Wednesday, 25 May 2016

River Surfing in Cairns?

Just after writing about how the mouth of Barr Creek can have violent episodes, I was able to actually able to film another creek having one.  Deep Creek at Clifton Beach is about one quarter the size of Barr Creek and longshore often blocks its mouth with sand during the dry season.  Last week the mouth of the creek was a mere trickle and then we had a little bit of rain.



The creek came up so fast that a little old lady who went for a walk along the beach could not get back.  I had helped her wade back across by walking out into the sea and crossing where the force of the water was less.

Some people arrived and were quite worried that their big dogs would jump in and get washed away but dogs have skinny legs and seem to be able to walk through really fast water so they relaxed.  Then everyone just started having fun.  A man was trying to body surf the waves but his dog kept climbing on his back and riding him like a surf board which was really annoying.  .

This time I could stand in right beside the channel and film the biggest of the waves just in front of me and I saw a lot of things that I had never seen before and you can see some of them in the video.  The standing waves move slowly upstream.  When they get really big, they collapse into rolling surge of slosh and which is washed downstream leaving the surface of fast flowing water almost flat.  After a few seconds, a new set of standing waves formed and over twenty seconds grew to full size and then as before collapsed.  I always thought that ocean swells moved through the flood and made the standing waves peak but this is not the case.  The standing waves were in this case a self-generating cycle.  In some of the standing waves, vortices are making patterns in the crests of the waves.

In my search for information about standing waves I discovered that if Barr Creek was in America, people would go nuts about it.  The Waimea River on the Hawaiian Island of Oahu is their version of Barr Creek

   

Does Barr Creek really compare to the Waimea River?  They are almost perfectly matched - look at this.

Barr Creek with 500 m scale bar (images Google Earth)
Waimea River in Oahu, Hawaii
The difference is that Barr Creek has a maximum drop to the sea of about 3.5 m whereas the Waimea River drops down by 6-7 m as the beach is built up high by 10 m waves (their king tides are only 0.5 m!).  Barr Creeks standing waves top out at about a 1.5 m high whereas the Waimea River standing waves seem to be twice as big at full power.  Like the Waimea River, one has to wait until the mouth is blocked and the creek fills up with freshwater and when it is on the point of bursting out.  Kids sometimes dig a channel on a very low tide and off it goes.  Unfortunately this is not a common event.  However Barr Creek has several tide powered events every year and where the water drops about a metre between the creek and the sea and there is still plenty of action.  These  events are bigger than my video of Deep Creek.  The best time to play is when the creek is running fresh during the wet season and you do not have to worry about box jelly fish but you always have to look for crocodiles.


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



Saturday, 7 May 2016

Critically Endangered Littoral Rainforests

Littoral rainforests are rainforests that grow beside the sea, usually on sand ridges or coral cays. In Australia, rainforests that grow on headlands have also been included under the littoral rainforest category for the purposes of environmental regulation.  Over the years, much native vegetation has been cleared from picturesque coastal locations and littoral rainforests on the east coast of Australia are now classified as critically endangered under the Commonwealth Environment Protection and Biodiversity Conservation Act and are strongly protected. 

Cardwell lilies and oak leaf fern
Wangetti Beach littoral rainforest
Between Cairns and Port Douglas are some of the largest remaining areas of littoral rainforest and in this post we will explore these forests.  The most noticeable thing about these forests is that they are very green places full of ferns and lilies.  In fact some of the littoral rainforests near Cairns look more like temperate oak forests than Australian forests.  When I was watching the movie Christmas Dragon which is set in medieval times somewhere in Europe (actually Provo, Utah), the look and feel of some of the forests was almost identical to the forest of Wangetti Beach shown in this post.  Botanists have noticed these strange similarities and have studied them to reveal some of the secrets of forest ecology.  Plant physiognomy is the study of this phenomenon.

