Scour Processes Observed in Field Data

River channels scour and aggrade due to complex, interrelated natural processes that are dynamic over space and time. Channel scour is particularly difficult to predict for bridge crossings, where scour processes are usually most dynamic. Field measurements of channel scour at bridges are needed to improve the understanding of scour processes and the ability to predict scour depths accurately. An extensive data base of more than 380 pier-scour measurements was collected over the last several years in cooperative studies between the U.S. Geological Survey (USGS), the Federal Highway Administration, and several State Departments of Transportation. This paper uses a subset of that national data base to evaluate selected scour processes in 139 measurements of local scour measured in live-bed and clear-water conditions. Channel scour was measured for 90 bridge piers at 44 bridges in 12 States. Scour depth is influenced by pier width in a relation that is linear in logarithmic space, with a slope of about 1.0. The maximum observed pier-width to scour-depth ratio is 2.1 for piers aligned to streamfiow. Flow depth and scour were found to have a relation that is linear in logarithmic space, and does not cease at some critical ratio of flow depth to pier width. Froude number appears to be a poor explanatory variable for the effect of flow intensity on scour depth.