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Development and Evaluation of Clear-Water Pier and Contraction Scour Envelope Curves in the Coastal Plain and Piedmont Provinces of South Carolina

U.S. Geological Survey Scientific Investigations Report 2005—5289, 98 pages (Published 2006)
(ONLINE ONLY)

By Stephen T. Benedict and Andral W. Caldwell

This report is available online in pdf format (5 MB): USGS SIR 2005—5289

ABSTRACT

The U.S. Geological Survey in cooperation with the South Carolina Department of Transportation collected clear-water pier- and contraction-scour data at 116 bridges in the Coastal Plain and Piedmont Physiographic Provinces of South Carolina. Pier-scour depths collected in both provinces ranged from 0 to 8.0 feet. Contraction-scour depths collected in the Coastal Plain ranged from 0 to 3.9 feet. Using hydraulic data estimated with a one-dimensional flow model, predicted clear-water scour depths were computed with scour equations from the Federal Highway Administration Hydraulic Engineering Circular 18 and compared with measured scour. This comparison indicated that predicted clear-water scour depths, in general, exceeded measured scour depths and at times were excessive. Predicted clear-water contraction scour, however, was underpredicted approximately 30 percent of the time by as much as 7.1 feet.

The investigation focused on clear-water pier scour, comparing trends in the laboratory and field data. This comparison indicated that the range of dimensionless variables (relative depth, flow intensity, relative grain size) used in laboratory investigations of pier scour, were similar to the range for field data in South Carolina, further indicating that laboratory relations may have some applicability to field conditions in South Carolina. Variables determined to be important in developing pier scour in laboratory studies were investigated to understand their influence on the South Carolina field data, and many of these variables appeared to be insignificant under field conditions in South Carolina. The strongest explanatory variables were pier width and approach velocity. Envelope curves developed from the field data are useful tools for evaluating reasonable ranges of clear-water pier and contraction scour in South Carolina. A modified version of the Hydraulic Engineering Circular 18 pier-scour equation also was developed as a tool for evaluating clear-water pier scour. The envelope curves and modified equation offer an improvement over the current methods for predicting clear-water scour in South Carolina.

Data from this study were compiled into a database that includes photographs, measured scour depths, predicted scour depths, limited basin characteristics, limited soil data, and modeled hydraulic data. The South Carolina database can be used to compare studied sites with unstudied sites to evaluate the potential for scour at the unstudied sites. In addition, the database can be used to evaluate the performance of various methods for predicting clear-water pier and contraction scour.

CONTENTS

Abstract

Introduction

Purpose and Scope

Previous Investigations

Description of Study Area

Approach

Data Collection

Data Assumptions

Clear-Water Scour Conditions

Assumption of Large Floods

Site Selection

Techniques for the Collection and Interpretation of Field Data

Data Collection in the Coastal Plain

Data Collection in the Piedmont

Development of the Predicted Bridge-Scour Database

Estimating Hydraulic Data

Estimates of the 100-Year Flow

Historical Flows

Predicted Clear-Water Pier Scour

Predicted Clear-Water Contraction Scour

Development of the South Carolina Pier-Scour Envelope Curve

Variables Influencing Pier Scour

Justification for Using the 100-Year Flow to Estimate Hydraulics

Time and Flow Duration

Flow Velocity

Flow Depth

Sediment Size

Pier Shape

Pier Skew

Pier Width and the South Carolina Pier-Scour Envelope Curve

Envelope Curves for Laboratory and Field Data

Equation for the South Carolina Pier-Scour Envelope Curve

Evaluation of Selected Methods for Predicting Clear-Water Pier Scour in South Carolina

The HEC-18 Pier-Scour Equation

The South Carolina Modified Pier-Scour Equation

The South Carolina Pier-Scour Envelope Curve

Guidance for Evaluating Pier-Scour Depth in South Carolina

Evaluating Scour Depth at Pier Widths Less Than or Equal to 6 Feet

Evaluating Scour Depth at Pier Widths Greater than 6 Feet

Evaluating Scour-Hole Top Width

Development of the South Carolina Clear-Water Contraction-Scour Envelope Curves

Clear-Water Contraction Scour in the Coastal Plain and Piedmont

Comparison of Measured and Predicted Contraction-Scour Depths

Development of the Contraction-Scour Envelope Curve Using the Geometric-Contraction Ratio

Development of the Contraction-Scour Envelope Curve Using Flow Velocity

Comparison and Limitations of the Contraction-Scour Envelope Curves

Selecting a Reference Surface for Clear-Water Contraction Scour

Pier Scour Within Clear-Water Contraction-Scour Areas

Estimate of Clear-Water Contraction-Scour Hole Location

The South Carolina Clear-Water Pier- and Contraction-Scour Database

Summary

Selected References

Appendix 1

Appendix 2

REPORT AVAILABILITY

This report is available online in pdf format (5 MB): USGS SIR 2005-5289
To view the PDF document, you need the Adobe Acrobat® Reader installed on your computer. (A free copy of the Acrobat® Reader may be downloaded from Adobe Systems Incorporated.)

Suggested citation:

Benedict, S.T., and Caldwell, A.W., Development and evaluation of clear-water pier and contraction scour envelope curves in the Coastal Plain and Piedmont Provinces of South Carolina: U.S. Geological Survey Scientific Investigations Report 2005–5289, 98 p.

For more information, contact the South Carolina Publications Unit.

USGS South Carolina Publications South Carolina Water Science Center

 USGS South Carolina Publications • South Carolina Water Science Center