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WIREs Water
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The hydraulic description of vegetated river channels: the weaknesses of existing formulations and emerging alternatives

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Currently, many of the methods used to predict the effect of vegetation on river flow suffer from one or both of the following problems: (1) a strong dependence on parameters that have a poor physical basis and which are only readily determined using empirical means; and (2) a poor conceptual basis, in terms of the way they represent the effects of vegetation on the flow, especially in higher dimensionality numerical models. This limits their contribution to problems that extend beyond basic hydraulic prediction (e.g., of water levels) to ecosystem understanding. In this study, we show how use of coupled biomechanical–hydraulic models may lead to a much‐improved representation of a range of open‐channel flow processes. Preliminary experiments over hypothetical vegetation canopies are producing very encouraging results and may provide the means for an improved representation of vegetation in higher dimensionality numerical models that may result in a better justification and more reliable identification of the conveyance parameters needed for both flood identification and the characterization of habitat. WIREs Water 2014, 1:549–560. doi: 10.1002/wat2.1044 This article is categorized under: Water and Life > Conservation, Management, and Awareness Science of Water > Hydrological Processes
Schematic model of canopy flow showing the development of the characteristic canopy velocity profile (blue, dashed line) and associated turbulence structure, due to the difference in above‐canopy (U2) and within‐canopy (U1) flow velocities.
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A plan view of the near‐bed Reynolds stress calculated at 0.02 z/h. Panel (a) represents the time‐averaged Reynolds stress and panel (b) represents the instantaneous Reynolds stress calculated from a Large Eddy Simulation predicting flow at a resolution of 50 Hz, flow in these images is from bottom to top.
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A plan view of the turbulent kinetic energy (TKE) calculated at 0.02 z/h. Panel (a) represents the time‐averaged TKE and panel (b) represents the instantaneous TKE calculated from a Large Eddy Simulation predicting flow at a resolution of 50 Hz, flow in these images is from bottom to top.
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A plan view of the vortex structures at ∼0.02 z/h calculated using FTLE. Individual wake structures can be observed forming around individual stems but the reattachment length is greater than the separation distance between stems forming larger‐scale turbulent structures. Flow is from bottom to top.
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The predicted vortex structures identified by finite‐time Lyapunov exponents (FTLE) down the midline of a hypothetical vegetation canopy. Vortex structures have been tracked for 1 second. As the variable increases (tends toward red) the vortex attractors are stronger demonstrating vortex ridges. A roller vortex can be seen to be developing as the structure moves off from the top of the canopy. Flow is from left to right.
[ Normal View | Magnified View ]

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Water and Life > Conservation, Management, and Awareness
Science of Water > Hydrological Processes

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