Investigation of Pier Scour in Coarse-Bed Streams in Montana, 2001 through 2007

Determination of pier-scour potential is an important consideration in the hydraulic analysis and design of highway bridges that cross streams, rivers, and other waterways in the United States. A primary goal of ongoing research in the field of bridge scour is to improve scour-prediction equations so that pier-scour depth is neither underpredicted nor excessively overpredicted. Scour depth for piers in noncohesive, non-uniform streambeds with a mixture of sand, gravel, cobbles, and boulders (coarse-bed streams) generally is less than the scour depth in finer-grained (mostly sand) streambeds under similar hydraulic conditions. The difference in scour depth is attributed to formation of an armor layer.