SAM SMITH never lets the weather spoil his days at the beach. A former engineer for Gold Coast City, he鈥檚 been watching the same stretch of Queensland shoreline for 30 years, and he鈥檚 stood on the dunes during five cyclones. 鈥淚鈥檝e seen a lot of dirty stuff up here on the Gold Coast,鈥� he says.
Dirty stuff notwithstanding, everybody wants a beachfront property on this stretch of coast, in romantically named spots such as Rainbow Bay, Currumbin, Kurrawa, Coolangatta, and, er, Nobby鈥檚 Beach. But build your house on sand and you might be in for trouble. Beaches are shifty places at the best of times, and if a storm comes along you want solid sea walls to protect you-right? Well, not necessarily. Smith knows the beaches here better than anyone, and he鈥檚 convinced they are masters of self-defence.
In the course of an ordinary seaside holiday, you probably won鈥檛 see your beach change shape. But when a severe storm strikes, things happen fast. A beach鈥檚 first defensive gambit is to build sandbars. Storm waves lift great quantities of sand from the seabed, and drag more back from the visible beach. After repeated lifting and settling, this sand humps itself up into one or more undersea sandbars. As waves roll over these bulwarks they break, losing half their energy in the process. Waves that have been tripped up by two or three sandbars end up with a lot less punch.
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In a big storm, though, waves hitting the beach still have enough oomph to do damage. And here, Smith believes, the beach deploys a final line of defence. When a storm is upon them, the dunes can be carved into high, strong, vertical scarps that reflect the energy of the incoming waves. Like a jujitsu master, the dune turns the storm鈥檚 strength against itself.
Though dune scarps are a familiar beach feature, most researchers don鈥檛 think they do much to fight erosion. Smith believes that鈥檚 because the researchers live in the wrong places. 鈥淵ou鈥檝e got to be there at the time to see how the beaches behave,鈥� he insists. 鈥淢ost coastal researchers are based far from the sea-the US Army Corps of Engineers are in Vicksburg, 200 miles from the ocean. The poor guys don鈥檛 have enough warning to get down to the beach and see the mud hit the fan. I can walk to the sea in five minutes.鈥�
His relationship with the beach began in 1971, when he started working for Gold Coast City. Nearly 30 years on, he still maintains a routine. 鈥淚 go on the beach every morning at about six, and log 25 parameters-wave height, period, direction, harmonics, which way the drift is going, slope of beach, how the sand is being sorted鈥︹�� But every now and then he gets a special reward-a cyclone. 鈥淚f you鈥檙e round here long enough, you really get your nose rubbed in the sand. I鈥檝e been in winds of up to 50 knots, and above 40 knots the wind strips sand off even the wet part of the beach. You get very wet. It鈥檚 unpleasant. You get frightened at times.鈥� But being there means that Smith has seen the beach under violent assault.
As the cyclone approaches, he says, the sea turns grey with vast volumes of churned-up sand. Then the big storm swell starts to take its toll. The top of the beach is washed out and deposited as a new sandbar out beyond the waterline. Waves rushing up the flattened foreshore cut into the dunes, forming a scarp.
Once the scarp reaches a height of 2 or 3 metres it becomes a rampart against the storm. When a wave hits, the water shoots straight up in the air, then falls back down, generating a new wave that runs out to sea. 鈥淚t鈥檚 a very efficient way of reflecting wave energy,鈥� says Smith.
After the storm, the dune re-forms. The waves subside, sand from the bar is pushed back onto the shore and the wind blows it back to the dune. Dune plants such as the spinifex grass that colonises the Gold Coast help to hold the dune together during the storm, and then to trap sand again afterwards. Smith calls dunes 鈥渁 kind of coastal engineering Phoenix鈥�.
It鈥檚 a neat story, but is it right? Andrew Short of the University of Sydney, author of the Handbook of Beach and Shoreface Morphodynamics, broadly agrees with these conclusions, though he is amused at Smith鈥檚 鈥渙blique view鈥� of beach processes. 鈥淲hile the scarp is in place and reflecting waves it is certainly slowing down, but not stopping, the erosion of the dune and underlying beach.鈥�
Even if Smith is right, a mystery remains: how do the scarps stand up? How does a pile of wind-blown sand form a tall, near-vertical wall strong enough to support the weight of someone standing right on its edge? In a paper submitted to Shore and Beach, Smith and Art Trembanis, a doctoral student at the Virginia Institute of Marine Sciences in Gloucester Point, say that one key ingredient is water. This won鈥檛 come as a surprise to anyone who鈥檚 ever made a sandcastle. But it was only in 1998 that this observation was put on a quantitative footing. In experiments with tiny spherical glass beads, Lyd茅ric Bocquet of the 脡cole Normale Sup茅rieure in Lyon and his colleagues found that in a humid atmosphere water percolates into the gaps between the beads. The water glues them together, allowing a scarp angle of up to about 70掳.
That alone isn鈥檛 enough to explain hard, vertical beach scarps, 鈥渂ut sand grains are not spherical beads鈥�, Smith points out. 鈥淲hen you look at one under a microscope and compare it with a picture of a big boulder, you can鈥檛 tell the difference.鈥� There are other substances in there too. 鈥淧ut a handful of dune sand in your mouth, and it tastes like table salt.鈥� The salt content can be huge, up to 30 per cent. So maybe salt crystals and other mineral deposits, along with the diatoms and tiny plants that live in the sand, are helping to lock the rough sand grains together. At this stage, it鈥檚 largely guesswork.
鈥淲e鈥檝e gone as far as we can go until someone does some proper comparative chemistry and physics,鈥� says Smith.
If these factors didn鈥檛 combine to harden the sand, and no scarp formed, Smith thinks the consequences would be disastrous. 鈥淗ere on the Gold Coast we鈥檇 lose the dunes in a moderate storm,鈥� he says, 鈥渁nd we鈥檇 have 30-storey buildings being undermined 100 metres inland.鈥� The lesson for planners, says Smith, is that dunes need room to maintain themselves. We shouldn鈥檛 put buildings right on top of them or mine them for sand.
Smith recognises that what happens on the Gold Coast might not happen everywhere. Sand properties differ from place to place, so the beach behaviour will differ too. And behind a big continental shelf, for example on the eastern seaboard of the US, storm surges cause more damage than waves. Scarps won鈥檛 protect Florida from hurricanes.
Even so, there are plenty of beaches where self-defence works better than engineering solutions. Left to themselves, says Trembanis, beaches survive because they yield to the sea, retreating or advancing as conditions change. 鈥淪ea walls can be overtopped and undercut as well as flanked and corroded,鈥� he says. 鈥淏eaches just roll with the punches.鈥