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U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2004-5213


Potential-Scour Assessments and Estimates of Scour Depth Using Different Techniques at Selected Bridge Sites in Missouri

By: Richard J. Huizinga and Paul H. Rydlund, Jr., in cooperation with the Missouri Department of Transportation

ABSTRACT

The evaluation of scour at bridges throughout the state of Missouri has been ongoing since 1991 in a cooperative effort by the U.S. Geological Survey and Missouri Department of Transportation. A variety of assessment methods have been used to identify bridges susceptible to scour and to estimate scour depths. A potential-scour assessment (Level 1) was used at 3,082 bridges to identify bridges that might be susceptible to scour. A rapid estimation method (Level 1+) was used to estimate contraction, pier, and abutment scour depths at 1,396 bridge sites to identify bridges that might be scour critical. A detailed hydraulic assessment (Level 2) was used to compute contraction, pier, and abutment scour depths at 398 bridges to determine which bridges are scour critical and would require further monitoring or application of scour countermeasures.

The rapid estimation method (Level 1+) was designed to be a conservative estimator of scour depths compared to depths computed by a detailed hydraulic assessment (Level 2). Detailed hydraulic assessments were performed at 316 bridges that also had received a rapid estimation assessment, providing a broad data base to compare the two scour assessment methods. The scour depths computed by each of the two methods were compared for bridges that had similar discharges. For Missouri, the rapid estimation method (Level 1+) did not provide a reasonable conservative estimate of the detailed hydraulic assessment (Level 2) scour depths for contraction scour, but the discrepancy was the result of using different values for variables that were common to both of the assessment methods. The rapid estimation method (Level 1+) was a reasonable conservative estimator of the detailed hydraulic assessment (Level 2) scour depths for pier scour if the pier width is used for piers without footing exposure and the footing width is used for piers with footing exposure. Detailed hydraulic assessment (Level 2) scour depths were conservatively estimated by the rapid estimation method (Level 1+) for abutment scour, but there was substantial variability in the estimates and several substantial underestimations.

TABLE OF CONTENTS

Abstract

Introduction

History of Scour Studies in Missouri

Purpose and Scope

Study Area

Potential-Scour Assessments (Level 1)

Estimates of Scour Depth Using Different Techniques

Rapid Estimation Method (Level 1+)

Special Variables Needed for Application in Missouri

Determination of Average Bed Location for Referencing Average Depth

Modification of Discharge

Road Overflow

Presence of Additional Bridges

Sites Located in the Mississippi Alluvial Plain Region

General Procedure of Level 1+ Assessments in Missouri

Scour Predictor Equations

Final Qualitative Assessment

Detailed Hydraulic Assessments (Level 2)

Hydraulic Analysis

Contraction Scour

Pier Scour

Abutment Scour

Reports

Comparison of Level 1+ Results to Level 2 Results

Contraction Scour

Main Channel Contraction Scour

Overbank Contraction Scour

Discussion of Contraction Scour Depth Estimate Comparisons

Pier Scour

Abutment Scour

Validation of Level 1+ Procedures Using Level 2 Results

Average Main Channel Velocity

Difference in Water-Surface Elevation

Determination of Road Overflow

General Limitations of Scour Depth Estimation Methods

Verification of Scour Depth Estimates Using Field Data

Scour Depth as a Function of Flood Duration in Cohesive Soils

Assumption of Soil Homogeneity

Accuracy and Consistency of Variables

Summary

References

FIGURES

1–3. Maps showing—

1. Highway districts and major physiographic regions of Missouri

2. Distribution of potential-scour assessment (Level 1) sites in Missouri

3. Distribution of rapid estimation method (Level 1+) sites in Missouri

4. Definition sketch of average flow area, average bed, and average flow depth as used in rapid estimation assessment (Level 1+) in Missouri

5. Map showing distribution of detailed hydraulic assessment (Level 2) sites in Missouri

6–15. Graphs showing—

6. Contraction and local pier scour depths for simulated maximum discharge of 1,000,000 cubic feet per second through Horse Island Chute bridge near Chester, Illinois

7. Total scour depths for three discharge simulations through Horse Island Chute bridge near Chester, Illinois

8. Comparison of discharge through bridge used in rapid estimation assessment (Level 1+) with that used in detailed hydraulic assessment (Level 2)

