Very Large GSCs for coastal protection

With travel restrictions being lifted in Queensland last week I was able to visit the Maroochy Groynes construction site, which are now installing very-large Geotextile Sand Containers (GSCs).

Over the last four years JBPacific has investigated the local coastal processes and economics of the groynes and have now redesigned the existing geobag structures to withstand larger waves and have a longer lifetime. The Maroochy groynes are some of the earliest GSC structures in Australia, and although they are considered successful they needed new designs to withstand the constant wave forces. Unlike other GSCs in protected areas, they are exposed to swell from the Pacific Ocean with nearshore waves reaching 2.5m in extreme conditions.

The redesign considered various shapes, new ways to interlock the bags, but settled on larger bags sizes which were tested under extreme conditions in the Water Research Laboratory (WRL), Sydney. But the designs were just the easy part. Halls Contractors are now putting the theory into practise, having developed new methods, tools and construction processes to carry and position the very-large GSCs.

So how bag are they?

  • Some coastlines with small waves get away with the use of 0.75m3 sandbags (~ 2t = 2009 Ford Taurus)

  • The biggest ‘of the shelf’ GSC size are a 2.5m3 unit (4.5t = African elephant).

  • The new Maroochy designs are using 4.5m3 bags (8.5t = A Tyrannosaurus Rex).

Why is this important?

Seawalls built using GSCs can be an economical coastal defence against erosion and wave overtopping. In particular, they can be installed in areas such as the Torres Strait and Pacific Islands where local rock sources are unavailable. But with increasing sea levels and larger waves the standard 2.5m3 units are not always suitable. The new 4.5m3 bags may therefore provide a greater standard of protection for many areas effected by extreme coastal processes, now and in the future.

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