Prior to 1986, Australian Rainfall and Runoff specifically required that overtopping of rockfill dams be prevented, by designing the spillway to have the capacity of the Maximum Probable Flood [PMF]. The perceived need to vary from this public safety standard followed increases in estimates of the Probable Maximum Precipitation [PMP] and thus increases in the PMF. These revisions to the design flows required existing dams in Australia to be upgraded, at significant cost to the community, if the public safety standard was to be maintained. The ‘no-overtopping’ standard was abandoned by principal engineering professional bodies and by government authorities in water engineering.
There was considerable controversy at the time of the change.

This paper offers perspectives, from recent flood events, as to the implications of the practice of designing rockfill dams and other dams to be overtopped by very large floods. The paper explains how these implications may be of a more physical, emotional, psychological and mathematical nature than the economic justifications given for the reduction on standards and guidelines effected 30 years ago. The paper describes how these implications may be adversely affecting the risk management of floods through rockfill dams intended by current procedures.

A review of standards and guidelines applied to rockfill dams by engineering professional bodies is recommended. The inclusion of engineering professional bodies in that review, wider than the authors of standards and guidelines, is advocated.