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
1

II
i
I
IJ
I
I
I
I
i I
I
I
I
I
I
I
I
I
I

There may be saddles near the dam, or on
tributary creeks, suited to lower cost
supplementary spillways such as a fuse
plug or unlined supplementary spillway
with control crest.

The economic and hydraulic effectiveness
of a combination of main spillway and
supplementary spillways can be assessed
from the site topography and an economic
study.

A2.4.2. Existing Dams.

The options to increase the safe flood
capacity of an existing dam will relate to
the site topography, type of dam and order
of increase in flood capacity required.

Some options, which can include
combinations, are:

widen the spillway
raise the dam to provide more flood
storage (Googong, AC1)
deepen the spillway for increased capacity
and install gates, or fabridams to retain
storage to FSL.
combine with augmentation of storage
(Glenbawn, NSW).
improve spillway hydraulic capacity by
installation of a labyrinth crest
arrangement (Little Para, SA); while these
provide better performance at lower flows,
they can drown out at bigger flows.
construct supplementary spillways,
including fuse plug arrangements.
(Oberon, NSW).
for concrete dams, strengthen with
prestressing to take the additional flood
load (Cataract, NSW; Glenmaggie,
Victoria).
provide supplementary spillway capacity
over the crest and on the downstream
batter with a concrete cascade (Melton,
Victoria).

The dissipation downstream, and effect on
the dam, will need careful consideration
for all arrangements to increase flood
capacity, particularly if the increased
flows are to be taken by the existing

spillway dnrte and dissipator. Model tests
are desirable for all spillway proposals.

It may be feasible to reduce the required
flood capacity by reducing the
consequences and impacts of a dam failure
to meet risk criteria, Refer non-structural
options, sub-section 7.3.

A2.4.3. Unlined Spillways.

The spillway chute can be unlined in
suitable foundation material, relative to
the possible frequency of flow, with the
acceptance of continuing future
maintenance. The acceptability of
potential erosion and sediment deposition
from infreqnent floods needs to be
carefully checked against EP A
requirements.

The quarry, suitably located, combined
\vith use of the material for the dam, can
provide an economical arrangement as an
unlined spillway, sloped (Pindari, NSW)
or stepped as a cascade (Dartmouth, Vic,
Googong, AC1).

Experience at Dartmouth and Copeton has
shown considerable rock washout can
occur requiring continuing remedial work.

Supplementary and emergency spillways,
which will only operate under rare events,
can generally be unlined, but it is desirable
to provide a fixed control crest or silL

A2.5. FLOOD ROUTING &
FREEBOARD ESTIMATION.
(Refer to sub-section 4.5, Freeboard, and
Fig 4.1).

A2.5.L Flood Routing.

Procedures for reservoir flood routing are
given in ARR87, section 7.5, and for flood
routing and freeboard estimation in USBR
(1981), ICE (1996) and US Corp, of Engs
(1984). Where there are reservoirs in
cascade, an inflow hydrograph, including
possible dambreak, will have to be

ANCOLD Guidelines on Selection of an Acceplable flood Capacily for Dams A2-7