Scour is the result of the erosive action of flowing water, entraining and removing boundary material from channel beds and/or banks and around bridge foundations. In gravel-bed rivers the interaction of the heterogeneous, large gravel particles with the approaching flow can generate coherent turbulent structures in the flow. In addition to increasing the shear stress applied by the flow onto the bed, these structures create a highly variable bed shear stress field increasing the gravel-bed mobility compromising bridge foundation integrity. 

The majority of the formulas used in current engineering practice for predicting scour depth around bridge foundations have been developed for sand-bed rivers, which are characterized by near-uniform bed material. Parameters such as the heterogeneity of bed material have been excluded from their formulations, and the empirical coefficients appearing in these formulas have been derived from laboratory experiments conducted with near uniform sand-sized sediment. When applied in gravel-bed rivers, these formulas significantly overestimate the scour depth. The research methodology accounted for the unique characteristics of gravel-bed rivers to minimize significant scour depth prediction errors. NCHRP Research Report 1031 provides a deployable methodology to predict scour at new bridge foundations and evaluate scour at existing bridge foundations in gravel-bed rivers.