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House of Assembly - Fifty-First Parliament, Third Session (51-3)
2009-06-03 Daily Xml
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Parliamentary Committees

PUBLIC WORKS COMMITTEE: WELLINGTON WEIR

Ms CICCARELLO (Norwood) (11:01): I move:

That the 320th report of the committee, entitled Wellington Weir—Preliminary Works, be noted.

The Murray-Darling Basin is experiencing the worst drought period on record and inflows are at record lows. This has had a major impact on water levels in the river system. As the drought has severely reduced the river flows to South Australia, the water level of the Lower Lakes has dropped to levels not seen since the construction of the barrages in the 1930s.

With the combined effects of evaporation and the absence of flushing flows, water quality in the Lower Lakes has declined to the point where, with minor exceptions, it is now too saline for irrigation or human consumption. The water quality in Lake Albert is generally too saline even for stock use. However, under the worst case scenario used for forecasting flows and levels in the river, the pool level below Lock 1 is expected to continue to fall.

As water levels in the Lower Lakes have fallen and exposed the sediments, the risk of the water bodies becoming acidified has emerged as a serious threat to water quality and the environment. The Murray-Darling Basin Ministerial Council's strategy seeks to avoid acidification of lakes Alexandrina and Albert by maintaining the lakes above alkalinity and water level management triggers.

However, if these triggers are reached and there is insufficient fresh water inflow, minimum quantities of sea water will need to be introduced through the barrages. If sea water is introduced to the lakes, there is a clear potential for salt to migrate upstream towards Murray Bridge, unless a weir is in place below Wellington. In this event, there are limited options for access to each abutment. On the eastern side access is only possible through Wellington Lodge, and on the western side access is only possible through Nalpa Station. The works forming part of this project are:

construction of approximately three kilometres of access road to the eastern abutment at an estimated cost of $1.5 million;

construction of approximately 7.5 kilometres of access road and 1.8 kilometres of causeway to the western abutment at an estimated cost of $7 million.

The roads are designed as conventional unsealed roads approximately six metres wide, with sufficient road base to ensure durability for the estimated traffic for construction, maintenance and removal of the weir.

The causeway required as part of the western access is a continuation of the road for approximately 1,800 metres, with a crest approximately six metres wide. Access is required to both abutments to allow for the expedient construction of the temporary weir and its expedient removal, should that be required.

The Murray-Darling Basin Ministerial Council strategy for the Lower Lakes requires any decision to let sea water into the lakes to be made on the basis of monitoring both the alkalinity of the water in the lakes and water levels in both lakes.

The key aim of this project is to be prepared for the possible construction of a temporary weir. It is required to minimise the delivery period should it be necessary to proceed with the construction of a temporary weir below Wellington. With continued minimum inflows, the critical water level trigger point is expected to be reached by February 2010. This date is the worst case for planning purposes. On that basis, if a decision is made to proceed with construction, closure of the temporary weir could be achieved by the end of 2009 (to provide a buffer before the February 2010 date), but only if the preliminary works for the weir are progressed immediately.

The primary concern is protection from possible adverse water quality impacts from the Lower Lakes. The preliminary works are not expected to deliver benefits in their own right, but significant benefits to the South Australian community will eventuate should it be necessary to construct a temporary weir to ensure water security for South Australians dependent on water from the River Murray downstream of Lock 1.

The committee considered a number of issues of concern in relation to the need and potential impact of this proposal. It was suggested that salt being carried downstream in the River Murray will separate from the water and be deposited at the Wellington weir if that structure is built and will have adverse consequences for water offtakes at that point. However, the water coming down the river has salinity that is uniformly mixed and such water does not separate naturally. SA Water does not anticipate any salinity problems as a result of a weir being built.

The member for Hammond raised concerns about water quality and the possibility of algal blooms if a weir is constructed at Wellington. However, SA Water has advised that there is no evidence of any water quality issues on any other lock or weir on the river, and there is no reason why one at Wellington should be different, in terms of water quality.

The critical point at which the alkalinity in the water is insufficient to neutralise the acid formed will happen when the level in Lake Alexandrina drops to minus 1.5 metres. Some self-regeneration of reed beds will help to manage the acidity, and trials are occurring in relation to reversing the acidification process by biological means. However, a huge area is involved that has never been treated in that way in the past. Further, it is unclear whether the results will be available before a decision has to be made to commence construction of a weir.

It has also been put to the committee that soil acidification can be avoided by the purchase of 60 gigalitres of extra River Murray water and that this amount of tradeable water is available in the system. However, SA Water has advised that 60 gigalitres was the volume of water required to stop the lakes going acidic before the winter of 2009. Up to 200 gigalitres of water is needed to take the lakes through to winter in 2010 in the absence of good winter inflows in 2009, and the purchase of that volume of water is not realistic.

It has also been put to the committee that the introduction of sea water into the Lower Lakes will change their ecology forever and that they will never get back to a proper fresh water system. However, the Murray-Darling Basin Commission has looked at the longer term outlook and, even under climate change, the Lower Lakes will go back to a fresh water system—so much so that a weir is likely to be needed so irregularly that a permanent structure has not been considered.

The committee is aware that much of the area around the location of the proposed Wellington weir is too soft to support a significant structure without sinking. SA Water has undertaken bore hole testing along the proposed causeway route through to Pomanda Island and is satisfied that settlement of the road or the causeway will not be an issue. It is quite a different foundation to the foundation materials on the proposed alignment of the weir, which is much softer.

The committee has been assured by SA Water that no decision has been reached to construct a temporary weir at Wellington. The state's preferred option is not to build a weir. This preference, however, relies upon sufficient inflows to avoid the worst case scenario that underpins this project. The committee is told that three months can be saved by doing this work now. That allows for any decision with respect to the weir to be delayed by three months. These works are an important element in pushing a final decision out as far as possible. Based upon the evidence presented to it, pursuant to section 12C of the Parliamentary Committees Act 1991, the Public Works Committee recommends the proposed public work.

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