Flow and transport through aquatic vegetation is characterized by a wide range of length scales: water depth ($H$), plant height ($h$), stem diameter ($d$), the inverse of the plant frontal area per unit volume (${a}^{\ensuremath{-} 1} $) and the scale(s) over which $a$ varies. Turbulence is generated both at the scale(s) of the mean vertical shear, set in part by $a$, and at the scale(s) of the stem wakes, set by $d$. While turbulence from each of these sources is dissipated through the energy cascade, some shear-scale turbulence bypasses the lower wavenumbers as shear-scale eddies do work against the form drag of the plant stems, converting shear-scale turbulence into wake-scale turbulence. We have developed a $k$–$\varepsilon $ model that accounts for all of these energy pathways. The model is calibrated against laboratory data from beds of rigid cylinders under emergent and submerged conditions and validated against an independent data set from submerged rigid cylinders and a laboratory data set from a canopy of live vegetation. The new model outperforms existing $k$–$\varepsilon $ models, none of which include the $d$ scale, both in the emergent rigid cylinder case, where existing $k$–$\varepsilon $ models break down entirely, and in the submerged rigid cylinder and live plant cases, where existing $k$–$\varepsilon $ models fail to predict the strong dependence of turbulent kinetic energy on $d$. The new model is limited to canopies dense enough that dispersive fluxes are negligible.
A series of laboratory experiments was conducted using arrays of rigid cylinders in a sandy bed as a proxy for mangrove roots and benthos. Synchronous colocated measurements of velocity and suspended sediment concentration were recorded within the array to investigate the effect of array density on sediment resuspension under different wave conditions. The measured increase in turbulent kinetic energy resulting from flow‐vegetation interactions is directly linked to the observed increase in sediment resuspension. The observations emphasize the role of turbulent kinetic energy generated by the flow‐vegetation interactions, rather than bed shear stress by the mean velocity, as the main driver of resuspension within the array. We test a modified Shields parameter analysis, as well as analytical predictions previously developed for unidirectional currents, which accurately predict resuspension thresholds under oscillatory flow conditions.
Traditional bed shear stress‐based models (e.g., Rouse model) derived from the classic parabolic profile of eddy viscosity in open‐channel flows fail to accurately predict suspended sediment concentration (SSC) in flows with aquatic vegetation. We developed a two‐layer, turbulence‐based model to predict SSC profiles in emergent vegetated flows. Turbulence generated from vegetation, bed, and coherent structures caused by stem‐bed‐flow interaction are considered into the near‐bed turbulent kinetic energy (TKE) to calculate the effective bed shear velocity, ubeff*. The model, validated by experimental data, further showed that the thickness height of the near‐bed layer (effective bottom boundary layer), Hb, varies with flow velocity and canopy density. Two additional models are provided to estimate Hb and ubeff*. The model is expected to provide critical information to future studies on sediment transport, landscape evolution, and water quality management in vegetated streams, wetlands, and estuaries.
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