It was on May 28, 1932 that the last load of boulder clay was dumped onto the 30-kilometer (18.6 mile) dam, which was built to keep out the North Sea and create an inland lake and polders, land reclaimed from the sea.

Crown Prince Willem-Alexander will lead anniversary commemorations for the dam, formally known as the Afsluitdijk, on Thursday.

The celebrations come as climate change forces the Dutch to undertake new work to improve the dam which ensures vital protection for a country where 26 percent of the land is below sea level.

There had been plans since the 18th century to close off the Zuiderzee, a salt water inlet of the North Sea, said Hans Vos, a regional official of the department of waterways and public works.

Vos said that the Dutch had to wait for Cornelis Lely, a young civil engineer, "to come up with such an incredibly ambitious but feasible plan" for the dam.

"After dramatic floods in 1916, Lely managed to convince the parliament: 'never again'."

And between 1927 and 1931 up to 5,000 men worked on the Afsluitdijk, which is an average of 85 meters (279 feet) wide and incorporates two sluices and a lock for ships to pass through.

Cornelis Lely never saw the dam, however, as he died before it was finished.

"The first consequence of the dam was the protection of the country but the Afsluitdijk also allowed polders to be built, link the northwestern Netherlands with the Friesland province and shorten the route to Berlin and Scandinavia," explained Fred Delpeut, director of the regional waterways body.

"Three years after the dam was closed the Zuiderzee, since renamed the IJsselmeer, became fresh water instead of salt water which means it is an important drinking water and irrigation reservoir for new polder towns like Lelystad or Almere," Delpeut said.

In the last few years another role has become increasingly important.

"The sluices in the dam allow us to regulate the water level in the IJsselmeer as climate change affects the fluctuations: it rains a lot more in winter and the summers here are much drier that 75 years ago," according to Delpeut.

To adapt to the climate change that Dutch engineers include in their long time predictions a third series of sluices to be added between 2008 and 2013. A project estimated to cost some 25 million euros would strengthen the defences so that the Netherlands could fend off the super storm that could statistically hit once every 10,000 years.

The sluices allow fresh water to be dumped into the North Sea at low tide when water levels in the IJsselmeer are rising.

"The politicians take this very seriously. They are not very hard to convince of the necessity of the changes," Delpeut said.

The Dutch already know that even these adjustments will not be enough.

"Already now we could be faced with a theoretically dangerous combination: a storm on the North Sea when the IJsselmeer water level is high," said Eric Regeling, a senior water management official.

"It is not yet urgent but we will need to make decisions in the next five to 10 years," he added.

One possible option is raising the dam, which could be 10 metres (33 feet) higher without compromising its stability. Or there could be a new smaller dam to protect the Afluitdijk itself.

Water protection in the Netherlands is a never ending job. In front of the monument for the men who built the Afsluitdijk is a bronze plaque which bears the inscription: "A people that live, build for their future".

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