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Evolution of the Gold Coast Seaway & Broadwater

Evolution of the Gold Coast Seaway & Broadwater

The Gold Coast Seaway, also known as Southport Seaway, is one of the two main tidal inlets on Australia’s east coast that serves as the main navigation entrance from the Pacific Ocean into the Gold Coast Broadwater and the southern part of Moreton Bay. It is one of the most significant engineering feats in Australia, which also includes the world-first sand bypassing system, constructed between 1984 and 1986.

The GC Seaway is known to provide safe navigation for vessels and boaties; however, this has not always been the case. Prior to its construction, the Nerang River entrance had a long history of instability. The main reasons for the construction of the Gold Coast Seaway, the sand bypassing system, and Wave Break Island were to stabilise the entrance, preventing shoreline erosion to the north, maintaining a decent beach width to the south, and overall, to provide a safe navigable channel.

The reasons for the entrance’s instability included the combination of northern drift of sand, strong south-easterly winds, and wave climate, which resulted in over 500,000 cubic metres of sand movement along the south-east Queensland coast each year. For instance, in the early 1800s, the Nerang River entered the Pacific Ocean where The Star Casino is now located in Broadbeach, and by 1930, the entrance moved to Main Beach where Sea World is today.

Emeritus Professor Rodger Tomlinson, foundation director of the Griffith Centre for Coastal Management, looked back on the history of the entrance, its early evolution, the main reasons that resulted in a call for a stable entrance, and some concerns that still need to be addressed.

“A potted history includes the first sightings of what was thought to be the entrance by Europeans in 1823. However, there are suggestions that the entrance may have been closed with the Nerang discharging up into Moreton Bay for a while,” shares Prof. Tomlinson.

He explained that the big change was the breakthrough at Jumpinpin in 1898, when the sea permanently broke through at Tuleen, creating the Jumpinpin Bar, and dividing Stradbroke Island in two, forming South and North Stradbroke Islands. It is believed that the breakthrough of the ocean at Jumpinpin started with a shipwreck.

“This changed the balance between tidal flow and alongshore sand movement at the Nerang entrance, with the tidal flow from the Broadwater now effectively being half of what it was before. This meant that the Nerang entrance migrated to the north under the influence of the alongshore sand movement creating the Spit. Of course, during major storms or floods, the entrance would break through to the south, and the cycle of migration would continue. This resulted in the build-up of the shoals inside the Broadwater and the entrance channel and ebb-tidal delta constantly changing and not reaching a stable condition. With the growth of the Gold Coast and increased boat usage, there were calls for a stable entrance,” adds Prof. Tomlinson.

According to estimates, it was believed that the entrance would be opposite Runaway Bay by 2050, and the town of Currigee on South Stradbroke Island would have been in danger if the entrance had not been stabilised. Unfortunately, it was already too late for Moondarewa, a coastal town on the southern tip of Stradbroke Island that disappeared in the late 1930s as a result of the northward migration of the entrance prior to the construction of the Seaway.

There were a number of factors that contributed to the decision to constructing a stable seaway. Prof. Tomlinson lists, “The need to make the entrance stable for navigation, potential interest in property development on South Stradbroke Island, and stabilisation of Main Beach against erosion. The entrance stabilisation was a recommendation in the 1970 Delft Report into coastal erosion.”

In 1983, the Queensland Government took action to stabilise the river mouth. During the initial planning, it was evident that the entrance would require an artificial sand bypass system across the river mouth in perpetuity, and that the most economical solution would be to form a new entrance through the Spit by building two breakwaters that can withstand the cyclonic conditions they might encounter. Model studies also showed that the entry of ocean swells can be reduced by orientating the breakwaters, 15 degrees north to east, and increasing the length of the southern wall. All these factors contributed to the need to construct an island, the Wave Break Island, that can capture storm waves, and protect the Broadwater’s western foreshore.

All the above-mentioned models were evaluated to bypass sand across the entrance. As not even a similar engineering feat existed in the world at the time, the task was more than challenging. The selected system involved pumping sand from the southern wall through a pipeline, under the seaway, and discharging it onto the ocean beach of South Stradbroke Island.

The Nerang River Entrance Stabilisation Scheme project cost $50 million, used one million cubic tonnes of rock, two layers of 20- and 25-tonne concrete cubes, and dredged 4.5 million cubic metres of sand. Currently, the sand bypassing system pumps 500,000 cubic metres of sand annually.

The Seaway provides for a safe transit between the ocean and the Broadwater for recreational and commercial craft, but sand build-up remains a concern. “The main stabilisation comes from the Seaway training walls, with the bypassing primarily there to transfer sand across to South Stradbroke Island so that it doesn’t erode,” explains Prof. Tomlinson. “The original intention was that this would keep all marine sand away from the entrance ensuring navigation. However, the system is not 100% efficient and sand has been gradually building up on the ebb delta. A very detailed analysis in 2014 undertaken for the Gold Coast Waterways Authority, and a qualitative research project this year shows continued overall growth, and the localised areas of shallowing.”

The sand bypass is effective in preventing erosion of South Stradbroke Island, and although it does not prevent sand build-up in front of the entrance, it has reduced the rate at which this occurs. Prof. Tomlinson warns that if there are no major dredging activities at the Seaway, we can expect a continuation of the growth of the ebb-tidal delta and the localised shoals that are a function of the weather conditions. “For example, our research shows that during storm conditions, the movement of sand on the offshore bar tends to go past the end of the bypassing jetty and deposits on the ebb-tidal delta.”

At the time of the construction of the sand bypassing system, engineers had no reference to any similar engineering feat. The scheme has been followed and used in various countries including South Africa, Japan, and South America. “The bypassing was the first of its design, although other types of bypassing have been used elsewhere such as fixed dredges in channels or on the beach. Following the Seaway, we have an identical system at the Tweed River in New South Wales. There is also a very similar system in Brazil,” adds Prof. Tomlinson.

Boaties should be careful when navigating the Gold Coast Seaway and be aware of some threats that they might face out on the water. As sand “leaks” around the jetty and onto the ebb delta, localised build-up can be a threat. Such as all waterways with mobile marine sand, the channels will shift, and shoals can grow – Curlew Island is a case in point. As mentioned earlier, the processes on the ebb tidal delta show how localised deposition can create a navigation hazard, whether it is waves breaking unexpectedly or simply the reduction in depth.

While navigation threats still exist, the Gold Coast Seaway is considered a safe entrance. The sand bypassing system, construction of Wave Break Island, and ongoing dredging have stabilised the entrance without damage to surrounding beaches.

 

 

By Patrick Molnar