water in new south wales - april 8, 2011... nsw office of water | issue 8 – april 8, 2011 page 3...

NSW Office of Water | Issue 8 – April 8, 2011

INTRODUCTIONThe NSW Office of Water, together with State Water Corporation and in consultation with local communities and stakeholders, are continuing to manage releases from the Menindee Lakes to the Lower Darling River and Great Darling Anabranch as floods from northern NSW and Queensland pass through the Darling River system.

This is the eighth information paper produced to provide details on the inflows, levels, operations and management of the Menindee Lakes. It is expected that a final community information paper for this event will be issued in early May 2011.

The Queensland flood peaks of January 2011 have recently passed into and through the Menindee Lakes. This means flood operations are now focused on the Lower Darling, and on minimising the risk of river bank slumping as levels fall.

CURRENT FLOW MANAGEMENTUpstream of the lakes, the Darling River at Wilcannia has fallen nearly 4 metres to 6.6 metres (16,000ML/d) after a flood peak of 10.5 metres (38,500ML/d) on 21 March. The fall in the upper Talyawalka Creek at the Barrier Highway has been even greater. It is now at 2.8 metres (2,000ML/d) after a peak of 30,500ML/d on 19 March. Upstream along the Barwon-Darling system flows are between 3,000ML/d and 6,000ML/d. Inflows of about this rate can be expected to continue to reach Menindee through May and into June 2011. Upstream of Wilcannia, the Darling River at Tilpa is currently 5,500 ML/d and falling slowly.

Inflows to the Menindee Lakes peaked at almost 50,000ML/d in the last week of March. Releases from the lakes were held at 30,000ML/d, which meant the total volume of water in the lakes during this period was increased by an average of 15,000-20,000ML/d. In early April inflows began reducing so the release from the lakes was also reduced to maximise the storage in the lakes while minimising flooding around Menindee.

After a peak of 37,000ML/d at Weir 32, releases were reduced to about 30,000ML/d and held at that level from 18 March following heavy local rainfall in mid-March. The reduction in flows at Weir 32 was re-commenced on 4 April as peak inflows passed into the lakes. High inflows have persisted for a few days longer than expected as a result of water draining from the floodplain; this required the flows at Weir 32 to be held at about 27,000ML/d for a few days before the reduction could continue. It is expected that within two weeks flows into the Lower Darling will be below 20,000ML/d.

Additional rainfall that had been predicted for early-April by the Bureau of Meteorology did not eventuate and there has been no rainfall at Menindee since 21 March. With a clear forecast for at least the next week, the release strategy is to reduce flows at Weir 32 to below 20,000ML/d over the next two weeks.

The total volume stored in the Menindee Lakes is now 2,017,000 megalitres which is 116.5 per cent of capacity. A maximum of around 2,035,000ML will be reached this weekend after which levels will begin fall for the remainder of April. The maximum capacity of the Menindee Lakes system is 2,050,000ML when surcharged to 118 per cent of full supply.

Community information Management of Menindee Lakes 2010-2011April 8, 2011

www.water.nsw.gov.au NSW Office of Water | Issue 8 – April 8, 2011 page 2

Management of Menindee Lakes 2010-2011 – April 8, 2011

WHAT THIS MEANS As expected, all the lakes are now near maximum surcharge capacity. Lake Wetherell, the last of the lakes to be filled, is at 122 per cent of capacity, while Lakes Pamamaroo, Menindee and Cawndilla are currently at 128 per cent, 116 per cent and 112 per cent respectively.

The Bureau of Meteorology’s long term outlook for autumn (April to June) is that there is a good chance (60 per cent) that median rainfall conditions will be received. There is no immediate demand for water to be released from the lakes for irrigation in the Lower Darling or to supply South Australia as there is still high flow in the Murray River. Storage levels are likely to remain surcharged above full supply through autumn and winter.

Total inflow to the lakes from this event has now exceeded 3,000,000ML, which makes this the third largest event by volume in the last 25 years, behind 1998 and 1990. Unlike these other events however, the storages were full at the start of the 2010–11. This means that the flows in the Lower Darling in terms of volume have been comparable in magnitude to these larger events.

Updated satellite imagery indicates a significant reduction in flows around Wilcannia and upstream. Water can be seen trapped in billabongs and lakes near the main river channel and along anabranches, and this will sustain dependent ecosystems for many months. High flows are continuing in the lower parts of the Talyawalka Creek system and in the top of the Lower Darling; however, these will recede in the next few weeks.

