Dike
The term dike (from Middle Low German dîk “dike”, “dam”, “pond” (artificially created water, originally = cut out)) describes hydraulic engineering protection systems along coasts and rivers . They are asymmetrically profiled structures , which lie as a dam alongside a river or the seashore and are intended to protect the low, low relief , immediately adjoining hinterland from flooding. In the southern German-speaking area, dykes protect floodplains separated from the river bed , where they are simply referred to as dams .
On the North Sea coast , dikes have also been built to reclaim new land for several centuries. In this context, the term “ dike” in the mudflats is still used today . The areas created in the past by the construction of the dykes are regionally differentiated and called Koog , Polder or Groden. The soils of these protected areas have sediment deposits transported from the sea level and are referred to as marsh (s) . In today's time, which is characterized by an advanced environmental awareness of the population, environmental policy has become a strongly formative factor. This advocates stronger regulation of land management. So today the reclamation of new land has taken a back seat.
Delimitation of the terms
In technical parlance, a dike is only temporarily dammed with water. That is why it is often located near a natural body of water (e.g. river , sea ) and serves to ward off temporary dangers ( flood protection ). In contrast, a dam is permanently dammed. This means that it can usually be found on an artificial body of water ( dam , dam , canal ). There is also the dam for the permanent control of the water in a body of water. This term specification is not used in the English-speaking area and in Austria.
Categorizations
According to the geographic location of the dike, two basic types are distinguished. These are river and sea dikes.
Another distinction is made, especially in the case of sea dikes, on the basis of the degree of protection. While low summer dykes only protect the adjacent farmland from the everyday level fluctuations in the course of the tides , winter dykes are larger and also offer protection against storm surges. The shallower summer dykes can still be found on the Halligen to protect the pastures, while the settlements are particularly protected from storm surges by higher-lying terps . On the mainland, too, there are occasionally old summer dikes, today they are mostly in the second or even third row due to the construction of newer sea dikes. Up until the 1970s, they offered flood protection in unpopulated outer dike areas used for agricultural purposes, for example the outer dike areas in the state of Kehdingen , which are now under nature protection . These summer dykes had a flat construction, as they were regularly flooded by storm surges and should not suffer any major damage from the tidal wave. Winter dykes were located further inland, were higher and stronger and protect the populated hinterland from the autumn and winter storm floods.
A further categorization takes place on coasts based on the location of the dike at sea. Foreland dikes are determined by the fact that the dike is protected by a more or less pronounced so-called foreland, while Schardiche lie directly on the water's edge and for this reason the dike foot must be secured in a special way. Since foreland dikes have greater dike security, attempts are made at critical points to create foreland on a sharp dike in a shorter time by building so-called parapet fields .
In addition, there are various, regionally shaped terms that, among other things, make the type and function of the dike clear. These include the terms:
- Closing dike
- Thigh dike
- Stackdeich
Function of the dike
In most cases, a dike protects land that is just above sea or water level from flooding due to excessively high water levels at z. B. Storm surges . As a characteristic landscape element is used primarily in the Netherlands and the German North Sea coast ( East Friesland , Oldenburg , Dithmarschen , North Friesland ), the tidal Elbe (circles Cuxhaven and Stade ) and for example on the German Baltic coast (. In the provost and Fehmarn ) the coastal protection . Inland, dykes on rivers serve to protect against flood events. A detailed assessment procedure must be carried out for effective disaster control while saving resources (both financially and in terms of environmental policy). This serves to optimize the building project in terms of size, type and condition of the structure.
History of dyke construction
The first dikes have been found in Mycenaean Greece . Up until the Middle Ages, people in Northern Europe were not familiar with large-scale dike construction.
