Wetland restoration is one way to reduce sediment in the Tamar: how will it work?

Small and large scale wetland programs were investigated in the Tamar Estuary Management Taskforce sediment report and estimated the costs associated with both proposals, along with other options such as the Tamar Lake proposal and no intervention.

UTAS academic Vishnu Prahalad has worked on a number of contract research projects with the federal government and NRM groups regarding wetland restoration and environmental outcomes.

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He said restoring wetlands was an idea he'd been pushing the state government on for a number of years, because of its ability to reduce sediment build-up and also reduce flood risk.

"Launceston is built on a floodplain...there's been a lot of human intervention over the years and the land has been heavily modified to the extent that the community does not identify a lot of it as wetlands," he said.

Restoring wetlands was a potential option identified as "priority two" in the recently released Tamar Estuary 10-year-plan.

However, the scale and cost of the wetlands proposal was contingent on a number of factors, such as the progress of $8 million in federal funding committed by the Labor party during the election campaign.

The plan also failed to mention where and how much wetlands would be restored, however that's likely to be part of a later implementation plan.

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Accumulation of sediment in the Tamar, particularly close to the Launceston marina, has been a primary concern of the community for decades.

Human intervention, such as the creation of Trevallyn Dam for power, along with Launceston's combined stormwater and sewerage system and historic agricultural practices for effluent and fertiliser has added to the concentration of heavy metals and effluent discharging into the estuary.

Extra contaminated product has added to the existing sediment levels, which has increased the visible mud flats people can see at low tide.

Mr Prahalad said wetlands acted as a natural filter and could help trap sediment among the plant life as it gets flushed in and out with the tide.

"The freshwater picks up the sediment and drags it into the wetlands floodplain area and some of it gets trapped there, so there's less going out than coming back in," he said. "If you restore the floodplains it will absorb the water and will also take flood pressure off the city."

The 10-year-plan for the Tamar Estuary notes restoring wetlands as a viable option for reducing sediment accumulation.

"Scientific modelling tells us that increasing the amount of water that can move upstream with the tides may have an impact on reducing sediment in the upper estuary," it reads.

"One way this might be achieved is to regenerate areas of wetland on the North Esk River. Over time, this could reduce the visible mudflats and improve recreational access at low tide."

However the plan only notes that the government is "currently investigating" the best way to increase the tidal prism" to ensure broad environmental benefits while reducing the size of the mudflats.

It also notes State Growth is "independently progressing" options for a site-specific dredging program, which was committed to during the state election.

More detail can be found in the Tamar Estuary Management report, released last year, on options for managing sediment in the estuary.

It examined a large and small scale wetlands restoration program and what it would look like in the context of Launceston's available land.

The small scale program identified two parcels of land in the North Esk floodplain area which are already in public ownership and would be most suitable for immediate wetland and floodplain restoration.

"The council-managed North Esk Trail has an existing network of shallow basins, constructed as part of Landcare's Ribbon of Blue project. A section of this area has become overgrown with willows and other woody weeds in recent years. Wetland restoration in this section will require a significant weed removal program and replanting," the report reads.

With regard to a large-scale wetland restoration, there is a large area on the North Esk floodplain that is undeveloped land. It's a mixture of public open space, utility, agricultural land and privately owned swamp and wetland.

Scotch Oakburn College's Environmental Association has actively restored approximately 10 hectares of wetland within the floodplain. Of the 255 hectares of undeveloped land, about 120 hectares would require restoration. Floodplain and wetland restoration in this area will require removal of informal tidal levees, removal of fill and construction of wetlands and interconnected waterways.

Construction of additional shallow basins or to restore large areas of wetland would require excavation of contaminated soils, which need to be treated before disposal.

These are known to be potential acid sulfate soils and are likely to contain elevated concentrations of nutrients and heavy metals.

Excavated excess sediments would need to be transported off site, treated for acid sulfate and then disposed of, most likely in land fill as Level 2 waste.

The volume of sediments to be excavated and treated would depend on the design and scope of the proposal.

However, Launceston has a well-documented lack of silt ponds, used in the past to decontaminate dredge-spoil or contaminated soil.

Ponds located at Ti Tree Bend could be part of the land proposed to be restored and would be removed to be rehabilitated as wetlands.

Despite this, they don't have the capacity to store the volume of material to be excavated during any rehabilitation proposal, documents received under Right to Information last year have shown.

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.

Mr Prahalad said it was hard to say how much wetland would be needed but he said "the more the better" would be his preference.

"There has been a push to restore wetlands for 10 years, it's the sensible thing to do," he said.

Mr Prahalad said Launceston, along with many other areas across the state and country was "at a crossroads" with regard to the impact of climate change on flood-prone areas and it was imperative action was taken now.

"Heavy rainfall and huge tides are amplifying the flood risk in Launceston...we have got an opportunity to restore the floodplains and we need to do it now," he said.

Any wetlands restoration project would require the support of all three levels of government and requires further funding outside the $140 million obtained during the Launceston City Deal.

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