Strange combinations of normal events can synergise to do as
much damage to the coast as cyclones. As climate change kicks in and slight increases in sea level and slight intensifications of trade wind systems occur, these synergies become more significant as these events are likely to cause somewhat more damage and take somewhat longer for nature to repair. In fact in recent years, most of the damage to the Queensland coastline in places that were not directly hit by cyclones, seems to be due to synergistic events.
A nice wide beach is just what you want on coming into the cyclone season
About 8 weeks later the beach was entirely gone leaving nearby houses highly exposed (23 Dec 2011)
Coming into the cyclone season, I was happy that the southern
end of Holloways Beach had a 25 m wide crest that would provide an effective
buffer against most cyclones. A few
weeks later this buffer had been entirely eaten away completely and trees were being
undermined and some were falling into the sea.
The culprit was a combination of high tides and strong winds which is a
regular and not very destructive occurrence, synergising with the meandering of
the small tidal creek to create the most efficient beach erosion system I have
ever seen.
The eroding sand cliff was over 2 m high in most places
In about 60 days, the mouth of Barr Creek moved by approximately
300 m and many thousands of tonnes of sand was removed from the beach. In previous post I have covered how Barr Creek
which is tiny tidal creek can become a serious geomorphological force when high
tides combine with longshore drift. In
this post, I will describe how this same system became locked into a highly
destructive cycle that resulted in the creek mouth rapidly migrating northward along
the beach and eating away the protective foredune in the process.
Same position, looking in the opposite direction with the creek mouth in its normal state
Where Barr Creek crosses the beach, it meanders just like
any other creek that flows through sandy ground. Straight sections of creek begin to curve and
curves grow more pronounced until they cut through the crest of the beach and
new mouth forms. Usually the meandering
curves are restricted to the protected estuary side of the beach with the creek
straightening just in time to pour into the sea. Low swells and high chop often surge into the
creek mouth on the incoming tide and dissipate their energy in the shallow
waters over the sandbanks within the estuary.
As the sides of wave exposed creek mouth have a profile similar to the
beach, the waves do little other than swash a few handfuls of sand from the
beach into the creek channel and add a little more velocity to the inflowing
tide. This is the normal condition of
Barr Creek.
Sometimes the beach near the mouth is stable for long enough for beach vegetation and even trees to grow
Once every ten to fifteen years the creek has meandered to
its maximum curvature and has created a high sand cliff that faces the ocean
but which is protected from wave attack as the channel lies behind the beach
crest. High waves near the peak of a king
tide can now wash right over the thin remaining beach crest that lies between
the channel and the sea. This delivers
enormous amount of sand into creek channel resulting in continual narrowing of
the channel at the same time as a king tide is trying to flood into the
estuary.
Washed over beach crest and narrowed channel, March 2015
The narrowed channel results in
very fast currents that sweep away sand from the base of the bank on the landward side of the channel, creating a sand cliff. The
cliff retreats as unsupported sand falls into the channel and is also removed
by the current. At the same time, the
washed over beach crest on the other side is being lowered and allowing waves
to surge over the top, where they cross the creek and slam directly into the
sand cliff. Each time part of the cliff
collapses; it is swept away by the extraordinarily strong currents in the
channel. The powerful wave and current
attack on the sand cliff which faces the ocean is what eats the beach.
Beach has just reached a critical state where waves can attack the outer bend (8 Aug 2011)
Creek is now locked into a northward migration (25 Sept 2011) - GoogleEarth Images
View of creek mouth on 24 Sept 2011 with kids playing in the current
Sand cliff or scarps also form on regular beaches during
cyclones but they are usually much smaller being on 0.5 to 1 m high rather than
the 2.5 m high sand cliff created where the channel has cut through the beach
crest. Also sand washed from the beach
into the sea usually forms a protective offshore submerged sandbar that helps
to reduce the level of wave attack on the beach. In the case of Barr Creek sand eroded from
the sand cliff helps to maintain the extreme currents that transport so much
sand and this helps lock in the destructive cycle.
Post Cyclone Larry (Cat 4) erosion scarp - the landfall was 100 km further south
Cyclone Larry seas were rough but not exceptional and the beach could easily endure
So far we have covered why the beach retreats so rapidly but
we have not covered why the mouth of the creek moves along the beach at a high
rate. The whole process is driven by tidal
currents moving sand. Without the
currents, the sand would just be swashed up and down the beach as it normally
is and the beach would remain much the same.
When the mouth is migrating, longshore drift keeps delivering sand to
the creek mouth forcing the creek mouth ever further to the north. Outgoing tides passing through the narrowed,
north pointing channel also deliver large amounts of sand to the creek
mouth. The result is long tapering sand
spit that rapidly extends on the seaward side of the channel that constraint
the channel to the base of the sand cliff.
In the next incoming tide vast amount of sand are swept over the sand
spit into the channel forcing the channel back against the sand cliff repeating
the process that eats the beach. So the
key element to the migration of the creek mouth is the rapid extension of the
sand spit which is washed over at high tide.
The surprising thing is that the process is self-generating and can
repeat for at least 30 days.
When Barr Creek started to destroy mature beach trees, the council cut through the sand spit (23 Dec 2011)
The new mouth seen 6 months latter, however a 250 m long erosion scarp is still visible to the north
This geomorphological process converted a relatively safe
beach into a highly exposed beach that could have enabled a cyclone to eat the
whole beach reserve and threaten houses.
Events before my time may have been even more spectacular. Long term residents say that the mouth of the
creek was once a few hundred metres north of where it is now. In a 1952 aerial photo, there is a hint of
this being the case as the creek mouth seen in the photo has pushed more than
250 m north of its normal position.
Behind the current foredune is a freshwater lagoon which may have been
created by the creek as locals say that the creek flowed behind the houses for
a period.
1952 Aerial photo showing the creek mouth eating the beach to the north
Behind the beach is a swale with a freshwater swamp full of Bullrush, the smooth patch in the above photo.
Another reason for understanding the behaviour of creek
mouths is that they are often dredged to maintain channels for navigation or to
provide sand for beach replenishment.
Currently the Moon River just north of Yorkeys Knob is being dredged and
is causing terrible beach erosion in Half Moon Bay and now Richters Creek is
being dredged to provide sand for replenishing Holloways Beach. Each time the creek mouth is dredged, there
is a massive impact on coastal processes in the vicinity of the creek mouth and
I wonder if it these impacts are actually adding to the erosion of Holloways
Beach in the long term. These systems
are complex and take years to respond to changed conditions so it would be easy
to misinterpret action and response.
Most creek mouths are pretty stable and have been in the same position for as long as we have records. Creeks like Barr Creek that have wandering mouths are comparatively rare. Currently I know of only Barr Creek and Hartleys Creek at Wangetti Beach which are unstable and have mouths that regularly move by more than 100 m. Larger creek mouths such as Richters Creek are also subject to similar processes. As the watercourse becomes larger, the balance between the forces of tides and waves changes and it is probable that river mouths rarely migrate the way that smaller watercourses can.
Postscript
It took a few years for the beach to fully recover. Whilst the face of the beach quickly recovers, there was a half metre deep hollow at rear of the beach as neither tides or wind penetrated to the rear of the beach to bring in sand. Eventually, a combination of very high tides and rough weather resulted in waves that could swash right over the beach and into hollow. As the swash drained back to Barr Creek along the hollow the water surged across the beach in one direction only and this quickly brought in enough sand to fill the hollow. We have also recently had good sand supply from the Barron River and the beach has now grown tall enough to support large areas of beach creepers, something which I have not seen on the beach before.
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
