Lower Lakes, Coorong and Murray Mouth



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12. APPENDIX B: DETAIL ON LOGIC TO SUPPORT WATER REQUIREMENTS (SECTION 3.2.)

Lakes Alexandrina and Albert overview


The Lower Lakes annually require sufficient environmental flows to achieve the following (Muller 2010; Lester et al. 2011b):

• sufficient volumes to maintain lake levels above 0.0 metres AHD to prevent exposure of sulfidic sediments

• sufficient volumes to vary lake levels seasonally between 0.35 m and 0.75 m above sea level annually, with higher lake levels of between 0.5 m and 0.83 m above sea level every third year (Muller 2010)

• barrage outflows sufficient to maintain salinity in Lake Alexandrina below 1,000 EC 95% of the time and below 1,500 EC 100% of the time

• a total average annual barrage outflow of 2,000 GL per three year rolling period and not less than 650 GL in any one of the three years will ensure the above target salinities are met in the Lower Lakes

• the river flows to the sea annually – higher security to low flow regime.


Lower Lakes water level


In order to achieve icon site ecological targets and to protect the key species of Lakes Alexandrina and Albert, lake level variation is required over the course of a year. In the past, the lakes required surcharging in early September to ensure irrigation supplies, evaporative losses and other uses could be met over summer. However, drought conditions between 2006 and 2010 have resulted in the installation of the Lower Lakes pipeline, which allows irrigation to be achieved in some (e.g. Langhorne Creek), but not all (e.g. Lake Albert fringes), areas adjacent to the Lakes regardless of lake level. SA Water off-takes on the river channel below Lock 1 have also been lowered, allowing water extraction to occur over the entire proposed operating range. It is therefore now possible to manage lake levels for ecological purposes, not solely for supply demands, although consultation will be required with consumptive users without access to the Lower Lakes pipeline. For optimal ecological outcomes, Muller (2010) suggests both an annual lake level envelope, with a return interval of one (every year), in conjunction with an occasional flooding envelope, with a return interval of three (once every three years).

Annual lake operating envelope (ARI = 1)


Muller (2010) proposes an annual variable lake level, fluctuating between 0.35m AHD – 0.70m AHD (Figure ).

A minimum lake operating level of 0.35 m AHD allows for key fringing wetlands, channels and aquatic habitats, particularly around Hindmarsh and Mundoo islands, to remain inundated. These wetland environments are inhabited by key threatened fish species such as Murray hardyhead, Yarra pygmy perch and southern pygmy perch (refer Target F2), and diverse submerged aquatic vegetation (refer Target V3). This range of lake levels also ensures operation of most fishways is achievable year-round, especially for the smaller vertical slots, and in particular, Hunters Creek fishway (refer Target F1). A lowering of levels is needed to establish fringing lakeshore vegetation (Target V3), which prevents lakeshore erosion. Under a varied lake operating regime, shoreline mudflats near the barrages are exposed and re-inundated (Target M1), providing suitable conditions for benthic invertebrates (Target I1), an important food source for migratory waders (Target B1).

An upper lake operating level of 0.70 m AHD in spring allows for ephemeral fringing wetlands of Lake Alexandrina and Lake Albert to be inundated briefly, creating a wetting and drying cycle. The wetting period promotes the growth of aquatic vegetation (Target V3) and stimulates a release of aquatic invertebrates from the egg-bank of the sediments, providing food for waterbirds (Target B1) and fish. Higher lake levels allow for larger barrage releases to be made during this period, providing greater connectivity between the Lakes and the Coorong (Target W3) for diadromous fish (Target F1). Refer to Table for a description of monthly ecological outputs corresponding to an upper and lower lake level.

this figure shows the target ‘operating envelope’ for lakes alexandrina and albert at an annual return interval of 1 (every year), showing upper and lower limits.

Figure : Proposed ideal operating envelope for Lakes Alexandrina and Albert at an Annual Return Interval of 1 (every year), showing upper and lower limits


Table : Rationale around ideal operating envelope for the lakes at an Annual Return Interval of 1 (every year).



Month

Upper

Lower

Rationale

(m AHD)

January

0.60

0.45

Gradual drawdown over late summer to expose mudflats and promote diverse vegetation, whilst supporting completion of life cycles for vegetation and dependent fauna.

