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Clear-Water Abutment and Contraction Scour in the Coastal Plain and Piedmont Provinces of South Carolina, 1996-99

U.S. Geological Survey Water-Resources Investigation Report 03-4064, 137 pages (Published 2003)

Stephen T. Benedict
In cooperation with the South Carolina Department of Transporation

This report is available online in pdf format (8 MB): USGS WRIR 03-4064
Appendix A contains a description of the SCBSD automated reports, raw data tables, and variable definitions.

ABSTRACT

The U.S. Geological Survey, in cooperation with the South Carolina Department of Transportation, collected observations of clear-water abutment and contraction scour at 146 bridges in the Coastal Plain and Piedmont of South Carolina. Scour depths ranged from 0 to 23.6 feet. Theoretical scour depths were computed at each bridge and compared with observed scour. This comparison showed that theoretical scour depths, in general, exceeded the observed scour depths and often were excessive. A comparison of field data with dimensionless relations for laboratory data showed that the range of dimensionless variables used in laboratory investigations was outside of the range for field data in South Carolina, suggesting laboratory relations may not be applicable to field conditions in South Carolina. Variables determined to be important in developing scour within laboratory studies were investigated to understand their influence within the South Carolina field data, and many of these variables appeared to be insignificant under field conditions found in South Carolina. The strongest explanatory variables were embankment length, geometric-contraction ratio, approach velocity, and soil cohesion. Envelope curves developed with the field data are useful tools for assessing reasonable ranges of scour depth in South Carolina. These tools are simple to apply and are an improvement over the current methods for predicting theoretical scour.

Data from this study have been compiled into a database that includes photographs, figures, observed scour depths, theoretical scour depths, limited basin characteristics, limited soil data, and theoretical hydraulic data. The database can be used to compare studied sites with unstudied sites to assess the potential for scour at the unstudied sites. In addition, the database can be used to assess the performance of various theoretical methods for predicting clear-water abutment and contraction scour.

The South Carolina Bridge-Scour Database

Selected data generated from this study has been compiled into a database called the South Carolina Bridge Scour Database (SCBSD), and can be viewed using Microsoft Access. The SCBSD includes photographs, figures, selected field data, variables used to compute theoretical scour, computed scour depths, limited basin characteristics, limited soil data, and selected hydraulic data estimated with the WSPRO model (Shearman, 1990). These raw data are compiled in various data tables within the database, and automated reports have been developed to allow extraction of selected data for a chosen bridge. The SCDOT assigns a unique 12-digit number for each bridge under its jurisdiction and these numbers are used in the database as the primary search variable for extracting selected data from the raw data tables for display in the automated reports. Appendix A contains a description of the SCBSD automated reports, raw data tables, and variable definitions.

The SCBSD is a valuable tool for use in investigating clear-water abutment scour and contraction scour. For one who is assessing scour at bridges in South Carolina, the SCBSD provides a tool for making site comparisons. Sites under investigation, but not included in the current study, can be compared with sites in the SCBSD to gain insights about the range of anticipated scour depths. The SCBSD also provides a source of data to assess various theoretical methods for predicting clear-water abutment and contraction scour. Most equations for predicting scour are driven by hydraulic variables, such as flow depth and velocity. These variables can be extracted from the SCBSD and used in various theoretical equations to compute theoretical scour depths. The theoretical scour depths can then be compared with observed scour, as well as the field-data envelope curves, to assess the chosen equation's performance. (Hydraulic data in the SCBSD may need to be manipulated to obtain specific variables required for a given theoretical equation.)

One should keep in mind that the hydraulic data in the SCBSD were generated from a model and, therefore, do not necessarily represent the flow conditions that created the observed scour. As a result, some error is likely to be introduced into the comparison of theoretical scour with observed scour because of inaccuracies in the hydralic data. However, the abundance of data points in the SCBSD will allow such comparisons to show the general trends of the theoretical equation and will provide some indication of the equation's performance.

The SCBSD provides only limited information at each study site and, therefore, cannot be relied upon to provide a complete understanding of the sites. If more detailed information is required to understand conditions at a given site, then other data sources, such as topographic maps and bridge plans, should be consulted. Under certain circumstances, site visits may be required to gain a full appreciation of the observed scour and the conditions that created it.

