Damned Marram: Not all grass is good for your surfing
Spend long enough on any urban beach and you'll eventually see them. They'll materialise on the fourth Sunday of every month, or some such pre-determined time, and toil away removing the Bitou Bush and Privet while restoring the sand dunes to their original state.
Whether it be Bushcare or Landcare, or even Dunecare, community-based groups do a wonderful job of rehabilitating the coastal interface, however the notion that the beaches are reverting 'back to their original state' is a fallacy, and one proven by the plant that is always allowed to stay.
If you're a surfer in Australia then chances are you've walked through this plant. It's part of the journey from the car park to the beach and yet most of it is introduced from overseas.
That's right. Marram grass, that dry, spindly grass scattered across most Australian coastal sand dunes, isn't native to Australia. It was introduced to Australia from Europe in the late 1800's with a very specific purpose: to help stabilise coastal dunes.
The reason Marram is used is because it's very good at what it does. In fact it's better at stabilising dunes than the native Spinifex grass. Marram spreads rapidly, it overtakes native sand dune species, and it mitigates the natural changes to coastal dunes caused by large swell.
The problem, at least from a surfer's point of view, is that Marram grass is too effective.
The purpose of sand dunes is to provide a natural defence mechanism against high wave energy. When a large swell hits the coast, waves 'grab' sand from the dunes, transport it offshore thereby causing the swell to break further out to sea. The resultant shift in sand buffers the dunes from the full impact of the storm.
Once conditions settle down, the sand is slowly deposited back towards shore and onto the foredune.
Native sand dune species have been part of this feedback system for many thousands of years, helping to trap and create dunes that are naturally lower and less hummocky in shape. In contrast, when Marram grass is introduced to a beach the dunes take a higher, steeper-faced and asymmetric profile.
Marram roots extend further into dunes than those of native grasses. The result of this is that when Marram-infested dunes are eroded by storm waves, large steep gouges are taken from the dune leaving it even more susceptible to further erosion.
If you live near a beachbreak think about the last time a large storm swell hit and how it cut into the dunes. Did you have to clamber down vertical metres of dune to get to sea level? If so, there's a very good chance it was caused by Marram grass or its close cousin Sea Wheatgrass.
While Marram grass is very effective in protecting coastal property, its use has a negative effect on the surf. Steeper dunes and beach profiles cause backwash and wobble as energy from receding waves pushes back out to sea. The most obvious example of this is Curl Curl on Sydney's Northern Beaches, though all beaches with Marram are affected to some degree.
Studies along Manawatu Beach in New Zealand found that foredunes formed with Marram grass have heights over 6 metres and slopes up to 28 degrees, while dunes formed with native grass were only 0.5m in height and had slopes of just 14 degrees.
Aside from backwash energy, there are other side-effects from Marram grass.
The establishment of taller dunes also locks in sand and vegetation creating permanence in a zone that has for millenia been in flux. Recently there has been consternation from surfers at North Narrabeen where Birdwood Dune has built up to an unprecedented height. Similar worries have been expressed at Woonona near Wollongong, where the local dune is now heavily vegetated. The sand contained in both those dunes, and many others around the coast, should be moving between the beach and the surf zone.
The result is an abundance of sand above the high tide mark and less sand below it.
Of course, the reason councils and community groups persist with Marram grass is because it is excellent at stabilising dunes and protecting expensive properties behind it. And although it's an introduced species it manages to balance the ever-changing coastal interface to a degree that most people – save the odd grumble from surfers like us – are happy with.
Just don't ever say that the coast is in its original condition. //CRAIG BROKENSHA & STU NETTLE
(Photo of erosion at Wanda by The Leader/John Veage)