EP0434521B1 - High water spillway for barriers and similar works - Google Patents

Abstract

For the purpose of effecting a quasi-permanent raising of the normal water level of an impounded reservoir and thereby augmenting its storage capacity except during the passage of major floods, the invention consists of installing on the sill of the spillway a water level raising means comprising at least one heavy element, the said means or water level raising elements being capable of resisting the water loads when spilling moderate heads (for discharging the floods of shorter recurrence intervals) by virtue of their own weight but breaching by overturning at a predetermined head corresponding to a level not higher than a predetermined maximum water level in order to discharge larger floods.

Description

The present invention relates to a spillway spillway for dams and similar structures, of the type comprising a spillway threshold whose crest is located at a first predetermined level lower than a second predetermined level corresponding to a maximum level or highest water level , for which the dam is designed, the difference between said first and second levels corresponding to a predetermined maximum flow of an exceptional flood, and a mobile rise closing the weir. Such a type of structure is generally known in the technical field under consideration.

The current state of practice in the design and construction of dams with overhanging thresholds means that these structures must be dimensioned for flood conditions (millennial for example) leading to significant heights of overhanging blade (of the order of 1 to 5m depending on the works).

With an equal dimensioning of the bodies of evacuation of the floods, the barrier with free overflow threshold offers compared to a work provided with valves the best safety vis-a-vis the hydrological hazard, which remains one of the major risks for the dams.

In return, the adoption of a completely free overhanging threshold leads to a loss of the useful retaining edge corresponding to the maximum height of the overhanging blade, that is to say the aforementioned difference of said first and second predetermined levels. This loss can represent, in particular for small or medium-sized works, a significant part of the useful volume of the reservoir (this part can reach or exceed 50%).

• 1°/ augmenter de façon quasi-permanente la capacité de stockage d'un barrage à seuil déversant libre;
• 2°/ maintenir et/ou accroître la sécurité de fonctionnement propre aux ouvrages à seuil déversant, en permettant de façon fiable le passage des crues exceptionnelles, tout en tolérant un déversement des crues de faible ou moyenne importance, sans intervention extérieure et sans modification majeure de l'ouvrage.

The problem that the present invention seeks to solve can be summarized by the following two main objectives, which can be sought simultaneously or alternatively:

• 1 ° / almost permanently increase the storage capacity of a dam with a free overflow threshold;
• 2 ° / maintain and / or increase the operational safety specific to overflowing threshold works, by reliably allowing the passage of exceptional floods, while tolerating a spill of low or medium-sized floods, without external intervention and without major modification of the book.

Various devices have already been proposed and currently exist to increase the storage capacity of a reservoir. In majority, these devices are essentially constituted by valve systems, which close the overflow threshold when the valves are closed. The valves, of whatever nature, conventional or inflatable, of automatic or manual operation, are generally of a fairly high investment cost and they require periodic maintenance and maneuvers. They also require continuous human monitoring or a controlled mechanism reacting to the water level of the reservoir, a mechanism which is often expensive and sophisticated and which is never completely immune to failure. Finally, for an equal discharge capacity, the operational safety and reliability of a gated structure is lower than that of a structure with a free overflow threshold (not gated).

Some devices exist, which temporarily increase the storage capacity of a reservoir, such as sandbags or cofferdams (also called flash boards). However, these devices remain of limited scope and, because they require human intervention prior to each flood, they present a significant operating hazard.

There is also, on certain large embankment dams, a section of fusible dam, leveled at a lower coast than that of the rest of the structure and operating according to the principle of the erosion of its constituent materials, erosion which is generated by a extreme rise in the level of the reservoir during a flood of very exceptional importance. The purpose of this fusible dam is to avoid the uncontrolled and catastrophic discharge of an extreme flood on the whole of a structure, by concentrating the effects of the flood on a section specially designed to break by erosion and thus offer additional evacuation capacity. After the rupture of the fusible dike, major repair work would be necessary to allow normal operation of the structure again.

To the knowledge of the applicant, it therefore seems that no existing device satisfactorily meets the objectives indicated above, with simple operation and for a moderate investment cost.

According to the present invention, the above-mentioned problem is solved by the fact that said rise comprises at least one rigid and massive rise element, which is placed on the crest of the overflow threshold and is held in place thereon by gravity, said element having a predetermined height, which is smaller than the difference of the first and second predetermined levels and which corresponds, for a water level substantially equal to said maximum level, to an average flood having a predetermined flow lower than said predetermined maximum flow, said riser being dimensioned in size and weight so that the moment of the thrust forces applied by water to the riser reaches the moment of gravity forces which tend to hold the riser in place on the threshold overflowing, and that consequently said rising element is unbalanced and driven out when the water reaches a third predetermined level higher than the top of the rising element, but at most equal to the second predetermined level.

Under these conditions, it is clear that the storage capacity of the dam is increased by an amount corresponding to the height of the raising element. The raising element (s) can be manufactured at a very moderate cost compared to the valves and, in the case where they are installed on the overflow threshold of an already existing dam, this installation can be done without the need for d '' make major modifications to the overflow threshold of the dam as we will see below. It is also clear that for medium-sized floods, as long as the water level does not reach said third predetermined level, which can be determined so as to be in practice equal to or slightly lower than said second predetermined level (maximum level or highest water level), the water will be able to pass over the said elevating element or elements to evacuate the flood, without this resulting in destruction of the elevation and, consequently, without it this results in a decrease in the increased storage capacity of the dam. On the other hand, if, in the case of an exceptional flood, the water level reaches said third predetermined level, the rising element or elements are automatically unbalanced and driven by the water, under the sole action of the thrust forces water, therefore without any external intervention, thus restoring the discharge threshold to its full evacuation capacity corresponding to the maximum height of the discharge blade for which the dam was designed.

Although, theoretically, this is not absolutely essential, a stop of predetermined height is preferably provided on the threshold overflowing at the foot of the raising element, on the downstream side thereof, to prevent it from sliding downstream on the threshold, without however preventing it from tipping over the stop when the water level reaches said third predetermined level. Of course, in this case, the height of the stop is taken into account as will be seen below for the dimensioning in size and weight of the raising element or elements.

A seal may be disposed between the overflow threshold and the base of the riser, near the upstream edge of said base. However, such a seal is not absolutely essential if, in the absence of a seal, water leaks between the raising element and the overflow threshold are low and if the area of the overflow threshold on which the said elevation element (s) rests is suitably drained so that no appreciable underpressure can be established under the said elevation element (s). On the other hand, as will be seen below, means may be provided for automatically establishing an underpressure under the said raising element or elements when the water level reaches said third predetermined level, in order to promote the imbalance and the tilting of said or said rising elements when it becomes essential to evacuate an exceptional flood.

