Modification of selected South Carolina bridge-scour envelope curves

Abstract

Historic scour was investigated at 231 bridges in the Piedmont and Coastal Plain physiographic provinces of South Carolina by the U.S. Geological Survey in cooperation with the South Carolina Department of Transportation. These investigations led to the development of field-derived envelope curves that provided supplementary tools to assess the potential for scour at bridges in South Carolina for selected scour components that included clear-water abutment, contraction, and pier scour, and live-bed pier and contraction scour. The envelope curves consist of a single curve with one explanatory variable encompassing all of the measured field data for the respective scour components. In the current investigation, the clear-water abutment-scour and live-bed contraction-scour envelope curves were modified to include a family of curves that utilized two explanatory variables, providing a means to further refine the assessment of scour potential for those specific scour components. The modified envelope curves and guidance for their application are presented in this report. Description of Study Area South Carolina has an area of about 31,100 square miles (mi 2) and is divided into three physiographic provinces-the Blue Ridge, Piedmont, and Coastal Plain. The Coastal Plain province is divided into upper and lower regions (fig. 1). The study area for this investigation includes most of South Carolina but generally excludes the Blue Ridge and the tidally influenced area of the lower Coastal Plain. The Piedmont covers about 35 percent of South Carolina and lies between the Blue Ridge and Coastal Plain (fig. 1). Land-surface elevations range from about 400 feet (ft) near the Fall Line (Coastal Plain boundary) to about 1,000 ft at the Blue Ridge boundary. The general topography includes rolling hills, elongated ridges, and moderately deep to shallow valleys. The drainage patterns are well developed with well-defined channels and densely vegetated floodplains. Streambed slopes in the Piedmont range from approximately 0.00015 to 0.0100 foot per foot (ft/ft) (Guimaraes and Bohman, 1992). The geology of the Piedmont generally consists of fractured crystalline rock overlain by moderately to poorly permeable silty-clay loams. Alluvial deposits along the valley floors generally consist of clay, silt, and sand, and form varying degrees of cohesive soils (Guimaraes and Bohman, 1992). The stream-channel sediments typically consist of sandy materials overlying decomposed rock or bedrock.

Assessing potential scour using the South Carolina bridge-scour envelope curves

“…The application and limitations of these envelope curves were documented in four reports (Benedict, 2003;Benedict and Caldwell, 2006, 2009, 2012. Because each report addresses different components of bridge scour, it was recognized that there was a need to develop an integrated procedure for applying the envelope curves to help assess scour potential at riverine bridges in South Carolina.…”

“…It is rare, however, that such data are available at or near a site of interest, making the evaluation of predicted scour as compared to field data difficult if not impossible. Realizing the value of historic scour measurements, the U.S. Geological Survey (USGS), in cooperation with the South Carolina Department of Transportation (SCDOT), conducted a series of three field investigations (Benedict, 2003;Benedict and Caldwell, 2006, 2009, 2012 to collect historic scour data with the goal of understanding regional trends of scour at riverine bridges in South Carolina.…”

The South Carolina bridge-scour envelope curves

“…The phrase "secondary envelope curve" also is used in this report for the same purpose. A description of these envelope curves and the techniques used to develop them can be found in Benedict and Caldwell (2012).…”

“…The modified envelope curves included the clear-water abutment-scour and live-bed contraction-scour envelope curves. Benedict and Caldwell (2012) referred to these family of curves as "secondary envelope curves" to indicate their modification from the original curves. The phrase "secondary envelope curve" also is used in this report for the same purpose.…”

“…This provides further confirmation that the patterns displayed in the field data are reasonable. On the basis of the work of Ballio and others (2009), Benedict and Caldwell (2012) modified the South Carolina abutment-scour envelope curves (Benedict, 2003) to include a family of envelope curves (secondary or modified envelope curves) that displays the combined effect of embankment length and the geometric-contraction ratio on abutment-scour depth. This was accomplished by grouping selected field data from South Carolina and the NBSD (table 8) into categories based on embankment-length and developing modified envelope curves for each embankment-length category.…”