What should be done for New Orleans and Venice?

A tale of two cities and rising sea levels

Because Venice is also threatened by the sea, comparisons between the situation there and the flooding of New Orleans are inevitable. But how much are the two cities alike and what lessons can be learned from New Orleans that informs the situation in Venice? I am a recently retired professor of coastal ecology at Louisiana State University and I have worked for many years on both the Mississippi delta and the wetlands of Venice Lagoon. The motivation for these studies was to determine the implications of sea level rise on the wetlands.

The catastrophe for New Orleans was not an act of God. Human activity for over a 100 years created the conditions that led to this inevitable tragedy. The city is built mostly in reclaimed wetlands that shrank and subsided when drained, to the extent that most of the city is below sea level, parts by as much as five metres.

Human-induced subsidence is an important factor that has contributed to both lowering of Venice and loss of wetlands in the lagoon and the Po delta. The dramatic deterioration of 25% of the wetlands of the Mississippi delta removed a very important buffer against storms. This deterioration is due primarily to human activity. The river that built and nourished the delta with freshwater and silts over thousands of years now has dikes to its mouth and without river input most wetlands sink below the water and die.

Similarly, in Venice, the Piave and Brenta rivers now bypass the lagoon and their sediments no longer nourish wetlands; the lagoon is losing more sediment to the Adriatic than it is gaining.

Many scientific studies clearly showed that a storm like Katrina would lead to catastrophic flooding and devastation in New Orleans and the media had made this known. This is similar to Venice where there is widespread understanding of the problems of acqua alta for the city. But in the case of both cities, the authorities have been slow to act. Venice is essentially as unprotected from a storm surge as it was at the time of the great flood of 1966.

So what can be learned from New Orleans that is valuable for Venice? First and foremost, the kind of catastrophic flooding and destruction resulting from Katrina is not possible in Venice; no single storm there could have winds in excess of 250 kph and a storm surge of six metres. However, the insidious slow rise in water levels due to both sinking of the land and rising sea levels is a problem for both cities.

There are two basic solutions for coastal cities to the problem of rising sea level. One is to encircle the city (or the country in the case of the Netherlands) with dikes. This is what has been done in New Orleans. The other is to build a movable barrier that allows tidal ebb and flow but stops very high storm surges, as with the Thames Barrier, and the proposed solution for Venice, the MOSE. The problem with a movable barrier is that it relatively soon becomes unworkable if the sea level is rising, and rising it is. The Intergovernmental Panel on Climate Change’s (IPCC) best estimate for sea level rise is nearly one half metre in this century. Thus, the MOSE will have to be closed more and more often. And once the barrier is in place, there will be pressure to use it more often to stop flooding from more moderate floods.

There is also the additional problem that operation of the MOSE will lead to more rapid deterioration of the wetlands of the lagoon. These marshes can survive only if the soil surface grows upward at the same rate that sea level is rising. We found in our studies that most new sediment is deposited on the marsh surface during scirocco winds because these lead to re-suspension of sediments on the lagoon bottom, and high water levels flood the marshes and lead to deposition of these sediments on the marsh surface. The MOSE will eliminate these strong pulses of sediments to the marshes. Thus protection of the city from flooding will negatively affect the marshes of the lagoon.

What should be done now, for both New Orleans and Venice? This is not an easy question to answer. In New Orleans, the height of dikes will be increased to protect against higher storms surges, but rainfall associated with a major hurricane can approach a half metre in 24 hours, so protecting the city must involve raising most of the structures in the city above flood level. We must also rebuild the delta wetlands that are so important in protecting the city.

In Venice, the MOSE will protect the city for several decades but will almost certainly lead to more rapid deterioration of the lagoon marshes. Controlled diversion of waters of the Brenta and Piave rivers back into the lagoon is a way that marshes can be sustained. This is currently being done with success in the Mississippi delta.

It seems almost impossible to raise the level of the city of Venice. In the long term, it may be that only completely encircling the city, and perhaps the lagoon, will protect the city. But this will change the nature of both city and lagoon. The lesson of the Netherlands is instructive here, both positively and negatively.

Finally, the coming energy crisis will greatly affect the way both New Orleans and Venice will function in the future. Whatever is done in both Venice and New Orleans must be done in a way that is sustainable and energy efficient.

Venice is over 1,000 years old, while New Orleans is slightly less than 300. Can these cities last another 500 years? The answer is clearly no if we do things as in the past. But if we look to the future and take into consideration global climate change, energy availability, and integration with the surrounding natural environment, the answer is perhaps yes.

The writer is the Distinguished Professor Emeritus of Louisiana State University.

Originally appeared in The Art Newspaper as 'A tale of two cities and rising sea levels'