Aquatic vegetation modifies hydrodynamics, turbulence structure, sediment transport, and ecological processes in marine ecosystems. Recent turbulence models for vegetated flows have focused on open channel unidirectional flows. However, the unsteadiness and turbulent structure of oscillatory flows often prevent the direct application of such models in wave-dominated environments. We investigate Turbulent Kinetic Energy (TKE) connected to the flow structure in oscillatory flows through aquatic vegetation. Using an oscillatory tunnel, we test vegetation densities up to $\phi=0.10$ with wave periods between 2.1-5.3 s and wave amplitudes between 2-10 cm. Our measurements show a nonlinear relation between the TKE inside the canopy and vegetation density due to the change from the stem- to canopy-scale dominated regime. We observe that $ah\geq 0.8$ marks a threshold for this transition: a reduction of wake TKE inside the canopy and an increase of shear TKE at the top of the canopy. This transition is characterized by increasing frequency and intensity of sweeps and ejections near the bed and at the canopy top. We developed a two-equation predictor for TKE at the top of the canopy using the “short-cut” TKE transfer first proposed by \citeA{finnigan2000turbulence} where canopy-scale eddies convert TKE into stem-scale eddies via the work against vegetation drag. For near-bed TKE, we adapt \citeA{tanino2008lateral}’s model to predict the maximum TKE values on oscillatory flows. These two predictors provide easy-to-use tools suitable for wave-dominated environments to accurately estimate TKE levels inside the canopy for estimating sediment transport rates and mass exchange across the canopy.

Aquatic vegetation alters the hydrodynamics of natural waters, such as rivers, lakes, and estuaries. Plants can generate turbulence that propagates throughout the entire water column, which affects gas transfer mechanisms at both air-water and water-sediment interfaces, driving changes of dissolved oxygen (DO), an important indicator of water quality. We conducted a series of laboratory experiments with rigid cylinder arrays to mimic vegetation using a staggered configuration in a recirculating race-track flume. Walnut shells were chosen as the sediment substrate, which interacts with DO in water. 2D planar Particle Image Velocimetry was used to characterize the flow field under various submergence ratios, highlighting the effect of vegetation on turbulence quantities. Gas transfer rates were determined by measuring the DO concentration during the re-aeration process based on the methodology proposed by the American Society of Civil Engineers. Our data provide new insight on Air-Water-Vegetation-Sediment interactions in streams as a function of submergence ratio, array density, and flow turbulence. A modified surface renewal model using turbulence production as an indicator of gas transfer efficiency is used to predict surface gas transfer rates. A delayed time of re-aeration between the bulk and the near-bed region was observed and varies with flow velocities and submergence ratios, which controls the oxygen flux from water to sediment. Future studies are required to investigate the cause of the delayed time to incorporate sediment oxygen demand in a substrate-to-surface transfer model.

Gravity currents frequently occur when excess suspended sediments are flushed along a river and discharged into greater natural water environments such as lakes, reservoirs, and estuaries. Gravity and turbidity currents have been broadly investigated, but the effect of aquatic vegetation on their propagation in natural waters still presents several open questions. We conducted a series of laboratory experiments to investigate how flexible vegetation affects the propagation and flow structure of gravity currents on a constant slope. We used both rigid cylinders and flexible synthetic plants to mimic natural submerged vegetation canopies. By varying density configurations of the vegetation array and comparing the outcomes of rigid cylinders and flexible plants, the data showed distinct patterns based on array density and plant morphology. A two-layer current was created when the array density is large enough to redirect the flow, as opposed to sparser conditions where the denser fluid passes swiftly through the array. Flexible vegetation further suppresses the propagation speed of gravity currents compared to arrays of rigid cylinder with the same density, highlighting the importance of the multi-scale processes driven by complex plant morphologies that are not represented by rigid cylinder arrays.

Turbulence generated by aquatic vegetation in rivers, lakes, and estuaries, can significantly alter the flow structure throughout the whole water column, affecting gas transfer mechanisms at the air-water interface, driving changes in indicators of water quality. We conducted a series of laboratory experiments with rigid cylinder arrays to mimic vegetation using a staggered configuration in a recirculating Odell-Kovasznay type race-track flume. 2D planar Particle Image Velocimetry (PIV) was used to characterize the mean flow field and turbulent flow statistics, to characterize the effect of emergent and submerged vegetation in terms of turbulent kinetic energy, Reynolds stresses, and turbulent shear production. The surface gas transfer rate was determined by measuring the dissolved oxygen (DO) concentration during the re-aeration process in water based on the methodology proposed by the American Society of Civil Engineers (ASCE). Our data provide new insight on how stem- and canopy- scale turbulence affect the surface gas transfer rate at different submergence ratios and array densities. The relation between mean flow velocity and turbulent shear production in these scenarios is used to develop a modified surface renewal (SR) model using turbulent shear production as an indicator of gas transfer efficiency, which allows us to more accurately predict surface gas transfer rates in vegetated flows.

Turbulence generated by aquatic vegetation plays a vital role in the interfacial transfer process at the air-water interface and sediment-water interface (AWI and SWI), impacting the dissolved oxygen (DO) level, a key indicator of water quality for aquatic ecosystems. We investigated the influence of vegetation, under different submergence ratios and plant densities, on the interfacial gas transfer mechanisms. We conducted laboratory experiments in a unidirectional recirculating flume with simulated rigid vegetation on a sediment bed. Two-dimensional planar Particle Image Velocimetry (2D-PIV) was used to characterize the mean flow field and turbulent quantities. Gas transfer rates at the AWI were determined by monitoring the DO concentration during the re-aeration process in water. SWI interfacial transfer fluxes were estimated by measuring the DO concentration difference between the near-surface and near-bed values. Compared to previous observations on a smooth bed without sediment, the presence of sediment enhances the bottom roughness, which generates stronger bed-shear turbulence. The experimental result shows that turbulence generated from the bed does not affect the surface transfer process directly. However, the near-bed suspended sediment provides a negative buoyancy term that reduces the transfer efficiency according to the predictions by a modified Surface Renewal model for vegetated flows. The measured interfacial transfer fluxes across the SWI show a clear dependence on the within-canopy flow velocity, indicating that bed shear turbulence and within-canopy turbulence are critical indicators of transfer efficiency at SWI in vegetated flows. A new Reynolds number dependence model using near-bed turbulent kinetic energy as an indicator is proposed to provide a universal prediction for the interfacial flux across the SWI in flows with aquatic vegetation. Our study provides critical insight for future studies on water quality management and ecosystem restoration in natural water environments such as lakes, rivers, and wetlands.