The invention can be applied both to the spillway of an existing dam and to that of a dam under construction. In the first case, the crest of the overhanging threshold is preferably leveled at a level lower than said first predetermined level and the one or more rising elements are placed on the leveled threshold. In this case, the storage capacity of the dam can be kept equal to that which it had before the overflow threshold was lowered, or it can be increased depending on whether the elevating element (s) is given a height such that its or their apex finds at said first predetermined level, or at a level higher than this, but lower than said third predetermined level. Whatever the height of the elevation element (s), within the limits indicated above, greater security is obtained than with the undisturbed overflow threshold, since the opening which is obtained after tilting of the element (s) the rise has a greater height than in the case of an undisturbed overflow threshold, thus making it possible to evacuate a higher flood flow than the maximum flow of the exceptional flood for which the dam was originally designed.

Similarly, in the design of a new dam, it will be possible to adopt a greater difference between the first and second predetermined levels (which contributes to increasing safety) without fear that this would lead to a reduction in the storage capacity of the dam, since this storage capacity can be maintained, or even increased, without reducing security, by providing one or more augmentation elements in accordance with the present invention.

In the case where several rising elements are provided, each rising element or a group of rising elements can be dimensioned so as to tilt for a predetermined water level lower than that at which another element or group of elements rise will tilt, the latter itself being dimensioned so as to tilt for a lower water level than that to which a third element or group of rise elements will tilt, and so on. In this way, a progressive increase in the evacuation capacity is obtained, if necessary, depending on the extent of the flood.

It will also be noted that, if one or more rising elements have been tilted and driven out by a flood exceptional, they can be easily and economically replaced by other elements of rise, without having to carry out important repairs, after the flood has been evacuated.

Other characteristics and advantages will appear during the description which follows of various embodiments of the present invention given by way of example, with reference to the appended drawings in which:

Figure 1 is a perspective view showing a structure, such as a dam, and its spillway spillway with free overflow threshold, to which the invention can be applied.

Figures 2a and 2b show, in vertical section and on a larger scale, the crest of the free overflow threshold of the dam of Figure 1 for two different water levels.

Figure 3 is an elevational view of the weir of Figure 1, seen from the downstream side and equipped with a fusible link according to the present invention.

FIG. 4 is a plan view of the weir of FIG. 3.

FIGS. 5a to 5e are views in vertical section making it possible to explain the operation of the fuse riser of the present invention, before, during and after the passage of a flood.

Figure 6 is a graph showing the various forces which, in service, can be applied to a riser according to the present invention.

FIG. 7 is a graph representing the variations of the moments of the driving and resistant forces as a function of the height of water above the overflow threshold, as well as the variations of the flow of water discharged as a function of the height of the overflow blade.

Figures 8a to 8c are cross-sectional views for comparing heights maximum of overhanging blades in the case of the present invention for rising elements having different heights (FIGS. 8a and 8b) and in the case of a known free overhanging threshold (FIG. 8c).

Figure 9 is a vertical sectional view showing a lifting element of the present invention, which is associated with a triggering device for tilting.

FIGS. 10a to 10c show, on a larger scale, various protective devices which can be provided at the upper end of the trigger device of FIG. 9.

Figures 11a to 11g show, in perspective, various embodiments of a rising element according to the present invention.

Figures 12 to 14 show, in vertical section, other alternative embodiments of the raising element of the invention.

FIG. 15 shows, in perspective, a detail of the rising element of FIG. 14.

Figure 16 shows, in perspective, another embodiment of the raising element of the invention.

FIG. 17 is a front view, downstream side, of the raising element of FIG. 16.

Figure 18 is a plan view of the riser of Figures 16 and 17.

Figure 19 is a sectional view along line XIX-XIX of Figure 18.

Figures 19a and 19b are views similar to that of Figure 19, showing variants.

Figure 20 is a view similar to Figure 19 showing another variant.

Figures 21 and 22 are plan views showing two other variants.

The structure 1 shown in FIG. 1 can be an embankment dam or a concrete or masonry dam. However, it should be noted that the invention is not limited to the type of dam shown in FIG. 1, but on the contrary it can be applied to any type of known dam with a free overflow threshold.

In figure 1, the reference number 2 designates the crest of the dam, the number 3 its downstream facing, the number 4 its upstream facing, the number 5 a spillway spillway, the number 6 the weir threshold 5 and the number 7 an evacuation channel. The spillway 5 can be located in the central part of the dam 1 or at the end of it or even excavated on a bank without this altering the possibility of using the invention.

For a structure with a free overflow threshold, the level RN of the normal reservoir in operation (see also Figure 2a) is that of the crest 8 of the overflow threshold 6. This level RN determines the maximum volume of reservoir that can be kept by the reservoir formed by the dam. The vertical distance R, called revenge, between the crest 8 of the weir and the crest 2 of the dam is the sum of two terms, namely, on the one hand, an increase h₁ in the water level due to a flood, up to a maximum level RM or highest water level (PHE), allowing the discharge of the maximum flood (figure 2b) for which the structure is dimensioned, and, on the other hand, an additional height h₂ intended to protect the ridge 2 of the dam against the oscillations of the water level at its maximum level RM (effect of wind, waves, etc.).

In a conventional dam with a free overflow threshold like the one shown in FIG. 1, the reservoir portion located between the normal retention level RN and the maximum level RM is not stored and is therefore lost for operation. One of the aims of the invention is to enable the level of normal operation of the reservoir to be raised almost permanently. increase its storage capacity, except during exceptional floods.

To this end, the invention provides for placing on the overhanging threshold 6 an increase 10, constituted by at least one solid element 11, for example five elements 11a-11e as shown in FIGS. 3 and 4, said increase 10 or the elements 11 being able to support, without breaking, the water load corresponding to a moderate spill (allowing the passage of the most frequent floods) while resisting by the effect of gravity, and being made fusible by tilting for a predetermined water load corresponding to a level N at most equal to the maximum level RM and then allowing the passage of the strongest floods.

Of course, the number of elevating elements 11 is not limited to five elements as shown in Figures 3 and 4, but may be smaller or larger depending on the length of the weir 5 (measured in the longitudinal direction of the dam) . Preferably, the number of elevating elements 11 is chosen so as to obtain low unit masses allowing easy installation and replacement of said elevating elements.

Each elevation element 11 is placed on the overflow threshold 6 and is held thereon by gravity. Preferably, each rising element 11 is retained, against any sliding downstream, by a stop 12 located at the foot of the element 11, on the downstream side thereof. The stop 12 can for example be embedded in the threshold 6, as shown for example in Figure 5a, and it can be discontinuous as shown in Figures 3 and 4. However, if desired, the stop 12 could be continuous. As will be seen below, the height of the stop 12 is predetermined, but it can be variable according to the forces involved and according to the water level from which it is desired to initiate the tilting of each rising element 11.

