Étang (body of water)

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An étang is a standing body of water that is not very deep and has a relatively small surface (a few tens of hectares). It arises on impermeable soils .

The Étang, picture by Théodore Fourmois , painted in 1867
Pond in northern Germany; In this example, water evaporation is reduced by reducing the free surface area; this is partially offset by evapotranspiration , i.e. evaporation through plants.
Municipality Weiher in Tucquegnieux ( Meurthe et Moselle ,), which was obviously created artificially with few natural embankments and the predominantly leisure activity, in particular the angling is used
Étang of Aureilhan (Landes)
Quarry pond in the province of Hainaut , Belgium

definition

There are different definitions for an étang , the closest is certainly the term pond . The étang is a body of water of natural or human origin within the mainland. It is perennial , so it carries water all year round and is connected to the natural water system, but can be drained for cultivation. The size is below that of a lake, but is larger than a pond or pond .

In France in particular, the terms étangs de pêche and étang de chasse (fish and hunting ponds) also exist.

etymology

The term étang comes from the French and goes back to the 12th century. Estanc had the meaning of a water surface with embankments that prevent it from running out. The separation between mare and étang was not always clear, the terms were sometimes used synonymously .

Formation and development of the étang

The majority of the étangs are of human origin, be it as a result of a dam or weir in a flowing water , be it through the deepening of a wetland with a natural water inflow (rain, spring ), or through digging until the groundwater level is reached. In Europe, the emergence of numerous étangs can be traced back to the construction of dams and weirs, which in the Middle Ages were very often carried out or directed by monasteries. The Étangs often served as water reserves and, above all, as fish ponds for a well-developed pond management .

The construction of an étang requires a water inflow and a water-impermeable soil, or a connection to the groundwater. The inflow can consist of a channel that feeds water from a natural watercourse. Often such a watercourse, such as a stream , is used to form a chain of étangs, one of which flows into the other. The weirs usually consist of heaps of earth, gravel and clay , more rarely a wall.

If the étang is surrounded by trees or in a zone of erosion , regular maintenance is essential to prevent silting . In order to correct an increased nutrient input, the aquatic plants and the reed beds , but also the neighboring trees have to be cut back. Muskrats can cause damage from holes they dig in weirs and embankments.

Emptying the Étangs by means of pumping , jacks or bungholes, with or without Monks . The uppermost part of the weir is often bricked up and provided with a grid so that the water can overflow during floods without destroying the weir or even being washed away.

Pond bungs and monks

A pond bung can be a natural water outlet, with or without a slider or monk , with or without a siphon .

A monk is a type of well, either made of concrete poured in place or made of prefabricated structural elements. The simplest have a side made of boards that are inserted into grooves in the side walls. The height of the top board also defines the highest water level and serves as an overflow during high water. Sometimes monks are executed with two layers of boards, with a filter in between, which also has to be cleaned regularly. In the lower part of the monk there is a pipe (pond bung) that leads through the dam into the Étang basin.

Often the monks are also subdivided in height and provided with pipes or jacks, which makes it possible to empty the étang in layers. Since the coldest water is close to the ground and the warmest on top, the temperature of the water in the étang can be regulated by emptying a suitable layer .

history

Originally, the Étangs were created primarily for fish farming . Since the church prescribed around two hundred fasting days per year for monks and nuns , there was a great need for fish that were allowed as food. For this reason, fish farming was particularly encouraged by the monasteries. In swampy areas (such as the Bresse ), the construction of étangs was a way of draining the cultivated areas. At the same time, thanks to the Étangs, it was possible to bridge the fallow year in three-field farming.

For the operation of the mills, the étangs with the associated watercourses ( canal, ruisseau, bief ) were used to drive the mill wheels. With these water reserves, the water levels could be increased in a targeted manner.

With the invention of the steam engine, the étang lost its importance a little, only a few mills or sawmills still use a mill wheel - and this only in emergencies such as power outages and the like. Ä. Today the étang is mainly used as a recreational opportunity, for fishing, hunting for waterfowl or still for fish farming.

Physical characteristics

Contrary to popular belief, limnological studies have shown that at times very strong thermal currents take place in ponds that are more than 2 meters deep.

