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Management of the Barwon-Darling floods of November 2011–June 2012

www.wa t e r . n sw . g o v . a u

Publisher: Department of Primary Industries, a division of NSW Department of Trade and Investment, Regional

Infrastructure and Services

Title: Management of the Barwon-Darling Floods of November 2011 – June 2012

First published July 2012

ISBN 978 1 74256 314 5

More information: www.water.nsw.gov.au – Bunty Driver 02 6024 8871

Author: NSW Office of Water – Brian Graham and Michael Wrathall

Unit : Water Management and Implementation

Location: Parramatta NSW Australia

© State of New South Wales through the Department of Trade and Investment, Regional Infrastructure and Services 2012. You may copy, distribute and otherwise freely deal with this publication for any purpose, provided that you attribute the Department of Trade and Investment, Regional Infrastructure and Services as the owner.

Disclaimer: The information contained in this publication is based on knowledge and understanding at the time of writing (July 2012). However, because of advances in knowledge, users are reminded of the need to ensure that information upon which they rely is up to date and to check currency of the information with the appropriate officer of the Department of Primary Industries or the user’s independent adviser.

NOW 12_265

http://www.water.nsw.gov.au/

Management of the Barwon Darling floods November 2011 – June 2012

Contents Overview .......................................................................................................................................1

Background ..................................................................................................................................2

Rainfall ........................................................................................................................................4

River Flow in the Barwon-Darling...............................................................................................8

Flows in the Talyawalka ............................................................................................................15

Menindee Lakes Operation .......................................................................................................17

Rising Water Levels ..................................................................................................................17

Peak Water Levels....................................................................................................................19

Falling Water Levels .................................................................................................................20

Environmental Outcomes..........................................................................................................21

Communications ........................................................................................................................23

Conclusions and Outlook..........................................................................................................25

Table of Figures and Tables

Figure 1 Menindee Lakes Storage – management responsibility ......................................................... 2

Figure 2 Menindee Lakes storage volumes 2000-2012 ...................................................................... 3

Figure 3 Murray-Darling Rainfall Totals (mm) ........................... 5 Week ending 27 November 2011th

Figure 4 Rainfall Timeline...................................................................................................................... 5

Figure 5 Murray-Darling Rainfall Totals (mm) ................................ 6 Week ending 4 February 2012th

Figure 6 Murray-Darling Rainfall Totals (mm) .................................... 7 Week ending 4 March 2012th

Figure 7 Barwon-Darling River System ................................................................................................. 8

Figure 8 Barwon-Darling Satellite Image and Flow Summary ............................................................. 9

Figure 9 Flow Hydrographs at Key Locations Upstream of Bourke .................................................... 11

Figure 10 Flow Hydrographs in the Condamine-Culgoa in 2010, 2011 & 2012.................................. 12

Figure 11 Flow Hydrographs at Key Locations Downstream of Bourke.............................................. 13

Figure 12 The Talyawalka Creek System ........................................................................................... 15

Figure 13 Darling River and Talyawalka Creek Satellite Image.......................................................... 16

Figure 14 Menindee Lakes General Layout ........................................................................................ 17

Figure 15 Darling River Flows ............................................................................................................. 20

Figure 16 Menindee Lakes Storage Volume ....................................................................................... 20

Figure 17 Great Darling Anabranch flows ........................................................................................... 22

i NSW Department of Primary Industries, July 2012


Overview This report summarises the management of extensive flooding throughout the Barwon-Darling and Lower Darling River systems during the summer of 2011/12. It describes the rainfall patterns, the flows that were generated, the flood planning process, and the operations at the Menindee Lakes that were undertaken by the NSW Office of Water and State Water Corporation.

This report also summarises the environmental outcomes and the extensive communications between agencies, the community, stakeholders and landowners.

Management of the lakes by the NSW Office of Water and State Water Corporation aims to meet multiple objectives. These include ensuring infrastructure safety, minimising the social and economic impacts of flooding, providing environmental outcomes and maximising the volumes of water stored in the lakes at the end of the flood.

The floods of summer 2011/2012 were amongst the largest events in the northern Murray Darling Basin in recent years. Significant rainfall and river flow statistics include the:

wettest November in 133 years of records at Moree

largest 8-day rainfall total in 133 years of records at Wilcannia

record flooding in the Balonne River at St George in Queensland

largest peak flow since 1976 in the Gwydir River at Gravesend

equal highest water level since 1976 in the Darling River at Bourke

Lakes Pamamaroo (left), Wetherell (centre) and Tandure (right) looking upstream

Photo: 28 February 2012 – Courtesy Barry Philp

1 NSW Department of Primary Industries, July 2012


Background The Menindee Lakes water storage scheme in western NSW is owned by the NSW government and managed in accordance with the Murray Darling Basin Agreement.

Under the Agreement, NSW is responsible for managing flows through the system whenever total storage volume falls below 480,000 megalitres in the Menindee Lakes Scheme, and until it returns to greater than 640,000 megalitres.

