How to fight a rising sea

The Dutch enjoy a hard-earned reputation for building river dikes and sea barriers. Over centuries, they have transformed a flood-prone river delta into a wealthy nation roughly twice the size of New Jersey.

If scientific projections for global warming are right, however, that success will be sorely tested. Globally, sea levels may rise up to a foot during the early part of this century, and up to nearly three feet by century's end. This would bring higher tidal surges from the more-intense coastal storms that scientists also project, along with the risk of more frequent and more severe river floods from intense rainfall inland.

Nowhere does this aquatic vise squeeze more tightly than on the world's densely populated river deltas.

So why is one of the most famous deltas – the Netherlands – breaching some river dikes and digging up some of the rare land in this part of the country that rises (barely) above sea level?

In the Biesbosch, a small inland delta near the city of Dordrecht, ecologist Alphons van Winden looks out his car window at a lone excavator filling a dump truck with soil. He considers the question and laughs. "We do have a hard time explaining this to foreigners," he says.

The work here represent a keystone in the country's ­climate-adaptation plans, Mr. van Winden says. Indeed, nowhere are adaptation planning efforts to address rising sea levels and flooding more advanced than in the Netherlands.

To be sure, the country's economic wealth and long experience dealing with threats from seas and rivers give it an advantage over other low countries that face rising waters, such as Bangladesh, Vietnam, and the tiny tropical island nation of Tuvalu in the South Pacific. But many of the approaches the Netherlands is taking can and are being slowly adopted even in countries far poorer, specialists say.

The excavation work here is one example of what van Winden calls "soft approaches" to flooding in this small nation where competing interests jostle for every square foot of land. By buying out the few farmers remaining in this region, breaching the dikes they built to protect their land, and digging additional water channels, the Dutch government aims to reduce peak flood flows at Dordrecht and other cities downstream. No longer will tightly constricted river and canal channels hold high water captive. Big floods will overspread the Biesbosch, reducing the threat of water spilling over the top of levees that guard densely populated cities to the west.

The Biesbosch may also be critical to the future of farming on the productive southwest coast. There, most of the area's fresh water sources are close to the coast – and vulnerable to salt-water contamination from a rising North Sea. This could make farming difficult, if not impossible. The Biesbosch, however, hosts three large reservoirs, each surrounded by a 20-foot-high dike. Fresh water piped from these reservoirs, some 50 miles inland, could keep coastal areas supplied.

1.4 billion live near seacoast

Globally, some 21 percent of the world's 6.6 billion people live within 20 miles of a seacoast – and nearly 40 percent within 60 miles, says Robert Nich­­olls, a professor of civil and environmental engineering at the University of Southampton in England.

Seacoast populations who face the greatest risk from floods, storms, and sea-level rise live on river deltas, says the UN-sponsored Inter­governmental Panel on Climate Change.

In the IPCC's latest set of reports on the impact of global warming, released earlier this year, scientists looked at data from 40 of the globe's river deltas, home to 300 million people. If current trends continue through 2050, flooding in the Nile, Mekong, and Ganges-Brahmaputra river deltas could each displace more than 1 million people. Up to a million more may be forced to head for higher ground in each of another nine deltas, including the Mississippi River delta. Up to 50,000 could be forced to relocate in each of 12 other deltas, including the Rhine River delta – an area known more widely as the Netherlands.

Besides global warming, scientists say the challenges these regions face have other causes as well. Levees, sea walls, drainage canals, dams, and other land-use patterns have taken a toll. Deltas tend to subside (sink) naturally, accentuating the rise in sea level. Past engineering projects can actually limit the ability of natural processes to replenish the land mass of deltas.

A patch of the Netherlands between Rotterdam and Gouda, called Zuid­plas­polder, highlights the issue in a way that New Orleans might recognize. The 19-square-mile area is bounded by dikes and the Gouwe River. Face the river, and the landscape looks like a typical river plain. But turn and face Zuid­plas­polder, and you see a steep decline dropping more than 20 feet. The huge dimple in the delta stretches as far as the eye can see.

It's the lowest spot in Europe, some 23 feet below sea level.

"And it's all subsidence," says Wille­mien Croes, a planner with the provincial government of South Holland. Over the centuries, residents dug up thick layers of peat to warm their homes in Gouda, Rotterdam, and Amsterdam, she says. Much of Zuidplaspolder then filled with water. Farmers pumped it dry, grew crops, and raised dairy herds on the rich clay and peat. When the soil settled, farmers ringed the area with dikes for protection.

The area's low elevation and the anticipated increased future risk of floods, combined with development pressures from Rotterdam and Gouda, have turned this area into one of the country's biggest adaptation challenges. But it's hardly alone: Some 60 percent of the country, accounting for 70 percent of its gross domestic product, lies below sea level.

