In coastal engineering, no one works from a blank slate. When a foreshore is redeveloped, the engineers charged with the transformation will see their work shaped by the existing infrastructure.
This is true even in a country like Australia, where coastal construction has been carried out over decades rather than centuries.
Even the influence imposed by previous construction might falter before the might of the natural forces that shift and push an area’s coastal geography.
“The main issue that coastal engineers face globally is that we’re generally resolving problems left from previous infrastructure, and that’s mostly because sand is moving along the coastline continuously,” University of Queensland School of Civil Engineering Professor Tom Baldock told create.
“Transport of littoral sand along the coast is the same worldwide, and if you interfere with that, that’s when you cause major problems with your down-drift coastal infrastructure in terms of chronic erosion problems that keep going forever.”
For instance, the coast around northern New South Wales and Queensland sees sand moving up from mid-NSW, across the Tweed River and up and out past Moreton Island.
“Then it goes up along the Sunshine Coast and eventually that sand works its way all the way up K’gari Island,” Baldock said.
“When you interrupt that, then you end up five, 10, 20 or 50 years later with long distances down the coastline with chronic erosion problems that you can’t fix because you don’t have a sand supply.”
The Mooloolaba foreshore
These are the factors coastal engineers must keep in mind when redeveloping this stretch of the Queensland shoreline.
And one such redevelopment is taking place right now in Mooloolaba, on the Sunshine Coast.
The Mooloolaba Foreshore Revitalisation Project is part of Sunshine Coast Council’s master plan shaping the region through to 2035. Its features include 6500 m2 of beachfront parkland, new community facilities and a new terraced seawall, constructed to withstand such future coastal hazards as storm surges and rising sea levels.
This will replace the existing vertical seawall, which dates to the 1960s, and which the council says can no longer be relied upon to adequately protect the foreshore in the case of multiple extreme weather events.
It’s an effort in providing refreshed amenities to community and business while also ensuring the infrastructure can withstand the impact of cyclones, storms or the effects of climate change.
Baldock said that designing for urban growth should go hand in hand with designing for resilience.
“You are always trying to make resilient systems and, in most of our engineering, we’re working with a lot of historical existing infrastructure or precedents or legal systems that influence those choices,” he said.
“But the best enduring solution is the most resilient one you can get for the cost benefit that’s appropriate. So, in the coastal environment, the general approach would be that where you can rely on natural systems to give you coastal protection and maintain the beaches, those systems are extraordinarily efficient at coastal protection.”
A shoreline like that of the Sunshine Coast, where the population extends right to the water and has lived there for generations, will need more than natural systems, which is the case with Mooloolaba.
But integrating those hard defences with natural processes ends up being more effective and more friendly to the bottom line. Then, the coastal infrastructure only needs to be relied upon to bolster the coastline’s natural defences.
“Your hard defences are there as a last resort, which is what we have on the Gold Coast and what will be in Mooloolaba in front of that revetment,” Baldock said. “The idea is that the revetment doesn’t get impacted by waves except in the most severe situations. The beach is doing the work most of the time.”
Read more: More than just seawalls – a trigger-based approach to coastal defences
Hard defences
A terraced design for the new Mooloolaba seawall will replace the existing vertical wall. A University of New South Wales Water Research Laboratory report conducted for the Sunshine Coast Council noted that terraced seawalls are preferred because they maximise public amenity, connect the beach with the foreshore, and are more accessible.
“You’ve got steps that provide access down [to the beach] and people can sit on that,” Baldock said. “That’s your primary defence and it’s a hard defence. You decide the elevation of the top of those steps partly based on the access, but you need to conceivably build that a bit higher in some locations and then come down again to the amenities on the other side.
“That’s certainly the case on more exposed coastlines, and you might even have a revetment or a wall along the top of your promenade to provide extra protection against impact.”
As well as enhancing public safety, that revetment can also act as a windbreak. But balance is important too; an imposing vertical wall would be effective structurally but be counterproductive to the foreshore’s use as a public space.
“You wouldn’t want to end up with a solid rock wall all the way along the Mooloolaba seafront,” Baldock said.
“That would not be visually appealing, but it would also make access difficult. It’s about access as much as visual amenity; you want people to be able to access the beach easily, but you also want the access along the top of the promenade to be easy.”
Even so, the recent experience of Tropical Cyclone Alfred emphasises how important practical considerations are in engineering like this. The Sunshine Coast Council’s UNSW report highlights the importance of ensuring the development can withstand major cyclone wave events.
“They’ve got major coastal hazards from storm waves initially and we would have seen that with TC Alfred,” Baldock said.
“Often those systems would be linked with a storm surge that will bring the waves closer to the shore and cause coastal flooding in canals. When you have a lot of breaking waves, that can also raise the water level close to the beach and give you an extra break-induced rise in the water level.
“That all then goes to the coastal impact on the shore, whether that’s from erosion of your beaches and dunes, or whether that’s overtopping your hard structures or overtopping of your natural structures like dune systems.”
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