Ring dykes
The first form of dyke were ring dikes , which were enclosed around a field or pasture. The oldest examples from the Roman Iron Age can be found in West Friesland. These early levees were barely more than four feet tall, but similar in profile to today's levees. They mainly served to protect agricultural areas from the occasional summer storm surges, which otherwise would have deteriorated the arable soils in the already salty marshland. As a rule, these low dykes also provided protection against the storm surges of that time in winter, as they ran much lower before the dyke lines were built. In this way, the dammed areas could sweeten out and enable improved agriculture .
From the 12th century the individual ring dikes were gradually connected until they formed a closed dike line, the so-called Golden Ring , at the end of the 13th century . The Golden Ring surrounded the whole of Friesland and extended from East Friesland via Butjadingen , Dithmarschen and North Friesland , and the lower reaches of the rivers were bordered by dams . With the continuous dike construction of sluices became necessary, which ensured the drainage of the areas behind the dykes.
The closed line of the dike did not only improve protection. While the flood was able to spread over a large area over the marshes before the closed dyke line was built, the flood now accumulated in front of the dykes and ran up significantly higher. The dykes, which were still low, broke more often and had to be renewed and raised at great expense. If sections of the dyke were fallow, large parts of the Sietland behind were flooded. Since these parts of the country were often below sea level, it was a problem that could hardly be solved to dry the areas again.
In historical times, ring dikes existed far from the sea inland, such as B. around Grafenrheinfeld in the flat valley of the Main south of Schweinfurt .
Stack dykes
In the late Middle Ages, the technique of stack dikes prevailed. The first stack dykes emerged in North Holland around 1440 and spread further east from there. From 1499 they can be traced for East Frisia and from 1590 in the country of Wursten . Stack dykes were mainly created because the earth required for the construction of the dykes was becoming increasingly scarce, as, for example, land losses during storm surges made it necessary to move the dyke line backwards. The lake-side berm was therefore dispensed with and the earth required there was replaced by a vertical wall made of wood, which often rose several meters above the mudflats. Stack dykes, also known as logs in East Frisia , were both complex to build and expensive to maintain, as the wood had to be renewed every 30 to 50 years due to a lack of impregnation . The wood, which hardly occurs in the march anyway, had to be bought and delivered from far away. In addition, it turned out that they could be washed down more easily. When the waves crashed against the stack dike, the water splashed up and pelted the dike, so that it quickly softened.
After the Burchardi flood in 1634, the German and Danish coasts switched to dykes with a flatter profile. When the storm surges of 1717 and 1825 showed that the dykes were too low, they were adjusted to the new flood height. The Hamburg storm surge of 1962 showed that they were still too low in many places.
The final end of the stack dykes came with the appearance of of Dutch sailing ships from Asia entrained Schiffsbohrwurms . From 1730 this worm-looking shell attacked the wooden parts of the stack dyke and literally ate them up. Entire dyke lines had to be replaced. Sluices and docks, which were also made of wood, also suffered from the mussel infestation. Since there were no remedies against the shipworm, they returned to the Bermedeicht technology in the 18th century. The quay walls of the port facilities and the sluices were gradually replaced by stone material that was often won by looting large stone graves.
Dike service obligations
Up until the 18th century, dike maintenance was the sole responsibility of those who owned land behind the dyke. The medieval Spadelandrecht allocated each his / her piece of dike and the corresponding Spadeland (foreland from which the earth was taken for the dyke construction). The residents also had to pay for wood and embroidery with the dike needle. In particularly endangered areas or when long dykes protected relatively little or little productive land, the dyke duties often meant unbearable burdens for the farmers. As a sign that they were giving up, they stuck the spade into their dike and left the country - in accordance with the spade law . Those who wanted to appropriate the land had to pull out the spade and demonstrate that they were ready to take on the duties on the dike. A dispute over the responsibilities in dike maintenance or new dykes was the order of the day, for example when the residents of a rear pool were not willing to help protect the new pool.
After the Burchardi flood, dyke construction and drainage began in North Friesland to be led by commercial entrepreneurs who were lured to Germany with Oktroy from the Netherlands. The survivors of the storm surge, who could not do the re-dike themselves, were expropriated.