These sinking lowlands are protected along the coast by sand dunes, dikes, and sea barriers that stretch across the mouths of estuaries. These natural and engineered defenses have protected millions from the North Sea since a devastating storm surge hit the country in 1953. But these defenses have come at an ecological cost. Unlike river deltas such as the Mississippi's, which grew as sediment washed downriver from deep in the North American interior, the Dutch delta was built by the sea. Currents swirling through the Strait of Dover since the end of the last ice age eroded the white cliffs and deposited the material along the Dutch coast.

That process has slowed substantially, says van Winden, who works for Stroming, an environmental consulting firm in Nijmegen. Although the delta drains three of Europe's major rivers – the Rhine, Meuse, and Scheldt – the rivers never carried enough sediment to build the delta, and don't carry enough silt to maintain it today. From that standpoint, he says, over the long term "we are living beyond our means."

Dutch humble in face of rising threat

Faced with the twin threats of increased river flooding from inland storms and higher ocean storm surges as the climate warms and sea levels rise, the country aims to meet these challenges with a variety of approaches, ranging from complex engineering to "natural." But it's doing so with increased humility, given the levee failures in New Orleans after hurricane Katrina in 2005.

"If you want a caricature of the Netherlands, it's: 'We have the dikes; we are 100 percent safe. So just go on with your life,' " says Pieter Bloemen, who runs the government's Adaptation Program for Spatial Planning and Climate. But these days, "even we proud Dutch, with climate change in the back of our heads, have to think about broken dikes. That's a big paradigm shift."

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.

"When you do an economic assessment of the damage," he says, "and what you can afford to [spend to] avoid that damage, a better safety level would be a recurrence of 1 in 100,000 years." One storm like that could cost the country up to a year's worth of gross domestic product – €500 billion ($730 billion).

In 1990, the government decided to maintain the country's existing coastline by replenishing its extensive phalanx of coastal dunes using enormous deposits of sand that lie far offshore – another geological gift delivered over millennia from the English Coast to the Netherlands.

Three years ago, the government added that it will strive not only to maintain the coastline at its current position, but also to maintain the shape of the current offshore slope to a depth of about 130 feet. Today, that means dredging and depositing nearly 16 million cubic yards of sand along the coast each year. So, as the sea level rises, the dunes will, too, says Joost Stronkhoorst, with the National Institute for Coastal and Marine Management at The Hague.

Offshore sand deposits are large enough to allow the Dutch to accommodate a rise in sea level up to 16 feet, he says. But the line of coastal dunes is not unbroken. The gaps are spanned by barriers that in some cases will require 20 feet added to their height given sea-level-rise scenarios out to 2100.

In some cases, that's not possible. The northern coastal town of Petten shows why. It's tucked hard against the back of a sea dike that traces its origins to the Middle Ages – and sits 14 feet below the level at which waves crash on the other side.

To build up a dike, you must expand its base, explains Roel Posthoorn, with the Dutch nature trust Natuurmonumenten, as he stands on the crest of the dike on a blustery fall afternoon. The presence of the village eliminates the chance to expand the dike's base inland. And churning North Sea currents already sweep away precious coastal sand from the seaward edge of the dike's base, preventing planners from trying to expand the dike seaward.

Possible solution: artificial reefs

Here, Mr. Posthoorn says, the long-term solutions may lie in building an offshore reef to reduce the height of the waves slamming into the dike. Or, as some are now beginning to suggest, perhaps the large deposits of sand offshore should also be used to build the country's coast westward by nearly a mile.

In the meantime, groups like Natuur­mon­­umenten are working to meet two of the country's adaptation goals by trying to prevent further development behind sea dikes like this one and converting the land to nature reserves. These "climate buffers" are another tool in the Netherlands' kit for coping with global warming.

Adaptation experts generally agree that scientists, engineers, and policymakers already know what needs to be done to adapt to global warming. For the most part, they say, it means doing what they already know how to do to reduce risks from natural hazards – it's just doing more of it and a better job of it.

As if to underscore the point, Henk Wolfort, a researcher at Alterra, an institute at Waganingen University that focuses on sustainable development, shows a set of maps illustrating the evolution of watery areas and polders in the country since the 14th century.

"Our problems are not so very different from the problems the people in the Middle Ages had," he says. Even back then, techniques like building on mounds or widening the space between river dikes to accommodate flooding were well understood. The lesson? In a high-tech age, some of the effective adaptation approaches may come from a decidedly low-tech time.

"I think the people in the Netherlands have forgotten about those old ideas because they have relied on technological solutions," he says. "Now they see that technical solutions don't provide 100 percent safety. So perhaps we should think about the old solutions again."