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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Appendix 2. COMMENTARY
(References in italics refer to the sections of the main guidelines)

A2.L BACKGROUND,
(Ref. Chapter 1, Introduction),

A2.U. ANCOLD.

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1
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AN COLD set up a WoIking Party in 1992 to
revise the 1986 ANCOLD Guidelines on Design
Floods for Dams in the light of developments in
assessment of existing dam spillways and extreme
flood estimation, and to integmte the ANCOLD
Guidelines on Risk Assessment published in 1994
(RA94).

The recommendation for national guidelines,
leading to the ANCOLD 1986 guidelines, was
made at the 1981 Workshop on Spillway Design
(AWRC, 1982). The diversity and lack of
knowledge on spillway design pmctices
throughout Australia was brought to attention
with the results of the joint IEAust (National
Committee on Hydrology & Water Engineering)
and ANCOLD survey of Australian dam design
flood data (ANCOLD-IEAust, 1981).

An initial draft revised ANCOLD Guidelines on
Design Floods for Dams (ANCOLD, 1995), was
distributed at the 1995 ANCOLD Annual Dams
Conference in Christchurch for testing and
comment to the Working Party.

Testing identified a need to review risk to life
criteria and clarify the risk procedures in RA94
(Murley, 1996).

Draft Guidelines with the revised title Selection
of Acceptable Flood Capacity (AFC) for Dams,
were prepared and distributed (ANCOLD, 1997),
having regard to proposed changes to RA94; the
then draft of the ANCOLD Guidelines on
Assessment of the Consequences of Dam Failure,
and the then draft revision of Chapter 13, ARR87,
Estimation of Large and ElI.ireme Floods.

Subsequently, further discussions on the risk
procedures and criteria indicakd significant
changes required for RA94. A Position Paper on
the AFC Guidelines was then issued (AN COLD,
1998) to highlight proposals for finalising the
AFC guidelines. This has been, subject to the
review and revision of RA94 (in hand); and
ARR87 Chapter 13, now issued as Book VI
(Nathan & Weinmann, 1999).

A2,l.2. FLOOD PROBABILITIES.
(Refer sub-section 1.4.4.)

It is recommended when communicating risks to
managers and the community to relate the risk to
the period of an assumed life for the dam,
(allowing for periodic review and remedial works
as necessary), or a period of occupancy or
personal memory of an area. Authorities assume a
dam life between I 00 and 300 years for
accounting purposes.

Table A2.1. below indicates probabilities of
Annual Exceedance Probability (AEP) relative to
periods.

The statistical relationship is:

Y -year probability flood = [ I - (1 - AEP)' ]

Where:

AEP is the annualexceedance probability of the
flood,and

Y is the number of years in the period.

ANCOW Guidelines on Selection of an Acceptable flood Capacity for Dams A2-3