Stopping

History

The stoppings probably exist since prehistory (water reserve drinkable, of irrigation, fish ponds, piscicultures) but it is with the Middle Ages that they strongly developed in Europe to supply the water mills. It seems that they sometimes could be pressed on sediments accumulated upstream of embâcles natural, or on the spot of stoppings of beavers whose toponymy preserves traces (for example in France with the word level and beaver (old name of beaver) which could be dependant, or with names of communes such as Beuvry (one of the old names of beaver) or the beuvrière (the “castorière” ). The old charts, of Cassini for example carry testimony of the many stoppings of small rivers made by the local peasants or monks, to preserve water and to raise there fish or for the steeping of the flax or hemp.

It should be noted that by preserving volumes of water and a height of more important water in dry season, these stoppings also could plug the estival fluctuations of the tablecloths (because all things being equal, it is the height of water which controls the speed of percolation, cf Loi of Darcy).

Some examples of dam in the world

the Stopping Hoover with the the United States (1931-1935)
the stoppings of Aswan on the the Nile, in Egypt
the Dam Inga on the Congo, in Democratic republic of Congo
the Barrage of Itaipu at the border enters the Brésil and the Paraguay
the Centrale Robert-Bourassa to the Quebec, Canada
the Barrage of the Three Throats in China
the stopping of the Large-Dixence , in Suisse, more precisely in Were worth, in the valley of Hérens
the Barrage Atatürk on the Euphrate in Turkey
the stopping Daniel-Johnson on the Manicouagan with the Quebec, Canada
the stopping of Nourek (300 m) to the Tadjikistan is most of the world.

Techniques of construction

General information

A stopping is subjected to several forces. Most significant are:

the Hydrostatic push exerted by water on its facing exposed to the water reserve;
the uplifts (Pushed of Archimedes), exerted by water percolant in the body of the stopping or the foundation;
possible forces caused by seismic acceleration.

To resist these forces, two strategies are used:

to build a sufficiently massive work to resist by its simple weight, that it is rigid (gravity dam out of concrete) or flexible (stopping in fill);
to build a stopping able to defer these efforts towards resistant banks or a rock foundation (arch dam, multiple arch dam…)

Elements of calculation

A stopping is subjected to a horizontal force related to the pressure exerted by water on its immersed surface. The hydrostatic pressure p in each point is function height of water above this point.

p = ρ X G X H

with:

ρ : density of water, approximately 1000 kg.m^-3

G : gravitation, approximately 10 m.s^-2

H : height of water above the point considered.

The resulting force F is the sum of the hydrostatic pressures being exerted on the immersed surface of the stopping.

F = $\ int_ {S} ^ {} p.dS$

This formula is not integrated " with the main" for the stoppings with complicated geometry. On the other hand, an analytical expression can be obtained for an element of stopping weight (a " plot" , of width L, and constant immersed height H).

F = ρ . G . L $\ int_ {0} ^ {H} h.dh$

with:

H : total height of the stopping in contact with water.

L : total width of the stud considered.

from where:

F = ρ . G . L . 1/2 . H²

One sees in this formula that the push exerted by water on a stopping increases with the square rise (what is true for any type of stopping). It does not depend to in no case volume of water stored in reserve.

Calculations above relate to only the rigid material stoppings (concrete, masonry…), whatever their type (weight, vault, buttresses…). On the other hand integration by studs can relate to only the stoppings of the type weight or buttresses . For the vaults, efforts being deferred laterally, a calculation by stud not taking into account that the vertical forces does not apply. On the other hand, with regard to the common excavation stoppings (ground, ground, ripraps, fill…), calculations are related with calculations of stability of slope of the slopes which must take into account the state saturated or not with these fill.

Hydraulic studies

In Hydraulique the small-scale Modèle is very much used for the studies of Mécanique of the fluids of the Ouvrage S such as port S, Digue S, Barrage S, etc One uses in these cases the probability of the Nombre of Froude.

