Environmental Fluid Mechanics

, Volume 6, Issue 5, pp 477–488 | Cite as

Prediction of near-field shear dispersion in an emergent canopy with heterogeneous morphology

Original Article


The evaluation of longitudinal dispersion in aquatic canopies is necessary to predict the behavior of dissolved species and suspended particles in marsh and wetland systems. Here we consider the influence of canopy morphology on longitudinal dispersion, focusing on transport before constituents have mixed over depth. Velocity and longitudinal dispersion were measured in a model canopy with vertically varying canopy density. The vertical variation in canopy morphology generates vertical variation in the mean velocity profile, which in turn creates mean-shear dispersion. We develop and verify a model that predicts the mean-shear dispersion in the near field from morphological characteristics of the canopy, such as stem diameter and frontal area. Close to the source, longitudinal dispersion is dominated by velocity heterogeneity at the scale of individual stems. However, within a distance of approximately 1 m, the shear dispersion associated with velocity heterogeneity over depth increases and eclipses this smaller-scale process.


Canopy Diffusion Dispersion Element array Near field Shear Vegetation Velocity 



Cross-sectional area


Volumetric frontal area density


Drag coefficient


Stem diameter


Vertical turbulent diffusion coefficient




Gravitational constant


Water depth


1 for upper canopy layer, 2 for lower layer


Dispersion coefficient


Stem density


Order of magnitude


Volumetric flow rate


Stem Reynolds number




Time-averaged fluid velocity

\(\langle u\rangle \)

Time- and horizontally averaged fluid velocity


Time- and spatially averaged fluid velocity


Distance in the direction of flow


Transverse coordinate


Height above bed


Scale constant


Effective vertical cloud width

\(\Delta \langle u\rangle\)

Difference between maximum and minimum velocities


Surface elevation


Kinematic viscosity


Concentration standard deviation


Spatial concentration variance


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Copyright information

© Springer Science+Business Media B.V. 2006

Authors and Affiliations

  1. 1.Ralph M. Parsons Laboratory, Department of Civil and Environmental EngineeringMassachusetts Institute of TechnologyCambridgeUSA

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