As shown in FIG. 4, a conventional seal 13, for example made of rubber, is provided at each of the two ends of the riser 10 between it and the lateral flanks 14 of the weir 5. When the riser 10 is formed by several elements 11, seals 13 are also provided between the vertical side walls, two by two facing each other, adjacent elevation elements 11 as is also visible in FIG. 4. Preferably, a seal 15 is also provided between the overflow threshold 6 and the base of the elevating elements 11 near the upstream edge 16 of said base as is for example visible in Figures 4 and 5a. Although FIG. 5c represents the seal 15 carried by the raising element 11, the seal 15 could also be installed in a groove provided in the overflow threshold 6. As shown in FIG. 4, the seals 13 and the seal 15 , when the latter is provided, are arranged in the same vertical plane. Instead of providing the joint 15 or in addition to it, a drainage system can be arranged in a known manner in the overflow threshold 6, in the zone thereof underlying upward 10, in order to dry out this area and to avoid that under normal pressure, pressure is applied to the elevating elements 11.

As shown in FIG. 5a, the rise 10 of the present invention makes it possible to raise the level of the normal restraint of the level RN (level of the normal restraint of the free overflow threshold 6, that is to say without the rise 10) up to the level RN 'corresponding to the height of the rise 10 above the threshold 6. As will be explained below, each rise element 11 is dimensioned so as to be self-supporting for a water load below a level predetermined N, itself at most equal to the maximum level RM already mentioned above. Thus, assuming for example that said predetermined level is equal to the level RM, as long as the level water remains below the RM level for low or medium floods and is between the RN 'and RM levels, the water pours over the rise 10 as shown in Figure 5b, without the increase is not destroyed. In this case, after evacuation of the flood, the water level drops to level RN 'or to a lower level if water is drawn into the reservoir.

On the other hand, if the water level reaches, in the aforementioned hypothesis, a predetermined level N equal or slightly lower than the maximum level RM in the case of a strong flood or exceptional flood, at least one element 11 of the rise 10 is unbalanced under the thrust of the water and rocks around the stop 12 as shown in FIG. 5c, and the element or elements 11 which are tilted are evacuated by the water of the flood at least to the bottom of the weir 5, thus allowing the evacuation of the strongest floods. After evacuation of a strong flood having caused the tilting of the rise 10, the overflow threshold 6 is found in the state shown in FIG. 5d, the water level having returned to the level of the normal reservoir RN or to a level even lower. We can possibly provide some spare elements 11, permanently available on the dam site, to allow a repair of the surge 10 if necessary and thus restore the level of the normal retention at level RN 'as shown in Figure 5e . It should be noted, however, that the non-replacement of one or more elements 11 after an exceptional flood which has caused the tilting of at least one element 11 does not reduce the operational safety of the structure.

The risks of malfunction due to floating bodies can be easily eliminated by upstream protection using conventional techniques adaptable to each particular case. Protection can for example be constituted by floating lines on the reservoir, at a certain distance upstream of the weir, or by stop devices fixed on the upstream facing of the dam.

We will now give a numerical example of the design of a fusible link according to the present invention. Usually, the dams and overflow weirs are dimensioned so that the level of the lake (level of the reservoir) reaches the maximum level RM for the exceptional flood envisaged (project flood). This flood may for example be the flood occurring only one year in a thousand (millennial flood).

d'après laquelle on peut voir que H est sensiblement égal à 2 m dans l'hypothèse faite plus haut. Toujours dans cette hypothèse, en l'absence de dispositif de vannes ou de hausses, le niveau du seuil 6 du déversoir 5 est arasé à 2 m en-dessous du niveau maximal RM pour permettre l'évacuation de la crue millénale, et on perd donc un volume utile d'eau correspondant à une tranche de 2 mètres.To fix the ideas, it will be assumed that the flow of this project flood is for example 200 m³ / s and that the free spillway 6 has a length of 40 m. Under these conditions, the height H of the water layer necessary to evacuate the flow of the project flood corresponds to 5 m³ / s per linear meter of threshold. This height H can be calculated by the following formula: $${\text{Q = 1.8 H}}^{\text{3/2}}$$

according to which we can see that H is substantially equal to 2 m in the hypothesis made above. Still in this hypothesis, in the absence of a valve or rising device, the level of the threshold 6 of the weir 5 is leveled 2 m below the maximum level RM to allow the evacuation of the millennial flood, and we lose therefore a useful volume of water corresponding to a 2 meter section.

For the determination of the height of the elevating elements 11, the invention is based on the observation that the maximum flow achieved on average over 20 years is much lower than that of the project flood. It can be around 50 m³ / s in the example chosen here. According to formula (1) this flow then corresponds to a sheet of water having a height of about 0.8 m. If we admit that elevating elements 11 can be destroyed on average every 20 years, we can then give the elevation elements a height of 2 m - 0.8 m = 1.2 m, thus allowing the passage above the elevation elements 11 of a water sheet of 0.8 m height corresponding to the flow 50 m³ / s. In this case, the level of the normal reservoir RN 'is raised to 1.20 m above the level of the normal reservoir RN of the overflow threshold 6 free, that is to say without the elevating elements 11. If we choose elevation elements 11 having a height greater than 1.2 m, the height of the admissible sheet of water will be less than 0.8 m and it will be necessary to allow the destruction of the elevation elements, for example every 10 years , but the level of normal restraint will be further increased. On the other hand, if we choose elevation elements 11 having a height less than 1.2 m, we can admit a sheet of water having a height greater than 0.8 m, the elevation elements then being destroyed than every 50 or 100 years, but the level of normal withholding will then be lower than in the previous cases. The choice of the height of the elevating elements 11 is therefore essentially an economic choice. In general, it is probably desirable to fix the time interval between two successive total destructions of the fusible riser at approximately 20 years, which would lead to a theoretical height of 1.2 m for the augmentation elements in the example considered here.

It is also advantageous that the destruction of all the raising elements 11 does not occur exactly for the same level of water. We can provide for example that a single element such as element 11c of Figures 3 and 4 is destroyed when the water reaches a first level N1 located about 10 cm below the maximum level RM, that at least one other element 11, such as elements 11b and 11d, are destroyed when the water reaches a second level N2 located approximately 5 cm below the maximum level RM, and that the other elements 11, such as the elements 11a and 11e, are destroyed when the water reaches said maximum level RM.

In this way, the destruction of the first element 11c by a medium-sized flood may be sufficient for the flow of the flood without additional rise in the water level, which avoids the destruction of the other elements 11a, 11b, 11d and 11e . However, the margin of 10 cm which is thus taken is added to the maximum permissible overhanging blade height, so that the height of the rising elements and, consequently, the slice of water gained (RN'-RN) becomes equal to 1.1 m (2m-0.8m-0.1m) in the example considered here.

The tilting of the elevation element (s) 11 and, consequently, their destruction depends on the balance between, on the one hand, the motor moment, that is to say the moment of the forces which tend to overturn the element considered, and, on the other hand, the resisting moment, that is to say the moment of the forces which tend to stabilize said rising element. If a trigger device, directly linked to the water level, is not provided to trigger the tilting of the raising element with precision for a predetermined water level, the water height corresponding to the above-mentioned equilibrium cannot be fixed only with a margin of uncertainty of up to 0.2 m. Under these conditions, it is necessary, for safety, to reduce the height of the elevation element (s) 11 by an amount corresponding to this margin of uncertainty, for example 0.2 m. However, it is possible to avoid having to reduce the height of the lifting elements by providing a trigger device which will be described later with reference to FIG. 9.

