A lot of people at the Murray-Darling Basin Authority believe it is getting worse, but finding the science behind the belief can prove difficult.
A statement from the Murray-Darling Basin Ministerial Council said the capacity had declined by about 20 per cent over 20 years.
A consultant's report to the MDBA last year noted it was generally thought that channel capacity of the choke had reduced from 10,600 Ml/day in 1996 to below 9500 Ml/day (downstream from Yarrawonga Weir) at present.
Where does this point of view come from?
A consultant's report published last year by Water Technologies found the evidence in: specific water surface gauge heights at Picnic Point, periodic flow trials conducted by the MDBA, an apparent reduction in commence-to-flow rates for several small effluent creeks along the choke into Moira Forest and an increased frequency of overbank flow events resulting from releases from Yarrawonga that previously would have been accommodated within the channel.
There is evidence of sand deposits on the bed of the river and there is also some evidence of the river widening at The Narrows near Barmah due to erosion.
Last year’s study not only raised questions over whether the capacity was changing, but also referred to an earlier study from 2009 which concluded: “There is no evidence of a general decline in hydraulic capacity within the Barmah Choke Reach that could be expected to continue into the future.”
In 2009, a report to the MDBA found ". . . a detailed analysis of flow and level data at key locations was undertaken, finding no evidence of a continuing decline in capacity.”
The report noted that the capacity at that time was about 10,600 Ml/day.
The 2009 report, by Sinclair Knight Merz, correctly forecast that there was a danger of downstream shortfalls due to the demands of both environmental watering and irrigation.
The discussion over the capacity of the choke is important because further studies will be looking at how the capacity will be addressed and whether there should be some work undertaken in the river, or whether that part of the river should be circumvented by a pipeline or channel.
The MDBA is facing a possible shortfall in delivery capacity downstream of the choke and is under pressure to come up with a solution.
A spokesperson for the MDBA noted recently:
“As reported in the Water Technology report there is a large volume of sand in the channel at the Barmah Choke. The report made use of a number of data sources including Lidar (laser measurement), aerial photography, channel cross-sections and bathymetry.’’
The Murray-Darling Ministerial Council has agreed to commission a feasibility study to explore the options to optimise the capacity of Barmah Choke and to reduce the risk of delivery shortfall in the Murray River.
The feasibility study is one part of a broader suite of work across the river system to address water delivery shortfall across the southern basin, including the tributaries. It will build on similar reviews that have been undertaken in the past to find a cost-effective option which may be acceptable to communities.
Perhaps we should heed the boffins at Water Technology who found in their 2020 report: “More data, analysis and investigation are required to first establish whether channel capacity is decreasing, the degree of reduction in channel capacity (if it’s occurring) and the likely trajectory for change based on an investigation of the process.”
An excerpt from the Water Technology report, 2020:
“It is noted that the Barmah Choke Study: Investigation Phase Report (MDBA, 2009), a hydrologically focused report, concluded that there is no evidence of a general decline in hydraulic capacity within the Barmah Choke Reach that could be expected to continue into the future.
“What is the magnitude and trajectory of change of these threatening processes? The magnitude of bed aggradation in the upper reach cannot be quantified with the data currently available. It can be speculated that, if bed aggradation is occurring due to excess sandy material being slowly transported through the system, this may be a temporary issue (albeit likely long-lasting). However, with no knowledge on the depth of this sand, its upstream longitudinal extent or even its source, it is not possible to establish a trajectory for this process. It is also important to note that we cannot even state with any confidence that channel capacity is decreasing to a point which threatens conveyance.”
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