediments 

with a relative sediment size less than 900 (figure 18); therefore, there is insufficient overlap 

between laboratory and field data to make a valid comparison. The field data show a cluster of 

ripple-forming sediments near a relative sediment size of 1,000 that is below the maximum scour 

for nonripple-forming sediments; however, the maximum relative depth of scour for ripple135

RELATIVE VELOCITY (Vo / Vc)

0.0

0.5

1.0

1.5

2.0

2.5

Relative Scour

<2

2 - 3

3 - 4

>4

COEFFICIENT OF

GRADATION

RELATIVE DEPTH OF SCOUR (ys/b)

135

RELATIVE VELOCITY (Vo / Vc)

0.0

0.5

1.0

1.5

2.0

2.5

Relative Scour

<2

2 - 3

3 - 4

>4

COEFFICIENT OF

GRADATION

RELATIVE DEPTH OF SCOUR (ys/b)

Figure 17. Effect of gradation and relative velocity on relative depth of scour for field data, with 

hand-drawn envelope curves for selected gradation classes. 
46 

forming sediments with relative sediment sizes of 4,000 exceeds the nonripple-forming 

sediments. 

100 101 102 103 10 2 3 4 5 67 2 3 4 5 67 2 3 4 5 67 2 3 4 5 67

RELATIVE SEDIMENT SIZE (b/D50)

0.0

0.5

1.0

1.5

2.0

RELATIVE DEPTH OF SCOUR (ys/b)

NONRIPPLE FORMING

RIPPLE FORMING

LIMIT OF LABORATORY DATA

Figure 18. Effect of relative sediment size on relative depth of scour for field data. 
47 

Ettema recognized that maximum depth of scour, determined as 2.4 times the pier width, was 

affected by the gradation of the bed material.(47) Ettema used a series of laboratory experiments 

to develop a correction factor to account for the gradation of the bed material on the maximum 

depth of scour. Hand-drawn envelope curves in figure 19 show that the relative scour depth is 

greater for ripple-forming sediments than for nonripple-forming sediments when the gradation 

coefficient is less than about 2.5. For gradation coefficients greater than 2.5, there is a reduction 

in the relative depth of scour for all observations. The reduction in the relative depth of scour is 

larger for ripple-forming sediments than for nonripple-forming sediments. An increase in the 

coefficient of gradation for a constant median grain size results in an increase in the coarser size 

fractions of the bed material; therefore, an increase in the coarse size fractions of the bed 

material reduces the depth of scour, and the depth of scour is dependent on the size distribution 

of nonuniform bed material. The larger reduction in scour for ripple-forming sediments may be 

caused by armoring of the scour hole by the coarser size fractions, but the small amount of data 

on ripple-forming sediments for the larger gradations makes any conclusions questionable. 

0 2 4 6 8 10

GRADATION COEFFICIENT

0.0

0.5

1.0

1.5

2.0

2.5

RELATIVE DEPTH OF SCOUR (ys/b)

NONRIPPLE FORMING

RIPPLE FORMING

COEFFICIENT OF GRADATION (σ)

RELATIVE DEPTH OF SCOUR (ys/b)

0 2 4 6 8 10

GRADATION COEFFICIENT

0.0

0.5

1.0

1.5

2.0

2.5

RELATIVE DEPTH OF SCOUR (ys/b)

NONRIPPLE FORMING

RIPPLE FORMING

COEFFICIENT OF GRADATION (σ)

RELATIVE DEPTH OF SCOUR (ys/b)

Figure 19. Effect of the coefficient of gradation on relative depth of scour for field data with 

hand-drawn envelope curves of ripple- and nonripple-forming sediments. 
48 

Depth of Approach Flow 

Most researchers agree that for constant velocity intensity, local pier scour increases as depth of 

flow increases, but as the depth of flow continues to increase, the scour depth becomes almost 

independent of flow depth. (See references 44, 47, 69, 73, 74, 75, and 76.) Chiew(44) plotted data 

that he collected along with experimental data from Shen et al.,(9) Ettema,(47) and Chee(76) and 

concluded that the flow depth does not affect scour if the depth is greater than four times the pier 

width. From this research, Melville and Sutherland developed the Ky factor in their prediction 

equation (table 3).(2) The relation between relative flow depth and relative scour depth for the 

field data is shown in figure 20. Although the curve for the Ky factor envelops the data to the 

right, the data do not follow the trend of the curve. Most laboratory data are collected at or near 

incipient motion. To better compare the field data with the laboratory data, field data with 

sediment transport conditions near incipient motion (0.8 < Vo/Vc <1.2) were selected and plotted 

in figure 21. Again, the field data do not follow the trend observed in the laboratory data; they 

indicate that the relative depth of scour tends to increase with increasing relative flow depth. 

0.0 0.5 1.0 1.5 2.0 2.5 3.0

RELATIVE DEPTH OF SCOUR (ys/b)

0

4

8

12

RELATIVE FLOW DEPTH (yo/b)

CURVE FROM

MELVILLE AND SUTHERLAND (1988)

0.0 0.5 1.0 1.5 2.0 2.5 3.0

RELATIVE DEPTH OF SCOUR (ys/b)

0

4

8

12

RELATIVE FLOW DEPTH (yo/b)

CURVE FROM

MELVILLE AND SUTHERLAND (1988)

Figure 20. Effect of relative flow depth on relative depth of scour with field data compared to the 

relation presented by Melville and Sutherland.(2)
49 

 

DEVELOPMENT OF SCOUR PREDICTION METHODOLOGY 

Assessment of Basic Variables 

Logically, pier width, pier shape, flow depth, approach velocity, and bed material characteristics 

are important variables in determining the depth of scour; however, most of the design equations 

presented in table 3 do not contain all of these variables. The Mississippi equation, which was 

one of the top equations (table 5), is based on only pier width and flow depth. Therefore, it is 

important to evaluate the significance of each variable on the depth of scour and the potential 

interaction among the variables. A combination of scatter plots and multiple regression analysis 

will be used for this evaluation. 

0.0 0.5 1.0 1.5 2.0 2.5 3.0

RELATIVE DEPTH OF SCOUR (ys/b)

0

4

8

12

RELATIVE FLOW DEPTH (yo/b)

CURVE FROM

MELVILLE AND SUTHERLAND (1988)

0.0 0.5 1.0 1.5 2.0 2.5 3.0

RELATIVE DEPTH OF SCOUR (ys/b)

0

4

8

12

RELATIVE FLOW DEPTH (yo/b)

CURVE FROM

MELVILLE AND SUTHERLAND (1988)

Figure 21. Effect of relative flow depth on relative depth of scour for field conditions near incipient 

motion (0.8<Vo/Vc<1.2) compared to the relation presented by Melville and Sutherland.(2)
50 

The effect of an individual variable on the depth of scour in the field is complicated by the 

interactive response of the variables to the dynamic conditions. Evaluating the effect of a 

particular variable on the depth of scour in the laboratory is easier than in the field. In the 

laboratory, all variables can be held constant and one variable changed; in the field, all of the 

variables interact and adjust to the changing flow conditions. 

Figure 22 shows a scatterplot matrix of basic variables reported in the field data with a linear 

least squares smooth through the data. Drainage area, slope, and pier width appear strongly 

correlated with scour depth. Pier width directly affects the strength of the vortex system, which 

erodes the material from around the base of the pier. Correspondingly, pier width shows the 

strongest correlation with scour depth. It is surprising that drainage area and slope have a 

stronger correlation with the scour depth than do approach depth or approach velocity. This 

strong correlation appears to be caused by the correlation of the pier width and approach depth 

with drainage area and slope (figure 22); thus, for these data, drainage area or slope may 

represent a combined effect of pier width and approach depth. There is also a positive correlation 

between depth of scour, approach depth, and approach velocity; however there is significant 

scatter in the data, indicating that these variables are less significant than pier width. The size and 

Log.DAM

Log.Slope

Log.WidthM

Log.DepthM

Log.VelM

Log.D50M

Log.D84M

Log.D95M

Log.Sigma

Log.ScourM

DRAINAGE AREA

SLOPE

PIER WIDTH

APPROACH DEPTH

APPROACH VELOCITY

D50

D84D95

GRADATION

DEPTH OF SCOUR

DRAINAGE AREA

SLOPE

PIER WIDTH

APPROACH DEPTH

APPROACH VELOCITY

D50

D84

D95

GRADATION

DEPTH OF SCOUR

Log.DAM

Log.Slope

Log.WidthM

Log.DepthM

Log.VelM

Log.D50M

Log.D84M

Log.D95M

Log.Sigma

Log.ScourM

DRAINAGE AREA

SLOPE

PIER WIDTH

APPROACH DEPTH

APPROACH VELOCITY

D50

D84D95

GRADATION

DEPTH OF SCOUR

DRAINAGE AREA

SLOPE

PIER WIDTH

APPROACH DEPTH

APPROACH VELOCITY

D50

D84

D95

GRADATION

DEPTH OF SCOUR

Figure 22. Scatterplot matrix and frequency distribution of basic variables and depth of scour, 

log-transformed. 
51 

distribution of the bed material also affects the depth of scour, but the slope of the linear smooth 

is small and the scatter of the data indicates a low correlation with the depth of scour. The bed 

material size is well correlated with the approach velocity and slope, which is what would be 

expected; coarse bed streams have higher slopes and higher velocities. The bed material size 

classes are strongly correlated with each other, but are not linearly correlated with the gradation 

coefficient. The strong linear correlation between bed material sizes could cause colinearity 

problems in the results of multiple linear regression if different bed material size variables are 

included in the same equation. 

Weighted multiple linear regression analysis was used to assess the importance of each variable 

on the depth of scour, while accounting for the interaction between variables. Bed material sizes 

were evaluated in separate equations because of their strong colinearity. All variables were 

transformed logarithmically to improve the linearity and distribution of the data. Weighted 

multiple linear regression computes coefficients and exponents that minimize the sum of squares 

of the residuals while taking into account weights assigned to each observation. If the weights for 

all observations are equal, approximately one-half of the data are underestimated and about onehalf are overestimated (figure 23). This approach, while yielding a combination of variables that 

fits the middle of the data, is not appropriate for design. An envelope curve is more appropriate 

0.0 0.2 0.4 0.6 0.8

LOGARITHM OF THE COMPUTED SCOUR DEPTH

-1.0 -0.5 0.0 0.5

LOGARITHM OF THE OBSERVED SCOUR DEPTH

WEIGHTED REGRESSION

UNWEIGHTED REGRESSION

Figure 23. Example of difference between unweighted regression and weighted regression in 

developing a design curve. 
52 

for design. To fit an envelope curve, the regression was completed with equal weights, then the 

weights were adjusted so that more weight was applied to those points that were underestimated 

and defined the upper boundary of the data (figure 23). The weighting function (w), shown in 

equations 38 and 39, was determined by trial and error to produce a reasonable envelope curve. 

for residuals > 0 (38) 

for residuals < 0 (39) 

It was observed that small adjustments in the weighting function significantly change the sum of 

squared errors, the number of observations underestimated, and the statistical significance of 

each of the variables in the equation formulation. Therefore, the equations developed from this 

weighted regression approach should not be treated as the optimal envelope equation for these 

data, but serve as indicators as to which variables should be considered in the development of 

design methodology. 

Regression analysis showed that the inclusion of bed material size characteristics in the equation 

improved the sum of squared errors (table 7). Unweighted regression indicated that only the bed 

material size was important and the gradation of the bed material was not statistically significant 

at the 0.1 level. The equation using the D84 sediment size resulted in the lowest sum of squared 

errors. It is surprising that when the bed material was removed from the equation the approach 

velocity was not statistically significant in the unweighted regression. It is also interesting that 

there is reasonable consistency in the exponents for pier width and approach depth, but the 

exponents on velocity vary by a factor of 10. 

The weighted regression analysis produced different results than did the unweighted analysis. As 

with the unweighted regression, the equations containing bed material characteristics all 

produced lower sum of squared errors than did the equations without bed material characteristics. 

For the weighted analysis, all of the variables in each analysis were significant, and the equation 

using D50 produced the lowest sum of squared errors. While these equations may not be the 

optimal approach to predicting the depth of scour, they clearly show that bed material 

characteristics are important in determining the depth of scour. 

Assessment of Current Methodology 

A K4 factor was added to the HEC-18 pier scour equation in the third edition of HEC-18 to 

account for bed material size characteristics.(6) FHWA derived the relation for that version of K4

from preliminary laboratory data provided by Molinas, and it was intended as an interim 

adjustment factor until more detailed analyses were available (see HEC-18-K4 equation in table 
53 

3). Table 5 indicates that the sum of squared errors was only reduced from 822 to 791 by the 

inclusion of the K4 term presented in the third edition of HEC-18. 

Mueller developed a relationship for K4 based on field data (see HEC-18-K4Mu in table 3).(5)

Mueller used the Chinese equation for determining the approach velocity for incipient motion 

(equation 2) at the pier for the median grain size, but extended it to the D95 size fraction. The 

fourth edition of HEC-18 adopted Mueller’s K4 but restricted the lower limit to 0.4 and required 

a value of 1 if D50 were less than 2 mm, or D95 less than 20 mm. These restrictions were applied 

to the evaluation of this factor in table 5 (HEC-18-K4Mu). Table 5 indicates that Mueller’s K4

factor as adopted in the fourth edition of HEC-18 reduces the sum of squared errors significantly 

from 822 to 448. Although Mueller’s 1996 K4 factor worked quite well for the field data 

available for evaluation, the formation of the equation causes it to be indeterminate for some 

situations and behave contrary to logic in others. The equation becomes indeterminate if the 

velocity for incipient motion of the D50 grain size is smaller than the approach velocity needed to 

scour the D95 grain size at the pier. The equation behaves contrary to logic if the D50 grain size is 

held constant and only the D95 is varied. In this situation, K4 increases as D95 increases. In the 

field, variables tend to change together as a system, whereas in the laboratory selected variables 

can be held constant and other variables can be changed arbitrarily. For the field data used by 

Mueller to develop the K4 factor, an increase in D95 always corresponded to an increase in D50

(figure 22).(5) Under these conditions, the velocity intensity term proposed by Mueller provides a 

reasonable envelope curve, but it can produce unexpected results due to the arrangement of the 

variables.(5) 
54 

Table 7. Summary of weighted and unweighted regression results using basic variables. 

Exponents 

Basic Variables 

Used in 

Analysis 

Weighted Coefficient

Pier 

Width 

Approach

Depth 

Approach 

Velocity D50 D84 D95 

b, yo, Vo, D50, σ No 0.229 0.678 0.207 0.298 -0.114 – – b, yo, Vo, D50, σ Yes 1.19 0.765 0.166 0.058 -0.0284 – – 

b, yo, Vo, D84, σ No 0.247 0.651 0.166 0.319 – -0.128 – b, yo, Vo, D84, σ Yes 1.34 0.712 0.147 0.140 – -0.038 – b, yo, Vo, D95, σ No 0.270 0.619 0.176 0.300 – – -0.123 b, yo, Vo, D95, σ Yes 1.46 0.640 0.178 0.157 – – -0.019 b, yo, Vo No 0.414 0.784 0.295 N.S. – – – 

b, yo, Vo Yes 1.25 0.900 0.207 0.029 – – – 

SSE—sum of squared errors N.S.—not significant at 0.1 level – —no value
55 

Mueller’s K4, as adopted by the fourth edition of HEC-18, is compared with the expanded data 

set presented in this report in figure 24.(5) The equation envelops the data, with the exception of 

five points. Four of the points underpredicted the observed scour by less than 5 percent. Of the 

five points lying above the envelope curve in figure 24, four observations are from streams in 

Ohio. The bridge scour data sets from Ohio contained extensive bed material data, which were 

collected annually during low flow at most sites. These data included composite samples 

collected at the bridge and in the approach cross sections and local samples collected at each pier 

(inside the scour hole, if one were present). The bed material size reported with the scour 

measurement was usually the sample collected at the pier for the low flow preceding the scour 

measurement. All four observations for Ohio plotted below the envelope curve if the composite 

samples were used. 

Molinas derived a new correction to the HEC-18 equation from his final laboratory data set.(1)

Although several of the terms are similar to those used by Mueller and Jones,(78) Molinas 

redefined the equations for computing the incipient motion velocity and the approach velocity 

causing incipient motion at the pier (see table 3). Although this new correction provided a 

significant decrease in the sum of squared errors (from 822 to 495), it also significantly increased 

the number of observations that were underpredicted (from 13 to 65). Figure 13 showed that 

most of these underpredictions occurred at D50 less than 2 mm. If the correction developed by 

Molinas is only applied to D50 greater than 2 mm, its performance was enhanced greatly. The 

10-1 100 10 78 2 3 4 5 6 78 2 3 4 5 678 2 3 4 5 1

MUELLER VELOCITY INTENSITY

10-1

100

9

2

3

4

5

6

7

8

9

OBSERVED SCOUR / COMPUTED SCOUR

K4 PROPOSED BY MUELLER (1996)

AS ADOPTED BY HEC-18, 4TH EDITION

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

− ′

− ′

c c95

o c

V V

V V

10-1 100 10 78 2 3 4 5 6 78 2 3 4 5 678 2 3 4 5 1

MUELLER VELOCITY INTENSITY

10-1

100

9

2

3

4

5

6

7

8

9

OBSERVED SCOUR / COMPUTED SCOUR

K4 PROPOSED BY MUELLER (1996)

AS ADOPTED BY HEC-18, 4TH EDITION

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

− ′

− ′

c c95

o c

V V

V V

Figure 24. Relation between the ratio of the observed depth of pier scour to the depth of 

pier scour computed by the HEC-18 equation (idealized K4) and the K4 proposed by 

Mueller(5) as adopted by the fourth edition of HEC-18.(77) 
56 

sum of squared errors rose to 609, but the number of observations underpredicted dropped from 

65 to 21, and the sum of squared errors for the underpredictions was reduced from 17 to 2.47. 

Development of New Methodology 

Patterns in the performance of the HEC-18 equation clearly show the need for a K4 term to 

correct the depth of scour, particularly for coarse bed materials. The HEC-18 equation showed 

no difference in its performance for clear water or live bed conditions (figure 25). Armoring of 

the scour hole could cause overpredictions by the HEC-18 equation for coarse bed material. The 

ability of the flow to transport the D95 sediment size at the pier (estimated using equation 2) was 

used to determine whether an armor layer would form in the scour hole and limit the depth of 

scour. Figure 26 shows that there is little difference in the idealized K4 term (observed depth of 

scour/HEC-18 computed depth of scour) for conditions where the armoring potential is high. 

Mueller observed the HEC-18 equation consistently overpredicted scour in coarse bed 

materials.(5) Figure 27 clearly shows that for this data set, the magnitude of the overprediction 

increases with the median bed material size. A wide variation in the depth of scour for sand is 

indicated by the long whiskers in the box plot. 

The K4 term was developed by evaluating both the whole data set and only the portion with 

median grain sizes coarser than sand. Figure 28 shows that the depth of scour computed from the 

HEC-18 equation overpredicts by a larger ratio as the bed material size increases. It is interesting 

that there is also a negative trend in the approach velocity; this trend would indicate that the 

HEC-18 equation may have too high an exponent on velocity. The K4 term should be 

dimensionless to maintain the dimensional homogeneity of the HEC-18 equation. Numerous 

combinations of variables were investigated, and the best correlation was found with the median 

size of the sediment relative to the pier width (b/D50). The equation for the envelope curve using 

this variable combination is: 

 (40) 
57 

Figure 26. Box plot of the variation in the ratio of the observed depth of pier scour to the 

depth of pier scour computed by the HEC-18 equation (idealized K4) for low and high armor 

potential conditions. 

Figure 25. Box plot of the variation in the ratio of the observed depth of pier scour to the depth 

of pier scour computed by the HEC-18 equation (idealized K4) for clear water and live bed 

conditions. 
58 

Figure 27. Box plot of the variation in the ratio of the observed depth of pier scour to the 

depth of pier scour computed by the HEC-18 equation (idealized K4) for sediment size 

classes. 
59 

Figure 28. Relation between the ratio of the observed depth of pier scour to 

the depth of pier scour computed by the HEC-18 equation (idealized K4) 

and selected variables. 
60 

Figure 29 shows the envelope curve for K4 developed from the b/D50 ratio. This curve is 

applicable for all grain sizes and appears to explain some of the underprediction for the HEC-18 

equation for the sand sizes. If this correction is applied to all observations, the 13 observations 

that HEC-18 originally underpredicted (table 5) are corrected, but the sum of squared errors 

increases to over 2,800. The large increase in the sum of squared errors is caused by the large 

scatter below the curve for values of K4 above 1. If the correction is limited to reducing the depth 

of scour (K4<1), the sum of squared errors is reduced to 611, but 14 observations are 

underpredicted. The sum of squared errors for the 14 observations underpredicted is 2.16, the 

same as the HEC-18 equation before this correction (table 5). Use of only bed material size to 

develop a dimensionally dependent equation reduced the sum of squared errors to 520; this 

reduction does not seem sufficient to justify the use of a dimensionally dependent equation to 

compute a K4 term. 

Although the K4 based on b/D50 does not perform as well as the HEC-18-K4Mu equation in table 

5, the basis for this new approach is supported to an extent by the work of Sheppard, who found 

that b/D50 was an important parameter based on his laboratory research.(64) 

Figure 29. Relation between relative errors in computed scour using the 

HEC-18 equation and relative bed material size. 
61 

Importance of Sampling Bed Material 

The four observations from Ohio above the envelope curve in figure 24 could be reduced or 

eliminated by use of the grain sizes associated with a composite sample or an average of all 

available composite samples. While this analysis highlights the sensitivity of Mueller’s 1996 K4

to the bed material samples, it also illustrates the importance of and uncertainty associated with 

determining bed material characteristics for field conditions. 

The potential variability associated with characterizing the bed material can be illustrated using 

the Ohio bridge scour data sets. These data sets contain 419 samples of bed material, of which 

149 represent composite samples for an entire cross section and the rest are point samples near 

the piers. Table 8 shows that individual samples can vary greatly; the average coefficient of 

variation is just over 1.0 for all samples, including the composite samples. If only the composite 

samples are considered, the variability is reduced, but the average coefficient of variation 

exceeds 0.7. This magnitude of variability is probably responsible for much of the scatter in the 

relation between depth of scour and bed material characteristics. Even if the perfect scour 

equation were developed, the variability of bed material characteristics used for input could 

result in a wide range of scour predictions, depending on the sensitivity of the equation to the bed 

material characteristics. Therefore, as scour equations are improved by accounting for the effect 

of bed material characteristics, there will be a commensurate need to ensure that sampling 

procedures provide representative characteristics of the bed material. If representative bed 

material characteristics are not obtained, the potential improvements in scour prediction will not 

be realized. 
62

Table 8. Summary of variability in bed material data from sites in Ohio. 

