Saturday, 5 March 2016

Beach Recovery at Yorkeys Knob

In the 1950's the sea was threatening to eat the houses on esplanade at Yorkeys Knob.  The original cause of this crisis took place in 1927, when the Barron River had carved a new channel through the cane fields to connect Thomatis Creek which flowed to the Barron with Richters Creek that went to the sea.  This new route was seven kilometres shorter than the old course of the Barron River.  Some years later, the mouth of the Barron also moved north by nearly 2 km after breaking through the beach during a flood.  Rivers pump sand out into the sea, yet only when the near shore waters near river mouths become almost choked with sand, do waves to return some of this sand to the beach.  It has taken nearly eighty years for the flow of sand back to the beach to be restored and beach to grow to its maximum width.

Changes in Yorkeys Knob Beach 1952-2015 (click here to enlarge)
Sand mining from the bed of the Barron River continued until the 1990's and intercepted both the 20 000 tonnes of sand coming down the river each year and consumed an additional 70 000 tonnes per annum of previously deposited bed load.  This consumption of the sand supply had a terrible effect on the beaches particularly Machans Beach and Holloways Beach, which lie between the old mouth of the Barron River and the new mouth at Richters Creek.  The fight to protect Holloways Beach and Machans Beach are covered in previous posts (see Coastal Protection in the subject index).  In contrast, Yorkeys Knob Beach is positioned to receive the high volume of sand that takes the shortcut to the new mouth and as soon as sand mining stopped began to grow quickly.

The growing width of Yorkeys Knob Beach provides more than a happy story of how a beach was saved, it is a chance to answer a swag of questions about how coastal features form as they have literally been forming in front our photographic eyes.  These questions are important to land management as well as being scientifically interesting.  Developers like to claim that dense coastal vegetation is just regrowth and that they should be able to remove some of it to create views.  Dense vegetation developing where previously there was open sea could affect the public by blocking cooling sea breezes and possibly by allowing increased mosquito activity.  Conversely, a wider beach with dense vegetation provides a much better buffer against storms.  Scientifically interesting aspects include the development of landscape features such as chenier ridges form and how and why some areas become impoverished grassland yet metres away a ferny rainforest is created.  Topics of scientific interest will be covered in future posts.

In overview, it appears that between 2002 and 2008, the beach got wider by approximately 5 metres  per year.  From 2008 to 2016, the position of the beach has been stable as sand is now able to escape around the rocks at the northern end of the beach.  Recently the beach has been getting higher rather than wider, with the foredune growing about one metre in height.  As the beach grew, a series of small ridges and swales were created that are now stabilised by dense grass and herb cover.  In places trees colonised, mostly in lines that were probably created by high tides washing seeds up onto the foredune.  Many of these seedlings have grown to become trees that are approaching maturity.  In places the dense wall of regenerating trees is lifting off the ground and an open understorey is developing where is breezy and open but shady.
2010 (left) and similar 2016 views (right).  The foredune has become much higher and has continuous vegetation
Another important observation is the resurgence of native plants.  In my first intensive survey of this area, much of the vegetation on the low dunes was composed of introduced species.  There was para grass, guinea grass, Tridax daisy, Hyptis, Singapore daisy, Chinese violets and Mossman River grass, which has nasty burrs that penetrate our skin.  Most of theses exotics have waned and been replaced by a vigorous sward of native grasses and herbs.  Healthy natural vegetation looks better than weed infested areas.  Native plants seem to fit together, each providing a different visual texture and each occupying a defined area.  In contrast exotic species tend to run rampant and form smothering tangled masses which 'lack natural design'.  The exotics are still present but have only a minor presence.  The photos below show how they were.
Top: Tridax daisy and Mossman River Grass (the nasty one)| Bottom: Singapore daisy and Hyptis
For me, seeing these changes is not a matter of good memory.  Since approximately 2004, I have been photographing a very wide range of subjects using geotagged photos.  I am possibly one of the leading practitioners in the world when it comes to using photos to track ecological change, yet after more than fifteen years of development, I am still working hard to make a system that makes it easy monitor the environment with photos.  If anybody thinks that they can do time series research without having developed or acquired powerful tools for this purpose, they will have a very hard time matching photos in future.  In another post I will describe how to use photos for monitoring.  Most of the hard work is done by a database application that I have been developing for many years.  The information in this post comes mainly from my personal photo collection, aerial imagery from Google Earth (thanks Google) and some really old aerial imagery that I have scanned in.  To make the beach fit better on the page. the aerial photography has been rotated.

Coverage of geotagged photos - you can never have enough

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