Siltation ponds on the West Tamar Highway would struggle to store one years' worth of dry material from a targeted dredging program in the upper kanamaluka/Tamar according to tender documents, but could be the only available option.
Other ponds, next to Ti Tree Bend, have already been earmarked for wetlands to store water for the combined sewerage system.
Tender advice documents to the City of Launceston last year, obtained under Right to Information, detail the storage capacities of the ponds, dredge material estimates and a requirement that the council make all of the ponds available for the dredging contractor.
But in the Tamar Estuary River Health Action Plan, which includes federal government funding, the Ti Tree Bend ponds would be converted to a 10-hectare wetland to increase the water storage capacity of the sewage treatment plant.
The wetland could be a key component in helping to reduce the amount of sewage entering the river during heavy rainfall events.
It means the West Tamar silt ponds were the only available location for dredge material to be stored, unless the wetland proposal was abandoned, or new silt ponds were built somewhere near the upper Tamar.
MORE ON THE PROPOSAL TO DREDGE THE UPPER TAMAR:
In the tender advice documents, the three West Tamar ponds have a remaining capacity of 46,000 cubic metres, but about 85 per cent of dredge material contains water, meaning they have a capacity for 6900 cubic metres of dry sediment.
The material must be stored in the ponds for two to three years to dewater before it can be disposed of.
The documents also showed that an "emergency restoration campaign" for dredging - that is, a one-off dredge to provide targeted access for river users - would need to remove 20,400 cubic metres of dry sediment, almost three times greater than the West Tamar ponds' capacity.
This included dredging parts of the channel however, which has largely returned to an adequate depth since natural flows were allowed to continue, reducing the amount required to be dredged.
It leaves capacity for, on average, an area of 69 metres by 100 metres to a depth of one metre, which The Examiner understands would not allow for full access to The Seaport marina, rowing club and yacht club, or make a visual impact for nearby businesses.
Once dredged, the siltation would also start to return to the river and require ongoing dredging.
Restoring the siltation ponds for use was estimated to cost $482,000, a one-off dredge of 21,000 cubic metres could cost $727,000 and an annual basic dredging program to reduce sedimentation build-up would, on average, cost $6.6 million per year over 10 years including disposal of material.
The Liberal party promised $4 million over two years for a targeted dredging program, and up to $1 million annually for a new Tamar management model, based on advice from the Tamar Estuary Management Taskforce. During the announcement, Premier Peter Gutwein said 200,000 cubic metres of sediment could be dredged "for $4 million", citing "the information we've received from operators that manage these types of programs".
Such an amount greatly exceeds the remaining storage capacity of the Ti Tree Bend and West Tamar ponds combined.
TEMT was due to release a detailed report into options for managing sedimentation in the Tamar in the first quarter of 2021, but this did not occur. There remains no indication of when this report will be released.
The Examiner asked the City of Launceston if there were still plans to use the Ti Tree Bend ponds for a wetland, if they were being considered for dredge material and if the West Tamar ponds could handle a dredging program.
The questions were forwarded to the Department of State Growth, TEMT and, ultimately, landed with the state government which did not provide specific answers.
"As part of the government's election commitment to improve the Tamar River, we will within our first 100 days commence the environmental approval process for the targeted site-specific dredging program, to enable better use of the river," a government spokesperson said.
"The approval process will consider how to undertake the dredging and handle the dredged material."
The City of Launceston refused to provide the tender advice documents before the election claiming they were prepared for TEMT, although TEMT is not mentioned in the documents.
Other documents obtained under RTI - also not released before the election - provide the latest details of metals contained within the sediment, which exceeds national dredging guidlines for zinc, arsenic, cadmium, copper, mercury, nickel and tributyltin. These would be released into the water way during dredging.
Testing of the siltation ponds show they still exceed EPA Tasmania contaminated soil management guidlines for cadmium, chromium, manganese and the toxic equivalent concentration. They were last used for dewatering in 2009 and remain "moderately to strongly acidic".
The dredging program would need to be given the approval of the EPA.
Labor supported dredging the upper Tamar, subject to the results of the TEMT report. The Greens planned to replace TEMT with an authority that would be supported by "a scientific advisory panel".