Levee

The term dyke (from Middle Low German Dîk " pond " ( artificially applied water ), originally = Ausgestochenes ) denotes hydraulic engineering protection systems along coasts and rivers. These are buildings that are as embankment alongside a river or the sea shore and the low and weak in relief immediately adjacent thereto, partially designated as a march to protect the hinterland against flooding. In Austria, a dike is referred to rather than levee. In addition, dikes can be used for land reclamation and is then called diking of Watts. The incurred in the past by the dikes along the North Sea coastal areas are regional variations called polder, polder or Groden. In the times of advanced environmental awareness of the population and the stronger in terms of environmental policy, policy regulation, the land reclamation is faded into the background.

• 3.1 ring dikes
• 3.2 Stack dikes
• 3.3 dyke duties 3.3.1 examples of medieval laws
• 3.3.2 dike associations

• 5.1 Weakening influences
• 5.2 flow
• 5.3 Failure Mechanisms 5.3.1 Internal erosion and slope failure
• 5.3.2 Hydraulic heave and Piping
• 5.3.3 Erosion of the embankment surfaces

Definition of terms

In technical parlance, the dike is in a natural waters ( river, sea ) and is used to prevent temporary hazards ( flood protection). In contrast, the dam is usually on an artificial bodies of water ( dam, dam, canal ), or the durable steering the water in a natural waters ( Leitdamm ).

Categorizations

According to the geographical location of the dike two basic types are distinguished. It is by river and sea dikes

A further distinction is made mainly for lake and insubric lake dikes based on the degree of protection. While lower summer dikes merely provide the protection of adjacent lands from everyday level fluctuations during the tidal gradient, winter dikes are made wider. The latter also provide protection during storm surges. Summer dikes are today mostly still to be found on the islets, where pastoralism is dominant and the protection of the settlements is ensured by mounds. Summer dikes can be but occasionally still be seen in the second and third row on the mainland. Until the 1970s they often provided a flood protection in arable called outer dike areas that were largely uninhabited, such as in the area of ​​the country Kehdingen. The summer dike has a flat structure, as it is regularly flooded by the water and thereby is not subject to significant damage caused by the tsunami. Winter dikes, however, are larger in size and also protect in the home seekers mainly in autumn and winter, the coastal storm surges the hinterland.

On coasts a further categorization is based on the location of the dyke to the sea. Foreshore dikes are determined that the dike is protected by a more or less pronounced far, so-called foreland, while band dikes are right on the water's edge and has to be secured for this reason the dike foot in a special way. Since foreshore dikes have a larger dike safety, an attempt at critical point to be caused by the construction of so-called Lahnungsfeldern in less time foreland at a sharp lying dyke.

There are also diverse, regionally influenced terminology, among other subjects, the nature and function of the dike clearly. This includes the terms:

• Afsluitdike
• Leg dike
• Stack dike

Function of the dike

A dike protects in most cases land which is just above the sea or water level, from flooding due to high water levels at level eg storm surges. As a characteristic element of the landscape it is mainly due to the Dutch and the German North Sea coast ( East Friesland, Oldenburg, Dithmarschen, North Friesland ), as well as on the German Baltic coast (eg in the provost and Fehmarn ) coastal protection. In the inland dikes on rivers serve as protection against floods. For effective disaster protection at the same time -saving resources ( both financial as well as environmental) is to carry out a detailed design method. This serves to optimize the construction projects in terms of size, type and condition of the building.

History of dikes

First dikes are detected from the Mycenaean Greece. In northern Europe, people knew to the Middle Ages no dikes, but they built their houses and settlements on artificially raised from earth settlement mounds, called terps.

Ring dikes

The first dike ring form were dikes, which closed around a cropland or pasture. These early dikes were barely higher than 1.20 meters, but similar to today's dikes in the profile. They served mainly to protect agricultural land from the occasional summer storm surges that would otherwise degrade the arable land in the already salty march on. In general, these low dikes protected even in winter against the tide storm surges, as these aufliefen much lower before the building closed dike lines. The bedeichten surfaces could aussüßen so and allowed an improved agriculture.

