Bridge Scour in Nonuniform Sediment Mixtures and in Cohesive Materials

This report presents the summary and synthesis of the various components of the experimental study entitled "Effects of Gradation and Cohesion on Bridge Scour" conducted at Colorado State University between the dates 1991 through 1996. As a result of this effort, in excess of 250 new pier scour data was collected and a new equation was developed expressing pier scour in terms of the dimensionless excess velocity factor, flow depth, pier diameter and a correction factor for the coarse fractions present in mixtures was derived. The new method was tested with available data from previous research. This equation shows that gradation effects are not constant through the entire range of flow conditions but vary with flow intensity. Additionally, a new method to adjust FHWA's Colorado State University pier scour equation for initiation of motion and sediment size was developed. Abutment scour experiments resulted in over 384 new points and 2 new abutment scour equations. The first equation was derived from a 0.1 mm uniform sand mixture and defines an envelope relationship. The second equation applies to mixtures with coarse fractions. A coarse size fraction compensation factor Wg is presented to account for the presence of varying amounts of coarse material in sediment mixtures under different dimensionless flow intensities. These new equations represented the experimental data accurately but have not been tested with field data. Effects of cohesion on pier and abutment scour were studied systematically, and in excess of 200 new data points were collected covering a range of flow and cohesive parameter values. Relationships were developed to explain the variability of bridge scour in cohesive materials for various cohesive material properties.