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

,1
II

1

11
I
I
I
I
I
I
I
I
'I
I
I
I
I
I
I
I

A2.3, IMMINENT FAILURE FLOOD
(IFF, SCF, DCF).

A2.3.1. The Term Imminent Failure Flood
(IFF)

The term IFF was introduced in the 1986
guidelines, with the IFF/RDF ratio as an
arbitrary index to assist in a preliminary
check for spillway inadequacy, and for
priority of detailed investigations for
remedial measures.

It was not intended that IFF be estimated
on a rigorous hydrological basis, or be
used as a desigu tool. However it was
interpreted in these ways leading to many
discussions on the estimation of IFF to
rigorous hydrological methodology.

In risk context, IFF, as defined in 1986,
does not give a correct, or consistent
interpretation of dam failure (or safety). It
was accepted that the actual failure stage
could not be estimated accurately, and the
procedure for IFF was causing confusion.
During the revision process of the 1986
guidelines, there was considerable
pressure to replace this term with a more
suitable term.

A2.3.2. The term Safety Check Flood
(SCF) was,adopted in ICOLD papers as
the assessed safe flood limit, based on
engineering analyses and judgement, with
the emphasis on maintaining dam integrity
(Berga, 1995). This was then proposed in
early draft AFC Guidelines to replace IFF.
The definition was similar to IFF and in
effect was the same stage (AN COLD,
1997).

However it was subsequently pointed out
that strictly this term was also incorrect as
it could imply the dam would be safe at
that flood stage. This may not be so and,
as for IFF, it is not possible to exactly
calculate the safe level.

A2.3.3. Dam Crest Flood.

The term Dam Crest Flood (DCF) was
then proposed, simply as a convenient and
unambiguous indicator to provide a
consistent basis for preliminary
assessment and consideration during the
risk process, with no assumption on
failure or safety.

A2.3.4. Safety and Failure Stages.

It is not feasible to assess an exact dam
safety, or failure, condition. Estimates can
be made for all these conditions using
deterministic methods, but these in effect
tend to be the "sign off level" beyond
which the designer is not prepared to
consider the dam safe.

The flood of record is another practical
indicator, and if it is a higher level than
the estimated sign off level, it is usually
taken as the flood stage which safely can
be assumed.

While DCF may be reasonably safe for an
embankment dam against an overtop
failure, there could be piping through the
upper dam section if the core does not go
to the crest, or if the upper crest section is
not adequately protected with filters to
prevent piping, or there is cracking due to
drying of the crest.

It is also possible that a smaller flood than
the DCF could cause scour at the spillway
andlor toe of the dam causing breaching of
the embankment, or instability of a
concrete dam.

On the other hand, concrete dams can
generally accept some overtopping, but
again it is difficult to define this level
exactly. The flood of record may indicate
the dam can safely sustain a higher level
of flood load than the estimated "sign off
level".

ANCOLD Guidelines on Selection of an Acceptable flood Capacity for Dams A2-5