From the 12th century the single ring dikes were gradually connected until a closed dike line, the so-called Golden ring formed at the end of the 13th century. The Golden Ring surrounding all of Friesland and stretched from East Frisia on Butjadingen, Dithmar and North Frisia, as are the lower courses of the rivers of dams were framed. With the continuous Bedeichung the system of sluices was necessary that ensured the drainage of the underlying behind the dikes areas.

However, the closed dike line did not only improve the protection. Could the flood spread over a large area before the construction of the closed dike line across the marshes, so the flood now accumulated before the dikes and ran much higher. The still low levees broke frequently and had to be expensively restored and increased. Fallow dike sections, so much of the underlying Sietlandes were flooded. As these parts of the country were frequently below sea level, it presented a little problem to be solved to determine the areas dry.

Stack dikes

In the late Middle Ages, the technique of stack dikes prevailed. The first stack dikes originated around 1440 in North Holland and spread from there to the east. From 1499 they have been documented Ostfriesland, 1590 in Wursten. Stack dikes resulted primarily because the earth needed for the construction of the dikes was always scarce, such as loss of land, a transfer back to the dike line made ​​it necessary during storm surges. It thus waived the offshore berm and replaced the necessary ground there by a made ​​of wood, vertical wall, which often rose several meters above the watts. Stack dikes, called in East Frisia also Holzungen, however, were both expensive in construction, as well as expensive to care, because the wood had to be replaced every 30 to 50 years due to lack of impregnation. The already in march hardly occurring wood had to be purchased and delivered from afar. In addition, proved that they could be easily washed away. When the waves surged against the stack dike, the water splashed into the air and rained down on the dike, so that it softened quickly.

After the Burchardiflut 1634 they switched to the German and Danish coast again to dikes with a flatter profile. As we saw with the floods of 1717 and 1825, that the levees had been too low, they were adapted to the new flood height. In the Hamburg- flood of 1962 proved that they were in many places still too low.

The final end of the stack dikes came with the appearance of the sailing ships of the Dutch introduced from Asia, Schiffsbohrwurms. From 1730 these like a worm -looking shell came over the wooden parts of the stack dikes and gnawed it downright. Whole dike lines had to be replaced. Sluices and port facilities that were also made ​​of wood, also suffered from the mussel case. Since there was no remedy against the shipworm, they returned in the 18th century back to the Bermedeichtechnik. The quays of the port facilities and the sluices were gradually replaced by stone material, which was often obtained by the plunder of megalithic tombs.

Dike duties

The dike preservation was the sole task of those who owned land behind the dike to the 18th century. The medieval Spadenland right divided to each his piece dike and associated Spadenland ( foreshore, from which the earth was taken for dike construction ). Also for wood and embroidery with the needle dike local residents had come up. In particularly sensitive areas or when long dikes protecting relatively little or little productive land, the dike duties often meant unbearable burdens on farmers. As a sign that they gave up, they put the spade in her dike piece and left the country. If the country wanted to acquire, had to pull out the spade and demonstrate the fact that he was ready to assume the duties on the dike. Dispute over the responsibilities in the dike maintenance or Neueindeichung was on the agenda, for example, when the inhabitants of a rear Koog were not willing to participate in the protection of the new Koog.

After Burchardiflut began in North Friesland, to be guided dikes and drainage of commercial entrepreneurs, who were lured with Oktroy from the Netherlands to Germany. The survivors of the storm surge that could not afford even the Wiedereindeichung were expropriated.

Examples of medieval laws

In Stedinger dike law of 1424 offending dike holders were severely punished. Who, for example, trees that were planted to protect dykes, damaged, where the hand was cut off, who held his levee route in poor condition and thus the destruction broke over the country, was buried alive together with wood and stones of his house in it. Who damaged the dike willfully or maliciously, was burned. Those who could not meet its obligations for the maintenance of dikes or wanted, had to abandon his country after the spade law.

