FLOOD04994

• County: Statewide
• Community: na
• Basin: Statewide
• Title: Guidelines on Selection of Acceptable Flood Capacity for Dams
• Date: 3/1/2000
• Prepared By: Australian National Committee on Large Dams
• Floodplain - Doc Type: Educational/Technical/Reference Information

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AZ.8. CONSTRUCTION FLOODS. (Tefer
sub-section 9.4)

AZ.8J. General Considerations.

The question of appropriate provisions for
constructions floods is not
straightforward, depending on the site and
whether it is a new dam or remedial wolks
to an existing dam,

The risk assessment process can assist by
providing a comprehensive consideration
of the risks and consequences, but the use
of risk to life criteria poses a question on
applicability to short term construction
periods as against application to the basic
structure long term life hazard,

AZ.8.2. 1986 ANCOLD Guidelines.

Appendix A, Floods During Construction
in the 1986 Guidelines on Design Floods
for Dams, was basically related to
construction of new dams and assumed it
appropriate that the river diversion flood
risk during the relatively short
construction period should be to an
equivalent order of safety to the dam
recommended design flood relevant to
consequences over its life.

The Recommended Design Flood ranges
relative to IFHC were adapted to an
assumed 100 year dam life, converted to
dam construction diversion periods of I, Z
and 5 years, with I: 10000 AEP adopted
for High IFHC, and rounded as follows:

RDF AEP for period

IFHC 1 year Z years 5 years
High I in 100 I in ZOO I in 500
Signt I in 100 I in ZOO 1 in 500
to to to
1 in 10 I in ZO I in 50
Low I in 10 I inZO 1 in 50
to to to
I in 1.6 I in Z.5 1 in 5.5

At that stage, while IFHC included risk to
life, there was no consideration of specific
risk to life criteria, but just a general risk

to the commmUty, and of comse to the
dam under construction and the site.

It was also pointed out in the 1986
Guidelines that for proposed dams, the
cofferdam could be designed to withstand
overtopping, with an equivalent effect on
the diversion flood provisions. Also, once
the main dam was constructed to a higher
level than the cofferdam, the dam then
controlled the safety level.

It was noted that if for the relatively short
construction period seasons of lower flows
are selected, then analysis of partial series
floods may be more appropriate than
annual flood series.

AZ.8.3. Proposed Dams.

In the case of a new dam under
construction, the dam break threat to the
community relates to the volume of water
stored during the construction phases. A
major concern is the economic threat to
the contractor and owner for damages and
disruption. It is also likely that there will
be insurance cover for damages, but
possibly not third party.

owners need to consider the acceptability
and defensibility of the risks imposed on
the population by the dam during
construction.

Whilst it has been traditional to accept
higher risks during construction than those
that are acceptable for the completed dam
in service, it is not clear that such practice
would be found defensible in the event of
a failure that resulted in loss of life.

AZ.8.4, Existing dams.

No specific guidance was given in the
1986 Guidelines to construction floods for
the case of remedial wolks to an existing
dam, which has stored water, with a
potentially full dam break threat to the
community, plus a responsibility to
maintain supplies.

A2-i2 ANCOLD Guidelines on Selection of an Acceptable flood Capacity for Dams