Similarly, NSW is responsible for managing flood flows through the Menindee Lakes whenever levels are expected to exceed full supply level (FSL) of 1,731,000 megalitres. During floods the lakes can be surcharged to 118 percent of full supply capacity: 2,050,000 megalitres.

Figure 1 Menindee Lakes Storage – management responsibility

Menindee Lakes Storage

NSW

control

MDBA

control

NSW

control

28%

37%

FSL: 100%

Max: 118% 2,050,000ML

1,731,000ML

480,000ML

640,000ML

Following the flood peak, when the lakes are surcharged, some storage volume is typically used to provide a safe rate of recession of river flows in the Lower Darling to minimise the potential for river damage through bank slumping.

After floods in 2000, the volume of water stored in the Menindee Lakes fell below 480,000 megalitres in March 2002 and management responsibility transferred from the (then) Murray-Darling Basin Commission to NSW. As a consequence of continuing drought the volume in the Menindee Lakes fell to 37,000 megalitres (2 percent of capacity) in January 2004. Between 2004 and 2007 there were a number of small inflows that increased storage volumes however they dropped again to 50,000 megalitres (3 percent of capacity) in December 2007.

Responsibility for the management of the Menindee Lakes passed to the (now) Murray-Darling Basin Authority (MDBA) in April 2010, after flows from floods in northern NSW and central Queensland in January 2010 reached the lakes and lake storage volumes exceeded 640,000 megalitres.



Aerial view of the Menindee Lakes system looking downstream

Photo: 12 April 2012 – Courtesy Brian Graham

However, by late October 2010, responsibility for the operation of the Menindee Lakes passed from MDBA back to NSW, when the storage rise beyond full supply level was forecast and flood management operations commenced. Responsibility for the operation of the Menindee Lakes remained with NSW over the 2011 winter while storage volumes exceeded full supply level.

Flood operations of 2010-2011 ceased in June 2011 when outflows from the Lakes to the Lower Darling River reduced to seasonal minimums. Low evaporation rates and continuing inflows throughout the winter meant that the Menindee Lakes remained above full supply level through to November 2011.

Figure 2 Menindee Lakes storage volumes 2000-2012

Menindee Lakes Storages (2000-2012)

0

500,000

1,000,000

1,500,000

2,000,000

2,500,000

02

/01

/00

02

/01

/01

02

/01

/02

02

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/01

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/01

/05

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/01

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/01

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/01

/08

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/01

/10

02

/01

/11

02

/01

/12

Sto

rag

e v

olu

me

(M

L)

Actual Max SurchargeFull Supply MDBA/NSW control 1MDBA/NSW control 2



The normal operating protocols for the Menindee Lakes require, when the lakes are surcharged, that releases be made to reduce water levels to full supply level between 1 November and 31 December. In 2011/12, this was to be achieved by releasing a large environmental flow delivery to South Australia, beginning in late-November 2011.

Menindee Lakes Storage Operating Rule

2. (v) Menindee Lakes Surcharging

(a) surcharging of the Menindee Lakes … is permitted between 1 March and 31 October provided that flow at Weir 32 was at least 500ML/d;

(b) any surcharge above the full supply level … existing at 31 October is to be drawndown gradually so that all the surcharge has been released by 31 December;

(c) additional surcharge is permitted between 1 November and 1 March if the flow at Weir 32 would otherwise exceed the downstream channel capacity of 20,000ML/d.

(Above rules adopted by the MDBC at Meeting 8, 8/9 August 1989.)

While the Lakes were being reduced to full supply level, flooding in northern NSW again triggered flood operations on 13 December 2011 and NSW Office of Water and State Water Corporation began local flood operations at Menindee Lakes for the second consecutive summer.

In 2011/12 there were three distinct events in late-2011 and early-2012 that produced flood flows in northern NSW and southern Queensland. This required the management of large releases from the near full Menindee Lakes to provide airspace in the storage scheme to capture the expected peak inflows, and reduce the peak flow level at Menindee to minimise the social and economic impact.

Rainfall Two intense rainfall events in the upper reaches of the Barwon-Darling catchment during November 2011 and February 2012 combined to produce large inflows into the river system.

A third event, with heavy rainfall in the Menindee region during late-February and early-March 2012, produced significant local inflows to Menindee Lakes.

The first of these events occurred in northern NSW in the upper reaches of the Border Rivers, Gwydir and Namoi catchments, centred near Moree (Figure 3 below).

Over the four-day period 23-26 November, 221mm of rainfall was recorded at Moree. This event alone represented the wettest November measured in Moree, from records extending back to 1879. As a result, major flooding was experienced in the Gwydir and Mehi Rivers at Moree. During this same period, 154mm was recorded further north at Mungindi and 138mm toward the south in Tamworth.



