A field investigation of diffusion within a submerged plant canopy

226 Pages·1.8 MB·English

by  Tarrell, Alvin E

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A Field Investigation ofDiffusion Within a Submerged Plant Canopy
by
AlvinE. Tarrell
B.S. ChemicalEngineering
University ofNebraska- Lincoln, 1984
Submitted in partial fulfillment ofthe requirements forthe dual degrees of
MASTER OF SCIENCE IN OCEANOGRAPfflC ENGINEERING
at the
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
and the
WOODS HOLE OCEANOGRAPfflC INSTITUTION
and
MASTER OF ENGINEERING IN CIVIL AND ENVIRONMENTAL ENGINEERING
at the
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
September 1997
© 1997 Massachusetts Institute ofTechnology and Woods Hole Oceanographic Institute
All rights reserved -*_0, .///?

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A Field Investigation ofDiffusion Within a Submerged Plant Canopy
by
Alvin E. Tarrell
Submitted to the Departments ofOcean Engineering
and Civil and Environmental Engineering
on July 25, 1997 in partial fulfillment ofthe requirements
forthe degrees ofMaster ofScience in Oceanographic Engineering
and Master ofEngineering in Civil and Environmental Engineering
ABSTRACT
Saltwater marshes and wetlands are important buffers at the land-sea interface. Among
the most biologically active ecosystems on Earth, natural and man-made wetlands are
important interceptors ofpollutants and nutrients bound for the coastal ocean. The
transport, dilution, and deposition processes occurring within the marsh are key factors in
determining this interception, and these are in turn determined largely by tidally driven
flows as influenced by marsh vegetation and other physical characteristics. Vegetation
type and density are ofprimary importance in these processes, both for pollutant/nutrient
uptake concerns and in determining hydrodynamic characteristics ofthe marsh.
This study examines the effect ofvegetation density and ambient flow on diffusivity
within a tidal marsh canopy, specifically Spartina alterniflora. Vegetation densities from
0-1.4% stem coverage and flows from 2-12 cm/sec were investigated using Rhodamine
WT
tracer, with resultant measured diffusivities ranging from approximately 0.5-3.0
cm /sec. Diffusivity was found to be a strong function ofambient current, but a much
weaker function ofvegetation density. Presence ofvegetation caused transverse and
vertical diffusivities to be essentially isotropic over all vegetation densities, as opposed to
the order ofmagnitude difference found in earlier non-vegetated studies. Only slight
vegetation coverage was found to be necessaryto produce this isotropy, with little
additional change as stem density increased.
Thesis Supervisor: Heidi Nepf
Title: Assistant Professor ofCivil and Environmental Engineering