Scientists
Weigh Options for Rebuilding New Orleans As experts ponder
how best to rebuild the devastated(毁坏)city, one question is whether to wall
off--or work--with--the water. Even before the death toll from
Hurricane Katrina is tallied, scientists arc cautiously beginning to discuss the
future of New Orleans. Few seem to doubt that this vital heart of U.S. commerce
and culture will be restored, but exactly how to rebuild the city and its
defenses to avoid a repeat catastrophe is an open question. Plans for improving
its levees and restoring the barrier of wetlands around New Orleans have been on
the table since 1998, but federal dollars needed to implement them never
arrived. After the tragedy, that’s bound to change, says John Day, an ecologist
at Louisiana State University (LSU) in Baton Rouge. And if there is an upside to
the disaster, he says, it’s that "now we’ve got a clean slate to start
from." Many are looking for guidance to the Netherlands, a
country that, just like bowl-shaped New Orleans, sits mostly below sea level,
keeping the water at bay with a construction of amazing scale and complexity.
Others, pointing to Venice’s longstanding adaptations, say it’s best to let
water flow through the city, depositing sediment to offset geologic
subsidence--a model that would require a radical rethinking of architecture.
Another idea is to let nature help by restoring the wetland buffers between sea
and city. But before the options can be weighed, several
unknowns will have to be addressed. One is precisely how the current defenses
failed. To answer that, LSU coastal scientists Paul Kemp and Hassan Mashriqui
are picking their way through the destroyed city and surrounding region,
reconstructing the size of water surges by measuring telltale marks left on the
sides of buildings and highway structures. They are feeding these data into a
simulation of the wind and water around New Orleans during its ordeal.
"We can’t say for sure until this job is done," says Day, "but the
emerging picture is exactly what we’ve predicted for years." Namely, several
canals--including the MRGO, which was built to speed shipping in the 1960s--have
the combined effect of funneling surges from the Gulf of Mexico right to the
city’s eastern levees and the lake system to the north. Those surges are to
blame for the flooding. "One of the first things we’ll see done is the complete
backfilling of the MRGO canal," predicts Day, "which could take a couple of
years." The levees, which have been provisionally repaired, will
be shored up further in the months to come, although their long-term fate is
unclear. Better levees would probably have prevented most of the flooding in the
city center. To provide further protection, a mobile clam system, much like a
storm Surge barrier in the Netherlands, could be used to close off the mouth of
Lake Pontchartrain. But most experts agree that these are short-term
fixes. The basic problem for New Orleans and the Louisiana
coastline is that the entire Mississippi River delta is subsiding and eroding,
plunging the city deeper below sea level and removing a thick cushion of
wetlands that once buffered the coastline from wind and waves. Part of the
subsidence is geologic and unavoidable, but the rest stems from the levees that
have hemmed in the Mississippi all the way to its mouth for nearly a century to
prevent floods and facilitate shipping. As a result, river sediment is no longer
spread across the delta but dumped into the Gulf of Mexico. Without a constant
stream of fresh sediment, the barrier islands and marshes are disappearing
rapidly, with a quarter, roughly the size of Rhode Island, already
gone. After years of political wrangling, a broad group pulled
together by the Louisiana government in 1998 proposed a massive $14 billion plan
to save the Louisiana coasts, called Coast 2050 (now modified into a plan called
the Louisiana Coastal Area project). Wetland restoration was a key component.
"It’s one of the best and cheapest hurricane defenses," says Day, who chaired
its scientific advisory committee. Although the plan was never
given more than token funding, a team led by Day has been conducting a pilot
study since 2000, diverting part of the Mississippi into the wetlands downstream
of the city. "The results are as good as we could have hoped," he says, with
land levels rising at about 1 centimeter per year--enough to offset rising sea
levels, says Day. Even if the wetlands were restored and new
levees were built, the combination of geologic subsidence and rising sea levels
will likely sink New Orleans another meter by 2100. The problem might be solved
by another ambitious plan, says Roel Boumans, a coastal scientist at the
University of Vermont in Burlington who did his Ph.D. at LSU: shoring up the
lowest land with a slurry of sediment piped in from the river. The majority of
the buildings in the flooded areas will have to be razed anyway, he says, "so
why not take this opportunity to fix the root of the problem" The river could
deposit enough sediment to raise the bottom of the New Orleans bowl to sea level
"in 50 to 60 years," he estimates. In the meantime, people could live in these
areas Venice-style, with buildings built on stilts. Boumans even takes it a step
further: "You would have to raise everything about 30 centimeters once every 30
years, so why not make the job easier by making houses that can
float." Whether that is technically or politically
feasible--Day, for one, calls it "not likely"--remains to be seen, especially
because until now, the poorest residents lived in the lowest parts of the city.
Any decision on how best to protect the city in the future will be tied to how
many people will live there, and where. "There may be a large contingent of
residents and businesses who choose not to return," says Bill Good, an
environmental scientist at LSU and manager of the Louisiana Geological Survey’s
Coastal Processes section. It is also not yet clear how decisions about the
reconstruction will be made, says Good, "Since there is no precedent of
comparable magnitude." Every level of government is sure to be involved, and
"the process is likely to be ad hoc." Even with the inevitable
mingling of science and politics, we still have "a unique chance to back out of
some bad decisions," says Good, who grew up in New Orleans. "I hope that we
don’t let this once-in-history opportunity slip through our fingers in the rush
to rebuild the city." Another ambitious plan is to shoring up the lowest land with a slurry of sediment ______