How to fight a rising sea

Zuidplaspolder is a case in point. As the lowest real estate in one of the Netherlands' most vulnerable provinces, it has become a test bed for factoring water and climate change into zoning and development plans. In the next 20 years, 15,000 to 30,000 new housing units will be built here. Anticipating this growth, in 2004, officials from provincial and local governments joined with nongovernmental organizations to develop a master plan for the polder. (A polder is a large tract of land containing farms and villages encircled by dikes. The dikes offer flood protection, but they also turn the polders into enormous bathtubs with bottoms that slowly, inexorably sink.)

The new homes that rise in the polder may look nothing like those in the villages the Dutch are used to, Mr. Bloemen says. To deal with floods, homes on this higher ground could be designed to float in place or built on stilts. They may sport tall ground floors, with living space and utilities placed on higher floors. Entire villages might be built to float in place, linked by buoyant sidewalks and roads.

In addition, he adds, officials may ask developers to use a technique that dates back centuries: building houses, even whole villages, on mounds. That low-tech approach is appearing in other parts of the world, too. Oxfam International is working with villages in Bangladesh to build individual homes and even small villages on flood-resistant mounds.

In the Netherlands, river floods are a top item on the climate-change adaptation must-fix list. To be sure, the country has tried to be forward-looking in tackling flood control and sea-level rise, notes Hans Balvoort, with the Netherlands' Ministry of Public Works, Transport, and Water Management. It typically uses a 50-year planning horizon. But a wake-up call came in the 1990s, "when, for the first time, rainfall was so heavy and intense that our pumping systems could not cope," he says.

Powerful pumps long ago replaced the signature windmills as the way to keep the polders from flooding. "On such a large scale," he says, the inability of pumps to keep pace with rainfall was "something we had not experienced before."

Moreover, for two winters during that decade, flooded rivers rose so high that officials evacuated some 250,000 people out of concern that levees might not hold. Instead of building large numbers of new levees, he continues, scientists, engineers, and officials looked for other ways to store flood waters over the short term to reduce the risk.

The Biesbosch project, with its dike removal, or "depoldering," is one approach. The government also is working on a range of other strategies to give flooded rivers more room to flow. They might spread dikes farther apart, excavate land between river and dikes (to capture overflow), deepen central river channels, remove jettylike groins that now force most of the flow into the center of a river, remove other obstructions, and even add new channels to the flood plain or restore old ones.

Storm surge is biggest coastal worry

The government plans to spend €2.2 billion ($3.2 billion) to make these changes to its rivers. Meanwhile, along the coast, the big worry is not about any average increase in sea level, which scientists project to rise here between 35 and 85 cm (14 to 33 inches) by 2100. Instead, the biggest concern is the change in storm-surge patterns that will ride atop that rise, says Pier Vellinga, who heads the climate program at Wageningen University.

As if to highlight this point, last weekend Britain and the Netherlands closed their sea barriers in the face of a storm in the North Sea that sent a 13-foot surge bearing down on their coasts.

Planners in other countries often design for a once-every-hundred-years storm. While that approach can be useful, the challenge is that climate change may throw those projections out of whack. For example, some researchers say that in the US Northeast, midcentury coastal winter storms could lead to flood levels every three or four years – floods of a severity that used to occur only once every 100 years. Netherlands planners aim for a 10,000-year storm for the country's most vulnerable areas. And even that may be inadequate, Dr. Vellinga says.

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