Types of stoppings

Stopping weight

A stopping weight is a stopping to which the proper mass is enough to resist the pressure exerted by water. These are stoppings often relatively thick, whose form is generally simple (their section are connected in the majority of the cases to a right-angled triangle.

One counts two big families of gravity dams, the stoppings weight-concrete, and the stoppings in fill (the latter generally not being qualified besides gravity dam, but of stopping in fill).

Even if the stoppings vaults or with buttress require less materials than the stoppings weight, the latter still are very much used nowadays. The concrete gravity dam is selected when the rock of the site (valley, banks) is sufficiently resistant to support such a work (if not, one resorts to the stoppings in fill), and when the conditions to build a stopping vault are not met (cf below). The choice of the technique is thus initially geological: a rather good rock foundation is necessary. But it is also necessary to have the construction materials (aggregates, cement) for.

The technology of the gravity dams evolved/moved. Until the beginning of the 20th century (1920-1930), the stoppings weight were built in masonry (there exists much of stoppings of this type in France, in particular for the water supply of the inland waterways). Later, it is the conventional concrete which was essential. Since 1978, a novel method replaced the conventional concrete. It is about the Concrete Compacted with the Roller. It is a concrete (aggregates, sand, cement, water) with little water, which has a granular consistency and not liquidates. It is set up like a fill, with earthmovers. It presents the main advantage being much less expensive than the traditional concrete.

The stopping of the Large-Dixence in Suisse is a gravity dam.

Stopping arches

The water drive is deferred on the sides of the valley by means of arched cement wall horizontally, and a sometimes vertically (it then is qualified vault with S curve ).

The technique of arch dam requires a rather narrow valley (even if stoppings vaults were sometimes built in rather broad valleys, pushing this technology with its limits) and a good rock of foundation. Even when these conditions are met, the arch dam is often competed with today by the concrete gravity dams or the rock-fill dam, whose implementation can be more mechanized.

By the matter little used, it is obviously a very satisfactory technique economically.

However, the greatest catastrophe of stopping lived in France (Malpasset, with the top of Frejus, the December 2nd 1959) related to an arch dam in the course of setting in water; it is the foundation (and not stopping itself) which did not support the efforts applied by reserve.

Before this accident (and, for some, today still), the vault is regarded as surest of the stoppings. Malpasset is the only known case of rupture of an arch dam.

One meets also stoppings with several vaults like the stopping of Hongrin in Suisse.

Stopping buttresses or multivoûtes

When the supports are too distant, or when the local material is so compact that an extraction proves almost impossible, the technique of the stopping with buttresses makes it possible to carry out a stopping with great saving in materials.

The flat wall or multivoûtes (Vézins, Migoëlou or Bissorte) out of concrete is pressed on reinforced concrete buttresses embedded in the foundation, which defer the water drive on the lower foundations and banks. One of the examples most important of this type is the stopping Daniel-Johnson with the Quebec, Canada.

Mobile needle stoppings

The mobile stopping or with constant level , has a limited height; it is generally built downstream from the course of the rivers, preferably at the place where the slope is weakest. One generally uses this type of stopping in the installation of the estuaries and the deltas.

According to the type of construction mobile stopping perhaps: thumb|250px|right|System White beet: ''' 1 ''' =aiguille, ''' 2 ''' =appui, ''' 3 ''' =passerelle, ''' 4 ''' =fermette, ''' 5 ''' =pivot, ''' 6 ''' to =heurtoir, ''' 7 ''' =radier

the pin weir , creates by the engineer Charles Poirée into 1834 who, taking as a starting point the old the sluice, extended the system over all the width of the course; improving considerably the river navigation as of half of the 19th century.

the system Poiré consists of a curtain of vertically put beams side by side barring the bed of the river. These beams or needles of a section from 8 to 10 cm and long from 2 to 4 m, according to the stoppings, come to rest against a stop (or stop) of the to erase (on the bottom) and on a metal footbridge consisted of sluice-gate trusses .

