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

5. HYDROLOGIC SAFETY

5.1. Failure Modes.

Flood failure due to inadequate spillway
capacity may include such failure modes as:
. overtopping and breaching of an
embankment dam, or unraveling of a
rockfill dam;
. stability or sliding failure of a concrete
dam under the additional flood
loading, and/or scour of abutments and
toe;
o overtopping of spillway walls, causing
erosion of the spillway stilling basin,
or toe of the dam, to cause
undermining and failure of the dam.

Failure of an upstream dam could create a
flood which could initiate a flood type
failure of a downstream dam in a cascade
effect.

A flood could initiate a p'pmg failure, or
failure following earthquake damage, bllt these
cases are not specific to spillway capacity.

5.2. AFC Options.

5.2.1. Existing Dams.

Options to be' considered where the actual
flood capacity of an existing dam is less than
the reqnired AFC can include:
. the "do nothing'~ case;
o performing remedial works to upgrade
flood provisions;
o using non-structural alternatives such
as: flood warning systems; planning
controls and relocation of properties
(effectively reducing consequences of
failure); and restrictions on reservoir
operation (effectively, reduces the
probability of dam failure). Some of
these options may be used as interim
measures once a spillway inadequacy
has been identified, while detailed
investigations are carried out.
o staging remedial works over a period
of years for progressive improvement
in safety, depending on consequences
of failure and fund availability. This
may achieve optimum overall

incremental improvements to safety
over a portfolio of dams for effective
use oflirnited funds.
o upgrading the flood capacity in
conjunction with storage
augmentation.
o breaching and abandoning the dam,

5.2.2. Proposed dams.

In deterministic design, it is considered
prudent to provide conservative flood capacity
to allow for the long-term future with possible
changes in development, social considerations
and consequences, flood estimates and
technology, In most cases the incremental cost
of providing conservative spillway capacity
will be relatively smaiL

1
i

.
1
~
t
t
j
,
j

In a risk study, options could include non-
stmctural solutions, such as dam break
warning and evacuation plans, but risk
management should consider the long term
safety aspects.

6. RISK PROCEDURE FOR AFC

6.1. General Risk Process.

6.1.1. Context
Initially management must set the conttp:t in
regard to corporate aims and responsibilities
for risk management.

Management has to consider the broader issues
of business risk in regard to a potential dam
failure with responsibility for services and
protection of the community and environment.

6.1.2. Load cases

Preferably the initial risk study should
consider all load cases to identify potential
critical failure events and priorities for
detailed investigations.

The detailed procedures for the risk assessment
process should be based on the current
ANCOLD Guidelines on Risk Assessment, The
comments below highlight aspects relevant to
hydrologic safety.

:
i

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

1