The NSW Office of Water is a separate office within the Department of Environment, Climate Change and Water

INTRODUCTIONFlood flows from the widespread rains across much of northern NSW in November and December are slowly making their way through the Barwon-Darling River system.

The peak from these flows is expected to reach the Menindee Lakes in late January and early February 2011.

Since the last information sheet was released before Christmas, there has been major flooding in the Condamine-Balonne River system in southern Queensland. The flows from this event will make their way slowly into the Barwon-Darling system. The peak from the event, that has just passed St George, is expected to reach the Menindee Lakes about mid-March.

Further, in the past few days torrential rainfall and more widespread flooding has occurred in southern Queensland, including the Condamine-Balonne, Intersecting Streams and Border Rivers. While it is too early to predict with confidence the flows that will enter the Barwon-Darling from these systems, it is likely to rank with some of the highest inflows in the past 25 years. This is expected to extend the duration of high flows reaching the Menindee Lakes and increase the peak inflows.

The river system is full. Heavy rain and flooding is continuing in the upstream catchments and local rainfall events continue to increase river flows and the volumes that will pass into the Menindee Lakes and into the Lower Darling River.

Community information Management of Menindee Lakes 2010-2011January 13, 2011

Menindee Weir, December 2010 (Courtesy Barry Philp State Water)

Looking downstream as the water flows out of Menindee

Lakes, December 2010 (Courtesy Barry Philp State Water)

www.water.nsw.gov.au NSW Office of Water | Issue 2 – January 13, 2011 page 2

Management of Menindee Lakes 2010-2011 – January 13, 2011

Prior to Christmas it looked like the forecast inflows would be able to be stored in the Menindee Lakes without needing to increase releases above current levels. This is no longer the case and releases into the Lower Darling River will need to be increased in the coming weeks to enable the future flood peaks to be attenuated, to minimize the number of dwellings inundated and to reduce the potential for cutting the Menindee to Pooncarie Road.

The NSW Office of Water, together with State Water Corporation are managing releases from the Menindee Lakes to the Lower Darling River and Great Anabranch to minimize flooding of dwellings around Menindee, maximizing environmental benefits downstream and making sure that the lakes are full when the flooding passes.

FLOW MANAGEMENTThe flows progressing through the Barwon-Darling River were generated mainly from the December 2009 floods in the Namoi, Castlereagh and Macquarie Rivers and are slowly receding.

The flow at Bourke has just peaked at 12.34m with flows around 88,000 megalitres per day (ML/d) and is receding extremely slowly compared to previous floods. This is expected to produce a peak flow of at least 30,000 ML/d at in the Darling River at Wilcannia in late January-early February, flowing into the Menindee Lakes in mid-February.

Flows commenced to pass into the Talyawalka creek system upstream of Menindee last week as flows in the Darling River at Wilcannia increased above 23,000 ML/d. These flows are expected to reach the Barrier Highway in the coming week. As the current flows are expected to be followed by further high flows in coming weeks it is expected that significant flows will pass through the Talyawalka system and reconnect with the Lower Darling River downstream of Menindee.

Rainfall over the past week has caused very large rises (near record rises in some areas) in the Border Rivers upstream of Mungindi that will result in significant flooding at Mungindi and, together with significant flows in the Moonie River, will result in flooding above Walgett.

The main flood in the Condamine River has just peaked at St George at around 290,000 ML/d. This peak is slightly lower than the peak recorded in March 2010, but the recession is expected to be slower and there is more volume passing into the Barwon-Darling than occurred in March.

Again, heavy rain and widespread flooding over the past week is expected to cause a second peak at St George, that will contribute further significant volumes in the Barwon-Darling River

Much of this water will flow into the Culgoa, Birrie, Bokhara and Narran Rivers, with the flow from the first of these events expected to peak in the Darling River at Bourke in late January. Levels at Bourke are expected to stay high through to at least mid-February.

These flows will then pass down the Darling River, and are expected to reach Menindee in late February to early March, with early estimates of peak inflows to the lakes between 30,000 and 40,000 ML/d.

Old Menindee Rd with the Darling River on the left hand side and floodplain on the right, January 2011 (Courtesy Barry Philp State Water)

The waters of the Darling River cover Crick Park (Courtesy Barry Philp State Water)

The waters of the Darling River cover Crick Park, January 2011 (Courtesy Barry Philp State Water)

www.water.nsw.gov.au NSW Office of Water | Issue 2 – January 13, 2011 page 3

Management of Menindee Lakes 2010-2011 – January 13, 2011

Hydrograhic teams from the NSW Office of Water are currently working in Queensland, assisting in monitoring flows in the Condamine-Balonne system. This work will provide much needed flow information in Queensland and will assist in flow forecasting downstream in NSW.

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

Boggabilla 7.30 39,280 Falling

Mungindi 6.26 9,636 Rising

Collarenebri (total) 2.04 16,721 Rising

Walgett 9.22 19,132 Falling

Brewarrina (total) - 57,575 Falling

Bourke 12.34 87,511 Peaking

Tilpa 11.39 49,795 Rising

Wilcannia 9.33 27,649 Rising

Weir 32 6.44 22,993 Steady

Pooncarie 6.74 14,297 Rising slowly

Burtundy 6.56 15,621 Rising slowly

St George 12.52 268,362 Falling (Peak 290,000 on 9/1/11)

River data (12 January 2011)

As a consequence of continuing rain and flooding upstream, higher inflows and higher total volumes are now expected to reach the Menindee Lakes than previously estimated.

