June 11th 2010
The ongoing price of Nelly Bay Harbour
“This problem is a direct result of the building and design of Nelly Bay Harbour,” was how Townsville City Council's Environment and Sustainability Committee Chair,Cr Vern Veitch described the ongoing erosion which has eaten deeply into the beach foreshore and has been the subject of a Shoreline Erosion Management Plan (SEMP) which has been given preliminary approval by Council's Environment and Sustainability Committee.
“We need it (SEMP) to get funding from a higher level of government. It's a professional study which suggests solutions to erosion at Nelly Bay,” said Cr Veitch.
Developed in consultation with the Townsville Port Authority, DERM, the Department of Employment and Economic Development, GBRMPA and the Department of Transport and Main Roads; adoption of the plan by full council later this month would allow the city to proactively plan for and manage erosion into the future.
Magnetic Times attended an impromptu meeting on the beach last year, after king tides and a storm had gouged out another large section of Nelly Bay, in which Cr Veitch and Cr Trevor Roberts heard the concerns of residents. The SEMP was developed since that meeting and an option that was discussed then which has been further developed in the SEMP is the construction of a “training wall” or groyne which would run approximately 70m out from the beach and 30 to 40 metres parallel to and south of the present breakwater. No decision has been made as to what materials might be used for the training wall but sand-filled geotextile bags or concrete cubes are considered options.
The idea is to “place sand to create a stable beach orientation in a fillet of sand against the southern flank of the training wall”. Although subject to further consideration the sand is expected to be sourced from lower Gustav Creek.
The recommended shoreline Erosion Management Plan for Nelly Bay
(courtesy Coastal Engineering Solutions)
The training wall, would, theoretically, still allow the movement of water under the bridge joining the breakwater with the mainland but is expected to stop the sand making its way beneath the bridge and into the harbour. Sand from Gustav creek would still be required to be transported to Nelly Bay beach and costings in the SEMP put this at an ongoing $60,000 per year.
Cr Veitch, said, “The causes of the problem are twofold. The moved headland (Nelly Bay Breakwall) two thirds of the way along the beach creates an erosion cutting point. The other part of the problem is wave set-up. The water going in has to get out somewhere and it goes sideways along the beach through to the harbour and it takes sand with it..”
“The problem is a direct result of the building of Nelly Bay harbour. Whenever you put in hard infrastructure you will change the pattern of beach sand distribution.”
Cr Veitch's observations are in line with predictions made by James Cook University's Marine Modelling Unit which Magnetic Times', (then) journalist, Wendy Tubman reported in February 1999, before the the present development was started, “The marina has altered the pattern of beach sediment transport in Nelly Bay ... there is some evidence that sand has begun to build up along the southwestern edge of the main breakwater. Because waves that will move this sand toward the southwest away from the marina no longer exist, sand will probably continue to move toward the marina. This change in sediment movement characteristics may cause loss of sand on the beach towards the middle of Nelly Bay, several hundred metres southwest of the marina”.
The SEMP asks that Council decide on a “design event”: the level of storm they might wish to seek “immunity” from and set a minimum 1 in 50 year event as the basis for costings. They also requires a decision as to the line of defence – the line at which replenished sand should aim to hold the erosion to.
According to SEMP early estimates are that it would take $754,000 to protect against a 1 in 200 year event – the rating achieved by the storm in February 2009.
While not ruling out stone or concrete to be employed as part of the training wall, Cr Veitch was adamant that a rock wall along the beach was no solution. “Rocks do not stop erosion they just move it somewhere else. Rocks bounce energy they don't absorb it. It just takes bigger waves to move them and in severe storms rocks are just like bigger grains of sand,” he said, adding that Cyclone Ului achieved just this at the Mackay breakwater in March this year. Cr Veitch also claimed that the rockwall approach costs about $1 million per 10 metres of beach to construct.
“This agreement is not a quick fix, it represents a long-term commitment to addressing the underlying causes of beach erosion and an ongoing commitment to protecting these shorelines.”
Adoption of this plan will not prevent our beaches from being eroded but it puts in place a strategic and manageable plan to defend our coast,” Cr Veitch said.
Story and photo at top: George Hirst
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