It is possible, for the flow of 50m³ / s considered in this example, to reduce the height of the maximum permissible overhanging blade to less than 0.8 m before tilting the lifting elements, by ensuring that the crest line of the elevating elements 11, considered individually or together, is no longer arranged parallel to the crest of the overhanging threshold 6, but along a non-straight line, for example a broken or curved line, to lengthen the length of overflow of the aforementioned speed. If this length is doubled, the flow of 50 m³ / s is then distributed over 80 m instead of 40 m and the height of the corresponding maximum admissible blade is reduced from 0.8 m to 0.5 m. This allows, all other things being equal, to raise the height of the elevating elements 11 by 0.3 m and to consequently increase the volume of water stored in the reservoir. Various forms of raising elements allowing the length of the discharge to be lengthened will be described later with reference to FIGS. 11e to 11g.

FIG. 6 shows the various forces which, in service, can be applied to a lifting element 11 of the present invention. For the description which follows, it will be assumed that the element 11 has a parallelepiped shape and has a width L and a height H₁. In FIG. 6, RM designates the maximum level as before, B designates the height of the stop 12 above the threshold 6, H₂ designates the height of the maximum permissible overhanging blade above the elevation element 11 and z indicates the water level. The driving forces which tend to tilt the raising element 11 are the thrust P of the water on the upstream face of the raising element 11 and the underpressure U which is possibly exerted on the base surface. said raising element and which is due to the existence of possible leaks at the seals or to the presence of a trigger device which will be described later. The resistive forces, which tend to stabilize the riser 11, are the sum W of the self-weight of the riser 11 and the weight of the column of water possibly present above said rising element.

• a) si : 0 < z < 3 B : $$\text{P =}\frac{\text{1}}{\text{2}}{\text{. Υ}}_{\text{W}}\text{. z²}$$
  
  $$\text{U =}\frac{\text{1}}{\text{2}}{\text{. Υ}}_{\text{W}}\text{. z . L}$$
  
  $${\text{W = Υ}}_{\text{b}}\text{. H₁ . L}$$
  
  $$\text{Mm = 0}$$
  
  $$\text{MmU =}\frac{\text{1}}{\text{3}}{\text{. Υ}}_{\text{W}}\text{. z. L²}$$
  
  $$\text{Mr =}\frac{\text{1}}{\text{2}}{\text{. Υ}}_{\text{b}}\text{. H₁ . L² +}\frac{\text{1}}{\text{2}}{\text{. Υ}}_{\text{W}}\text{. z² . (B -}\frac{\text{z}}{\text{3}}\text{)}$$
  
• b) si : 3 B < z < H₁ : $$\text{P =}\frac{\text{1}}{\text{2}}{\text{. Υ}}_{\text{W}}\text{. z²}$$
  
  $$\text{U =}\frac{\text{1}}{\text{2}}{\text{. Υ}}_{\text{W}}\text{. z . L}$$
  
  $${\text{W = Υ}}_{\text{b}}\text{. H₁ . L}$$
  
  $$\text{Mm =}\frac{\text{1}}{\text{2}}{\text{. Υ}}_{\text{W}}\text{. z² . (}\frac{\text{z}}{\text{3}}\text{- B)}$$
  
  $$\text{MmU = Mm +}\frac{\text{1}}{\text{3}}{\text{. Υ}}_{\text{W}}\text{. z. L²}$$
  
  $$\text{Mr =}\frac{\text{1}}{\text{2}}{\text{. Υ}}_{\text{b}}\text{. H₁ . L²}$$
  
• c) Si : H₁ < z : $$\text{P =}\frac{\text{1}}{\text{2}}{\text{. Υ}}_{\text{W}}{\text{. <figure-callout id="1" label="H" filenames="imgf0003.png,imgf0007.png" state="{{state}}">H</figure-callout>}}_{\text{1}}^{\text{2}}{\text{+ Υ}}_{\text{W}}\text{. H₁ . ( z - H₁)}$$
  
  $$\text{U =}\frac{\text{1}}{\text{2}}{\text{. Υ}}_{\text{W}}\text{. z . L}$$
  
  $${\text{W = Υ}}_{\text{b}}{\text{. H₁ . L + Υ}}_{\text{W}}\text{. ( z - H₁ ) . L}$$
  
  $$\text{Mm =}\frac{\text{1}}{\text{2}}{\text{Υ}}_{\text{W}}{\text{.<figure-callout id="1" label="H" filenames="imgf0003.png,imgf0007.png" state="{{state}}">H</figure-callout>}}_{\text{1}}^{\text{2}}\text{.(}\frac{\text{H₁}}{\text{3}}{\text{- B) + Υ}}_{\text{W}}\text{.H₁.(z-H₁)(}\frac{\text{H₁}}{\text{2}}\text{- B)}$$
  
  $$\text{MmU = Mm +}\frac{\text{1}}{\text{3}}{\text{Υ}}_{\text{W}}\text{. z. L²}$$
  
  $$\text{Mr =}\frac{\text{1}}{\text{2}}{\text{Υ}}_{\text{b}}\text{. H₁ . L² +}\frac{\text{1}}{\text{2}}{\text{Υ}}_{\text{W}}\text{. (z - H₁) . L²}$$
  

To calculate the values of P, U and W, as well as the values of the corresponding motor and resistance moments with respect to the stop 12, it is necessary to consider several cases as a function of the water height z above the threshold 6 The values of P, U and W and of the corresponding motor and resistive moments are summarized below for the different cases, said values being given per unit of length of the rising element 11.

• a) if: 0 <z <3 B: $$\text{P =}\frac{\text{1}}{\text{2}}{\text{. Υ}}_{\text{W}}\text{. z²}$$
  
  $$\text{U =}\frac{\text{1}}{\text{2}}{\text{. Υ}}_{\text{W}}\text{. z. L}$$
  
  $${\text{W = Υ}}_{\text{b}}\text{. H₁. L}$$
  
  $$\text{Mm = 0}$$
  
  $$\text{MmU =}\frac{\text{1}}{\text{3}}{\text{. Υ}}_{\text{W}}\text{. z. L²}$$
  
  $$\text{Mr =}\frac{\text{1}}{\text{2}}{\text{. Υ}}_{\text{b}}\text{. H₁. L² +}\frac{\text{1}}{\text{2}}{\text{. Υ}}_{\text{W}}\text{. z². (B -}\frac{\text{z}}{\text{3}}\text{)}$$
  
