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

I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I

the. uncertainties in estimating the actual
failure stage.

7.1.2. Embankment dams:

(i) The bank may be able to SlUVive under
some overtopping, depending on the dam
arrangement and the combination of depth and
duration of overflow. Inspection of the dam
and plans and historical information can assist
in making ajudgement.

In deterministic assessment it is usually
assumed the dam will fail as soon as
overtopping develops.

In risk assessment it may be the judgement
that, in considering the dam and overtopping
duration,a pdf with say overtopping of 100
mm to 200 mm would be the safety limit; in
others 300 mm to 600 mm (Appendix 3, sub-
section A3.6).

(ii) The other case to consider is the possibility
of piping through the top section of the dam
before the flood surcharge level reaches the
crest. This depends on the core and filter
details, and crest protection against cracking.

7.1.3. Concrete Dams.

An assumption is required on stability over a
range of flood surcharge levels from an
estimated "safe" flood surcharge level to a
maximum level, which may well be above the
crest, This can be estimated using a
deterministic assessment, which could be the
flood of record. For a risk study, again a pdf
can be assumed.

7.1.4. Rockftl/ Dams.

An assumption is required for the overtopping
range before unravelling would occur. With
mesh to the crest on the downstream face,
some overtopping can be assumed (refer sub-
section 4.5.3).

With concrete faced rockfill dams, an
assessment will be also required on the
integrity of any parapet or wave wall under
surcharge load.

7.1.5. Scour Impacts.

Downstream scour at the spillway or toe could
cause breaching of an embankment or
instability/sliding of a concrete dam. This will
require assessments for conditional
probabilities of breaching over the range of
spillway flows.

7.2. Risk to Life criteria.

(Refer comments on criteria and estimation of
potential loss of lift Appendix 3).

7.2.1. ANCOLD Key Factor.

ANCOLD RA94 adopted risk to life criteria as
the prime safety requirement. The practicality
and acceptability of meeting risk to life criteria
should be assessed.

Options should also be considered, in
conjunction with ALARP, in regard to
incremental economic benefits and potential
social 'and environmental impacts, which could
be so severe as to indicate the need for a risk
level lower than required by the risk to life
criteria,

7.2.2. Uncertainties.

While estimates of potential loss of life are
critical for assessment of risk to life criteria,
methodology is still developing, and the few
models available have significant uncertainty
factors in the loss of life estimates (refer
Appendix 2, sub-section A3.2),

The associated estimation of warning time also
requires careful consideration on the
practicality of dam break warning and
evacuation plans as discussed in sub-section
7.3,

7.2.3. Estimation of risk to life

The relative probabilities of failure for all
critical load events can be estimated from
event trees, or scoping risk studies. These can
be combined with estimates of possible loss of
life, to compare with individual and societal
risk to life criteria.

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