These sluice-gate trusses can swivel to be erased on the bottom in the event of raw and to leave the unrestricted passage to water. The sluice-gate trusses are connected between them by a bars support which retains the needles and a bars meeting , moreover they constitute the footbridge of operation.

the needles at their top present a form which allows an easy seizure. Nevertheless it is a tiresome, long and dangerous work (it takes several hours and the work of several men to conclude the task). Although this type of stopping is replaced by more modern and automatic techniques; on certain still existing stoppings, the wood needles are replaced by aluminum needles filled of Polystyrène (for buoyancy in the event of fall in the river), with a weight quite less and more easily manoeuvrable.

has obliteration on the river bed (Seuil (stopping)) to allow the total flow or intermediate position to create a Déversoir.

Mobile stoppings with leaf

has leaf or carries to vertical axis , like the Dutch stoppings modern of (Maeslantkering), or them doors with the Léonard de Vinci closing the port-channel of Cesenatico to prevent the strong tides from invading the grounds.

has leaf with horizontal axis with possibility of escaping into air when the flow becomes critical, which avoids constituting an obstacle with the water run-off in times of rising. This type of stopping is generally employed to prevent salt water from going up the estuary, as with Volta Scirocco in Italy.

the fixed part corresponds to a platform (or foundation raft) tight.
large a Valve with sector , which in position of total closure determines a leaf which is pressed on the platform, while in position of complete rising, it leaves the completely free flow.
a shutter valve, assembled on the generator higher of the valve than sector, which makes it possible to regulate the flow in the Déversoir and the upstream desired water level of the stopping.

the water run-off can occur by the lower part of the leaf when the lower valve with sector is raised (what also makes it possible to clean the surface of the platform), or by the top in Déversoir, when the valve higher than shutter is lowered.

mobile Stopping with gravity , of a very simple operation theoretically, the valve with gravity comprises only few machine elements. It is about a leaf, kind of hollow envelope articulated around a hinge fixed on a concrete base.

In position rest the envelope fills of water and goes down from its own weight on the foundation raft.
In active position, of the air injected drives out water and makes it possible the leaf to go up by gravity. The height depends on the quantity of air insufflated.
Such a process is in application in the Projet Mose which must protect the Lagune from Venice of high waters of the Adriatic (Acqua Alta).

mobile Stopping with valves , of an operation comparable with the stopping with mobile with gravity above with the difference close which it is driven by two hydraulic actuating cylinders located on both sides valve. It respects its function perfectly: to control the flow of the river to maintain a level appreciably constant in the level upstream. Its principal disadvantage is to be excessively dangerous for the nautical tourist. The fish can go up it only when the river is in high waters and the completely lowered valve.

Stopping in fill

One calls stoppings in fill all the stoppings made up of a common excavation, that it is very fine (clay) or very coarse (ripraps).

This family gathers several categories, very different. The differences come from the types of materials used, and the method employed to ensure the sealing.

The homogeneous stopping is a stopping in fill built with a sufficiently tight material (clay, silt). It is the oldest technique of stoppings in fill.

The stopping with argillaceous core comprises a clay core (which ensures the sealing), shouldered by refills made up of more permeable materials. This technique has at least two advantages on the homogeneous stopping: (1) the materials of refill are more resistant than the argillaceous materials, one can thus build stiffer slopes and (2) one controls best the flows which percolent in the body of the stopping.

Some cousins of the stoppings with core: stoppings in fill with tight central wall (trench wall slurry out of concrete, asphaltic concrete wall).

More recent, the family of the stoppings with mask upstream. The sealing is ensured by a " masque" , built on the face upstream of the stopping. This mask can be out of reinforced concrete (it currently builds many and very dam out of ripraps with reinforced concrete mask), out of concrete asphaltic, or made up of a thin membrane (most frequent: membrane PVC, bituminous membrane).