Rainfall Event 1

Figure 3 Murray-Darling Rainfall Totals (mm) Week ending 27th November 2011

(Source: Australian Bureau of Meteorology)

Figure 4 Rainfall Timeline

Event 1 Event 2 Event 3

The second significant rainfall event influencing flows in the Barwon-Darling system occurred in a diagonal band from southern Queensland near Mitchell to north-east NSW at Port Macquarie (Figure 5). Following a wet final week in January, an intense low pressure system remained stationary over this region for several days, resulting in large falls at the beginning of February 2012.

Over the three days of 1-3 February 2012, 185mm of rainfall was recorded at Moree and 182mm at Tamworth. In Mitchell, 241mm of rainfall was recorded during this period while St George also recorded 185mm. This event produced another significant peak in the Gwydir, as well as very large flows in the Condamine-Balonne system and other western Queensland rivers.

Nov-11 Dec-11 Jan-12 Apr-12 Feb-12

~200mm in Northern NSW

~200mm in S QLD ~240mm in

Western NSW / Nthn NSW

Mar-12



Rainfall Event 2

Figure 5 Murray-Darling Rainfall Totals (mm) Week ending 4th February 2012


The third major event occurred in a diagonal band through north-western, central and south-eastern NSW as well as north-eastern Victoria (Figure 6). Very large falls caused by a near stationary trough of low pressure were recorded throughout much of the State.

Record 8-day rainfall totals, were recorded at a number of locations including Wilcannia which recorded 238mm in the period 27 February – 4 March 2012. Severe flooding resulted in the south of the State, while much of the upper Barwon-Darling catchment was spared the most intense rainfall.



Rainfall Event 3

Figure 6 Murray-Darling Rainfall Totals (mm) Week ending 4th March 2012


The following table summarises these three rainfall events.

Table 1: Barwon-Darling Rainfall Totals (mm) - Summary of Significant Events

Dates Location River System Total rainfall (mm)

Border Rivers ~150

Gwydir ~220

1 23 – 26 Nov 2011 Northern NSW

Namoi ~140

Southern QLD Condamine-Balonne ~240

Gwydir ~180

2 1 – 3 Feb 2012

Northern NSW

Namoi ~180

3 27 Feb – 4 Mar 2012 Western NSW Darling ~240



River Flow in the Barwon-Darling

Figure 7 Barwon-Darling River System

The first two rainfall events (Table 1) occurred more than two months apart and produced distinct river flow peaks on the Darling River at Bourke. Respective hydrographs are plotted at Figure 8.

The first peak at Bourke on 11 January 2012 was driven predominantly by high flows in the Gwydir, Namoi and Border River catchments in late November and early December.

The most intense rainfall fell in the Gwydir Valley. A peak of 160,000 megalitres per day was recorded on the Gwydir River at Gravesend upstream of Moree. Further downstream, the Gwydir wetlands filled and spilled into the Barwon River channel. This provided substantial attenuation of peak flows. There was also a significant contribution of flows in the Barwon- Darling River from the Border Rivers and from local runoff upstream of Collarenebri.

The second and much larger flood peak at Bourke on 6 March 2012 was produced when flows from tributaries in Queensland and northern NSW, caused by heavy and widespread rainfall in late-January and early-February 2012, merged in the Barwon-Darling.

Included in this event, was a record flood peak of approximately 320,000 megalitres per day that passed through St George along the Condamine and into the Balonne River on 8 February and another flood peak of 290,000 megalitres per day on the Gwydir River at Gravesend upstream of Moree, the largest since 1976.

Gwydir River floodwaters entered the previously inundated wetlands resulting in a peak of 226,000 megalitres per day in the Barwon River through Collarenebri on 11 February. Additional contributions from the Namoi River to the Barwon River flows resulted in a peak at Walgett of 284,000 megalitres per day on 13 February.

The floods in each of the tributary systems can be seen in a satellite image taken on 9 February 2012. Overlaid are the respective flow hydrographs in (Figure 8). Flow peaks are summarised in Table 2.



Figure 8 Barwon-Darling Satellite Image and Flow Summary

(Source: NASA/GSFC, MODIS Rapid Response 2012)

Warrego flood

(Feb 12)

Gwydir flood

(late-Nov 11)

Balonne flood

(Feb 12)

Gwydir flood

(Feb 12)

Image dated 9 Feb 2012



Table 2 Peak Flows Entering the Barwon-Darling River

Peak Flow (GL/d) Event Date River Location

Gauges Combined @Bourke

17/12/2011 Barwon Collarenebri 88.5 88.5

11/12/2011 Namoi Goangra 21.5

15/12/2011 Namoi (Pian Ck) Waminda 1.9

~22

12/01/2012 Culgoa Mundiwa 5.5

1

14/01/2012 Bokhara Bokhara 1.3

6.5

73.6

11/02/2012 Barwon Collarenebri 226 226

13/02/2012 Namoi Goangra 54.9

13/02/2012 Namoi (Pian Ck) Waminda 19.7

74.6

28/02/2012 Culgoa Mundiwa 44.3

2

03/03/2012 Bokhara Bokhara 8.5

47.0

234

In both events, the greatest contribution to peak flows at Bourke came from the Barwon River upstream of Collarenebri; which was fed from the Gwydir and Border Rivers. Despite the attenuating effect of the Gwydir wetlands, significant peak inflows were still recorded in the main Barwon river channel.