February

0.55

0.40

February drawdown not to drop below +0.4 m AHD to ensure upstream migration of congolli (and common galaxias) juveniles from estuarine to fresh waters.

March

0.50

0.40

Summer/autumnal drawdown to expose mudflats and promote diverse vegetation but still support biotic movement between fresh and estuarine waters.

April

0.50

0.35

Autumnal drawdown to promote diverse vegetation can drop to +0.35 m AHD assuming that functional connectivity between the lakes and the island anabranches remains at this level.

May

0.50

0.35

Autumnal drawdown to promote diverse vegetation but maintain connectivity to islands and fringing wetlands.

June

0.50

0.35

Winter low point prior to inflows commencing, supports overwintering reeds which die-off and need their new shoots to grow at a rate that matches rising water levels in spring.

July

0.50

0.40

Fill typically begins in mid- to late winter. It may naturally have occurred very quickly but this proposed rate of fill would allow growth of new shoots of reeds and other vegetation to match rising water levels (highly turbid water) and thus promote diversity.

August

0.60

0.45

Highest rate of fill in late winter and early spring. Native fish, frogs and other fauna need access to vegetation from now onwards into spring/summer to obtain food and shelter for recruitment and protection from predation.

September

0.65

0.50

High water in spring for native fish, frogs and other fauna to access habitats and successfully recruit.

October

0.70

0.50

High water in spring for native fish, frogs and other fauna to access habitats and successfully recruit.

November

0.70

0.50

High water in spring for native fish, frogs and other fauna to access habitats and successfully recruit.

December

0.65

0.45

Commencement of summer drawdown to expose mudflats and promote diverse vegetation.

Occasional lake flooding envelope (ARI = 3)


Muller (2010) also proposes a one-in-three year higher lake level event, at which lake levels can fluctuate within a higher envelope of 0.50 m AHD - 0.95 m AHD (Figure 10). This follows a more natural pre-regulation regime, and was also informed by native species’ flow requirements.

Occasional flooding, at levels >0.70 m AHD in spring and summer, allows extensive floodplain inundation in the key breeding season for threatened fish (Target F2), waterbirds (Target B1) and frogs. Long-lived vegetation such as the swamp paperbark (Melaleuca halmaturorum), often situated high on the floodplain, is also benefited by occasional inundation (Target V3). It ensures year-round operation of all fishways is achievable (supporting targets W3 and F1), with larger releases likely during high water levels in spring and summer. This may also coincide with an export of salt from the Lakes and the River Murray system as a whole, freshening in the Coorong (Target W1), and the maintenance of an open Murray Mouth (Targets W2 and W4). Refer to Table for a description of monthly ecological outputs corresponding to an upper and lower lake level for the one in three year scenario.



this figure shows the target ‘operating envelope’ for lakes alexandrina and albert at an annual return interval of 3 (every 3 years), showing upper and lower limits.

Figure : Proposed ideal operating envelope for Lakes Alexandrina and Albert at an Annual Return Interval of 3 (once every three years), showing upper and lower limits.
Table : Rationale for ideal envelope for lake levels that would occur at an Annual Return Interval of 3, showing upper and lower limits

Month

Upper

Lower

Rationale ARI = 3

(m AHD)

January

0.85

0.60

Gradual drawdown over late summer to expose mudflats and promote diverse vegetation, whilst supporting completion of life cycles for floodplain vegetation and dependent fauna. Optimal outcomes for small-bodied native fish if water levels are  +0.8 m AHD from August to February (Bice pers. comm.).

February

0.80

0.60

Summer drawdown to expose part of the mudflats and promote diverse vegetation but stay above +0.6 m AHD so as to support faunal access to littoral vegetation and upstream migration of congolli (and common galaxias) juveniles from estuarine to fresh waters.

March

0.75

0.55

Summer/autumnal drawdown to expose mudflats and promote diverse vegetation, whilst still supporting high levels of connectivity between lakes, island anabranches and the Coorong.

April

0.70

0.55

Autumnal drawdown to promote diverse vegetation but maintain connectivity to islands and fringing wetlands.

May

0.70

0.50

Autumnal drawdown can drop to +0.50 m AHD to promote diverse vegetation but maintain connectivity to islands and fringing wetlands

June

0.70

0.50

Winter low point prior to inflows commencing, supports overwintering reeds which die-off and need their new shoots to grow at a rate that matches rising water levels in spring.

