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PRESERVING
A FRAGILE HABITAT
In the 1930s, the Corps of Engineers standardized the fractured, discontinuous levee, thus cutting off the estuary from the natural flooding that breathed life into the ecosystem. Isolation of this estuary caused shifts in animal and plant populations, including the local fisheries industry. Once the harmful effects on the estuary were recognized, the Corps set about a three-decade-long design and construction of a freshwater diversion to rejuvenate the ecosystem. The Caernarvon Freshwater Diversion opened in 1991, though officials were not entirely sure how to optimize its use to the environment’s full benefit. For the past four years, coastal expert Jaye Cable and her team of researchers have tromped through the marshlands and boated down the channels of the Breton Basin taking samples of sediment and measuring water flow in this delicate ecosystem. Continuous recording gauges were placed by the team in several key bayous to measure water level, water velocity, salinity, total suspended solids, and temperature. Cable also uses a boat-mounted Acoustic Doppler Current Profiler to determine the basic direction and magnitude of water flow during the planned pulsed events of the diversion. Sediments are more effectively delivered to the estuary under certain meteorological and river conditions. Factors such as wind direction and velocity, river stage, sediment load, and inter-annual sea level changes can influence sediment transport. For example, if the diversion gates are open and strong winds are blowing northward from the Gulf of Mexico, freshwater and sediment distribution to lower areas of the estuary may be hindered.
Home to numerous aquatic species, including oysters and brown and white shrimp, the fisheries industry is a substantial part of the local economy. In addition to contributing to environmental preservation, Cable’s project to optimize the diversion operation could help enhance the fisheries industry in the Breton Sound Basin. At the beginning of the project, Cable and other researchers, including LSU’s John Day and Brian Fry, asked state officials to run the diversion at high and low pulses during each spring. These pulses of water were designed to more closely mimic natural flooding events to the extent possible. Over time, they have found that the pulses of water are more beneficial to sediment delivery to the estuary than previously realized. They also discovered that the estuary’s long-term sedimentation rate is keeping pace with sea level rise. “We are not fighting a losing battle,” says Cable. Cable emphasizes, however, that the diversion’s primary function is to stop salinity intrusion into the estuary headwaters. From her perspective, the goal is to preserve what currently exists, not to restore any land loss that may have been caused by coastal erosion or subsidence. Such an aim would require a higher-cost and differently designed diversion. The data gathered and subsequent analysis will shape the final report Cable and her team will present to DNR in the fall of 2005, helping to preserve a vital Louisiana ecosystem. ON
THE WEB:
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During
the Great Mississippi Flood of 1927, New Orleans bankers decided
to dynamite the levee south of the city to prevent flood waters
from destroying New Orleans; however, the break in the levee sent
flood waters across the Breton Sound Basin estuary, which lies
in St. Bernard and Plaquemines parishes, devastating the area.
Funded
by grants from the EPA, USDA, and the Louisiana Department of
Natural Resources, Cable, an associate professor of Oceanography
and Coastal Sciences and in the Coastal Ecology Institute, is
searching for ways to yield the maximum environmental benefit
from the diversion. In addition to sustaining life, the freshwater
flow from the diversion also keeps marine water from creeping
too far in shore, which can shift the plant and animal balance
in the basin.