Field Observations and Evaluations of Streambed Scour At Bridges

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 contents or use thereof. This report does not constitute a standard, specification, or regulation.

The U.S. Government does not endorse products or manufacturers. Trade and manufacturers' names appear in this report only because they are considered essential to the object of the document.

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Technical Report Documentation Page

1. Report No.

FHWA-RD-03-052

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
9818 Bluegrass Parkway
Louisville, KY 40299

10. Work Unit No. (TRAIS)

11. Contract or Grant No.

DTFH61-93-Y-00050

13. Type of Report and Period Covered

Final Report: 1993-1999

12. Sponsoring Agency 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: 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 page)

Unclassified

21. No. of Pages

134

22. Price

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

SI* (Modern Metric) Conversion Factors

LIST OF FIGURES

1	Illustration of a specific gage plot showing stream degradation
2	Example of short-term scour and fill with no long-term changes
3	Illustration of reference surface for contraction scour
4	Illustration of reference surface for contraction scour
5	Illustration of a reference surface for clear water contraction scour with material deposited immediately downstream of the scour hole
6	Illustration of reference surface sketched on a cross section pilot
7	Scatterplots of computed versus observed scour, in meters (m), for selected pier scour equations.
8	Evaluation of residuals for the Froehlich equation
9	Evaluation of residuals for the Froehlich Design equation
10	Evaluation of residuals for the HEC-18-K4 equation
11	Evaluation of residuals for the HEC-18-K4Mu equation
12	Evaluation of residuals for the Mississippi equation
13	Evaluation of residuals for the HEC-18-K4Mo equation
14	Box plot illustrating the effect of pier shape on relative depth of scour
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
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
17	Effect of gradation and relative velocity on relative depth of scour for field data, with hand-drawn envelope curves for selected gradation classes.
18	Effect of relative sediment size on relative depth of scour for field data
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
20	Effect of relative flow depth on relative depth of scour with field data compared to the relation presented by Melville and Sutherland(2)
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
22	Scatterplot matrix and frequency distribution of basic variables and depth of scour, log-transformed.
23	Example of difference between unweighted regression and weighted regression in developing a design curve
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)
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
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.
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
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
29	Relation between relative errors in computed scour using the HEC-18 equation and relative bed material size
30	Illustration of flow contracted by an embankment constructed in a floodplain
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)
32	Comparison of measured and computed contraction scour at S.R. 15 over the PearlRiver near Burnside, MS, at the left (south) relief bridge (1 ft = 0.305 m)
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.)
34	Measured cross sections at U.S. Route 12 over the Pomme de Terre River in Minnesota.
35	Comparison of observed and model velocity distributions at U.S. Route 12 over the Pomme de Terre River, Minnesota, for April 5, 1997
36	Comparison of observed and model velocity distributions at U.S. Route 12 over the Pomme de Terre River, Minnesota, for April 9, 1997
37	Plan view of Swift County Route 22 over the Pomme de Terre River, Minnesota,(no scale)
38	Sketch of flow conditions at Swift County Route 22 over the Pomme de Terre River, Minnesota (not to scale)
39	Cross sections collected along the upstream edge of Swift County Route 22 over the Pomme de Terre River, Minnesota
40	Comparison of observed and model velocity distributions for April 5, 1997, at Swift County Route 22 over the Pomme de Terre River, Minnesota
41	Comparison of observed and model velocity distributions for April 9, 1997, at Swift County Route 22 over Pomme de Terre River, Minnesota

