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Close Read on Coastal Erosion
Excerpt from p. 11-13
Coastal Erosion
One of the greatest threats facing Louisiana today is coastal
erosion. Losing approximately 35 square miles of land a year, the
state is literally shrinking as the marshes disappear into the Gulf.
In the last 50 years, an area the size of Rhode Island has vanished.
On average, an area the size of two football fields (about 2.5 acres)
disappears every hour. However, Hurricane Katrina destroyed
approximately 64,000 acres in one day. In many places where
cows grazed on pastures just a few years ago, there is now only
water.
Going…Going…Gone?
As millions of tons of sediment deposit onto a river delta,
the increasing weight causes the entire area to sink. As long as the
river continues to deposit sediment, the marsh builds up faster than
it sinks. But if the river changes course and abandons the delta, the
process reverses. The marsh continues to sink, but no new
sediment is deposited. Ocean waves and hurricanes accelerate
erosion, and the marsh eventually disappears into the Gulf.
Global warming impacts our coastal marshes too. Largely
because pollutants are disrupting the atmosphere, the earth’s
climate appears to be warming. If this continues, polar ice caps
may begin to melt, causing ocean levels to rise and coastal marshes
to slowly disappear.
Despite these naturally destructive forces, Louisiana’s
coastal marshes remained healthy and stable until the 20th century.
Though marshes were destroyed in some areas, river sediment
produced new marshland in others. Erosion is actually a natural
process, but human activity has accelerated it dramatically.
Human Interference
About 100 years ago, human activity began accelerating the
natural erosion process. Extensive levee systems were constructed
along rivers for flood control. These levees extend to the Gulf and
keep the rivers flowing within their channels. As a result, the
rivers can no longer overflow their natural levees to deposit fresh
sediment in the coastal marsh. Instead, the rivers shoot the
sediment straight into the Gulf’s deep water. New deposits of
sediment are no longer available to refresh the marshland.
Another problem is that less sediment reaches the Gulf
today than in earlier years. Rivers have been dammed to make
reservoirs, and these dams trap sediment. In fact, only about half
as much sediment reaches the Gulf today as did 100 years ago.
The search for oil is another major cause of coastal erosion.
Numerous canals and pipelines have been constructed in the
coastal marshes to extract oil. These often allow saltwater
intrusion. Saltwater from the Gulf of Mexico moves into the
marshes along these avenues and kills the marsh grasses that hold
the soil in place. Also, vast holes created by oil drilling lie under
the crust alone cannot support the weight of the marsh, so it
steadily sinks.
Slowing Coastal Erosion
Measures are being taken to stop coastal erosion. Grasses
have been planted in some areas to help hold the soil in place, and
sand is being pumped onto barrier islands to keep them from
eroding away. In 1991, the U.S. Army Corps of Engineers
completed the Caernarvon Diversion Project at a cost of $26
million. Located 15 miles downstream from New Orleans, this
structure allows Mississippi River water to flow through the levees
into coastal marshes.
Twenty-three miles upstream from New Orleans is the
Davis Pond Freshwater Diversion. Completed in 2002 at a cost of
$120 million, the project allows Mississippi River water to flow
into Barataria Bay. Over time the sediment-rich Mississippi water
should help rebuild the marshes in those areas.