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Abstract Aquatic vegetation has the potential to increase suspended sediment capture while also increasing sediment resuspension and bedload transport. Suspended sediment can induce density stratification, which modulates the turbulence in the water column. We derive a Rouse‐based formulation for suspended sediment concentration (SSC) including the effect of sediment‐induced density stratification. We perform Large Eddy Simulations of vegetated and non‐vegetated channels to explicitly highlight the effect of stratification on SSC profiles. We found that the impact of stratification is dominant in the near‐bed region within the bottom boundary layer, affecting both sediment resuspension and bedload transport. Stratification reduces the likelihood of both dominant sweep and ejection events in the near the bed region which may affect sediment entrainment and bedload transport. Modifications to existing models of sediment entrainment and bedload transport are suggested to account for the effects of sediment induced stratification in vegetated and non‐vegetated channels. 

Abstract Aquatic vegetation plays an important role in natural water environments by interacting with the flow and generating turbulence that affects the air‐water and sediment‐water interfacial transfer. Regular and staggered arrays are often set as simplified layouts for vegetation canopy to study both mean flow and turbulence statistics in vegetated flows, which creates uniform spacing between vegetation elements, resulting in preferential flow paths within the array. Such preferential paths can produce local high velocity and strong turbulence, which do not necessarily happen in natural environments where vegetation is randomly distributed. How the randomness of the canopy affects interfacial processes by altering spatial turbulence distribution, which can potentially lead to different turbulence feedback on the interfacial transfer process, remains an open question. This study conducted a series of laboratory experiments in a race‐track flume using rigid cylinders as plant surrogates. Mean and turbulent flow statistics were characterized by horizontal‐ and vertical‐sliced PIV. Based on the measured flow characteristics under different stem diameters and array configurations, we propose a method to quantify the randomness of the vegetation array and update a sediment‐water‐air interfacial gas transfer model with the randomness parameter to improve its accuracy. The updated model agrees well with the dissolved oxygen experimental data from our study and data from existing literature at various scales. The study provides critical insight into water quality management in vegetated channels with improved dissolved oxygen predictions considering vegetation layout as part of the interfacial transfer model. 

Abstract Dissolved Oxygen (DO) fluxes across the air‐water and sediment‐water interface (AWI and SWI) are two major processes that govern the amount of oxygen available to living organisms in aquatic ecosystems. Aquatic vegetation generates different scales of turbulence that change the flow structure and affect gas transfer mechanisms at AWI and SWI. A series of laboratory experiments with rigid cylinder arrays to mimic vegetation was conducted in a recirculating race‐track flume with a lightweight sediment bed. 2D Planar Particle Image Velocimetry was used to characterize the flow field under different submergence ratios and array densities to access the effect of vegetation‐generated turbulence on gas transfer. Gas transfer rate across AWI was determined by DO re‐aeration curves. The effective diffusion coefficient for gas transfer flux across SWI was estimated by the difference between near‐bed and near‐surface DO concentrations. When sediment begins to mobilize, near‐bed suspended sediment provides a negative buoyancy term that increases the critical Reynolds number for the surface gas transfer process according to a modified Surface Renewal model for 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 SWI in flows with aquatic vegetation. This study provides critical information and useful models for future studies on water quality management and ecosystem restoration in natural water environments such as lakes, rivers, and wetlands. 

Abstract We present a critical analysis of experimental findings on vegetation–flow–sediment interactions obtained through both laboratory and field experiments on tidal and coastal environments. It is well established that aquatic vegetation provides a wide range of ecosystem services (e.g. protecting coastal communities from extreme events, reducing riverbank and coastal erosion, housing diverse ecosystems), and the effort to better understand such services has led to multiple approaches to reproduce the relevant physical processes through detailed laboratory experiments. State‐of‐the‐art measurement techniques allow researchers to measure velocity fields and sediment transport with high spatial and temporal resolution under well‐controlled flow conditions, yielding predictions for hydrodynamic and sediment transport scenarios that depend on simplified or bulk vegetation parameters. However, recent field studies have shown that some simplifications on the experimental setup (e.g. the use of rigid elements, a single diameter, a single element height, regular or staggered layout) can bias the outcome of the study, by either hiding or amplifying some of the relevant physical processes found in natural conditions. We discuss some observed cases of bias, including general practices that can lead to compromises associated with simplified assumptions. The analysis presented will identify potential pathways to move forward with laboratory and field measurements, which could better inform predictors to produce more robust, universal and accurate predictions on flow–vegetation–sediment interactions. © 2020 John Wiley & Sons, Ltd. 

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.