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Water en land

Mens en Milieu

Coastal protection   Sea dikes   
Sea dike Koehool, Friesland, Ecomare

Sea dikes

Sea dikes have been built for protecting vulnerable parts of the coast. 34 of the 353-kilometer long Dutch North Sea coast are protected by dikes. All of the polders bordering the Wadden Sea and the delta waters are also protected by dikes. Along with breakwaters, sea dikes are the 'rocks' on the further sandy coastline and therefore have their very own assortment of flora and fauna. Due to progressing land reclamation, some old sea dikes now lie inland. Sometimes, extra dikes were built behind sea dikes, to serve as backup. These dikes are called 'sleeping' dikes.

On Texel

Land reclamation on Texel 1250-1900, Ecomare

Frisian monks began building dikes on Texel in the 13th century. Some of the old dikes are now found on farmlands since marshes located on the seaside of the dike have since been reclaimed. An example is the Waal and Burger dike. In 1500, this sea wall was constructed for the polders located east of De Koog. It's difficult to imagine when standing on the dike, with all the grasslands now surrounding the dike.

  • The first dikes

    The history of dike construction in the terpen and wierden countryside goes way back in time. The oldest known sea dike in the Netherlands is around 2000 years old and lies in the Frisian township of Peins. It was built from sods of grass piled on top of each other. The oldest sea dike is located in North Groningen, built around 1200 A.D.
    The early dikes were very low; you could see. One could easily look over them. They were mainly made from clay and had a slanting incline. The dikes were later raised and made wider.

  • Eelgrass protects the Netherlands

    During the Middle Ages, many dikes were made from eelgrass pressed together. These dikes probably protected up to several farms and farmland. In the 10th century, larger dikes were constructed to protect several villages. Even later, ring dikes were built to protect entire regions, such as the Frisian Oostergo and Westergo around 1100. Even for these long dikes, eelgrass was often used, re-enforced when possible with sand and clay.

    Much more land for farming became available when the dikes were built, which was a great improvement for the farmers. The construction of dikes was less favorable for trade: villages which had an opening connection with the Wadden Sea were suddenly closed off from open water. Consequently, a network of canals and waterways were laid to allow for shipping. Small cities and 'zijl' villages (where the sluices were situated) developed along the coastline. 

    In the early 16th century, the dikes were enforced with rows of poles. Double rows were placed along the seaside of the dike. Eelgrass or reeds were laid in between and then covered with stones. This created a steep wall on the seaward side, meant to break or hold back the waves. Breakwaters were also built perpendicular to the dike. However the great shipworm, imported from Asia around 1739, began its destructive work. Sleeping dikes were built landward of the dikes in order to prevent large disasters.

  • Stone Age

    In the early 19th century, the government wanted to protect the sea dikes with Drentse or Norwegian stones, but there was a lot of resistance because they were so expensive. Beginning in 1863, after the tidal flood of 1825 and the continuing damage from the great shipworm, dikes were built with a sloping incline and stones. Basalt was applied at critical spots. One thought by 1888 that this work was finished. 

    However, in 1916, the sea attacked and caused an enormous amount of damage, particularly along the Zuiderzee coasts. The dikes were apparently not high or strong enough. A plan evolved to close off a major part of the Zuiderzee from the Wadden Sea. The remaining sea dikes in the wadden and delta regions would be raised to 'Zuiderzee height' (4.30 meters above Amsterdam Ordnance Datum, NAP). Once agian, by ai40 when this work was accomplished, one thought that they were finished.

    The rest is recent history. In 1953, many dikes in Zeeland and several in the wadden region were breached during a heavy storm. The disaster took many lives. The Delta Plan was agreed upon not long afterwards: all sea channels in the delta region, with the exception of the Westerschelde, would be closed off from the sea. The remaining sea-defending dikes, including those in the wadden region, would be raised to Delta Height (7.65 above NAP). And again, having completed the works, one hopes this will be sufficient.