 All Samples Composite Sample

Site 

Number 

of 

Samples Minimum Maximum Mean SD COV 

Number 

of 

Samples Minimum Maximum Me

41 27 0.02 24.00 7.16 7.27 1.02 8 0.13 15.50 42 17 0.13 11.50 1.91 3.21 1.68 7 0.31 1.00 43 17 0.03 54.00 13.85 17.53 1.27 7 1.40 28.00 44 17 0.70 60.00 16.89 14.38 0.85 8 0.70 17.90 145 17 0.17 13.00 5.02 4.23 0.84 7 2.80 13.00 46 3 0.16 1.60 0.73 0.76 1.04 1 0.43 0.43 47 19 0.83 51.00 19.33 15.29 0.79 8 2.35 42.00 148 18 0.01 1.09 0.26 0.30 1.15 8 0.09 0.40 59 17 0.06 7.80 1.39 2.11 1.52 7 0.16 7.80 60 23 0.70 47.90 13.20 10.20 0.77 8 1.00 47.90 161 47 0.01 59.00 18.44 14.51 0.79 7 10.20 28.00 262 17 0.54 46.00 15.28 12.78 0.84 8 10.00 46.00 263 12 0.03 32.00 8.40 9.02 1.07 6 3.50 32.00 164 28 0.01 27.00 8.74 7.96 0.91 8 5.10 20.50 165 17 0.10 2.20 0.92 0.64 0.69 7 0.12 2.00 66 28 0.10 21.00 4.63 6.17 1.33 8 0.41 15.00 67 18 0.47 47.00 15.42 11.98 0.78 8 6.40 31.50 168 6 0.48 17.50 4.75 6.44 1.35 2 4.19 17.50 169 21 0.02 20.60 10.47 7.89 0.75 7 2.20 20.00 170 17 0.03 16.00 2.12 3.88 1.84 5 0.89 3.25 71 16 0.03 8.00 2.20 2.45 1.12 7 0.90 8.00 79 17 0.11 28.00 7.73 9.28 1.20 7 0.84 18.00 

SD—standard deviation COV—coefficient of variation – —no value
63 

CHAPTER 6: SCOUR CAUSED BY FLOW CONTRACTION AT BRIDGES

GENERAL 

Contraction and local abutment scour are a result of flow acceleration caused by encroachment 

of highway embankments and abutments onto the main channel and (or) floodplain. The 

accelerated velocity produces an increase in sediment transport from the bridge section until 

equilibrium conditions are reached or the bed becomes armored. Contraction scour is usually 

associated with flow acceleration in the contracted opening parallel to the channel and can occur 

throughout the bridge cross section. Abutment scour only occurs near the abutment and is 

associated with the localized flow curvature and acceleration near the ends of the bridge 

abutments as the flow separates from the channel boundary. 

Contraction and abutment scour often are treated independently because they are caused by 

different processes as the flow accelerates through a contracted opening; however, the separation 

of contraction and abutment scour is often difficult in field observations of scour. Treating the 

components of scour as independent and additive may be appropriate for wide, shallow bridge 

crossings, but may not be appropriate for narrow bridge openings. HEC-18 treats contraction and 

abutment scour as separate and additive because the equations used to compute these scour 

components were developed with this assumption. 

DISCUSSION OF PREDICTIVE EQUATIONS 

General 

Most scour equations are either theoretically derived or empirically developed from small-scale 

laboratory research. Controlled laboratory settings tend to oversimplify or ignore many of the 

complexities present in the natural setting. Laboratory research is typically conducted in straight 

laboratory flumes with uniform flow, noncohesive uniform bed material, and limited verification 

of results with field data. Although contraction and abutment scour may be interrelated, 

laboratory tests typically study these components independently. Before applying a scour 

prediction equation, it is important to recognize the limitations of the equations, the conditions 

for which the equations were developed, the methods used to develop them, and how the 

underlying data were interpreted. 

Contraction Scour 

Contraction scour has traditionally been classified as live bed or clear water. The live bed 

condition is characterized by bed material’s being transported into the contracted opening from 

upstream of the bridge. Live bed scour is typical of scour that occurs in the main channel portion 

of a waterway in high-flow conditions. Clear water contraction scour occurs when the flow 

conveyed to the bridge crossing is not transporting bed material; thus, all material that is 

transported from the contracted section is sediment being scoured. Scour occurring on vegetated 

floodplains may be classified as clear water scour despite the potential for the shear stress in the 

approach section to be greater than the critical shear stress of the material comprising the 

floodplains. 
64 

Four approaches can be used to estimate the elevation of the scoured bed in a contracted section: 

1. Regime relations for channel size and shape. 

2. Empirical relations similar to at-a-station hydraulic geometry relations for width and 

depth. 

3. Numerical sediment transport models. 

4. Contraction scour equations based on particular sediment transport and uniform flow 

formulas. 

These methods do not compute the depth of contraction scour directly, but rather the equilibrium 

depth of flow in the contracted section. 

Regime equations attempt to quantitatively describe the shape, width, and depth of channels that 

are “in regime,” or have reached equilibrium—a state in which they are neither aggrading nor 

degrading. Much of the research on the proposed regime equations was based on canals in India 

during the 1930s and 40s. Regime equations depend highly on the similarity of the site 

conditions for which they were developed. Because the equations were predominately based on 

conditions in India and the western United States, they usually are not applicable to areas outside 

these regions. 

Hydraulic geometry relations are similar to regime equations and use at-a-station relations to 

describe the change in geometry of a single cross section for varying discharge. As with regime 

equations, these relations are empirical, and the limitations are dependent on the similarity 

between the site conditions being analyzed and those for which the equations were developed. 

Numerical sediment transport models can also be used to compute the depth of scour in a 

contracted bridge opening. Models are available from several Federal agencies and some private 

companies. The primary difference among the models is whether they are two-dimensional, onedimensional, or one-dimensional stream-tube models. The cost of applying the models and the 

difficulty of setting up the models to accurately replicate field conditions have limited their use 

for prediction of contraction scour. Numerical models are sometimes used where site conditions 

are complex and (or) the estimated depth of scour has a significant effect on the design and cost 

of a proposed bridge. 

Semi-empirical contraction scour equations are based on sediment transport and uniform flow 

formulas. These relations are commonly used and are recommended by FHWA in HEC-18.(6)

Because these equations are widely used, a more detailed discussion is provided. Following the 

traditional body of research on contraction scour, these equations are divided into the live bed 

and clear water classifications. 

Live Bed Contraction Scour Equations 

Straub was the first to develop an approach to predict contraction scour that most others would 

follow.(79) He assumed that the bed in the contracted section would scour until it reached a depth 

at which the local transport capacity was equal to the amount of material being supplied from 
65 

upstream (sediment discharge continuity). He selected the DuBoys sediment transport equation 

to compute the amount of material supplied to the reach and the local transport capacity in the 

contraction. Straub estimated the energy dissipation rate (friction slope) in the contracted and 

uncontracted reaches using Manning’s equation. This assumption is reasonable where flow 

curvature is small and pressure gradients are small compared to boundary stresses. The 

hydraulics in a short contraction, such as a bridge crossing, require the consideration of 

additional energy losses not accounted for in a roughness coefficient based on the channel 

composition and configuration.(80,81,82) Straub’s equation, based on sediment discharge 

continuity, water discharge continuity, and the Manning equation, has the general form of 

equation 41: 

 (41) 

where 

y2 is the depth in the contracted section; 

y1 is the depth in the uncontracted section; 

Q2 is the discharge in the contracted section; 

Q1 is the discharge in the uncontracted section; 

EQ is the exponent on the ratio of discharges; 

b1 is the bottom width in the uncontracted section; 

b2 is the bottom width in the contracted section; 

Eb is the exponent on the ratio of bottom widths; 

n2 is the Manning’s n in the contracted section; 

n1 is the Manning’s n in the uncontracted section; 

En is the exponent on the ratio of roughness coefficients; and 

τ represents one or more shear-stress variables. 

For most applications, the shear stress-based function is assumed equal to unity. 

Laursen(61) followed Straub’s approach but used his own sediment transport equation.(83)

Richardson and Richardson modified Laursen’s live bed equation by removing the ratio of 

Manning’s n in equation 41.(84) They concluded that Laursen’s equation did not account correctly 

for the increase in transport that would occur if a plane bed existed in the contracted opening 

with a dune bed configuration in the approach section. For this situation, Laursen’s equation 

would predict less scour than if the roughness coefficients were equal. The Manning’s n ratio in 

Laursen’s equation does, in fact, behave properly. The basic principle of estimating contraction 

scour is the assumption of achieving equilibrium sediment transport. With a plane bed 

configuration more sediment can be transported at a reduced depth than in a dune bed 

configuration; therefore, equilibrium sediment transport can be achieved at a shallower depth. To 

achieve a plane bed configuration, the streambed had to progress through the dune bed 

configuration in the contracted section. A deeper scour may have occurred at a lower flow with a 

dune configuration in the contracted section than at a higher flow with a plane bed configuration. 

Therefore, to predict the maximum depth of scour for design purposes, a constant Manning’s n

should be assumed in the approach and bridge sections. This yields the same result as that 

proposed by Richardson and Richardson.(84)
66 

Culbertson et al. examined all of the previously published research and concluded that none of 

the existing equations were applicable to all channels or to abrupt contractions, such as bridges 

and road embankments.(85) They developed their own derivation of Straub’s equation using 

Colby’s transport relations for sand bed streams.(86) Culbertson et al. recognized the difference 

between a long contraction and a bridge contraction and believed that the equations that were 

developed based on the long-contraction assumption would provide, at best, rough estimates of 

contraction scour at bridges.(85) In conclusion, Culbertson et al. believed that: “Although 

laboratory research on alluvial channels may lead to more reliable predictions of scour and fill 

based on hydraulic theory and empirical equations, the scour and fill problem is inherently 

complicated, and evaluations based on field experience are needed.” (p. 43)(85)

Table 9. Summary of live bed contraction scour equation exponents. 

(See equation 41 for definitions of terms.) 

Equation 

(Q2/b2)/ 

(Q1/b1) b1/b2 Q2/Q1

Straub(79) – 0.43 0.86 

Straub(79) – 0.642 – 

Griffith(87) – 0.637 – 

Neill (ed.)(23) 0.67–0.85 – – 

Laursen(61) – 0.6–0.7 0.86 

Komura(88) – 0.85 – 

Komura(88) – 0.667 – 

Culbertson et al.(85) – 0.667 – 

The approach to developing live bed contraction scour equations is very similar among all 

researchers and differs primarily by the method of determining the sediment transport capacity. 

Table 9 shows a summary of the exponents of the ratios common to the equations developed by 

other researchers. There is good consistency in the exponents, considering that each researcher 

used a different sediment transport equation. In the derivation of the live bed contraction scour 

equations, the sediment transport equation is applied to both the contracted and uncontracted 

sections, and only the difference in the transport rates between these sections affects the 

computed depth of scour. Thus, the depth of contraction scour does not appear to be sensitive to 

the selection of the transport equation. 

The discharge in the contracted and uncontracted sections is typically determined using a onedimensional step-backwater computer model such as Water-Surface Profile Computations 

(WSPRO)(89) or Hydrologic Engineering Center-River Analysis System (HEC-RAS).(90) Streamtube and one-dimensional models distribute the flow in a cross section based on conveyance. The 

flow distribution in a contracted opening is more dependent upon momentum than on roughness, 

making conveyance-based flow distributions inaccurate. Experience in applying backwater 

models indicates that distribution of the flow by conveyance may lead to overestimating the 

depth of contraction scour. At the uncontracted approach cross section, a conveyance-based flow 

distribution may place more flow in the floodplain than what actually was present, resulting in 

the main channel’s being too low. Increased flow in the floodplain can result because: (1) the 
67 

one-dimensional model assumes a constant slope for the whole cross section when, in reality, the 

downstream water-surface slope varies across the section; (2) the one-dimensional model does 

not account for flow-path lengths between main channel and the floodplains; and (3) the onedimensional model does not account for the lateral resistance of the flow moving from the main 

channel to the floodplains. Conversely, the conveyance-based flow distribution may place too 

much flow in the main channel at the bridge because the conveyance tubes fail to represent the 

accelerating curvilinear flow separating from abutments and (or) road embankments. A reduction 

of the main channel flow in the uncontracted section coupled with an increase of main channel 

flow at the bridge section could lead to an overprediction of depth of contraction scour. 

Clear Water Contraction Scour Equations 

Clear water scour occurs where the boundary shear stress in the uncontracted section is less than 

or equal to the critical tractive force of the bed material, thus, preventing the supply of material 

into the contracted section. Laursen assumed that the maximum limit of clear water scour occurs 

when the boundary shear stress is equal to the critical tractive force.(91) This assumption is 

common among all of the proposed clear water contraction equations. The critical shear stress of 

a channel with a specific grain size is commonly estimated from the Shields diagram. The critical 

velocity for incipient motion can be computed from the Shields parameter by substituting the 

Manning equation for the slope term of the shear-stress equation and then using Strickler to 

approximate Manning’s n.

(92) A critical velocity equation (equation 3) can be obtained by 

rearranging terms and solving for velocity. By setting the velocity in the contracted section equal 

to the critical velocity and solving for depth, the following generic clear water contraction scour 

is obtained: 

 (42) 

where 

V2 is the average velocity in the contracted opening in meters per second; and

Dm is the mean grain size of the bed material, in meters. 

On the basis of research on the effective size of bed material for riprap design and resistance to 

erosion presented in Richardson et al.,(93) Richardson and Richardson suggest that 1.25 D50 be 

used for Dm.

(84) 

The adequate determination of the critical velocity or critical shear stress and the corresponding 

scour is not well established for channels having cohesive bed material, bed material that varies 

with depth, heavily vegetated floodplains, previously developed scour holes, and armored beds. 

Laursen, Neill, White, Iwagaki, and Shields have all developed clear water scour relations, but 

all are based on assumptions that tend to oversimplify the conditions present in most field 

situations. (See references 94, 91, 95, 96, and 97.) The clear water scour equations were 

developed for flat beds, but only after a scour hole develops the hydraulics change, affecting the 

accuracy of the prediction equations. Proper representation of bed material is also critical to the 

application of clear water scour equations. Bed material samples should represent both the 
68 

surface and subsurface material. Clear water contraction equations can be applied to each 

consecutive layer at sites where layered soils are present by modifying parameters as each layer 

is eroded and the next is exposed. Until recently, the effects of armoring were not accounted for 

in determining the depth of scour. Froehlich(98) presented a method based on the active-layer 

approach by Borah(99) to estimate the depth of scour required to obtain an armor layer of 

sufficient size and thickness to limit clear water scour. 

Abutment Scour 

The current knowledge of prediction of scour at abutments is derived from regime theory 

equations, equations used to estimate the depth of scour for spur dikes, and equations developed 

from small-scale physical model studies conducted in laboratory flumes. Unfortunately, none of 

these approaches have resulted in a satisfactory prediction equation. These approaches’ inability 

to accurately predict scour at abutments is a result of the simplifying assumptions on which the 

research is based and the complexity of abutment scour in field conditions. The configuration of 

bridge abutments and associated embankments is complex when placed in the context of river 

hydraulics. 

Field Conditions 

The geometric configuration greatly affects the way flow is directed around the abutments. The 

abutment may be located in the channel, at or near the top bank, or on the floodplain. The 

configuration of the abutment may be vertical, have wing walls at various angles, or have a spill 

slope protected with riprap or some other armoring material. Although abutments with spill 

slopes are usually protected, the armoring can fail or be undermined by scour causing the 

abutment configuration to change during a flood. The embankments may not be perpendicular to 

the approach flow but may be angled either upstream or downstream. Drainage ditches along the 

toe of the embankment are common and complicate the flow patterns around the abutment. 

The natural flow distribution in a river can also have a significant effect on the depth of scour at 

an abutment. The distribution of the approach flow blocked by the embankment is dependent on 

the roughness and topography of the floodplain and alignment of the main channel. The flow 

distribution and direction can change significantly during a flood hydrograph. Such complexity 

and the variability of these conditions between sites are major obstacles in developing a reliable 

method for predicting scour at abutments. 

Discussion of Equations 

Some abutment scour equations are based on data for scour at the end of spur dikes. The most 

notable equation of this type was published by Richardson and Huber as an equation for 

predicting scour when the embankment length to flow depth ratio exceeds 25.(100) This equation 

uses only the Froude number and depth of flow and does not account for the length of the 

embankment. 

Most equations for predicting scour at abutments are based on small-scale physical model 

studies. Literature documenting the laboratory experiments reveals that the approach section of 
69 

the flume usually had a constant depth with a uniform velocity distribution. The roughness of the 

channel was also typically uniform throughout the approach and bridge sections, and the bed 

material was generally composed of uniform sand. The abutments were represented by solid, 

nonerodible obstructions protruding from the sides of the flume with ends that varied in 

configuration to represent typical shapes of embankments and abutments at contracted bridge 

openings. These conditions are different from the conditions that occur in the field. 

Several researchers have attempted to account for some conditions commonly found in the field. 

Dongol, Melville, and Sturm and Janjua have used models that incorporate the floodplain, some 

channel geometry effects, and nonuniform flow distributions.(101,102,103) Unfortunately, the 

amount of field data on abutment scour that can be used to evaluate the validity of the laboratory 

studies is limited. 

The laboratory research, although not in agreement, has typically used some combination of the 

following variables to predict scour at abutments: (1) embankment length, (2) abutment shape, 

(3) depth of flow, (4) velocity, and (5) discharge. Because of the simplicity of many of the 

laboratory experiments, the variables in equations developed from these experiments are 

ambiguous when these equations are applied to field conditions. In simple flume studies, the 

flow depth is uniform everywhere, and there is no way to define what depth is controlling the 

depth of scour (the depth at the abutment, the depth in the approach upstream of the abutment, or 

an average depth of flow blocked by the abutment). In the field, these may all be different 

values; however, in the laboratory with a uniform bed they are all the same. The velocity term is 

also a good example of potential ambiguous variables in the field. The velocity in the contracted 

opening adjacent to the abutment (which would represent the accelerated curvilinear flow) would 

be different from the unobstructed approach velocity upstream of the abutment or the average 

velocity of the approach flow blocked by the length of the embankment. Flume studies often use 

the total length of the embankment from the flume wall to the abutment as the embankment 

length; however, this approach fails to account for the flow separation and recirculation zone that 

forms along the upstream edge of the embankment (figure 30). The effective length of the 

embankment on the depth of scour depends on the distribution of the approach flow and the 

floodplain roughness and geometry. It is important that laboratory research emulate the 

conditions in the field so that equations developed are representative of field conditions. 
70 

EVALUATION OF PUBLISHED FIELD OBSERVATIONS 

General 

Numerous papers and reports have been written on the various aspects of scour at bridges; 

however, complete and reliable field data are rare. Table 10 summarizes a literature survey that 

yielded 29 references that might have contained field data for abutment and contraction scour. A 

brief summary of selected references is presented. 

Summary of Selected References 

Research on backwater and discharge computations at single and multiple bridge openings, at 

bridges with openings that have been excavated, and at bridges with guide banks provided data at 

60 bridges. (See references 81, 104, 105, 106, 107, and 108.) These data were collected at sites 

with heavily vegetated floodplains where complete real-time detailed data sets would be difficult 

to collect. The USGS, Mississippi District, has summarized data for two of the sites (figures 31 

and 32) and made comparisons with computations based on contraction scour equations 

recommended in HEC-18.(109) Lithologic logs available for the site at Edinburg, MS, clearly 

showed the elevation of a hard clay layer (figure 31). No detailed bed material data were 

available for the site at Burnside, MS (figure 32); however, attempts to collect a sample of 

surface bed material indicate a hard surface; small amounts of clay were recovered.

Figure 30. Illustration of flow contracted by an embankment constructed in a 

floodplain. 
71 

Table 10. Summary of published field data for contraction and abutment scour. 

Reference Abutment Contraction 

Inglis(34) – G 

Lane and Borland(110) – G 

Wilson(104) – A 

Colson and Wilson(105) – A 

Colson and Wilson(106) – A 

Norman(16) – X, C 

Schnieder et al.(81) – A 

Colson and Schnieder(107) – A 

Blodgett(111) – A 

Blodgett(26) – – 

Wilson, Jr.(108) – A 

Crumrine(112) – A, G 

Blodgett and Harris(113) – X 

Ahmed et al.(114) – A, X 

Butler and Lillycrop(115) – G 

Fischer(17) X – 

Holnbeck et al.(18) X X, C 

Fischer(116) – A 

Brabets(117) – X, C 

Fischer(118) – X, C 

Fischer(118) – A 

Monk(119) – A, G 

Hagerty et al.(120) – A, G 

Crumrine et al.(121) – G 

Hayes(122) – X, C 

Jackson(42) – X 

Parker(123) G – 

Benedict and Caldwell(124) G G 

Mueller and Hitchcock(125) A, G A, G 

X—data presented 

A—additional analysis required 

G—general observations 

C—comparison with computed scour included
72 

Figure 31. Comparison of measured and computed contraction scour at State Route (S.R.) 16 

over the Pearl River near Edinburg, MS, at the left (east) relief bridge (1 ft = 0.305 m). 

23,650 23,700 23,750 23,800 23,850 23,900 23,950

MDOT STATIONING (FT)

285

290

295

300

305

310

315

320

325

330

335

340

345

350

355

ELEVEVATION (FT)

1963 “as-built” bridge cross section

April 13, 1979 (Discharge = 73,900 ft3/s)

Computed contraction scour

1963 “as-built” bridge cross section

April 13, 1979 (Discharge = 73,900 ft3/s)

Computed contraction scour (Discharge = 73,900 ft3/s)

ELEVATION, IN FT

STATIONING, IN FT

23,650 23,700 23,750 23,800 23,850 23,900 23,950

MDOT STATIONING (FT)

285

290

295

300

305

310

315

320

325

330

335

340

345

350

355

ELEVEVATION (FT)

1963 “as-built” bridge cross section

April 13, 1979 (Discharge = 73,900 ft3/s)

Computed contraction scour

1963 “as-built” bridge cross section

April 13, 1979 (Discharge = 73,900 ft3/s)

Computed contraction scour (Discharge = 73,900 ft3/s)

23,650 23,700 23,750 23,800 23,850 23,900 23,950

MDOT STATIONING (FT)

285

290

295

300

305

310

315

320

325

330

335

340

345

350

355

ELEVEVATION (FT)

1963 “as-built” bridge cross section

April 13, 1979 (Discharge = 73,900 ft3/s)

Computed contraction scour

1963 “as-built” bridge cross section

April 13, 1979 (Discharge = 73,900 ft3/s)

Computed contraction scour (Discharge = 73,900 ft3/s)

ELEVATION, IN FT

STATIONING, IN FT

Figure 32. Comparison of measured and computed contraction scour at S.R. 15 

over the Pearl River near Burnside, MS, at the left (south) relief bridge 

(1 ft = 0.305 m). 
73 

Norman presented detailed data and analyses of seven sites in Alaska; these are the most detailed 

and complete data sets available in the literature.(16) These data include concurrent measurements 

of uncontracted and contracted sections during floods, bed material data, velocity data, and 

detailed discussions of the site and data collection procedures. The local scour portions of these 

data are included in the national bridge scour database.(21,22) A summary table for the three sites 

with contraction scour measurements and a comparison with computed values of mean depth in 

the contracted section is presented in table 11. 