July

0.75

0.55

Fill typically begins in mid- to late winter. It may naturally have occurred very quickly but this proposed rate of fill would allow growth of new shoots of reeds and other vegetation to match rising water levels in highly turbid waters.

August

0.80

0.60

Highest rate of fill in late winter and early spring. Native fish, frogs and other fauna need access to vegetation from now onwards into spring/summer to obtain food and shelter for recruitment and from predation.

September

0.85

0.65

Some species require occasional inundation of whole floodplain (e.g. long-lived vegetation), which is assumed to occur at average water levels of +0.83 m AHD given that is when flow commences over the islands and into the Coorong. NB: water levels can exceed +1.0 m AHD in some areas.

October

0.95

0.70

Some species require occasional inundation of whole floodplain (e.g. long-lived vegetation) at lake levels of ≥+0.85 m AHD.

November

0.95

0.70

Some species require occasional inundation of whole floodplain (e.g. long-lived vegetation) at lake levels of ≥+0.85 m AHD. Extended floodplain inundation compared to ARI = 1 to allow fish and plants to complete life cycles.

December

0.90

0.65

Commencement of summer drawdown to expose mudflats and promote diverse vegetation.



Lakes Alexandrina and Albert salinity


An average annual salinity threshold of 1,000 EC for Lake Alexandrina is recommended by Muller (2010), based on salinity tolerances of key ecological indicators, many of which are consistent with icon site targets. This is considered to be the maximum threshold and has been selected due to the salinity relationship between Lake Alexandrina and Lake Albert. A maximum average salinity of 1,000 EC in Lake Alexandrina traditionally coincides with a salinity of around 1,700 EC in Lake Albert (Heneker 2010). As 1,700 EC is the upper end of salinity tolerance for many key freshwater species, including species of fish and aquatic vegetation, these figures have been selected as average annual upper salinity thresholds for both Lake Alexandrina and Lake Albert (Lester et al. 2011b).

Lakes Alexandrina and Albert volumes

Volumes required to achieve target lake levels


It is very difficult to determine the volume of water required for the Lower Lakes (and the entire weir pool below Lock 1) to achieve a certain water level, as this is dependent on inflows and starting lake levels. As a guide, the following table describes fill volumes required to raise Lower Lakes levels in 10 cm increments (Table ). This table may be used to estimate water requirements to lift lake levels to within the above-described operating ranges, particularly when levels fall below 0.35 m AHD.

Volumes required to achieve Lower Lakes salinity targets


In order to achieve an average annual salinity of 1,000 EC in Lake Alexandrina, rules for required cumulative annual barrage discharges have been determined through extensive modelling (Henneker 2010, cited in Lester et al. 2011b).

B1 is the minimum barrage outflow volume in any given year

B2 is the minimum cumulative barrage outflow volume over two years

B3 is the minimum cumulative barrage outflow volume over three years.

1. B1

2. B2 – Fx-1



3. B3 – Fx-1 – Fx-2

The rules associated with maintaining a salinity threshold in Lake Alexandrina of 1,000 EC equates to a minimum barrage outflow volume that is required in a given year (Fx) is equal to the greater of:

1. 650 GL

2. 4,000 GL – Fx-1

3. 6,000 GL – Fx-1 – Fx-2 (where Fx-2 is equal to the lesser of the actual outflow 2 years prior to the current year and 2,000 GL).

The above rules for lake operation and barrage releases, which are required to achieve optimal lake levels and salinities, can be related back to the LLCMM icon site ecological targets which are specific to the Lower Lakes. This is documented in Table .



Table : Required volume (GL) to raise water levels in the weir pool from Lock 1 to the barrages by 10 cm from the corresponding starting water level

Starting Level (m AHD)

Increase in Surface Area (ha)

Volume (GL)

0.6

243

87

0.5

266

86

0.4

403

86

0.3

641

85

0.2

1,122

85

0.1

1,407

83

0

1,333

82

-0.1

1,613

80

-0.2

1,762

79

-0.3

1,789

77

-0.4

1,873

75

-0.5

2,881

73

-0.6

2,351

70

-0.7

1,654

68

-0.8

1,691

66

-0.9

1,783

65

-1

2,107

63

-1.1

3,086

60

-1.2

2,680

57

-1.3

2,881

55

-1.4

4,071

51

-1.5

3,351

47

-1.6

2,082

45

-1.7

1,792

43

For example, to increase water level from 0.0 m AHD to 0.1 m AHD 83.35 GL are required, this input would increase the surface area by 1,407.18 ha.



