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How to fight a rising sea

What the Netherlands has done – and is urgently planning to do – in the face of climate-driven sea-level rise holds important lessons for the rest of the world.

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"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).

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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."

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