Estuary fix caused new woes: report

A FAILED seven-year program to remove natural but unsightly mudflats from the Tamar Estuary instead caused boats to run aground and water contamination, a report has found.

The Launceston City Council is now considering dredging the kanamaluka/Tamar Estuary after raking ceased this year, having been deemed an expensive flop.

The council, in partnership with the Launceston Flood Authority, had been using a boat to rake sediment in a trial to remove unattractive mudflats from sight, protect against floods and improve boat navigation channels.

Media were briefed on the findings of a review of the sediment raking program yesterday.

Testing and data collection showed that raking in the estuary caused sediment to resettle in the navigation channel, causing boats to run aground. It also released pollutants into the water as dormant nutrients and heavy metals rose from the disturbed sediment in plumes.

Data showed the pollutants had “substantial and sustained impacts on water quality” for at least two to three weeks. Unsightly mud shoals were largely removed, but reappeared in other areas downstream.

The report found raking did not have a significant effect on sediment levels to protect against flooding, with sediment returning quickly to the same level or higher than before raking. The raking trial started in 2012 and ended this year.

Environment scientist and consultant Dr Rebecca Kelly worked on the report.

“As a scientist, I’m not sure I’m allowed to say whether it was worth it or not, but it certainly didn’t achieve the objectives it set out to achieve,” she said.

The Tamar Estuary Management Taskforce separately commissioned a Trevallyn Flow Releases Study in 2018, to see if water releases from Trevallyn Dam could remove sediment from the upper estuary, both with and without sediment raking.

The study demonstrated that even a release of all the water in Trevallyn Dam down the South Esk River would have a negligible effect on silt mobilisation and would cost Hydro Tasmania about $100,000 in lost electricity generation and $190,000 with raking.

The total cost of the program has not been tallied.