Flows from the Namoi River valley contributed about 20-25 percent of total flows recorded in the Barwon River at Walgett. Despite very large flows in the Condamine-Balonne and other Queensland tributaries, the contribution of Queensland rivers to the Barwon-Darling flood peaks was relatively small (10 to 20% at Bourke). For these 2011/2012 events, the Castlereagh and Macquarie valleys contributed negligible flow to the Barwon-Darling system.



Figure 9 Flow Hydrographs at Key Locations Upstream of Bourke

2012 Flood: U/S Bourke

0

50,000

100,000

150,000

200,000

250,000

300,000

350,000

26-Nov-11 10-Dec-11 24-Dec-11 07-Jan-12 21-Jan-12 04-Feb-12 18-Feb-12 03-Mar-12 17-Mar-12 31-Mar-12

ML

/d

St George (Balonne-Culgoa)

Brenda (Culgoa)

Boggabilla (Border)

Collarenebri (Border/Gwydir)

Goangra (Namoi)

Bourke (Darling)

Peaks of

230,000ML/d Peak of

320,000ML/d

Throughout 2010, 2011 and 2012, flows from the Condamine-Balonne system, including the Culgoa River, contributed significant volumes of water into the Barwon-Darling River. The three largest flow peaks ever recorded in the Culgoa River (Brenda gauge) were on 19 January 2011, 31 January 2011 and 15 February 2012.

The peak inflow to the Barwon River from the Condamine-Balonne system was about 50,000 megalitres per day (ML/d). This was less than the 2011 event (70,000 ML/d) but larger than the 2010 event (24,000 ML/d). All events had comparable peak flows upstream at St George (Figure 10). This demonstrates that each flood is unique and must be managed on a real-time basis as antecedent and current conditions can have a significant impact on flows.

The primary difference between the 2011 and 2012 events was the smaller volume of water contained within the single peak in 2012 (2,600 GL) at St George compared to collective volumes of the two major peaks in 2011 (8,800 GL).

There were significant attenuation of flood peaks in the Barwon-Darling between Walgett and Brewarrina. A peak of 284,000 ML/d at Walgett reduced down to a maximum flow close to 200,000 ML/d at Brewarrina. Despite this smaller peak, the total volume recorded flowing past each site was similar, so the majority of overbank flow eventually returned again to the Barwon River channel.



Figure 10 Flow Hydrographs in the Condamine-Culgoa in 2010, 2011 & 2012

2010 Flood

0

50,000

100,000

150,000

200,000

250,000

300,000

2/2/10 16/2/10 2/3/10 16/3/10 30/3/10 13/4/10 27/4/10

ML

/d

0

10,000

20,000

30,000

40,000

50,000

60,000

70,000

80,000

90,000

100,000St George

Brenda

Mundiwa

Peak of

294,000ML/d

Peak of 30,000ML/d

Peak of

19,000ML/d

2011 Flood

0

50,000

100,000

150,000

200,000

250,000

300,000

6/12/10 20/12/10 3/1/11 17/1/11 31/1/11 14/2/11 28/2/11ML

/d 0

10,000

20,000

30,000

40,000

50,000

60,000

70,000

80,000

90,000

100,000St George

Brenda

Mundiwa

Peak of 94,000ML/d

Peak of

65,000ML/d Peak of

290,000ML/d

2012 Flood

0

50,000

100,000

150,000

200,000

250,000

300,000

26/11/11 17/12/11 7/1/12 28/1/12 18/2/12 10/3/12 31/3/12

ML

/d

0

10,000

20,000

30,000

40,000

50,000

60,000

70,000

80,000

90,000

100,000St George

Brenda

Mundiwa

Peak of

80,000ML/d

Peak of

320,000ML/d Peak of

44,000ML/d



Downstream of Bourke, there were also some minor inflows to the Barwon-Darling River from the Warrego and Paroo rivers generated by heavy rainfall events in late-January and early-February 2012. The Warrego experienced very large flows in its upper reaches (up to 300,000 ML/d at Wyandra) and significant volumes (over 500,000 ML) of water spilled west to the Paroo River via the Cuttaburra channels. Although local rainfall in March put more water into the Paroo, the vast majority either infiltrated into the flood plains or evaporated before reaching the Darling River. Flows from the Warrego that reached the Darling below Bourke peaked at just under 7,000 ML/d, representing only 3 percent of the peak flow recorded at Bourke.

