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We performed a three-dimensional high-resolution Large Eddy Simulation using a sinusoidal ripple-like bedform geometry with spacing and amplitude consistent with equilibrium conditions under the prescribed oscillatory forcing. The simulation results are validated qualitatively and quantitatively with previous laboratory experiments and numerical simulations. Our numerical simulations allow to analyze the presence of "ribs", strong flow rotational features in the spanwise direction developing perpendicular to the ripple crest. We show that ribs tend to appear on the upslope and crest of ripples and display a dominant spacing, ranging from 0.5λ to λ, during wave phases associated to flow acceleration. When the vortex is transported over the downstream ripple during the flow deceleration, ribs appear for the second time over the upslope of the downstream ripple, with the dominant spacing about 1/8 ripple wavelength. The presence of flow three-dimensionality corresponds to a large variability of the shear stress over ripples in the spanwise direction, which could ultimately favor the development of three-dimensional geometry. The threedimensional vortex dynamics are evaluated using the concept of vortex strength. Peak vortex strength is observed at the maximum free stream velocity, but the largest variation of the vortex strength and vortex size in the spanwise direction occurs prior to the flow reversal. The variability of the vortex structures might provide a favorable setting for the development of ribs and might relate to the spacing of ribs.

High-resolution large eddy simulations and complementary laboratory experiments using particle image velocimetry were performed to provide a detailed quantitative assessment of flow response to gaps in cylinder arrays. The base canopy consists of a dense array of emergent rigid cylinders placed in a regular staggered pattern. The gaps varied in length from [Formula: see text] to 24, in intervals of 4 d, where d is the diameter of the cylinders. The analysis was performed under subcritical conditions with Froude numbers [Formula: see text] and bulk Reynolds numbers [Formula: see text]. Results show that the gaps affect the flow statistics at the upstream and downstream proximity of the canopy. The affected zone was [Formula: see text] for the mean flow and [Formula: see text] for the second-order statistics. Dimensionless time-averaged streamwise velocity within the gap exhibited minor variability with gap spacing; however, in-plane turbulent kinetic energy, k, showed a consistent decay rate when normalized with that at [Formula: see text] from the beginning of the gap. The emergent canopy acts as a passive turbulence generator for the gap flow for practical purposes. The streamwise dependence of k follows an exponential trend within [Formula: see text] and transitions to a power-law at [Formula: see text]. The substantially lower maximum values of k within the gap compared to k within the canopy evidence a limitation of gap measurements representative of canopy flow statistics. We present a base framework for estimating representative in-canopy statistics from measurements in the gap.

We carried out high‐resolution Large Eddy Simulations to study the three‐dimensional hydrodynamic characteristics over regular and irregular wave‐generated ripples. Two types of irregularities in ripple crestlines are considered, namely ripples with a termination defect and with a bifurcation defect. Over regular ripples, the flow and vortex structures vary in the spanwise direction, displaying evident three‐dimensional vortex structures. Compared to regular ripples, the presence of a bifurcation or a termination defect implies local variations in ripple height and spacing. As a result, the maximum velocity, vortex strength and the position of the vortex center over ripples neighboring a defect are all affected by the presence of the defect. The maximum velocity at the ripple crest neighboring a defect is linearly related to the position of the vortex center. The presence of defects induces an asymmetric distribution of shear stress and sediment flux that will ultimately affect bedform evolution.

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