Sea dikes
Sea dikes have been built for protecting vulnerable parts of the coast. Along 34 of the 353 kilometer North Sea coast of the Netherlands, a number of dikes have replaced dunes that were too weak or too narrow. Most of the sea dikes lie along the Wadden Sea and the delta waters. In addition to breakwaters, sea dikes are the 'rocks' along 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 on land. Sometimes, extra dikes were built behind sea dikes, to serve as backup. These dikes are called 'sleeping' dikes.
Subjects:
the first dikes
dike structure
dike underwater
dike above water
are dikes water-resistant?
height of dikes
dike cutoffs
dike breaches
coastal protection
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. 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.
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. The oldest sea dike is located in North Groningen, built around 1200 A.D.
The early dikes in Friesland were very low. One could easily see across them. They were mainly made from clay and had a slanting incline. The dikes were later raised and made wider. In the beginning of the 16th century, the dikes were enforced with rows of poles. Eelgrass or reeds were place between the double rows that were placed along the seaside of the dike and then covered with stones. A steep wall was created on the seaside which had 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 behind the dikes in order to prevent large disasters. The government wanted to protect the sea dikes at the beginning of the 19th century with stones from Norway, but there was a lot of resistance because they were so expensive. After the tidal flood of 1825 and the continuing damage from the great shipworm, dikes were built starting in 1863 with a sloping incline and stones. Basalt was applied at critical spots. This work was finished in 1888 and the dikes were raised for the last time after 1953 within the framework of the Delta Act.
Construction of a modern dike
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 at times.
Sinking works
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
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 water-resistant?
No dike is water resistent. Because the dike facing is porous on the seaside, water is always seeping from the sea to the landside through the dike. Therefore, the dike must be built in such a way that the water encounters a lot of resistance, whereby a so-called 'normal dropline' is formed. This hanging line indicates the groundwater level within a dike and must run from sea level down to the ditch level. A normal hanging line forms when well compact sand is used: sand that leaves little room for water between the sand grains.
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 of 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. Managers of nature areas along sea dikes are often pleased with a certain amount of salty seepage, because brackish water environments are created with its unusual plants and animals.
Height of dikes
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 superstorm which can occur once in every 10,000 years. In theory, such a dike has a chance of being flooded once every 10,000 times. This dike height is calculated from historcal data on maximum water levels.
Sea dikes in the Netherlands were raised twice in the course of the 20th century. All dikes were raised where necessary to a height of 4.30 meter above Normal Amsterdam Level (NAP) as part of the Zuiderzee Works. After the flood disaster in 1953, it was decided that all the sea-walls had to be raised to 7.65 meters above NAP. This is referred to as 'raising the dikes to delta height'. The lower half of the figure above shows the fortification and heightening on the inland side of the dike. In some cases, due to landscape or technical reasons, fortification took place on the seaside of the dike.
Sea dikes in the Netherlands
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 Hondsebosse Zeewering), by Den Helder and on the north point of Texel (the Bolwerk).
Since 1990, the construction of new North Sea dikes ended when the policy choice for sand nourishments became the most important form of coastal protection.
Sea dikes lie just about everywhere along the delta rivers and the Wadden Sea, protecting the inland-lying polders from extremely high water levels. The exceptions to this rule are the beach plains on all the Wadden Islands, the salt marshes on Vlieland, Terschelling, Ameland and Schiermonnikoog and the uninhabited islands.
Dike cutoffs
Old dikes have been disected in many places to allow traffic to drive through. This is called a dike cutoff. The hole 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.
Dike breaches
Dike breaches can happen during heavy storms that raise the water level and simultaneously cause hefty swell. 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 a large length of the weak dike must be replaced, then the new dike is often built on the seaside.
Safety of dikes
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. The standard is to withstand extreme situations that can occur on the average of once per 4000 years. A periodic survey by Rijkswaterstaat and the Dike Board Hollands Noorderkwartier in early 2006 showed that number of dikes are too low and unstable. The Afsluitdike from 1932 is deteriorating and needs urgent re-enforcement. The principle assumption is that this dike should be able to withstand a super storm which could happen once in every 10,000 years. Therefore, the Afsluitdike must be raised from 3.5 to 5.5 meters above NAP, which will cost between 100 and 400 million euros. The Hondsbossche and Pettemer seawalls are also weak chains. And 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. Where the millions of euros will come from to re-enforce the dike is still unclear.
The eight weak links in the North Sea dikes, including the Hondsbossche and Pettemer seawall, will be tackled between 2007 and 2010. Ninety-six million euros have been made available for the task.
Only 18% of the dikes in Friesland and Groningen are in good shape, according to the Water Board Wetterskip Fryslan. Calculations have shown in theory that a large breach in the Wadden Sea dike could kill 1000 people and created 100 billion euros in damage. Ninety million euros is needed to fix the dikes.
In order to increase the safety of the Afsluitdike, eight plans were presented to the Ministry for Waterworks in July 2008. One of them includes making salt marshes on the wadden side, which would also provide extra nature and recreational options. Several of the plans will be chosen for further development and the Parliament hopes to make a final choice by December 2008.
Hondsbossche Seawall: 'the weakest chain'
A weak spot is the Hondsbossche Seawall by Petten. 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 for coastal defence scheme. 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.
A steel dam wall was hammered into the crown of the seawall in 2005 as a temporary solution; this was also done by the Pettemer seawall. The grass on the top of the dike was also replaced with basalt-shaped cement blocks in a checker board pattern.
Along the Marsdiep
The strong current in the Marsdiep scour away the stones at the foot of the dike near Den Helder. The dike foot must be reinforced by dumping extra natural stones. Such an operation has been assessed at 1.45 million euros.
Dikes in the future
A trial dike was 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 able to guard 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 and one can quickly take measures to prevent a breach and flooding. If the experiment is successful, other dikes will also be provided with sensors.
Marsh buffers
The managing boards are considering constructing marsh buffers in Zeeland. A wide marsh which lies in front of the main dike can grow as sea-level rises due to subsidence. The province of Zeeland is interested in this option as well as other alternative methods of flood control.
Wave break simulator
An alternative to dike improvement is reinforcing it on the inland side. As part of an European Project, experiments were performed with a wave break simulator in Delfzijl in 2007. The apparatus can dump 14,000 liters of water at one time over the inside of the dike. In this way, different kinds of materials that can enforce the dike can be tested. The experiment in Delfzijl showed that the present inland side of the dikes are 50 times more sturdy than first believed. Experiments will also be performed in other parts of the country.
Managing the sea dikes in the Netherlands
According to the Act for flood defence, managing the dikes in the Netherlands rests by the Water Board Districts. For example, the Hoogheemraadschap for the Uitwaterende Sluizen in Hollands Noorderkwartier ('Water Board for the Draining Locks in Holland's Northern Quarter') manages the wadden dikes. A few sea dikes along the coast and the Afsluitdike form an exception to this rule. For various reasons, these dikes come under Rijkswaterstaat.
Weblinks
Dutch site on coastal protection
http://www.zeeweringen.nl.
Source: de Vleet, Ecomare