TO INSTALL SCBSD:
The SCBSD was developed using Microsoft Access 2000. The electronic file for the database requires approximately 638 megabytes of computer storage. To install the database, the file "scour.mdb" should be copied to a directory of the user's choice. After copying this file to the computer, the properties of the file should be changed from "Read-only" by (1) right clicking on the file and selecting "Properties" on the popup menu,
(2) unselecting the "Read-only" option in the Properties menu box, and then
(3) clicking "OK" at the bottom of the menu box.

TO INVOKE SCBSD
The file "scour.mdb" should be opened in Access. Upon opening this file, a menu box titled, "Bridge-Scour Data Reports," will appear. This menu box lists the 10 automated reports described in Appendix A and allows the user to generate the automated reports for a selected bridge. Near the top of this menu box is a drop-down menu designated by a menu button with a downward pointing arrowhead. Clicking on this button will produce a list of sites included in this study. From this list, the user should select a bridge of interest. Once a specific bridge has been selected, the user can view the various automated reports, for the bridge of interest, by selecting the "View Selected Record" menu button that is to the right of the report of interest. Selecting the "View All Reports" menu button will generate, in numeric order, the corresponding report for all bridges.

TO OBTAIN SCBSD (638MB), Please e-mail Pubs Information

CONTENTS

Abstract

Introduction

Purpose and Scope

Acknowledgments

Previous Investigations

Description of Study Area

Data Assumptions

Justification for the Assumption of Large Flood Flows

Justification for the Assumption of Equilibrium-Scour Conditions

Basin Equilibrium Scour

Site selection

SCDOT List of Bridges with Observed Scour

Selection of Previously Studied Level 2 Bridge-Scour Sites

Selection of Piedmont Sites Influenced by the August 1995 Flood

Selection of Piedmont Bridges with Wide, Flat Floodplains

Selection of Coastal Plain Sites Influenced by the September 1999 Flood

Techniques for the Collection and Interpretation of Field Data

Data Collection in the Coastal Plain

Data Collection in the Piedmont

Flow Model Data

Sediment Samples

Contraction Scour in the Abutment Area

Pier Scour in the Abutment Area

Estimating Hydraulic Data

Development of WSPRO Models

Estimates of Hypothetical Flows

Historical Flows

Flood of October 1992

Flood of August 1995

Flood of September 1999

Development of Theoretical Bridge-Scour Database

Theoretical Pier Scour

Theoretical Live-Bed Contraction Scour

Theoretical Clear-Water Contraction Scour

Theoretical Abutment Scour

Variables Influencing Abutment Scour

Time and Flow Duration

Flow Velocity

Flow Depth

Sediment Size

Sediment Gradation

Embankment Length

Abutment Shape

Embankment Skew

Channel Geometry

Geometric-Contraction Ratio

Conclusions

Abutment Scour in the Piedmont

Comparison of Observed and Theoretical Abutment-Scour Depths

Piedmont Floodplain Geometry as an Indicator of Potential Scour

Embankment Length as an Indicator of Potential Scour

Geometric-Contraction Ratio as an Indicator of Potential Scour

Floodplain Land Cover as an Indicator of Potential Scour

Abutment Scour in the Coastal Plain

Abutment-Scour Hole Patterns

Comparison of Observed and Theoretical Abutment-Scour Depths

Embankment Length as an Indicator of Potential Scour

Geometric-Contraction Ratio as an Indicator of Potential Scour

Bridges 240 Feet or Less in Length.

Floodplain Land Cover as an Indicator of Potential Scour

Effect of Upstream Channel Alignment on Abutment Scour

Guidance for Assessing Abutment-Scour Depth Using the Envelope Curves

Selecting a Reference Surface for Abutment-Scour Depth

Estimating the Embankment Length and the Geometric-Contraction Ratio

Selecting the Appropriate Abutment-Scour Depth Envelope

Single-Bridge Openings

Multiple-Bridge Openings

Contraction and Pier Scour within the Abutment-Scour Area

Limitations of the Abutment-Scour Depth Envelope Curves

Guidance for Assessing Abutment-Scour Hole Location, Width, and Shape

Lateral Reference for Scour Hole

Scour-Hole Top Width

Scour-Hole Longitudinal Location

Scour-Hole Shape

Clear-Water Contraction Scour in the Piedmont

Comparison of Observed and Theoretical Contraction-Scour Depths

Geometric-Contraction Ratio as an Indicator of Potential Scour

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

Permissible Velocities for Clear-Water Contraction Scour

The South Carolina Bridge-Scour Database

Summary

Selected References

REPORT AVAILABILITY

This report is available online in pdf format (8 MB): USGS WRIR 03-4064
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.)

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