Friday, 16 January 2015

Finishing off the Machans Beach Seawall

After a few close calls with category 5 cyclones, a heavy duty rock wall is being constructed for protection.  Machans Beach seawall is reaching the final stages of construction and the causeway that was used by trucks to take rock out for placement on the wall is now itself being pulled up and placed on the wall to create a much thicker layer of rock armour.  As the causeway is about 2 km long, removing the causeway is a huge job. 
View of seawall construction
Causeway is located on left and is separated from rock wall by about 5 m
A pair of excavators are working together at the end of the causeway.  One fishes up rocks from the end of the causeway and places then on the side of the causeway between the excavators.  These rocks are then picked up by the second excavator and placed on the seawall.  Each stone is carefully placed and tamped down to ensure that it is unlikely to move.  It is a strange thing as the stability of seawalls increases when the amount of open space between the rocks increases, so tightly placing the stones possibly weakens the wall.  However people will climb over the wall so tightly placing stone improves public safety. 
Excavators passing stones from end of cause then on to rock wall
Tamping down a rock
At the top of the wall is a broad lip that is out of reach of the excavators removing the causeway and rock is placed on the lip directly by a smaller excavator equipped with a rock grab that lifts rock directly from a conveniently parked semi and places/drops the rocks onto the wall directly.  A white sheet of geo-fabric prevents soil from washing into the gaps between the rocks. 

Lifting rock from a semi and placing it on the seawall
The truck has to negotiate some tight spaces when delivering rock to the esplanade but with a bit of toing and froing, it gets around the corners.

A heavy responsibility, you can't damage the pub
Gradually sections of finished seawall are emerging. There is also a hidden part of the seawall, the base of the causeway will remain in place to provide support to the toe of the seawall.

Finished seawall
As the causeway was built over a small beach that ran along the base of the old seawall, the local sand supply to Holloways Beach may have been interrupted.  The southern part of the beach fared quite well as it is can receive sand from Barr Creek and from an off-shore sandbar that has migrated in but the northern part of Holloways Beach is again in trouble.  Fortunately, it seems that the sand supply is recovering and the coastline around the Barr Creek Mouth appears to have prograded by as much as 20 m.  It is hard to tell what contribution the seawall has as the beach also progrades due to floods pushing sand out of the creek and prevailing winds moving sand along the beach but these events have temporary effect and sand erodes away again.
Where the people are is new beach

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