Table 11. Comparison of measured mean depth to calculated mean depth at Alaskan bridges 

where contraction was present during flood flows (modified from Norman(16)). 

Computed Depth 

of Scour (m) 

Location 

Section 

Number

Measured 

Depth (m) Straub Laursen Komura 

1 4.51 Uncontracted section 

2 4.78 4.94 4.97 5.09 

4 5.49 6.10 6.25 6.89 

Susitna River, bridge 254, 

8/11/71 

5 5.61 6.16 6.34 7.04 

Tazlina River, bridge 573, 1 2.80 – – – 

9/4/71 2 3.14 2.99 2.99 3.60 

Tanana River, bridge 524, 1 3.96 Uncontracted section 

8/13/71 2 4.69 5.09 5.09 2.99 

Fischer presented post-flood analysis data on contraction scour occurring at S.R. 2 over the 

Weldon River in south-central Iowa.(17) The bridge is 68 m long and is supported by two 

monolithic piers and spill-through abutments. The area consists of rolling hills that surround a 

wide river valley with a floodplain width of 670 m at the site. Bed material at the site consists of 

banks of sandy, silty clay with sand and occasional boulders in the streambed underlain by 

glacial clay. The peak discharge of 1,930 m3

/s was determined by a combined contractedopening and road-overflow measurement of peak discharge using the techniques outlined by 

Matthai and Hulsing.(80,126) The estimated peak discharge was about four times the 100-year 

design flood. High-water marks indicated that the water-surface fall through the bridge was 1.45 

m; the depth of flow over the roadway was about 0.8 m. Bed profiles measured along the 

upstream and downstream edges of the bridge after the flood showed that the spill-through slope 

at the left abutment was scoured about 4 m to approximately the top of the glacial clay. The main 

channel portion of the bridge did not show appreciable scour when compared to the cross section 

from the construction plans; however, the bed elevation was very close to the upper limit of the 

glacial clay and may have refilled with sand before the post-flood measurements. The average 

velocity through the submerged bridge opening was estimated to be 3.0 m/s at the peak, but it 

was likely greater when the entire discharge passed through the bridge before overtopping. 

Downed trees, sand deposition on the floodplain, cobble deposition in the main channel, and 

eroded banks were evidence of the high velocity that passed through the bridge. None of these 

characteristics were observed upstream of the bridge. 
74 

Holnbeck et al. presented a case study of contraction and abutment scour at U.S. 87 over Razor 

Creek in Musselshell County, MT.(18) Razor Creek is an ephemeral stream with a drainage area 

of 44.3 km2. The bridge, constructed in 1955 and recently replaced, had a span of 22.9 m with 

two timber-pile bents at 7.6-m spacing. The streambed has a sand and gravel layer ranging in 

thickness from 1.2 to about 4.4 m, and a grain size distribution of D10, 0.065 mm; D50, 2 mm; 

and D90, 20 mm. Very dense, tan sandstone and weathered shale underlie the sand and gravel to a 

depth of at least 10.7 m. Peak discharges at the bridge resulting from thunderstorms in July 1986 

and June 1991 were estimated by the USGS using the width-contraction, indirect measurement 

method.(80) These two floods are believed to be about three and four times the estimated 100-year 

peak discharge, respectively. Contraction scour is probably not significant at this site for floods 

less than the 1991 flood because the indirect measurement of the 1986 peak discharge indicated 

no water-surface drop through the bridge, but a water-surface drop of 2.4 m was observed for the 

1991 flood. The components of total scour were computed from hydraulics estimated from a 

WSPRO model using the Laursen live bed contraction scour equation, the Froehlich live bed 

abutment scour equation, and the HEC-18 pier scour equation (table 12). The observed scour at 

the abutments included loss of riprap that had been placed before the 1991 flood, which may 

have reduced the depth of scour. The presence of sandstone in the area near the right abutment 

may have also limited scour depth in that area. 

Table 12. Comparison of computed and measured scour at U.S. 87 on Razor Creek, MT, June 

1991 (modified from Holnbeck et al.(18)). 

Computed Scour (m) 

Location Contraction Local Total 

Observed 

Scour (m) 

Left abutment 0.70 2.50 3.20 0.85 

Right abutment 0.70 3.66 4.36 2.23 

Pile bents 0.70 1.43 2.13 0.94 

Brabets presented an analysis of scour at 12 bridges located along the Copper River Highway 

from Flag Point to the Million Dollar Bridge in Alaska.(117) The lower Copper River is a complex 

and dynamic river system. The bridges analyzed are over distributary channels in the 

approximately 777-km2

 delta, and these channels constantly scour and fill, causing lateral 

channel migration. The bed material at bridges 331 and 1187 is medium gravel. Spur dikes were 

constructed at the left upstream abutment of bridge 331 and at both upstream abutments of 

bridge 1187. An approach section surveyed in May 1992 at each bridge was used to evaluate the 

contraction scour at this site (table 13). 
75 

Table 13. Measured and predicted mean depth of flow at bridges 331 and 1187 on the Copper 

River Highway, Alaska, in May 1992 (modified from Brabets(117)). 

Measured Mean 

Depth of Flow (m) Predicted Mean Depth of Flow, y2 (m) 

Bridge y1 y2 Straub Laursen Komura 

331 2.86 3.29 4.21 4.08 4.82 

1187 2.04 2.38 2.50 2.47 2.68 

Fischer presented a case study of scour caused by flooding in September 1992 at S.R. 14 over 

Wolf Creek in Iowa.(118) The bridge is a 30.5-m single-span steel structure supported by vertical 

wall concrete abutments with wingwalls. The floodplain is primarily agricultural and is 

approximately 400 m wide near the bridge. Dense woods line both the upstream and downstream 

sides of the highway near the bridge. Along the right upstream floodplain is woody riparian 

vegetation along the stream; pasture sits further back on the floodplain. The estimated peak 

discharge of 2,200 m3

/s was determined based on high-water marks and modeling of the flow 

using WSPRO. Fischer evaluated gravel deposits left when the road was overtopped and 

estimated the fall through the bridge at incipient road overtopping was 3.4 m.(118) The hydraulic 

analyses indicated that the flow may have been supercritical through the bridge opening at that 

time. The water-surface elevation continued to rise following the overtopping and completely 

submerged the bridge section; the fall through the bridge was reduced to 1.94 m. Information 

from the highway department indicates that backwater caused by channel constrictions 

downstream eventually reduced the fall to about 0.3 m and eventually submerged the bridge 

section. Using the streambed profile from the 1946 bridge plans as a reference, estimates were 

that 6 m of contraction scour occurred during this event. Using the methods outlined in 

Richardson et al., the depth of contraction scour was computed to be 9.1 m.(59) A pond about 

75 m long and 50 m wide downstream of the bridge remained after the flood had receded. This 

large area was scoured by the high-velocity flow exiting the contracted opening. 

Hayes analyzed records at gauging stations in Virginia and Maryland and identified four sites 

with contraction scour.(122) Routine streamflow measurements do not contain an approach-andexit section to use as a reference surface. Hayes analyzed bed-elevation trends in the record and 

selected only stations where no long-term trends were present; he used the cross section 

geometry that existed before the bridge was constructed as the reference surface for these stable 

sites. Each site was visited and the appropriate cross sections surveyed and bed material samples 

collected. WSRPO was used to model the hydraulics at each site to allow computation of 

contraction scour according to the procedures recommended by Richardson and Davis.(6) Table 

14 presents the data and computed depths of contraction scour. The sites on Big Pipe Creek and 

the Northeast Branch Anacostia River are coarse bed material streams with a D50 of about 20 

mm. The other two sites are sand bed streams. It can be seen from the results in table 14 that the 

equations frequently underestimate, and no gross overestimation of scour occurred. 
76 

Table 14. Comparison of measured mean depth to calculated mean depth at bridges where 

contraction was present during flood flows (modified from Hayes(122)). 

Site Date 

Discharge 

(m3/s) 

Mean 

Velocity

(m/s) 

Measured 

Contraction 

Scour (m) 

Laursen Clear 

Water 

Scour (m) 

Laursen Live 

Bed 

Scour (m) 

S.R. 194 over Big Pipe Creek near Bruceville, MD 

 12/30/48 65.1 1.23 1.04 0.18 – 

 1/5/49 51.0 1.18 1.10 0.12 – 

 6/23/72 98.8 1.15 1.07 0.43 – 

 3/19/75 54.9 1.18 0.73 0.00 – 

 9/25/75 109.0 1.43 0.64 0.58 – 

 10/23/90 75.3 1.23 0.82 0.27 – 

 10/23/90 74.5 1.10 0.82 0.34 – 

S.R. 410 over Northeast Branch Anacostia River at Riverdale, MD 

 4/20/40 65.7 1.06 0.27 0.00 – 

 4/20/40 54.9 1.05 0.21 0.00 – 

 8/9/42 83.5 1.14 0.30 0.06 – 

 7/27/45 66.5 1.03 0.34 0.00 – 

 7/27/45 59.5 0.99 0.30 0.00 – 

 4/27/52 55.2 1.04 0.34 0.00 – 

 4/13/61 58.3 1.47 0.40 – – 

 8/25/67 86.4 2.02 0.03 0.85 – 

S.R. 614 over Pamunkey River near Hanover, VA 

 8/25/69 447.3 0.80 2.13 – 1.89 

 6/1/71 265.9 0.66 1.40 – 1.13 

 10/29/71 272.1 0.68 1.34 – 1.19 

 6/25/72 512.5 0.89 2.26 – 1.98 

 3/21/75 311.4 0.80 0.85 – 1.25 

 4/19/83 223.1 0.60 0.76 – 0.91 

 8/20/85 334.1 0.76 1.25 – 1.16 

 5/31/90 419.0 0.87 0.58 – 1.65 

 1/14/91 216.6 0.55 0.85 – 0.88 

 4/12/93 253.7 0.59 1.07 – 1.01 

 4/13/93 233.0 0.54 1.04 – 1.01 

 4/19/93 250.3 0.59 1.10 – 1.07 

 11/30/93 331.3 0.73 1.31 – 1.31 

 3/31/94 546.4 0.91 2.10 – 1.92 

S.R. 42 over Youghiogheny River at Friendsville, MD 

 2/3/50 91.7 1.38 0.18 0.00 – 

 1/11/57 98.2 1.35 0.30 0.00 – 

 5/8/67 142.4 1.85 0.30 0.00 – 

 9/14/71 118.9 1.70 0.21 0.00 – 

 2/25/77 109.3 1.55 0.24 0.00 – 

 11/5/85 362.4 3.05 0.03 0.73 – 

 7/13/90 212.3 2.29 0.30 0.15 – 

 4/1/93 136.5 1.71 0.15 0.00 – 
77 

Jackson presented field data and associated analysis for sites in Ohio that were originally 

selected to minimize contraction and focus on pier scour; thus, contraction scour was only 

observed at three sites.(42) Data were collected from the bridge deck, so no concurrent approachand-exit sections were measured. Jackson assessed the stability of the streambed elevation and 

concluded that the streambed was relatively stable and that use of the low-water approach-andexit sections as the reference surface did not introduce significant error (less than 0.2 m). The 

data are presented in table 15, but no comparison with computed scour was presented. 

Table 15. Contraction scour data published by Jackson.(42)

Measurement Site Date 

Measured 

Contraction 

Scour (m) 

Sediment 

Transport 

Condition 

U.S. 33 over Clear Creek 

near Rockbridge, OH 

 1/28/94 1.5 Clear water 

S.R. 128 over Great Miami 

River at Hamilton, OH 

 7/18/92 2.3 Live bed 

S.R. 22 over Todd Fork at 5/17/90 0.8 Live bed 

Morrow, OH 12/18/90 0.9 Live bed 

 

Benedict and Caldwell presented an interim summary of a study to measure and analyze the 

characteristics of clear water scour at bridges in the Coastal Plain and Piedmont areas of South 

Carolina.(124) The data are being collected during low water and reflect the maximum scour 

resulting from all historical floods at a given site. Field data collection techniques allow the 

identification and adjustment of scour depths for any infilling that may have occurred. Although 

no real-time hydraulic data are being collected, the investigators plan to use WSPRO to model 

and compare the measured scour depths with computed depths of scour obtained by applying the 

methods recommended in Richardson and Davis.(6) 

The Coastal Plain, which comprises about 63 percent of South Carolina, is characterized by thick 

sand deposits, relatively flat stream slopes (0.0002 to 0.004 m/m), and heavily vegetated 

floodplains. The lower Coastal Plain contains many swamps that have a heavily vegetated 

floodplain drained by a network of shallow, poorly defined channels. The root masses of the 

vegetation significantly impede the transport of bed sediments, thus creating clear water scour at 

bridge contractions. The typical scour hole at sites with smaller bridges (bridge length of 61 m or 

less) was observed to develop as a single, long, and deeply cut channel extending from just 

upstream of the bridge to as much as 60 m downstream. Flows from around the left and right 

abutments overlap, and scour is concentrated near the middle of the bridge. As the bridge length 

increases, the observed pattern of scour changes to two separately cut holes near the abutments. 

The patterns described are common, but the scour patterns vary depending upon site-specific 

conditions. The maximum clear water scour observed at 63 sites in the Coastal Plain ranged from 

0.4 to 7.2 m. 

The Piedmont represents about 35 percent of South Carolina; it is characterized by clayey soils, 

moderately steep stream slopes (0.001 to 0.01 m/m), and incised channels on heavily vegetated 

floodplains. Clear water contraction and abutment scour occurs where the abutments are set back 

from the main channel and the embankments contract the flow in the floodplain. The typical 
78 

clear water contraction scour pattern on an overbank is a shallow swale running parallel with and 

directly under the bridge. The typical clear water abutment scour pattern is a shallow swale 

oriented upstream to downstream near the abutment toe. Deviation from these patterns is 

infrequent where the soils are predominantly clay. The maximum clear water scour observed at 

47 sites in the Piedmont range from 0 to 1.4 m. Although sandy floodplains are rare in the 

Piedmont, the clayey soils at some sites have a high sand content. These sites typically have wide 

floodplains with a pronounced contraction at the bridge. The maximum clear water scour 

observed at 18 Piedmont sites with sandy floodplains ranges from 0.9 to 5.5 m. 

The summary of data presented by Benedict and Caldwell provides field validation of concepts 

commonly accepted, but previously not documented by field data.(124) The paper does not present 

any of the raw data, as the analysis of these data is ongoing by the USGS, South Carolina 

District. The results of the project will provide valuable field-based insight into clear water scour 

processes and will inform a substantial database for future research. 

Of the 29 references, only Norman presented detailed data collected during floods.(16) Nearly all 

of the sites presented in the literature would require the compilation of the raw data and 

additional analysis to obtain complete abutment and contraction scour data sets. Only two 

references included data on abutment scour.(18,118) Seven papers presented data on contraction 

scour, and 14 discussed sites that could yield contraction scour data if additional data were 

available to supplement the limited data presented in the papers. 

The comparison of field data with computed scour showed mixed results. Papers by Norman, 

Holnbeck et al., Brabets, and Fischer showed that the contraction scour equations typically 

overpredicted the observed scour and in a few instances, severely overpredicted the scour. (See 

references 16, 18, 117, and 118.) The paper by Hayes showed that the Laursen contraction scour 

equation underpredicted a number of measurements, and no severe overprediction was 

present.(122) The accuracy of the contraction scour equation may depend greatly upon the degree 

of contraction, the flow distribution, the configuration of the approach, and how well the 

hydraulic model represents the true flow distribution. 

ANALYSIS OF REAL-TIME DATA 

General 

During record flooding in the Minnesota River Basin in April 1997, USGS, in cooperation with 

FHWA, deployed the USGS bridge scour data collection team to collect real-time scour 

(contraction and local) measurements at contracted bridge openings. An analysis of two sites that 

were surveyed during the April 1997 flooding is presented. Both contracted bridges span the 

Pomme de Terre River, where an estimated 200-year discharge was measured at the USGS 

Appleton gauging station (05294000) located approximately 19 km downstream of the U.S. 

Route 12 bridge. The compiled field data (channel and floodplain bathymetry, water discharge, 

water-surface elevations, roughness, and bridge geometry) were used to calibrate a stepbackwater model at each site. The hydraulics and predicted depth of scour based on the 

calibrated model were compared with the field measurements. 
79 

U.S. Route 12 over the Pomme de Terre River 

Site Description 

U.S. Route 12 crosses the Pomme de Terre River about 20 km west of Danvers, MN. The 

single-span steel-truss structure was constructed in 1933 with a maximum span length of 26.9 m. 

The bridge has vertical wall abutments with wing walls; each abutment and wing wall rests on 

concrete footings supported on timber piling. Neither abutment was riprapped, nor were there 

any other scour-protection measures. A field investigation conducted in 1995 by a local 

engineering firm, BRW, Inc., revealed no evidence of significant scour at the abutment face.(127)

The upstream floodplain consists of a mixture of open agricultural land with scattered trees and 

brush, with a park on the upstream left bank. The area downstream of the bridge is more heavily 

wooded and is classified on the USGS topographic map as a wetland area. The streambed 

material near the bridge generally consists of fine-grained, organic, silty sand with some gravel. 

A sieve analysis of a surficial bed material sample indicated a median diameter of 0.15 mm. 

Based on the soil borings and blow counts documented in the bridge plans, the bed material 

appears to become harder and denser as depth increases. Because samples were not collected and 

analyzed, it is difficult to ascertain the makeup of the soils at depths below the surface. 

During the April 1997 flood the bridge experienced both contraction and abutment scour. A large 

scour hole developed at the right abutment, scouring below the abutment cutoff wall and 

resulting in failure of the fill material behind the abutment. Slumping of the embankment slope 

and some deformation of the approach highway were observed. Although scour measurements 

showed a scour hole 2 m below the footing of the left abutment, no deformation was observed 

near the left abutment. These conditions resulted in closure of the bridge. Because of the age and 

scheduled replacement of the bridge, it was not repaired but was replaced with a new structure 

after the flood. 

Discussion of Field Data 

Data were collected during the flood (on April 5 and 9, 1997) at U.S. Route 12 over the Pomme 

de Terre River. A crewed boat was deployed during the initial visit on April 5, 1997. The use of 

the crewed boat and an ADCP allowed bathymetry and three-dimensional velocities to be 

measured at the bridge and in the approach and exit sections extending about 100 m upstream 

and 70 m downstream. Heavy vegetation and submerged obstructions in the floodplains limited 

data collection to the main channel. Measurements on April 9 were limited to data collected from 

the bridge deck. Channel bathymetry was measured along the upstream and downstream faces of 

the bridge and at selected locations beneath the bridge using an echo sounder deployed on a 

knee-board. Velocity magnitudes and water discharge were measured using a vertical axis 

current meter. Water-surface elevations were measured by taping down from the top-of-curb on 

the bridge both upstream and downstream, near the left abutment. On April 5, the water-surface 

elevation was 310.70 m above National Geodetic Vertical Datum (NGVD) of 1929 at the 

upstream edge of the bridge, and the total discharge was 141.6 m3

/s. By April 9, the watersurface had risen to an elevation of 311.5 m above NGVD of 1929, and the discharge had 

increased to 162.8 m3/s. 
80 

The direction of flow through the bridge was controlled by the configuration of the upstream 

floodplains. The channel upstream of the bridge was straight, but the left floodplain was much 

wider and carried considerably more flow than the right floodplain. The flow from the left 

floodplain skewed the flow through the bridge opening to about 50° on average. Figure 33 is a 

sketch of spot elevations and the flow direction on April 9, which shows the severe skew of the 

flow to the bridge opening. 

The appropriate reference surface was determined from an analysis of cross sections collected by 

BRW, Inc., on June 5, 1995 and by the USGS during the flood on April 5, 1997. On these two 

dates, cross sections collected approximately 90 m upstream from the bridge showed only about 

0.15 m difference in the channel-bottom elevation. The flood cross section was the lower of the 

two. Downstream from the bridge, the cross section surveyed on June 5, 1995 (approximately 

23 m downstream) had less than 0.3 m in variation when compared with the cross section 

surveyed on April 5, 1997 (approximately 61 m downstream), but was 0.5 m higher than the 

cross section surveyed on April 5, 1997 (approximately 30 m downstream). It is possible that the 

April 5, 1997, cross section could have been affected by the scour at the bridge section; thus, it 

was not considered in setting the reference surface. The WSPRO bridge section surveyed by 

BRW, Inc., on June 5, 1995, showed from 0.3 to 0.6 m of abutment scour in the cross section; 

however, the center of the channel at the bridge appeared to be representative of consistent 

channel slope from the upstream section to the downstream section. Because little general scour 

was observed at the upstream and downstream sections, the mean elevation of the unscoured 

portion of the WSPRO bridge section (elevation 307.9 m) was used as the contraction scour 

reference surface. 

Figure 33. Illustration of U.S. Route 12 over Pomme de Terre River, Minnesota, showing 

spot elevations and surface current patterns on April 9, 1997. 

(Elevations are in meters referenced to NGVD of 1929.) 
81 

A summary of the contraction scour data is shown in table 16. The contracted section on April 5, 

1997, was measured under the bridge from data collected by an ADCP. The maximum erosion of 

the streambed was 2.3 m from the defined reference surface; however, when the entire streambed 

below the bridge was averaged the depth of contraction scour was only 0.9 m. The hydraulic data 

presented for April 5, 1997, were collected with the ADCP. The ADCP data had a significant 

amount of invalid data that were estimated in final processing. Channel banks were not clearly 

delineated in the approach section, creating a degree of uncertainty in the approach discharge. 

Overall, it is suspected that the approach discharge is ±20 percent, and the total discharge is ±10 

percent. Measurements made with a sounding weight on April 9, 1997, were collected during the 

discharge measurement along the upstream face of the bridge, and no approach data are 

available. An echo sounder mounted on a knee-board was also used to make measurements on 

April 9, 1997. The board was floated from upstream to downstream under the bridge; the 

measurements reflect the depths at the upstream or downstream face of the bridge. 