Forest scene from Christmas Dragon
Ferny dry rain forest
Forest scene from Wangetti Beach with scrub fowl nest in distance
In most littoral rainforests, tall trees are the exception and the rainforests composed of low twisted trees or even dense thickets.  Beach scrub is an old name for this type of vegetation.  Sand has little water holding capacity so during droughts there is almost no soil moisture left for the trees.  Only trees that can survive for months with very little water do well and even they can suffer greatly.  During severe dry seasons many plants will have canopies filled with leaves that have died but which remained attached.  In a nutrient poor environment trees try to hold on to their leaves and during droughts leaves often die slowly from tip to base.  Littoral rainforests are home to dry rainforest species, many of which can also be found in dry rocky areas such as on the sides of rocky headlands.

Large Mimusops tree
The largest tree in the forest was this Mimusops elengi (Spanish cherry, Red Coondoo)
Red Coondoo, Spanish Cherry
Base of this large mimusops tree is almost a metre thick
Littoral rainforest trees can be very slow growing.  At East Point in Mackay, littoral rainforest trees were scattered across a wide sandy plain.  Using the earliest aerial photo I have ever seen (1949) and an aerial photo from 50 years later, I discovered that almost all the trees present in 1949 were still present and that no new trees had become established.  Most of these trees were only 2 or 3 m high and were wind pruned and spreading.  In a garden, the same type of tree (Cupaniopsis anacardioides) would need less than 10 years to grow to this size.  

Littoral rainforests can be quick to colonise new beach ridges that form near river mouths, such as at Yorkeys Knob.  However the species present in the newly vegetated area are different and tend to be softer, faster growing species such as beach hibiscus.  Areas that fail to be colonised by rainforest trees tend to remain treeless forever.  Most littoral rainforests outside the wettest parts of Queensland have natural open spaces within.  When there is not enough rainfall or fertility to support continuous rainforest, the rainforest organises itself into patches which accumulate resources and open areas which provide resources, mainly groundwater. 

A patch of new forest beside a desolate space on the foredune
The bare patches can persist even in old forest - this space has moss on the ground
Fruit eating birds make up most of the wildlife in littoral rainforests as the forest patches are too small and spread out to support rainforest specialists.  Many migratory species fly along the coast and spend time in the rainforests including fly-catchers and cuckoos.  Some birds of rainforest edges such as black butcherbirds and scrub fowl live permanently in the littoral rainforests but can move between patches.  Orange-footed scrub fowl (Megapodius reinwardt) make enormous mound nests on the forest floor that can be more than 3 m high. 

Megapodius reinwardt nest, Wangetti Beach
3 m high nest mound of an orange-footed scrub fowl
In wind exposed areas, the forest canopy is often wind pruned into a wedge of vegetation that points toward the sea.  The wind also cuts paths or tunnels through the littoral rainforest.  Salt spray is as much of a factor in wind pruning as is fast moving air.

A wedge of littoral rainforest allow the forest to creep into the wind
A herb flat has fingers extending the forest on the hind dune due to wind tunneling
If the wind gets into the forest it can clears out foliage between the canopy and the ground
Threatening processes is the name applied to events that destroy or degrade ecosystems.  For littoral rainforests, the main threatening process was local councils clearing vegetation to provide recreation areas.  Forty years ago it was common to clear tracks through coastal vegetation by bulldozer.  Vegetation has failed to reclaim these disturbed areas even without any traffic or wind tunneling, so littoral rainforests basically do not recover from disturbance. Perhaps man made gaps persist indefinitely for the same reason that natural gaps persist.

However now that clearing is banned, weeds are becoming the main issue.  Unfortunately one of the most destructive plants is the iconic coconut palm.  The palms interfere with the wind flows that shape littoral rainforests.  Beneath coconut palms the soil is so full of palm roots that digging a hole with a spade is very hard and the palms consume all the soil resources.  Coconuts also acts like umbrellas preventing rain from falling on and washing the foliage of samplings and shrubs below so their foliage is burned by salt from salt spray carried in by the wind from breaking waves.  Finally, plants below coconut palms are subject to falling fronds.  Within ten to twenty years of the arrival of coconut palms, the littoral rainforest can be replaced with a messy thicket of coconuts.  Captain Cook, never found coconut palms in Australia and after his stay in Tahiti, he knew what they were and considered them worth stopping for to add to the menu.  Coconuts are an introduced plant that has multiplied during the last twenty years to become a serious pest of littoral rainforests.  

Cocos nucifera
Understorey plants being buried by coconut trash