U.S. Department of Transportation
Federal Highway Administration
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Federal Highway Administration Research and Technology
Coordinating, Developing, and Delivering Highway Transportation Innovations
REPORT 
This report is an archived publication and may contain dated technical, contact, and link information 
Publication Number: FHWAHRT12022 Date: May 2012 
Publication Number: FHWAHRT12022 Date: May 2012 
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The study described in this report was conducted at the Federal Highway Administration's TurnerFairbank Highway Research Center (TFHRC) J. Sterling Jones Hydraulics Laboratory in response to State transportation departments' requests for new design guidance to predict bridge pier scour for coarse bed material. The study included experiments at the TFHRC J. Sterling Jones Hydraulics Laboratory and analysis of data from the Colorado State University and the United States Geological Survey. This report will be of interest to hydraulic engineers and bridge engineers involved in bridge foundation design. It is being distributed as an electronic document through the TFHRC Web site (https://www.fhwa.dot.gov/research/)
Jorge E. PaganOrtiz
Director, Office of Infrastructure
Research and Development
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Technical Report Documentation Page
1. Report No.
FHWAHRT12022 
2. Government Accession No.  3 Recipient's Catalog No.  
4. Title and Subtitle
Pier Scour in ClearWater Conditions with NonUniform Bed Materials 
5. Report Date May 2012 

6. Performing Organization Code  
7. Author(s)
Junke Guo, Oscar Suaznabar, Haoyin Shan, and Jerry Shen 
8. Performing Organization Report No.


9. Performing Organization Name and Address Genex Systems, LLC 
10. Work Unit No. (TRAIS) 

11. Contract or Grant No. DTFH6111D00010 

12. Sponsoring Agency Name and Address
Office of Infrastructure Research and Development 
13. Type of Report and Period Covered
Laboratory Report, 

14. Sponsoring Agency Code


15. Supplementary Notes The Contracting Officer's Technical Representative (COTR) was Kornel Kerenyi (HRDI50). 

16. Abstract
Pier scour design in the United States is currently accomplished through application of the Colorado State University (CSU) equation. Since the Federal Highway Administration recommended the CSU equation in 2001, substantial advances have been made in the understanding of pier scour processes. This report explains a new formulation for describing scour processes and proposes a new equation for pier scour design. A critical review of selected studies is summarized. A simplified scour mechanism is proposed in terms of a pressure gradient resulting from the flowstructure, flowsediment, and sedimentstructure interactions. An equilibrium scour depth equation is proposed based on this understanding of the scour mechanism and is validated and refined by a combination of laboratory and field data. The proposed equation is primarily applicable to clearwater scour conditions with nonuniform coarse bed materials. 

17. Key Words
Bridge scour, CSU equation, Hager number, Local scour, Pier scour, Sediment mixtures, Nonuniform bed material, Coarse bed materials 
18. Distribution Statement
No restrictions. This document is available to the public through the National Technical Information Service; Springfield, VA 22161 

