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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9. FLOOD MANAGEMENT
CONSIDERATIONS.

A number of other matters associated with
flood provisions and flood control are
commented on below, with general indications
for detailed consideration. It is considered that
specific guidelines are not appropriate as most
cases are site and situation specific.

9.1. Gated Spillways.

9.1.1. Dam Failures.

Data on dam failures show not only a high
proportion of dam failures due to floods, but
also a significant number of these failures due
to malfunction and operational
mismanagement of gates under extreme storm
and flood conditions (Reference ANCOLD
Guidelines on Dam Safety Management,
1994), Unfortunately limited data is available
on gate operation problems with Australian
dams.

9.1.2. Gated Operation.

The onset of a never before experienced flood
is unlikely to be quickly recognised by
operators from experience limited to historical
records used for flood operation. Case studies
available indicate a serious possibility of
human misjudgement, and even panic, under
extreme danger situations.

It is therefore possible that a potential dam
break situation will not be recognised in time
and/or quickly lead to an alert to the
appropriate emergency authority.

There will also be reluctance by owners and
operators to fully open gates because of the
possibility of causing premature major
flooding downstream (or prematurely trigger a
major evacuation) with potential legal
ramifications if the dam did not subsequently
fail.

9.1.1. Aspects tf' be considered include: .

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Gates - mechanical, electrical, and
manual control equipment; power back
up; ability to raise gates at a rate
relative to an extreme flood rise;
reliability; maintenance; testing; and
potential malfunctions.
Operational procedures - level of
experience of operators; potential
human errors and misoperation,
including management decisions;
effects of changes in staff; procedures
for flood control, pre-releases;
freeboard; multiple events;
Potential failure modes; assumptions
on numbers of gates operating; effect
of debris, and earthquake;
Manuals for operation, emergency,
contingencies; regular review,

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9.2. Joint Probabilities.

Book VI (Nathan & Weinmann, 1999)
includes guidelines on the incorporation of
joint probabilities associated with inflow
floods and initial reservoir level, and of inflow
floods with flood flows in downstream
tributaries. Matters include:
(i) The joint probabilities of concurrent
winds and reservoir levels with extreme flood
conditions relate to assessment of potential
risk of failure due to overtopping. It may be
sufficient to adopt a mean storage level
associated with, say, the top 10% of inflow
floods.

However, if the likely initial drawdown has an
appreciable impact on outflow floods, then a
more rigorous approach based on the explicit
consideration of the joint probabilities of
inflows and storage levels is recommended.

The analyses should be based on either a long
period of representative historic data or else on
synthetic infonnation derived from simulation
models.

The influence of seasonality and
correlation between inflows and initial
reservoir levels can be incorporated into
the joint probability analyses, and guidance

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