ANALYSIS OF PIER SCOUR PREDICTIONS AND REAL-TIME FIELD MEASUREMENTS

The variability and complexity of site conditions make the development of methodology for predicting scour at bridge piers difficult. Laboratory investigations often oversimplify or ignore many of the complexities that are common in the field. The U.S. Geological Survey, in cooperation with the Federal Highway Administration and many State highway agencies, has collected and compiled 493 field measurements of local pier scour at 79 sites located in 17 States. The pier-scour measurements were used to evaluate 26 published pier scour equations. No single equation conclusively was better than the others, but the top six equations generally appear to be the Froehlich Design, HEC-18, HEC-18-K4, HEC-18-K4Mu, HEC-18-K4Mo (>2mm), and Mississippi equations. However, comparison of the scour predicted from these equations with the observed scour clearly shows that there is variability in the field data that is not correctly accounted for in the equations. Analysis of laboratory and field data indicated the importance of bed-material characteristics as an explanatory variable in the predictive equations. A new correction to the HEC-18 equation to account for the relative bed-material size is presented.