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 (Tsakiris et al., 2014; Sarkar et al., 2016), 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 (Dey and Raikar, 2005; Ettema et al., 2011; Guo et al., 2012; Manes and Brocchini, 2015). As a result, 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. Due to these limitations, when applied in gravel-bed rivers, these formulas significantly overestimate the scour depth (Dey and Raikar, 2005; Holnbeck, 2011).
Recognizing the shortcomings of current scour prediction methodologies, researchers have attempted to include the effects of gravel particle heterogeneity using empirically derived correction coefficients. However, the failure of existing scour prediction methodologies to fundamentally account for the unique characteristics of gravel-bed
rivers translates to significant scour depth prediction errors, even after correction.
Development of a scour prediction methodology that accounts for gravel-bed rivers will be valuable for bridge owners and practicing engineers and will provide more accurate predictions of the scour depth. The objective of this research is to develop a rigorously tested and rapidly deployable methodology for bridge owners that will accurately predict scour at new bridge foundations and evaluate scour at existing bridge foundations in gravel-bed rivers by (1) providing reliable data for fluid-induced forces, required to generate incipient motion in the surface and subsurface layers of gravel-bed rivers; and (2) combining fluid-induced erosion forces, and incipient motion data to predict scour depths at bridge foundations with gravel-bed rivers that have ¼ inch gravel to 12 inch cobble mixtures with varying sand content up to 20%.
Anticipated final deliverables include: (1) a final report that documents the new scour depth prediction method, and details its development, range of applicability, and the data requirements for immediate use by bridge owners; (2) at least one case study illustrating the use and step-by-step computational procedures and application of the new methodology; (3) an executive summary of the project; (4) recommendations for incorporation into the current scour design methods; (5) a PowerPoint presentation describing the background, objectives, research approach, findings, and conclusions; (6) presentation of findings at the National Hydraulic Engineering Conference and one meeting of the AASHTO Program Delivery and Operations or Cross Discipline Committee; and (7) a stand-alone technical memorandum titled “Implementation of Research Findings and Products”.