The Étang , like bodies of water in general (German: See, Weiher, Tümpel - French: Lac, Étang, Mare), form a very sensitive ecosystem that reacts directly to environmental pollution of all kinds. Flora and fauna contain diverse species that are dependent on management (filling - using - draining) and many of these species are protected today. On the part of nature conservation associations, as well as government agencies, great attention is now paid to the preservation and care of the étangs in France .

Water balance

The water balance of an étang is basically obtained by subtracting runoff and evaporation from precipitation and inflow. Due to the fact that the étangs have a strong vegetation, which leads to a further, difficult to measure evaporation, the creation of a water balance for an étang is much more difficult than for a basin , a reservoir or a retention basin .

  1. The water loss consists not only of the runoff , but also of the overflow as well as the seepage in the clay soil and the embankments ( Darcy law ), but not least also of evaporation and evapotranspiration . because of plants in and on the banks of the Étang , especially tree roots that penetrate the slopes. In some cases, water loss due to cold and frost must also be taken into account
  2. When it comes to water inflow , the following must be taken into account: inflow through one or more streams and canals, but also meteor water and the inflow from the groundwater.

The terms of the water balance equation change depending on whether the water surface is free or more or less overgrown and in this case depending on the type of plant ( marsh plants , reeds, water lilies, etc.).

Environment and ecology

In addition to natural coastal lakes and lagoons , the étang is a geographical object that is hydrographically and biogeographically more or less artificial, and the ecological quality of which is controversially discussed. The benefit is particularly controversial in fishing circles, as the nobler fish species ( salmonids ) that live in flowing waters are generally preferred to fish in stagnant waters ( carp , pike ). The desire to renature the Étangs and to convert them back into flowing waters therefore emerges again and again from these circles . On the other hand, the Étangs and their embankments form welcome retreats for flora and fauna, which would still largely exist without human intervention. If the beaver had not been exterminated as a natural inhabitant, a largely similar landscape would have resulted from its buildings.

Furthermore, the value of the étang - much more so than that of the pond or pool - was long misunderstood in France, except in certain areas where there is still a pronounced fishing industry (e.g. Dombes , sometimes also Bresse ). The inventory of wetlands that the water authorities are currently compiling will reveal larger connections in terms of morphometry , geomorphology , hydrography , ecology , fish farming , physics , socio-economy and cultural history . In the Limousin alone , 22,792 bodies of standing water (including around 16,000 étangs ), which have usually existed for more than a thousand years, were recorded and cataloged. This work leads to the fact that the Limousin is the first still water region in France, with the high Limousin in particular being the region of France that is most densely covered with standing water.

The construction of an étang, and in particular its shape and depth, affect its environment. It can be a purely artificial remodeling or it can have positive ecological effects. The diversion of a stream, the change in the flow speed, the runoff into a previously cooler, cleaner stream, as well as the warming of the water in summer, can cause profound changes in fauna and flora.
In a large number of étangs - intentionally or accidentally - plants, algae and bacteria, but also fish, crustaceans, snails and mussels, which are allochthonous or even invasive (see invasion biology ).

In terms of hydrology , the étang is a fairly constant water reserve, with the help of which the water flow can be regulated, in particular by supplying it at low water or drought, but also by retaining it during high water. The groundwater layer is usually protected and preserved through natural seepage. There is more danger in the case of étangs that are used for leisure purposes, as the withdrawal of water from the natural water balance has a negative impact on balance (in Florida, ponds are created as storage basins and the water is used to irrigate the lawns of golf courses).

In addition to evaporation, the evapotranspiration of the reed beds or the plants on the bank slopes must be taken into account. In hot zones with strong sunshine and strong winds, together with the activities of plankton , there are day-night rhythms that can lead to excess salt . This is the case, for example, in the Camargue , as Heurteaux showed in complex studies in 1969.