Between Bourke and Wilcannia, flow peaks are typically attenuated when water spills out of the main channel onto the floodplain. The first peak at Bourke had reduced by 46 percent by the time it reached Wilcannia, while the larger second peak reduced by 60 percent. The total combined volume including both events fell by almost 30 percent. This is comparable to historical losses that can be as high as 30 to 40 percent by volume. The losses experienced in the floods of 2012 were about twice those experienced in 2011.

While typically referred to as losses, overbank flows that pass into billabongs, effluent streams and anabranches provide enormous environmental benefits to the health and productivity of the floodplains.

Figure 11 Flow Hydrographs at Key Locations Downstream of Bourke

2012 Flood: D/S Burke

0

10,000

20,000

30,000

40,000

50,000

60,000

70,000

01-D

ec-1

1

15-D

ec-1

1

29-D

ec-1

1

12-J

an-1

2

26-J

an-1

2

09-F

eb-1

2

23-F

eb-1

2

08-M

ar-1

2

22-M

ar-1

2

05-A

pr-1

2

19-A

pr-1

2

03-M

ay-1

2

17-M

ay-1

2

31-M

ay-1

2

14-J

un-1

2

0

50,000

100,000

150,000

200,000

250,000

Wilcannia

Talyawalka @ Barrier Hwy

Lakes Inflow (est)

Menindee u/s Weir 32

Burtundy

Bourke Town

230,000 ML/d

53,000 ML/d

The two distinct peaks at Bourke in January and February could still be distinguished at Wilcannia and the Menindee Lakes. However, the distinct recession following the first peak, witnessed at Bourke, was almost entirely offset at Wilcannia by rising water of the fast approaching second peak.

Whilst the floods had been destructive and sharply peaked at their upstream sources, flows along downstream reaches had slowed and attenuated, resulting in extensive inundation of the Darling River floodplain downstream of Bourke. Many low-lying floodplain areas and ephemeral streams were inundated for the second time in two years. Some higher floodplains were wetted for the first time in a decade due to the higher water levels triggered by the second flood peak.

Additional inflow to the Menindee Lakes came from local rainfall in March. This had a noticeable and immediate impact on storage volumes (Figures 15 & 16). Wilcannia experienced 238mm of rainfall in 7 days, while Menindee only recorded 72mm during the same period. During this time,



the gates at the Menindee Main Weir were operated to maintain releases at a steady rate. This minimised the impact of localised flooding on Menindee township.

At Wilcannia, flow in the Darling River was overbank (greater than 25,000 megalitres per day) for a total of four months (Figures 8 & 11). Total flow past Wilcannia (including the main channel and Talyawalka Creek) peaked at 93,000 megalitres per day on 7 April 2012 (Figure 15).

In total about 5,700 GL of water passed Wilcannia from mid December 2011 to late May 2012. During the same period in 2010/11, 5,000 GL was recorded. 4,700 GL entered the Menindee Lakes scheme some 120 kilometres downstream in 2011/12 compared with 4,000 GL in 2010/11.

Table 3 Barwon-Darling Historical Floods

Year Max height at Bourke (m)

Total volume at Bourke (GL)

Number of days at Bourke

Max height at Wilcannia (m)

Total flows at Wilcannia (incl.

Talyawalka Ck) (GL)

Max height Weir 32 (m)

1971 13.73 7,700 115 11.0 5,800 7.6

1974 14.09 8,200 110 11.07 6,450 7.63

1976 14.17 14,000 180 11.59 10,500 8.07

1983 13.27 7,200 120 10.65 5,500 7.06

1988 12.57 3,500 80 10.19 2,900 5.10

1990 12.99 9,000 250 11.00 8,150 7.37

1998 13.78 9,700 110 10.83 6,700 7.45

2001 12.28 3,300 140 9.75 2,250 6.21

2010 10.78 2,370 120 9.43 2,400 5.44

2011 12.56 5,800 110 10.50 5,000 7.10

2012 13.78 8,000* 165* 10.63 5,600* 7.10

* Incorporates two distinct rainfall events occurring two months apart

Note: Largest historical flood events are highlighted in bold

Compared to previous Barwon-Darling flood events, the 2012 event measured at Bourke was the fifth largest in terms of volume since the 1970s and the equal 3rd largest in terms of river level (see Tables 3 & 4).

The flood peak travelled at a similar speed to previous flood events in the Barwon-Darling system, but faster than the 2011 event. This is probably due to the wetter channel and floodplain from inundation in recent years. The large transmission losses between Bourke and Wilcannia was probably due to the improved floodplain vegetative cover from a sequence of good seasons that attenuated the peak flows and a number of higher flood runners, billabongs and anabranches that were previously dry and were inundated by the larger flood peak in this event.



Flows in the Talyawalka

The Talyawalka Creek is a complex anabranch system of the Darling River. Flows leave the Darling River upstream of Wilcannia, and divide into a number of streams. Some of the flows make their way back to the Darling River between Wilcannia and upstream of Menindee, and others join the Lower Darling River below Menindee. Others discharge into terminal lakes and flood runners, such as Teryawynia Lake (Figures 12 & 13).

