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Date:    Sun, 2 May 1999 17:14:11 EDT
From:    C 
Subject: Constructed wetlands - hydrodynamics

----------------------------Original message----------------------------

Nepf seeks to design 'constructed wetlands' to fight water
pollution

Tech Talk
Massachusetts Institute of Technology,
Cambridge, Mass.

http://web.mit.edu/newsoffice/tt/1999/apr28/nepf.htmllm99.htmlct_parties/Pl


WEDNESDAY, APRIL 28, 1999
By Denise Brehm
News Office


Eelgrass and cattails may evoke images of supernatural creatures,
but they're actually ordinary plants being used for some pretty
extraordinary work -- to help clean up human-made messes at the
marshy juncture where wetlands meet open waters.  Indeed, the
United States annually spends $100 billion to clean up these
problems.

Ecologists, botanists and environmental engineers are studying
how this vegetation can be planted in certain configurations and
densities to create constructed wetlands that absorb and
dissipate runoff pollution before it makes its way into the open
waters of lakes and oceans.  The design of these systems is
currently limited by a poor understanding of the impact of
vegetation on the water movement.

Heidi Nepf, associate professor of hydrodynamics in the
Department of Civil and Environmental Engineering, is addressing
this limitation through field and laboratory research.  She and
four graduate students have created a model that describes the
hydrodynamics -- or water movement -- through vegetation that may
enable more accurate prediction of wetland filtering capacity,
and thus improve wetland design and management.

"Coastal wetlands protect surface water quality by acting as
natural filters for both dissolved and particulate-born nutrients
and contamination.  It is only by understanding how the water
moves that we can understand where particles and contaminants are
going, and how plant/flow interaction affects the fate of the
nutrients and contaminants in wetlands," said Dr. Nepf, who
recently published two papers on her research in Water Resources
Research (February) and Limnology and Oceanograpy (April).

Sometimes the plant canopy "captures" the contaminants by
creating still regions that allow particulate material to
accumulate around stems and leaves.  But if these regions of
still water are suddenly disturbed by stronger flows, as may
happen in storms, those same contaminants could be let loose all
at once, with the potential to cause even more problems.

Another effect of the plant/flow interaction is the creation of
small wakes behind plant stems.  This bit of turbulence may
improve the plants' uptake of elements like phosphorous and
nitrogen, or it may accelerate the uptake of chemicals by
microbial communities living on the plants' surfaces.  These
processes are key to wetland function, and they are controlled
in part by the characteristics of the flow.

"When you stir your coffee, you get curls coming off the back of
your spoon.  With plants, that's reversed.  The plants are still
and the water moves around them, leaving little wakes behind,"
Dr. Nepf said.

Dr. Nepf and co-author Evamarie Koch, a seagrass biologist at the
Horn Point Environmental Laboratory at the University of
Maryland, found that secondary flows created by the interaction
of plant stems with the primary flow can have an accumulated
effect in surprising ways.

For instance, in their paper in Limnology and Oceanography, they
show how pressure created by the water flowing against the base
of the stem can cause water to be pushed into the soil under a
plant, through its roots and emerge again in a vertical flow on
the opposite side of the stem.  This vertical flow of water could
carry nutrients out of the sediment and deliver them to the
growing portions of the plant tissue.

The paper in Water Resources Research, which is based on the
laboratory and field work of four MIT graduate students -- Al
Tarrel, Jennifer Sullivan, Christophe Mugnier and Becky
Zavistoski -- presents for the first time the model that
describes the relationship between vegetative drag, turbulence
and mixing within a canopy that may improve wetland design and
management.

"We're still at an early stage in this work where physicists are
just beginning to seriously interact with ecologists and
biologists," said Dr. Nepf, a physicist, who cautions that they
don't yet have an exact model of the perfect wetland.

To do their studies, Dr. Nepf and the graduate students used
wooden dowels, strips of plastic and rubber bands to make model
beds that mimic different types of aquatic grasses in a
66-foot-long flume at MIT's Parsons Lab.

The flume, which looks like a very long fish tank, currently
holds 1,500 gallons of water and 850 small "plants" -- actually
short pieces of wooden dowel stuck in holes in the bottom of the
tank, each with six slender strips of plastic affixed with rubber
bands.  The plastic strips wave about in the tank exactly as
seagrass blades would in a coastal embayment.  A hydraulically
driven paddle produces waves in the flume; a four-beam laser
doppler measures the turbulence as the water moves around and
over the plastic plants growing out of the bottom of the tank.

For each of their three experiments -- on emergent grasses (reeds
or cattails), stiff submerged grasses (Bermuda grass) and, most
recently, submerged and flexible eelgrass -- Dr. Nepf and her
students spend many weeks determining the appropriate materials
to make a scaled model of plants growing in either fresh or
saltwater.  By choosing materials of the appropriate density,
geometry and elasticity, they are able to build model grasses
that bend and move just like the real thing.

The current flume experiment of submerged eelgrass was designed
and built by graduate student Marco Ghisalberti, who scaled
exactly the imitation bed of six-inch-high eelgrass to the actual
three-foot-high grasses.

This research was initially funded by the MIT Doherty
Professorship and is currently funded by a National Science
Foundation Career Award.


For more information, contact:

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