4. New dams

In many countries, the exceptional floods may be very high, and the level of safety required increases the capacity, and thus the cost, of new spillways, which may represent a large part of the total dam cost. Well adapted risk analyses and new solutions for spillways may allow substantial savings to be made.

Thirty years ago, most dams were designed for a design flood with an annual probability such as 10^-3Above the relevant reservoir level, a margin of safety of some metres (the freeboard) was retained below the embankment crest, but the true probability of overtopping (and failure) was not analysed precisely.

It is now usual, and advised by ICOLD Bulletins, also to consider a 'check flood' of very low annual probability (such as 10^-5 ) or a theoretical 'probable maximum flood' and to ensure that this check flood (often about twice the design flood) may be spilled without dam failure; but it is usually accepted that the corresponding reservoir level may be close to the embankment crest, as the flood failure usually requires an overtopping of the embankment crest of some hours by a nappe depth of 0.20 to 0.50 m. The freeboard is thus considerably reduced for the check flood.

Gated spillways

Gated spillways are most often preferred for capacities of more than 1000 m³/s, radial gates being the most usual. The reservoir may be operated at the level of the design flood, but the cost of gates is high and adapted to the check flood, that is, to about three times the flood of 10^-2 annual probability. Consequently, for a century two-thirds of the spillway capacity will probably never be used; however, this solution is not completely safe because the gates require careful maintenance and operation and redundancy of operating devices (including power supply).

Incidents during heavy rainfall are not uncommon, and total jamming of gates has caused the failure of some large dams: this risk appears to be the largest risk associated with gated dams, and jamming may be caused by mechanical or electrical problems, access, lack of operators, or wilful damage. In the case of total jamming of gates, new dams should thus be designed to discharge at least the annual flood over the gates or through an emergency spillway. This is obtained if the freeboard above the gates is half of the gates height, the flow over the gate being (1/3)^1.5 , that is, about 20 per cent of the flow with all the gates open.

To avoid the operating cost of gates, various solutions involving automatic gates have been used worldwide, but their reliability is questionable as there are two risks: unnecessary opening, or total jamming. Consequently, they should be used only for small dams, or for a part of the total spillage capacity.

Free-flow spillways

To avoid the cost or risks associated with gates, two-thirds of the world's large dams and particularly most spillways with a discharge capacity of less than 1000 m³/s are free-flow spillways. Their operation is simple and safe, but the drawback of the usual shapes such as the Creager shape is the rather low specific flow: the flow per metre of spillway length (in m³/s/m) is about 2.2 h^1.5 , h being the nappe depth in metres. A flow of 1000 m³/s under a 3 m depth thus requires 90 m of spillway length. Apart from the cost of a long spillway for embankment dams, the maximum nappe depth reduces the useful reservoir depth; and reducing, for instance, by 3 m a reservoir depth of 30 m, in fact reduces by about 30 per cent the live storage (or increases the dam cost by 20 per cent). The reduction in hydropower output may also be substantial. Multiplying by 3 or 4 the specific flow of the spillway when using the PK. weir solution will very often be a major improvement. Examples are given next for spillways of 200 m³/s, 1000 m³/s and 5000 m³/s discharge capacity.

For 200 m³/s, a traditional spillway would be, for instance, 35 m long and the nappe 2 m deep. With a P.K. weir, the length may be reduced to 10 m, avoiding a side spillway to embankment dams. This would require about 60 m³ of reinforced concrete in precast elements, less than US$ 20 000 in many developing countries.

Figs. 4, 5 and 6. Variations to the hydraulic shape of the inlet and outlet.

For 1000 m³/s of capacity, instead of a 90 m spillway 3 m deep, it is possible to use a 20 m-long spillway of the same depth or a 40 m-long spillway with a nappe depth of 2 m, using 300 m³ of reinforced concrete in precast low-cost elements.

For a spillway with a design flood of 2500 m³/s and 5000 m³/s of check flood, a traditional design would use, for instance, four radial gates 12 m wide and 10 m high, with a freeboard of 5 m used for the check flood. The total length of the spillway, including piers, will be 60 m. In case of total jamming of the gates, the maximum flow over the gates will be 1000 m³/s, approximately the annual flood. Four alternatives using P.K. weirs are suggested next:

A P.K. weir about 80 m long with the sill level at the same level as the top of the gates. For the check flood, the level would be the same. The nappe depth of the P.K. weir would be, for the 100 year flood, about 1.5 m. it would be thus necessary to buy more land than with the gated dam, but the overall cost would be much lower, and the safety improved. This weir requires 2000 m³ of reinforced concrete.

A PK. weir 50 m long and two flap gates, 6 m high and 12 m wide. This would add flexibility for managing the reservoir and controlling floods. The gates could be automatic, and the water level reduced for usual floods.

A P.K. weir 50 m long and two low gates, 40 m² each: these gates could be placed 20 or 30 m below the dam crest to control the reservoir and possibly to flush sediments.

A PK. weir 40 m long and two radial gates, as in the basic solution. The operating levels are the same as for the basic solution for all floods. This solution is substantially less expensive than the basic one. The total cost of the civil works is about the same, and half of the cost of the gates is avoided. In the event of total jamming of the gates, it would be possible to discharge the design flood.

For most new dams, at least one solution using P.K. weirs will thus be less expensive than traditional solutions, while maintaining or improving the safety. As there are no patents, these solutions may easily be implemented with the resources of each country.
P.K. weirs will probably be used more often for new dams, increasing the flow of a Creager weir by about four times and requiring 0.4 m³ of reinforced concrete per m³/s of total spillway capacity. The construction can be done by the main contractor, and the cost per m³/s would be about US$ 100 in developing countries (to be spent in local currencies). The cost may be US$ 300 to 500 in industrialized countries. This will make it possible to divide the spillway length of a free flow spillway by four or to reduce by 60 per cent the loss of storage. The combination of P.K. weirs with gates may also be very attractive.

Fig. 7 (below left): construction using pre-cast elements.
Fig. 8 (below right): the use of a fuseplug to increase spillway capacity