Like all bodies of water, étangs or even golf ponds have a great attraction for water birds due to their reflection . Researchers from the University of Florida have investigated whether these bodies of water could create a refuge for water birds. At 183 golf ponds (on 12 golf courses in southwest Florida) 10,474 birds and 42 species were counted over two years (Jan. – April 2001 and 2002). The vegetation and hydrology of the ponds were compared in order to uncover connections with the presence or absence of individual species. The number of birds in relation to the water area was surprisingly low (less than 2 animals per hectare). For comparison, on Lac du Der alone , which is also artificially created, up to 68,000 cranes were counted in the same period and over 270 bird species are observed every year. The researchers have subsequently proposed adding value to the gulf ponds by making changes in vegetation and hydrology to meet the needs of waterfowl.

When the artificial étangs are suddenly emptied , especially when they are used for fish farming , as in the Dombes , mineral and organic substances are released into the deeper waters, which change the water quality. In the étang itself, the remaining plant residues are cleaned by wind and rain and after refilling the water is better cleaned by the plants.

Certain étangs contain toxins that come from human settlements, industries or agriculture ( artificial fertilizers , pesticides ). Fish and hunting ponds often have a significant amount of lead (fishing lead and lead shot) that has been stored on the ground for decades (lead shot was only banned in France in 2005, in other countries up to 20 years earlier). This lead residue can poison sediments and lead to lead poisoning in birds, which in turn can lead to meat poisoning .

Freshwater étangs

The impermeable soils, together with slight slopes, characterize areas with frequent étangs , to which the following regions belong in France:

Some of the étangs reach areas that can be classified as lakes. Outside France, Belgium should be mentioned, with the Étangs of Ixelles .

Salzétangs

One often speaks of Salzétangs, which are actually lagoons (French: la lagune or le lagon), which most of the time are connected to the sea via a low point . The following should be mentioned: Étang de Thau , Étang de Berre , Étang de l'Or and Étang de Bolmon .

These étangs are characterized by large fluctuations in salinity, which can be attributed to the strong solar radiation and the wind (→ high evaporation rate) as well as the low tidal range in the étang (→ large water influx). The Étang de Thau has a tidal range of a few centimeters, but the Mediterranean at Sète 14 cm. As a result, between 750,000 and 3,750,000 cubic meters of water are exchanged between the Mediterranean and Étang at every tide .

Fish ponds

Étangs , especially if they can be easily emptied and managed, have long allowed extensive fish production. The pond economy already developed in Europe in the Middle Ages, especially as a result of the great demand from the monasteries. In China , the pond economy developed before our era.

Other areas of aquaculture such as shrimp farming are also based on pond management.

Other areas of application

One method of wastewater treatment is to pass contaminated water through a series of étangs or lagoons where the contaminants can settle.

Pond management in the Bresse

The area of ​​the Bresse is covered by around 2,500 still waters, which almost without exception arose in the Middle Ages under the direction of monasteries and nobles. A detailed set of rules for the management of the Étangs was drawn up early on .

Due to the social structure in the Bresse, which was characterized by clerical and noble lords, a special form of three-field economy developed . The peasants were usually self-employed who had to pay a tithe for their fiefs . Now the feudal lords had different needs, in addition to the provision of bread grain. The noble feudal lords needed oats because they were the only ones who owned horses. Their feudal takers, the farmers, did the field work at most with carts of cows or oxen. The clerical feudal lords, on the other hand, like monasteries and bishops, had a great need for fish in order to be able to bridge the approximately 200 days of fasting. The extermination of beavers can also be traced back to the strict rule of fasting , as they were allowed to be eaten as aquatic animals even on fasting days.

The farmers of the Bresse started to incorporate the étangs into their production rhythm. Early in the first year they put fish in the étang , which remained and grew in it all year and the following. In Advent to the end of the fasting period of the third year was Étang fished out and then emptied. In spring was in Étang oats sown, which was harvested in the summer. Then the étang was filled again and the rhythm began again.

The over-fishing was done as follows: In the middle of the Étangs least one trench was drawn at the definition, which ended in a major depression at discharge. If the water was now drained, the fish had no other option than to escape into the longitudinal ditch, from where they could be scooped out.

Through this type of management, the farmers achieved an additional yield of fish in spring and oats in summer every three years, with which either the tithing could be paid or which brought in additional sales proceeds.