The dam Mattmark in Suisse is an example of this type of stopping. In France, the stopping of Greenhouse-Ponçon (second greater reserve of Europe) is a stopping in fill.

Other types of stoppings

There exist other categories of stoppings, in general of more reduced size.

The stoppings of mining deads are stoppings built with residues of mining to create a storage section of these deads. The stoppings are progressively assembled exploitation of the mine. They are connected with the stoppings in fill.

The stoppings of mountain are works intended to fight against the effects of torrential erosion. They are works built across the torrents. They can stop (partially or completely) solid transport; they can also fix the profile longitudinally talweg by decreasing the aggressiveness of the flows.

Life of the stoppings

Maintenance of the stoppings

A stopping is not a simple more or less solid wall. It is not inert and makes the seismological monitoring object and technique under several criteria. The work saw, works and is tired according to the efforts to which it is subjected.

For reasons of maintenance of the works, the stoppings are regularly inspected. Each year, the appearance of the stopping is examined, and periodically (every 10 years in France) the water reserve is emptied in order to give access at the same time to the lower part of the work and equipment (pipeline water, grids, valves, etc).

The interesting works the Public safety are also auscultated, by sensors making it possible to measure their behaviors (measurements of displacements, of pressure of water, flow…). On its state safety depends on the populations installed downstream.

For as much the probability of rupture is extremely weak: statistically, a rupture per annum on a world park of 16.000 stoppings, China excluded. In Europe, the probability is even lower. In fact the danger is highest at the time of the first filling, the risk being however much less low for the concrete works than for those in fill.

In France, the stoppings built in the Alps, in the Years 1950 and 1960, to most extremely of the golden age of the Hydro-electric power, arrived today in a phase of ageing which requires increasingly high expenses of maintenance. EDF estimates that the majority of the hydraulic works reach only half of their life expectancy but announced an important investment plan for maintenance and the rehabilitation.

Catastrophes

A design defect or of maintenance can lead to a catastrophe: if the stopping yields whereas the water reserve is relatively important, a tidal wave can break on the populations living downstream, more or less channeled by the topography of the river on which the stopping was established. (see the article Catastrophe). In France, such a catastrophe took place in 1959 close to Frejus, with the Barrage of Malpasset.

The film the Madness of the men (2001), reports the vexations of the Barrage of Vajont, in Italy, with the beginning of the year 1960. Drawn from an actual fact, the film shows the causes and the sequence of the events which led to a landslide of 270 million cubic meters in water of the lake of reserve of the stopping. The gigantic wave which followed made 2.000 victims, the October 9th 1963.

Seisms

The seisms belong to the events likely to harm the stability of the stoppings.

However, historically, the ruptures caused by seisms far from many are compared with those due to design defects.

In France, dam is the data-processing simulation object of behavior in the case of the strongest seism known in the area (often estimated according to old strong documents). Thus the seism of reference in the Pyrenees east that of 1634, magnitude estimated at 8). Such a seism would cause catastrophic damage in the cities of the Western South, but would thus be supported by all dam.

The most frequent ruptures related to works in fill of moderated size, built with materials sandy or silteux, or based on grounds of this nature; it can indeed develop in this case a phenomenon called liquefaction, which makes lose any resistance to sand or the saturated silt.

Dismantling of the stoppings

The dismantling of a stopping is not business of ecology, but of attack of the limit of life of the stopping, even if that allows, partly to the river ecosystems to function in a more satisfactory way. The initial investment carried out by the manufacturer, always to satisfy a need of public service (drinking water, irrigation, electricity) with means of sustainable development, do not have vocation to be abandoned or destroyed. One will note the absence of financing of these dismantlings for the piscicultural use (primarily of leisure), and the absence of planning of durable means of replacement of the energy production thus lost.

