Local Scour Around Elliptical Pier Groups in Non-Uniform Sediment with Submerged Flexible Vegetation

Local scour around bridge-pier groups is governed by interactions among pier geometry, flow interference, and sediment mobility. This study investigates these processes for elliptical pier groups in non-uniform sediment protected by submerged flexible vegetation under steady, clear-water laboratory conditions. Single- and three-pier configurations were tested in tandem and side-by-side arrangements at 2D and 3D (D = pier’s diameter) spacings. Experimental data included velocity profiles, scour morphology, and turbulence characteristics. In tandem groups, strong sheltering at 2D concentrated scour at the front pier (max 69 mm), while rear pier scour was minimal (9 mm). At 3D, sheltering weakened, leading to more independent scour. In side-by-side groups, contraction dominated, with maximum scour decreasing from 72 mm at 2D to 60 mm at 3D. Sediment gradation further moderated erosion through surface armoring. Notably, within the tested densities, the flexible canopy reduced near-bed momentum and bed shear stress toward or below critical sediment-motion thresholds, strongly suppressing scour under the tested laboratory canopy. These findings clarify the coupled roles of pier interference, sediment armoring, and vegetation-induced resistance, advancing nature-based local scour mitigation within the investigated constraints.