Figure 1 Darling River flood operations - Image: MODIS Australia6 Subset - Terra 250m Bands 7-2-1 Image for 2011/095 (5 April 2011) Source: NASA/GSFC, MODIS Rapid Response 2011.

NSW Office of Water - Menindee advice

What this means As expected, all the lakes are now near maximum surcharge capacity. Lake Wetherell, the last of the lakes to be filled, is at 122 per cent of capacity, while Lakes Pamamaroo, Menindee and Cawndilla are currently at 128 per cent, 116 per cent and 112 per cent respectively.

The Bureau of Meteorology’s long term outlook for autumn (April to June) is that there is a good chance (60 per cent) that median rainfall conditions will be received. There is no immediate demand for water to be released from the lakes for irrigation in the Lower Darling or to supply South Australia as there is still high flow in the Murray River. Storage levels are likely to remain surcharged above full supply through autumn and winter.

Total inflow to the lakes from this event has now exceeded 3,000,000ML, which makes this the third largest event by volume in the last 25 years, behind 1998 and 1990. Unlike these other events; however, the storages were full at the start of the 2010/11. This means that the flows in the Lower Darling in terms of volume have been comparable in magnitude to these larger events.

Updated satellite imagery indicates a significant reduction in flows around Wilcannia and upstream. Water can be seen trapped in billabongs and lakes near the main river channel and along anabranches, and this will sustain dependant ecosystems for many months. High flows are continuing in the lower parts of the Talyawalka Creek system and in the top of the Lower Darling; however, these will recede in the next few weeks.

Figure 1 Darling River Flood Operations

Image: MODIS Australia6 Subset - Terra 250m Bands 7-2-1 Image for 2011/095 (5 April 2011)

Source: NASA/GSFC, MODIS Rapid Response 2011.

Darling River

Poopelloe Lake

Teryawynia Lake

Paroo River

Menindee Lakes

WILCANNIA

Talyawalka Creek

MENINDEE

Darling Anabranch Lower

Darling

Bulk of flood water



Peak of flooding around Menindee township.

Photo: 12 March 2011 – Courtesy Barry Philp State Water, Menindee.

The main weir at Menindee looking upstream, gates above the water. Flows of about 33,000ML/d into the Lower Darling River. A peak flow of nearly 36,000ML/d occurred on 9 March 2011.

Photo: 22 February 2011 – Courtesy Barry Philp State Water, Menindee.

Photo taken from the main weir at Menindee looking upstream toward Lake Wetherell.

Photo: 23 December 2010 – Courtesy Barry Philp State Water, Menindee.



RIVER LEVELS

Location Height (m) Flow (ML/d) Comment

Bourke 4.71 5,160 Steady (Peak 88,000 on 14/2/11)

Talyawalka (BH) 2.82 2,096 Falling (Peak 30,500ML/d on 19/3/11)

Wilcannia 6.61 16,093 Falling (Peak 38,500 on 21/3/11)

Weir 32 6.67 27,214 Falling slowly (Peak 37,000 on 11/3/11)

Pooncarie 7.30 21,881 Falling slowly (Peak 23,000 on 30/3/11)

Burtundy 7.35 19,610 Steady

River data from 7 April 2011

Darling River Flows and Menindee Storage Volume

Darling River Flows and Menindee Storage Volume

0

20,000

40,000

60,000

80,000

100,000

01-O

ct-1

0

15-O

ct-1

0

29-O

ct-1

0

12-N

ov-1

0

26-N

ov-1

0

10-D

ec-1

0

24-D

ec-1

0

07-J

an-1

1

21-J

an-1

1

04-F

eb-1

1

18-F

eb-1

1

04-M

ar-1

1

18-M

ar-1

1

01-A

pr-1

1

15-A

pr-1

1

29-A

pr-1

1

Flow

(ML/

d)

1,500,000

1,600,000

1,700,000

1,800,000

1,900,000

2,000,000

2,100,000

2,200,000

Stor

age

Volu

me

(ML)

Bourke Flow

Weir 32 Release

Wilcannia Flow

Forecast Conditions -approximate only.