Figure 12 The Talyawalka Creek System

Flows up to 2,000 ML/d in the Talyawalka Creek at the Barrier Highway upstream of Wilcannia typically stay within the Creek channel and do not return to the Darling River. As flows increase, an increasing proportion returns to the Darling River. At around 18,000 ML/d, approximately one-third of the flow continues down the Talyawalka Creek, one-third returns to the Darling River above Menindee, and one-third passes onto the floodplain above Menindee. Above 18,000 ML/d a higher proportion of flow tends to return to the Darling River above Menindee, resulting in larger inflows to the Lakes.

The first flood event in 2011/12 triggered a peak flow in the Talyawalka Creek of about 8,000 ML/d (on 13 February 2012). This resulted in the watering of the main channel, other low-lying anabranches and some lakes and waterholes within the Talyawalka Creek system through late-January and February 2012. The total volume of this first event was less than 200,000 megalitres (measured at the Barrier Highway).



Figure 13 Darling River and Talyawalka Creek Satellite Image

(Source: NASA/GSFC, MODIS Rapid Response 2012)

Minor contribution

from the Paroo

Flood

peak

Spill to

to

es

Talyawalka via

ws

Lak

Seven Mile Ck

Inflo

the

Return flow from

Talyawalka to Darling

Up to 6,000ML/d on

Talyawalka Ck

Return flow

below Menindee

township

Image dated: 18/04/2012

The second flood event was much more significant, peaking at 53,000 ML/d on 7 April 2012. Combined with a peak flow in the Darling River channel at Wilcannia of 40,000 ML/d, a total of up to 93,000 ML/d passed Wilcannia at the peak of this event. While more extensive inundation was experienced in the floodplain of Talyawalka Creek, the majority of this larger flow returned to the Darling River channel and either flowed away again at Seven Mile Creek or flowed via the main channel of the Darling River to the Menindee Lakes. Approximately 1,300,000 megalitres passed the Barrier Highway via Talyawalka Creek as a result of this second peak.

High flows exceeding 25,000 ML/d in the main Darling River channel continued for a total three months. Despite extensive ‘losses’ filling many Talyawalka floodplains, lakes and billabongs, flows passed through the entire length of the system, with significant return flows back into the Lower Darling River downstream of Menindee, below Weir 32.



Menindee Lakes Operation

Rising Water Levels The Menindee Lakes storages were surcharged to 105 percent of full supply level during 2011 and were slowly falling when the first large rainfall event occurred in late November 2011. Regulated releases had just begun and were increasing through early December 2011. When upstream flows began entering the Barwon River channel, initial forecasts indicated that releases greater than channel capacity of the Lower Darling River, approximately 9,000 ML/d (recorded at Weir 32), would be necessary to keep the Lakes within full surcharge capacity.

Releases were increased to 16,000 ML/d by 24 December and again to 22,000 ML/d by 7 January 2012. This caused inundation of some downstream access routes and lower lying parts of properties. It was estimated at the time that this release rate would create sufficient airspace to contain future inflows from the forecast flood inflows, recognising that further heavy rainfall and inflows was possible.

With drying conditions in late-December 2011 and early-January 2012, releases were eased in late-January to below inundation levels to provide some relief to local affected properties. However, in late-January and early-February, heavy rainfall in upstream catchments required another increase in releases to the lower Darling River on the understanding that inflows could be amongst the highest recorded in the past 25 years.

Figure 14 Menindee Lakes General Layout



By mid-February 2012, releases approached 35,000 ML/d, as measured on the Lower Darling River at Weir 32, and were kept at this level for two months to minimise downstream flood inundation (Figure 15). This rate of release was required to create the airspace in the Menindee Lakes storage scheme needed to absorb forecast inflows of over 50,000 ML/d.

This release rate was similar to the maximum regulated releases in 2011, where low-lying dwellings close to the river were inundated, access to some other properties became more difficult, some crops were submerged, and the Menindee to Pooncarie Road was cut.

Water was preferentially released from Lake Wetherell which was drawn down to about 50 percent of full supply level. On 27 February 2012, the main weir gates were fully removed and inflows of around 33,000 ML/d were being passed unimpeded.

Two days later on 29 February, heavy local rainfall centred on Wilcannia resulted in a near instantaneous increase in inflows to the Lakes. Rather than leaving the gates out of the water and allow additional flooding at Menindee township, the gates were replaced and operated dynamically to limit outflows (and flooding) to the current levels of 35,000 ML/d at Weir 32. Some airspace was consumed during the process in order to manage the flood threat and prevent additional inundation at Menindee.

On 7 March 2012, the main gates were again removed clear of the water, passing all inflows until 1 April. One of the significant environmental benefits of removing the main gates is to enable fish passage from the Lower Darling to the river upstream of the Menindee main weir.

