Air slots have been used since the 1950s and ‘60s to prevent damage associated with cavitation in outlets and on spillways. They have allowed the construction and trouble-free operation of high head spillways. In spite of this, cavitation related damage and impractical slot geometry still exists and development is needed to overcome such problems.

Air slots were first proposed in:

• Italy for the Calacuccia tunnel (1954).

• US for the Grand Coulee and Yellow Tail dams (1960 and 1971 respectively).

• Soviet Union for the Nurek and Bratsk dams (late 1960s).

• Mexico for the Infiernillo dam (1969).

• Canada for the Mica dam (1971).

Air slots were first field tested in Guri dam in Venezuela where a severely damaged zone at the foot of the spillway was protected with a ramp; in the Brazilian dams of Foz do Areia and Emborcacao in which the spillways operated for long periods of time with large discharges and without damage; and in the Yellowtail and Bratsk dams.

Generally, the use of aerators has been accepted as a solution to prevent cavitation damage, but for some reason aerators have not always been installed in time to prevent damage.

In Mexico the delay in providing aerators in the tunnels of the El Infiernillo dam resulted in some damage during relatively large discharges. Experience of the behaviour of aerators under various conditions has demonstrated they are a safe and reliable means of protecting spillways or low level outlets in which there are high water velocities.

Trigomil dam

Trigomil dam is located in the state of Jalisco on the Ayuquila river in Mexico. It is a multi-purpose, roller compacted concrete (RCC) dam 107m high and a 255m wide. It impounds a reservoir with a total storage volume of 325Mm3. The spillway is located in the centre of the dam and has a slope, after the ogee crest, of 0.8:1 that ends in a circular flip bucket with a radius of 13m and a 25° exit angle.

Due to the high head of the scheme an analysis was considered necessary. This favoured the insertion of an aerator slot 52m below the crest elevation in order to protect the lower part of the spillway from cavitation. A slot with a ramp was selected for the spillway, with slots of the same geometry perpendicular to the flow in the spillway sides to allow the air to enter. This was Mexico’s first dam to have a functioning aerator.

The aerator has a self-draining cross-section and since it has either a horizontal or downstream slope, avoids the problems of partial filling associated with rectangular slots. The selected upstream ramp is 4m long and 0.4m high; which seems an excessive height but the poor concrete finish on the spillway surface justified an increase in the downstream zone protected by air.

Two scale models were built: one 1:75 model to observe the general flow pattern and a 1:25 sectional model with a width of 0.5m to study the aerator in more detail and to perform measurements such as water depths, the length of the jet and the pressures at the bottom. The final geometry was accepted and a prototype aerator was constructed in 1991.

During construction in January 1992, when the ogee of the spillway was still unfinished, a sudden El Niño flood event caused overtopping of the works in the spillway. The flood even dragged down some equipment but the dam was only slightly damaged. Two years later more heavy rain fell and the spillway, now fully finished, worked with a maximum discharge of 650m3/sec.

An inspection after the flood did not find any signs of even incipient damage associated with cavitation, so it seems the spillway is fully protected from this kind of damage with the selected aerator.

Preventing damage

The use of aerators, provided they are correctly designed and located, is an excellent way to prevent damage due to cavitation in high head spillways or bottom discharges.

The slot geometry used in the Trigomil dam has obvious advantages over rectangular slots. It does not suffer from partial water filling and the rounded downstream corner avoids air flow separation, improving air passage.
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