Wednesday, December 1, 2010
The North Sea Protection Works
For many, the image out of the Dutch fight against the North Sea rests in the figure of a young boy valiantly saving his town by using a finger to plug a hole in the dike. But this familiar hero is a fictional one, a creation of American author Mary Mapes Dodge in her book, Hans Brinker. In reality, heroism falls on all the Dutch, who for more than a millenium have been wresting precious agricultural lands from the sea and fighting to hold on to them. Their greatest achievement-a colossal fun in the dike-if the vast and one dress project known as
that Netherlands North Sea protection works.
Because much of the Netherlands lies below sea level, normal tides would daily inundate about half the country if previous generations of industrious Dutch had not raised dikes and dams. Severe storms often cause tidal waters to crash into the dikes and inundate rivers and estuaries. Although all of the coastal areas are threatened, two particularly vulnerable ones are the large tidal inlet formerly known as the Zuider Zee and the delta created by the Rhine and Meuse rivers in the southwestern corner of the country.
Dutch engineers purse propose that the Zuider Zee be dammed and drained in the 19th century, but the government was reluctant to tackle such an immense project. Then, in 1916, a furious storm hit the northern provinces. The difficulties of wartime agricultural production were compounded, and the way was paved for the damming of the Zuider Zee.
The dam enclosing the Zuider Zee was built in two sections using traditional materials. Beginning in 1923, workers laid boulder clay in parallel layers and filled the space in between with sand, stones, and handmade, mattresses fashioned from brushwood. To curtail erosion, larger mattresses ballasted with chains and stones were sunk in the estuaries channels. Dredges, cranes, tugboats, and barges were engaged in the erection of the main dam, 300 feet wide at sea level and 25 feet high at the level of its causeway. As the tide turned on the final day of construction, fill tumbled into the dam's last gap,
transforming the inlet into a freshwater lake, renamed the Ijsselmeer. The finished dam contains sluices for draining excess water and locks for maintaining shipping.
After the damming came the draining. In all, more than a half million acres of polders, or reclaimed farmland, emerged from the bottom of the former Zuider Zee. Young Dutch farmers clamoring for the right to settle the new polder lands, because farms on new, unobstructed land were far more suitable for modern, mechanized farming methods than traditional farms in older areas.
In 1953, the "storm of the century" howled across the North Sea and into the Netherlands, testing the strength of the Zuider Zee enclosure. It held, with damage to the causeway heavy in places. The country's unprotected southwestern provinces felt the full brunt of the storm, with water surging over seawalls and up the delta's wide waterways. More than 1,800 people lost their lives, and livestock numbering in the hundreds of thousands perished. The country then realized that the long-intended plan to safeguard the southwestern delta, the Delta Plan or Delta Project, must be mobilized.
The plan would undergo a many incarnations. The last one involved a damming four estuaries in the middle of the delta while leaving open channels to Rotterdam in the north and Antwerp, Belgium, in the south. A two-mile-long surge barrier in the Oosterschelde estuaries was the most complex and sophisticated piece of the project.
Originally, the Oosterschelde was to be a closed barrier. But lobbying by fishermen and conservationists resulted in the switch to a movable barrier. To facilitate construction, engineers fashioned islands on three sandbars in the estuaries and constructed work harbors, material yards, and work sites there. A dam connected two of the islands, effectively creating three channels in the estuaries, each to receive a section of the surge barrier.
The movable barrier consists of 65 concrete piers weighing 18,000 tons apiece. The piers support 300- to 500-ton steel gates and their hydraulic machinery, as well as a roadway above and load-bearing beams below. Constructed on the work islands, the piers and their mechanisms had to be lifted into precise positions in the estuary. But the type of equipment needed for such gargantuan and specialized tasks did not exist anywhere in the world; it had to be invented.
The Oosterschelde barrier also honored traditional methods. As part of the measures taken to stabilize the sea floor, mattresses were laid under each pier to prevent erosion. They were not the hand-built weaving of trees and brush used to close the Zuider Zee, however. Instead, they were high-tech sandwiches of sand and gravel between space-age fabric covers. The Oosterschelde project finished in 1986. Since then, the Dutch have taken additional measures, including the completion in 1997 of the barrier that protects the port of Rotterdam.
"In terms of magnitude," an American trade journal wrote, the North Sea project "approaches of the Great Wall of China. In terms of complexity and technical sophistication, it approaches the lunar shot. It is unique, expensive, and quite unlike any other civil engineering project to be found on this planet."
Source: The Wonders of the World, National Geographic Society