Monday, May 27, 2024

Launceston Council explore sediment options

There has been a renewed focus on the health of Launceston’s kanamaluka/Tamar Estuary in recent months, on the back of concerns from the business community, and rowing and yachting groups.

The Tamar Estuary Management Taskforce is undertaking new work in this area in order to help guide future decision making.

The TEMT was established three years ago as part of the Launceston City Deal, resulting in the development of the six-year $140m River Health Action Plan.

The City of Launceston played a lead role both in advocating for the estuary’s inclusion in the City Deal, and in the creation of the River Health Action Plan.

At the most basic level, there are two challenges facing the estuary – water quality and sediment management.

The River Health Action Plan is primarily focused on the former, and is already making significant gains in terms of water quality improvements in our local waterways.

The taskforce has overseen the roll-out of programs aimed at diverting pathogens from our waterways, including stock catchment fencing initiatives and sewage intrusion into stormwater projects.

The sewage intrusion project alone will remove more than five Olympic swimming pools of sewage from the estuary every year by finding and fixing pipe cross-connections. This project has been a collaboration between NRM North and the City of Launceston, which have been the lead agencies, along with the West Tamar and Meander Valley Councils.

“However, more recent concerns we’ve seen in the press centre around the issue of sediment management rather than water quality,” Council said in a statement.

Historically, the upper estuary was dredged. Dredging in the upper estuary started as early as the 1890s, and was carried out at various times until the 1960s when the port authority relocated to Bell Bay.

“Dredging comes at a significant cost, and there must also be considerations for where dredged material can be disposed of safely, without environmental harm,” Council said.

In more recent years the Launceston Flood Authority trialled sediment raking, which involves mechanically stirring up silt from the riverbed to be carried away by tidal currents.

Sediment raking was undertaken by the Launceston Flood Authority between 2012 and 2018 under a five year permit which, on expiration, was extended for a further 12 months.

However at the conclusion of the trial a comprehensive review of its efficacy found the sediment raking had not achieved its primary goal of a net loss of sediment from the upper estuary for the purposes of flood defence.

It also found the raking program had resulted in potential environmental harm as high toxicant levels were detected in the water column, as far downstream as Clarence Point, as well as infilling of the navigation channels in the upper estuary. As a result of this data, the Environmental Protection Authority indicated that it would not grant a further license for the program to continue into the future.

Since the raking program was halted, there has been a continued accumulation of sediment in the upper estuary. Sediment accumulation will continue until equilibrium – a balance between the sediments and the tidal processes – is achieved.

Recent bathymetric surveys indicate that these processes are already restoring the navigation channels.

“This is a natural process, but has resulted in renewed attention and concerns from some Launceston residents,” Council says.

“There are essentially two schools of thought on sediment management. Some people would like to see continued intervention to remove visible mud from the upper estuary. Others argue the upper estuary should be left to return to equilibrium and that perhaps we as a community should reassess what a healthy estuary looks like.

“We continue to see community discussion around these kinds of issues, as well as other proposals like a Tailrace canal.

“In order to allow us to make informed decisions about sediment management into the future, the Tamar Estuary Management Taskforce is currently undertaking a scientifically robust, evidence-based evaluation of the wide range of sedimentation management options that have been proposed for the estuary. The review will also assess the relative costs of those options.”

The TEMT expect to complete this work in the first quarter of 2021.

“Our understanding of the kanamaluka / Tamar Estuary has improved dramatically in recent decades, but we all have much to learn and a number of misconceptions remain in the general community,” Council said.

“There is still much work to be done to ensure we have an estuary the City can be proud of, and the Council looks forward to seeing the TEMT review later this year.

“The data gathered for this assessment will allow for a sensible community discussion on ways forward, and an improved understanding of what our goals should be when it comes to sediment management.”

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