Thursday, 24 July 2014

A Clean New Land

It is a surreal experience to go to a place that you know well and to find that it is entirely different.  Ellie Point has never been attractive.  It is a stark, shadeless place with an incredible sandfly population and appears to be the final resting place of any rubbish that the good folks of nearby Cairns throw into the sea.  It it a place that I wished could be better and maybe I have gotten my wish.  I found my self standing on clean new land where only three years ago, I was motoring my boat.

A new beach facing the City of Cairns
Ellie Point with a new beach - it would be more than 400 m long and would extend 100 m into the sea
In my life time, the coast line has only ever seemed to retreat, but now I am beginning to see this trend reversed.  The long period of beach erosion was caused in part by human appetite for sand.  For many years, sand was dredged from the nearby Barron River as it was transported toward the sea.  More than 20 000 tonnes per year were taken.  As a result, the Cairns northern beaches were starved of sand supply and began to erode.  Seawalls are still being built to protect houses from disappearing into the sea where the beaches have been all but lost.  Even as I write, I can hear machinery crunching on massive rocks to build a new seawall for Machans Beach on the northern side of the Barron River.  Sand was also mined from the sand flats at Ellie Point.  This was unfortunate as it delayed the development of sand flats around the recently moved Barron River mouth.  The new mouth emptied into deeper water and sand was carried by the flow of the river into deeper water where waves could not bring it back to the beach.  For these reasons sand extraction from rivers was banned (~1990) and sand mining at Ellie Point ceased.  Sand mining is now only allowed in so called in-active sand deposits which are old beach ridges or alluvial deposits which are located mainly under sugarcane fields.

View from the new beach back to the previous shoreline across a shallow sandy basin
Geology maps tell me that the oldest beach ridges are about 5 km inland and that they are about 5000 years old.  This suggests that the coastline should be prograding at an average rate of about 1 m per year.  In recent years, the beach at Yorkeys Knob has been prograding even faster as it receives sand from both the Barron River and Richters Creek.  Only this year have there been clear suggestions that beaches are again growing within the delta.  Shallow waters over sandflats now extend out hundreds of metres from the beach and waves can been seen refracting around the higher sandbars, resulting in sand being moved toward the beach.  However within the delta, prevailing winds and waves can also strip sand from one area and deposit it in another so prograding beaches are often matched with nearby areas of coastal erosion.
Just to the north the coastline is regressing and trees are falling into the sea.
There is another reason why sand is not mined at Ellie Point.  It contains acid sulfate materials.  Behind the new beach is a backwater that has filled with mangrove detritus.  When this material is buried, the organic matter feeds bacteria which combine sulfates from seawater with iron from seawater or from the mangrove detritus to create iron sulfide.  On exposure to air, the sulfides oxidise and become sulfuric acid which can eat concrete and steel and burn living organisms.  Whilst the process of acid sulfate soil formation is well known, as far as I can tell, it has never been visually documented.  Hopefully, I can fill this gap.  Another post describes the acid and hydrogen sulfide being produced by organic matter that was buried in the old Barron River mouth 40 years ago.

In the basin behind the new beach is a vast deposit of mangrove detritus
Where it has been buried by sand, black rivulets issue from the ground

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