The Tasmanian Government, the Launceston Flood Authority (LFA) and Hydro Tasmania have announced a trial to assess the effectiveness of a controlled release of water from the Trevallyn Dam coordinated with silt raking operations on the removal of silt from the Tamar estuary.
A build-up of sediment has an adverse impact on the effectiveness of the Tamar flood levee protection scheme, designed to prevent flooding of the low-lying areas of Launceston should there be a significant flood event. The levees were recently upgraded at a cost of $58 million.
Hydro Tasmania and the LFA will work together with the State Government to conduct a cost-benefit analysis of different options and identify opportunities for collaboration including to identify the best use of natural flood events to support silt raking operations.
The three-day trial water release from Trevallyn Dam will be undertaken by Hydro Tasmania to coincide with the series of strongest tide events that begun on 27 August. The LFA will coordinate silt raking operations during the water release.
The cost and economic benefits of providing water from hydro-generation infrastructure to support silt raking operations in years with low natural inflows and few flood events will be assessed following the upcoming trial. Bathymetric surveys will be carried out before and after the trial water release to measure the extent of sediment removed.
The announcement came with the release of a report from the Tamar Estuary Sediment Tracer Study, undertaken by the Australian Maritime College in association with specialist company ETS Worldwide and NRM North. The $100,000 study was funded by the Federal Government through the support of Bass Liberal MHA Andrew Nikolic.
The study was carried out between September and November 2014 when rainfall was low and there were no significant spill events over Trevallyn Dam. The 2014 raking campaign during this dry period removed 100,000 cubic metres of sediment which is not considered sufficient to maintain the ongoing efficacy of the flood levees. In contrast, a similar raking effort during 2013 moved 246,000 cubic metres from the Upper Reaches of the Tamar.
In 2013 when more silt was removed, there was higher rainfall and greater natural flood events, resulting in six significant spills over Trevallyn Dam, three of which were more than 500 cubic metres per second at their peak.
The study provides valuable data on the dispersal and final destination of fine sediment material disturbed during raking, to inform future programs. The study results indicate that in higher rainfall years, raking removes more silt and is therefore effective and cost efficient.
It is hoped that kayakers will be able to take advantage of the water released during the trial.
A. Mark Thomas
M&M Communciations