Related terms

Individual evidence

  1. ↑ Building instructions for monks (French)  ( page no longer available , search in web archives )  Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice.@1@ 2Template: Dead Link / ftp.fao.org  
  2. ↑ Still waters at the Conservatoire d'Espaces Naturels de Bourgogne
  3. ↑ GE Harbeck: A practical field technique for measuring reservoir evaporation utilizing mass-transfer theory. (= US Geol. Survey Prof. Paper 272-E ). 1962.
  4. ^ WB Langbein, CH Hains, RC Culler: Hydrology of stock-water reservoirs in Arizona. (= US Geol. Survey Circ. 110). March 1951.
  5. ^ GE Harbeck, Kohler, MA Koberg, GE et al .: Water-loss investigations; Lake Mead studies. (= US Geol. Survey Prof. Paper. 298). 1958.
  6. ^ MA Kohler, TJ Nordenson, DR Baker: Evaporation maps for the United States. (= US Weather Bureau Tech. Paper 37, pls. 2.4 ). 1959.
  7. ↑ G. Katul, K. Novick: evapotranspiration. In: Encyclopedia of Inland Waters. 2009, pp. 661-667.
  8. ^ WS Eisenlohr, Jr .: Water loss from a natural pond through transpiration by hydrophytes. In: Water Resources Research. v. 2, no. 3, 1966, pp. 443-453.
  9. ^ WS Eisenlohr, Jr .: Determining the water balance of a lake containing vegetation. In: Internat. Assoc. Sci. Hydrology Pub. v. 1, 70, 1966, pp. 91-99.
  10. ^ WD Willis, HL Parkinson, CW Carlson, HJ Haas: Water table changes and soil moisture loss under frozen conditions. In: Soil Sci. v. 98, no. 4, October 1964, pp. 244-248.
  11. ^ JJ Marciano, GE Harbeck: Mass-transfer studies, in Water-loss investigations - Lake Hefner studies. technical report. (= US Geol. Survey Prof. Paper. 269). 1954, pp. 16-70.
  12. ^ WS Eisenlohr, Jr .: Water loss from a natural pond through transpiration by hydrophytes. In: Water Resour. Res. 2 (3), 1966, pp. 443-453, doi: 10.1029 / WR002i003p00443 .
  13. ↑ a b c d Pascal Bartout: Pour un référentiel des zones humides intérieures en milieu tempéré: l'exemple des étangs en Limousin (France). Dissertation 2006. 201, ISBN 978-613150618-5 .
  14. ↑ Water authority of the Ministry of the Environment
  15. ↑ P. Kerambrun: Cycle nycthémeral de la salinité dans un étang de Camargue. Téthys, 1970 - Marine d'Endoume station
  16. ^ Pierre Heurteaux: Report des eaux souterraines avec les sols halomorphes et la végétation en Camargue. In: La Terre et vie. Number 4.
  17. ^ A b c C. LeAnn White, Martin B. Main: Waterbird use of created wetlands in golf-course landscapes. In: Wildlife Society Bulletin. 33 (2), 2005, pp. 411-421. doi : 10.2193 / 0091-7648 (2005) 33 [411: WUOCWI] 2.0.CO; 2
  18. ^ LPO, Crane Census Results on Lac du Der , accessed September 22, 2014.
  19. ↑ LPO, Guide des oiseaux du lac du Der et des environs, French, English and Dutch (36 pages).
  20. ↑ Audouin Jacques: Hydrologie de l'étang de Thau (Hydrology of the Thau lagoon). In: Revue des Travaux de l'Institut des Pêches Maritimes. Vol. 26, N. 2, 1962-03, pp. 5-1041962-03. ISSN  0035-2276 . Accès via les archives d'Ifremer
  21. ^ Atlas of the still waters in Saône-et-Loire, Conservatoire d'Espaces Naturels de Bourgogne
  22. ↑ Explication des statuts, coutumes et usages observés dans la province de Bresse, Bugey, Valromay et Gex Philibert Collet, 1698.

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Web links

bibliography

  • L. Touchart: Définition de l'étang: le point de vue de la geographie limnologique. In: Studia Universalis Babes-Bolyai Geographia. LI (1), Cluj-Napoca 2006, pp. 117-132.

fr: Étang