Saturday, 21 May 2016

How the Creek Ate the Beach

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.  The narrowed channel results in very fast currents that sweep away sand from the base of the high sand cliff on the landward side of the channel.  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 housed 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.

Holloways Beach South in March 2016



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