Abstract
The U.S. Geological Survey, in cooperation with the South Carolina Department of Transportation, used ground-penetrating radar to collect measurements of live-bed pier scour at 78 bridges in the Piedmont and Coastal Plain physiographic provinces of South Carolina. The 141 measurements of live-bed pier-scour depth ranged from 0.5 to 5.1 meters. Using hydraulic data estimated with a one-dimensional flow model, predicted live-bed scour depths were computed with scour equations from the Hydraulic Engineering Circular 18 and compared with measured scour. This comparison indicated that predicted pier-scour depths generally exceeded the measured pier-scour depths. At times, predicted pier-scour depths were excessive with overpredictions as large as 7.0 meters. Relations in the live-bed pier-scour data also were investigated, leading to the development of an envelope curve for assessing the upper-bound of live-bed pier scour using pier width as the primary explanatory variable. The envelope curve developed with the field data has limitations, but it can be used as a supplementary tool for assessing the potential for live-bed pier scour in South Carolina. This paper will present findings related to the field investigation of live-bed pier scour. A companion paper presents findings related to live-bed contraction scour that was studied during the same field investigation.
Highlights
The U.S Geological Survey (USGS), in cooperation with the South CarolinaDepartment of Transportation (SCDOT), investigated historic scour at 235 bridges in the Piedmont and Coastal Plain physiographic provinces of South Carolina (Benedict, 2003 ; Benedict and Caldwell, 2006; Benedict and Caldwell, 2009)
The general objectives of these studies were to (1) collect field measurements of historic abutment, contraction, and pier scour at sites that could be associated with major floods, (2) use the field data to assess the performance of the scour-prediction equations listed in the Federal Highway Administration Hydraulic Engineering Circular No 18 (HEC-18) (Richardson and Davis, 200 I), and (3) develop regional envelope curves to help assess scour potential in the Piedmont and Coastal Plain regions of South Carolina
Ground penetrating radar has been used successfully to locate and estimate scour depths associated with historic live-bed scour (Placzek and Haeni, 1995; Webb and others, 2000) and the shallow (6 meters or less) freshwater granular bottom streams of South Carolina provide a favorable environment for the application of ground-penetrating radar (GPR). (See Benedict and Caldwell (2009) for details regarding the application and limitations of GPR for the investigation.) To provide some assurance that measured historic scour in this investigation would reflect scour resulting from high flows, USGS streamflow gaging records were reviewed and a strategic set of 78 bridge sites located in the Coastal Plain and Piedmont physiographic provinces of South Carolina were selected for data collection (Figure I)
Summary
FIELD DATA A primary objective of the investigation by Benedict and Caldwell (2009) was to develop a database of live-bed pier-scour field measurements that could be used to assess regional scour trends in South Carolina as well as evaluate the performance of the HEC-18 (Richardson and Davis, 2001) pier-scour equation. (See Benedict and Caldwell (2009) for details regarding the application and limitations of GPR for the investigation.) To provide some assurance that measured historic scour in this investigation would reflect scour resulting from high flows, USGS streamflow gaging records were reviewed and a strategic set of 78 bridge sites located in the Coastal Plain and Piedmont physiographic provinces of South Carolina were selected for data collection (Figure I).
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