• b) if: 3 B <z <H₁: $$\text{P =}\frac{\text{1}}{\text{2}}{\text{. Υ}}_{\text{W}}\text{. z²}$$
  
  $$\text{U =}\frac{\text{1}}{\text{2}}{\text{. Υ}}_{\text{W}}\text{. z. L}$$
  
  $${\text{W = Υ}}_{\text{b}}\text{. H₁. L}$$
  
  $$\text{Mm =}\frac{\text{1}}{\text{2}}{\text{. Υ}}_{\text{W}}\text{. z². ((}\frac{\text{z}}{\text{3}}\text{- B)}$$
  
  $$\text{MmU = Mm +}\frac{\text{1}}{\text{3}}{\text{. Υ}}_{\text{W}}\text{. z. L²}$$
  
  $$\text{Mr =}\frac{\text{1}}{\text{2}}{\text{. Υ}}_{\text{b}}\text{. H₁. L²}$$
  
• c) If: H₁ <z: $$\text{P =}\frac{\text{1}}{\text{2}}{\text{. Υ}}_{\text{W}}{\text{. <figure-callout id="1" label="H" filenames="imgf0003.png,imgf0007.png" state="{{state}}">H</figure-callout>}}_{\text{1}}^{\text{2}}{\text{+ Υ}}_{\text{W}}\text{. H₁. (z - H₁)}$$
  
  $$\text{U =}\frac{\text{1}}{\text{2}}{\text{. Υ}}_{\text{W}}\text{. z. L}$$
  
  $${\text{W = Υ}}_{\text{b}}{\text{. H₁. L + Υ}}_{\text{W}}\text{. (z - H₁). L}$$
  
  $$\text{Mm =}\frac{\text{1}}{\text{2}}{\text{Υ}}_{\text{W}}{\text{.<figure-callout id="1" label="H" filenames="imgf0003.png,imgf0007.png" state="{{state}}">H</figure-callout>}}_{\text{1}}^{\text{2}}\text{(}\frac{\text{H₁}}{\text{3}}{\text{- B) + Υ}}_{\text{W}}\text{.H₁. (Z-H₁) (}\frac{\text{H₁}}{\text{2}}\text{- B)}$$
  
  $$\text{MmU = Mm +}\frac{\text{1}}{\text{3}}{\text{Υ}}_{\text{W}}\text{. z. L²}$$
  
  $$\text{Mr =}\frac{\text{1}}{\text{2}}{\text{Υ}}_{\text{b}}\text{. H₁. L² +}\frac{\text{1}}{\text{2}}{\text{Υ}}_{\text{W}}\text{. (z - H₁). L²}$$
  

In the above-mentioned formulas, P, U, W, L, H₁, B and z have the meanings already indicated above. Mm is the motor moment in the absence of underpressure U, MmU is the motor moment in the presence of an underpressure U, Υ _w is the density of water and Υ _b is the average density of l 'rising element.

In the graph in FIG. 7, the plots A, C and D respectively represent the variations of Mr, Mm and MmU as a function of the water height z above the threshold 6, and the plot E represents the variation of the flow of discharged water Q as a function of the height H of the overflow blade [Q = 1.8. H ^3/2 , H being equal to (z - H₁) before tilting of the rising element 11 and to z after tilting of said element]. Traces A, C, D and E were obtained from the formulas indicated above and for H₁ = 1.2 m, L = 1.1 m, B = 0.15 m, Υ _w = 1 and Υ _b = 2.4.

By considering the plots A and C, we see that the motor moment Mm (without underpressure U) reaches the same value as the resistive moment Mr for a value of z approximately equal to 2.4 m. In other words, in the absence of an underpressure U, the tilting of the raising element 11 will occur when the water level reaches a height of 2.4 m above the threshold 6. Similarly , considering the plots A and D, we see that in the presence of an underpressure U, the motor moment MmU reaches the same value as the resistive moment Mr for a value of z of about 2 m, that is ie for the maximum level RM in the numerical example considered here. In other words, in the presence of an underpressure U, the tilting of the raising element 11 will take place when the level of the water will reach the maximum level RM. From formulas (17) and (19), we see that if we had wanted that, in the absence of U-pressure and without changing the value of the height H₁ of the rising element 11, the tilting of the latter occurs for a value of z equal to 2 m, therefore for the maximum water level RM, it would have been necessary to decrease the value of Υ _b and / or the value of L and / or the value of B compared to the values indicated above.

From the above, it can be seen that, by an appropriate dimensioning in size and weight of the lifting element 11 and by an appropriate dimensioning of the stopper 12, it is possible to cause the raising element 11 to tilt for a predetermined water level. It can also be seen that if the raising element 11 has been dimensioned to switch to a predetermined water level in the absence of an underpressure at its base and if the seal between the raising element and the threshold 6 is not perfect, an underpressure will be exerted on the base of the raising element, which will cause it to tip over for a water level below the aforementioned predetermined water level. A leak is therefore not catastrophic but rather constitutes a safety factor insofar as it helps the tilting of the lifting element.

This can be used to bring about the tilting of the raising element 11 in an even safer and more precise manner with regard to the level of water at which the tilting occurs. Indeed, it may be advantageous to take measures so that the underpressure U applied to the raising element remains zero or very low as long as the water level remains below a predetermined level, and so that a substantially higher value pressure is abruptly applied to the rising element 11 the instant the water level reaches said predetermined level, the dimensioning of the elements being such that at this instant the motor moment suddenly passes from a value Mm a little smaller than the value of the resistant moment Mr to a value MmU substantially greater than the value of said resistive moment M. For this purpose, it is possible for example to use a trigger device such as that shown in FIG. 9. The trigger device shown in FIG. 9 essentially consists of a vent pipe 21 which, in service normal, puts the area underlying the rising element 11 in relation to the atmosphere, the upper end 21a of the vent pipe 21 being situated at a level N equal to the level for which it is desired that the tilting of the elevation element 11 occurs. The pipe 21 can be straight and pass through the raising element 11 as shown in solid lines in FIG. 9, or it can be bent as shown in phantom in 21 'in FIG. 9, so that its end upper is offset upstream relative to the elevating element 11, or the vent pipe can be partially embedded in the threshold 6 as also shown in phantom in 21 '' in Figure 9. In the case where several rising elements 11 are provided and must tilt for different water levels, such as the levels N₁, N₂ and RM (FIG. 3) at least one vent pipe 21 is associated with each raising element and each pipe 21 extends upwards to a level N equal to the level N₁ or N₂ or RM for which the corresponding element must tilt. Naturally, in this case, the zones of the threshold 6 which are underlying the raising elements which have to tilt for different water levels, must be isolated from each other by the appropriately arranged seals.

The upper end of each vent pipe 21 can be fitted with a protection device against the floating bodies, so as not to be blocked by them, or a wave protection device, so that one or more successive waves do not inadvertently trigger the tilting of the elevating element 11. Such protective devices are shown in Figures 10a to 10c. The protection device of FIG. 10a essentially consists of a funnel 22, the upper edge 23 of which is at a level higher than the level N and which comprises at least one small hole 24 at a level lower than the level N. In FIG. 10b, the protection device consists of the pipe 21 itself, the upper end of which is bent in the form of a siphon 25. Finally, the protection device of FIG. 10c consists of a bell 26 , which covers the upper end 21a of the vent pipe 21 and the apex 27 of which is at a level slightly higher than the level N.

