One of most dynamic sections of coastline near Cairns is Barr Creek, which lies between Machans Beach and Holloways Beach. This tiny little creek is only 1.2 km long, yet it can turn ordinary tides into dramatic events. Barr Creek is unlike all of the other creeks in the region as it is an ancient abandoned mouth of the Barron River. The other creeks have catchments on side the mountain ranges and their channels are maintained by flooding rains draining back to the sea. Barr Creek barely has any catchment at all, just fringing mangrove swamps. In theory, when turbid seawater fills into the Barr Creek at high tide, sediment should settle out of the still water and rapidly fill the creek in. This is exactly what happened in the nearby Blind Barron, a mouth that was abandoned approximately 70 years ago. Yet Barr Creek maintains itself.
|The mouth of Barr Creek migrates from side to side|
|Most of Barr Creek can be seen from the beach|
|Barr Creek 1 km upstream is almost too small to turn a small boat|
Underneath the surface the outgoing flood is a turbulent maelstrom. The sandy bottom is not smooth but a complex pattern of mega ripples and swirling vortexes that vacuum sand of the bottom and inject it into the flowing water. A single tide can transport enough sand to create an alluvial fan that extends 100 m into the sea and which would require more than 2500 tonnes of sand by rough calculation. A continually rolling wave is located at the seaward edge of the alluvial fan. Inshore waters are shallow so the alluvial fan ends with a steep drop of only half a metre.
Strong longshore drift removes the alluvial fan within a few days to weeks. The same longshore drift brings new sand to the creek mouth where it is sucked up the creek to form an alluvial fan that faces upstream. Sand accumulated in the creek mouth in this way provides material for forming a fan in the sea when the tide runs out.
|Aerial view showing both alluvial fans. The fan in the sea is 85 m long and is mainly from one tide.|
|It is amazing how big the alluvial fans can get. At the front-left are a few mega-ripples|
|Kids love the high banks where the creek cuts into the beach|
|Getting rolled along by the incoming tide is fun too!|
On a particularly violent day when outgoing tide was reinforced with a freshwater flood, I was out there having fun when a rescue helicopter came out and started circling around me. I was trying to surf the standing waves on a competition surf ski which can be paddled to about 15 km/h. But the water was much faster and I would paddle upstream like mad whilst getting sucked backward through the flood until I caught a standing wave and hopefully surfed. Of course you can’t see what is coming when you are going backwards which makes it very exciting and it is almost impossible to go through the roller backwards without coming off. Beyond the roller is flat calm sea so you just get back onto the surf ski, paddle to the beach and go again. Of course the rescue helicopter shows up just at the point of wipe out and thinks I am in trouble and started preparing to rescue me, which was embarrassing.
|Barr Creek mouth widened to 60 m after a cyclone when Barron River water flowed through fields into Barr Creek|
For more information on coastal processes, see the subject index