Field Observations and Evaluations of Streambed Scour at Bridges

The variability and complexity of site conditions make it difficult to develop methodology for predicting scour at bridges. Laboratory investigations often oversimplify or ignore many complexities 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 field data on scour at bridges at 79 sites located in 17 States. These data have been analyzed to isolate pier scour, contraction scour, and abutment scour. The national data base contains 493 local pier scour measurements, 18 contraction scour measurements, and 12 abutment scour measurements. The pier scour measurements were used to evaluate 26 published pier scour equations. The Froehlich Design, HEC-18, HEC-18-K4, HEC-18-K4Mu, HEC-18-K4Mo (>2mm), and Mississippi equations proved to be better than the other equations for predicting pier scour for design purposes. However, comparison of the scour predicted from these equations with the observed scour clearly shows that variability in the field data is not correctly accounted for in the equations. Relations between dimensionless variables developed from laboratory experiments did not compare well with the field data. Analysis of the pier scour data indicated the importance of bed-material characteristics as a variable in the predictive equations. A new K sub 4 term for the HEC-18 pier-scour equation was developed based on the relative bed-material size (b/D50) where b=pier width and D50 is the median bed material. A review of published literature found 29 references to abutment and contraction scour data; however, only a few provided complete data sets. Published comparisons of observed versus computed scour were inconclusive. A detailed comparison of computed contraction and abutment scour with field observations for two sites in Minnesota was also inconclusive. The current methodology for computing scour depth provides reasonable estimates of the maximum total scour, but the individual estimates of contraction and abutment scour did not compare well with the observed data. The accuracy of the contraction and abutment scour equations may depend on the degree of contraction, the flow distribution in and configuration of the approach, and how well the hydraulic model represents the true flow distribution.