  • Constructing of a modern dike
    Construction of a modern dike, Ecomare

    A sea dike is built up from a dike body and a facing, which must protect the dike body from erosion caused by waves, currents and breaking ice. Because new dikes often settle, the material must also be reasonably flexible. In earlier days, dikes were made from eelgrass, osier branches, wooden poles, natural stone and sometimes even bricks. Nowadays, cement blocks, cement poles and asphalt are often used.

  • Dike under water

    The scheme above shows a cross-section of a dike. The underwater part of the dike is not visible but is very important. The underwater bank (left in the illustration) must protect the underside of the dike from eroding away by currents and waves. This protective layer can be made from synthetic cloth with stones thrown on top. The so-called underwater submerged sloop follows. This part of the dike consists of synthetic cloth with a layer of large stones. Lots of animals live on the stones, such as crabs, anemones, sea squirts and shellfish. Seaweed and algae also grow on the stones. The underwater quay supports the dike body and is often built from sand or gravel. Steel slags (waste product from the steel industry) are also used.

  • Sinking works
    Willow matting, Ecomare

    Sinking works are often applied in combination with dikes, due to the positive effect the sand fixation has on the dikes. Normally speaking, the sea currents scour away the sand in front of a dike. When sinking works are laid, this scouring occurs further seaward, fortifying the stability of these dikes and making them less sensitive to erosion. Basalt blocks or other heavy material are thrown on top to keep these sinking works in place.

  • The dike above water
    'Toe construction' of the Hondsbossche Seawall, Ecomare

    Rows of poles are often seen at the foot of the dikes. These poles, which can be two meters long, form the 'toe construction' and make sure that the other layers do not shift downward. A layer of basalt or cement blocks are then placed on a layer of coarse grainy material. There is lots of room between the stones so that water can easily sink down. The water is quickly led back to sea by the layer of grainy material. This construction absorbs the waves and protects the dike body.

    The asphalt layer begins above the filter layer. This is a water-resistant layer, also referred to as 'closed facing', which protects the dike body from erosion and sagging. The crown and landside of the dike consists of a clay layer overgrown with grass. This clay layer protects the dike from water and holds down the underlying layers of sand. The layer of grass is essential: the roots give it a sturdy structure, so that the clay cannot wash away. The grass also keeps the clay from drying out.

  • Are dikes waterproof?
    Dropline: normal and too high, Ecomare

    No dike is waterproof. Because the dike facing is porous on the seaside, seawater is always seeping to the landside through the dike. The water level under a dike runs from sea level to ditch level.

    Seepage occurs when the dike allows too much water through. Salty groundwater comes to the surface on the landside of the dike. Seepage is undesirable because the clay layer in the dike can be damaged by the pressure of the water, thereby weakening the dike.

    Farmers are not happy with salty seepage because the salt hinders the growth of the crops. However, managers of nature areas along sea dikes are often pleased with a certain amount of salty seepage, because a brackish water environment is created with its unusual plants and animals.

  • Height of dikes
    Strengthening the dike , Ecomare

    How does one decide how high a dike should be? The dikes are developed in such a way that their height is equal to the maximum water level during a super storm which can occur once every 10,000 years. In theory, such a dike has a chance of flooding once every 10,000 years. This dike height is calculated from historical data of maximum water levels. All sea-defensive dikes in the Netherlands were raised in the second half of the 20th century to this 'delta height', 7.65 meters above NAP.

  • Sea dikes in the Netherlands
    Sea dike with dandelions, Ecomare

    North Sea dikes can be found at the tips of the Zeeuws-Vlaanderen and the delta islands, slightly north of the Hook of Holland, by Petten (the Hondsebossche Zeewering), by Den Helder and on the north point of Texel (the Bolwerk). Since 1990, the construction of new North Sea dikes ended when policy chose for sand nourishments as the most important form of coastal protection.

