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
I

wind waves and set-up which could
cause significant overtopping with a
threat to the dam integrity;
. dams where failure would result in
catastrophic consequences; this
involves owner responsibility, but
noting freeboard provides additional
safety at additional cost;
. proposed dams should normally be
provided with freeboard because of the
potential for major future changes, and
failure consequences, downstream.
The incremental cost of freeboard is
generally relatively small for a
proposed dam.

4.5.3. Concrete and Rockjill Dams.

The AFC for concrete and rockfill dams can be
adopted with what is termed "negative,
freeboard" with the following considerations:
. concrete dams: allowing some
overtopping to the theoretical limit for
sliding, or instability under
overturning or downstream scour
effects (the flood of record can be an
indicator); alternatively, a risk
probability study can be done;
. rocktill dams 'W;th steel, mesh
protection on the downstream face
right up to the crest; some overtopping
allowed at the theoretical limit for
unraveling. (lCOLD Bulletin No 89,
1993); ANCOLD 1994 suggests 3m
for deterministic design, but a range
would be used with a probability
distribution function (pdt) for a risk
study.

4.5.4. Embankment Dams.

The normal deterministic approach is for the
maximum flood surcharge level to preferably
be at or below the top of the impervious core
for embankment dams. For a risk study a pdf
can be assumed.

While AFC is nominally the critical case to
determine the crest level, a check should be
made for the case of SDF still water level plus
a conventional "dry" freeboard for winds
relevant to the SDF event for operational

conditions. This could indicate the crest level
should be set higher than the AFC level; the
SDF and supplementaIy spillway provisions
should then be re-assessed.

4.5.5. Freeboard Estimation & Flood
Routing.

References for estimation of wind wave and
other factors relative to freeboard, and for
flood routing are given in Appendix 2, sub-
section A2.5. (the Bureau of Meteorology can
provide advice on relevant winds for the
condition being assessed).

4.5.6. Economic considerations.

The decision on freeboard provision must take
into account that the provision of "dry"
freeboard can in effect increase the DCF, and
safety level, at least an order of magnitude,
with associated reduction in risk, but increase
in cost.

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