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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a5$es&ed for each dam (FSSR 10, lnst.
Hydrology 1983).

Comments on flood management,
including gates, are given in section 9.

A2.5.2. Freeboard Provision,

The deterministic basis for freeboard, to
set the crest level, relates to the estimation
of the "dry freeboard" component above
the routed flood level.

The need for freeboard was initially
recognised with embankment dams
(USBR, Design of Small Dams) to
provide safety against overtopping and
damage due to wind waves, both during
the normal operations and floods. It was
also recognised that there were
uncertainties in flood estimation.
Procedures were developed to estimate
effects from waves and other factors.

Other factors considered were settlement
of the crest, reducing freeboard, and crest
cracking and weaknesses in the upper
section of the banle

Minimum freeboard allowances are often
adopted (lCE 1996). Some Australian
authorities previously considered the
provision of freeboard as in effect an extra
factor of safety; for example, the crest was
set for a flood 20% larger than the design
flood.

The inflow design flood practice for State
Rivers & Water Supply Commission, Vic
for many years was to specify an
"embankment flood" (in effect a
forerunner of IFF) an order of magnitude
higher (1:100000 AEP) than the 1:10000
AEP design flood.

A2.5.3, Factors to Assess.

Wind effects and fetch for wind wave run-
up and wind set-up. Procedures are given
in Saville et al (1962), USBR (1981) US
Corps ofEngs (1976) and Donelan I
JONSWAP method (CIRlAICUR, 1991).

Other factors include assessment of
potential wave effects from landslides,
seismic action (Davidson & McCartney,
1975) and seiche, surges, (Kovacs 1984,
Shand, 1984).

Assessment of pre-flood reservoir level;
wind speeds, direction and duration
relative to fetch, during the flood event
(the Bureau of Meteorology can provide
information); and, if the dam is gated,
potential malfunction or misoperation of
gates during the flood.

A2.5.4. Detemiinistic Basis,

European Countries, and many other
countries adopt as a deterministic basis,
full level, or even spilling, as the pre flood
level, with bottom outlets not operative,
and if the spillway is gated, generally a
check with a proportion of gates
inoperative (ICE, 1996; SANCOLD,
1991).

A2.5.5. Risk Basis.

An important consideration is the degree
of correlation between extreme rainfall
events and extreme wind of a critical
direction, and between these factors and
prior storage level. Often it will be
necessary for persons knowledgeable in
hydro-meteorology to reason, from first
principles, the existence or other wise of
significant correlations, because of a lack
of extreme event data that wonld enable
calculation of the correlations.

For rigorous risk assessment, all the
factors can be included using probability
distribution functions and statistical
procedures.

For a scoping study, assumptions can be
made, with simple response factors. The
Risk Supplement, Appendix 3, sub-section
A3.6, includes an example of a simple
system response with gates.

A2-8 ANCOLD Guidelines on Selection of an Acceptable flood Capacity fOr Dams