Examples of medieval-early modern ordinances
In Sachsenspiegel ownership is fixed at land loss or gain. Those who evade their duties on the dike lose their inheritance.
In the Stedinger dike law of 1424, defaulting dyke keepers were punished with severe penalties. For example, those who damaged trees that were planted to protect the dykes had their hands cut off, those who kept their dyke stretch in a poor condition and thus the ruin broke in on the land, were buried alive in it with the wood and stones of their house. Anyone who willfully or maliciously damaged the dike was burned. Those who could not or did not want to meet their duties to maintain the dykes had to give up their land according to the Spatenrecht.
In the Bremen dike order of 1473 it says:
- Every owner of a piece of land behind the dyke is required to work and has to work on the dykes through manual and span services as well as financial contributions.
- Everyone who works on the dyke has to make an honest change. No one is allowed to curse or make blasphemous speeches while the dike is being worked on. Nobody can evade the dyke obligations.
With a water protection ordinance , King Maximilian I had his own water tax levied in the Regional Court of Bozen in 1497 in order to have the flood-damaged protective structures on the local rivers ( “wasserpaw” ) renewed.
Dike associations
Whereas in the past dykes had to be built by the inhabitants of the coastal areas themselves, coastal protection in industrialized countries has been a state responsibility since industrialization . The state (federal and state governments) is responsible for the new construction and, if necessary, expansion (especially raising dikes). The dyke associations are responsible for maintaining the dykes . All landowners whose land is so deep that it has to be protected from flooding by dykes are required to dyke. The current shape of the coast on the North Sea was created through the construction of dykes and the dykes of new land. This is symbolized in the saying Deus mare, Friso litora fecit (“God created the sea, the Frisian the coast”).
While summer dykes were normal 40 years ago, today they can mostly only be seen in the area of the second dyke line. The only exception to this today are the Halligen in the North Frisian Wadden Sea . They are no longer built today, as the foreland at winter dykes is often reserved for nature conservation as a protective shallow water zone and this protects the hinterland sufficiently from storm surges. Today they are used either as pasture or as a nature reserve. In addition, the flow is slowed down by the foreland vegetation to such an extent that the dike slope on the water side does not necessarily have to be secured with rubble stones, but rather thick grass is sufficient to prevent washouts. Winter dykes, which used to be built behind the summer dykes, protected the populated land at that time.
The Civil Code (BGB) has left the dike law to the respective state law for regulation (Art. 66 EG to the BGB).
Dike construction today
Dyke constructions on the coasts are meanwhile ingenious systems that do not only consist of the main dike. In front of the dike, a dike foreland can break the waves and reduce the speed of the incoming water. Upstream summer dykes have a supportive effect here. Nevertheless, the main dike behind it is of greatest importance. Downstream dykes that are sometimes present - the second, third, ... dyke lines are referred to in this regard according to the arrangement - form a risk buffer in the event of flooding as a result of dike breaches during storm surges or targeted flooding in the event of extreme flood events on rivers. In this context, so-called sleeping dikes are often used .
In terms of its structure, the dyke is today a mostly asymmetrical structure in its cross-section on the coast. Newly created dykes usually consist of a sand core covered by a one to two meter thick layer of cohesive material ( clay soil ). Subsequently raised dykes have a greater thickness of the clay layer, since for reasons of soil mechanics they cannot be rebuilt with sand. The surface is planted with grass to avoid erosion by wind and water and to increase stability . In order to keep the sward short and dense and to trample the ground, sheep graze on dikes, which avoids being driven on by mowing machines. Dyke stretches that are at great risk from the impact of waves, mostly Schardiche that are exposed to the water without a protective foreland, have an asphalt surface, for example at the Eider Barrage .
The height and width of the dyke depends on the respective landscape and the risk of flooding. The river dikes on the Lower Elbe have in certain sections z. B. a height of 8 to 9 m, in some places the sea dikes are even higher and sometimes over 100 m wide.