It may be advantageous, in order to improve the safety of an existing structure, the overflow threshold 6 of which was initially leveled, depending on the project flood initially chosen, at a level determining the level of the normal reservoir RN (FIG. 8c), to level the threshold 6 a few decimeters below its current coast (corresponding to RN) and to place on the leveled threshold 6 a fuse increase 10 in accordance with the present invention, composed of at least one increase element 11 dimensioned in size and weight as described above to switch around the stop 12 when the water level reaches a predetermined level at most equal to the maximum level RM corresponding to the project flood. Under these conditions, the probability of opening of the increase 10 is not modified but, in the event of an exceptional flood, the flow section available after total destruction of the increase 10 is notably increased for the same water level in restraint, this which makes it possible to safely pass a flood having a flow rate much higher than that of the flood for which the structure was originally dimensioned. In the case where the height chosen for the elevating elements 11 is equal to the leveling height of the threshold 6 (FIG. 8a), one simply obtains an increase in the safety of the structure, without changing the level of the normal reservoir RN , compared to the existing structure before the threshold 6 is lowered (Figure 8c). However, it is possible both to increase the safety of the structure and to raise the level of the normal restraint to a level RN 'by giving the elevating elements 11 a height such that their apex is at a level higher than the level RN, but below the maximum level RM (Figure 8b).

In the above description, it has been assumed that each raising element 11 is constituted by a block having roughly a parallelepiped shape. The block 11 can be a monolithic block, of reinforced or unreinforced concrete, with a flat upper face (FIG. 11a) or curved (FIG. 11b). According to another embodiment, each raising element 11 can be constituted by a hollow block as shown in FIG. 11c, comprising one or more cells filled with ballast 32, such as for example sand, gravel or other materials weighing in bulk. A cover (not shown) can be provided for closing the cell or cells 31 after they have been filled with ballast. The embodiment of FIG. 11c is particularly suitable when the rise 10 must comprise several rise elements all having the same height, but having to tilt for different water levels. In this case, it suffices to adjust the weight of each of the elevating elements 11 by an appropriate amount of ballast to obtain the tilting of the element of corresponding increase for the desired predetermined water level.

According to another embodiment of the present invention, each raising element 11 may be constituted by an assembly of plates, made of concrete, steel or any other suitable rigid and heavy material. As shown in Figure 11d, the plate assembly may include a rectangular base plate 33, horizontal or substantially horizontal, and a rectangular plate 34, vertical or substantially vertical, which rises from the downstream edge of the base plate 33. It will be noted that, in this case, the weight of the water column situated above the base plate 33 contributes, as a resisting force, to stabilize the rising element as long as the water level n 'has not reached the predetermined level at which the tilting of said rising element occurs.

As shown in FIGS. 11e to 11g, the assembly of plates can comprise several substantially rectangular 34, vertical or substantially vertical, plates which are joined by their lower edge to the base plate 33 and which are joined in pairs by their edges vertical so as to form a sort of screen. All the plates 34 have the same height, but they can have the same width (FIG. 11e) or different widths (FIGS. 11f and 11g). In this case, each rising element has a non-rectilinear crest line, for example a sawtooth line (figure 11e), or a truncated sawtooth line (figure 11f) or even a crenelated line (figure 11g). Unlike FIG. 11d, in which the raising element 11 is seen from the downstream side, in FIGS. 11e to 11g, the raising element 11 is seen from the upstream side. The embodiments shown in FIGS. 11e to 11g are interesting because they make it possible to increase the length of discharge, which, for a same water level, reduces the height of the overflow blade necessary for the evacuation of the lowest flood flows, therefore the most frequent, without causing the destruction of the rise and without compromising safety, as has already explained above. In addition, this makes it possible to correspondingly increase the height of the lifting elements and, consequently, to the same extent the level of the normal restraint. For example, a slot arrangement like that of FIG. 11g, tripling the length of discharge, makes it possible to reduce by half the height of the discharge blade at low flow rates, which allows a corresponding increase in the storage capacity of the reservoir without reduce the possibility of evacuation of exceptional flood flows.

Instead of using 34 flat plates, one could also use curved or corrugated plates to increase the length of discharge.

FIG. 12 represents, in vertical section, a rising element 11 similar to those of FIGS. 11d to 11g, equipped in addition with a vent pipe 21 having the same function as that of FIG. 9. In FIG. 12 , the horizontal plate 33 is fixed to the vertical plate 34 so as to be at a distance above the threshold 6, and it comprises, on the upstream side, a flange 33a directed downwards. The seal 15 is disposed between the flange 33a and the threshold 6. Below the plate 33 is thus formed a chamber 35, into which the pipe 21 opens at its lower part. An orifice 36 is provided at the base of the plate 34, the orifice 36 having a smaller section than that of the pipe 21.

With the rising element of figure 12, when, in service, the water level is close to the level N, but lower than this one, the possible waves on the surface can cause water entries in the pipe 21. These water inlets will partially fill the chamber 35 which, at the same time will empty through the orifice 36. This avoids that an underpressure is applied to the plate 33 because of the waves, as long as the water level has not reached the level N at which it is desired that the tilting of the raising element 11 occurs. The chamber 35 and the orifice 36 therefore make it possible to increase the precision of the level at which the tilting occurs. Of course, it is possible to provide under the element 11 of FIG. 9 a chamber similar to the chamber 35, as well as a drainage orifice for this chamber similar to the orifice 36.

Figure 13 shows, in vertical section, an elevation element 11 composed of several modules 11g to 11j which are stacked on top of each other. Preferably, the modules have shapes such that they fit into each other so as not to slide relative to each other, in service, under the pressure of the water. The modules can all have the same vertical dimension or different vertical dimensions; for example, the upper module 11j has a smaller vertical dimension than those of the other modules. With such a construction of the rise element, not only are the operations for setting up the rise easier, but it is also possible to give the rise different heights depending on the season, without requiring special human supervision .

FIG. 14 shows a modular elevating element 11 like that of FIG. 13, but formed by an assembly of plates 33, 34 and 37. The plates 33 and 34 are rigidly fixed together, while the plate 37 can be mounted removably on the plate 34 to enhance the latter. The plates 34 and 37 can be held together by at least two pairs of plates 38, one pair of which is visible in FIGS. 14 and 15, and which are rigidly fixed to one of the two plates 34 and 37. Instead of the plates 38 one can also use bars extending over the entire length of the plates 34 and 37. A seal 39 is provided between the plates 34 and 37. Of course, instead of having only two vertical plates 34 and 37 , more can be expected.

