Management of Menindee Lakes 2011-2012 Issue 7 – 5 April 2012

Introduction
The flood peak in the Darling River is now approaching Wilcannia. Upstream at Tilpa the river level is falling and no
further rises are anticipated in this event. Downstream at Menindee, high inflows to the lakes system are expected
throughout April and into May, however, maximum outflows will be limited to current levels. The NSW Office of
Water and State Water Corporation are continuing to manage operations at Menindee Lakes in anticipation of the
forecast inflows.
This information paper updates current flow conditions and operations, as well as provides information on what can
be expected through April and May as the flood waters pass through the Darling River system.
In short, residents along the Darling River, from downstream of Tilpa to Burtundy, can expect an extended period of
high flow and widespread rural inundation, similar to events of 1971, 1990 and 1998.
Residents and authorities are reminded to check with the NSW Office of Water in Buronga, approvals that
might be necessary before undertaking any earthworks to protect infrastructure or crops.

River Operations
Darling River Flows and Menindee Storage Volume
The flow in the Darling River main channel at Wilcannia is currently at 39,000 megalitres per day and the flow in the
Talyawalka Creek is over 50,000 megalitres per day. This combined flow is expected to approach a maximum of
about 100,000 megalitres per day over the next few days causing major flooding. This is slightly lower than the
early forecasts but comfortably within the range of planning expectations.
Downstream at Menindee, releases from the lakes system have been made in preparation of the anticipated
significant inflows. The lakes are currently 82 percent full and can hold a further 600,000 megalitres under
surcharge conditions. A large proportion of this available airspace will be used to manage forecast inflows of
between 60,000 and 70,000 megalitres per day during April.
The main weir gate has been re-positioned in the water to limit outflows (measured at Weir 32) to a maximum of
35,000 megalitres per day, making town flooding and conditions immediately downstream of Menindee no more
severe than what is currently being experienced. This will also cause lake levels to rise throughout April and May.
The NSW Office of Water will aim to begin reducing outflows from the lakes as soon as possible to allow water
levels to fall and alleviate flooding in the Menindee town area and downstream. However this is not expected until
May. Minimising outflows will also ensure that the lakes are full at the end of this flood event to provide maximum
resource availability into the future.
Lower Darling River Flows
Downstream flooding could be similar to that experienced during the 1998 flood which had a comparable peak flow
at Bourke of 230,000 megalitres per day (13.78m gauge height). Menindee releases to the Lower Darling in that
event reached 46,500 ML per day through Weir 32 (7.45m gauge height or 10.0m at the Menindee Town gauge).
The targeted peak flow for this 2012 event is 35,000 megalitres per day through Weir 32 and with flows from the
Talyawalka, combined flows in the Lower Darling immediately downstream of the Menindee Lakes will be as high
as 50,000 megalitres per day. Historically, flows of this size, generally flow evenly to the Lower Darling and the
Great Anabranch.
As far as possible the NSW Office of Water will reduce lake outflows at the time of peak Talyawalka inflows below
Weir 32 to minimise the influence of the Talyawalka on the Lower Darling. If that can be achieved then a flow pulse
of 22,000 - 24,000 megalitres per day in the Lower Darling will not be experienced but rather steady flow conditions
of around 18,000 to 20,000 megalitres per day produced by the Menindee outflows for the past few weeks, will be
seen.
Water levels in the Lower Darling River at Pooncarie and Burtundy are both rising very slowly. The NSW Office of
Water will aim to keep peak flow in the Lower Darling below that of the 1998 event, and no more than about 24,000
ML per day (7.7m gauge height) at Pooncarie and 22,000 ML per day (7.7m gauge height) at Burtundy.
Great Darling Anabranch Flows
Flow in the Lower Darling at the Great Anabranch effluent has been relatively steady throughout March at around
18,000 megalitres per day, commensurate with the steady flows through Weir 32. In the Anabranch at Wycot the
flow gradually rose through March to reach 13,000 megalitres per day and some 4.2 metres. At the peak of flow,
expected in late April/May, levels are not expected to exceed 5.2 metres. A few thousand megalitres per day is now
flowing in the lower reaches of the Anabranch and joining the Murray River. It is anticipated that this full
connectivity through the Anabranch system will last at least through May, with significant flow volumes expected to
reach the Murray River.
Combined Murray and Murrumbidgee River Flows
The flood peak in the Murrumbidgee River is current downstream of Hay where the river is now falling from 12.9
metres. Peak flow of around 40,000 megalitres per day is expected at Balranald next week. This water will then
enter the Murray River and produce flows at Euston Weir of up to 60,000 megalitres per day from mid April.
It is expected that the Murray peak flow will pass Wentworth in mid to late April with the Darling River contributing
steadily flows of around 18,000 to 22,000 megalitres per day during this period. Flows from the Great Darling
Anabranch will be much longer in arriving at the Murray and have minimal impact on peak flows.
It is expected that high flows to South Australia will persist from mid-late April to early

 

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