The land side of the dike must have a dyke weir path ( dyke defense path ) through which, for example, sandbags can be transported. Its attachment improves the resistance when washing over. In the hinterland and on the inland side of Kögen there is often a second line of dikes, which mostly consists of old dikes such as earlier summer dikes and sleeping dikes . The latter are the historic dikes of the former coastline. After the coastline was relocated seaward through land reclamation , it was usually left standing in order to keep the flooding damage as small as possible in the event of a dike breach . The areas between the first and second line of the dyke are mostly sparsely populated because the old town centers are behind the old dykes. Outside dyke areas with summer dykes are created on rivers in order to lower the water level in the event of strong meltwater runoff as a possibly flooded compensation basin. In the outer dike area the vegetation of the grassland and in the polderland the vegetation of the arable land breaks the waves of the water.
- The attachment of new dyke stretches with straw is called embroidery .
- An entrance or a driveway to the dike is called an act .
- A closable passage for a traffic route is called a Stöpe or Schart .
During land reclamation , land wax is diked into Kögen. The sea dike must be equipped with a sewer for regular drainage .
A strong threat to the stability of the structure, especially in the case of river dikes, is the undermining of the facilities in some places by the burrowing activity of muskrats . Civil engineering and hydraulic engineering incur high costs for repair and maintenance work. The muskrat can therefore be hunted all year round.
In Germany, DIN 19712 “River dikes” is used for building new dikes.
Dike cross-section of modern river dikes
Cross-section:
Development from stack dike to dress dike
Cross-sections:
Conversion from the dressing dike to the sandpit dike
Cross section:
danger points of the modern sandpit dike
Information board DIN 19712
Modern dyke construction on a flood retention basin
Dike damage and failure
Debilitating Influences
River and sea dikes only offer protection against flooding if all layers and components of the structure are undamaged. Even minor, at first sight insignificant damage can spread in the event of a flood and be the starting point for the dike to fail.
A big problem on sea dikes is plant debris washed up during storm surges and rubbish, tea , which is harmful to the sward of the dike. The grass suffocates under a tea blanket and is replaced by herbaceous growth. Animal pests, especially mice, can also find shelter in the loose floating debris. Small animals burrowing on river dikes are also a danger, especially if these passages dig into the dike. These cavities, which can reach deep into the core, encourage strong seepage currents and internal erosion in the event of flood pollution.
Large vegetation on dykes can promote internal erosion at the roots. Even more problematic is the uprooting of trees by the wind, which leaves a crater as a failure point. There should therefore be no trees on or immediately next to dikes.
In order to identify damage or weakening at an early stage, dyke associations carry out regular inspections and inspect all components that are important for their function as part of a dyke exhibition .
Flow through
In the event of flooding, the dike is burdened by the high water level. Compared to the river or sea level, the groundwater level in the hinterland is significantly lower. There is a difference in water pressure between the air and water sides of the dike. In the dike body, the two pressures are equalized. Since the dyke construction materials (especially sand, but also clay and clay) can not technically be completely sealed, there is always an inland water flow through the dyke. With the help of the comparatively dense, cohesive materials, this flow is slowed down so much that, as a rule, no damage occurs. The boundary between the flowed through and the non-flowed dyke material is called the seepage line and is usually in the form of a slightly curved line between the flood and groundwater levels. A dike foot drainage can ensure that the seepage line does not end in the rear of the dike, which would mean a water leak there. Instead, the seepage water is safely discharged through the drainage system and erosion is prevented by a filter structure.
Failure mechanisms
The constant flow of water in the dike and in the subsoil can lead to a dike breach in conjunction with damaged areas or poor planning. There are several mechanisms, some of which work together and cause the dike to fail.
