The sand spit off the coast south-east of Townsville that we call Cape Bowling Green has the important job of sheltering the Ramsar-listed (internationally important) wetlands of Bowling Green Bay National Park and the communities of Cungulla and Jerona from the forces of the open ocean.
Frighteningly, that long, thin sandy cape is currently under threat of being washed away entirely - and the Burdekin Falls Dam could be partly to blame.
Confusing concept? Using the research of Dr Eric Wolanski and imagery from John Connell and Chris Hopper, this article will help you wrap your head around this important local issue.
Our story starts about 2500 years ago, when coarse sediment and sand that was carried down the Burdekin River during flooding events was discharged at the mouth of the river to form a sand spit that later became known as Cape Bowling Green. While sand was regularly eroded away by waves and coastal winds, more sand would be discharged during wet season flooding events, replenishing the cape.
Jump forward to 1987, when the construction of the Burdekin Falls Dam was completed, with the purpose of irrigating farmland in North Queensland.
As you can expect, the dam led to a drastic decrease in peak discharges of the Burdekin River. Now, about 95% of the heavy sediment that used to make its way to the river mouth is trapped by the Burdekin Falls Dam. This has had two major observable impacts. Firstly, the river below the dam has become significantly more turbid (i.e. less clear), as fine sediment that usually attaches to coarse sediment and sinks to the riverbed now remains suspended in the water (more on this elsewhere). Secondly, as you may imagine, the sand of Cape Bowling Green has not been replenished at the usual rate, leading to a disruption to that delicate erosion/replenishment balance mentioned above.
Field surveys and analysis of aerial and satellite imagery indicate that the peninsula width has diminished significantly, and, if this trend continues, Cape Bowling Green is likely to breach. The most vulnerable point on the peninsula is now only 15m wide, compared to 24m in 2018 and 108m in 2011. The length of the breach zone is now about 300m long, compared to 50m in 2018.
In the breaching zone, erosion cliffs are now observable, and all of the trees in that area have been killed. Furthermore, waves have been overtopping the 2.5-3m high dunes at some points, reducing the dunes by about a metre and removing important protective vegetation. Waves have also left erosion scars one metre deep, all the way from the sea to the mangrove fringe in the bay.
These erosion scars have no protective vegetation; creepers are trying to colonise and stabilise these breaches. Quad bikes use these gaps in the sand dune to traverse the peninsula, further damaging any protective vegetation and enabling further erosion.
Dr Eric Wolanski stands next to quad bike tracks on Cape Bowling Green.
The impacts of the decrease in coarse sediment to replenish the sand spit have been exacerbated by the increasing intensity of coastal winds (see the figure below), which is yet to be properly investigated.
According to Dr Wolanski, if and when the breach fully develops - possibly during a storm - the top 4km of Cape Bowling Green will become a sand island and probably erode away with the waves and disappear altogether over time. The Cape Bowling Green peninsula will then be much smaller and sea waves will readily penetrate Bowling Green Bay, affecting the Bowling Green Bay Ramsar-listed wetlands and the small townships of Cungulla and Jerona.
The nature and extent of the impacts from changed exposure is currently unknown, and there is no research being undertaken to examine this. To help prevent potentially catastrophic impacts to Bowling Green Bay, further research, investment and preventative measures are required. NQCC has been bringing attention to this issue and Dr Wolanski's findings, and advocating for action - before it is too late.