Work has recently been completed on the fishways and navigable passes at Locks 7 and 8 on the river Murray in Australia. IWP&DC discovers why this work was necessary
On August 23 2004, Australian Minister for the Environment, the Hon. Ian Campbell, on behalf of the Murray-Darling Basin Ministerial Council (MDBMC), officially opened the fishways and navigable passes at Locks 7 and 8 on the river Murray. The $6.6M works, which have been undertaken through the Murray-Darling Basin Commission (MDBC) with financial contributions from the Australian government and the states of New South Wales, Victoria and South Australia, are part of the Living Murray project, an MDBMC initiative developed to restore the health of the river Murray and the Murray-Darling Basin.
Use of the Murray-Darling Basin’s water resources has brought great benefit to Australia. The basin supports about $10B worth of farm production annually (or 41% of Australia’s total gross value of agricultural production) and supplies water for irrigation and other uses that support two million people. However, changes made to the river flow patterns over the years have caused the health of the Basin’s rivers to decline, the river Murray in particular.
A scientific study of the Basin’s rivers (Norris R. H. et al., 2001) revealed that:
• Vegetation and wildlife are significantly impaired along 40% of the total river length in the Basin.
• Numbers of native fish, birds, plants and aquatic insects are declining due to loss of habitat, lack of flooding and loss of natural water temperature and flow signals that tell them when to breed.
• Nutrients and sediment have increased.
The declining health of the river Murray has become increasingly apparent over the past decade and has been discussed at length by the MDBMC and stakeholders, as well as in the media and community. The MDBMC has recognised a need to spend $150M modifying dams, weirs and locks as one of a number of measures necessary to achieve the best environmental outcomes from the water currently available to the river Murray system.
The locks and weirs on the river were originally built in the 1920s and 1930s to enable paddle steamers to navigate the river and to provide water for irrigation during periods of low flow. The structures consist of three parts:
• The lock chamber – provides the facility to pass boats from one pool level to the next during regulated flow conditions.
• The fixed weir – comprised of concrete piers with concrete stoplogs and used for flow regulation.
• The navigable pass – located between the lock chamber and the fixed weir, the navigable pass consists of steel trestles hinged to the concrete sill on the riverbed, supporting a steel superstructure, with steel needle beams and timber Boule panels used to maintain pool levels. The whole structure can be dismantled to allow navigation during periods of high flow when the lock chamber is no longer operable.
Over the years a number of components of the locks and weirs such as concrete stoplogs, cranes, lock gates and hydraulics and controls for lock gates have been replaced to keep them up-to-date.
Locks and weirs 7 and 8,which are managed by SA Water Corporation, have been upgraded over the last 15 months as part of a major programme of works along the river Murray from Locks 1 to 10 and Lock 15 over the next five years. They were selected for the first contract as they represented the most frequent operational risks.
During periods of high river flows, the lock chamber becomes submerged and inoperable, requiring the navigable pass to be removed in order to allow river craft to cross the weir. Before the current upgrade, it was difficult to remove the navigable pass and quite dangerous to put back in place. Divers had to be used during reinstatement and they operated in hazardous conditions involving fast flowing, turbulent and murky water.
Increasing standards for occupational health and safety highlighted the need to make the navigable pass sections safer to operate. In 1996 River Murray Water, in conjunction with the States’ Constructing Authorities, began investigating ways to eliminate the need for divers.
Over the next two years engineering consultants developed a range of concepts. The preferred option involved building lower concrete piers, over which the vessels could pass when the river was too high for the lock to be used. Removable steel units house the concrete stoplogs, which form the water-retaining barrier.
The project was managed by SA Water with a construction steering committee and with input during design development from a project reference group made up of skippers, lockmasters, environmental managers, houseboat hirers, marine safety officers, tourism operators and fishers formed to provide comment on the options and help assess impacts.
The upgrade of the navigable pass eliminates the need for divers and enables the navigable pass to be removed and replaced using the existing lock crane. It has significant occupational, health and safety benefits for lock operators as well as improving the structural integrity of the weir. The modifications also improve the flexibility of operation, allowing the weirs to meet the needs of environmental flow requirements as well as reducing the operation and maintenance costs.
Fishways 7 and 8
Native fish populations in the river Murray and across the basin have suffered serious declines. Populations are estimated to have fallen generally across the basin to one tenth of their levels pre-European settlement and are still declining, although it is recognised that numbers can vary greatly in different localities. Of the basin’s 35 native fish species, eight are listed as threatened. The Trout Cod is rated as critically endangered and the natural range of other species has dwindled sharply.
The locks and Weirs on the river (14 weirs, 13 with locks and five barrages) have created virtual ‘aquatic islands’ where native fish have not been able to move along the river corridor, except during times of floods. Another 4000 barriers along other streams and rivers in the basin also need to be investigated. More than 80% of the native fish are under 20cm in length and even the great Murray Cod cannot jump the weirs. (Within these sections of the river there are areas that have seen an increase in the numbers of specific – usually larger – species, due to restocking programmes).
As part of an objective of returning native fish populations to 60% of their estimated pre-European settlement levels within 50 years, new ‘vertical slot’ fishways were built at Locks 7 and 8. The fishways – the first major structural programme to be funded by the Living Murray Implementation Programme – is part of the MDBMC’s Native Fish Strategy, which will provide 2300km of fish passage from the Sea to the Hume dam over five years at a cost of $25M.
The ‘sea to Hume dam’ fishways project is a vast cooperative effort – funded from four governments, is built by a constructing Authority from one state (SA), has on-ground works located in Victorian and NSW land with design and ongoing monitoring from experts within all jurisdictions.
The fishway at Lock 8 has been in operation for several months and close monitoring has shown that a wide range of species of native fish have effectively moved through the structure.
The new fishways are a vast improvement on previous fishway designs used in the basin which were based on northern hemisphere 1920s designs and suited stronger, surface swimming exotic fish like Salmon rather than Australian native fish. With this new design, fish can swim up through a series of pools in a gently sloping structure, allowing them to take their time, moving from pool to pool according to their swimming abilities.
While Lock 7 and Lock 8 fishways are completely new structures, work has also been done to modify an existing fishway at Euston weir. Since its completion, there has been a significant increase in the migration of different species and sizes that have not previously been able to move through the original fishway.
Ongoing monitoring of the fishways will be important and scientists from four states will be involved over a number of seasons and years to get a clear picture of the effectiveness of these fishways.
Locks 9 & 10
Following the completion of works on Locks 7 and 8, SA Water is now planning works at Locks and Weirs 9 and 10 on the Murray at Kulnine and Wentworth. The proposed navigable pass modifications at locks and weirs 9 and 10 and new fishways are similar in nature to the modifications undertaken at Lock and Weir 8.
The modification to the navigable passes require the existing trestle structures to be replaced with reduced height concrete piers and removable steel deck units (which will be removed and reinstated by the existing lock cranes) and concrete stoplogs. The stoplogs and steel deck unit will be removed during flood flows to allow navigation through the modified navigable pass, once water levels reach 1.9m over the reduced height concrete piers. The navigable pass will be similar in appearance to the adjacent fixed section of the weir.
The modifications to the pass will require the use of temporary structural works including cofferdams to create dry areas for the contractor to work in. Throughout construction, water levels upstream of the weirs will be maintained at normal levels and locks will remain open for river traffic.