Internal erosion and slope failure
In the event of internal erosion, the water flow in the dike causes the smallest grains of soil to be loosened and transported away with the water. Ever larger cavities are created, which further weaken the dike. This erosion becomes visible through parts of the airside embankment breaking off. The risk of a dike breach due to internal erosion does not only exist when the flood has reached its apex, but especially when the water level is falling rapidly. The water in the dike then flows out of the dike on both sides and slope breaks can occur on both the air and the water side. Burrowing animal passages strongly promote internal erosion.
Hydraulic ground failure and piping
Due to the high water pressure difference between the two sides of the dike, water flows in aquifer-conducting layers below the dike. Sealing layers of clay in the hinterland can be raised by the water pressure and suddenly break open or tear, which is known as hydraulic ground failure. The escaping water ( called smoke water ) usually erodes fine grains of soil, which are floated up to form funnels around the breakpoints. From the point of departure, the erosion proceeds backwards through the ground towards the water side of the dike (“retrograde erosion”). Here, tubular cavities form, which is why this mechanism also Piping (from the English word pipe for pipe , tubing ) is called.
Erosion of the slope surfaces
Water flowing over waves can erode the air side of the dike if there is no intact grass revetment. Waves, debris and ice floes on the water-side embankment can also lead to the erosion of dyke material. The water flowing over or through can leave scouring behind, which sometimes remain as brackish .
Dike defense
Dykes must be easily accessible so that the emergency services can get to the site without any problems.
Analysis of the dike quality
For this purpose, regular inspections are carried out as part of a dike exhibition .
In addition to the Deichschau i. d. As a rule, the information that is available and can be obtained with justifiable economic effort is used by examining existing documents, such as
- Damage assessment / cadastre,
- Measurement,
- Geotechnical / geophysical investigations.
In the Netherlands, on a trial basis, analysis cables are already being laid in new dikes, which automatically measure the quality of the inside of the dike, such as water ingress, and report it automatically to a monitoring center.
Administration in Germany
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The states of Bremen, Hamburg, Mecklenburg-Western Pomerania, Lower Saxony and Schleswig-Holstein have important dykes. They are maintained by dike associations or state companies.
Dykes in German literature
A classic literary example on the subject of dyke construction is Theodor Storm's novella Der Schimmelreiter . Due to the origin of the poet, the piece is often referred to as the North Frisian National Epic.
literature
- Marie Luisa Allemeyer: "No country without a dike ...!" The lifeworlds of a coastal society in the early modern period . In: Publications of the Max Planck Institute for History, Vol. 222 . Vandenhoeck & Ruprecht, Göttingen 2006, ISBN 3-525-35879-2 .
- Albert Brahms : Initial reasons for dyke and water construction, or thorough instructions on how to build efficient, durable dams against the violence of the greatest sea floods, and how to keep them in an undiminished condition at any time, so that the land can be protected against perishable incursions and floods sey. 2 volumes, Tapper, Aurich 1754 and 1757. Unchanged reprint 1767 and 1773, published by the Marschenrat . Reprint: Verlag Schuster, Leer 1989, ISBN 3-7963-0273-4 .
- Michael Ehrhardt: “Always towards the great water.” On the history of the dikes in Wursten . Landschaftsverband Stade, Stade 2007, ISBN 978-3-931879-35-8 .
- Michael Ehrhardt: "A guldten band of the country". The history of the dikes in the Old Country . Landschaftsverband Stade, Stade 2003, ISBN 3-931879-11-9 .
- Norbert Fischer : In the face of the North Sea - On the history of the dykes in Hadeln . Stade 2007, ISBN 978-3-931879-34-1 .
- Norbert Fischer: water shortage and marsh society. On the history of the dikes in Kehdingen . Landschaftsverband Stade, Stade 2003, ISBN 3-931879-12-7 .
- Albert Panten : The early dike construction in North Friesland: Archaeological-historical investigations . 2nd Edition. Association of the North Frisian Institute, 1995, ISBN 3-88007-158-6 .