In the embodiment represented in FIGS. 16 to 19, the parts of the raising element 11 which are identical or which play the same role as those of the preceding embodiments, in particular those represented in FIGS. 11f and 12 , are designated by the same reference numbers. As can be seen in Figures 16, 17 and 19, the upstream plates or panels 34a of the riser 11 are vertical and have a rectangular shape, while the downstream plates or panels 34b have a trapezoidal shape and are inclined from upstream to downstream, their upper edge being more downstream than their lower edge. Finally, the side and intermediate plates or panels 34c are vertical and have a trapezoidal shape. In this way, when a flood pours over the rising element, the flow of water is improved over the rising element and the length of the discharge perimeter is further increased compared to the shape of the execution of figure 11f. As already indicated above, the extension of the length of the discharge perimeter makes it possible, for a given flow rate, to reduce the height or the thickness of the discharge blade and, consequently, to increase correspondingly the height of the lifting elements.

The plates or panels 34a, 34b and 34c, as well as the bottom plate 33 are preferably made of steel, but they could also be made of concrete, plastic or any other suitable material. As can be seen in figure 19, the plate bottom 33 rests on and is anchored to a sole 41. The sole 41 is preferably made of concrete, for example reinforced concrete. As shown in FIG. 18, the sole 41 has, seen from above, a trapezoidal perimeter, the large base of which is on the upstream side and the small base of the downstream side. Thus, when several raising elements 11 are juxtaposed, the raising element which must tilt first is not hampered in its tilting movement by the adjacent elements.

As shown in Figure 19, the sole 41 itself rests on a frame 42 having a trapezoidal perimeter corresponding to that of the sole 41. The support frame 42 can be made for example of concrete, loaded or not, reinforced concrete , steel, plastic, or any other suitable material. Two stops 12 are provided near the ends or at the ends of the downstream side of the support frame 42. These two stops 12 can be made in one piece with the support frame 42. As shown in FIG. 19 the support frame 42 is placed on threshold 6, previously leveled in the case of an already existing threshold or previously fitted out in the case of a new structure. The threshold 6 is then reconstituted by a cement grout 6a serving to anchor the support frame 42 of which only the upper face is flush with the reconstituted threshold to receive and support the elevating element 11.

As is more particularly visible in FIG. 19, the underside of the sole 41 is hollowed out so as to define a chamber 35 between it and the top of the threshold 6. At least one notch 36 is formed in the sole 41 on the front side of it. The notch 36 defines a drainage orifice making it possible to evacuate any water present in the chamber 35.

As can be seen in Figure 18, the crest line of the riser 11 has two waves which respectively define two cells on the upstream side of the riser. In one of the cells is disposed an added well 43, the bottom of which is pierced with an orifice which is coincident with two other orifices respectively drilled in the bottom plate 33 and in the sole 41 and which form a passage 44 the interior of the well 43 in communication with the chamber 35.

As can be seen in FIG. 18, the well 43 has a horizontal section of approximately rectangular shape and greatly elongated in the upstream-downstream direction. This elongated shape makes it possible to obtain a very long discharge perimeter when the water level reaches the upper edge of the well. At its upper end, on the upstream side, the well 43 has a vertical extension 45. The extension 45 forms a deflector which improves the flow regime of the water and at the same time serves to deflect any floating bodies to prevent them from enter well 43.

The well 43 can be made of steel, concrete, plastic or any other suitable material and, depending on the material used, it can be fixed to the bottom plate 33 and the downstream plate 34b by welding, by gluing, by bolting or by any other suitable fixing means.

As can be seen in Figures 17 and 19, the well 43 extends upward to a level N 'higher than the level RN' corresponding to the crest line of the rising element 11 and defining the level of normal restraint.

We will now describe the operation of the elevating element 11 shown in Figures 16 to 19. When the water level reaches the level RN 'during a raw, the water begins to flow over the crest line of the rising element, but it remains stable because the moment of the thrust forces relative to the stops 12 remains less than the sum of the moments of the weight clean of the rising element and the weight of the water above the bottom plate 33. Then, when the water level reaches the level N ', the water begins to flow into the well 43 and, from there, through the passage 44, into the chamber 35, from which it escapes through the drainage orifice 36. In steady state, the flow of water entering the well 43 reduced by the flow d water leaving through the orifice 36 establishes a certain height of water z₂ in the well 43. If we designate by z₁ the thickness of the sheet of water pouring into the well 43 (FIG. 19), by L _d la length of the discharge perimeter at the top of well 43, the flow rate Q₋ = of water entering the well is given by the formula: $${\text{Q₋ = 1.8.z₁}}^{\text{3/2}}{\text{. L}}_{\text{d}}$$

Similarly, if we designate by s the passage section of the drainage orifice 36, the flow rate Q₋ of the water leaving through the orifice 36 is given by the formula: $${\text{Q₋ = 2.7. s. z₂}}^{\text{1/2}}$$

In steady state Q₋ = Q₋ and the height of water z₂ stabilizes in well 43. According to formulas (20) and (21), z₂ can be expressed as a function of z₁ as follows: $$\text{z₂ = 0.44.}\frac{{\text{<figure-callout id="2" label="L d" filenames="imgf0001.png,imgf0002.png" state="{{state}}">L </figure-callout>}}_{\text{d}}^{}}{\text{s²}}{\text{. <figure-callout id="1" label="z" filenames="imgf0003.png,imgf0007.png" state="{{state}}">z</figure-callout>}}_{\text{1}}^{\text{3}}$$

The formula (22) concretely highlights the sensitivity of the system: for a small variation in height z₁ of the sheet of water pouring into well 43, there is a significant amplification effect on the height of water z₂ in well 43. This height of water z₂ in well 43 therefore creates on the upper wall of the chamber 35, that is to say on the underside of the sole 41, an underpressure or lifting force which tends to tilt the raising element around the two stops 12. It is therefore possible to ensure that that, when the water level reaches a predetermined level N, the water in the well 43 quickly reaches a level sufficient to cause the tilting of the raising element.

In the variant embodiments shown in FIGS. 19a and 19b, the bottom plate 33, the plates 34a, 34b and 34c as well as the sole 41 are made in one piece, for example in concrete (FIG. 19a) or in a plastic material (figure 19b). For the rest, the elevation element 11 shown in figures 19a and 19b is identical to that of FIG. 19.

FIG. 20 shows an elevation element 11 similar to that of FIG. 19, but without a well 43. In addition, the elevation element 11 of FIG. 20 does not have an opening in the sole 41 and in the plate bottom 33, that is to say that the opening 44 of Figure 19 is absent. The water supply to the chamber 35 is here effected by a pipe 46, which is embedded in the threshold 6. One end of the pipe 46 has a vertical part 46a which opens into the chamber 35, and its other end is connected to a water intake, which is located at any point upstream of the dam and which can have a shape such as that shown in FIGS. 10a to 10c or even a shape similar to that of well 43.

