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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(vi) Risk cost analyses. Thcs<:; are
generally assessed on annualised
bases, a methodology applicable to
recurring incidents. This should be
recognised in reviewing risk
outcomeS and risk costs. A dam
failure is not an annual repetitive
event, but a single catastrophic
event, with once off total loss of
life and damages, not recurring
annualised losses (NRC, 1985).
(vii) While useful for comparative
assessment of options, annualised
information can be misleading
when applied to extreme flood
events with low notional
probabilities, but catastrophic
results. It tends to indicate
relatively low net annual direct
cost benefits, and very high costs
. to save a statistical life (Cost to
save a life), compared with
repetitive failure cases in other
industries.
(viii) Note that there are uncertainties in
the risk estimates and that not all
critical impacts can be quantified
in dollar terms.
(ix) The owner has the responsibility
to make the decisions in regard to
business risk and responsibilities
to the community; the risk data are
there to assist with decisions, not
prescribe them,

1.4. Dams and Floods.

1.4. I. Flood failures.

In considering dam hydrologic safety, records
show dams continue to fail. Floods, often
combined with malfunction or misoperation of
gates, and with management/operator errors,
are a major cause of failure, particularly for
overtopping of embankment dams (some 70%
of Australia's large dams).

The problem facing owners who have financial
constraints, but major liabilities with dam
ownership, is how much flood protection, and
what level of safety, should be provided

against floods. Extreme' floods are random
events which can, but may never occur during
the operating life of a dam, say 100 to 300
years, but have the potential to cause
catastrophic consequences.

1.4.2. Community expectations.

It should be made clear to the community
that it is not practicable to provide complete
protection against every flood that could
occur.

The community downstream of a dam can also
have the (mistaken) expectation that dams are
primarily designed and operated to protect
them against all floods. People tend to have
short term memories on severe flood events,
and do not realise the events experienced and
remembered are probably no worse than a I in
20 or 50 Annual Exceedance Probability
(AEP) event, compared with potential extreme
floods of say I in 1000 AEP and greater,

1.4.3. Spillway Safety Levels.

Spillways are basically designed to protect the
dam against floods only to a specific design
flood event. Dams do provide some protection
to the community, particularly by attenuating
the more frequent floods, and protection from
the additional impacts of a dam failure at
floods up to the design event. Gated spillways
provide more flexibility, but with the penalty
of potential operational problems.

Some dams may be designed specifically for
flood protection, or include a flood attenuation
component, but again this can only be to the
safe limit for dam safety, not any feasible
flood, or flood sequence.

The annual exceedance probabilities adopted
for dam floods appear very low relative to
other risks because an imposed involuntary
critical risk, such as a dam failure, should only
add a small proportional risk to the
background risks already imposed on a
community (Refer Appendix 3, sub-section
A3.1.3). The consequences of a dam failure to
the community, and what is often not realised,
to the environment, are likely to be
catastrophic.

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ANCOLD Guidelines on Selection of an Acceptable flood Capacity for Dams