Table 16. Summary of contraction scour measurements at U.S. Route 12 over the Pomme de 

Terre River in Minnesota. 

Measurement 

Number Date Location Equipment 

Scour

Depth

(m) 

Accuracy 

(m) 

1 4/5/97 Centerline ADCP 0.9 0.6 

2 4/9/97 Upstream Sounding weight 3.2 0.6 

3 4/9/97 Upstream Echo sounder 3.8 0.6 

4 4/9/97 Downstream Echo sounder 1.4 0.6 

Contracted Section Uncontracted Section 

Width 

(m) 

Depth 

(m) 

Discharge

(m3/s) 

Average 

Velocity

(m/s) 

Width

(m) 

Depth

(m) 

Discharge 

(m3/s) 

Average 

Velocity

(m/s) 

1 26.8 3.7 142 1.5 21.3 2.4 51.0 1.0 

2 26.8 7.3 163 0.8 – – – – 

3 26.8 7.2 163 0.9 – – – – 

4 26.8 5.3 163 1.2 – – – – 

The reference surface used to determine the depth of abutment scour was the concurrent ambient 

bed; therefore, the depth of abutment scour reported is additional local scour below the depth of 

contraction scour (table 17). The data collected on April 5, 1997, were collected with an ADCP 

using a weighted average of all four beams as the measured depth. Because a weighted average 

was used, it is possible that the local abutment scour was not accurately measured, and no values 

are reported. The cross sections measured on April 9, 1997, all showed a similar pattern with 

abutment scour holes on each side and a sharp mound in between the scour holes but skewed 

toward the left bank (figure 34). It appears that the abutment scour holes may have overlapped. 

The highest elevation in the center of the cross section was subtracted from the reference surface 

to obtain the depth of contraction scour. The abutment scour was reported as the depth below the 

highest elevation in the center of the cross section. All velocities presented in table 17 were from 

the discharge measurement made along the upstream side of the bridge. Although no abutment 
82 

scour was observed on April 5, 1997, the velocities at the abutments were much higher (left = 1.6 

m/s, and right = 1.8 m/s). 

Table 17. Summary of abutment scour data for U.S. Route 12 over the Pomme de Terre River in 

Minnesota. 

Date Abutment Location Equipment

Scour

Depth

(m) 

Accuracy

(m) 

Embankment 

Length 

(m) 

Velocity 

at 

Abutment 

(m/s) 

Depth 

at 

Abutment

(m) 

4/9/97 Right Upstream 

Sounding 

weight 2.4 0.6 307 1.3 9.1 

4/9/97 Right Upstream 

Echo 

sounder 2.1 0.6 307 1.3 9.4 

4/9/97 Right Downstream 

Echo 

sounder 3.4 0.6 307 1.3 8.2 

4/9/97 Left Upstream 

Sounding 

weight 0.9 0.6 121 1.2 7.6 

4/9/97 Left Upstream 

Echo 

sounder 0.5 0.6 121 1.2 7.6 

4/9/97 Left Downstream 

Echo 

sounder 1.8 0.6 121 1.2 6.7 

Model Calibration 

The HEC-RAS model, a one-dimensional step-backwater model, was calibrated to represent the 

field hydraulics as accurately as possible. The bathymetry from the April, 1997, flood was used 

to build the calibration models for the two sets of data (April 5 and 9, 1997). Because bathymetry 

data on April 9, were limited to the upstream and downstream edges of the bridge, the cross 

sections collected on April 5, were used to build the HEC-RAS model for April 9. The majority 

of the floodplain bathymetry utilized in building the models was taken from a full valley section 

found in the original bridge plans and adjusted to be consistent with topographic maps. 

The water-surface elevation observed in the field rose 0.76 m between April 5 and 9. The model 

only showed a 0.3 m change and was unable to reproduce the observed change without 

unreasonable changes to the model input. This large hydraulic variation may be attributed to the 

U.S. Route 12 bridge reach being under a backwater condition because of some unidentified 

downstream condition. Large ice drifts were observed during both site visits, indicating the 

potential for the formation of a debris and (or) ice dam downstream of the data collection area. 

Analysis of the Appleton gauging station records was of little assistance because the gage was 

washed out on April 6, 1997, by the failure of a small upstream dam. The water-surface elevation 

at the upstream side of County Route 22 located about 10 km upstream changed only 0.2 m over 

the same period; therefore, the model was considered adjusted despite the apparent discrepancy 

with the water-surface elevation observed on April 9. 
83 

One of the most important factors in using one-dimensional models at contracted bridges is the 

ability for the model to accurately represent the velocity distribution laterally across the stream 

and floodplain. Figures 35 and 36 depict how the velocity distribution varied between the model 

and field measurements, using the geometry from April 5 and 9, 1997. The distribution shown in 

figure 35 reveals that the flow in the field was indeed skewed toward the right abutment. HECRAS did not duplicate this skewed flow pattern, but rather computed a relatively uniform flow 

distribution across the cross section because the model assigned flow tubes of equal conveyance 

through the geometrically uniform bridge section. For the scoured channel bathymetry, HECRAS did a better job of reproducing the observed velocity distribution (figure 36), although the 

model does not recognize the region of reverse flow that occurred adjacent to the left abutment. 

The HEC-RAS computed velocities are greater near the deeply scoured region adjacent to the 

right abutment because the slope and roughness are constant across the cross section, so the 

conveyance becomes dependent upon the depth of flow.

290 295 300 305 310 315 320

301

303

305

307

309

311

4/5/97 CENTERLINE

4/5/97 UPSTREAM

4/9/97 UPSTREAM

4/9/97 DOWNSTREAM

4/9/97 UPSTREAM (SOUNDING WEIGHT)

6/5/95 DOWNSTREAM

ELEVATION, IN M ABOVE SEA LEVEL

STATION, IN M

313

Left

Abutment

Right

Abutment

290 295 300 305 310 315 320

301

303

305

307

309

311

4/5/97 CENTERLINE

4/5/97 UPSTREAM

4/9/97 UPSTREAM

4/9/97 DOWNSTREAM

4/9/97 UPSTREAM (SOUNDING WEIGHT)

6/5/95 DOWNSTREAM

ELEVATION, IN M ABOVE SEA LEVEL

STATION, IN M

313

Left

Abutment

Right

Abutment

Figure 34. Measured cross sections at U.S. Route 12 over the Pomme de Terre River in 

Minnesota. 
84 

306

308

310

312

314

316

318

320

ELEVATION IN M

290 295 300 305 310 315 320

0.0

0.5

1.0

1.5

2.0

2.5

FIELD - UPSTREAM 5 m

MODEL - UPSTREAM EDGE

FIELD - CENTERLINE

CROSS SECTION

STATION, IN M

VELOCITY, IN M/S

306

308

310

312

314

316

318

320

ELEVATION IN M

290 295 300 305 310 315 320

0.0

0.5

1.0

1.5

2.0

2.5

306

308

310

312

314

316

318

320

ELEVATION IN M

290 295 300 305 310 315 320

0.0

0.5

1.0

1.5

2.0

2.5

FIELD - UPSTREAM 5 m

MODEL - UPSTREAM EDGE

FIELD - CENTERLINE

CROSS SECTION

STATION, IN M

VELOCITY, IN M/S

Left

Abutment

Right

Abutment

Figure 35. Comparison of observed and model velocity distributions at U.S. Route 

12 over the Pomme de Terre River, Minnesota, for April 5, 1997. 

0.0

0.5

1.0

1.5

2.0

300

302

304

306

308

310

312

314

316

318

320

322

324

326

ELEVATION IN M

290 295 300 305 310 315 320

FIELD - UPSTREAM 5 m

MODEL - UPSTREAM EDGE

CROSS SECTION

STATION, IN M

VELOCITY, IN M/S

0.0

0.5

1.0

1.5

2.0

300

302

304

306

308

310

312

314

316

318

320

322

324

326

ELEVATION IN M

290 295 300 305 310 315 320

0.0

0.5

1.0

1.5

2.0

300

302

304

306

308

310

312

314

316

318

320

322

324

326

ELEVATION IN M

290 295 300 305 310 315 320

FIELD - UPSTREAM 5 m

MODEL - UPSTREAM EDGE

CROSS SECTION

STATION, IN M

VELOCITY, IN M/S

Left

Abutment

Right

Abutment

Figure 36. Comparison of observed and model velocity distributions at U.S. 

Route 12 over the Pomme de Terre River, Minnesota, for April 9, 1997. 
85 

Assessment of Scour Computations

The calibrated model was used to assess how accurately the scour for this flood could have been 

predicted. The bathymetry in the calibrated model was replaced with the original bathymetry 

extracted from the BRW, Inc., WSPRO model, which represented the pre-flood condition. The 

discharges from both April 5 and 9, were then run through the HEC-RAS model with the original 

bathymetry to determine the hydraulic parameters required to compute scour at the bridge. The 

contraction scour was computed in HEC-RAS by allowing the model to use the default equation 

(live bed or clear water) depending upon the hydraulic conditions. Table 18 compares the 

observed contraction scour to that computed by the model. 

The computed depth of contraction scour was less than the observed value for all measurements. 

The contraction scour observed on April 9, 1997, may not be typical live bed contraction scour 

because depth of contraction scour could be affected by overlapping abutment scour holes. The 

abutment scour was computed in HEC-RAS by both the Froehlich equation and the Hydraulic in 

the River Environment (HIRE) equation, which are the two equations recommended in HEC-18. 

The HIRE equation is only applicable (but not required) if the embankment length-to-flow depth 

ratio at the abutment is greater than 25. In this case, the embankment to flow depth (L/a) ratio is 

33. Table 19 compares the observed abutment scour to that computed by the model. 

The data summarized in table 19 show the overprediction of scour that is common for abutment 

scour computations. Although the abutment scour equations overpredicted the local scour and 

the contraction scour equation underpredicted the contraction scour (table 18), when added 

together they predicted the total scour with reasonable accuracy and actually underpredicted the 

scour observed at the upstream edge of the bridge on April 9, 1997. These are somewhat 

surprising results that should be viewed with caution because the skew of the flow through the 

bridge could not be accounted for in the one-dimensional model, and the individual components 

were both in error. The agreement, therefore, may be somewhat coincidental. 

Table 18. Comparison of observed to computed contraction scour at U.S. Route 12 over the 

Pomme de Terre River in Minnesota. 

 Location Depth of Scour (m) 

Date (Edge of Bridge) Equation Computed Observed 

4/5/97 Upstream Live bed 0.4 0.9 

4/9/97 Upstream Live bed 0.6 3.8 

4/9/97 Downstream Live bed 0.6 1.4 
86 

Table 19. Comparison of observed to computed abutment and total scour at U.S. Route 12 over 

the Pomme de Terre River in Minnesota. 

 Observed 

Based on 

Froehlich 

Equation 

 

Based on HIRE 

Equation 

Date Abutment 

Location 

(Edge of 

Bridge) Equipment 

Local

Scour

Depth

(m) 

Total 

Scour

Depth

(m) 

Local 

Scour 

Depth 

(m) 

Total 

Scour 

Depth 

(m) 

Local 

Scour 

Depth 

(m) 

Total 

Scour 

Depth 

(m) 

4/9/97 Right Upstream Sounding weight 2.4 5.6 4.6 5.2 10.8 11.4 

4/9/97 Right Upstream Echo sounder 2.1 5.9 4.6 5.2 10.8 11.4 

4/9/97 Right Downstream Echo sounder 3.4 4.8 4.6 5.2 10.8 11.4 

4/9/97 Left Upstream Sounding weight 0.9 4.1 4.0 4.6 5.2 5.8 

4/9/97 Left Upstream Echo sounder 0.5 4.3 4.0 4.6 5.2 5.8 

4/9/97 Left Downstream Echo sounder 1.8 3.2 4.0 4.6 5.2 5.8 

Swift County Route 22 over the Pomme de Terre River 

Site Description 

Swift County Route 22 crosses the Pomme de Terre River near Artichoke Lake, MN, and is 

located 10 km upstream from the U.S. Route 12 bridge. This bridge has two piers in the main 

channel with the abutments set at the edge of the main channel. The spill-through slopes at the 

abutments were protected by riprap and formed the banks of the main channel. The bridge is 

located in a very sinuous reach of the river with two large meanders immediately upstream and 

downstream of the bridge (figure 37). The floodplains are composed of farmland and forest. 

During the flooding in April 1997, the USGS visited this site three times. During all three visits, 

the floodplain flow was concentrated in the right floodplain. This concentration of flow in the 

right floodplain is likely caused by the channel alignment upstream of the bridge. No defined 

point of reattachment along the right embankment was found during the flood. Flow was toward 

the main channel along the entire length of the right embankment. The flow separated from the 

right embankment, nearly perpendicular to the main channel flow, and joined the main flow just 

left of the rightmost pier (figure 38). During the visit on April 5, the flow from the right 

floodplain was so intense that a standing wave formed upstream of the bridge where the 

floodplain and main channel flow began mixing. The area from the rightmost pier to the right 

abutment was primarily slack and reverse flow. The depth of flow at the right abutment 

progressively deepened from 4.5 m on April 4, to 6 m on April 9. On April 9, a portion of the 

right embankment slumped, forcing Swift County officials to temporarily close the bridge until 

riprap was placed to protect the bridge. 
87 

BRIDGE

FL

N

OW

Figure 37. Plan view of Swift County Route 22 over the 

Pomme de Terre River, Minnesota (no scale). 

AREA OF 

STANDING MIXING

WAVE

BOILS AND

REVERSE

FLOW

AREA OF 

STANDING MIXING

WAVE

BOILS AND

REVERSE

FLOW

Left 

Right Bank

Bank

Figure 38. Sketch of flow conditions at Swift County Route 22 over the 

Pomme de Terre River, Minnesota (not to scale). 
88 

Discussion of Field Data 

Data collection efforts were restricted to data that could be collected from the bridge deck for all 

three site visits during the flood (April 4, 5, and 9, 1997). All bathymetry data were collected by 

floating an echo sounder attached to a knee-board across the river; the sounder was controlled by 

a hand line from the bridge. The board was allowed to float downstream; streambed elevations 

were collected as far as 30 m downstream from the bridge. Data collected upstream of the bridge 

were restricted to the upstream edge of the bridge deck and the area around the upstream end of 

the right wing wall. Data could not be collected in the floodplains because of heavy vegetation. 

Velocity magnitudes and water discharge were measured during two of the three site visits using 

a vertical-axis current meter deployed along the upstream edge of the bridge. Water-surface 

elevations were measured at the upstream edge of the bridge from the top of the bridge deck 

between the left most pier and the left abutment. Table 20 summarizes the hydraulic data 

collected during the flood. Additional bathymetry data were collected 21 m upstream from and 

30 m downstream from the bridge after the flood during a low-water site visit on July 15, 1997. 

Figure 39 shows the elevation and geometry changes experienced by the streambed at the bridge 

during the period of data collection. 

The rightmost pier may have had some effect on the depth of scour at the right abutment, yet it is 

difficult to determine the pier’s effect on the depth of local abutment scour. Limited 

measurements upstream of the rightmost pier showed the scour hole extended beyond the 

influence of the pier. The effect of the abutment is believed to be the dominant scouring factor; 

therefore, all scour is credited to the abutment with none reported for the pier. The observed 

velocity in the area at the right abutment dropped considerably as the scour-hole depth increased. 

The velocity at the left abutment held steady through the data collection period, as did the depth 

and shape of the scour hole. All abutment scour measurements were collected from the upstream 

edge of the bridge. 

Table 20. Summary of hydraulic data collected at Swift County Route 22 over the Pomme de 

Terre River in Minnesota. 

Water-Surface Elevation 

(m, NGVD of 1929) 

Velocity 

(m/s) 

Date Upstream Downstream 

Discharge 

(m3/s) Average Maximum 

4/4/97 317.02 316.93 – – – 

4/5/97 317.15 317.06 132 1.3 2.5 

4/9/97 317.34 – 146 1.2 1.8 
89 

Contraction scour is typically computed as the difference in average bed elevation between the 

uncontracted and contracted sections, adjusted for bed slope. Because field measurements could 

not be collected in the uncontracted section during the flood, a cross section collected in 1991 

and included in the bridge plans was used as a reference surface. All contraction scour 

measurements were made along the upstream edge of the bridge. As shown in figure 39, there is 

less than 0.3 m difference in the bed elevation near the center of the channel (beyond the limits 

of the abutment scour holes) between the 1991 cross section and those collected during and after 

the 1997 flood. A value of zero for contraction scour is reported. 

The reference surface used to determine the depth of abutment scour was the concurrent ambient 

bed; therefore, the depth of abutment scour reported is additional local scour below the depth of 

contraction scour, which for this site was negligible. A reference surface at 313.7 m above 

NGVD of 1929 was used to measure local abutment scour. A summary of the abutment scour 

data is presented in table 21. 

180 190 200 210

310

311

312

313

314

315

316

317

4/4/97

4/5/97

4/9/97

BRIDGE PLANS

STATION, IN M

ELEVATION, IN M

PIER PIER

180 190 200 210

310

311

312

313

314

315

316

317

180 190 200 210

310

311

312

313

314

315

316

317

4/4/97

4/5/97

4/9/97

BRIDGE PLANS

STATION, IN M

ELEVATION, IN M

PIER PIER

Left

Abutment

Right

Abutment

Figure 39. Cross sections collected along the upstream edge of Swift County 

Route 22 over the Pomme de Terre River, Minnesota. 
90 

Model Calibration 

The data collected on April 5 and 9, 1997, and July 15, 1997 were utilized to build and calibrate 

the HEC-RAS model. Because no bathymetry data were collected during the flood in either the 

approach or exit sections, low-flow cross sections measured before and after the flood were used. 

The bathymetry data collected on July 15, 1997, along with geometry taken from the bridge 

plans, were the basis for the cross sections upstream and downstream of the bridge crossing. 

Despite the added hydraulic complexities introduced by the meander of the channel near the 

County Route 22 bridge, the HEC-RAS model predicted the water surface at the bridge within 

0.06 m of what was measured in the field on April 5 and 9. When an ineffective flow area 

representing the recirculation zone between the right abutment and the rightmost pier was 

included, the model predicted the water-surface elevation at the bridge within 0.03 m of what 

was observed in the field. 

The velocity distributions from the model and the field compared favorably, although the onedimensional model could not replicate the two-dimensional features of the flow field. Figures 40 

and 41 show the velocity distributions for the model, using the geometry from April 5 and 9, and 

field measurements collected with a vertical-axis current meter along the upstream edge of the 

bridge. The one-dimensional model results did not compare well with the April 5, 1997, 

observations (figure 40). Although the model estimated the peak velocity near the rightmost pier 

reasonably well, the model velocities were too high near the right bank and in the center of the 

main channel and too low along the left bank. Figure 41 shows that the model did a better job 

redistributing the flow after the scour had fully developed. The errors displayed should be 

expected when using a conveyance method to distribute flow that is complex and dominated by 

two-dimensional effects of the contraction. Since data were not available for the approach 

section, no comparisons could be made upstream from the bridge. 

Table 21. Summary of abutment scour field data for Swift County Route 22 over the Pomme de 

Terre River in Minnesota. 

Date Abutment Location Equipment

Observed

Scour 

Depth 

(m) 

Accuracy

(m) 

Embankment

Length 

(m) 

Velocity 

at 

Abutment 

(m) 

Depth 

at 

Abutment

(m) 

4/4/97Right Upstream 

Echo 

sounder 1.2 0.3 157 – 4.2 

4/5/97Right Upstream 

Echo 

sounder 1.2 0.3 162 2.5 4.8 

4/9/97Right Upstream 

Echo 

sounder 3.0 0.5 166 1.0 6.4 

4/4/97Left Upstream 

Echo 

sounder 0.9 0.3 44 – 4.0 

4/5/97Left Upstream 

Echo 

sounder 0.9 0.3 47 1.5 4.4 

4/9/97Left Upstream 

Echo 

sounder 0.6 0.3 50 1.6 4.3 
91 

180 190 200 210

-1

0

1

2

312

314

316

318

320

322

324

ELEVATION, IN M

OBSERVED

MODEL

CROSS SECTION

VELOCITY, IN M/S

STATION, IN M

PIER PIER

180 190 200 210

-1

0

1

2

312

314

316

318

320

322

324

ELEVATION, IN M

OBSERVED

MODEL

CROSS SECTION

VELOCITY, IN M/S

STATION, IN M

PIER PIER

Left

Abutment

Right

Abutment

Figure 40. Comparison of observed and model velocity distributions for April 5, 

1997, at Swift County Route 22 over the Pomme de Terre River, Minnesota. 

Figure 41. Comparison of observed and model velocity distributions for April 9, 1997, 

at Swift County Route 22 over the Pomme de Terre River, Minnesota. 
92 

Assessment of Scour Computations 

The calibrated model was used to assess how accurately the scour for this flood could have been 

predicted. The original geometry of the bridge section was taken from the bridge plans and input 

into the calibrated HEC-RAS model. The approach-and-exit cross sections were modified to be 

consistent with the streambed elevations from the bridge plans. The ineffective flow area 

between the rightmost pier and the right abutment was assumed to be effective since it is unlikely 

that it would have been assumed ineffective without field observations. The discharges from both 

April 5 and 9, 1997, were then modeled with the original bathymetry to determine the hydraulic 

parameters needed for scour computations. The analysis did not include the data collected on 

April 4, 1997, because no hydraulic measurements were made during that site visit. 

The contraction scour was computed in HEC-RAS by allowing the model to use the default 

equation (live bed or clear water) depending upon the hydraulic conditions computed by the 

model. The model correctly predicted little or no contraction scour for the prescribed discharges. 

Abutment scour was computed in HEC-RAS by both the Froehlich and the HIRE equations. The 

data contained in table 22 show that the Froehlich equation did a good job predicting abutment 

scour, when compared to the fully developed scour holes on April 9, 1997. Because the 

equations predict maximum depth of scour, the Froehlich equation correctly overpredicted the 

depth of scour, when compared to the scour holes at the right abutment measured on April 5, 

1997, which had not fully developed. The HIRE equation overpredicted scour for all situations. 

Table 22. Comparison of observed to computed abutment scour at Swift County Route 22 over 

the Pomme de Terre River in Minnesota. 