In the case where it is desired that the elevating elements 11 switch individually or in groups, for example two elevating elements each time, and this for successive water levels whose values are increasing, it will suffice to give the well 43 of the various elevation elements 11 of different heights corresponding to the successive levels for which the tilting of rising elements must occur. In the case of FIG. 20, it is also necessary to provide several pipes 46, as shown, one for each element 11 or for each group of elements 11 which must swing together for a predetermined level.

Although the elevation element 11 depicted in Figures 16-18 has a two-wave crest line, its crest line could have a smaller or greater number of waves, for example a wave as shown in the figure 21 or three waves as shown in Figure 22. Although Figure 22 shows only one well 43, it could be provided for example two wells respectively disposed in the end cells as shown in phantom in this figure.

To achieve the rise 10, it is possible to juxtapose several elements 11 all having the same number of waves or having different numbers of waves.

In conclusion, the height of the rise 10, therefore of its element (s) 11, depends on an economic choice, on the desired progressiveness in the tilting of the various rising elements, on the precision of the water level at which the tilting (precision which can be improved by providing a water additive trigger device at the base of the raising element, as described above) and of the shape of the crest line of the raising, line which can be straight, broken, curved or wavy. In the digital example described above, the height of the resulting rising elements can vary between 0.9 m and 1.5 m, allowing, depending on the options taken, to gain between 45 and 75% of the slice of water that would be lost without the use of the fuse holder.

From the above, it is clear that the fusible link of the present invention makes it possible to substantially and almost permanently increase the storage capacity of a dam or other structure to free overflow threshold, while maintaining or increasing the operational safety specific to structures with free overflow threshold, reliably allowing the evacuation of exceptional floods by automatic opening (tilting of at least one element of the rise) without any monitoring or any human intervention or control device. It is also clear that the surge can be manufactured and installed on the weir sill of a dam or other structure for a lower cost than that of previously known valves, and without major modification of the weir sill.

It is understood that the embodiments of the present invention which have been described above have been given for purely indicative and in no way limitative, and that numerous modifications can be easily made by those skilled in the art without however, depart from the scope of the present invention as defined by the appended claims. Thus, in particular, the seal 15 located at the base of the raising element may not be located near the upstream edge of said base, but at any other desired location under the base.

Claims (16)

1. Overflow spillway for dams and similar structures comprising an overspill sill (6) whose crest (8) is set at a first predetermined level (RN), lower than a second predetermined level (RM) corresponding to the maximum reservoir level (PHE) for which the dam (1) is designed, the difference between the said first and second predetermined levels (RN and RM) corresponding to a predetermined maximum discharge of a design flood, and a moveable water level raising means (10) on the sill (6) of the spillway (5) characterised in that the water level raising means (10) comprises at least one rigid heavy element (11) tresting on the crest (8) of the spillway sill (6) and held in place thereon by gravity, the said element (11) having a predetermined height (H₁) which is less than the difference between the first and second predetermined levels (RN and RM) and which corresponds, for a headwater level substantially equal to the said maximum level (RM), to a mean flood with a smaller predetermined discharge than the predetermined maximum discharge, the said element (11) being of such size and weight that the moment of the forces applied by the headwater on the element (11) comes to equal the moment of the gravity forces tending to maintain the element in place on the sill (6) so that consequently the element (11) is destabilized and expelled when the water reaches a third predetermined level (N) higher than the top of the element (11) but not higher than the second predetermined level (RM).
2. Overflow spillway as claimed in Claim 1, characterised in that an abutment (12) of predetermined height (B) is provided on the spillway sill (6) at the downstream toe of the water level raising element (11) to prevent the said element (11) from sliding downstreamwards on the said sill (6).
3. Overflow spillway as claimed in Claim 1 or 2, characterised in that, in the case of an existing spillway (5), the crest (8) of the overspill sill (6) is lowered to a lower level than the said first predetermined level (RN) and the water level raising element (11) is installed on the lowered sill and is given a height such that its top is at a level (RN') at least equal to the said first predetermined level (RN) but lower than the said third predetermined level (N).
4. Overflow spillway as claimed in any of Claims 1 to 3, characterised in that a seal (15) is provided between the overspill sill (6) and the base portion of the water level raising element (11) near the upstream edge (16) of the said base portion.
5. Overflow spillway as claimed in any of Claims 1 to 4, characterised in that the said water level raising element (11) has the material form of a substantially parallelepipedic solid block.
6. Overflow spillway as calimed in any of Claims 1 to 4, characterised in that the said water level raising element (11) has the material form of a substantially parallelepipedic hollow block containing a weighting material (32).
7. Overflow spillway as claimed in any of Claims 1 to 4, characterised in that the said water level raising element (11) consists of an assembly of plates (33, 34) which comprises at least one substantially horizontal base plate (33) and at least one substantially vertical and substantially rectangular plate (34) rising from the base plate (34).
8. Overflow spillway as claimed in Claim 7, characterised in that the vertical plate (34) rises from the downstream edge of the base plate (33).
9. Overflow spillway as claimed in Claim 7 characterised in that the said plate assembly comprises a plurality of substantially rectangular and substantially vertical plates (34) with their lower edges joined to the base plate (33) and their contiguous vertical edges joined together so as to form a sort of accordeon screen.
10. Overflow spillway as claimed in any of Claims 1 to 9, characterised in that the said water level raising element (11) exhibits a non-rectilinear crest line.
11. Overflow spillway as claimed in any of Claims 1 to 10, characterised in that a plurality of water level raising elements (11) are located side-by-side along the crest (8) of the spillway sill (6) with seals (13) between adjacent side faces of the said elements (11).
12. Overflow spillway as claimed in Claim 11, characterised in that the size and weight of the water level raising elements (11) are such that at least a first one (11c) of said water level raising elements (11) is destabilized when the headwater reaches the said third predetermined level (N1), said third level being lower than said second predetermined level (RM), that at least a second one (11b, 11d) of said water level raising elements (11) is destabilized when the headwater reaches a fourth predetermined level (N2) between the second and third predetermined levels (RM and N1), and that at least a third one (11a, 11e) of said water level raising elements (11) is destabilized when the headwater reaches a fifth predetermined level higher than the fourth level (N2) but not higher than the second predetermined level (RM).
13. Overflow spillway as claimed in any of Claims 1 to 12, characterised in that a hole (36) is provided on the downstream side of said element (11) to drain the underside space between the said water level raising element (11) and the spillway sill (6).
14. Overflow spillway as claimed in Claim 13, characterised in that it comprises at least one duct means (21, 43) which under normal service conditions maintains the said underside space between the raising element (11) and the sill (6) at atmospheric pressure, said duct means (21, 43) having an upper end which is at a level equal to or substantially equal to the said third predetermined level (N) and vertically above or upstream of water level raising element (11).
15. Overflow spillway as claimed in Claim 14, characterised in that the underside of said water level raising element (11) is recessed to form a definite chamber (35), and the lower end of the duct means (21, 43) opens into the said chamber (35).
16. Overflow spillway as claimed in any of Claims 1 to 15, characterised in that the water level raising element (11) comprises several stacked parts (11g-11j, 34, 37).

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