LIST OF TABLES

1	Number of bridges damaged and destroyed by scour, 1985-95
2	Summary statistics for selected pier scour measurements
3	Summary of selected local pier scour equations.*
4	Summary of exponents for variables used in selected equations
5	Summary of the performance of the selected pier scour equations
6	Coefficients for the effect of pier shape relative to the scour that would be expected at a circular pier
7	Summary of weighted and unweighted regression results using basic variables
8	Summary of variability in bed material data from sites in Ohio
9	Summary of live bed contraction scour equation exponents
10	Summary of published field data for contraction and abutment scour
11	Comparison of measured mean depth to calculated mean depth at Alaskan bridges where contraction was present during flood flows (modified from Norman(16)
12	Comparison of computed and measured scour at U.S. 87 on Razor Creek, MT, June 1991
13	Measured and predicted mean depth of flow at bridges 331 and 1187 on the Copper River Highway, Alaska, in May 1992
14	Comparison of measured mean depth to calculated mean depth at bridges where contraction was present during flood flows
15	Contraction scour data published by Jackson.(42)
16	Summary of contraction scour measurements at U.S. Route 12 over the Pomme de Terre River in Minnesota
17	Summary of abutment scour data for U.S. Route 12 over the Pomme de Terre River in Minnesota
18	Comparison of observed to computed contraction scour at U.S. Route 12 over the Pomme de Terre River in Minnesota
19	Comparison of observed to computed abutment and total scour at U.S. Route 12 over the Pomme de Terre River in Minnesota. 86
20	20.Summary of hydraulic data collected at Swift County Route 22 over the Pomme de Terre River in Minnesota
21	Summary of abutment scour field data for Swift County Route 22 over the Pomme de Terre River in Minnesota
22	Comparison of observed to computed abutment scour at Swift County Route 22 over the Pomme de Terre River in Minnesota
23	List of sites
24	Pier scour observations

LIST OF SYMBOLS

b is the pier width.

is the effective pier width defined as bcos alpha plus L sin alpha

b₁is the bottom width in the uncontracted section.

b₂is the bottom width in the contracted section.

c_ais 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.

D₁₀is 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.

D₃₅is the grain size of bed material for which 35 percent is finer.

D₅₀is the grain size of bed material for which 50 percent is finer; the median grain size.

D₈₄is the grain size of bed material for which 84 percent is finer.

D₉₀is the grain size of bed material for which 90 percent is finer.

D₉₅is the grain size of bed material for which 95 percent is finer.

D₉₉Is the grain size of bed material for which 99 percent is finer.

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

D_mis the mean grain size of the bed material.

DA is the drainage area.

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

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

E_nis the exponent on the ratio of roughness coefficients or live bed contraction scour equation.

E_Qis 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_ois the flow Froude number defined as V_o/(gy_o)^0.5.

F_pis 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 HEC-18 equation for sediment size by Molinas; see table 3.⁽¹⁾

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

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

K_scis 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⁾

K_S₂is a coefficient for pier shape used by Larras and is 1.0 for cylindrical piers and 1.4 for rectangular piers.⁽⁴⁾

K_uis 1.0 for SI units and 1.81 for customary English units in the critical velocity equation.

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

K₁is 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.

K₂is a coefficient to correct for the skew of the pier to the approach flow, defined as (cos α + (L/b)sin α)^0.65.

K₃is 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.

K4MuK₄ coefficient derived by Mueller.⁽⁵⁾

K4MoK₄ coefficient derived by Molinas.⁽¹⁾

is a coefficient to correct for flow alignment defined by Melville and Sutherland (1988).⁽²⁾

L is the length of the pier.

Q₁is the discharge in the uncontracted section.

Q₂is the discharge in the contracted section.

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

V_ois the approach velocity for pier scour.

V_cis the critical (incipient-transport) velocity for the D₅₀size particle.

V_cxis the critical (incipient-transport) velocity for the D_x size particle.

V_Ris 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 D₅₀ 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_cmis the critical (incipient-transport) velocity for the coarse size fraction defined by Molinas; see table 3.⁽¹⁾

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

V₂ is the velocity in the contracted section.

y_ois the approach depth of flow for pier scour.

y_sis the depth of scour.

Y₁is the depth in the uncontracted section.

Y₂is 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).

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Page Owner: Office of Research, Development, and Technology, Office of Infrastructure, RDT

Scheduled Update: Archive - No Update

Technical Issues: TFHRC.WebMaster@dot.gov

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 bridges--United States--Mathematical models, Bridges--United States--Foundations and piers, Streambeds, Bridge abutments, Bridge piers, Contraction

Updated: 04/23/2012

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