 Local Scour Depth 

Date Abutment Location Equipment Observed 

(m) 

Froehlich 

Equation 

(m) 

HIRE 

Equation

(m) 

4/5/97 Right Upstream 

Echo 

sounder 1.2 2.9 3.8 

4/5/97 Left Upstream 

Echo 

sounder 0.8 0.7 2.8 

4/9/97 Right Upstream 

Echo 

sounder 3.0 3.3 4.1 

4/9/97 Left Upstream 

Echo 

sounder 0.6 0.9 3.1 
93 

CHAPTER 7: SUMMARY AND CONCLUSIONS 

The analysis and prediction of scour at bridges is complex. Scale models and design 

methodology must account for the variability of site conditions and the potential interaction of 

the various components of scour. Scour at bridges has traditionally been classified into the 

categories of degradation, contraction scour, and local scour (abutment and pier). These 

categories do not explicitly account for the natural scour and deposition that occurs in a river 

during a flood or series of floods. Data collected at bridges during floods must be carefully 

analyzed to determine the magnitude of local and contraction scour. If appropriate reference 

surfaces are not selected, the scour reported may reflect scour caused by other processes. 

Researchers and design engineers have agreed that field data on bridge scour are needed to 

validate laboratory experiments and to ensure the reliability of design methodology. The USGS, 

in cooperation with FHWA and many State highway agencies, has collected and compiled data 

on scour at bridges. The national database has been expanded and now contains 493 local pier 

scour measurements, 18 contraction scour measurements, and 12 abutment scour measurements 

from 79 sites located in 17 States. 

Various researchers have proposed many pier scour equations, but none have accurately and 

conservatively predicted the scour observed in the field. Most equations are based on scaled 

laboratory experiments that did not account for the complexity of the field conditions: Relations 

among dimensionless variables developed from these laboratory experiments did not compare 

well with the field data. The Froehlich Design, HEC-18, HEC-18-K4, HEC-18-K4Mu, HEC-18-

K4Mo (>2 mm), and Mississippi equations proved to be better than other equations for 

predicting pier scour for design purposes. The comparison of the scour depths predicted from 

these equations with scour depths measured in the field clearly showed that processes are 

reflected in the field data that are not correctly accounted for in these equations. Analysis of the 

pier scour data indicated the importance of bed material characteristics as an explanatory 

variable in the predictive equations. A new K4 term for the HEC-18 equation was developed 

based on the relative bed material size (b/D50). 

Although caused by the same geometric contraction of the floodplain, contraction and abutment 

scour have traditionally been studied and treated separately. Contraction scour equations are 

based on theoretically developed equations that do not account for the complexity of the flow 

conditions in the field. Likewise, most abutment scour equations are developed from laboratory 

experiments that oversimplify or ignore many complexities common at highway bridges. Field 

data are needed to evaluate published approaches to computing contraction and abutment scour. 

A review of the literature found 29 references with mention of contraction and (or) abutment 

scour data, but only one presented detailed data collected during floods. Only two references 

included data on abutment scour. Published comparisons of field data with computed scour 

showed mixed results. Four papers showed the contraction scour equation typically 

overpredicted the observed scour and in a few instances severely overpredicted the scour; 

however, one paper showed the Laursen contraction scour equation underpredicted a number of 

measurements, and no severe overprediction was observed. The accuracy of the contraction and 

abutment scour equations recommended in HEC-18 may depend greatly upon the degree of 
94 

contraction, the flow distribution, the configuration of the approach, and how well the hydraulic 

model represents the true flow distribution. 

Comparison of computed abutment and contraction scour depths with depths measured in the 

field for U.S. Route 12 and Swift County Route 22 over the Pomme de Terre River in Minnesota 

provides insight to the capabilities and limitations of using one-dimensional models and the 

available abutment and contraction scour equations to predict scour at contracted bridge 

openings. The application of the methods outlined in HEC-18 to these sites showed a variability 

of results similar to the comparisons published in the literature. HEC-RAS and the equations 

recommended in HEC-18 provided reasonable predictions for maximum total scour at the two 

bridges; however, the magnitudes of the individual components (abutment and contraction) of 

scour did not compare well with the field data. Although field data in the approach sections were 

inadequate to provide a comprehensive evaluation of the ability of a one-dimensional model to 

represent a complex two-dimensional flow field, the comparisons that could be made showed the 

one-dimensional model computed flow distributions comparable to the field data for the fully 

developed scour hole conditions, but were less accurate for initial conditions and in areas of 

highly curvilinear flow. 

Overall, the methodology for computing scour at bridges published in HEC-18 provides 

estimates that are generally conservative, in that the depth of scour is usually overpredicted. The 

complexity and variability of conditions at bridges make the development of predictive 

methodology difficult. The equations oversimplify most conditions, but modification of the 

methodology to account for site complexity and variability is not a simple task. New 

methodologies must balance the desire to fully explain complex processes with the need to 

provide procedures that are time and cost effective to apply. Additional field data and model 

studies are needed to continue to improve scour prediction methodology. 
95 

APPENDIX A: PIER SCOUR FIELD DATA 

Table 23. List of sites. 

Site Stream State 

Route 

Class 

Route 

Number/Name 

Drainage 

Area (km2) 

Slope in 

Vicinity (m/m) 

1 Susitna River AK State 3 29785 0.00040

2 Knik River AK Unknown Old Glenn Hwy. 3108 0.00069

3 Knik River AK State 1 – 0.00100

4 Tazlina River AK State 4 6915 0.00210

5 Tanana River AK State 2 34965 0.00060

6 Tanana River AK State 3 66304 0.00015

7 Snow River AK State 9 389 –

8 Red River AR U.S. 71 124398 –

9 Red River AR Interstate 30 135550 –

10 Red River AR U.S. 82 136428 –

11 South Platte River CO State 37 24859 0.00093

12 South Platte River CO County 87 31391 0.00132

13 Arkansas River CO County 613 – 0.00050

14 Rio Grande River CO U.S. 285 4118 0.00075

15 Leipsic River DE State 9 – –

16 Assawoman Bay DE State 54 – –

17 South Altamaha River GA Interstate 95 36260 –

18 Eel River IN State 59 2279 0.00035

19 Wabash River IN State 163 30355 0.00014

20 White River IN State 157 11375 0.00020

21 Red River LA State 3032 157213 0.00010

22 Red River LA State 3032 157213 0.00010

23 Youghiogheny MD State 42 764 0.00500

24 Big Pipe Creek MD State 194 264 0.00157

25 Choptank River MD State 287 – –

26 Pearl River MS State 25 8107 0.00019

27 Pearl River MS State 25 8107 0.00019

28 Homochitto River MS U.S. 84 469 0.00093

29 Pearl River MS U.S. 98 14815 0.00019

30 Pearl River MS U.S. 98 14815 0.00019

31 Clarks Fork Yellowstone River MT U.S. 310 4685 0.00700

32 Gallatin River MT U.S. 191 2137 0.00630

33 Yellowstone River MT U.S. 89 7366 0.00220

34 Badger Creek MT U.S. 89 619 0.00390

35 Otselic River NY State 23 396 0.00040

36 Chemung River NY State 427 6491 0.00075

37 Schoharie Creek NY State 30 1383 0.00200

38 Susquehanna River NY County 314 5781 0.00057

39 Genesee River NY County Bailey Road 2549 0.00090

40 Delaware River NY City 6 7951 0.00114

41 Great Miami River OH State 128 9402 0.00049
96 

Table 23. List of sites, continued. 

Site Stream State 

Route 

Class 

Route 

Number/Name 

Drainage 

Area (km2) 

Slope in 

Vicinity (m/m) 

42 Hocking River OH State 278 1492 0.00038

43 Honey Creek OH State 67 386 0.00140

44 Little Miami River OH State 350 1748 0.00084

45 Ottawa River OH County 122 337 0.00144

46 Scioto River OH State 4 1368 0.00008

47 Todd Fork OH State 22 679 0.00179

48 Killbuck Creek OH County 621 1197 0.00023

49 Pamunkey River VA State 614 2800 0.00012

50 Nottoway River VA State 653 3680 0.00016

51 Bush River VA U.S. 460 166 0.00110

52 Dan River VA U.S. 501 7071 0.00025

53 Tye River VA State 56 240 0.00290

54 Little Nottoway River VA State 603 – 0.00200

55 Reed Creek VA State 649 – 0.00010

56 North Fork Holston River VA State 633 – 0.00100

57 Mississippi River IL State 51/150 1835274 0.00030

58 Mississippi River MN State 3 95312 0.00022

59 Clear Creek OH U.S. 33 238 0.00190

60 Grand River OH State 84 1774 0.00109

61 Great Miami River OH State 41 2401 0.00023

62 Mad River OH U.S. 36 420 0.00136

63 Massies Creek OH U.S. 68 219 0.00357

64 Maumee River OH U.S. 127 5895 0.00022

65 Salt Creek OH U.S. 50 741 0.00082

66 Scioto River OH State 159 9969 0.00035

67 Sugar Creek OH U.S. 250 805 0.00087

68 Tuscarawas River OH City Walnut Road 1329 0.00008

69 Tascarwas River OH County 14 6216 0.00047

70 Wakatomika Creek OH State 16 363 0.00062

71 Walnut Creek OH County 17 559 0.00068

72 Minnesota River MN County 14 10619 –

73 Pomme De Terre River MN County 22 2165 0.00060

74 Brazos River TX County 2004 113810 0.00030

75 Mississippi River MO Interstate 255 – 0.00010

76 Mississippi River MO State 799 1805230 –

77 Middle Fork Crow River MN State 4 – 0.00100

78 Pomme De Terre River MN U.S. 12 2189 0.00050

79 Agulaize River OH State 198 518 0.00060
97

Table 24. Pier scour observations. 

Measurement Site State Date Time Pier Upstream/ Downstream Pier Type Pier Shape 

Pier 

Width 

(m) 

Pier 

Length 

(m) 

Skew 

(deg)

Velocity 

(m/s) 

Depth 

(m) 

Debris 

Effects Bed Material

D16 

(mm)

D50 

(mm)

(

1 2 AK 7/11/1965 15:30 5 Upstream Single Sharp 1.8 8.8 0 3.7 5.5 Unknown Noncohesive 0.92 5.00 2 3 AK 6/24/1966 – 1 Upstream Single Round 1.5 11.2 0 1.5 2.1 Unknown Noncohesive 0.39 1.80 3 3 AK 6/28/1966 – 1 Upstream Single Round 1.5 11.2 0 0.9 0.9 Unknown Noncohesive 0.39 1.80 4 3 AK 6/24/1966 – 2 Upstream Single Round 1.5 11.2 0 1.6 2.0 Unknown Noncohesive 0.39 1.80 5 3 AK 6/28/1966 – 2 Upstream Single Round 1.5 11.2 0 1.0 0.9 Unknown Noncohesive 0.39 1.80 6 3 AK 6/17/1966 – 3 Upstream Single Round 1.5 11.2 0 0.5 1.2 Unknown Noncohesive 0.08 0.58 7 3 AK 6/24/1966 – 3 Upstream Single Round 1.5 11.2 0 1.6 3.0 Unknown Noncohesive 0.39 1.80 8 3 AK 6/28/1966 – 3 Upstream Single Round 1.5 11.2 0 1.1 1.8 Unknown Noncohesive 0.39 1.80 9 3 AK 6/17/1966 – 4 Upstream Single Round 1.5 11.2 0 0.8 1.5 Unknown Noncohesive 0.08 0.58 

10 3 AK 6/24/1966 – 4 Upstream Single Round 1.5 11.2 0 2.0 3.2 Unknown Noncohesive 0.39 1.80 11 3 AK 6/28/1966 – 4 Upstream Single Round 1.5 11.2 0 1.2 2.4 Unknown Noncohesive 0.39 1.80 12 3 AK 6/17/1966 – 5 Upstream Single Round 1.5 11.2 0 0.9 1.2 Unknown Noncohesive 0.08 0.58 13 3 AK 6/24/1966 – 5 Upstream Single Round 1.5 11.2 0 1.8 3.0 Unknown Noncohesive 0.39 1.80 14 3 AK 6/28/1966 – 5 Upstream Single Round 1.5 11.2 0 1.1 2.3 Unknown Noncohesive 0.39 1.80 15 3 AK 6/17/1966 – 6 Upstream Single Round 1.5 11.2 0 0.3 0.5 Unknown Noncohesive 0.08 0.58 16 3 AK 6/24/1966 – 6 Upstream Single Round 1.5 11.2 0 2.1 2.6 Unknown Noncohesive 0.39 1.80 17 3 AK 6/28/1966 – 6 Upstream Single Round 1.5 11.2 0 1.1 1.5 Unknown Noncohesive 0.39 1.80 18 3 AK 6/17/1966 – 7 Upstream Single Round 1.5 11.2 0 0.2 0.6 Unknown Noncohesive 0.08 0.58 19 3 AK 6/24/1966 – 7 Upstream Single Round 1.5 11.2 0 1.8 3.0 Unknown Noncohesive 0.39 1.80 20 3 AK 6/28/1966 – 7 Upstream Single Round 1.5 11.2 0 1.0 2.0 Unknown Noncohesive 0.39 1.80 21 7 AK 9/23/1970 – 5 Unknown Single Round 1.0 0.0 0 1.6 1.5 Insignificant Noncohesive 3.00 7.60 22 1 AK 7/2/1971 – 1 Upstream Single Sharp 1.5 6.1 0 2.0 5.8 Insignificant Noncohesive 58.00 70.00 

23 1 AK 8/11/1971 – 1 Upstream Single Sharp 1.5 6.1 0 3.0 5.3 Insignificant Noncohesive 58.00 70.00 

24 1 AK 7/2/1971 – 2 Upstream Single Sharp 1.5 6.1 0 2.6 4.1 Insignificant Noncohesive 58.00 70.00 

25 1 AK 8/11/1971 – 2 Unknown Single Sharp 1.5 6.1 0 2.9 6.6 Insignificant Noncohesive 58.00 70.00 

26 1 AK 7/2/1971 – 3 Upstream Single Sharp 1.5 6.1 0 2.1 3.4 Insignificant Noncohesive 58.00 70.00 

27 1 AK 8/11/1971 – 3 Unknown Single Sharp 1.5 6.1 0 3.5 5.2 Insignificant Noncohesive 58.00 70.00 

28 1 AK 7/2/1971 – 4 Downstream Single Sharp 1.5 6.1 0 1.5 4.1 Substantial Noncohesive 58.00 70.00 

29 1 AK 8/11/1971 – 4 Downstream Single Sharp 1.5 6.1 0 2.9 5.3 Substantial Noncohesive 58.00 70.00 

30 5 AK 7/16/1971 – 1 Downstream Single Round 1.5 9.4 37 2.2 3.7 Moderate Noncohesive 5.00 14.00 

31 5 AK 7/16/1971 – 2 Downstream Single Round 1.5 9.4 37 2.2 3.7 Moderate Noncohesive 5.00 14.00 

32 5 AK 7/16/1971 – 3 Downstream Single Round 1.5 9.4 37 2.1 4.6 Moderate Noncohesive 5.00 14.00 

33 5 AK 7/16/1971 – 4 Downstream Single Round 1.5 13.5 37 1.7 4.3 Moderate Noncohesive 5.00 14.00 

34 6 AK 8/17/1967 – 1 Downstream Group Sharp 3.0 14.6 0 2.6 6.7 Moderate Noncohesive 12.00 15.00 

35 4 AK 9/2/1971 – 1 Upstream Group Round 4.6 0.0 0 2.9 3.7 Insignificant Noncohesive 68.00 90.00 1
98

Table 24. Pier scour observations, continued. 

Measurement Site State Date Time Pier Upstream/ Downstream Pier Type Pier Shape

Pier 

Width 

(m) 

Pier 

Length 

(m) 

Skew 

(deg)

Velocity 

(m/s) 

Depth 

(m) 

Debris 

Effects Bed Material 

D16 

(mm)

D50 

(mm) 

36 4 AK 9/4/1971 – 1 Upstream Group Round 4.6 0.0 0 3.5 4.6 Insignificant Noncohesive 68.00 90.037 8 AR 5/9/1990 15:00 8 Downstream Group Round 2.1 9.4 11 2.7 12.3 Unknown Unknown – 0.138 8 AR 5/9/1990 15:00 9 Downstream Group Round 2.1 9.4 8 3.9 13.0 Unknown Unknown – 0.139 9 AR 5/12/1990 14:00 4 Upstream Unknown Sharp 2.1 0.0 0 2.9 10.8 Unknown Unknown – 0.1

40 9 AR 5/12/1990 14:00 5 Upstream Unknown Sharp 2.0 0.0 0 0.7 8.1 Unknown Unknown – 0.1

41 10 AR 5/14/1990 14:00 7 Downstream Group Round 3.0 10.9 0 1.9 11.7 Unknown Unknown – 0.342 10 AR 5/14/1990 14:00 8 Downstream Group Square 3.0 10.9 14 2.3 13.5 Unknown Unknown – 0.343 10 AR 5/14/1990 14:00 10 Downstream Group Round 3.0 10.9 0 1.5 9.1 Unknown Unknown – 0.344 13 CO 5/23/1984 9:00 1 Upstream Single Round 1.2 6.4 12 1.0 2.1 Unknown Noncohesive 0.40 1.1

45 13 CO 5/23/1984 10:30 1 Downstream Single Round 1.2 6.4 12 1.0 2.1 Unknown Noncohesive 0.40 1.1

46 13 CO 6/5/1984 14:00 1 Upstream Single Round 1.2 6.4 0 1.6 2.3 Unknown Noncohesive 0.40 1.147 13 CO 6/5/1984 15:00 1 Downstream Single Round 1.2 6.4 0 1.6 2.3 Unknown Noncohesive 0.40 1.1

48 13 CO 9/27/1984 10:00 1 Upstream Single Round 1.2 6.4 19 0.8 0.7 Unknown Noncohesive 0.34 0.649 13 CO 9/27/1984 11:00 1 Downstream Single Round 1.2 6.4 19 0.8 0.7 Unknown Noncohesive 0.34 0.6

50 13 CO 5/23/1984 9:00 3 Upstream Single Round 1.2 6.4 0 0.7 0.6 Unknown Noncohesive 0.40 1.151 13 CO 5/23/1984 10:30 3 Downstream Single Round 1.2 6.4 0 0.7 0.6 Unknown Noncohesive 0.40 1.1

52 13 CO 6/5/1984 14:00 3 Upstream Single Round 1.2 6.4 0 1.0 1.0 Unknown Noncohesive 0.40 1.153 13 CO 6/5/1984 15:00 3 Downstream Single Round 1.2 6.4 0 1.0 1.0 Unknown Noncohesive 0.40 1.1

54 14 CO 5/22/1984 12:00 1 Upstream Single Sharp 0.9 27.4 26 1.6 1.2 Unknown Noncohesive 8.81 29.8

55 14 CO 5/22/1984 14:00 1 Downstream Single Sharp 0.9 27.4 26 1.6 1.2 Unknown Noncohesive 8.81 29.8

56 14 CO 9/25/1984 8:00 1 Upstream Single Sharp 0.9 27.4 36 1.1 0.3 Unknown Noncohesive 8.81 29.8

57 11 CO 5/21/1984 10:00 3 Upstream Single Sharp 0.5 7.1 20 1.4 1.8 Unknown Noncohesive 0.43 1.1

58 11 CO 5/21/1984 13:00 3 Downstream Single Sharp 0.5 7.1 20 1.4 1.8 Unknown Noncohesive 0.43 1.1

59 11 CO 5/21/1984 10:00 4 Upstream Single Sharp 0.5 7.1 20 1.8 1.6 Unknown Noncohesive 0.43 1.1

60 11 CO 5/21/1984 13:00 4 Downstream Single Sharp 0.5 7.1 20 1.8 1.6 Unknown Noncohesive 0.43 1.1

61 11 CO 6/26/1984 14:00 4 Upstream Single Sharp 0.5 7.1 43 1.0 1.4 Unknown Noncohesive 0.43 1.1

62 11 CO 6/26/1984 16:00 4 Downstream Single Sharp 0.5 7.1 43 1.0 1.4 Unknown Noncohesive 0.43 1.1

63 11 CO 10/3/1984 8:00 4 Upstream Single Sharp 0.5 7.1 0 0.6 0.6 Substantial Noncohesive 0.43 1.864 11 CO 10/3/1984 10:00 4 Downstream Single Sharp 0.5 7.1 0 0.6 0.6 Substantial Noncohesive 0.43 1.865 11 CO 5/21/1984 13:00 5 Downstream Single Sharp 0.5 7.1 20 1.4 1.3 Unknown Noncohesive 0.43 1.1

66 11 CO 6/26/1984 16:00 5 Downstream Single Sharp 0.5 7.1 43 1.0 1.0 Unknown Noncohesive 0.43 1.1

67 11 CO 10/3/1984 10:00 5 Downstream Single Sharp 0.5 7.1 43 0.8 0.7 Unknown Noncohesive 0.43 1.8

68 12 CO 5/18/1984 11:00 1 Upstream Single Square 0.3 7.3 26 0.8 1.0 Unknown Noncohesive 0.40 0.9

69 12 CO 5/18/1984 13:30 1 Downstream Single Square 0.3 7.3 26 0.8 1.0 Unknown Noncohesive 0.40 0.9

70 12 CO 6/25/1984 14:00 1 Upstream Single Square 0.3 7.3 15 0.7 0.6 Unknown Noncohesive 0.40 0.9
99

Table 24. Pier scour observations, continued. 

