Book 8 - Parliament of Victoria

FLOODS: WATER STORAGE MONITORING
1652 COUNCIL Wednesday, 1 June 2011
constraints of the design of these storages, including not
compromising their primary function of water supply to
urban and rural customers.
Each water storage facility has a filling curve for the
water authority to use a guide for maximum storage
levels during the period from May to October.
However, while the filling curve can provide flood
mitigation benefits, its primary function is to maximise
the storage potential for consumptive use. The
government does not propose to change the design
purpose of the major storages and is satisfied that the
current internal review process being undertaken by
rural water corporations will ensure that all practical
steps will be taken to minimise floods to the greatest
extent possible.
It is important to remember that Victoria has only
recently experienced its worst and longest drought on
record — over 13 years — with a number of major
irrigation storages being virtually emptied whilst others
were drawn down to record low levels. During this
period farmers experienced record low irrigation
allocations, forcing many to abandon farming, and
many towns faced prolonged periods of severe water
restrictions. It is therefore imperative that any proposals
to significantly increase flood mitigation beyond what
is currently undertaken not compromise the reliability
of existing entitlements, including supply to towns. The
potential for flood mitigation that can be provided by
storages varies significantly depending on the storage’s
size in comparison to flood inflows, the design of the
storage and whether the storage has the capacity to
absorb the flood inflows. I would like to cite a few
examples of how these factors affect flood mitigation.
As Mr Barber knows, Lake Eildon is the only
Goulburn-Murray Water storage which has a formal
filling curve that is defined in the governing bulk
entitlement. The purpose of the filling curve is to limit
the risk of not filling the storage in the May to October
period if releases are made to maintain airspace to
provide additional flood mitigation benefits. The
adopted risk is 5 per cent, and releases would be
undertaken to prevent the storage filling to above the
filling curve until October or November if conditions
turn wet. Operating to the filling curve will provide
approximately 300 000 megalitres of airspace to absorb
floods in the following two months.
Dams such as Lake Eppalock were built for water
supply irrigation purposes, and as such they are
managed to maximise the water resource available for
water entitlement holders. The operations of the dams
at lower levels to provide flood mitigation benefit
would be at the expense of water entitlement reliability.
Any change to that would require consultation with and
possible compensation for the affected entitlement
holders.
Honourable members interjecting.
Mr RAMSAY — Deputy President, is there another
debate happening on the other side of the chamber?
The ACTING PRESIDENT (Ms Crozier) —
Order! I ask members on my left to make their
conversations a little less audible.
Mr RAMSAY — While Lake Eppalock is not
operated to provide a flood mitigation benefit, by its
very nature as a fixed-crest spillway it does provide
significant flood mitigation. For instance, during the
January 2011 event the instantaneous peak flow was
approximately 140 000 megalitres per day. Due to the
fixed-crest spillway creating surcharge volume in the
lake, the peak outflow was reduced to
80 000 megalitres per day.
Mr Barber interjected.
Mr RAMSAY — Surcharge, Mr Barber. GMW can
only prerelease water when there is a high level of
certainty that the prerelease volume will be recovered
by the end of the event — for example, if there was a
reliable prediction of rainfall. Typically this would be
based on a four-day Bureau of Meteorology forecast.
Given the outlet capacity of only 1000 megalitres the
day, this means that very little airspace can be created
in the time following the prediction of rainfall before
the event occurs.
Lake Eppalock has a storage capacity of more than
312 000 megalitres. The historic record shows great
variability in inflows to Lake Eppalock. While there
was unprecedented rain in January, there remains
significant uncertainty as to the volume of inflow that
might be received in the coming months. GMW has
recently commenced work to review the impact on
entitlements of operating within the greater airspace.
The outcome of this work will be used to further inform
discussion with key stakeholders in the community.
Lake Buffalo and Cairn Curran Reservoir, the only
other gated spillway storages within GMW, have local
operating rules in place which are designed to manage
the storage levels within the limits of the bulk
entitlement. Local operating rules which allow for the
creation of airspace are driven by dam safety
considerations.
Southern Rural Water has existing storage filling curves
for Lake Glenmaggie, which still apply. However, it