THE removal of all or part of the Wallaga Lake causeway would not significantly improve flushing or water quality upstream of the bridge, a study has found.
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The study, carried out by WBM Oceanics Australia, looked at different options for changing the causeway to improve water quality in Wallaga Lake.
Those options included construction of a second opening at the southern end of the causeway; construction of a second opening and a dredged channel to the ocean entrance and construction of a second opening plus several smaller openings along the rest of the causeway.
Bega Valley Shire Council’s manager of administration and design, Doug Mein, said that while modelling had shown that all three options would improve tidal range upstream to approximately pre-causeway conditions, they would not improve flushing times to the extent that might have been anticipated.
“The news is not as good as was hoped for by people interested in the health of Wallaga Lake,” he said this week.
The WBM Oceanics Australia study was carried out on behalf of the Wallaga Lake estuary management committee with the assistance of State Government funding.
Committee spokesman, Sean Burke, said the aim of the study had been to assess the potential of a second opening in more detail because of the “widely held view” that the causeway was contributing to water quality problems in the lake.
An Estuary Processes Study undertaken in 1996 indicated that flushing might be improved with a second opening through the causeway.
Mr Burke said the Wallaga Lake estuary management committee had been established by Bega Valley Shire Council to prepare an estuary management plan for the lake.
He said the plan had since been adopted by the Bega Valley and Eurobodalla Shire Councils and was now being implemented.
Mr Burke said one of the strategies in the plan was to maintain and improve tidal exchange and a key action from that strategy had been to undertake detailed investigations to determine the benefits and costs of opening the causeway.
He said improvements to the causeway opening had been identified as desirable at community meetings and by most of the bodies represented on the estuary management committee.
Mr Mein said the first option (construction of a second opening with no additional work) had been identified as the most appropriate for improving tidal flushing in Wallaga Lake because it would best replicate pre-causeway “tidal flushing characteristics”.
It would also be the cheapest option at a cost of approximately $4-million.
But he said it would only be of benefit when the mouth was open and would be of “minimal advantage for flushing the upper reaches of the lake”.
Mr Burke said the construction of the causeway in the 1890s had a significant impact on the local community with the transport of produce and supplies made much easier between Bermagui and Tilba/Narooma (or “Noorooma” as it was back then).
While the opening of the bridge had been a cause of celebration at the time, Mr Burke said there had been some shallowing near the causeway over the decades and a belief had developed in the community that the causeway was contributing to water quality problems in the lake.
The study is currently on display at Council’s Bega and Bermagui offices and is also available for loan from the libraries in those towns.
