Environmental Fluid Mechanics

, Volume 6, Issue 5, pp 477–488

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

Original Article

DOI: 10.1007/s10652-006-9002-7

Cite this article as:
Lightbody, A.F. & Nepf, H.M. Environ Fluid Mech (2006) 6: 477. doi:10.1007/s10652-006-9002-7

Abstract

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.

Keywords

CanopyDiffusionDispersionElement arrayNear fieldShearVegetationVelocity

Abbreviations

A

Cross-sectional area

a

Volumetric frontal area density

CD

Drag coefficient

d

Stem diameter

Dz

Vertical turbulent diffusion coefficient

f

Function

g

Gravitational constant

h

Water depth

i

1 for upper canopy layer, 2 for lower layer

K

Dispersion coefficient

n

Stem density

O

Order of magnitude

Q

Volumetric flow rate

Red

Stem Reynolds number

t

Time

u

Time-averaged fluid velocity

\(\langle u\rangle \)

Time- and horizontally averaged fluid velocity

U

Time- and spatially averaged fluid velocity

x

Distance in the direction of flow

y

Transverse coordinate

z

Height above bed

β

Scale constant

Δh

Effective vertical cloud width

\(\Delta \langle u\rangle\)

Difference between maximum and minimum velocities

η

Surface elevation

ν

Kinematic viscosity

σx

Concentration standard deviation

\(\sigma_x^2\)

Spatial concentration variance

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