Table : Environmental water requirements of the Lower Lakes to achieve icon site ecological targets

TLM Objective (M/F/B)

Target

Biota

Optimum salinity*

Level (m AHD)

Frequency (yrs)

Annual Volume (GL)

Duration and Timing

B

Maintain or improve bird populations in the Lower Lakes

Waders

fresh - brackish

0.35 - 0.7
0.5 - 0.8

1 in 1
1 in 3

min 650GL/yr barrage outflow#

As per hydrographs (Figs 5 & 6)

Fish-eating birds

< 1,000 EC

0.35 - 0.7
0.5 - 0.8

1 in 1
1 in 3

min 650GL/yr barrage outflow#

As per hydrographs (Figs 5 & 6)

Water fowl (herbivorous)

< 1,000 EC

0.35 - 0.7
0.5 - 0.8

1 in 1
1 in 3

min 650GL/yr barrage outflow#

As per hydrographs (Figs 5 & 6)

M & F

Maintain or improve recruitment success of diadromous fish in the Lower Lakes (and Coorong)

Fish requiring both marine & freshwater habitats

fresh - brackish

>0.5 m AHD for fishway releases

1 in 1

minimum outflow of 52 GL/y fishway operation +attr flows

Fishways run for 12 months

F

Maintain or improve recruitment success of endangered fish species in the Lower Lakes

Murray hardyhead

brackish

0.35 - 0.7
0.5 - 0.8

1 in 1
1 in 3

min 650GL/yr barrage outflow#

As per hydrographs (Figs 5 & 6)

Southern pygmy perch

< 1,000 EC

0.35 - 0.7
0.5 - 0.8

1 in 1
1 in 3

min 650GL/yr barrage outflow#

As per hydrographs (Figs 5 & 6)

Yarra pygmy perch

< 1,000 EC

0.35 - 0.7
0.5 - 0.8

1 in 1
1 in 3

min 650GL/yr barrage outflow#

As per hydrographs (Figs 5 & 6)

O, F & B

Maintain or improve invertebrate populations in mudflats (Lakes)

Mudflat invertebrates

fresh - brackish

0.35 - 0.7
0.5 - 0.8

1 in 1
1 in 3

min 650GL/yr barrage outflow#

As per hydrographs (Figs 5 & 6)

Aquatic invertebrates

< 1,000 EC

0.35 - 0.7
0.5 - 0.8

1 in 1
1 in 3

min 650GL/yr barrage outflow#

As per hydrographs (Figs 5 & 6)

B

Facilitate frequent changes in exposure and submergence of mudflats (Lakes)

Mudflat invertebrates

N/A

0.35 - 0.7
0.5 - 0.8

1 in 1
1 in 3

N/A

As per hydrographs (Figs 5 & 6)

B

Maintain habitable sediment conditions (Lakes)

benthic invertebrates

N/A

0.35 - 0.7
0.5 - 0.8

1 in 1
1 in 3

min 650GL/yr barrage outflow#

As per hydrographs (Figs 5 & 6)

TLM Objective (M/F/B)

Target

Biota

Optimum salinity*

Level (m AHD)

Frequency (yrs)

Annual Volume (GL)

Duration and Timing

F & B

Maintain or improve aquatic and littoral vegetation in the Lower Lakes

Freshwater aquatic vegetation

< 1,000 EC

0.35 - 0.7
0.5 - 0.8

1 in 1
1 in 3

min 650GL/yr barrage outflow#

As per hydrographs (Figs 5 & 6)

F

Maximise fish passage connectivity between the Lower Lakes and Coorong

Fish requiring both marine & freshwater habitats

N/A

>0.5 m AHD for fishway releases

1 in 1

minimum outflow of 52 GL/y fishway operation + attr flows

Fishways run for 12 months

TLM Objectives: M refers to open Murray Mouth, B refers to enhanced migratory bird habitat and F refers to more frequent estuarine fish recruitment.

*Salinity ranges: Fresh 0 – 800 EC, Brackish (Oligohaline) 800 – 8000 EC. (USGS 2011).

#A minimum barrage outflow of 650GL is dependent on larger outflows in subsequent years (averaging 2,000GL over 3 years), as described above.

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