In the Bremen Dike order of 1473 stated:

In Sachsenspiegel the ownership of land loss or gain are specified. Who his duties eludes the dike, loses his inheritance.

Dike associations

Did dikes are built even earlier by the inhabitants of coastal regions themselves, the coastal protection is in industrialized countries since the industrialization of the state's responsibilities. The government (federal and state ) is for the new building and expansion, if necessary (especially dike increases) responsible. The maintenance of the dikes is up to the dike associations. Dike must take place for all landowners whose land is so low that it must be protected by levees from flooding. By dikes and embankments of new territory was the present form of the coast of the North Sea. This is symbolized in the phrase Deus mare, Friso litora fecit ("God made ​​the sea, the frieze the coasts ").

During summer dikes were the norm 40 years ago, they are usually uncovered only in the region of the second dike line today. An exception today form only the holms in the North Frisian Wadden Sea. You are no longer built today, because often the foothills of winter dykes protecting shallow water zone is reserved for nature conservation and this protects the hinterland sufficiently from storm surges. They are now used either as pasture or as a nature reserve. In addition, the flow is decelerated by the extent the foreshore vegetation, that the water-side levee slope must not necessarily be secured with bulk stones, but a strong grass cover sufficient to prevent washouts. Winter dikes that were formerly often built behind the summer dikes, then protected the populated country.

The Civil Code (BGB ), the dike right choice of each country's right to control (Article 66 EC on the Civil Code ).

Dikes today

Dikes along the coasts are now sophisticated systems consist not only of the main dike. So a dyke can pause the accrual of waves and reduce the speed of the oncoming water before the dike. Supportive also act upstream summer dikes here. Nevertheless, the underlying Hauptdeich comes to the most important. Sometimes existing downstream levees, it is spoken in this regard according to the arrangement of the second, third, ... dike line, form a risk buffer in case of flooding as a result of dam failures during storm surges or flooding targeted at extreme flood events in rivers. In this context, it is often spoken of so-called sleep dikes.

In his construction of the dike is now a cross in his profile on the coast mostly asymmetric structure. Newly created dikes usually consist of a sand core surrounded by a one to two meters thick layer of cohesive material ( clay soil ) is covered. Subsequently increased dikes have a greater thickness of the clay layer, since for reasons of soil mechanics can not be recreated with sand. The surface is planted to prevent erosion by wind and water and for increasing the stability of grass. In order to keep the sward short and dense and festzutrampeln around the bottom, dikes are grazed by sheep, which also avoids Befahrung by mowers. High-Risk of waves dike routes, mostly crowd dikes that are exposed to water without protective foreshore, have a asphalt surface, such as on Eidersperrwerk.

The dike height and width depends on the particular landscape and flood risk. The river dikes on the Elbe, in certain sections, for example, a height of 8 m, sea dikes are higher and sometimes over 100 m wide.

The land side of the dike has a Deichwehrweg ( dike defense ) have, can be conveyed via the for example the sandbags. Meanwhile attachment improves resistance when over-flush. In the hinterland and on the inland side of polders a second dike line, which usually consists of Altdeichen, sleep dikes often follows; completely new dike lines do not provide dikes. Dikes are often the old dikes along the former coastline after the coast line was laid through land reclamation in the earlier flood area. These prevent that, after a dike breach larger areas to be flooded. The areas between the first and second dike line are usually sparsely populated. On rivers outer dike areas are created with summer dikes to reduce as possible balancing reservoir flooded the water level in heavy water drainage. In the outer dike area of ​​the vegetation of the grasslands and the polder land of vegetation of arable land breaks the waves of the water.

• The attachment of new dike sections with straw is called embroidery.
• A rise or a driveway to the dike is called act.
• A lockable passage for a traffic route is called Stöpe or Schart.

In the land reclamation Landanwachs is dammed to polders. The dike shall be provided for the regular drainage with a Dredge.