It is now expected that releases will have to be increased to around 26,000 ML/d. It is intended to increase flows to this level in the week commencing 17 January. Until that time, releases from Menindee Lakes will be kept at current levels. Depending on the updated flow forecasts at that time, it is expected that releases will have to be increased, which unfortunately may inundate some houses and further reduce access to properties. It is also likely that there may be further increases in releases in the weeks to come.

This will be required so that peaks from current events reach Menindee Lakes can be attenuated to minimize the number of houses affected, and to prevent, where possible, cutting of access roads including the Menindee to Pooncarie Road.

Further, the rain throughout the catchments is continuing and local rainfall around Menindee and Wilcannia is adding to flows in the river.

Current releases from the Menindee Lakes are;

ML/d

Main weir 17,200

Lake Wetherell outlet 1,800

Lake Pamamaroo outlet 1,800

Lake Menindee outlet 2,300

Lake Cawndilla outlet 1,750

Total 24,850

www.water.nsw.gov.au NSW Office of Water | Issue 2 – January 13, 2011 page 4

Management of Menindee Lakes 2010-2011 – January 13, 2011

HOW WILL LAKE AND RIVER LEVELS CHANGEOver the past months, lake levels have been receding from being surcharged to 110.7 percent of capacity on 19 October, to 97.7 per cent at 11 January. Most of the reduction in levels has been at Lake Wetherell.

As inflows increase, the cumulative volumes in the lakes will increase, to a point where the lakes will again be surcharged.

After the current inflows peak in the last week of January-first week in February, inflows may decline slightly and the cumulative volumes stored in the lakes may also decline. This will help to absorb the peak inflows where possible and maintain a lower maximum release rate when the inflows from the current flood in the Condamine-Balonne system reach Menindee in late February-early March.

In late February-early March, lake levels will rise again and are expected to be surcharged above the nominal full capacity. (The lakes can be surcharged up to about 118 percent of capacity for short periods but it is a priority to reduce levels to nominal full supply as soon as possible).

River levels downstream of Menindee will remain relatively stable for the next week. Depending on the updated flow forecasts at that time, it is expected that releases will be increased, which unfortunately will inundate some houses and further reduce access to properties.

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

08-O

ct-1

0

15-O

ct-1

0

22-O

ct-1

0

29-O

ct-1

0

05-N

ov-1

0

12-N

ov-1

0

19-N

ov-1

0

26-N

ov-1

0

03-D

ec-1

0

10-D

ec-1

0

17-D

ec-1

0

24-D

ec-1

0

31-D

ec-1

0

07-J

an-1

1

14-J

an-1

1

21-J

an-1

1

28-J

an-1

1

04-F

eb-1

1

Flow

(ML/

d)

1,550,000

1,600,000

1,650,000

1,700,000

1,750,000

1,800,000

1,850,000

1,900,000

1,950,000

Stor

age

Volu

me

(ML)

Menindee Storage

Bourke Flow

Weir 32 ReleaseWilcannia Flow

Forecast Conditions -approximate only.

Menindee Storage

HOW THIS FLOOD COMPARES TO PREVIOUS EVENTSThis section is shown to enable the comparison of the current flood events with previous floods.

Year Max height at Bourke

(m)

Total Volume at Bourke

(GL)

Max height at Wilcannia

(m)

Total flows at Wilcannia

(GL)

Max height Weir 32 (m)

1996 12.39 2,500 9.85 2,400 5.18

2001 12.28 3,300 9.75 2,250 6.21

2010 10.78 2,370 9.43 2,400 5.44

www.water.nsw.gov.au NSW Office of Water | Issue 2 – January 13, 2011 page 5

Management of Menindee Lakes 2010-2011 – January 13, 2011

© State of New South Wales through the Department of Environment, Climate Change and Water, 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.Printed on 100% recycled Australian paper (ISO 14001)

NO

W 11_003

ENVIRONMENTAL MANAGEMENTAs part of the longer term management strategy of the Menindee Lakes, to preserve the significant environmental values of the floodplain of Lake Wetherell, releases over the past few months have been made preferentially from Lake Wetherell to dry as much of the floodplain as possible.

This has enabled much of the floodplain to dry out, keeping the floodplain vegetation alive and maintaining habitat for native species.

Releases are also continuing from Lake Cawndilla to the Great Anabranch, providing much needed environmental benefits, given that prior to October 2010, no flows had passed into the Great Anabranch for the previous 10 years. This is enabling the filling of some of the Anabranch Lakes that are listed in the Directory of Important Wetlands in Australia.

Consideration will also be given to removing the main weir gates from the river, allowing fish passage through the weir. This was last achieved in September and October 1998 for 41 days.

COMMUNICATION AND ADDITIONAL INFORMATIONThe Office of Water and State Water will continue to prepare and publish this series of information sheets every two weeks where possible.

A regional committee, that includes representatives of water users from the Menindee, Lower Darling and Great Anabranch has been established to ensure that available information is distributed about river levels and flow management.

The Office of Water and State Water will continue to work with the SES, local Police and Councils.

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

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