Development of Regional Envelope Curves for Assessing Limits and Trends in Scour

Current bridge-scour equations can have substantial uncertainty and the computed results must be scrutinized to assure that predicted scour is reasonable. A useful way to assess whether the predicted scour depth is reasonable is to compare it with field measurements of historic scour. However, historic bridge-scour data are limited and often unavailable for many regions of the United States, making such comparisons difficult or impossible. Recent field investigations by the U.S. Geological Survey in South Carolina demonstrate how post-flood and historic-scour measurements can be collected and organized into regional envelope curves to help assess the reasonableness of predicted scour. Using historic bridge-scour data collected at 167 bridges, envelope curves for clear-water abutment, pier, and contraction scour have been developed for South Carolina, and an investigation to develop envelope curves for live-bed contraction and pier scour is currently in progress. Although envelope curves do not give a precise determination of scour, they display the range and limits for historic scour, providing valuable insights into regional scour trends. An overview of the development of regional bridge-scour envelope curves is presented in the paper, and a general plan for accomplishing regional investigations is provided.