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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higher safety standard) than the AI'C options
related sole1y 10 risk to life criteria and risk
costs.

The selection of AFC involves consideration
of the estimated probability of dam failure, in
relation to the incremental consequences due
to dam failure under flood conditions, above
the natural flooding consequences prior to dam
failure. There could be practical and legal
difficulties in actually assessing, and applying,
the concept ofincremental impacts after a dam
failure.

The outflows up to the AFC, without dam
failure, are likely to cause severe damage and
disruption to the dam and appurtenant works,
and certainly to the community and river
valley upstream and downstream. This can be
regarded as acceptable for overall community
safety against the incremental impacts of dam
failure, provided the assessed damage will not
cause the dam to fail, releasing the stored
waters.

Under such flood conditions, without dam
failure, significant repairs may be required to
the dam, spillways and other appurtenant
works, and for downstream communities and
the river valley.

4.2. Inter relation of AFC, DCF & SDF.

The assessment of AFC and spillway
provisions involves consideration of two flood
stages:

(i) Dam Crest Flood (DCF), an indicator
for initial hydrologic safety assessment
and dambreak studies;

(ii) Spillway Design Flood (SDF), the
"serviceability" flood for operational
spillway hydraulic sizing and consideration
of optimum overall spillway provisions to
provide for the AFC.

4.3. Dam Crest Flood (DCF).

4.3.1. Definition
The Dam Crest Flood (DCF) is defined as
the flood event which, when routed through

the reservoir, results in a still water level in the
reservoir, excluding wave effects, which:
. for an embankment dam is the lowest
point of the embankment crest;
. for a concrete dam is the uppennost
level of the crest, excludIDg handrails,
and nonnally parapets;
. for a concrete faced rockfill dam, is
the lowest point of the crest.

The tenn Dam Crest Flood replaces the 1986
guidelines tenn "Imminent Failure Flood"
(IFF) which was not strictly correct and
inconsistent in application,

DCF is simply a convenient and unambiguous
indicator that allows the probability to be
detennined of the still water level in a
reservoir reaching a specified level (as defined
above).

DCF provides a consistent basis for initial
consideration in the process of dam flood
safety risk assessment. The DCF stage can be
used initially as a guide to potential spillway
requirement or inadequacy, and for dam break
studies; and subsequent! y in the selection of
AFC to meet risk criteria.

4.3.2. Parapet & Wave Walls.

There could be cases where a wave wall,
or parapet, on a concrete faced rockfill
dam (as an extension of the concrete
facing), concrete or embankment dam, has
been designed to take full flood surcharge
loading or assessed as capable of
supporting that load. In such a case, the
DCF could be considered at the top of the
wave wall or parapet (Fig 4.1),

4.3.3. Estimation of DCF and associated
AEP.

The flood event that, when routed through
the reservoir results in the specified reservoir
level (4.3.1), can be produced by an infinite
number of combinations of inflow flood and
initial reservoir level, rather than any single
design flood, as implied in the terminology
"inflow design flood" general(v used in dam
design flood guidelines.

ANCOLD Guidelines on Selection of an Acceptable flood Capacity for Dams 7