U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2004-5213
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 | ||
---|---|---|
Multiply | By | To obtain |
Length | ||
inch (in.) | 2.54 | centimeter (cm) |
inch (in.) | 25.4 | millimeter (mm) |
foot (ft) | 0.3048 | meter (m) |
Area | ||
square foot (ft2) | 929.0 | square centimeter (cm2) |
square foot (ft2) | 0.09290 | square meter (m2) |
square mile (mi2) | 259.0 | hectare (ha) |
square mile (mi2) | 2.590 | square kilometer (km2) |
Flow Rate | ||
foot per second(ft/s) | 0.3048 | meter per second (m/s) |
cubic foot per second (ft3/s) | 0.02832 | cubic 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.
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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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