19. Security Classification Unclassified 
20. Security Classification Unclassified 
21. No. of Pages 62 
22. Price 
Form DOT F 1700.7 (872)  Reproduction of completed page authorized 
SI* (Modern Metric) Conversion Factors
Figure 1. Equation. Laursen's equation
Figure 2. Equation. Laursen's equation for potential maximum scour
Figure 3. Equation. Approximate maximum scour
Figure 4. Equation. CSU equation
Figure 5. Equation. CSU equation for clearwater scour at circular piers
Figure 6. Equation. MelvilleChiew equation
Figure 7. Equation. Hager number
Figure 8. Equation. Oliveto and Hager timebased scour
Figure 9. Equation. Form for maximum scour
Figure 10. Equation. SheppardMelville equation
Figure 11. Equation. Critical velocity
Figure 12. Equation. Potential maximum scour derived from SheppardMelville
Figure 13. Equation. Scour for shallow water or wide piers
Figure 14. Equation. Scour for deep water or narrow piers
Figure 15. Equation. Scour proportional to flow and structure parameters
Figure 16. Equation. Scour proportional to flow and sediment parameters
Figure 17. Equation. Scour proportional to sediment and structure parameters
Figure 18. Illustration. Side view of flowstructure interactions in initial scour phase
Figure 19. Illustration. Plan view of flowstructure interactions in initial scour phase
Figure 20. Equation. Bernoulli equation with Prandtl boundary layer theory
Figure 21. Equation. Radial velocity
Figure 22. Equation. Tangential velocity
Figure 23. Equation. Modified Bernoulli equation
Figure 24. Equation. Pressure gradient
Figure 25. Equation. Conservation of vorticity
Figure 26. Illustration. Vortex processes in wake flow region
Figure 27. Equation. Application of Bernoulli's equation
Figure 28. Equation. Reduction of Bernoulli equation
Figure 29. Equation. 1/7th power law
Figure 30. Equation. Stagnation point pressure
Figure 31. Equation. Ratio for stagnation depth
Figure 32. Illustration. Plan view of initial scour phase
Figure 33. Illustration. Side view of equilibrium phase of scour
Figure 34. Illustration. Hydrodynamic force of sediment particle
Figure 35. Equation. Pressure gradient at particle
Figure 36. Equation. Pressure difference
Figure 37. Equation. Hydrodynamic force
Figure 38. Equation. Scour potential
Figure 39. Equation. Scour depth scale relation
Figure 40. Equation. Scour depth and blocking area
Figure 41. Illustration. Upstream view of equilibrium scour
Figure 42. Equation. Relative force strength with D_{50}
Figure 43. Equation. Scour depth
Figure 44. Equation. Scour depth with scaling parameter
Figure 45. Equation. Dimensionless scour depth
Figure 46. Equation. Dimensionless pier scour
Figure 47. Equation. Critical Hager number for pier scour
Figure 48. Equation. Critical particle Hager number
Figure 49. Equation. Approximation for weak scour
Figure 50. Equation. Approximation for strong scour
Figure 51. Illustration. FHWA tilting flume
Figure 52. Illustration. Test section with sediment recess at the pier location
Figure 53. Photo. Automated flume carriage in the J. Sterling Jones Hydraulics Laboratory
Figure 54. Photo. Automated point laser distance sensor (side view)
Figure 55. Photo. Automated point laser distance sensor (top view)
Figure 56. Photo. Bed bathymetry after a pier scour test
Figure 57. Photo. Result of a pier scour test with a graded sediment
Figure 58. Photo. Result of a pier scour test showing the armoring layer
Figure 59. Graph. Comparison of equations for maximum potential scour
Figure 60. Graph. Confirmation of equation form
Figure 61. Equation. Initial design equation
Figure 62. Equation. Design equation showing optimized parameters
Figure 63. Equation. Final design equation
Figure 64. Graph. Predicted versus measured relative scour: proposed equation
Figure 65. Graph. Predicted versus measured relative scour: CSU equation
Figure 66. Graph. Predicted versus measured relative scour: SheppardMelville equation
Figure 67. Equation. Final design equation with correction factors
Table 1. Similitude comparison of data sources
Table 2. Properties of bed materials tested at TFHRC
Table 3. Summary of pier scour tests conducted at TFHRC
Table 4. Bed material properties for CSU experiments
Table 5. Comparison of design equations
Table 6. Summary of TFHRC pier scour tests
Table 7. Summary of CSU laboratory test
Table 8. Field data measurements
B  Channel width, ft 
b  Pier diameter, ft 
D  Sediment size in general, ft 
D/Dt  Material derivative 
D_{*}  Sediment size (no dimension) 
D_{i}  Sediment size where i percent of the sediment is finer by weight, ft 
D_{j}  Size of sediment just under downflow jet, ft 
D_{50}  Median grain size, ft 
F  Froude number (no dimension) 
F  Hydrodynamic force, lb 
ƒ, ƒ_{i}  Functional symbol i = 1, 2, 3 (no dimension) 
F_{ƒ}  Friction 
g  Gravitational acceleration, ft/s2 
H  Hager number defined as (no dimension) 
H_{c}  Critical particle value of H corresponding to the Shields diagram (no dimension) 
H_{cp}  Critical value of H for pier scour (no dimension) 
h  Flow depth, ft 
K_{}1  Correction for pier shape 
K_{2}  Correction for attack angle of approach flow 
K_{3}  Correction for bed form 
K_{4}  Correction for armoring 
p  Pressure, lb/ft2 
Q  Flow 
R_{h}  Hydraulic radius, ft 
R  Radius of pier R = b/2, ft 
r  Distance from center of pier, ft 
t  Time, s 
u  Velocity vector, ft/s 
u  Velocity distribution, ft/s 
u_{r}  Potential velocity in radial direction, ft/s 
u_{ø}  Potential velocity along perimeter, ft/s 
ν  Kinematic viscosity, ft2/s 
V  Approach flow velocity, ft/s 
V_{c}_{}  Critical approach velocity at sediment threshold, ft/s 
V_{j}  Downflow jet with velocity, ft/s 
W  Submerged weight of sediment, lb 
y  Distance from bed, ft 
y_{s}  Scour depth, ft 
γ  Specific weight of water, lb/ft3 
ρ  Density of water, slug/ft3 
ρ_{s}  Density of sediment, slug/ft3 
σ  Sediment nonuniformity (gradation coefficient) defined as (no dimension) 
σ_{cp}  Nonuniformity coefficient for Hcp. 
τ_{o}  Bed shear stress, lb/ft2 
τ_{c}  Critical shear at sediment threshold, lb/ft2 
τ_{1}  Grain bed shear, lb/ft2 
Ø  Angle (no dimension) 
Ω  Vorticity (s1) 
∇^{2}  Laplace operator 
∇  Vector differential operator 