Strong threat to the stability of the structure, especially in river dams is, in some places the undermining of the facilities by the subversive activities of muskrats. For the repair and maintenance work to the civil and hydraulic high costs. The muskrat is therefore hunted all year.

In Germany the DIN 19712 finds "river dikes " for the dike construction application.

Cross section: Development from the stack dike dress calibration

Cross-sections: Remodeling of the dress calibration for sand core dike

Cross section: Danger points of the modern sand core dyke

Information panel DIN 19712

Modern dyke construction on a flood - retention basins

Dike damage and failure

Debilitating influences

River and sea dikes only offer protection against flood, when all layers and components of the structure are undamaged. Even small, subtle at first glance damage can escalate in the event of a flood and be the starting point for a failure of the dike.

A major problem of sea dikes are alluvial plant residues and waste, Teek, which is harmful to the turf of the dike during storm surges. Under a Teekdecke suffocates the grass and is replaced by weeds. In addition, animal pests, especially mice, find shelter in the loose flotsam. At river dikes burrowing small animals are also at risk, especially if they dig transitions in the dike body. These cavities, which may extend into the core to deep favor at a high water stress strong seepage and internal erosion.

In order to detect early damage or weakening, dike associations regularly conduct inspections and inspect as part of a dike show all important for the function of ingredients.

Flow

At high tide, the dike is burdened by the high existing water. Compared to the river or sea, the water table in the hinterland is significantly lower. There is a water pressure difference between air and water side of the dike. In the dyke body alignment between two pressures is done. Since the dike construction materials ( particularly sand, but also clay and clay) technically can not be completely sealed, it always comes to a landward flow of water through the dike. With the help of relatively dense, cohesive materials but this flow is so slow that a rule will not cause damage. The border between perfused and non-perfused dike material is called a phreatic and usually presents in the form of a slightly curved line between the flood and the water table. By Deichfußdrainage can be assured that the phreatic line does not end in the dike side, which would mean there a water outlet. The leachate is instead transferred securely through the drainage and prevent erosion by a filter assembly.

Failure mechanisms

The ever-present flow of water in the dike and in the subsoil can be damaged parts or defective design of lead in combination to a dike breach. There are several mechanisms which act together to some extent and cause a failure of the dike.

Internal erosion and slope failure

For internal erosion, water flow in the dike causes smallest soil particles are loosened and carried away with the water. There are now growing cavities that weaken the dike. Visible this erosion by breaking off parts of the air-side embankment. The risk of a dike breach by internal erosion is not only when the flood has reached its apex, but especially in a fast -falling water levels. The water in the dike then flows out to either side of the dike and slope breaks can occur both on the air, and on the water side. Transitions from burrowing mammals favor the internal erosion strong.

Hydraulic heave and Piping

Due to the high water pressure difference between the two sides of the dike, water flows in groundwater conducting layers below the dike. Sealing Auelehmschichten in the hinterland can be lifted by the water pressure and break suddenly or tear, which is called a hydraulic fracture. The escaping water ( seepage called ) mostly eroded fine soil particles that are floated to the departure points around to funnels. From the departure point from the erosion progresses backwards through the soil toward the water side of the dike ( " headward erosion "). Here, tubular cavities form, so this mechanism is also called Piping ( from the English word for pipe tube, pipe ).

Erosion of the slope surfaces

By pouring water waves over the air side of the dike can erode when no intact revetment made ​​of grass is present. Wave action, sediment and ice floes on the water-side slope can also lead to an erosion of the dike material.

Management in Germany

The states of Bremen, Hamburg, Mecklenburg -Western Pomerania, Lower Saxony and Schleswig -Holstein have a significant dyke systems. They are serviced by dike associations or state farms.

Dikes in the German literature

A literary classic example on the subject of dikes is Theodor Storm's novella The Ghost Rider. Due to the origin of the poet, the piece is often referred to as the Nordfriesisches national epic.