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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debris effects, In some cases a
preliminary inspection may be
sufficient;
. Prepare preliminary estimates of the
Incremental Flood Hazard Category
(IFHC) using ANCOLD Guidelines
on Assessment of the Consequences
of Dam Failure with estimates of
population at risk (PAR) and extent
of consequences for risk studies.
This could include preliminary
estimates of loss of life, economic
factors, business risks, and
subsequent detailed studies for risk-
reduction options,
. This study should consider whether
there is d limiting flood capacity
beyond which the incremental effects
of dam failure may be acceptable;
generally called the Base Safety
Condition, refer Appendix 2, sub-
section A2. 7.3.

(v). Prepare event/fault tree analyses.
These are done to assess the critical flood
failure scenarios and relative probabilities
to provide the bases for assessment of
options and risk costs and risk to life. Refer
RA94, sub-section 5.3 and reference Pate-
Cornell (I 984) for information on event and
fault trees, A scoping study could provide
sufficient information without a detailed
event tree analysis,

(vi) Estimate the potential loss of life.
This includes a detailed consideration' of
the PAR and operative emergency warning
and evacuation plans for comparison with
risk to life and other criteria, and risk
reduction options, Noting that the
population within a few kilometres
downstream of the dam are at the highest
risk, refer Appendix 3, Risk Supplement,
sub-section A3.3.

(vii) Assess the AFC options.
Do a preliminary assessment using the
failure probabilities and estimates of
possible loss of life, with options for
providing the flood capacity, to ,initially
compare with risk to life criteria within the
total risk from all loads;

(viii) Estimate the economic and other
factors.
An estimate of the risk costs will consider
the incremental cost effectiveness;
business risk costs; and other quantifiable
and non-quantifiable,' and
incommensurable impacts. It should be
realised that the environmental impact of
a dam failure would be likely to result in
major and irreversible changes to the
valley environment.

(ix) Prepare ALARP Assessments.
Consider all the incremental costs and benefits
of risk reduction options, and the practicalities,
both structurally and financially relative to risk
criteria,

(x) Collate all risk data.
Collate the data to compare cost effectiveness
of alternative spillway and non-structural
measures, and provide comprehensive
information to the decisionmaker.
The report should provide an Executive
Summary in clear language for managers
with a realistic appreciation of the
uncertainties and credence of risk study
outcomes.

(xi) Make sure the final decision provides
acceptable overall practical and cost
effective safety levels.
The owner should consider the criticality of all
load events on dam safety, including the
uncertainties in risk estimates. The final
decision must also consider business policy
and political, legal, community and regulatory
issues; and authority perceived integrity,
credibility and responsibility.

7. ASSOCIATED FACTORS.

7.1. Conditional probability of failure;
Hydrologic Safety Assessment.

7. 1. 1. System Response.

For risk assessment it is necessary for each
load event to assess conditional probability of
failure response curves and probability
distribution functions (pdt) to estimate the
total probability of failure, taking into account

ANCOLD Guidelines on Selection of an Acceptable flood Capacity for Dams 17