Measurement Site State Date Time Pier Upstream/ Downstream Pier Type Pier Shape

Pier 

Width 

(m) 

Pier 

Length 

(m) 

Skew 

(deg)

Velocity 

(m/s) 

Depth 

(m) 

Debris 

Effects Bed Material

D16 

(mm)

D50 

(mm)

D8

(mm

71 12 CO 6/25/1984 16:30 1 Downstream Single Square 0.3 7.3 15 0.7 0.6 Unknown Noncohesive 0.40 0.94 372 12 CO 10/2/1984 14:00 1 Upstream Single Square 0.3 7.3 15 0.7 0.2 Unknown Noncohesive 0.30 0.67 273 12 CO 10/2/1984 16:00 1 Downstream Single Square 0.3 7.3 15 0.7 0.2 Unknown Noncohesive 0.30 0.67 274 12 CO 5/18/1984 11:00 2 Upstream Single Square 0.3 7.3 26 1.1 1.0 Substantial Noncohesive 0.40 0.94 375 12 CO 5/18/1984 13:00 2 Downstream Single Square 0.3 7.3 26 1.1 1.0 Substantial Noncohesive 0.40 0.94 376 12 CO 5/18/1984 11:00 3 Upstream Single Square 0.3 7.3 14 1.2 1.0 Unknown Noncohesive 0.40 0.94 377 12 CO 5/18/1984 13:30 3 Downstream Single Square 0.3 7.3 14 1.2 1.0 Unknown Noncohesive 0.40 0.94 378 12 CO 6/25/1984 14:00 3 Upstream Single Square 0.3 7.3 20 0.8 0.3 Unknown Noncohesive 0.40 0.94 379 12 CO 6/25/1984 16:30 3 Downstream Single Square 0.3 7.3 20 0.8 0.3 Unknown Noncohesive 0.40 0.94 380 12 CO 5/18/1984 11:00 4 Upstream Single Square 0.3 7.3 23 1.2 1.3 Substantial Noncohesive 0.40 0.94 381 12 CO 5/18/1984 13:30 4 Downstream Single Square 0.3 7.3 23 1.2 1.3 Substantial Noncohesive 0.40 0.94 382 12 CO 6/25/1984 14:00 4 Upstream Single Square 0.3 7.3 16 1.0 0.3 Unknown Noncohesive 0.40 0.94 383 12 CO 6/25/1984 16:30 4 Downstream Single Square 0.3 7.3 16 1.0 0.3 Unknown Noncohesive 0.40 0.94 384 12 CO 10/2/1984 14:00 4 Upstream Single Square 0.3 7.3 16 0.7 1.0 Unknown Noncohesive 0.30 0.67 285 12 CO 5/18/1984 11:00 5 Upstream Single Square 0.3 7.3 16 1.2 1.9 Unknown Noncohesive 0.40 0.94 386 12 CO 5/18/1984 13:30 5 Downstream Single Square 0.3 7.3 16 1.2 1.9 Unknown Noncohesive 0.40 0.94 387 12 CO 6/25/1984 14:00 5 Upstream Single Square 0.3 7.3 11 1.1 0.4 Unknown Noncohesive 0.40 0.94 388 12 CO 6/25/1984 16:30 5 Downstream Single Square 0.3 7.3 11 1.1 0.4 Unknown Noncohesive 0.40 0.94 389 12 CO 10/2/1984 14:00 5 Upstream Single Square 0.3 7.3 11 1.1 0.9 Unknown Noncohesive 0.40 0.94 390 12 CO 10/2/1984 16:00 5 Downstream Single Square 0.3 7.3 11 1.1 0.9 Unknown Noncohesive 0.40 0.94 391 12 CO 5/18/1984 11:00 6 Upstream Single Square 0.3 7.3 16 1.2 2.7 Unknown Noncohesive 0.40 0.94 392 12 CO 5/18/1984 13:30 6 Downstream Single Square 0.3 7.3 16 1.2 2.7 Unknown Noncohesive 0.40 0.94 393 12 CO 6/25/1984 14:00 6 Upstream Single Square 0.3 7.3 8 1.1 0.5 Unknown Noncohesive 0.40 0.94 3

94 12 CO 6/25/1984 16:30 6 Downstream Single Square 0.3 7.3 8 1.1 0.5 Unknown Noncohesive 0.40 0.94 3

95 12 CO 10/2/1984 14:00 6 Upstream Single Square 0.3 7.3 8 1.1 0.6 Unknown Noncohesive 0.30 0.67 2

96 12 CO 10/2/1984 16:00 6 Downstream Single Square 0.3 7.3 8 1.1 0.6 Unknown Noncohesive 0.30 0.67 2

97 12 CO 5/18/1984 11:00 7 Upstream Single Square 0.3 7.3 14 1.6 2.8 Substantial Noncohesive 0.40 0.94 398 12 CO 5/18/1984 13:30 7 Downstream Single Square 0.3 7.3 14 1.6 2.8 Substantial Noncohesive 0.40 0.94 399 12 CO 6/25/1984 14:00 7 Upstream Single Square 0.3 7.3 13 1.1 0.7 Unknown Noncohesive 0.40 0.94 3

100 12 CO 6/25/1984 16:30 7 Downstream Single Square 0.3 7.3 13 1.1 0.7 Unknown Noncohesive 0.40 0.94 3

101 12 CO 10/2/1984 14:00 7 Upstream Single Square 0.3 7.3 13 0.7 0.7 Unknown Noncohesive 0.30 0.67 2

102 12 CO 10/2/1984 16:00 7 Downstream Single Square 0.3 7.3 13 0.7 0.7 Unknown Noncohesive 0.30 0.67 2

103 12 CO 5/18/1984 11:00 8 Upstream Single Square 0.3 7.3 11 1.3 2.9 Substantial Noncohesive 0.40 0.94 3

104 12 CO 5/18/1984 13:30 8 Downstream Single Square 0.3 7.3 11 1.3 2.9 Substantial Noncohesive 0.40 0.94 3

105 12 CO 10/2/1984 14:00 8 Upstream Single Square 0.3 7.3 11 0.7 0.8 Insignificant Noncohesive 0.30 0.67 2
100

Table 24. Pier scour observations, continued. 

Measurement Site State Date Time Pier Upstream/ Downstream Pier Type Pier Shape 

Pier 

Width 

(m) 

Pier 

Length 

(m) 

Skew 

(deg)

Velocity 

(m/s) 

Depth 

(m) 

Debris 

Effects 

Bed 

Material 

D16 

(mm) 

106 12 CO 10/2/1984 16:00 8 Downstream Single Square 0.3 7.3 11 0.7 0.8 Unknown Noncohesive 0.30 107 16 DE 9/24/1991 14:00 C Upstream Group Cylindrical 0.8 13.1 0 0.3 3.2 Unknown Unknown 0.09 108 16 DE 6/8/1992 9:00 C Upstream Group Cylindrical 0.8 13.1 0 0.3 3.1 Unknown Unknown 0.09 109 16 DE 6/8/1992 10:30 C Upstream Group Cylindrical 0.8 13.1 0 0.3 3.1 Unknown Unknown 0.09 110 16 DE 9/24/1991 14:00 E Upstream Group Cylindrical 0.8 13.1 0 0.3 8.0 Unknown Unknown 0.09 111 16 DE 6/8/1992 9:00 E Upstream Group Cylindrical 0.8 13.1 0 0.5 7.8 Unknown Unknown 0.09 112 16 DE 6/8/1992 10:30 E Upstream Group Cylindrical 0.8 13.1 0 0.5 7.8 Unknown Unknown 0.09 113 16 DE 9/24/1991 14:00 F Upstream Group Cylindrical 0.8 13.1 0 0.4 7.1 Unknown Unknown 0.06 114 16 DE 6/8/1992 9:00 F Upstream Group Cylindrical 0.8 13.1 0 0.5 7.6 Unknown Unknown 0.06 115 16 DE 6/8/1992 10:30 F Upstream Group Cylindrical 0.8 13.1 0 0.5 7.6 Unknown Unknown 0.06 116 16 DE 9/24/1991 14:00 G Upstream Group Cylindrical 0.8 13.1 0 0.3 3.8 Unknown Unknown 0.06 117 16 DE 6/8/1992 10:30 G Upstream Group Cylindrical 0.8 13.1 0 0.3 3.7 Unknown Unknown 0.06 118 16 DE 6/8/1992 9:00 H Upstream Group Cylindrical 0.8 13.1 0 0.2 1.5 Unknown Unknown 0.06 119 16 DE 6/8/1992 10:30 H Upstream Group Cylindrical 0.8 13.1 0 0.2 1.5 Unknown Unknown 0.06 120 16 DE 6/8/1992 9:00 I Upstream Group Cylindrical 0.8 13.1 0 0.2 0.3 Unknown Unknown 0.06 121 15 DE 11/28/1988 16:00 C2 Upstream Group Square 0.4 8.2 0 0.6 4.7 Unknown Unknown 0.06 122 15 DE 11/15/1990 13:30 C2 Upstream Group Square 0.4 8.2 0 0.7 5.3 Unknown Unknown 0.06 123 15 DE 12/2/1990 13:00 C2 Upstream Group Square 0.4 8.2 0 0.4 4.8 Unknown Unknown 0.06 124 15 DE 12/2/1990 14:00 C2 Upstream Group Square 0.4 8.2 0 0.7 4.2 Unknown Unknown 0.06 125 15 DE 12/4/1990 15:00 C2 Upstream Group Square 0.4 8.2 0 0.4 4.7 Unknown Unknown 0.06 126 15 DE 10/7/1991 13:00 C2 Upstream Group Square 0.4 8.2 0 0.5 4.6 Unknown Unknown 0.06 127 15 DE 6/10/1992 9:30 C2 Upstream Group Square 0.4 8.2 0 0.5 4.6 Unknown Unknown 0.06 128 15 DE 6/10/1992 10:30 C2 Upstream Group Square 0.4 8.2 0 0.5 4.2 Unknown Unknown 0.06 129 15 DE 6/10/1992 11:30 C2 Upstream Group Square 0.4 8.2 0 0.5 3.7 Unknown Unknown 0.06 130 15 DE 11/15/1990 13:30 C3 Upstream Group Square 0.4 8.2 0 0.5 4.0 Unknown Unknown 0.06 131 15 DE 6/10/1992 10:30 C3 Upstream Group Square 0.4 8.2 0 0.5 4.9 Unknown Unknown 0.06 132 17 GA 2/12/1990 14:45 11 Upstream Group Square 1.2 9.9 0 0.6 5.7 Insignificant Noncohesive 0.55 133 17 GA 2/12/1990 17:05 11 Upstream Group Square 1.2 9.9 0 0.5 5.5 Insignificant Noncohesive 0.55 134 17 GA 2/12/1990 13:00 12 Upstream Group Square 1.2 10.8 0 0.5 7.1 Insignificant Noncohesive 0.55 135 17 GA 2/12/1990 14:45 12 Upstream Group Square 1.2 10.8 0 0.7 6.4 Insignificant Noncohesive 0.55 136 17 GA 2/12/1990 17:05 12 Upstream Group Square 1.2 10.8 0 0.7 6.2 Insignificant Noncohesive 0.55 137 17 GA 2/12/1990 13:00 13 Upstream Group Square 1.8 10.8 0 0.5 8.0 Insignificant Noncohesive 0.55 138 17 GA 2/12/1990 14:45 13 Upstream Group Square 1.8 10.8 0 0.7 7.6 Insignificant Noncohesive 0.55 139 17 GA 2/12/1990 17:05 13 Upstream Group Square 1.8 10.8 0 0.7 7.0 Insignificant Noncohesive 0.55 140 57 IL 8/3/1993 – 11 Upstream Single Square 4.0 11.6 11 2.4 22.5 Insignificant Noncohesive 0.30 
101

Table 24. Pier scour observations, continued. 

Measurement Site State Date Time Pier Upstream/ Downstream Pier Type Pier Shape 

Pier 

Width 

(m) 

Pier 

Length 

(m) 

Skew 

(deg)

Velocity 

(m/s) 

Depth 

(m) 

Debris 

Effects 

Bed 

Material 

D16 

(mm) 

141 57 IL 8/12/1993 – 11 Upstream Single Square 4.1 11.6 4 2.0 22.4 Insignificant Noncohesive 0.30 142 57 IL 9/13/1993 – 11 Upstream Single Square 4.7 11.6 4 1.8 16.7 Insignificant Noncohesive 0.30 143 18 IN 11/13/1992 12:30 1 Downstream Single Round 0.9 10.2 0 0.3 5.1 Unknown Noncohesive 0.25 

144 18 IN 11/13/1992 10:30 2 Upstream Single Round 0.9 10.2 0 0.8 5.9 Unknown Noncohesive 0.25 145 19 IN 1/3/1991 11:00 8 Upstream Single Round 0.9 10.5 5 1.6 9.4 Unknown Noncohesive 0.24 146 19 IN 1/3/1991 11:45 8 Downstream Single Round 0.9 10.5 5 1.6 9.4 Unknown Noncohesive 0.24 147 20 IN 1/3/1991 12:30 2 Upstream Single Round 0.9 14.5 5 1.1 3.4 Unknown Noncohesive 0.24 148 20 IN 1/4/1991 12:00 2 Upstream Single Round 0.9 14.5 5 1.0 3.2 Unknown Noncohesive 0.24 149 20 IN 1/4/1991 13:00 2 Downstream Single Round 0.9 14.5 5 1.0 3.2 Unknown Noncohesive 0.24 150 20 IN 1/3/1991 12:30 3 Upstream Single Round 0.9 13.1 0 1.3 5.1 Unknown Noncohesive 0.24 

151 20 IN 1/3/1991 12:30 4 Upstream Single Round 0.6 12.8 10 1.6 6.6 Unknown Noncohesive 0.24 152 20 IN 1/4/1991 12:00 4 Upstream Single Round 0.6 12.8 10 1.9 6.2 Unknown Noncohesive 0.24 153 21 LA 5/19/1990 14:20 4 Upstream Single Sharp 4.3 16.5 0 2.6 11.6 Insignificant Noncohesive 0.2154 21 LA 5/19/1990 15:30 4 Downstream Single Sharp 4.3 16.5 0 2.6 12.3 Insignificant Noncohesive 0.2

155 21 LA 5/22/1990 13:35 4 Upstream Single Sharp 4.3 16.5 0 2.1 9.4 Insignificant Noncohesive 0.2

156 21 LA 5/22/1990 14:45 4 Downstream Single Sharp 4.3 16.5 0 2.1 9.4 Insignificant Noncohesive 0.2

157 21 LA 5/19/1990 14:20 5 Upstream Single Sharp 4.3 16.5 0 3.2 11.9 Insignificant Noncohesive 0.2

158 21 LA 5/19/1990 15:30 5 Downstream Single Sharp 4.3 16.5 0 3.2 12.6 Insignificant Noncohesive 0.2

159 21 LA 5/22/1990 13:35 5 Upstream Single Sharp 4.3 16.5 0 2.9 9.8 Insignificant Noncohesive 0.2

160 21 LA 5/22/1990 14:45 5 Downstream Single Sharp 4.3 16.5 0 2.9 9.8 Insignificant Noncohesive 0.2

161 22 LA 5/17/1990 13:30 4 Upstream Single Round 4.3 12.2 0 2.5 11.7 Insignificant Noncohesive 0.2162 22 LA 5/17/1990 15:10 4 Downstream Single Round 4.3 12.2 0 2.5 11.7 Insignificant Noncohesive 0.2163 22 LA 5/19/1990 10:25 4 Upstream Single Round 4.3 12.2 0 2.6 10.8 Insignificant Noncohesive 0.2164 22 LA 5/19/1990 12:55 4 Downstream Single Round 4.3 12.2 0 2.6 11.3 Insignificant Noncohesive 0.2165 22 LA 5/22/1990 8:55 4 Downstream Single Round 4.3 12.2 0 2.1 9.3 Insignificant Noncohesive 0.2166 22 LA 5/22/1990 10:30 4 Upstream Single Round 4.3 12.2 0 2.1 9.3 Insignificant Noncohesive 0.2167 22 LA 5/17/1990 13:30 5 Upstream Single Round 4.3 12.2 0 3.0 12.0 Insignificant Noncohesive 0.2168 22 LA 5/17/1990 15:10 5 Downstream Single Round 4.3 12.2 0 3.0 11.7 Insignificant Noncohesive 0.2169 22 LA 5/19/1990 10:25 5 Upstream Single Round 4.3 12.2 0 3.2 11.2 Insignificant Noncohesive 0.2170 22 LA 5/19/1990 12:55 5 Downstream Single Round 4.3 12.2 0 3.2 11.7 Insignificant Noncohesive 0.2171 22 LA 5/22/1990 8:55 5 Downstream Single Round 4.3 12.2 0 2.9 9.6 Insignificant Noncohesive 0.2172 22 LA 5/22/1990 10:30 5 Upstream Single Round 4.3 12.2 0 2.9 9.7 Insignificant Noncohesive 0.2173 24 MD 6/23/1972 17:30 1 Upstream Single Round 1.2 9.8 0 0.8 3.5 Unknown Unknown 13.0174 24 MD 9/25/1975 19:00 1 Upstream Single Round 1.2 9.8 0 1.3 3.1 Unknown Unknown 13.0175 24 MD 6/23/1972 17:30 2 Upstream Single Round 1.2 9.8 0 1.1 2.4 Unknown Unknown 13.0
102

Table 24. Pier scour observations, continued. 

Measurement Site State Date Time Pier Upstream/ Downstream Pier Type Pier Shape

Pier 

Width 

(m) 

Pier 

Length 

(m) 

Skew 

(deg) 

Velocity 

(m/s) 

Depth 

(m) 

Debris 

Effects Bed Material

D16 

(mm)

D

(m

176 24 MD 9/25/1975 19:00 2 Upstream Single Round 1.2 9.8 0 1.6 2.4 Unknown Unknown 13.00 2177 24 MD 5/29/1990 20:30 2 Upstream Single Round 1.2 9.8 0 1.0 1.9 Unknown Unknown 13.00 2178 24 MD 10/23/1990 15:30 2 Upstream Single Round 1.2 9.8 0 1.6 2.0 Unknown Unknown 13.00 2179 24 MD 10/23/1990 20:00 2 Upstream Single Round 1.2 9.8 0 1.6 3.1 Unknown Unknown 13.00 2180 25 MD 2/23/1989 11:00 CNTR Upstream Single Square 1.2 10.7 0 0.1 1.1 Unknown Unknown 0.18 

181 25 MD 3/25/1989 14:00 CNTR Upstream Single Square 1.2 10.7 0 0.4 2.2 Unknown Unknown 0.18 

182 25 MD 7/28/1991 10:30 CNTR Upstream Single Square 1.2 10.7 0 0.2 1.5 Unknown Unknown 0.18 

183 25 MD 2/23/1989 11:00 LEFT Upstream Single Square 1.2 10.7 0 0.7 1.9 Unknown Noncohesive 0.18 184 25 MD 3/25/1989 14:00 LEFT Upstream Single Square 1.2 10.7 0 0.8 3.1 Unknown Noncohesive 0.18 185 25 MD 7/28/1991 10:30 LEFT Upstream Single Square 1.2 10.7 0 0.8 2.2 Unknown Unknown 0.18 

186 25 MD 2/23/1989 11:00 RT Upstream Single Square 1.2 10.7 0 0.3 1.1 Unknown Unknown 0.18 

187 25 MD 2/23/1989 14:00 RT Upstream Single Square 1.2 10.7 0 0.6 2.1 Unknown Unknown 0.18 

188 25 MD 7/28/1991 10:30 RT Upstream Single Square 1.2 10.7 0 0.3 1.5 Unknown Unknown 0.18 

189 23 MD 7/13/1990 13:00 LEFT Upstream Single Sharp 1.5 12.7 0 2.3 2.4 Unknown Unknown 68.00 10190 23 MD 4/1/1993 9:30 LEFT Upstream Single Sharp 1.5 12.7 0 2.1 2.1 Unknown Unknown 68.00 10191 23 MD 7/13/1990 13:00 RT Upstream Single Sharp 1.5 12.7 0 2.6 3.0 Unknown Unknown 68.00 10192 23 MD 4/1/1993 9:30 RT Upstream Single Sharp 1.5 12.7 0 1.9 2.4 Unknown Unknown 68.00 10193 58 MN 3/28/1969 14:30 9 Upstream Single Square 5.0 9.8 0 0.7 5.9 Insignificant Noncohesive 0.20 

194 58 MN 4/3/1969 11:57 9 Upstream Single Square 4.8 9.8 0 0.8 6.5 Insignificant Noncohesive 0.20 

195 58 MN 4/9/1969 11:52 9 Upstream Single Square 4.5 9.8 0 1.2 8.5 Insignificant Noncohesive 0.20 

196 58 MN 4/15/1969 12:40 9 Upstream Single Square 5.3 9.8 0 1.6 11.8 Insignificant Noncohesive 0.20 

197 58 MN 4/23/1969 11:37 9 Upstream Single Square 5.4 9.8 0 1.4 9.5 Insignificant Noncohesive 0.20 

198 58 MN 4/28/1969 12:15 9 Upstream Single Square 5.5 9.8 0 1.1 8.7 Insignificant Noncohesive 0.20 

199 58 MN 6/4/1969 13:55 9 Upstream Single Square 5.1 9.8 0 0.5 5.7 Unknown Noncohesive 0.20 200 58 MN 3/28/1969 14:30 10 Upstream Single Square 3.3 9.8 0 0.7 4.3 Insignificant Noncohesive 0.20 

201 58 MN 4/3/1969 11:57 10 Upstream Single Square 3.3 9.8 0 0.8 4.8 Insignificant Noncohesive 0.20 

202 58 MN 4/9/1969 11:52 10 Upstream Single Square 3.3 9.8 0 1.2 6.7 Insignificant Noncohesive 0.20 

203 58 MN 4/15/1969 12:40 10 Upstream Single Square 3.1 9.8 0 1.6 9.8 Insignificant Noncohesive 0.20 

204 58 MN 4/23/1969 11:37 10 Upstream Single Square 3.7 9.8 0 1.4 9.8 Insignificant Noncohesive 0.20 

205 58 MN 4/28/1969 12:15 10 Upstream Single Square 3.7 9.8 0 1.1 7.9 Insignificant Noncohesive 0.20 

206 58 MN 6/4/1969 13:35 10 Upstream Single Square 4.1 9.8 0 0.5 4.7 Insignificant Noncohesive 0.20 

207 75 MO 7/14/1993 – 8 Upstream Single Square 2.8 9.0 0 0.0 12.6 Insignificant Noncohesive 0.28 

208 75 MO 7/17/1993 – 8 Upstream Single Square 2.8 9.0 0 1.9 14.3 Insignificant Noncohesive 0.28 

209 75 MO 7/19/1993 – 8 Upstream Single Square 2.8 9.0 0 2.0 15.1 Insignificant Unknown 0.28 210 75 MO 8/17/1993 – 8 Upstream Single Square 2.8 9.0 0 1.2 12.9 Insignificant Noncohesive 0.28 
103

Table 24. Pier scour observations, continued. 

