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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possible loss of power, communication,
and access, with chaos, well before a
dambreak condition.
To provide sufficient lead time for the
emergency authority, the dam operator has
to make an early decision to alert the
emergency authority that, in addition to
the flooding emergency, the dam may fail
(case studies of catastrophic incidents
indicate a reluctance by owners and
operators to advise the public of potential
dangerous problems).
This involves recognition by all that, for
protection of the community, plans should
be initiated as soon as possible, even
though the dam may not fail (this is
comparable to any potential risk situation,
such as a bomb scare; the people are
evacuated first, not wait until it is certain
whether it is a bomb or not);
The effectiveness of owner dam
emergency plans for threat recognition,
and action and advice to the relevant
emergency authorities;
For small catchments, there would be
flash flood conditions with no practical
time for pre-dam break emergency
arrangements or an alert.
The availability of power and
communications in an extreme flood
situation;
The effectiveness of the emergency
authority;
Community reactions and co-operation.
An effective on-going program of review,
update and education including the'
community at risk.

A2.7. FALLBACK OPTION.
(Refer to section 8, AFC screening
process) .

A2.7.1. Background.

The Working Party received many
comments that a fallback prescriptive
option could be relevant for Authorities
with limited funds, or dams where the
spillway capacity and/or raising the bank
could be upgraded at relatively low cost.
The October 95/97 drafts included
indicative fallback options.

The Worldng Party continued to be
concerned that a detenninistic fallback
option was not compatible with risk
assessment. A fallback might not provide
an appropriate optimum economical
solution on one hand, or an appropriate
risk safety level on the other hand.

However, at the present stage of
development of risk assessment
methodology, there may be concern, in
some situations, that the defensibility of a
risk assessment is such that a deteffi1inistic '
fallback requirement should be adopted.

Reference should be made to the current
Risk Guidelines for possible restriction
due to the criteria and methodology for
individual risk to life.

Also, ANCOLD risk criteria, particularly
the individual risk criteria, apply to total
risk from all causes and loads, and within
this there could be some flexibility on
acceptable flood risk.

The consideration of the Working Party in
1997 was to exclude a fallback option,
However, the fiffi1 conclusion from many
subsequent comments was that a fallback
option should be included in the AFC
Guidelines.

A2,7.2. Fallback provision.

The provisions in Table 8.1 are a
translation of the 1986 RDF table to the
current IFHC categories, still with PMF as
the upper safety limit for the Extreme
IFH C case, but transitioning to the PMP
design flood, as the current expert
hydrologist ''best estimate", for High A
and, with qualifications, as the upper limit
for High B and C IFHC.

Qualifications of alternative upper limits
have been given for High B and High C
IFH C cases because the procedure for
assignment of AEP to the PMP and PMP
Design Flood could result in imposing an
inappropriate higher standard on small
catchment dams compared with larger

A2-iO ANCOW Guidelines on Selection of an Acceptable flood Capacity for Dams