Frequently Asked Questions – 18/11/14 (Final)
Murray Mouth Sand Pumping
1
Question
Why is the Murray Mouth at risk of
closure?
Answer
The mouth has become severely constricted due to the natural build-up of sand. Sand enters the
mouth during periods of low flows in the river and is only flushed out when there are periods of
high flows. For example, the high flows in 2010-11, when South Australia received a peak flow of
94 gigalitres (GL) a day, provided a peak flow through the barrages of 75 GL per day, which was
sufficient to scour the mouth and flush sand back into the southern ocean.
Sand has been accumulating steadily in the mouth since late 2011 as the high river flows receded
and the discharge rate from the barrages has also reduced.
The quantity of sand presently inside the mouth now is similar to the volume present in 2002
when a dredging program commenced. This program was required for eight years to keep the
mouth open.
2
What is needed to keep the mouth
open?
While flows from the Murray-Darling Basin are the key requirement to maintain an open mouth,
there is no agreed, absolute flow required to achieve this, because it is not an exact science. An
open mouth depends on a range of factors, including flow and localised tidal activity.
Generally flows of around 75 GL a day are required to scour sand from the mouth and wash it
out to sea. As the flow over the barrages recedes, this results in an accumulation of sand.
Based on current water availability forecasts, it is likely that flows into the Lower Lakes over late
summer and into autumn will be lower than experienced in recent years, and will not be
sufficient to keep the mouth open.
3
Why is it important to keep the
mouth open?
An open mouth is necessary to allow for the exchange of water between the ocean and the
Coorong, to help reduce salinity levels in the Coorong. Tidal variations also help to maintain
habitat for small insects and other creatures that live in the sand and mud flats which are food
sources for migratory water birds.
An open mouth also allows some species of fish to travel between the ocean and the estuary to
meet feeding and breeding requirements.
When flows into the Coorong are low, a closed mouth would cause salinity and water
temperatures in the Coorong to increase, resulting in unfavourable conditions for native plants
and animals in the Coorong.
In times of higher flows, a closed mouth would trap salt and nutrients in the Coorong and
increase the likelihood of algal issues.
An open mouth also provides an opportunity to discharge salt from the River Murray system,
which is a critical objective of the Murray-Darling Basin Plan.
4
Why hasn’t the Basin Plan ensured
the mouth remains open?
The heightened risk of closure of the mouth highlights the importance of continuing to
implement the Basin Plan and recovering the full amount of environmental water required under
the Plan. Full water recovery under the Basin Plan is not expected until 2019.
As at 31 August 2014, around 1,500 GL of environmental water was available for the
Commonwealth Environmental Water Holder to use. The Basin Plan sets a target of 2,750 GL of
water recovery with the flexibility to increase this volume by another 450 GL.
Under the Basin Plan, it is expected that flows to the mouth will increase by around 30 percent,
and that it should remain open 95 percent of the time without dredging.
While full implementation of the Basin Plan will significantly reduce the times that the mouth will
close, in some years, due to the particular conditions, dredging may still be required to keep the
mouth open.
5
Why is the pending closure of the
mouth a whole of Basin issue and
not just South Australia’s problem?
The Commonwealth, South Australia, New South Wales and Victoria share the responsibility of
managing River Murray assets, such as the mouth, under a joint venture agreement. This
includes the funding of works and measures along the river system, such as dredging the mouth.
6
Have other options besides
dredging been considered to keep
the mouth open?
Dredging has been shown to be the most effective method for keeping the mouth open, in terms of
cost and environmental criteria, compared with a range of structural and other methods. Presently
there is insufficient water available within the River Murray system to provide flows sufficient to scour
the mouth and delay dredging.
7
How much will it cost to keep the
mouth open, and who will pay?
During the Millennium Drought, dredging operations to keep the mouth open cost approximately
$6 million a year. The Murray-Darling Basin Authority has provided $4 million in the event that
dredging may be required again this summer.
8
Will dredging stop people fishing
and boating at the mouth?
It is likely that ‘boating exclusion zones’ will be established near the mouth during sand pumping
activities so that contractors can carry out their work safely. This means that the public may be
excluded from boating in the vicinity of the mouth.
Information about boating access to the mouth will be provided through Notices to Mariners and
signage at Sugars Beach, Mundoo and Beacon 19 boat ramps.
It is likely that vehicle access to the mouth from Goolwa Beach will not be affected. Signage and
fencing will be used to divert drivers around the sand disposal points.
9
Will dredging the mouth affect
Encounter Bay Marine Park?
The mouth is situated on the southern ocean coastline and is exposed to extremely high wave
energy. This results in significant disturbance of the near-shore coastal waters by waves and
wave-induced currents. In addition, when the River Murray is discharging flows into the sea, this
adds turbidity and changes the water quality. These are normal ocean conditions near the
mouth.
On this basis, dredging the mouth and placing the spoil on the beach of the Sir Richard and
Younghusband Peninsulas, is unlikely to significantly increase the turbidity or negatively impact
on the water quality of the near-shore coastal waters or the Encounter Bay Marine Park.
10
What has South Australia done with
the water it has received to date for
River Murray operations and could
this water have been managed
differently to keep the mouth
open?
Managing the River Murray system is a complex task in which the needs of human consumption,
the environment, agriculture and industry must be taken into account.
Water levels within the Lower Lakes are actively managed to provide outcomes for the region.
This included a water level cycling event in July-August 2014 providing a peak discharge through
the barrages of about 25 GL a day. This helped slow the accumulation of sand.
In early November, the discharge rate through the barrages is 2 GL a day and the volume of
environmental water committed to the region for 2014-15 is not sufficient to maintain an open
mouth without dredging.
11
12
13
Are we using water at any
environmental watering sites
upstream that could be used to
keep the mouth open?
Has weir pool raising contributed to
the mouth closing?
Environmental water is being used to test the operation of the Chowilla regulator this spring. The
majority of this water is flowing through the Chowilla anabranch back into the River Murray to
deliver benefits along the main River Murray Channel before flowing through the mouth.
Has lake level cycling contributed to
the mouth closing?
Lake level cycling has been undertaken to reduce salinity in Lake Albert and has not contributed
to the mouth closing. Modelling studies undertaken by the CSIRO indicate that the volume of
water released through the barrages determines how open the mouth is.
The volume of water held behind Lock and weirs 1 and 2 was a small volume of around 15 GL.
This volume has a negligible impact on mouth openness. Some of this water will flow into the
Lower Lakes during November and December when the weir pool raising project is completed.
The timing of the releases may affect the depth of the mouth from one day to the next but the
average depth of the mouth over the year will be the same if the total volume released through
the barrages is the same.
14
Will the South East Flows
Restoration Project help solve the
issue?
The South East Flows Restoration Project will not solve issues at the mouth. The Project aims to
improve the health of South East wetlands and the Coorong by allowing more fresh water to flow
from the Upper South East Drainage System to the South Lagoon of the Coorong. The Project is
expected to increase median flows into the Coorong at Salt Creek from 29.7 to 56.2 GL a year.
This volume is only around three percent of the annual flow required to keep an open mouth.
15
Would a Coorong connector
improve the situation at the
mouth?
A connector between Lake Albert and the Coorong would not increase the total volume of water
being discharged from the Lower Lakes to the Coorong and would therefore not improve
conditions in the mouth.
16
Would removing the barrages fix
the issues at the mouth?
Removing the barrages would not improve Murray Mouth openness because the flow rate and
total volume of River Murray water flushing out the mouth would not change significantly.