Measurement Site State Date Time Pier Upstream/ Downstream Pier Type Pier Shape 

Pier 

Width 

(m) 

Pier 

Length 

(m) 

Skew 

(deg)

Velocity 

(m/s) 

Depth 

(m) 

Debris 

Effects 

Bed 

Material 

D16 

(mm) 

211 75 MO 9/16/1993 – 8 Upstream Single Square 2.8 9.0 0 1.1 11.0 Insignificant Noncohesive 0.28 212 75 MO 7/14/1993 – 9 Upstream Single Square 2.9 9.0 0 0.0 13.3 Insignificant Noncohesive 0.28 213 75 MO 7/17/1993 – 9 Upstream Single Square 2.9 9.0 0 2.3 14.6 Insignificant Noncohesive 0.28 214 75 MO 7/19/1993 – 9 Upstream Single Square 2.9 9.0 0 2.0 15.4 Insignificant Noncohesive 0.28 215 75 MO 8/17/1993 – 9 Upstream Single Square 2.9 9.0 0 1.4 12.9 Insignificant Noncohesive 0.28 216 75 MO 9/16/1993 – 9 Upstream Single Square 2.9 9.0 0 1.2 11.0 Insignificant Noncohesive 0.28 217 76 MO 7/15/1993 – 10 Upstream Single Sharp 5.5 20.7 0 2.6 20.0 Insignificant Noncohesive 0.55 218 28 MS 1/25/1990 9:00 3 Upstream Single Cylindrical 2.4 2.4 0 1.9 3.0 Insignificant Noncohesive 0.49 

219 28 MS 8/27/1992 10:10 3 Downstream Single Cylindrical 2.4 2.4 0 1.9 2.6 Substantial Noncohesive 0.49 

220 28 MS 12/21/1972 6:00 4 Upstream Single Cylindrical 2.4 2.4 0 2.1 3.9 Unknown Noncohesive 0.49 

221 28 MS 4/25/1973 5:30 4 Upstream Single Cylindrical 2.4 2.4 0 1.9 2.7 Unknown Noncohesive 0.49 222 28 MS 1/25/1990 9:00 4 Upstream Single Cylindrical 2.4 2.4 0 2.1 2.9 Insignificant Noncohesive 0.49 

223 28 MS 8/27/1992 11:10 4 Upstream Single Cylindrical 2.4 2.4 0 2.3 2.7 Insignificant Noncohesive 0.49 

224 28 MS 1/25/1990 9:00 5 Upstream Single Cylindrical 2.4 2.4 0 1.7 3.0 Insignificant Noncohesive 0.49 

225 28 MS 8/27/1992 10:55 5 Upstream Single Cylindrical 2.4 2.4 0 2.0 3.1 Insignificant Noncohesive 0.49 

226 26 MS 2/25/1991 14:30 12L Upstream Group Square 0.4 8.0 28 0.8 2.9 Insignificant Cohesive – 227 26 MS 5/1/1991 10:00 12L Downstream Group Square 0.4 8.0 23 0.8 5.1 Insignificant Cohesive – 228 26 MS 2/25/1991 14:30 14L Upstream Group Square 0.4 8.0 18 1.0 7.3 Insignificant Cohesive – 229 26 MS 5/1/1991 10:00 14L Upstream Group Square 0.4 8.0 16 1.3 6.4 Insignificant Cohesive – 230 26 MS 2/25/1991 14:30 15L Upstream Group Square 0.4 8.0 16 1.2 8.9 Insignificant Noncohesive 0.36 231 26 MS 5/1/1991 10:00 15L Downstream Group Square 0.4 8.0 14 1.3 8.8 Insignificant Noncohesive 0.36 232 26 MS 2/25/1991 14:30 16L Upstream Group Square 0.8 8.0 16 1.2 8.8 Insignificant Noncohesive 0.36 233 26 MS 5/1/1991 10:00 16L Upstream Group Square 0.8 8.0 8 1.4 8.2 Moderate Noncohesive 0.36 234 26 MS 1/31/1990 15:00 17L Upstream Group Cylindrical 1.8 6.2 11 0.9 5.3 Insignificant Noncohesive 0.26 

235 26 MS 2/25/1991 14:30 17L Upstream Group Cylindrical 1.6 6.2 16 1.0 6.7 Insignificant Noncohesive 0.26 

236 26 MS 5/1/1991 10:00 17L Upstream Group Cylindrical 1.4 6.2 11 1.1 6.5 Insignificant Noncohesive 0.26 237 26 MS 1/31/1990 15:00 18L Upstream Group Cylindrical 1.8 6.2 11 0.4 5.3 Moderate Noncohesive 0.26 

238 26 MS 2/25/1991 14:30 18L Upstream Group Cylindrical 1.6 6.2 16 0.6 6.4 Moderate Noncohesive 0.26 

239 26 MS 5/1/1991 10:00 18L Downstream Group Cylindrical 1.5 6.2 14 0.7 6.8 Insignificant Noncohesive 0.26 

240 27 MS 5/1/1991 11:20 15R Upstream Group Square 0.4 8.0 16 1.6 9.3 Insignificant Noncohesive 0.36 241 27 MS 2/25/1991 15:20 16R Upstream Group Square 0.8 8.0 16 1.2 8.4 Insignificant Noncohesive 0.36 242 27 MS 5/1/1991 11:20 16R Downstream Group Square 0.8 8.0 11 1.4 8.1 Insignificant Noncohesive 0.36 243 27 MS 2/25/1991 15:20 17R Upstream Group Cylindrical 1.7 6.2 16 0.9 7.1 Insignificant Noncohesive 0.26 

244 27 MS 5/1/1991 11:20 17R Upstream Group Cylindrical 1.5 6.2 8 1.0 6.6 Insignificant Noncohesive 0.26 245 27 MS 2/25/1991 15:20 18R Upstream Group Cylindrical 1.6 6.2 20 0.7 6.2 Insignificant Noncohesive 0.26 
104

Table 24. Pier scour observations, continued. 

Measurement Site State Date Time Pier Upstream/ Downstream Pier Type Pier Shape

Pier 

Width 

(m) 

Pier 

Length 

(m) 

Skew 

(deg) 

Velocity 

(m/s) 

Depth 

(m) 

Debris 

Effects Bed Material

D16 

(mm)

D

(m

246 27 MS 5/1/1991 11:20 18R Upstream Group Cylindrical 1.6 6.2 14 0.6 7.0 Insignificant Noncohesive 0.26 247 29 MS 1/30/1990 14:15 4 Downstream Single Square 1.6 8.2 14 2.1 6.8 Insignificant Noncohesive 0.39 248 29 MS 5/10/1991 14:45 4 Upstream Single Square 1.6 8.2 8 2.1 7.5 Insignificant Noncohesive 0.39 

249 29 MS 1/30/1990 14:15 5 Upstream Single Square 1.9 8.2 8 2.0 8.6 Insignificant Noncohesive 0.39 

250 29 MS 5/10/1991 14:45 5 Downstream Single Square 1.8 8.2 11 2.0 8.8 Insignificant Noncohesive 0.39 251 29 MS 1/30/1990 14:15 6 Upstream Single Square 1.7 8.1 0 1.1 8.0 Insignificant Noncohesive 0.39 

252 29 MS 5/10/1991 14:45 6 Downstream Single Square 1.7 8.1 11 1.6 9.2 Insignificant Noncohesive 0.39 

253 29 MS 1/30/1990 14:15 7 Upstream Group Square 1.2 7.0 0 0.6 7.0 Insignificant Noncohesive 0.39 254 29 MS 5/10/1991 14:45 7 Downstream Group Square 1.2 7.0 0 0.9 8.8 Insignificant Noncohesive 0.39 

255 30 MS 1/27/1990 14:55 4 Upstream Group Cylindrical 1.6 6.4 8 1.6 8.2 Unknown Noncohesive 0.39 256 30 MS 1/30/1990 15:00 4 Upstream Group Cylindrical 1.7 6.4 8 1.4 7.6 Unknown Noncohesive 0.39 257 30 MS 5/10/1991 10:45 4 Upstream Group Cylindrical 1.7 6.4 11 1.6 8.9 Unknown Noncohesive 0.39 258 30 MS 1/27/1990 12:30 5 Upstream Group Cylindrical 1.7 6.4 22 2.3 8.7 Unknown Noncohesive 0.39 259 30 MS 1/30/1990 15:00 5 Upstream Group Cylindrical 2.0 6.4 8 1.7 8.7 Unknown Noncohesive 0.39 260 30 MS 2/5/1990 17:35 5 Upstream Group Cylindrical 1.8 6.4 16 1.3 7.8 Unknown Noncohesive 0.39 261 30 MS 5/10/1991 10:45 5 Downstream Group Cylindrical 1.7 6.4 11 2.0 8.8 Unknown Noncohesive 0.39 262 30 MS 1/27/1990 14:55 6 Upstream Group Cylindrical 1.7 6.4 16 1.7 9.2 Insignificant Noncohesive 0.39 

263 30 MS 1/30/1990 15:00 6 Downstream Group Cylindrical 1.7 6.4 14 2.1 8.4 Unknown Noncohesive 0.39 

264 30 MS 2/5/1990 17:35 6 Downstream Group Cylindrical 1.8 6.4 18 1.3 7.7 Unknown Noncohesive 0.39 

265 30 MS 5/10/1991 10:45 6 Upstream Group Cylindrical 1.6 6.4 14 2.2 8.3 Unknown Noncohesive 0.39 

266 34 MT 5/21/1991 15:30 P1 Upstream Single Sharp 1.0 11.0 0 1.6 0.5 Moderate Noncohesive 0.35 267 34 MT 6/4/1991 12:40 P1 Upstream Single Sharp 1.0 11.0 0 1.3 0.5 Moderate Noncohesive 0.35 268 34 MT 6/21/1991 14:50 P1 Upstream Single Sharp 1.0 11.0 0 1.3 0.4 Moderate Noncohesive 0.35 269 34 MT 5/21/1991 15:30 P3 Upstream Single Sharp 1.0 11.0 0 1.6 0.4 Moderate Noncohesive 0.35 270 34 MT 6/4/1991 12:40 P3 Upstream Single Sharp 1.0 11.0 0 1.4 0.3 Moderate Noncohesive 0.35 271 34 MT 6/21/1991 14:50 P3 Upstream Single Sharp 1.0 11.0 0 1.6 0.5 Moderate Noncohesive 0.35 272 34 MT 5/21/1991 15:30 P4 Upstream Single Sharp 1.0 11.0 0 1.2 0.3 Moderate Noncohesive 0.35 273 34 MT 6/4/1991 12:40 P4 Upstream Single Sharp 1.0 11.0 0 0.8 0.1 Moderate Noncohesive 0.35 274 34 MT 6/21/1991 14:50 P4 Upstream Single Sharp 1.0 11.0 0 0.6 0.2 Moderate Noncohesive 0.35 275 31 MT 6/6/1991 11:15 1 Upstream Single Sharp 1.3 15.2 5 2.5 2.6 Insignificant Noncohesive 17.00 3

276 31 MT 6/10/1991 12:30 1 Upstream Single Sharp 1.3 15.2 5 2.1 2.0 Insignificant Noncohesive 17.00 3

277 31 MT 6/13/1991 12:30 1 Upstream Single Sharp 1.3 15.2 5 2.1 2.3 Insignificant Noncohesive 17.00 3

278 31 MT 6/18/1991 12:45 1 Upstream Single Sharp 1.3 15.2 5 1.5 1.4 Insignificant Noncohesive 17.00 3

279 31 MT 6/6/1991 11:15 2 Upstream Single Sharp 1.3 15.2 5 2.0 2.2 Insignificant Noncohesive 17.00 3

280 31 MT 6/10/1991 12:30 2 Upstream Single Sharp 1.3 15.2 5 1.8 1.7 Insignificant Noncohesive 17.00 3
105

Table 24. Pier scour observations, continued. 

Measurement Site State Date Time Pier Upstream/ Downstream Pier Type Pier Shape

Pier 

Width 

(m) 

Pier 

Length 

(m) 

Skew 

(deg) 

Velocity 

(m/s) 

Depth 

(m) 

Debris 

Effects Bed Material

D16 

(mm)

D5

(m

281 31 MT 6/13/1991 12:30 2 Upstream Single Sharp 1.3 15.2 5 2.1 1.9 Insignificant Noncohesive 17.00 3282 31 MT 6/18/1991 12:45 2 Upstream Single Sharp 1.3 15.2 5 1.1 1.2 Insignificant Noncohesive 17.00 3

283 32 MT 6/6/1991 13:35 1 Upstream Single Sharp 1.0 12.0 3 2.6 1.5 Insignificant Noncohesive 38.00 9

284 32 MT 6/18/1992 14:45 1 Upstream Single Sharp 1.0 12.0 3 1.6 1.0 Insignificant Noncohesive 38.00 9

285 32 MT 6/23/1993 – 1 Upstream Single Sharp 1.0 12.0 3 1.9 1.0 Insignificant Noncohesive 38.00 9

286 32 MT 6/6/1991 13:35 2 Upstream Single Sharp 1.0 12.0 3 3.2 1.7 Insignificant Noncohesive 38.00 9

287 32 MT 6/18/1992 14:45 2 Upstream Single Sharp 1.0 12.0 3 2.1 1.1 Insignificant Noncohesive 38.00 9

288 32 MT 6/23/1993 0:00 2 Upstream Single Sharp 1.0 12.0 3 2.1 1.2 Insignificant Noncohesive 38.00 9

289 33 MT 5/21/1993 16:10 1 Upstream Single Sharp 0.9 10.4 0 2.4 2.7 Unknown Noncohesive 28.00 7

290 33 MT 5/27/1993 10:00 1 Upstream Single Sharp 0.9 10.4 0 2.5 2.5 Unknown Noncohesive 28.00 7

291 33 MT 6/30/1993 10:30 1 Upstream Single Sharp 0.9 10.4 0 1.5 2.0 Unknown Noncohesive 28.00 7

292 33 MT 5/21/1993 16:10 2 Upstream Single Sharp 1.0 10.4 0 2.3 2.5 Unknown Noncohesive 28.00 7

293 33 MT 5/27/1993 10:00 2 Upstream Single Sharp 0.9 10.4 0 2.4 2.4 Unknown Noncohesive 28.00 7

294 33 MT 6/30/1993 10:30 2 Upstream Single Sharp 1.0 10.4 0 1.5 1.9 Unknown Noncohesive 28.00 7

295 33 MT 5/21/1993 16:10 3 Upstream Single Sharp 0.9 10.4 0 1.0 2.3 Unknown Noncohesive 28.00 7

296 33 MT 5/27/1993 10:00 3 Upstream Single Sharp 0.9 10.4 0 1.1 2.1 Unknown Noncohesive 28.00 7

297 33 MT 6/30/1993 10:30 3 Upstream Single Sharp 0.9 10.4 0 1.1 1.8 Unknown Noncohesive 28.00 7

298 36 NY 10/23/1970 – 1 Upstream Single Round 1.5 14.6 0 2.0 3.8 Unknown Noncohesive 11.00 2

299 36 NY 6/24/1972 – 1 Upstream Single Round 1.5 14.6 0 4.1 8.3 Unknown Noncohesive 11.00 2

300 36 NY 10/23/1970 – 2 Upstream Single Round 1.5 14.6 0 1.8 3.7 Unknown Noncohesive 11.00 2301 36 NY 6/24/1972 – 2 Upstream Single Round 1.5 14.6 0 3.9 9.5 Unknown Noncohesive 11.00 2302 36 NY 3/25/1980 – 2 Upstream Single Round 1.5 14.6 0 2.4 4.9 Unknown Noncohesive 11.00 2303 36 NY 10/23/1970 – 3 Upstream Single Round 1.5 14.6 0 1.5 3.4 Unknown Noncohesive 11.00 2304 36 NY 6/24/1972 – 3 Upstream Single Round 1.5 14.6 0 3.7 9.6 Unknown Noncohesive 11.00 2305 36 NY 9/28/1975 – 3 Upstream Single Round 1.5 14.6 0 3.2 5.8 Unknown Noncohesive 11.00 2306 36 NY 3/25/1980 – 3 Upstream Single Round 1.5 14.6 0 2.3 3.8 Unknown Noncohesive 11.00 2307 36 NY 10/23/1970 – 4 Upstream Single Round 1.5 14.6 0 1.0 2.4 Unknown Noncohesive 11.00 2308 36 NY 6/24/1972 – 4 Upstream Single Round 1.5 14.6 0 3.4 9.7 Unknown Noncohesive 11.00 2309 36 NY 9/28/1975 – 4 Upstream Single Round 1.5 14.6 0 2.9 5.7 Unknown Noncohesive 11.00 2310 36 NY 3/25/1980 – 4 Upstream Single Round 1.5 14.6 0 2.1 3.7 Unknown Noncohesive 11.00 2311 36 NY 10/23/1970 – 5 Upstream Single Round 1.5 14.6 0 0.8 2.0 Unknown Noncohesive 11.00 2312 36 NY 6/24/1972 – 5 Upstream Single Round 1.5 14.6 0 3.2 8.1 Unknown Noncohesive 11.00 2313 36 NY 9/28/1975 – 5 Upstream Single Round 1.5 14.6 0 2.7 5.4 Unknown Noncohesive 11.00 2

314 36 NY 3/25/1980 – 5 Upstream Single Round 1.5 14.6 0 2.0 3.6 Unknown Noncohesive 11.00 2315 36 NY 10/23/1970 – 6 Upstream Single Round 1.5 14.6 0 0.5 1.7 Unknown Noncohesive 11.00 2
106

Table 24. Pier scour observations, continued. 

Measurement Site State Date Time Pier Upstream/ Downstream Pier Type Pier Shape

Pier 

Width 

(m) 

Pier 

Length 

(m) 

Skew 

(deg)

Velocity 

(m/s) 

Depth 

(m) 

Debris 

Effects Bed Material

D16 

(mm)

316 36 NY 6/24/1972 – 6 Upstream Single Round 1.5 14.6 0 2.7 5.8 Unknown Noncohesive 11.00 317 36 NY 9/28/1975 – 6 Upstream Single Round 1.5 14.6 0 2.3 3.8 Unknown Noncohesive 11.00 

318 36 NY 3/25/1980 – 6 Upstream Single Round 1.5 14.6 0 1.6 2.6 Unknown Noncohesive 11.00 

319 40 NY 9/16/1992 – 1 Upstream Single Round 3.0 17.7 0 4.5 7.7 Unknown Noncohesive 14.00 

320 39 NY 10/21/1998 – 3 Upstream Single Sharp 1.8 6.7 0 2.9 6.4 Unknown Noncohesive 18.00 

321 35 NY 10/24/1990 – 1 Upstream Single Round 0.9 12.2 30 2.1 3.1 Insignificant Noncohesive 18.00 

322 37 NY 11/17/1989 – 1 Upstream Single Round 1.7 13.1 0 3.7 5.5 Insignificant Noncohesive 15.00 

323 37 NY 11/17/1989 – 2 Upstream Single Round 1.7 13.1 0 3.4 5.1 Insignificant Noncohesive 15.00 

324 38 NY 8/27/1991 – 2 Upstream Single Round 1.5 12.2 0 2.5 5.5 Unknown Noncohesive 14.00 

325 38 NY 8/27/1991 – 3 Upstream Single Round 1.5 12.2 0 2.7 5.6 Unknown Noncohesive 14.00 

326 38 NY 8/27/1991 – 4 Upstream Single Round 1.5 12.2 0 2.2 5.1 Moderate Noncohesive 14.00 

327 79 OH 7/13/1992 14:30 1 Upstream Single Round 0.8 9.4 40 0.5 3.2 Insignificant Noncohesive 0.01 

328 79 OH 7/17/1992 16:10 1 Upstream Single Round 0.8 9.4 50 0.3 2.2 Insignificant Noncohesive 0.01 

329 59 OH 1/28/1994 10:00 1 Upstream Single Round 1.2 9.4 0 1.1 3.2 Unknown Noncohesive 0.03 

330 60 OH 12/31/1992 11:05 1 Upstream Single Round 1.6 10.9 0 1.5 2.1 Insignificant Noncohesive 0.39 331 60 OH 12/31/1992 11:05 3 Upstream Single Round 1.6 10.9 85 0.8 1.8 Insignificant Noncohesive 0.09 332 41 OH 5/16/1990 10:00 2 Upstream Single Round 1.1 24.9 0 1.4 3.9 Insignificant Noncohesive 0.75 333 41 OH 7/18/1992 12:45 2 Upstream Single Round 1.1 24.9 8 1.5 3.7 Insignificant Noncohesive 0.75 334 41 OH 1/29/1994 9:10 2 Upstream Single Round 1.1 24.9 0 1.8 4.1 Insignificant Noncohesive 0.25 335 41 OH 5/16/1990 10:00 3 Upstream Single Round 1.1 24.9 0 1.4 4.1 Insignificant Noncohesive 0.14 336 61 OH 7/17/1992 11:40 1 Upstream Single Sharp 0.9 17.4 60 0.2 2.2 Insignificant Noncohesive – 

337 61 OH 11/13/1992 12:10 1 Upstream Single Sharp 0.9 17.4 60 0.2 1.8 Insignificant Noncohesive – 

338 61 OH 1/29/1994 10:55 1 Upstream Single Sharp 0.9 17.4 66 0.2 1.6 Unknown Noncohesive 0.5339 61 OH 7/17/1992 11:40 2 Upstream Single Sharp 0.9 17.4 66 0.2 2.3 Insignificant Noncohesive – 

340 61 OH 11/13/1992 12:10 2 Upstream Single Sharp 0.9 17.4 66 0.2 1.9 Insignificant Noncohesive 0.0

341 61 OH 1/29/1994 10:55 2 Upstream Single Sharp 0.9 17.4 66 0.3 1.7 Unknown Noncohesive 3.5342 61 OH 7/17/1992 11:40 3 Upstream Single Sharp 0.9 17.4 63 0.5 2.8 Insignificant Noncohesive 2.2

343 61 OH 11/13/1992 12:10 3 Upstream Single Sharp 0.9 17.4 63 0.5 2.4 Insignificant Noncohesive 1.6

344 61 OH 1/29/1994 10:55 3 Upstream Single Sharp 0.9 17.4 63 0.6 2.4 Unknown Noncohesive 4.0345 61 OH 7/17/1992 11:40 4 Upstream Single Sharp 0.9 17.4 62 0.8 3.0 Insignificant Noncohesive 5.0

346 61 OH 11/13/1992 12:10 4 Upstream Single Sharp 0.9 17.4 64 0.6 2.7 Insignificant Noncohesive 1.4

347 61 OH 1/29/1994 10:55 4 Upstream Single Sharp 0.9 17.4 60 0.7 2.4 Unknown Noncohesive 6.5348 61 OH 7/17/1992 11:40 5 Upstream Single Sharp 0.9 17.4 65 0.8 2.9 Insignificant Noncohesive 1.6

349 61 OH 11/13/1992 12:10 5 Upstream Single Sharp 0.9 17.4 65 0.6 2.6 Insignificant Noncohesive 2.7

350 61 OH 1/29/1994 10:55 5 Upstream Single Sharp 0.9 17.4 66 0.6 2.2 Unknown Noncohesive 4.0
107

Table 24. Pier scour observations, continued. 

