Laboratory Assessment of Scour Evolution Around Engineered Logjams in Gravel Bed Rivers

Engineered logjams (ELJs) are permeable flow deflection structures increasingly utilized by transportation and river restoration practitioners to stabilize banklines, protect roadways, and improve aquatic habitat. Localized flow acceleration near hydraulic structures causes clearwater bed scour at ELJs; prediction of associated scour extents is needed to ensure structure and river stability. Although many studies focus on sand-bed scour approximations at the maximum equilibrium condition, increasing focus is being given to scour processes in non-uniform beds, at permeable structures, and for scour evaluations prior to equilibrium. Critical needs still lie, however, in relating the simultaneous evolution of scour, velocity, and bed shear stress in non-uniform beds at permeable ELJ installations. The overarching goal of this research is to provide a fundamental understanding of the scour hole evolution around a permeable ELJ structure in gravel bed rivers under clearwater conditions. The study shows that the ELJ structure deflects conical scour hole formation away from the structure nose. The scour hole evolution rate is found to be governed by the interlocking of particles (i.e. the gradation of particles affects interlocking). This research utilizes non-intrusive flow and scour measurement techniques providing aerial velocity and scour volume information around the ELJ. The basis of this study is the hypothesis that the rate of scour evolution is not only dictated by the gradation of particles, but also by the feedback between the gravel-bed matrix, the structure, and the flow. It is the three-way interaction that is encapsulated in the development of a decay function that relates shear stress decay with scour hole development. Analysis of the decay function informs the development of a parametric function for maximum scour depth estimation. The overall findings of this research can guide future efforts of practitioners to determine critical conditions for clearwater scour and ultimately the life-expectancy of these structures. More research is needed to extend estimation of scour under live-bed conditions in gravel-bed rivers for permeable structures, while also enhancing the proposed scour evolution formula to incorporate measure of turbulent strength around the structure for the area, intensity, and frequency of key vortical structure developing around an ELJ structure.