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Spillways for Dams

Most dam spillways are open cut channels. Some dams have tunnel type spillways which tend to have smaller discharge capacities than open channels; in particular, tunnel spillways tend to choke at discharges approaching their maximum capacity, something which does not happen with open channel spillways.

The main decisions that have to be taken during the design of a spillway for a dam are:

  • how large a flood should the dam be capable of withstanding, ie what is the Probable Maximum Flood (PMF);
  • how much of the PMF does the spillway have to handle (ie maximum discharge capacity);
  • to what extent is it necessary to line the spillway with concrete;
  • to what extent is it necessary to provide energy dissipation structures at the downstream end of the spillway.

The first two decisions above refer to the fact that dam designers can handle flood inflows into the storage in two ways;

  • spillway discharge capacity, ie flood waters flow out of the storage through the spillway;
  • temporary surcharge storage (ie the reservoir level rises above the normal operating level, without overtopping the crest of the dam, temporarily storing flood water which will be gradually released after the flood is over.

The last two of the decisions above depend on the geology of the spillway site, specifically the erosion resistance of the rock mass. Spillways can be classified accordingly as follows:

  • unlined rock cut channels - no concrete lining or energy dissipation. Non-erodible rock is required for such spillways eg Pindari Dam, Glennies Creek Dam and Windamere Dam;
  • a short concrete lined chute discharging into an unlined rock channel - no energy dissipation eg Copeton Dam;
  • concrete lined chute terminating in a flip bucket energy dissipator which is designed to throw the discharging flood waters into an aerial trajectory so that the water impacts the ground some distance downstream from the actual spillway structure eg Lostock Dam;
  • fully concrete lined chute terminating in a full energy dissipator such as a submerged roller bucket or hydraulic jump stilling basin eg Burrendong Dam.

Burrendong Dam spillway showing fully lined chute and full energy dissipator.

To view a presentation on the history and geological investigations for the spillway at Copeton Dam, see Copeton Dam Spillway.

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