  • Dike cutoffs
    Cutoff in the old sea dike, Wim Bouwland,

    Old dikes have been dissected in many places to allow traffic to drive through. This is called a dike cutoff. The opening in the dike must be able to close, because these old dikes still fulfill a function for flood control. Therefore, a stop-log house or dike platform is always found next to such a cutoff, in which stop logs are stored to fill the hole in case of emergency. In the Dollard polders, you find dike cutoffs that use doors instead of logs to close the dike.

  • Dike breaches

    Dike breaches can happen during heavy storms that raise the water level and simultaneously cause hefty swells. If a hole forms in the dike, a current channel directly behind the dike is created. When this hole is more or less basin-shaped, it is called a 'wiel' in Dutch. In order to halt the breach, the hole must be filled with bags of sand, but this will only work for a short period of time. A dam or a temporary dike around the wiel can help if the sand bags do not work. In addition, a caisson or vessel full with sand or stone can be sunk into the hole temporarily. After the make-shift recovery of the hole, the temporary dike around the wiel can be rebuilt into a true dike. If the length of the weakened dike that needs to be replaced is long, then the new dike is often built on the seaside.

  • Safety of dikes
    Work activities on the Wadden Sea dike, Friesland, Ecomare

    The Act on Water Retaining Structures was enforced in 1995. Dike managers are required to test the dikes every five years to determine whether or not they still satisfy the safety standard and eventually report to the Minister of Public Works.
    A periodic survey by Rijkswaterstaat and the Dike Board Hollands Noorderkwartier in early 2006 showed that number of dikes is too low and unstable. The Afsluitdike (Closure Causeway) built in 1932 is deteriorating and needs urgent re-enforcement. The Hondsbossche and Pettemer seawalls are also weak chains. Furthermore, the Wadden Sea dike on Texel and a number of other dikes on the islands and in Friesland and North-Holland are too unstable. A part of the Frisian Wadden Sea dike has a six-kilometer long dike that is in very bad shape. 

  • Hondsbossche Seawall: 'the weakest chain'
    Hondsbossche Seawall, Ecomare

    The Hondsbossche Seawall by Petten is a weak spot. This dike was found unsafe when TNO tested its newest calculation models on wave energy during a super storm. Therefore, re-enforcement of the dike has been given high priority in the coastal defense plan.

    Raising the seawall is not the best option. The dike would have to be raised 1.5 to 3.5 meters. A study from the project bureau Kustvisie 2050 showed that a series of breakwaters in combination with large sand nourishments would cost less and be just as effective. In the meantime, breakwaters have been removed from the plans. In 2012, it was announced that the seawall would be re-enforced by dumping 20 million cubic meters of sand seaward of the dike.

  • Along the Marsdiep

    The strong current in the Marsdiep, the channel between Den Helder and Texel, scours away the stones at the foot of the dike near Den Helder. The dike foot must be re-enforced by dumping extra boulders. Such an operation has been assessed at 1.45 million euros.

  • Dikes in the future

    An experimental dike has been constructed in Bellingwolde (Groningen), which is filled with the most modern sensor technology. The condition of the dike can be checked electronically. The water boards must be capable of guarding the dikes 24 hours a day in the future. If the condition of the dike worsens, then such an apparatus will warn one ahead of time so that measures can be quickly taken to prevent a breach and flooding. If the experiment is successful, other dikes will also be provided with sensors.

  • Marsh buffers
    Marsh buffer,

    The managing boards are considering constructing marsh buffers in Zeeland. Sediment precipitation can widen the marsh stretching in front of the main dike as sea-level rises. The province of Zeeland is interested in this option as well as other alternative methods for flood control.

  • Managing the sea dikes in the Netherlands

    According to the Act for flood defense, managing the dikes in the Netherlands rests by the Water Board Districts. For example, the Hoogheemraadschap Hollands Noorderkwartier ('Water Board Holland's Northern Quarter') manages the wadden dikes on Texel. A few sea dikes along the coast and the Afsluitdike form an exception to this rule. For various reasons, Rijkswaterstaat is responsible for these dikes.