5. Increasing the operating level of free-flow reservoirs

If the nappe depth for the design flood is h, it will be possible to raise the operating level by 0.6 h if lowering the sill by 0.9 h and installing P.K. weirs, of which the height H will be 1.5 h.
As the value h is between 1 and 4 m for many existing dams, H will be usually between 2 and 6 m.
Increasing the operating level will require 2 m³ of reinforced concrete and 1.5 m³ of ordinary concrete per metre of spillway length for 1 m of reservoir level increase for most spillways.
The ratio between the area, of the reservoir in m² and the spillway length, L, in metres, is usually between 5000 and 50 000, often between 10 000 and 20 000. The provision of 2 m³ of reinforced concrete will save 10 000 to 20 000 m³ of water storage. Taking into account also the cost of ordinary concrete and sill lowering, the cost per m3 of extra water storage will be in the range of US¢5 in most developing countries (to be spent in local currencies), and possibly five times more in industrialized countries where the cost of labour is much higher.
Where the area of a free-flow reservoir is more than 2 or 3 per cent of the catchment area, the volume stored in the nappe depth reduces the downstream flow peak. It is necessary in this case to reduce the nappe depth saving by the P.K. weir so as not to reduce the dam safety.
It should also be underlined that using a P.K. weir for existing dams reduces the time for floods downstream of the dam to peak, when the reservoir is not full at the beginning of the flood.
Using PK. weirs in conjunction with gates as suggested in Section 7 avoids these drawbacks.

6. Increasing the capacity of existing free-flow spillways

It is often required to increase the spilling capacity of a free-flow spillway by 50 to 100 per cent, sometimes more. P.K. weirs can be used accordingly. For instance, if the nappe depth for the present design flood is h, lowering the sill by h and placing a P.K. weir to keep the same operating level will increase the flow by about 70 per cent, requiring, per extra m³/s, 0.5 m³ of reinforced concrete and 0.35 m3 of ordinary concrete, that is, a cost of about US$ 150 in developing countries and US$ 500 in industrialized countries.
It is possible to modify only a part of the spillway length according to the required flow increase.

7. Increasing storage and safety of existing dams

Safety authorities often wish to increase the spillway capacity, while dam owners mainly wish to increase the storage. It is possible to use PK. weirs to increase the storage by 30 per cent of the present nappe depth and to increase the maximum discharge capacity by 50 per cent. The value of extra storage may pay also for the extra safety.

8. Increasing storage and flood control by existing free-flow reservoirs

For many existing reservoirs of area s (in m²) and a maximum nappe depth h (in m), the volume s x h is lost for storage and may represent 20 to 50 per cent of the live storage and a significant part of the flood volume. It is also poorly used for reducing the flow peak of floods downstream of annual probability 10^-1 to 10^-2 , that is, the main usual flood damage because the nappe depth is much less than h for these floods.

A very attractive solution may provide the advantages of a fully gated solution, without its cost and drawbacks. The sill of the existing spillway can be lowered by a depth equal to h ; P.K. weirs 1.5 h high will be placed along two-thirds of the spillway and a flap gate 1.5 h high along one-third.

The gate, which may be automatic, would be open for most of the flood season. A study of hydrograms shows a great reduction in the flow peaks downstream for most floods. The storage will be completed at the end of the flood season, and increased by 0.5 s h. The maximum spilling capacity of the spillway is also increased. No permanent gate operator is necessary. The consequences of gate incidents will be greatly reduced compared with fully gated dams.

This solution, which has never been used, seems extremely attractive for thousands of existing dams, as the storage saving would also pay for an increase in dam safety and downstream flood control.

9. Emergency spillways

It is often necessary to add emergency spillways to existing gated spillways. Instead of adding costly gated emergency spillways, which do not avoid the risk of gates jamming, it could often be very advantageous to use a PK. weir spilling the nappe depth corresponding to the free­board. A PK. weir may discharge for instance, a specific flow of more than 50 m³/s/m for a nappe depth of 4 m.