Darling River below the main weir. Releases targeting a flow of 35,000ML/d at Weir 32.




Main Weir at Menindee with the gates first clear of the water

Photo: 27 February 2012 – Courtesy Barry Philp

Peak Water Levels During the first two weeks of April 2012, inflows increased to a peak of approximately 60,000 ML/d while releases were limited to 35,000 ML/d. The flood peak passed through Wilcannia during this time and, with over 600,000 ML of airspace available in the Lakes, releases into the Lower Darling River could begin to be slowly reduced. By minimising outflows through Weir 32 during the highest inflow period, storage levels rose quickly, but Menindee township and surrounding lands were protected from the extremes of the flood.

Following the passing of the flood peak, releases were reduced to 24,000 ML/d and then 22,000 ML/d at Weir 32 in order to alleviate many of the local flooding impacts. With inflows of over 50,000 ML/d continuing for two weeks to mid-April, the vast majority of available airspace in the Lakes was filled during April.

During early May, releases were again slowly reduced to balance the flood mitigation benefits while maximising the volume of water stored in the lakes. On 12 May 2012, the total storage volume peaked at 2,025 GL, which is 117 percent of full supply volume and only 25 GL below the maximum surcharge volume. At the time, the airspace remaining in the Menindee Lakes represented just over one day of inflows.

With inflows beginning to fall rapidly and the total storage having peaked, releases to the Lower Darling could be reduced further according to flow management protocols.




Figure 15 Darling River Flows

Darling River Flows

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Figure 16 Menindee Lakes Storage Volume

Menindee Lakes Storage

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Gates in water

Falling Water Levels After months of inundation the saturated river banks of the Lower Darling were susceptible to slumping if water levels were allowed to reduce too quickly. Although it is natural for rivers to rise and fall, a rapid reduction in water levels, particularly after a long period of inundation, significantly increases the risk of subsidence of the river bank and the potential to exacerbate the loss of River Red Gum trees into the river channel.

A review of historical practises adopted in the management of previous flood recessions was compared with current conditions. This led the Office of Water and State Water to implement a reduction of regulated releases from the Menindee Lakes to the Lower Darling River of a maximum of 1,000 ML/d each day at Weir 32 for flows between 20,000 ML/d and 10,000 ML/d.

This precautionary rate of recession was slower than the protocol which allows for a reduction in releases of up to 10 percent of the previous day’s flow.


Releases through Weir 32 below 10,000 ML/d were made at rates consistent with operating protocols, of 500 ML/d each day down to 5,000 ML/d, then 250 ML/d each day down to seasonal minimums, which for May to October is 200 ML/d, except when the lakes are surcharged, in which case the minimum is 500 ML/d.

Menindee Lakes Storage Operating Rules

Maximum rates of rise and fall downstream of Menindee Lakes

Maximum Rate of fall

Flow Range (ML/day) Rate of Fall (ML/day per day)

Minimum to 5,000 250

5,000 to 10,000 500

10,000 to 20,000 10% of prior flow

Above 20,000 Consistent with flood operation objectives & rate of fall of inflow

The natural inflow recession was steeper than the safe outflow proposed. This meant that storage levels in the lakes would reduce. Nevertheless, with the base flows expected over winter from water slowly draining from upstream floodplain storages, and low evaporation rates, lake levels are forecast to slowly rise and remain well above full supply level until downstream demand grows in Spring and Summer.

Most of the seasonal minimum release, 500 ML/d on average, will be made from Lake Menindee. However, to maintain stream health in the reach between Menindee Outlet and up to the main weir, some water will be released from Lake Pamamaroo, upstream of Menindee township.

Main Weir at Menindee with the gates clear of the water

Photo: 20 March 2012 – Courtesy Barry Philp

Environmental Outcomes The environmental benefits of the 2012 flood were substantial, particularly following the drought-breaking floods of 2011. The Talyawalka anabranch system was replenished for the second time in two years, with many lakes and billabongs filling. Lakes Wongalara, Poopelloe and



Teryawynia, to the south and east of the Darling River, were filled as were the majority of other small ephemeral lakes adjacent to the Talyawalka Creek (Figures 12 & 13).

Flows through Seven Mile Creek and those reaching the Talyawalka in its lower westward reaches provided in-stream benefits and provided for the continuing recovery of riparian vegetation after many years without water. The replenishment of these water bodies regenerated wetland and riparian habitat that will maintain the ecosystems for the coming years.

Lake Wetherell, that comprises the inundated floodplain that joins four smaller lakes (Figure 14), was drawn down to nearly 50 percent of capacity. This allowed some drying and aeration of important riparian zones, preventing drowning of vegetation by continuous inundation. This followed approximately 7 months of saturation as the Lake was held in surcharge after the 2011 floods.

The removal of the main weir gates from the main channel of the Darling River between 27 and 29 February and again between 7 March and 1 April 2012 allowed unrestricted fish passage through the weir. This total of 29 days was similar to that achieved in February and March 2011. Before then, the previous opening was some 13 years earlier, in September and October 1998, for a total of 41 days.