Menindee Storage



OUTLOOK FOR MENINDEE TOWNSHIP AND THE LOWER DARLING

Water levels have begun to drop around the Menindee township and will continue to fall slowly over the next couple of weeks. If there is no further rainfall, flows at Weir 32 should fall below 18,000 ML/d by 21 April. The outlook for the Lower Darling remains largely unchanged with flows at Pooncarie and Burtundy remaining fairly steady at around 20,000 ML/d and minor flooding at Burtundy continuing during April. In the absence of further storms and significant inflows, the river level at Burtundy is likely to begin falling during May.

Flows in the Murray River downstream of the Darling River junction are still high at 60,000ML/d, but are now falling steadily.

Current releases from the Menindee Lakes are:

ML/d

Main weir 23,200

Lake Wetherell outlet 2,800

Lake Pamamaroo outlet -

Lake Menindee outlet -

Lake Cawndilla outlet 1,500

Total 27,500

HOW THIS FLOOD COMPARES TO PREVIOUS EVENTSThe table below shows a comparison of the current flood events with previous floods.

YearMax height at

Bourke (m)Total volume at

Bourke (GL)Max height at Wilcannia (m)

Total flows at Wilcannia (incl.

Talyawalka Ck) (GL)

Max height Weir 32 (m)

1988 12.57 3,500 10.19 2,900 5.10

1990 12.99 9,000 11.0 8,150 7.37

1998 13.78 9,700 10.83 6,700 7.45

2001 12.28 3,300 9.75 2,250 6.21

2010 10.78 2,370 9.43 2,400 5.44

2011 12.56 5,800 10.5 5,000* 7.10

* Predicted values

ENVIRONMENTAL MANAGEMENTThere has been no rainfall in the local area since 21 March, but the landscape continues to flourish. Flows into the Lower Darling River and Great Darling Anabranch will recede from next week throughout April and into May. Nevertheless most billabongs and lakes, which have been dry for over 10 years, have now received flows for nearly six months since mid October 2010.

The Anabranch continues to flow its full length to the Murray and has been completely transformed from the arid conditions of recent years to a rejuvenated riverine environment. Lakes Nearie and Popiltah continue to receive water while Travellers Lake, Mindona Lake and Popio Lake can be seen near full in the bottom left hand corner of Figure 1.

There are two areas of environmental management of concern to the NSW Office of Water. The first is the potential for the emergence of blackwater as flows drain from floodplains back to the river, and the second is the potential for environmental damage through bank slumping.

Blackwater is a natural phenomenon where warm water, high in organic matter becomes depleted of oxygen. When affected water enters a confined pool, the resultant rapid reduction in dissolved oxygen levels can cause asphyxiation in fish. The substantial inundation of floodplains along the Darling River during this flood event has



© State of New South Wales through the Department of Primary Industries, 2011 This work may be freely reproduced and distributed for most purposes, however some restrictions apply. Contact the Department of Environment, Climate Change and Water for copyright information.

Disclaimer: While every reasonable effort has been made to ensure that this document is correct at the time of publication, the State of New South Wales, its agencies and employees, disclaim any and all liability to any person in respect of anything or the consequences of anything done or omitted to be done in reliance upon the whole or any part of this document. This information does not constitute legal advice. Users are advised to seek professional advice and refer to the relevant legislation, as necessary, before taking action in relation to any matters covered by this information sheet.

NO

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increased the risk of blackwater production as flows recede to be within channel, and floodplains drain back to the river. Extensive blackwater events have occurred this year in the Murray, Murrumbidgee and, more recently, the Darling River near Bourke.

Bank slumping, which can cause trees to topple into rivers, is a serious threat after long periods of inundation as we have now experienced in the Lower Darling. Saturated river banks cannot support their own weight if water levels are allowed to fall too quickly. Careful reduction of releases from the Menindee Lakes into the Lower Darling and the slow drop in river levels downstream is required to minimise the risk of damage.

The improved flow conditions in the Lower Darling continue to attract increasing numbers of larger vessels as visitors access this region by boat. Boat owners are asked to minimize the wash from vessels as this too can be damaging to riverbanks, especially over the coming Easter holiday period when wet banks are expected to be exposed.

COMMUNICATION AND ADDITIONAL INFORMATIONFlows and water levels are being closely monitored and information will be updated and distributed by the NSW Office of Water and State Water where necessary.

The NSW Office of Water and State Water Corporation will continue to work with the SES, local police and councils as required.

WHERE DO I GO FOR ADDITIONAL INFORMATION? NSW State Water office in Cawndilla Street Menindee.

SES free call 132 500

NSW Office of Water: call Bunty Driver T 03 5898 3910 or visit the website www.water.nsw.gov.au

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