- Jürgen W. Schmidt: On the distribution of loads during the dike construction work in the Prignitz area in the 18th and 19th centuries . In: Messages from the Association for the History of Prignitz . tape 10 . Perleberg 2010, p. 52-67 .
- Robert Stadelmann: Setting limits to floods. Schleswig-Holstein's coastal protection. West coast and Elbe. In: North Friesland . tape 1 . Husum 2008, ISBN 978-3-89876-312-7 .
Web links
- Dyke construction: literature from and about dykes in the catalog of the German National Library
- Dike construction and coastal protection in Lower Saxony
- List of dike associations in Lower Saxony
- List of dike associations in Bremen
- Internet presence of the I. Oldenburgische Deichbandes
- "Flood protection by electrocution" article on the investigation of dikes on the geosciences portal planeterde
- Dike and society
- Dike and North Sea coast
Individual evidence
- ↑ dike. duden.de, accessed on August 14, 2013 .
- ↑ Heinz Patt, Peter Gonsowski: Hydraulic . 2011, p. 98 , doi : 10.1007 / 978-3-642-11963-7 ( springer.com [accessed May 30, 2018]).
- ^ Committee for coastal protection works .: EAK 2002: Recommendations for the implementation of coastal protection works . Westholsteinische Verl.-Anst., Boyens, Heide 2002, ISBN 3-8042-1056-2 , p. 305 .
- ^ Jost Knauss: Dike . In: Holger Sonnabend (ed.): Man and landscape in antiquity. Lexicon of historical geography . Stuttgart 2006, ISBN 3-476-02179-3 , pp. 91 .
- ^ Karl-Ernst Behre : The history of the landscape around the Jade Bay . Brune-Mettcker, Wilhelmshaven 2012, ISBN 978-3-941929-02-9 , pp. 65 .
- ↑ Hansjörg Küster: History of the landscape in Central Europe: from the Ice Age to the present . CH Beck, Munich 1999, ISBN 3-406-45357-0 , p. 215 f .
- ^ Karl-Ernst Behre : The history of the landscape around the Jade Bay . Brune-Mettcker, Wilhelmshaven 2012, ISBN 978-3-941929-02-9 , pp. 51 .
- ^ Karl-Ernst Behre : The history of the landscape around the Jade Bay . Brune-Mettcker, Wilhelmshaven 2012, ISBN 978-3-941929-02-9 , pp. 70 .
- ^ Karl-Ernst Behre : The history of the landscape around the Jade Bay . Brune-Mettcker, Wilhelmshaven 2012, ISBN 978-3-941929-02-9 , pp. 72 f .
- ↑ See the place name Spadenland
- ^ Sachsenspiegel: About the dyke construction
- ^ Hannes Obermair : Bozen Süd - Bolzano Nord. Written form and documentary tradition of the city of Bozen up to 1500 . tape 2 . City of Bozen, Bozen 2008, ISBN 978-88-901870-1-8 , p. 234-235, No. 1324 .
- ↑ a b Explanation of terms - types of dike damage. (No longer available online.) Deichverband Dormagen, archived from the original on August 29, 2013 ; Retrieved August 14, 2013 .
- ↑ Ulrich Förster: Practical investigations into retrograde erosion on the experimental dike “IJkdijk” . In: Holger Schüttrumpf (Ed.): Internationales Wasserbau-Symposium Aachen 2012: Floods - a permanent task! January 2012 ( deltares.nl [PDF; 1.1 MB ]). PDF file; 1.1 MB ( memento from July 20, 2014 in the Internet Archive )
- ↑ Technical relief organization : Handbook Flood Protection Dike Defense Document page 26, PDF page 30
- ↑ Haselsteiner, Roland / Schmitt, Alexander / Kretzschmar, Jan: The dike condition analysis as the basis for upgrading and maintenance concepts for river dikes . In: "Dresdner Wasserbauliche Mitteilungen", issue 50, Dresden 2014, p. 188