Prior to the flood management, environmental account water was being released in early December to be passed into South Australia. By mid December these releases were overtaken and replaced by unregulated flood flows which, combined with unregulated Murray and Murrumbidgee flows, providing significantly greater volumes than regulated environmental water releases.

Figure 17 Great Darling Anabranch flows

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Between late-December 2011 and May 2012, flows into the Lower Darling were greater than 9,000 ML/d and a proportion passed naturally from the main channel of the river into the Great Darling Anabranch downstream of Menindee. The Anabranch flows were enhanced by releases of 1,500 ML/d from Lake Cawndilla, into Tandou Creek which flows into Redbank Creek and on to the Anabranch (Figure 17).

These flows passed the full length of the Anabranch for the second time in two years, having been preceded by a decade-long drought. A total volume of 1,000GL was measured past the Wycot gauge in the upper section of the Anabranch, between January and May 2012. All lakes, flood-runners and billabongs, many listed in the Directory of Important Wetlands in Australia, received water during this event.



Cawndilla outlet looking downstream releasing approximately 1,400 megalitres per day.


Communications Consultation with stakeholders and communities was a key focus of the NSW Office of Water and State Water Corporation in managing both the 2011 and 2012 floods.

The Office of Water established two management groups that included a technical working group and a community advisory group.

The technical working group comprised flood forecasting and management expertise from various agencies that managed and analysed flood data and advised on emerging developments and possible outlook scenarios.

The community advisory group comprised landowners and local water users, Councils, emergency services, and local industry representatives.

Weekly teleconferences were held separately with both groups to provide information exchange; sharing data, observations and testing management strategies.

Throughout the flood, the NSW Office of Water together with State Water published ten Community Information Communiqués. These were direct emailed to approximately 100 local stakeholders including local and regional media outlets.

The Community Information Communiqués provided information on Darling River flood flows and the management of the Menindee Lakes System, including predicted increases in river heights that allowed irrigators to adjust pumps and other infrastructure.

The regular release of Community Information Communiqués also enabled the broader community to watch the passage of floodwaters through the system and to understand the operations and the potential impact of those operations on downstream properties.

This information was also available on the Office of Water’s website.



First page of a Community Information Communiqué – 9 March 2012




Conclusions and Outlook The 2012 flood event at Menindee was one of the larger events experienced in recent years. It was more significant than the 2011 event, but smaller by volume than both 1998 and 1990, and much smaller than two of the largest events in 1976 and 1974.

State Water Corporation, as operator of the Menindee Lakes infrastructure, managed approximately 5,000GL through the system. With the Menindee Lakes storage scheme already full at the start of the event, the entire flood inflow volume had to be passed. Total volumes managed through the Menindee Lakes system was the equivalent to 10 times the volume of Sydney Harbour and three times the total storage capacity of Menindee Lakes themselves.

Peak inflows of up to 60,000ML/d were reduced to a maximum outflow of 35,600ML/d at Menindee, significantly reducing damage to houses and property that would otherwise have occurred.

The two distinct rainfall events in the upper parts of the catchment produced a long duration high river in the Darling River below Bourke. At Wilcannia, the Darling River pushed flow into the Talyawalka Creek system for the first four months of 2012, providing significant environmental benefits.

In addition, the long period of high river flows in the Lower Darling combined with releases from Lake Cawndilla ensured significant flows through the Great Darling Anabranch.

Table 4 Flood Volumes - 2012 Event vs Historical Events

Total volume in GL (% change from U/S volume) Location Distance (km)

Time (days)

2012 2011 2010 1998 1990

St George 0 0 2,600 8,800 3,600 - -

Bourke 650 27 8,000 5,800 (-34%) 1,100 (-69%) 9,700 9,000

Menindee 900 42 4,700 (-41%) 4,000 (-31%) 1,100 (0%) 5,000 (-48%) 7,000 (-22%)

Burtundy 400 25 2,900 (-38%) 3,400 (-15%) - 2,400 (-52%) 4,250 (-39%)

The establishment of the technical and community working groups enabled the exchange of information between agencies and communities. The regular publication of Community Information Communiqués provided the opportunity for downstream irrigators and property owners, as well as the broader community to anticipate the rising river levels, understand the necessity of specific operations and to make appropriate preparations for flooding.

In June 2012 the Menindee Lakes held around 1,920GL (111 percent of full supply capacity). The Lakes are slowly rising and likely to remain above full supply level over winter with base flows into the lakes more than offsetting losses. Rainfall over winter is expected to be about average however, should conditions return to dry, there is sufficient volume now stored in the Menindee Lakes to ensure high security water supply for licensed water users in the Lower Darling River valley for the coming two years, without any further inflows.

nsw department of industry - management of the barwon-darling … · 2015. 3. 6. · triggered...

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