10. P.K. weirs in canals

PK. weirs may be used on flat bottoms, as is the case, with traditional labyrinth weirs, with a better cost efficiency, especially for large specific flows.

11. P.K. weirs compared with fuse devices

Various fuse devices have been used for the same purposes as considered above for PK. weirs.
For new dams, PK. weirs will usually be more attractive than fuse devices, but some fuse devices may be more interesting for some existing dams, for example:

on top of arch dams, where placing PK. weirs may be difficult; or,
for cost reasons, as in the examples below.

Three fuse devices are considered, in the following paragraphs, with specific targets:

Thousands of small dams in the USA use flashboards for increasing the storage of existing free-flow reservoirs. They are usually vertical woodboards, standing against vertical steel pipes embedded in the spillway sill concrete. The elements are usually about 1 m high or less. They may be withdrawn by hand before the flood season, or are overtopped by small floods and bend for the large floods (simple steel plates directly embedded in the sill may be an alternative). This solution is not expensive, but not precise, and requires some maintenance. It could be used for increasing free-flow reservoir storage by 0.5 to 1 m but, for safety reasons, the height of flashboards would be limited to 25 per cent of the gap between the spillway sill and the embankment crest, and the elements would bend for a water level well below the crest.

They may thus be more attractive than P.K. weirs only where the required increase in reservoir level is lower than 1 m.

For increasing the spillway capacity of existing free-flow spillways, it is possible to lower the sill and place simple fuseplugs in ordinary concrete (see Fig.8).

The thickness of the various elements would be, for instance, such that a first element would tilt for the present design flood and the last one for the check flood. This solution is patented in most industrialized countries, but it can be patent-free in most developing countries where its cost may be even lower than the cost of P.K. weirs for increasing the free-flow spillway capacity. They are less attractive for new dams, or for increasing the storage. They require about 0.5 m³ of ordinary concrete for increasing the flow by 1 m³/s.

Fusegates are gravity elements which tilt when an uplift is created under them for a predetermined upstream level. Their layout may be straight or labyrinth shaped. Structural costs are lower than for P.K. weirs, but their design is more complex and their patent requires payment in hard currency. They may be attractive as compared with P.K. weirs for improving existing large spillways.The drawback of fuse devices is the loss of some elements and water, for instance, once per century. It may be attractive to use PK. weirs for one half of a spillway and fuseplugs or fusegates for the other half, and to design the fuse devices for tilting only for a flood of low annual probability, such as 10^-3

12. Summary and conclusion

P.K. weirs are simple solutions as safe and easy to operate as traditional free flow spillways and much more efficient. They may:

increase the specific flow fourfold;

allow specific flows of up to 100 m³/s/m;

reduce substantially the cost of most new dams and guarantee their safety;

increase the storage of many existing reservoirs for a cost in the range of US¢ 5/m³ in most developing countries, and US¢ 25 in industrialized countries;

improve the flood control by many existing dams; and,

increase the spilling capacity of many existing dams with 0.5 m³ of reinforced concrete per extra m³/s.

The efficiency and cost can easily be checked within each country, and detailed designs can be standardized. There are no patents, and expenses can be in local currencies.

F. Lempérière has been involved in the construction or design of fifteen hydraulic schemes on large rivers including the Rhine, Rhone, Nile and Zambezi. He is Honorary Chairman of the French Committee on Large Dams. He has been Chairman of the ICOLD Committee on Costs of Dams (1991-2001). He is Chairman of Hydrocoop, a non-profit­making international association for technical exchanges.

Hydrocoop, 4, Cité Duplan, 75116 Paris, France.

A. Ouamane has been a lecturer since 1986 at the Hydraulic Department of Biskra University, Algeria. He is Head of Research at the Hydraulic Laboratory. He has conducted various theoretical and experimental studies on shaft weirs, stepped weirs and labyrinth weirs.

Department of Hydraulics, University of Biskra, BP 918 RP 7000, Biskra, Algeria.