A Bay In Flux
by David Pescovitz
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UC Berkeley researcher
Mark Stacey calls the entire San Francisco Bay his "laboratory."
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UC Berkeley researcher
Mark Stacey calls the entire San Francisco Bay his "laboratory." The
professor of civil and environmental engineering analyzes the physics
of the 1,600 square mile waterway between the Pacific Ocean and
the Sacramento-San Joaquin Delta. By studying the estuary, from
centimeter scale turbulence to the seasonal transport of salt between
the ocean and the Bay, Stacey's research could impact everything
from the preservation of delicate ecosystems to the quality of
our drinking water.
"To understand how everything from nutrients and salt to contaminants
and invasive species move through the Bay, you have to understand
the basic physics of water flow," Stacey says.
Stacey's research is focused on ocean-estuary exchanges,
the flow between freshwater from upstream river systems and fluxes
from the open sea. In the San Francisco Bay, Stacey explains, the
relationship between the coastal ocean and the estuary is largely
unknown. But through a combination of boat trips around the Bay
and computer simulations, Stacey and his students are developing
predictive models of how water and, more importantly, the various
materials in the mix, is transported through the estuary system.
The largest harbor on the US Pacific Coast, the San Francisco Bay-Delta Estuary contains over 90 percent of the state's coastal wetlands and supports 750 species of fish, animals, and birds.
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For example, one of Stacey's Bay-based research efforts is
concentrated on the movement of salt in the water. The Bay's
salt field, or seawater front, shifts within the Bay in response
to myriad factors like tidal dynamics and freshwater inflows from
rivers. The salt field, Stacey says, is partially at the mercy
of the state's water managers. Opening reservoir valves upstream
pushes the salt field out of the Bay while scaling down the flow
from the reservoirs causes the salt to flow back into the Bay.
"Where the salt field sits during different seasons of the year
has implications for the health of the Bay's ecosystem," Stacey
says. "Certain species need it to be pushed downstream
to the ocean during the spring or back into the Bay during the
fall."
Still, the water managers must cater to the needs of the Bay's
human population as well. A vast majority of the state's
population drinks at least some water from the Delta, and too much
salinity in the Bay can jeopardize drinking water supplies.
"We'd like to manage our freshwater resources in a way that
the position of the salt field in the Bay doesn't compromise
either drinking water quality or the safety of the ecosystem," he
says.
Through
field studies, Mark Stacey (left) and his graduate
students also hope to gain an understanding of where
sediments dredged from the Bay--to clear the way for
boats, for instance-- eventually settle. This data
could help determine the availability of sediment necessary
for marshland restoration in the South Bay.
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To gather their
data, Stacey and his team frequently embark on Bay cruises aboard
a small boat outfitted with a variety of sensing
instruments. An Acoustic Doppler Current Profiler (ADCP) converts
the echoes of audio waves into a three-dimensional representation
of the current. Meanwhile, other sensors immersed in the water
keep track of temperature and salinity while also measuring chlorophyll
concentration, indicative of what's living in the water.
The key to gathering useful data is measuring flows and currents
on wide time scales, from "turbulent scales" lasting
only a few seconds to 12-hour tidal scales, to lunar and annual
cycles. Minor variations in the small scale dynamics can have a
big impact, Stacey says. Ignoring how the various size and time
scales are intertwined makes it impossible to adequately represent
the physical properties in computer models and test various hypotheses
about the ocean-Bay exchanges.
"The thing I like about field studies is that we're not idealizing," he
says. "We capture the real physics and then go into a computer
modeling stage."
Most recently, Stacey and his team began unraveling a long-standing
mystery of ocean-estuary exchange. As part of the Flux Observations
at the Golden Gate (FOGG) project, the researchers are not only
studying how much salt and other materials move between the Bay
and the ocean, but why this happens at all. The researchers compared
their measurements of the flux of salt coming and out of the Bay
against theoretical amounts predicted based on the flow of water
from the rivers upstream.
"Measurements have never been made this way before with such a high
level of detail," he says. "Finally, we can really peel
back the layers of these mechanisms to determine which ones are
most important.
The goal now is to identify the specific physical mechanisms--wind
or tides, for example--that are creating the flux.
"There are a vast number of unanswered but interrelated questions," Stacey
says. "It's very detailed work, but I get excited thinking
about how it may affect ecosystem dynamics and California water
policy."
Mark
Stacey’s home page
Flux Observations at the Golden Gate (FOGG)
Lab Notes is
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© 2003 UC Regents.
Updated 10/31/03.
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