Sunday, 3 April 2016

The Secret Power of Ordinary Coastal Processes

For a long time geomorphologists (people who study how landscapes evolve) thought that is was mainly day to day coastal processes that shaped our coastline.  Then came a realisation that many coastal features were in fact created by major events from super cyclones to tsunamis.  Major events then became the main focus of study.  But there seems to be another agent of change that possibly has as much effect as major events and this is synergies of ordinary events.  In more than twenty years of observations of the Cairns Northern Beaches, these synergies have caused more beach erosion than several major cyclones. In this post and a future post, I will describe two synergies that can move vast amounts of sand in periods of hours.

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
Barr Creek's secret is that it has flood-like outgoing tides.  No other creek in the region has these.  I suspect that on highest astronomical tides (~3.3 m), seawater expands over an almost flat ground surface in areas with mangroves and salt pan.  As tides get higher, the area that is flooded increases exponentially and the volume of water in the Barr Creek estuary would be much greater for an extreme high tide than a normal high tide.  When the tide turns, all of that extra water has to squeeze out of a creek mouth that is always being narrowed as longshore drift fills the mouth with sand at a rapid rate.  Longshore drift occurs when trade wind driven waves strike the beach at an angle and their swash pushes sand along the beach.  On the outgoing tide, waters flood out through the narrowed mouth at high speed creating standing waves that can be more than a metre high.  Ocean swells also force their way up into the outgoing flood and momentarily transform standing waves into raging peaks of water as they pass.  When conditions were like this we used to drop everything and go white water rafting (the video does not show a full power day).  After the flood has surged more than one hundred metres out to sea it plunges through a rolling wave than marks the end of the flood and the start of the sea.

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
Beaches often seem to be static places, adjusting only slowly to the balance between accretion and erosion, however the truth is that every wave moves more sand than a good man can shovel in a minute.  Usually the amount of sand added and removed is in balance.  It is only in special places where sand movements are not balanced that true level of movement is revealed.

Kids love the high banks where the creek cuts into the beach
Getting rolled along by the incoming tide is fun too!
There is a funny side to the Barr Creek tidal movements.  Years ago Holloways Beach had a pub and Machans did not.  So people would come across the creek at low tide then go home in the dark.  At night you can’t see power of the water or its depth.  Many people have been swept away attempting to cross the creek.  Lucky they could all swim.  The incoming tide is pretty nasty too and some people got swept up the creek into the mangroves and you can imagine the fun: drunk in the dark and staggering through the mangroves.

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 
In this post I have talked about how a synergy of ordinary events can create very powerful forces.  In a future post, I will show the damage this process can do when it combines with other ordinary events to create an extra-ordinary synergy.

For more information on coastal processes, see the subject index

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