Water India V
Key Note Adress
F. Lempérière (Hydrocoop France)
1) Most dams, and especially the great majority of irrigation dams, have free flow spillways. This solution is economical for catchments areas of few km2 with extreme floods under 100 m3/s ; but it is in fact expensive in most Asia and particularly in India for irrigation dams with catchments areas over 5 km2 for 3 reasons.
– The extreme flow to be used as safety check flood is most often in the range of 20 m3/s per km2 for catchments areas between 5 and 500 km2 and many dams of moderate storage should discharge at least 500 or 1000 m3/s requiring thus long spillways.
– The cost per m of the spillway is usually much more expensive than the cost of an earth fill dyke.
– The average depth of a reservoir is usually under 30 % of the dam height, i e in the range of 5m for a dam 20 m high ; for a specific flow of the spillway of 5 or 10 m3/s the nappe depth of a free flow spillway is 2 or 3 m, close to 50 % of the average reservoir depth ; reducing by half this nappe depth for a same flow may increase by 20 % the storage (and often much more)
There are two solutions for improving the cost efficiency of free flow weirs : labyrinth weirs multiplying by 3 the specific flow or fuse devices opening for exceptional floods.
2) New Labyrinth Weirs (Piano Keys Weirs).
Labyrinth weirs have been used since 50 years for 50 or 100 spillways : quite all have vertical walls and a trapezoidal lay out. Most were for specific flows of 5 to 20 m3/s/m with walls 2 to 4 m high, reducing the nappe depth by about 1 m. Few were for much larger flows with walls up to 9 m high.
These spillways have well operated without incidents but this solution has not been much developed for 3 reasons :
– The shape was often not optimized for cost saving and far from the best cost efficiency.
– The shapes were not standardized and each dam required specific studies and model tests, relatively expensive for rather small discharges.
– The designed labyrinth shapes could not be placed upon gravity dams usual cross sections, i e on most spillways.
Since 10 years, Hydrocoop, which is a not profit Association, have coordinated studies of new labyrinth weirs shapes and relevant hydraulic model tests mainly in France and Algeria (Biskra) but also in India (Roorke), China, Vietnam and Switzerland with three targets.
– Cost optimization taking in account hydraulics, structural design and construction efficiency.
– Standardized shapes which may be used anywhere without further studies ; but their discharge may be easily checked in any hydraulic laboratory.
– New Labyrinth shapes which may be placed upon usual gravity dams cross sections ; i.e upon most new or existing spillways.
Many shapes have been studied ; the most attractive have overhangs and a rectangular lay out which justifies their name of Piano Keys Weir (P.K. Weir).
One of the most attractive is represented in fig. 1. (Model B) with the following data :
– Length of walls about five times the length of spillway.
– Inlet width a about 1,2 the outlet width b.
– Slope P/B in the range of 0,4 to 0,5.
The specific flow is close (in m3/s/m) to 4,5 h√H, h being the nappe depth in m and H the maximum height of the walls (in m) with a saving in nappe depth of 0,5 H as compared with a Creager Weir of which the flow is about 2,2 h1,5. The discharge of the labyrinth is about 2√h x the discharge of Creager Weir. For an usual value h = O,5 H the discharge is multiplied by 2√2, i.e about 3.
This solution is extremely attractive for new dams with maximum discharge between 100 and 2000 m3/s because the length of the spillway may be divided by 3 or the nappe depth divided by 2. The quantity of reinforced concrete for usual P K Weir designs is in the range of 0,3 m3 per m3/s, less than half the quantity for traditional labyrinth weirs.
Two Implementations of P.K. weirs have been made by Electricite de France in 2007 and 2008. Many other implementations are planned.
To check the behavior and the performance of P.K.Weir type B several tests detailed on selected forms were then carried out recently at the Hydraulic Developments and Environment Laboratory of the University of Biskra. The tests carried out on fourteen small-scale models of P.K.Weir type B gave a basis to optimize the increase in the discharge of P.K.Weir according to relationship’s between the length, the height, the width and slope of apron, in particular according to the relationship between the length of the crest and its width N=L/W.
The flow on the P.K. Weir is completely different from the flow on the Creager weir; it is characterized by two types of flow according to the head on the sill of weir.
- For low and medium heads, the flow is characterized by two discharging nappes,the first in the form of a jet of the bottom which flow along the inclined apron of the downstream alveolus and the second in the form of a screen more or less thin according to the load on the weir.
- For the high heads the two discharging nappes become interdependent constituting so a single nappe, consequently the hydraulic efficiency decreases.
The discharge which passes by the P.K. Weir is widely superior to that of the Creager weir, in particular for the low and medium heads. It is from 3,5 to 2,5 times for the heads lower than the half of the height of the weir (h/P < 0,5) and reach a value of the order of 1,5 times for values of heads equal or superior to the height of the weir. This shows that the P.K. Weir can be a solution to evacuate high discharges under low heads.
The saving in head is about the same for many nappe depths, about 0,5 H and the increase of discharge (m3/s/m) close to 2 H1,5.
