Environmental Engineering Reference
In-Depth Information
may be an inadequate guide to the future when coastal fl ooding is
concerned.
Most of southeastern Louisiana was built over many thousands of
years by the deposition of sediment from the Mississippi River, which
naturally changed course and fl ooded innumerable times over the millen-
nia. The sediments that were deposited as the fl oods subsided were much
greater than the sediment lost to hurricane waves and created the land
that New Orleans sits on, as well as every patch of ground to the south.
Then people moved into the area, and, as people are wont to do,
deliberately upset the natural balance because of the human inclination
that always seems to be to conquer nature rather than learn to live with
it. Over time, thousands of acres of trees, bushes, and other vegetation
were paved over, decreasing the land's ability to absorb rain. Levees and
concrete barriers were built along the river to prevent the fl ooding that
had built the land, replenishing the swamps and marshes that had pro-
vided partial protection from Gulf hurricanes. The artifi cial restraints at
the river's sides increased water volume in the channel, causing rapid
erosion of Louisiana's land at its contact with the Gulf of Mexico. Since
the 1930s, an estimated 1,900 square miles of land have washed into the
Gulf, an area the size of a football fi eld lost every forty-fi ve minutes. The
area of land lost in the past seventy years is twice the size of Rhode
Island. Louisiana accounts for 80 percent of the nation's coastal land
loss, with rates ranging between 25 and 35 square miles per year.
Not all of southern Louisiana's land-loss problems have resulted from
human activities. Below the delta surface are many faults—breaks in the
earth's crust parallel to the coast—and the land on the south side of the
faults is dropping relative to the landward side at a rate of 0.2 inch each
year.
Increased hurricane intensities in recent years, due at least in part to
the burning of fossil fuels and climate change, have made the problem
worse. In August and September 2005, the one-two punch of Hurricanes
Katrina and Rita removed 200 square miles of Louisiana wetlands.
20
Environmentalists and ecologists have pointed out that the vast
earthen levee walls damage any wetlands they cross. Healthy tidal wet-
lands are not compatible with levee construction, and without healthy
wetlands, the land loss will continue. Since 1956, the New Orleans
metro area has lost 23 percent of its wetlands. Scientists estimate that
2.7 miles of wetlands can reduce storm surge by 1 foot.
21
The importance
of wetlands has been increasingly recognized during the past decade, and
the country as a whole gained wetlands at a rate of about 32,000 acres
per year (50 square miles) between 1998 and 2004.