In France, the Barrage of Poutès (Haute-Loire) could thus be dismantled. The first hydroelectric stopping to be dismantled is that of Kernansquillec with Plounévez-Moëdec in the Coasts of Armor. “In 1996, the demolition of the hydro-electric stopping, a first in France on a river with salmons, allowed the landscape absorbed to remake surface” http://www.riviere-du-leguer.com/pdf/35-LES_MOULINS_REMARQUABLES.pdf Source.

In the same way, because not satisfying more the obligations of public safety, the dam Pinay (drinking water, communal control of work) with St Chamond was put in safety in 2000 per boring of a sluice at the foot of the stopping.

Environmental consequences

A stopping can prevent the migration of watery species between the upstream and the downstream: obligatory in certain countries since a few years on the new works (in France, on the classified rivers " migrateurs" since the Law " Pêche" n° 84-512 of June 29th, 1984), the scales with fish are not always present, in particular for the old works or on the rivers where the presence of migrating species is not identified, and are thus not classified (these works were not made compulsory). Reciprocally, certain works are equipped without obligation, by the will of the owner. Moreover, certain scales with fish badly built can appear not very effective.

An alternative solution was carried out on the Garonne between Carbonne and Camon, where the sequence of five important stoppings would have required expensive equipment, and a way very testing for the migrating one. The fish are thus " piégés" at an end of the chain, identified and transported by tanker at the other end.

The ecosystem of an important zone is affected at the time of the installation of a stopping because of the flood of the zone in Amont, of the modification of the mode of water run-off of the zone in Aval and the modification of the water quality caused by reserve. A natural and balanced ecosystem is reconstituted in these zones more or less quickly (in the space of approximately 30 years, the ecosystem would be recreated with 99 %, this including the old drained zones). Nevertheless, if it is true that an ecosystem is recreated, it is never identical to that of origin: the disappearance of the currents upstream, and the very strong reduction in the flow downstream, generally causes the disappearance of certain species autochtones. On the contrary, a storage reservoir can also have positive effects: reception of migratory birds, places of reproduction of the piscicultural and invertebrate species, sites favorable to the fishing of leisure or, in certain cases, improvement of the conditions of flow in low water level. More and more, the hydroelectric stoppings take part in a support of low water level, making it possible to keep an estival life in the rivers, to mitigate the many taking away (authorized or not), to improve cooling of water, and the dilution of pollution downstream.

Since the same Law Fishes of 1984, all the obstacles on the French rivers must obligatorily leave in the river 1/40 of the module (average of flow), and 1/10 for all the new works or whose title is renewed. In order to put an end to this uneven situation (posing many problems of variation of the flows on the same river), the new Law on Water and Aquatic environments (" LEMA" , Loi n°2006-1772 of December 30th, 2006) fixed at January 1st, 2014 the deadline of delivery of 1/10 for all the works.

This LEMA however introduces the exception of the dams with a high head, ensuring the support of the electrical communication, to which the reserved flow could be limited to 1/20 (a list having to be fixed by decree). In the same way, on justification by an adapted study, the flow could be modulated over the year (reserved mode).

On the ecological level, the assessment would be very in favor if any positive effect were not mentioned (which is nevertheless the reason of their construction). Namely:

production of an renewable energy, without production of gas with greenhouse effect, only able to ensure the variation of production imposed by the consumer
restitution integral of water, alone its potential energy being used.

For the stoppings of irrigation or drinking water, the benefits for the man are also obvious, and must thus be weighed on the same basis, if not more, that the benefits or disadvantages carried in the aquatic environment or the fishing of leisure.

Knew?

the oldest stopping known, a 115 m length, was built in the valley of Garawi in Egypt towards 3000 av. J.C
As of 560 ap J.C., the Byzantine historian Procope de Césarée mentioned arch dam upstream in masonry (stopping of Daras).
the first arch dam modern was built by François Zola, father of Emile Zola, between 1843 and 1859 close to Aix-en-Provence.
At the 16th century, the Spaniards carried out dam in masonry. Most remarkable was that of Tibi, with 18km in the north of Alicante builds in 1594. High of 45 m, it is still used.

See too

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