9. Comparison of average main channel contraction scour depths determined by rapid estimation assessment (Level 1+) with those determined by detailed hydraulic assessment (Level 2)

10. Comparison of average overbank contraction scour depths determined by rapid estimation assessment (Level 1+) with those determined by detailed hydraulic assessment ( Level 2)

11. Comparison of average pier scour depths determined by rapid estimation assessment (Level 1+) with those determined by detailed hydraulic assessment (Level 2)

12. Comparison of average abutment scour depths determined by rapid estimation assessment (Level 1+) with those determined by detailed hydraulic assessment (Level 2)

13. Relation between unit discharge (q2) and average main channel velocity at bridge (V2) for selected sites in Missouri

14. Relation between average main channel velocity at bridge squared (V22) and difference in water-surface elevation between bridge and approach sections (Dh) for selected sites in Missouri

15. Comparison of 100-year peak discharge through bridge estimated using rapid estimation assessment (Level 1+) equation with that determined in detailed hydraulic assessment (Level 2) for bridges with impendent road-overtopping discharge less than 100-year peak discharge

16. Relation between scour depth estimated using the Colorado State University equation and the residual (measured scour minus the estimated scour depth) at selected sites in Missouri

TABLES

1. Number of bridges that received potential-scour (Level 1), rapid estimation (Level 1+), and detailed hydraulic (Level 2) assessments, grouped by Missouri Department of Transportation (MoDOT) district

2. Bridge site information, potential and observed scour index values, scour susceptibility, and subsequent rapid estimation (Level 1+) or detailed hydraulic (Level 2) assessments of bridges that received a potential-scour assessment (Level 1) in Missouri

3. Bridge site information and estimated scour depths from rapid estimation assessments (Level 1+ ) in Missouri

4. Summary of rapid estimation assessments (Level 1+) and resultant detailed hydraulic assessment (Level 2) candidates by Missouri Department of Transportation (MoDOT) district

5. Bridge site information and computed scour depths from detailed hydraulic assessments (Level 2) in Missouri

6. Summary of detailed hydraulic assessments (Level 2) by Missouri Department of Transportation (MoDOT) district

7. Summary of discharge data, measured scour depths, and scour depths estimated using various methods at selected sites in Missouri

 

Conversion Factors and Datum
MultiplyByTo obtain
 Length 
   
inch (in.)2.54centimeter (cm)
inch (in.) 25.4millimeter (mm)
foot (ft)0.3048meter (m)
   
  Area 
   
square foot (ft2)929.0square centimeter (cm2)
square foot (ft2)0.09290square meter (m2)
square mile (mi2)259.0hectare (ha)
square mile (mi2)2.590square kilometer (km2)
   
 Flow Rate 
   
foot per second(ft/s) 0.3048 meter per second (m/s)
cubic foot per second (ft3/s)0.02832cubic meter per second (m3/s)
   
Hydraulic Gradient
   
foot per mile (ft/mi) 0.1894 meter per kilometer (m/km)
   
  Unit Discharge* 
   
cubic foot per second per foot [(ft3/s)/ft] 0.09290 cubic meter per second per meter [(m3/s)/m]
   
  Acceleration 
   
foot per second squared (ft/s2) 0.3048 meter per second squared (m/s2)
   

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:
° F = (1.8 x °C) + 32

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows:
° C = (°F -32 ) / 1.8

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

*Unit discharge: The standard unit for unit discharge is cubic foot per second per foot [(ft3/s)/ft]. In this report, the mathematically reduced form foot squared per second (ft2/s), is used for convenience.

 


This report is available online in Portable Document Format (PDF). If you do not have the Adobe Acrobat PDF Reader, it is available for free download from Adobe Systems Incorporated.

View the report in PDF (4.1 MB)

(The printed report includes a CD-ROM of Tables 2, 3, and 5; these tables are shown below in Microsoft Excel format).

Table 2 in Excel Format (586 KB)

Table 3 in Excel Format (1.5 MB)

Table 5 in Excel Format (543 KB)

 

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For more information about USGS activities in Missouri contact:

Director

U.S. Geological Survey

Missouri Water Science Center

1400 Independence Road

Rolla, Missouri 65401

Telephone: (573) 308-3667

Fax: (573) 308-3645


or access the USGS Missouri Water Science Center home page at:  http://mo.water.usgs.gov/.


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Last modified: Thursday, January 10 2013, 06:40:06 PM
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