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DEVELOPMENT AND EVALUATION OF LIVE-BED PIER AND CONTRACTION SCOUR ENVELOPE CURVES IN THE COASTAL PLAIN AND PIEDMONT PROVINCES OF SOUTH CAROLINA

Project Number: 2519-9MV08
Project Chief: Andy Caldwell, Stephen Benedict
Cooperator: South Carolina Department of Transportation
Period of Project: 2004 to July 2009

INTRODUCTION

Collecting sub-surface channel and scour data at a small, wadeable stream using a ground penetrating radar system.

Collecting sub-surface channel and scour data at a small, wadeable stream using a ground penetrating radar system.

Since 1996, the U.S. Geological Survey (USGS) South Carolina Water Science Center, in cooperation with the South Carolina Department of Transportation (SCDOT), has been using historic scour data collected from field investigations to develop regional envelope curves for clear-water abutment, contraction, and pier scour. The USGS is currently analyzing historic live-bed scour data at selected sites in South Carolina with the goal of developing similar regional envelope curves for live-bed scour. The collection of historic scour data is relatively simple for clear-water scour because the scour holes do not refill. The measurement of historic live-bed scour, however, can be problematic because of the partial or total refilling of scour holes by channel sediment. To address this problem, a geophysical technique, ground-penetrating radar (GPR), was used to assess buried scour surfaces. From 2005 to 2007, the USGS used GPR to collect measurements of historic live-bed contraction and pier scour at 78 bridges in South Carolina. While there are limitations and uncertainty associated with scour measurements made with GPR, the data are yielding valuable insights into regional trends of live-bed contraction and pier scour in South Carolina.

Example of ground penetrating radar profile showing approximate location of contraction scour.

Example of ground penetrating radar profile showing approximate location of contraction scour.

PROBLEM

Current methods for estimating bridge-scour depths are based on equations derived from theory or developed in laboratory investigations and, in general, have not been verified for field conditions. In the late 1990's, USGS, in cooperation with the SCDOT, began field investigations of clear-water abutment, contraction, and pier scour in South Carolina (Benedict, 2003; Benedict and Caldwell, 2006). Field measurements were compared with the laboratory-derived equations listed in the Federal Highway Administration Engineering Circular No. 18 (HEC-18) (Richardson and Davis, 2001), and the equations were found to be poor predictors and often gave excessive scour depths. The field data were analyzed and scour assessing tools were developed providing an improved method for assessing clear-water abutment, contraction, and pier scour in South Carolina. The findings of these investigations indicated that a similar approach could be used to develop improved scour assessing tools for other components of bridge scour including live-bed pier scour and live-bed contraction scour and this is the focus of the current investigation.

OBJECTIVES AND SCOPE

Photo of collecting ground penetrating radar data at a river.

Collecting ground penetrating radar data at a river

The objectives of this project include (1) the collection of live-bed pier scour and contraction scour at selected bridges in the Piedmont and Coastal Plain of South Carolina using GPR; (2) a comparison of observed scour with theoretical scour in order to assess the current scour-prediction methods (Richardson and Davis, 2001); (3) the investigation of various physical relations that may help explain live-bed scour processes in South Carolina, and (4) if possible, the development of simple tools for assessing live-bed pier and contraction scour in South Carolina.

The scope of the project is the Coastal Plain (except in tidally influenced areas) and Piedmont Provinces of South Carolina. Field observations of live-bed pier scour and contraction scour were collected at selected sites in both provinces. Using hydraulic data estimated from the WSPRO model, theoretical scour will be computed at these sites and compared to observed scour in order to assess existing live-bed scour equations. Relations within the field data will be studied and attempts will be made to develop simple tools for assessing live-bed-water pier scour and contraction scour. If the development of such tools is not possible, the data will be tabulated and presented in a report.

CURRENT FINDINGS

Currently (2008), preliminary interpretation of GPR data at 25 sites has been made, including 12 sites in the Coastal Plain and 13 in the Piedmont. Preliminary findings indicate the following:

  • Complete refill of live-bed scour holes occurred infrequently and remnant scour holes that identify the location and approximate depths of historic scour often were identifiable in the channel bed topography as determined by the GPR
  • Fathometer traces will often provide similar estimates of the depth of refilled sediments within live-bed scour holes.
  • In general, contraction-scour depths in the Piedmont tend to be lower than those of the Coastal Plain. This trend is probably caused by the larger contractions of flow that exist at Coastal Plain bridges in contrast to those of the Piedmont.
  • While bridge-scour depths estimated from GPR data will have some associated error and at times will be unsuccessful in defining refilled scour holes, these data can provide a valuable understanding of historic scour at a site of interest.

BENEFITS

This project benefits the SCDOT by providing additional streamflow and scour data at bridges in South Carolina. These data will aid in the understanding of bridge-scour processes and potentially provide better methodology for predicting bridge scour at bridges in South Carolina. These data, when combined with the clear-water abutment-, contraction-, and pier-scour envelope curves developed by the USGS, can be used to make a full assessment of scour at bridges in the Piedmont and Coastal Plain (excluding tidal regions) of South Carolina. In addition, these methods may possibly be applicable to streams in other states.

A bridge scour database will be created that includes observed and theoretical scour along with other pertinent attributes. The results of the project will be published in a USGS Scientific Investigations Report.

REFERENCES

Benedict, S.T., 2003. Clear-water abutment and contraction scour in the Coastal Plain and Piedmont Provinces of South Carolina, 1996-99, U.S. Geological Survey, Water-Resources Investigations Report 03-4064.

Benedict, S.T., and Caldwell, A.W., 2006. Development and evaluation of Clearwater pier and contraction scour envelope curves in the Coastal Plain and Piedmont of South Carolina, U.S. Geological Survey Scientific Investigations Report 2005-5289.

Richardson, E.V., and Davis, S.R., 2001. Evaluating scour at bridges, Federal Highway Administration Hydraulic Engineering Circular No. 18, Publication FHWA-NHI-01-001.

Shearman, J.O., 1990. User's manual for WSPRO-A computer model for watersurface profile computations, Federal Highway Administration, Report no. FHWA-IP-89-027.

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