Measurement Site State Date Time Pier Upstream/ Downstream Pier Type Pier Shape 

Pier 

Width 

(m) 

Pier 

Length 

(m) 

Skew 

(deg)

Velocity 

(m/s) 

Depth 

(m) 

Debris 

Effects 

Bed 

Material 

D16 

(mm)

351 61 OH 7/17/1992 11:40 6 Upstream Single Sharp 0.9 17.4 60 0.8 2.9 Insignificant Noncohesive 1.00 352 61 OH 11/13/1992 12:10 6 Upstream Single Sharp 0.9 17.4 57 0.9 2.6 Insignificant Noncohesive 9.50 353 61 OH 1/29/1994 10:55 6 Upstream Single Sharp 0.9 17.4 60 0.7 2.3 Unknown Noncohesive 9.00 

354 61 OH 7/17/1992 11:40 7 Upstream Single Sharp 0.9 17.4 60 0.8 3.0 Insignificant Noncohesive 0.98 355 61 OH 11/13/1992 12:10 7 Upstream Single Sharp 0.9 17.4 60 0.7 2.5 Insignificant Noncohesive 7.80 356 61 OH 1/29/1994 10:55 7 Upstream Single Sharp 0.9 17.4 60 0.7 2.3 Unknown Noncohesive 4.20 

357 42 OH 12/31/1990 13:00 1 Upstream Single Round 0.8 9.1 22 1.5 5.3 Insignificant Noncohesive 0.86 358 42 OH 7/14/1992 10:50 1 Upstream Single Round 0.8 9.1 32 0.9 3.6 Insignificant Noncohesive 0.42 359 42 OH 7/17/1992 12:40 1 Upstream Single Round 0.8 9.1 20 1.2 3.5 Insignificant Noncohesive 0.42 360 42 OH 1/29/1994 13:50 1 Upstream Single Round 0.8 9.1 14 1.8 5.2 Unknown Noncohesive 0.68 361 42 OH 12/31/1990 13:00 2 Upstream Single Round 0.8 9.1 16 0.7 4.2 Insignificant Noncohesive 0.03 362 42 OH 7/14/1992 10:50 2 Upstream Single Round 0.8 9.1 12 0.6 2.3 Insignificant Noncohesive 0.05 363 42 OH 7/17/1992 12:40 2 Upstream Single Round 0.8 9.1 8 1.2 2.0 Insignificant Noncohesive 0.05 364 42 OH 1/29/1994 13:50 2 Upstream Single Round 0.8 9.1 0 0.8 3.7 Unknown Noncohesive 0.07 365 43 OH 2/2/1990 15:30 2 Upstream Single Round 1.0 10.9 0 1.2 2.0 Insignificant Noncohesive 9.30 366 43 OH 7/15/1992 14:25 2 Upstream Single Round 0.9 10.9 12 1.2 2.1 Insignificant Noncohesive 12.00 367 43 OH 7/17/1992 10:40 2 Upstream Single Round 0.9 10.9 0 1.2 2.1 Insignificant Noncohesive 12.00 368 48 OH 5/17/1990 12:15 1 Upstream Single Round 0.8 9.6 0 0.7 2.2 Insignificant Noncohesive 0.08 369 48 OH 7/23/1990 13:05 1 Upstream Single Round 0.8 9.6 0 0.9 2.8 Insignificant Noncohesive 0.08 370 44 OH 5/16/1990 14:30 1 Upstream Single Round 0.8 7.4 0 1.4 2.6 Insignificant Noncohesive 14.70 371 44 OH 12/19/1990 10:10 1 Upstream Single Round 0.8 7.4 0 1.5 2.3 Insignificant Noncohesive 14.70 372 44 OH 12/19/1990 10:10 2 Upstream Single Round 0.8 7.4 0 1.1 1.7 Insignificant Noncohesive 12.00 373 62 OH 1/5/1993 12:10 1 Upstream Single Round 1.6 11.3 0 1.0 0.9 Substantial Noncohesive 11.50 374 62 OH 7/2/1993 10:30 1 Upstream Single Round 1.6 11.3 0 1.2 1.6 Substantial Noncohesive 3.80 375 62 OH 1/28/1994 11:25 1 Upstream Single Round 1.6 11.3 0 1.4 2.0 Unknown Noncohesive 3.80 376 63 OH 1/28/1994 10:50 1 Upstream Group Square 0.3 14.4 14 0.3 1.8 Unknown Noncohesive 0.12 

377 64 OH 7/14/1992 10:45 2 Upstream Single Sharp 1.2 16.4 32 0.8 1.7 Insignificant Noncohesive 4.10 378 64 OH 7/18/1992 10:15 2 Upstream Single Sharp 1.2 16.4 37 1.1 3.8 Insignificant Noncohesive 4.10 379 64 OH 1/6/1993 13:10 2 Upstream Single Sharp 1.2 16.4 15 1.5 6.0 Insignificant Noncohesive 4.10 380 64 OH 7/14/1992 10:45 3 Upstream Single Sharp 1.2 16.4 36 0.9 1.5 Insignificant Noncohesive 0.34 381 64 OH 7/18/1992 10:15 3 Upstream Single Sharp 1.2 16.4 32 0.8 3.6 Substantial Noncohesive 0.34 382 64 OH 1/6/1993 13:10 3 Upstream Single Sharp 1.2 16.4 10 0.9 5.8 Moderate Noncohesive 0.18 383 45 OH 8/22/1990 10:40 1 Upstream Single Round 0.8 11.4 0 0.8 1.6 Unknown Noncohesive 0.90 384 45 OH 12/30/1990 14:30 1 Upstream Single Round 0.8 11.4 0 1.3 3.2 Unknown Noncohesive 0.90 385 45 OH 7/17/1992 14:15 1 Upstream Single Round 0.8 11.4 0 1.2 2.3 Moderate Noncohesive 0.66 
108

Table 24. Pier scour observations, continued. 

Measurement Site State Date Time Pier Upstream/ Downstream Pier Type Pier Shape

Pier 

Width 

(m) 

Pier 

Length 

(m) 

Skew 

(deg)

Velocity 

(m/s) 

Depth 

(m) 

Debris 

Effects Bed Material

D16 

(mm

386 45 OH 8/22/1990 10:40 2 Upstream Single Round 0.8 11.4 0 0.2 1.5 Insignificant Noncohesive 0.0387 45 OH 12/30/1990 14:30 2 Upstream Single Round 0.8 11.4 0 0.7 3.1 Insignificant Noncohesive 0.0

388 65 OH 1/28/1994 10:50 1 Upstream Single Round 1.3 8.3 18 1.5 4.5 Insignificant Noncohesive 0.1

389 65 OH 1/28/1994 10:50 2 Upstream Single Round 1.3 8.3 18 1.7 6.0 Unknown Noncohesive 0.4

390 46 OH 5/14/1990 16:05 1 Upstream Single Sharp 1.1 10.2 8 0.4 1.8 Moderate Noncohesive 0.0

391 46 OH 5/18/1990 10:50 1 Upstream Single Sharp 1.1 10.2 8 0.5 2.5 Insignificant Noncohesive 0.0

392 46 OH 12/31/1990 11:55 1 Upstream Single Sharp 1.1 10.2 8 0.8 4.2 Insignificant Noncohesive 0.0

393 66 OH 1/29/1994 11:50 28 Upstream Single Round 1.4 18.3 0 0.9 3.7 Insignificant Noncohesive 0.7

394 66 OH 1/29/1994 11:50 29 Upstream Single Round 1.4 18.3 8 1.5 5.9 Unknown Noncohesive 0.1

395 66 OH 1/29/1994 11:50 30 Upstream Single Round 1.4 18.3 10 1.3 5.7 Unknown Noncohesive 0.0

396 67 OH 1/14/1993 11:15 1 Upstream Single Round 0.9 18.8 0 1.3 1.7 Insignificant Noncohesive 0.7

397 67 OH 12/6/1993 8:20 1 Upstream Single Round 0.9 18.8 0 1.3 1.8 Unknown Noncohesive 6.1

398 69 OH 1/15/1993 10:45 2 Upstream Single Round 1.0 8.9 0 1.2 2.3 Insignificant Noncohesive 0.4

399 69 OH 12/6/1993 10:00 2 Upstream Single Round 1.0 8.9 0 1.5 2.4 Insignificant Noncohesive 1.3

400 69 OH 1/15/1993 10:45 3 Upstream Single Round 1.0 8.9 0 1.3 3.0 Moderate Unknown 4.4401 69 OH 12/6/1993 10:00 3 Upstream Single Round 1.0 8.9 0 1.4 3.1 Unknown Noncohesive 3.7

402 47 OH 5/16/1990 11:45 1 Upstream Single Round 1.1 11.9 0 1.6 1.6 Insignificant Noncohesive 0.6

403 47 OH 5/17/1990 9:05 1 Upstream Single Round 1.1 11.9 0 2.1 2.9 Insignificant Noncohesive 0.6

404 47 OH 12/18/1990 14:30 1 Upstream Single Round 1.1 11.9 0 1.8 3.1 Insignificant Noncohesive 0.6

405 47 OH 1/28/1994 12:50 1 Upstream Single Round 1.1 11.9 0 1.8 2.5 Insignificant Noncohesive 13.0

406 47 OH 5/16/1990 11:45 2 Upstream Single Round 1.3 11.9 0 1.7 2.0 Insignificant Noncohesive 4.5

407 47 OH 5/17/1990 9:05 2 Upstream Single Round 1.3 11.9 0 2.1 3.2 Insignificant Noncohesive 4.5

408 47 OH 12/18/1990 14:30 2 Upstream Single Round 1.4 11.9 0 2.0 3.4 Insignificant Noncohesive 4.5

409 47 OH 1/28/1994 12:50 2 Upstream Single Round 1.3 11.9 0 1.7 2.7 Insignificant Unknown 37.0410 71 OH 7/13/1992 12:30 1 Upstream Single Round 0.6 10.2 0 0.9 2.0 Insignificant Noncohesive 1.2

411 71 OH 7/17/1992 15:00 1 Upstream Single Round 0.6 10.2 0 1.2 3.1 Moderate Noncohesive 1.2412 74 TX 10/23/1994 – 6 Downstream Group Square 22.9 14.0 0 2.4 12.5 Substantial Cohesive – 413 51 VA 5/29/1990 12:00 2 Upstream Single Round 0.8 13.1 0 0.6 2.7 Unknown Unknown 0.2414 52 VA 10/24/1990 11:00 1 Upstream Single Round 1.0 25.3 0 1.6 6.2 Unknown Noncohesive 0.1415 52 VA 10/25/1990 11:00 1 Upstream Single Round 1.0 25.3 0 1.9 7.9 Unknown Noncohesive 0.1416 52 VA 4/22/1992 12:00 1 Upstream Single Round 1.0 25.3 0 1.3 5.8 Unknown Noncohesive 0.1417 52 VA 10/25/1990 11:00 2 Upstream Single Round 1.0 25.3 0 2.2 9.3 Unknown Noncohesive 0.1418 52 VA 4/22/1992 12:00 2 Upstream Single Round 1.0 25.3 0 1.7 8.4 Unknown Noncohesive 0.1419 54 VA 5/2/1989 13:00 1 Upstream Single Round 0.7 8.5 0 0.7 1.7 Unknown Unknown 0.3420 54 VA 8/24/1990 10:30 1 Upstream Single Round 0.7 8.5 0 0.7 1.8 Unknown Unknown 0.3
109

Table 24. Pier scour observations, continued. 

Measurement Site State Date Time Pier Upstream/ Downstream Pier Type Pier Shape

Pier 

Width 

(m) 

Pier 

Length 

(m) 

Skew 

(deg)

Velocity 

(m/s) 

Depth 

(m) 

Debris 

Effects Bed Material

D16 

(mm)

421 54 VA 8/24/1990 14:30 1 Upstream Single Round 0.7 8.5 0 0.6 1.6 Unknown Unknown 0.35 422 54 VA 3/29/1991 12:30 1 Upstream Single Round 0.7 8.5 0 0.4 0.9 Unknown Unknown 0.35 423 54 VA 3/29/1991 15:30 1 Upstream Single Round 0.7 8.5 0 0.5 1.4 Unknown Unknown 0.35 424 54 VA 3/29/1991 17:30 1 Upstream Single Round 0.7 8.5 0 0.5 1.6 Unknown Unknown 0.35 425 54 VA 5/2/1989 13:00 2 Upstream Single Round 0.7 8.5 0 0.2 1.0 Unknown Unknown 0.35 426 54 VA 8/24/1990 10:30 2 Upstream Single Round 0.7 8.5 0 0.3 0.9 Unknown Unknown 0.35 427 54 VA 5/2/1989 13:00 3 Upstream Single Round 0.7 8.5 0 0.2 0.9 Unknown Unknown 0.35 428 54 VA 8/24/1990 10:30 3 Upstream Single Round 0.7 8.5 0 0.3 0.8 Unknown Unknown 0.35 429 54 VA 8/24/1990 14:30 3 Upstream Single Round 0.7 8.5 0 0.2 0.6 Unknown Unknown 0.35 430 54 VA 3/29/1991 17:30 3 Upstream Single Round 0.7 8.5 0 0.5 1.1 Unknown Unknown 0.35 431 56 VA 5/29/1990 11:00 1 Upstream Single Round 0.6 9.0 0 1.2 2.2 Unknown Noncohesive 18.00 

432 56 VA 5/29/1990 11:00 2 Upstream Single Round 0.6 9.0 0 1.4 2.4 Unknown Unknown 18.00 433 56 VA 3/30/1991 10:00 2 Upstream Single Round 0.6 9.0 0 1.4 2.6 Unknown Unknown 18.00 434 56 VA 2/26/1992 10:00 2 Upstream Single Round 0.6 9.0 0 1.4 3.0 Unknown Unknown 18.00 435 56 VA 3/4/1993 13:30 2 Upstream Single Round 0.6 9.0 0 1.5 3.0 Unknown Unknown 18.00 436 56 VA 3/4/1993 17:30 2 Upstream Single Round 0.6 9.0 0 1.6 3.3 Unknown Unknown 18.00 437 50 VA 1/16/1991 10:30 CNTR Upstream Single Round 0.9 9.8 0 0.9 4.7 Unknown Unknown 0.35 438 50 VA 1/18/1991 10:30 CNTR Upstream Single Round 0.9 9.8 0 0.8 4.9 Unknown Unknown 0.35 439 50 VA 4/4/1991 14:30 CNTR Upstream Single Round 0.9 9.8 0 1.0 5.0 Unknown Unknown 0.35 440 50 VA 4/5/1991 12:00 CNTR Upstream Single Round 0.9 9.8 0 1.0 4.8 Unknown Unknown 0.35 441 50 VA 3/12/1992 11:30 CNTR Upstream Single Round 0.9 9.8 0 0.9 4.5 Unknown Unknown 0.35 442 50 VA 3/11/1993 9:30 CNTR Upstream Single Round 0.9 9.8 0 1.1 5.0 Unknown Unknown 0.35 443 50 VA 3/17/1993 9:30 CNTR Upstream Single Round 0.9 9.8 0 1.0 5.1 Unknown Unknown 0.35 444 50 VA 3/18/1993 12:00 CNTR Upstream Single Round 0.9 9.8 0 1.1 4.8 Unknown Unknown 0.35 445 50 VA 3/19/1993 11:00 CNTR Upstream Single Round 0.9 9.8 0 1.1 5.2 Unknown Unknown 0.35 446 50 VA 4/1/1993 10:00 CNTR Upstream Single Round 0.9 9.8 0 1.1 4.7 Unknown Unknown 0.35 447 50 VA 1/16/1991 10:30 LEFT Upstream Single Round 0.9 9.8 0 0.9 4.3 Unknown Unknown 0.3448 50 VA 1/18/1991 10:30 LEFT Upstream Single Round 0.9 9.8 0 0.8 4.4 Unknown Unknown 0.3449 50 VA 4/4/1991 14:30 LEFT Upstream Single Round 0.9 9.8 0 0.9 4.5 Unknown Unknown 0.3450 50 VA 4/5/1991 12:00 LEFT Upstream Single Round 0.9 9.8 0 0.9 4.3 Unknown Unknown 0.3451 50 VA 3/12/1992 11:30 LEFT Upstream Single Round 0.9 9.8 0 0.9 4.4 Unknown Unknown 0.3452 50 VA 3/11/1993 9:30 LEFT Upstream Single Round 0.9 9.8 0 1.0 4.6 Unknown Unknown 0.3453 50 VA 3/17/1993 9:30 LEFT Upstream Single Round 0.9 9.8 0 1.0 4.4 Unknown Unknown 0.3454 50 VA 3/18/1993 12:00 LEFT Upstream Single Round 0.9 9.8 0 1.0 4.6 Unknown Unknown 0.3455 50 VA 3/19/1993 11:00 LEFT Upstream Single Round 0.9 9.8 0 1.0 4.6 Unknown Unknown 0.3
110

Table 24. Pier scour observations, continued. 

Measurement Site State Date Time Pier Upstream/ Downstream Pier Type Pier Shape

Pier 

Width 

(m) 

Pier 

Length 

(m) 

Skew 

(deg)

Velocity 

(m/s) 

Depth 

(m) 

Debris 

Effects Bed Material

D16 

(mm)

456 50 VA 4/1/1993 10:00 LEFT Upstream Single Round 0.9 9.8 0 1.0 4.7 Unknown Unknown 0.3457 49 VA 5/31/1990 13:30 CNTR Upstream Single Round 0.9 10.7 0 1.4 8.7 Unknown Unknown 0.3458 49 VA 1/14/1991 12:00 CNTR Upstream Single Round 0.9 10.7 0 1.0 7.3 Unknown Unknown 0.3459 49 VA 12/13/1992 11:30 CNTR Upstream Single Round 0.9 10.7 0 1.0 4.9 Unknown Unknown 0.3460 49 VA 12/14/1992 10:00 CNTR Upstream Single Round 0.9 10.7 0 1.0 6.9 Unknown Unknown 0.3461 49 VA 12/15/1992 11:00 CNTR Upstream Single Round 0.9 10.7 0 0.9 6.7 Unknown Unknown 0.3462 49 VA 1/11/1993 12:00 CNTR Upstream Single Round 0.9 10.7 0 0.9 6.5 Unknown Unknown 0.3463 49 VA 1/12/1993 10:30 CNTR Upstream Single Round 0.9 10.7 0 0.9 6.6 Unknown Unknown 0.3464 49 VA 4/12/1993 12:00 CNTR Upstream Single Round 0.9 10.7 0 1.0 7.5 Unknown Unknown 0.3465 49 VA 4/13/1993 9:30 CNTR Upstream Single Round 0.9 10.7 0 1.0 7.4 Unknown Unknown 0.3466 49 VA 4/19/1993 10:00 CNTR Upstream Single Round 0.9 10.7 0 1.0 7.6 Unknown Unknown 0.3467 49 VA 5/31/1990 13:30 LEFT Upstream Single Round 0.9 10.7 0 0.5 6.5 Unknown Unknown 0.3468 49 VA 12/13/1992 11:30 LEFT Upstream Single Round 0.9 10.7 0 0.4 5.0 Unknown Unknown 0.3469 49 VA 12/14/1992 10:00 LEFT Upstream Single Round 0.9 10.7 0 0.4 5.3 Unknown Unknown 0.3470 49 VA 12/15/1992 11:00 LEFT Upstream Single Round 0.9 10.7 0 0.3 5.2 Unknown Unknown 0.3471 49 VA 1/11/1993 12:00 LEFT Upstream Single Round 0.9 10.7 0 0.3 4.8 Unknown Unknown 0.3472 49 VA 1/12/1993 10:30 LEFT Upstream Single Round 0.9 10.7 0 0.3 5.0 Unknown Unknown 0.3473 49 VA 4/12/1993 12:00 LEFT Upstream Single Round 0.9 10.7 0 0.5 5.5 Unknown Unknown 0.3474 49 VA 4/13/1993 9:30 LEFT Upstream Single Round 0.9 10.7 0 0.4 5.6 Unknown Unknown 0.3475 49 VA 4/19/1993 10:00 LEFT Upstream Single Round 0.9 10.7 0 0.5 5.8 Unknown Unknown 0.3476 49 VA 5/31/1990 13:30 RT Upstream Single Round 0.9 10.7 0 0.6 8.0 Unknown Unknown 0.3477 49 VA 12/13/1992 11:30 RT Upstream Single Round 0.9 10.7 0 0.4 5.9 Unknown Unknown 0.3478 49 VA 12/14/1992 10:00 RT Upstream Single Round 0.9 10.7 0 0.4 6.1 Unknown Unknown 0.3479 49 VA 12/15/1992 11:00 RT Upstream Single Round 0.9 10.7 0 0.2 5.7 Unknown Unknown 0.3480 49 VA 1/11/1993 12:00 RT Upstream Single Round 0.9 10.7 0 0.3 5.6 Unknown Unknown 0.3481 49 VA 1/12/1993 10:30 RT Upstream Single Round 0.9 10.7 0 0.3 5.9 Unknown Unknown 0.3482 49 VA 4/12/1993 12:00 RT Upstream Single Round 0.9 10.7 0 0.4 6.3 Unknown Unknown 0.3483 49 VA 4/13/1993 9:30 RT Upstream Single Round 0.9 10.7 0 0.3 6.4 Unknown Unknown 0.3484 49 VA 4/19/1993 10:00 RT Upstream Single Round 0.9 10.7 0 0.3 7.1 Unknown Unknown 0.3485 55 VA 3/29/1991 14:30 2 Upstream Single Round 0.6 9.1 0 1.1 0.8 Unknown Noncohesive 38.0

486 55 VA 6/5/1992 10:30 2 Upstream Single Round 0.6 9.1 0 1.7 3.2 Unknown Noncohesive 38.0

487 55 VA 3/24/1993 9:30 2 Upstream Single Round 0.6 9.1 0 2.0 3.2 Unknown Noncohesive 38.0

488 53 VA 5/3/1989 12:00 2 Upstream Single Round 0.6 12.5 0 0.6 0.5 Unknown Noncohesive 33.0

489 53 VA 5/7/1989 9:00 2 Upstream Single Round 0.6 12.5 0 1.6 0.7 Unknown Noncohesive 33.0

490 53 VA 4/22/1992 11:00 2 Upstream Single Round 0.6 12.5 0 1.6 1.7 Unknown Noncohesive 33.0
111

Table 24. Pier scour observations, continued. 

Measurement Site State Date Time Pier Upstream/ Downstream Pier Type Pier Shape

Pier 

Width 

(m) 

Pier 

Length 

(m) 

Skew 

(deg)

Velocity 

(m/s) 

Depth 

(m) 

Debris 

Effects Bed Material 

D16 

(mm)

491 53 VA 5/3/1989 12:00 3 Upstream Single Round 0.6 12.5 0 1.2 1.2 Unknown Noncohesive 33.0492 53 VA 5/7/1989 9:00 3 Upstream Single Round 0.6 12.5 0 1.6 1.5 Unknown Noncohesive 33.0493 53 VA 4/22/1992 11:00 3 Upstream Single Round 0.6 12.5 0 2.6 2.6 Unknown Noncohesive 33.0

113 

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