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HURRICANE Gustav mercifully struck New Orleans only a glancing blow this week, leaving the evacuated population with homes to return to. As New Scientist went to press, it looks as though some of the tens of thousands of people who left low-lying Louisiana, west of the city, may not have been so lucky. The plight of this poor area of the US exemplifies the problems facing coastal communities around the world as sea levels rise.
According to the US Army Corps of Engineers, which maintains the levees around and within New Orleans, the city’s defences are in much better shape now than they were three years ago when hurricane Katrina wrought havoc. That storm killed 1500 people, left tens of thousands homeless and did some $80 billion of damage.
The corps has added new floodgates, improved the design of the levees and is in the process of increasing their height. This week the levees held, but only just. By 2011, New Orleans should be impenetrable to evena “100-year storm” – one so unusually fierce that there is only a 1 per cent chance of it striking in any given year.
Yet even with that protection, the corps insists that evacuations should always be considered in future as an added safeguard. Is this a reasonable thing to ask the residents of New Orleans and nearby towns and cities – that once or twice a decade they must leave their homes and hope the wind, water and looters do not get in?
It is probably feasible to build a wall right across Louisiana to keep storm surges out, though the cost of construction and maintenance would be astronomical. Better then to try to encourage the natural defences in the area.
New Orleans and neighbouring towns used to be protected by a vast network of barrier islands and wetlands which took the power out of approaching storm surges. These have been steadily disappearing. Silt from the Mississippi river is now directed off the continental shelf and into the deeps of the Gulf of Mexico. The lack of silt, together with sea level rise and the creation of new channels, cut to assist the oil and gas industry, has wiped out at least 4800 square kilometres of wetlands since the 1930s.
These wetlands could be restored (though “hard” coastal defences would still be
needed in places). A plan to do this, costed in 2005, came out at $14 billion: not a small investment. But compare that with the bill for reconstructing New Orleans and the surrounding towns after Katrina and it starts to look like a bargain. There is a catch, though, the plan would mean redirecting the flow of the Mississippi, which would almost certainly restrict access for shipping into the busy port of New Orleans.
Without such action, the wetland barrier will continue to sink while sea level rises, leaving residents in the area more vulnerable by the year. Then there’s another potential
threat: research published this weeksupports the notion that the fiercest storms in the north Atlantic Ocean are growing fiercer (see page 20). This is a complex and controversial issue, and the latest findings do not prove
that coastal towns will be at greater risk in future; there is, for example, still a suggestion that these fierce storms are less likely to make landfall. Still, the long-term prospects for New Orleans in its present form do not look good.
There are radical ideas to transform the city inside the levees. Much of New Orleans is below sea level and, like the surrounding wetlands, is sinking. These areas could be flooded so they trap the silt, giving the land a chance to build back up. Buildings could sit on piles or floats so they have greater resistance to occasional flooding (Ecological Engineering, vol 26, p 317) . But the chances of such bold experiments being carried out in a poor city do not look high.
Yet without fresh ideas, the coast is going to move ever closer to people’s front doors. Levees will become unfeasibly high and Louisiana’s towns and cities will be inundated more frequently. Being in a hurricane-prone area makes these issues more urgent: questions are likely to be asked of parts of Florida before the week is out. But the problem of encroaching sea is not a localised one. What Louisiana and Florida are struggling with today, New York, Shanghai, Mumbai and London will have to face tomorrow. ●
“Is it reasonable to ask the residents of New Orleans to evacuate their homes once or twice a decade?”
Holding back the seaIf we don’t learn from New Orleans, we will fail to save other coastal cities
www.newscientist.com 6 September 2008 | NewScientist | 5
