IN response to Alan Birchmore's letter to the editor entitled 'Loving mud not acceptable, action needed' (The Examiner, January 13), I am a career scientist with 20 years of experience in environmental modelling and assessment, having worked as a senior academic at the Australian National University before setting up my own consultancy company in 2008 following my move to Tasmania.
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In 2018 I was asked to conduct a comprehensive review of the Launceston's sediment raking program when the original license ended and issuing a new permit by EPA was being considered.
I was asked to answer two key questions:
- Did sediment raking achieve its objectives? This considered the effectiveness of sediment raking in reducing overall levels of sediment in the upper estuary, impacts on visible mudflats and the navigability of channels.
- What impacts did sediment raking have on water quality?
I conducted a comprehensive review of bathymetric data covering the period from 2008 to the end of 2018, which included conditions before raking commenced and during the raking program.
I also considered two separate water quality data sets - data collected by the Launceston Flood Authority immediately before and after raking in the vicinity of the raking vessel, and a separate long term water quality data set collected by the TEER Program that consists of samples along the length of the estuary.
The review concluded that raking failed to reduce sediment in the upper estuary.
Mr Birchmore's use of sediment levels in 2017 to justify continued sediment raking falls into the 'lies damned lies and statistics' category of number selection.
The estuary experienced a flood of historic proportions down both the South Esk and North Esk in June 2016.
Floods of this magnitude induce major levels of scour within the estuary, with the 2016 flood no exception.
Bathymetric data suggest this flood removed approximately 265,000m3 of sediment from the upper estuary.
Following this, the estuary began its natural process of sediment accumulation - a process that was still active when sediment raking ceased.
To claim that sediment levels in 2017 relate to the success of sediment raking rather than a historically large flood event is wrong.
A better assessment of the effectiveness of sediment raking in reducing sediment levels is to consider the conditions that existed immediately before the 2016 flood following the extensive sediment raking campaigns of 2014 and 2015.
As is shown in the report sediment levels in the upper estuary at this time were higher than at any time previous in the data set, including during the dry period of 2008 - that is, raking failed in it objective to lead to net sediment reductions in the upper estuary.
The analysis also showed that while visible mudflats were reduced by sediment raking this came at the expense of reduced navigability in the upper estuary.
The raking program essentially pushed sediment from mudflats into the channel where it reduced the navigable depths, leading to images of the Home Point tourist boat stuck in the mud.
Sediment raking essentially created an optical illusion, hiding sediment normally visible in mudflats at low tide in the channels where it couldn't be seen.
Finally, the negative impacts of sediment raking on water quality, and consequently the environmental health of the estuary, were clear.
Mudflats are nature's storage for pollutants entering the estuary.
Mudflats are nature's storage for pollutants entering the estuary.
The mudflats around Launceston are known to contain heavy metals and high concentrations of nutrients.
Sediment raking essentially released these toxicants into the water column, with the LFA's data showing toxicants such as aluminium and arsenic increased by orders of magnitude in response to raking, well above levels that are considered safe.
Impacts on water quality were seen to persist for at least three weeks after each day of sediment raking and at least as far as Clarence Point (i.e. impacts were seen for the length of the estuary).
The improvements in water quality observed in the 2020 report card released by the TEER program also indicate that the estuary is now recovering from the negative impacts the sediment raking program had on the water quality, and subsequently the health of the estuary.
The sediment raking report was peer-reviewed and endorsed by 14 scientific and technical experts.
A full copy of the report can be found at www.launceston.tas.gov.au and a summary of the key findings at www.stategrowth.tas.gov.au
- Dr Rebecca Kelly (BEc, BSc (Hon 1 Mathematic), PhD, GAICD), chair of the Scientific and Technical Committee, Tamar Estuary and Esk Rivers Program.
