Physical model study of bridge contraction scour

Abstract

Because scour has contributed to the collapse of a large number of bridges, many researchers have tried to find a general method of predicting scour to prevent the failure of bridge foundations. However, existing scour depth equations recommended by the Federal Highway Administration (FHWA) in the United States generally give excessively conservative estimates of the scour depth. One reason is that these equations are based primarily on idealized and simplified laboratory experiments in rectangular flumes. In addition to idealized and simplified laboratory experiments, another possible reason for scour depth overprediction is the current practice of adding separate estimates of contraction scour and local scour when in fact these processes occur simultaneously and interact. This study focuses specifically on the hydraulic modeling of local pier scour and contraction scour. Scaled field site was constructed in the Hydraulics Laboratory at the Georgia Institute of Technology based on their Froude number similarity. We used several flow rates, including the 50- and 100-year peak flood discharge and the 1998 historical flood discharge from field measurements provided by the U.S. Geological Survey (USGS) and used them to compare scour measurements (contraction scour and local scour) in the laboratory and in the field. Based on the findings, a procedure for hydraulic modeling with scaled field geometry is suggested.