3) Collapsible devices (fuse plugs)
Various solutions of collapsible spillway devices have been used worldwide. The principle is usually that the device is overtopped without damage by the usual floods and collapse for floods in the range of 1/20 to 1/100 yearly probability.
Sand bags have been widely used but have many drawbacks, and may well not collapse when required being thus very dangerous. Flash boards (wood boards standing against vertical steel pipes) have been widely used in United States because their cost is very low but they do not collapse for a precise discharge, and are very exposed to the impact of floating debris and to willful damages : they are thus not much favoured now.
For floods up to a few hundred m3/sec, a simpler and very economical system, particularly efficient to increase storage or spillage capacity of existing dams, has been studied, optimized and developed by the Hydrocoop association under the name of « Concrete Fuse Plugs ».
Concrete Fuse Plugs system was successfully tested on models in various countries such as Algeria, China, France, and Vietnam under Hydrocoop coordination and two first realizations were initiated in may 2008 in Burkina Faso and in Vietnam and will be completed in autumn 2008.
3.1 PRINCIPLE OF FUNCTIONNING
The concrete fuse plugs are simple massive blocks placed side by side on a spillway sill. They are free standing and self stable until the water level in the reservoir reaches a certain elevation, but start tilting when this elevation is exceeded. They may be designed to tilt before being overtopped and, In this case, they have significant height as compared to their thickness.
The present paper only dealt with concrete fuse plugs allowing overtopping before tilting.
These fuse plugs are designed to be overtopped by a water nappe with a depth ”h” which may be important (can be up to several times the block height “H”) before tilting. This type of fuse plug has significant length and thickness as compared to its height. Its upstream upper corner could be trimmed to ease the flow discharge.
The elements (blocks) placed on the same sill may have the same height but different thickness, and therefore different mass, so that they tilt for different water elevations.
The water elevations can be predicted quite accurately as far as the magnitude of the uplift pressure under each block is well known. A simple way to solve this problem is to design the block so as to be sure that the uplift pressure is total: this can be achieved by designing a hollow section underneath the plug, which is widely open at the upstream side and completely closed and watertight at the downstream side as shown on Fig. 11.
3.2 GENERAL ARRANGEMENTS
In the case where very long blocks are used, they may be placed directly side by side. Otherwise, the model tests suggest that intermediate walls should be used between each fuse plug. These walls, which can be designed as a part of the spillway sill, will help maintain the water nappe of the fuse plugs adjacent to the tipped one. This will make sure that the water elevations triggering the tipping of each element can be calculated more accurately. These walls do not need to be very large or thick to be efficient. General arrangements are shown on Fig. 12 to 14.
At least 4 to 5 blocks of different thickness are usually required to obtain a progressive tilting of the various blocks in relation with the water level.
To allow correct tilting and avoiding sliding, each block is standing against small abutments fixed downstream in the spillway sill.
The extremities of the blocks must be designed to avoid friction against adjacent blocks or separating walls. This can be achieved by different ways such as for instance by chamfering the extremities so that one face is slightly longer than the other or by creating a small “tail” at the extremities which may also be used to place the water tight joint between blocks (see paragraph 3.3.1 and 3.3.2. here after).
The hollow section underneath the block may be closed on the side ends of the blocks at their bottom, so forming a “chamber” closed on three sides and opened upstream (see paragraph 3.3.1 here after). In case the hollow section is not closed, or in case of very long blocks, it may be necessary for the stability of the blocks, to use few unsealed supports at the upstream bottom of the blocks.
3.3. DETAILED ARRANGEMENTS
Several solutions may be used to design the extremities of the blocks. As examples, Fig. 15 shows a block with side ends closed, underneath chamber and downstream face longer than the upstream one, while Fig. 16 shows a block with tails.
3.3.2. Vertical seals
Usual rubber seals can be used for the vertical seals to be placed at the extremities of the block. Various solutions may be used: One side of the seal must be free to allow the block to tilt (for instance just laid against the “tail”).The other side (against intermediate wall or adjacent block) may be either bolted through steel plates at the end of the construction or directly embedded at concreting time or just laid on the “tail” (see Fig.17).
3.3.3. Nappe ventilation
It is advisable to assure a correct aeration of the water nappe to avoid vibration. Lack of ventilation may also leads to an early tilting of the block because the water pressure above the block may be reduced due to the air dragged in the nappe.
This ventilation is usually simple to achieve with pipes embedded into the intermediate walls.
3.3.4. Floating debris
Model tests have shown that floating debris have no significant effects on the water level at tilting time.
3.4. THEORETICAL CALCULATIONS
Theoretical calculations are easy before overtopping. After overtopping, problem remains to estimate the height of the water nappe above the blocks and the horizontal pressure against the upper part of the blocks and to take into account dynamical effects and friction of the water against the blocks. It means that theoretical calculation may allow making a preliminary design, but model tests remain suitable to precise the exact widths to be adopted to assure tilting at the required levels.
For this rough theoretical calculation, model tests already performed have shown that the pressure above the block may be considered as about 0.6h, i.e. a little less than the theoretical value of 2h/3 corresponding to a thick sill, and that dynamical effects and friction may be neglected.
