Tamar River pollutants would decrease if a project to make it a freshwater lake at its upper reaches was combined with TasWater’s plan to improve its sewerage system, according to Tamar Lake proponent Robin Frith.
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Tamar Lake Inc has completed feasibility studies into its proposal to install a barrage across the river at the south end of Long Reach, including 3D modelling for water pollution from Launceston to Tamar Island.
Nitrogen and phosphorus levels were lower in the Home Reach/Yacht Basin area under normal flows in modelling showing river conditions if the Tamar Lake was created.
However, the phosphorus and nitrogen levels in the North Esk area were even lower when the LSIP worked in parallel to the $320m barrage proposal.
The upper reaches of the river, if turned into a freshwater lake, would have a B-rating water quality, an improvement on the current D-rating, according to Tamar Lake Inc.
Earlier studies showed that water quality in the Tamar Lake would exceed NRM North’s target, it says.
Mr Frith said that results showed under normal flow conditions, 80 per cent of new pollutants entered the river down the Tailrace from catchments, or north of the Tailrace, including discharges from the Ti Tree Bend and Riverside sewage treatment plants.
In current conditions these moved back into the Home Reach area, whereas a Tamar Lake environment would transport them rapidly downstream, he said.
Flushing of the Home Reach, lower North Esk and Yacht Basin area would increase if Cataract Gorge flows grew from 2.5 cumecs to 10 to 20 cumecs, Mr Frith said.
“Our modelling shows that at least in the upper Tamar, the flushing in a Tamar Lake environment is more effective than in the current estuarine environment. Perhaps we should be viewing it more as a freshwater Tamar River and a Tamar Estuary.”
Tamar Lake Inc proposes a barrage would eliminate the impacts of asymmetrical tidal action on the river, and move the flocculation zone - where saltwater meets freshwater – to the north of the barrage. The sedimentation would move out into the Bass Strait once the barrage was opened on the ebb tide.
The group released a final report into its proposal to build a barrage in May, saying one of its key findings was that sediment accumulation was a historical legacy caused by natural hydrodynamic processes.
It said the report showed that without radical change in the hydrodynamics of the upper Tamar, the Home Reach/Yacht Basin area would remain muddy and shallow.
According to the report, the barrage would protect the city against sea level rise, and the region would benefit from jobs. Other key findings included that no listed species would be threatened, and there would be no negative effects on Launceston’s flood levels, according to Tamar Lake Inc.