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Publication Number: FHWARD03052 Date: May 2005 
This report describes the most comprehensive set of realtime 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
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Technical Report Documentation Page
1. Report No. FHWARD03052 
2. Government Accession No. 
3. Recipient's Catalog No. 

4. Title and Subtitle FIELD OBSERVATIONS AND EVALUATIONS OF STREAMBED SCOUR AT BRIDGES 
5. Report Date May 2005 

6. Performing Organization Code  
7. Author(s) David S. Mueller and Chad R. Wagner 
8. Performing Organization Report No. 

9. Performance Organization Name and Address U.S. Geological Survey Water Resources Division 
10. Work Unit No. (TRAIS) 

11. Contract or Grant No. DTFH6193Y00050 

13. Type of Report and Period Covered Final Report: 19931999 

12. Sponsoring Agency and Address Office of Engineering Research and Development 

14. Sponsoring Agency Code 

15. Supplementary
Notes 

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, HEC18, HEC18K4, HEC18K4Mu, HEC18K4Mo (>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 bedmaterial characteristics as a variable in the predictive equations. A new K_{4} term for the HEC18 pierscour equation was developed based on the relative bedmaterial size (b/D_{50}) where b = pier width and D_{50} 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 page) Unclassified 
21. No. of Pages 134 
22. Price 
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SI* (Modern Metric) Conversion Factors
b is the pier width. 
is the effective pier width defined as 
b_{1 }is the bottom width in the uncontracted section. 
b_{2 }is the bottom width in the contracted section. 
c_{a }is the pier location code in the Arkansas pier scour equation, c_{a}= 0 for main channel piers and c_{a}= 1 for piers on the banks of the main channel or on the floodplain. 
D_{10 }is the grain size of bed material for which 10 percent is finer. 
D_{16} is the grain size of bed material for which 16 percent is finer. 
D_{35 }is the grain size of bed material for which 35 percent is finer. 
D_{50 }is the grain size of bed material for which 50 percent is finer; the median grain size. 
D_{84 }is the grain size of bed material for which 84 percent is finer. 
D_{90 }is the grain size of bed material for which 90 percent is finer. 
D_{95 }is the grain size of bed material for which 95 percent is finer. 
D_{99 }Is the grain size of bed material for which 99 percent is finer. 
D_{i}or Dis the grain size of bed material for which i or x percent is finer. 
D_{m }is the mean grain size of the bed material. 
DA is the drainage area. 
D_{CFM }is an average of the coarse grain sizes used by Molinas; see table 3.^{(1)} 
E_{b}is the exponent on the ratio of bottom widths for live bed contraction scour equation. 
E_{n}is the exponent on the ratio of roughness coefficients or live bed contraction scour equation. 
E_{Q }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 &F_{o}is the flow Froude number defined as V_{o}/(gy_{o})^{0.5}. 
F_{p }is the pier Froude number defined as V_{o}/(gb)^{0.5}. 
G is the acceleration of gravity. 
kis the standard normal deviate of i. 
K is a multiplying factor that varies from 1.3 to 2.3 
K_{d } is a coefficient to correct for sediment size by Melville and Sutherland.^{(2)} 
K_{i } is a coefficient to correct the HEC18 equation for sediment size by Molinas; see table 3.^{(1)} 
K_{I }is a coefficient to correct for flow intensity defined by Melville and Sutherland.^{(2)} 
K_{s }is a coefficient to correct for pier shape defined by Melville and Sutherland.^{(2)} 
K_{sc }is a coefficient for pier shape in the Simplified Chinese equation, defined by Gao et al. to be 1 for cylinders, 0.8 for roundnosed piers, and 0.66 for sharp nosedpiers.^{(3)} 
K_{S}_{2 }is a coefficient for pier shape used by Larras and is 1.0 for cylindrical piers and 1.4 for rectangular piers.^{(4)} 
K_{u }is 1.0 for SI units and 1.81 for customary English units in the critical velocity equation. 
K_{y }is a coefficient to correct for flow depth defined by Melville and Sutherland.^{(2)} 
K_{1 }is a coefficient based on the shape of the pier nose, defined as 1.1 for squarenose piers, 1.0 for circular or roundnosed piers, 0.9 of sharpnosed piers, and 1.0 for a group of cylinders. 
K_{2 }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}. 
K_{3 }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. 
K_{4} is a coefficient to correct for bed material size and gradation; see table 3. 
K4MuK_{4} coefficient derived by Mueller.^{(5) } 
K4MoK_{4} coefficient derived by Molinas.^{(1)} 
is a coefficient to correct for flow alignment defined by Melville and Sutherland (1988).^{(2)} 
is a coefficient to correct for flow alignment defined by Melville and Sutherland (1988).^{(2)} 
L is the length of the pier. 
Q_{1}is the discharge in the uncontracted section. 
Q_{2 }is the discharge in the contracted section. 
S is the slope of channel in the vicinity of the bridge. 
V_{o }is the approach velocity for pier scour. 
V_{c }is the critical (incipienttransport) velocity for the D_{50 }size particle. 
V_{cx }is the critical (incipienttransport) velocity for the D_{x} size particle. 
V_{R }is a velocity intensity term used by Richardson and Davis; see table 3.^{(6)} 
is the approach velocity corresponding to critical velocity and incipient scour of the D_{50} in the accelerated flow region at the pier. 
is the approach velocity corresponding to critical velocity and incipient scour of the D_{x} in the accelerated flow region at the pier. 
V_{i} 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)} 
V_{cm }is the critical (incipienttransport) velocity for the coarse size fraction defined by Molinas; see table 3.^{(1)} 
V_{LP }is the live bed peak velocity defined by Sheppard.^{(7)} 
V_{2 } is the velocity in the contracted section. 
y_{o }is the approach depth of flow for pier scour. 
y_{s }is the depth of scour. 
Y_{1 }is the depth in the uncontracted section. 
Y_{2 }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 (ft^{2}/sec). 
Topics: research, infrastructure, hydraulics Keywords: research, infrastructure, hydraulics, Bridge scour, field data, contraction scour, abutment scour, pier scour, local scour, debri TRT Terms: Scour at bridgesUnited StatesMathematical models, BridgesUnited StatesFoundations and piers, Streambeds, Bridge abutments, Bridge piers, Contraction Updated: 04/23/2012