On the graph shown on Fig. 18, the bold line MV represents the moment of the vertical forces and the one for MH is for the moment of the horizontal forces for a total water level W = H + h . Tilting corresponds to the crossing of the two lines. The angle between the two lines is large enough to show that reasonable accuracy may be obtained. The dotted line MV1 corresponds to the depth of the block increased by 10%.
Model tests already performed have helped finding the ratio between the thickness of a block and water level at tilting time.
On a preliminary approach, the simplified formula h = E – 0.4 x H may be used for blocks having the same general shape as described above with an hollow section 10% of the block height H and with a concrete density in the range of 2.3 . A block with a thickness E equivalent to 1.5 times its height H, tilts for a nappe depth h, which is approximately its height H. Concrete density variation of 5% will lead to a variation of about 10% of the nappe depth at the tilting time
Blocks can be prefabricated or built in situ. In the case of blocks built in situ on an existing spillway, they can be built for instance as follows, using clay plug for downstream water tightness (see Fig.19):
- Levelling the spillway sill.
- Laying materials to form the chamber. Such materials may be of any kind provide they are easy to remove after block concreting. At the downstream extremity, clayey sand will be used so as to plug possible leakage at the seal location.
- Laying of a plastic membrane above the materials.
- Concreting the block.
- Taking out of the materials to empty the chamber (except the downstream plug) using few supports in case side ends are not closed.
Another solution for downstream water tightness may be, instead of using a clay plug, to lay on the spillway sill a rubber seal underneath the downstream extremity of the block (see Fig.20).
3.6. Scope of utilization
Concrete Fuse Plugs may be used for new dams. In such case, it is possible, with about the same quantity of concrete and cost, to double the flow of the extreme flood discharged through the spillway.
They may also be used to improve existing free overflow spillways either by increasing spillage (after lowering the sill) or by increasing reservoir storage (installing higher units) or by combining both, as shown on Fig. 21. and 22.
3.7 Conditions of utilization
When fixing the design criteria (i.e. to determine for what flood each block will tilt), the designer has to face two opposite constraints :
– for economical reason, to avoid that blocks tilt too often (with corresponding losses and replacement costs, though the cost to replace a tilted block is
cheaper than the cost of an original one and remains low as compared to the benefit of using the system for one year),
– for safety reason, to keep a reasonable distance between the dam crest and the water level at tilting time of the last block.
These criteria have of course to be fixed case by case, depending of the local data and of the objective pursued, but, most often,
– concrete fuse plugs should be designed to tilt for floods between about the 20 years flood and the 100 years flood or more.
– when heightening an existing free flow spillway, the block height should not be more than about 1/4 of the distance between the dam crest and the sill of the spillway.
3.8 Comparison with others systems
To upgrade existing free flow spillway, concrete fuse plugs are particularly interesting. Compact labyrinths weir with inclined shapes and hangovers (so called Piano Keys Weirs) may be more efficient but require more demolition works and more concrete, so their cost is higher and may become prohibitive for small dams.
For new dams, concrete fuse plugs remain a possible solution, but Piano Keys Weirs may be more attractive: Volumes of concrete are not very different and when the cost increase is not too important, it remains preferable to avoid the drawbacks of a fusible system.
In usual cases, less than 1 m3 of concrete blocks is necessary to increase the discharge capacity by 1 m3/sec: For an existing free flow spillway, it means that about 2 m3 of concrete should be removed and less than 1 m3 of blocks should be placed instead; For a new dam, it means that using 1 m3 of blocks instead of a usual Creager sill, can both save around 1 m3 of concrete and increase possible discharge capacity by 1 m3/sec.
4) Fuse gates :
The fuse gates device, developed since 15 years by Hydroplus (France) associates the principle of fuse devices with a labyrinth shape. Tilting of fuse elements is reached by uplift under the fuse gate for a precise level.
This solution is more expensive thean P.K. Weirs or simple concrete fuse plugs but it increases the efficiency. It has applied now to 50 dams worldwide (including 15 in Gujurat) and is mainly cost effective for large discharges. It has been used for discharges up to 30.000 m3/s.
5) Choice of solutions :
For new dams the Piano Keys Weir (P.K. Weir) will probably be very often the most attractive solution for extreme floods between 200 and 2000 m3/s. Between 50 and 200 m3/s the concrete fuse plugs may be cost effective.
Associating a P.K. Weir spillway with bottom gates for small discharges or with a surface spillway for very large discharges may optimize the management and the safety of the reservoir.
For adding an emergency spillway to a gated spillway, P.K. Weir may be the most attractive for a discharge up to 1000 or 2000 m3/s, fuse gates for very large discharges.
For improving existing free flow spillways, P.K. Weir may be used but concrete fuse plugs may be less expensive for discharges up to about 200 m3/s and fuse gates more efficient for large discharges.
Traditional Creager spillways are usually not the most cost efficient solution, except for discharges under 100 m3/s. New labyrinth shapes (P.K. Weirs) or simple fuse plugs or fuse gates may often be much more cost effective.