NRM NOrth Tamar

Muddying the waters – managing sedimentation in the kanamaluka / Tamar Estuary

28 October 2019



Anyone living in or around Launceston would be familiar with the flood of controversy surrounding the quality of water in the kanamaluka / Tamar estuary.

 

For the local community, sedimentation in the upper estuary and the presence of visible mudflats in and around the community are frequently raised as concerns, not only because of the less than desirable aesthetics, but also the impact on access and navigation in the channels and Seaport area.

 

Over the years, Hydro Tasmania has often been blamed for the current state of the estuary, with the Trevallyn Dam cited as the cause and even calls to remove the dam.

 

But the reality is more complicated than that and it’s becoming clear that taking such a drastic step would not only be prohibitively expensive, it would be a wasted effort in attempting to solve the sediment issues in the upper estuary.

 

Dredging up the past

 

While there might seem to be a link between the dam and sedimentation in the estuary, history tells us otherwise.

 

There was significant sedimentation of the upper estuary before the dam was built in 1955, and attempts have been made since European settlement to manage sediment through extensive dredging.

 

In fact, large-scale dredging programs were used to allow navigation of boats into the Port of Launceston from the 1880s to the 1960s.

 

Not only that, but removing the dam would lead to the closure of Trevallyn Power Station and would come at a significant cost.

 

The power station generated around 400 GWh or four per cent of the state’s electricity demand in 2018 – 19, which is enough to power all of the houses in Launceston and then some. And the foregone generation would cost around $30 million a year in lost revenue.

 

Going with the flow

Dam4

Another potential solution put forward has been the targeted release of flows from Trevallyn Dam. But, until now, there’s been little evidence of the effectiveness or otherwise of this option. 

 

In 2018, the Tamar Estuary Management Taskforce (TEMT) commissioned the Trevallyn Flow Releases Study to evaluate the ability of water releases from the dam to remove sediment from the upper estuary, both with and without sediment raking.

 

The independent study found that targeted water releases are not effective in managing sediment; in fact, even if all the water was released from Trevallyn Dam, sediment would be reduced by negligible amounts, and it would return within three months.

 

Naturally high-flow events such as flooding of the North and South Esk are significantly more effective at mobilising sediment than artificial releases of water through the dam.

 

But, the modelling predicts that the highest feasible targeted water release and the average annual natural spill event would both scour a negligible amount of sediment.

 

The evidence also shows that combining water releases with sediment raking is still ineffective in removing or managing sediment in the upper estuary.

 

And, for a small, temporary improvement in sedimentation, water releases are not cost effective – flow releases alone cost around $100,000 in lost revenue and, combined with sediment raking, the cost rises to approximately $190,000.

 

I can see clearly now

 

So, what next? There can be no doubt that this is an important issue that the local community wants to see addressed. But what is clear is that there is nothing to be gained from taking action without fully understanding the effectiveness of the options.

 

Importantly, that means taking into account the natural state and responses of the estuary. 

 

The kanamaluka / Tamar estuary is a drowned river valley – an estuary that has formed through sea level rise in a historical river valley.  Sedimentation is a natural process in drowned river valleys, and gradual infilling over long periods results in the formation of mudflats and deltas.  Tamar Cruise image

 

As well, the incoming tide is stronger than the outgoing tide, which results in a normal pattern of sediment accumulation in the upper estuary.

 

Of course, all of this means there is no quick fix and more work will be needed to find a solution that is feasible, cost-effective and, most importantly, that works.

 

As a member of the Tamar Estuary and Esk Rivers (TEER) Program, we’ve been working with stakeholders and contributing data and knowledge for several years to help improve outcomes.

 

While the way forward now rests with the City of Launceston, we will continue to work collaboratively with them and other stakeholders to help find evidence-based solutions for both the short and long term, which will provide the best results for Launceston and its community.

 

Top image courtesy of NRM North

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