Saturday, June 8, 2013

To Control Floods, The Dutch Turn to Nature for Inspiration

21 FEB 2013: REPORT

To Control Floods, The Dutch Turn to Nature for Inspiration

The Netherlands’ system of dikes and sea gates has long been the best in the world. But as the country confronts the challenges of climate change, it is increasingly relying on techniques that mimic natural systems and harness nature’s power to hold back the sea.

by Cheryl Katz

On a freezing winter day along the south-central coast of Holland, two beachcombers, hunched against the wind, stroll along a crescent of sand extending more than half a mile into the North Sea. Nearby, a snowkiter skims over the 28 million-cubic-yard heap of dredged sediment spreading along the shore. If all goes as planned, the mound will eventually disappear, rearranged by ocean currents into a 12-mile-long buffer protecting the coastline for the next two decades.

This is the Sand Engine, one of the latest innovations from Dutch masters of flood control technology and designed, as the national water boardRijkswaterstaat says, so that “nature will take the sand to the right place for us.” After having constructed the country’s vaunted system of sea gates and dikes, Dutch planners and engineers are now augmenting it with new technology enlisting nature to keep the water at bay.

“Normally, there is a lot of erosion here,” says hydraulic engineer Mathijs van Ledden, sweeping an arm toward the snow-covered spit snaking around an elongated lagoon. Van Ledden is a flood risk reduction specialist with Royal HaskoningDHV, a Dutch engineering consultancy involved in creating the Sand Engine, currently 2.2 miles wide. “This big reservoir of sand should re-nourish the rest of the coast in time,” he says, gesturing toward the skyline of The Hague, several miles away.

Comment by Anumakonda Jagadeesh

Excellent post.

Flood control is an important issue for the Netherlands, as about two thirds of its area is vulnerable to flooding, while the country is among the most densely populated on Earth. Natural sand dunes and man-made dikes, dams and floodgates provide defense against storm surges from the sea. River dikes prevent flooding from water flowing into the country by the major rivers Rhine and Meuse, while a complicated system of drainage ditches, canals and pumping stations (historically: windmills) keep the low lying parts dry for habitation and agriculture. Water control boards are the independent local government bodies responsible for maintaining this system.

In modern times, flood disasters coupled with technological developments have led to large construction works to reduce the influence of the sea and prevent future floods.

Historically Dikes played a crucial role in flood control in Netherlands.

The first dikes were low embankments of only a metre or so in height surrounding fields to protect the crops against occasional flooding. Around the 9th century the sea was on the advance again and many terps had to be raised to keep them safe. Many single terps had by this time grown together as villages. These were now connected by the first dikes.

After 1000 AD the population grew which meant there was a greater demand of arable land but also that there was a greater workforce available and dike construction was taken up more seriously. Major contributors in later dike building were monasteries. As the largest landowners they had the organization, resources and manpower to undertake these large construction works. By 1250 most dikes had been connected into a continuous sea defense.

The next step was to move the dikes ever more seawards. Every cycle of high and low tide left a small layer of sediment. Over the years these built up to such a height that it was rarely flooded. It was then considered safe to build a new dike around this area. The old dike was often kept as a secondary defense, called sleeper dike.

A dike couldn't always be moved seawards. Especially in the southwest river delta it was often the case that the primary sea dike was undermined by a tidal channel. A secondary dike was then built, called inlaagdijk. With an inland dike, when the seaward dike collapsed the secondary inland dike becomes the primary. Although the redundancy provides security, the land from the first to second dike is lost- over the years the loss can become significant.

Taking land from the cycle of flooding by putting a dike around it prevents it from being raised by silt left behind after a flooding. At the same time the drained soil consolidates and peat decomposes leading to land subsidence. In this way the difference between the water level on one side and land level on the other side of the dike grew. While floods became more rare, if the dike did overflow or was breached the destruction was much larger.

The construction method of dikes has changed over the centuries. Popular in the Middle Ages were 'wierdijken', earth dikes with a protective layer of seaweed. An earth embankment was cut vertically on the sea facing side. Seaweed was then stacked against this edge, held into place with poles. Compression and rotting processes resulted in a solid residue that proved very effective against wave action and needed only very little maintenance. In places where seaweed was unavailable other materials such as reeds or wicker mats were used.

Another system used much and for a long time was that of a vertical screen of timbers backed by an earth bank. Technically these vertical constructions were less successful as vibration from crashing waves and washing out of the dike foundations weakened the dike.

Much damage was done to these wood constructions with the arrival of the shipworm (Teredo navalis), a bivalve thought to have been brought to the Netherlands by VOC trading ships, that ate its way through Dutch sea defenses around 1730. The change was made from wood to using stone for reinforcement. This was a great financial setback as there is no natural occurring rock in the Netherlands and it all had to be imported from abroad.

Current dikes are made with a core of sand, covered by a thick layer of clay to provide waterproofing and resistance against erosion. Dikes without a foreland have a layer of crushed rock below the waterline to slow wave action. Up to the high waterline the dike is often covered with carefully laid basalt stones or a layer of tarmac. The remainder is covered by grass and maintained by grazing sheep. Sheep keep the grass dense and compact the soil, in contrast to cattle.

The sand engine is an experiment in the management of dynamic coastline. It is run off South Holland. A sandbar-shaped peninsula was created by man; the surface is about 1 km². It is expected that this sand is then moved over the years by the action of waves, wind and currents along the coast. For the first time in 2011 at the request of the Hoogheemraadschap van Delfland as part of the coastal management and the maintenance of the coastline by the highways and the province of South Holland, a peninsula was created between Ter Heijde and Kijkduin, where natural beaches and dunes are relatively narrow.

Marcel Stive may be considered as the father of this project. Another such project could provide a solution to the weak coast between Camperduin and Petten, which is called the Hondsbossche seawall. The experience of this first sand engine is interesting for future projects. The construction of the sand engine at Ter Heijde cost 70 million euros and is the first in the world of its kind. Joop Atsma, State Secretary for Infrastructure and Environment, presented the project in November 2011 and his purpose was to convince that the technique of sand engine could be useful on more locations along the Dutch coast.

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