Wednesday, 5 November 2014

Constructing Machans Beach Seawall

Machans Beach which is a sandy island in the mangrove-infested Barron River delta is getting major seawall upgrade.  Category 5 cyclones have become regular events during the previous decade with at least 5 nearly landing at Machans (Ingrid, Larry, Monica, Yasi and Ita).  When you live here you can remember them well as you have pack up your bags each time and flee.  Machans Beach was lucky and was not directly hit but a place that is very similar in almost all respects was hit-Tully Heads.

The old seawall is very steep and many of the rocks are not supported from below
Graphic of seawall design from public notice sign -showing the lower angle and extended toe.
Like Machans Beach, Tully Heads did not have a properly engineered seawall.  The seawalls were constructed over time in response to local erosion events or storm damage.  Clearly these sorts of seawalls do not stand up to category 5 cyclones and at Tully Heads, nearly all the homes in the first 2 rows were lost. Seawalls can also be dangerous and kids playing on seawalls are sometimes killed by shifting stones and there have been local incidents.  Upgrading the Machans Beach seawall became a priority project.
Gap bashed though seawall by Cyclone Yasi - Tully Heads

Rocks thrown up from seawall onto lawns
To begin at the beginning, Machans Beach needs a seawall because the beach eroded.  The beach eroded because someone cleared the vegetation between the headwaters of two creeks and during a flood, one of the creeks, Thomatis Creek flowed backwards from the Barron River, over land and into the headwaters of the Richters Creek.   As a result of that flood in 1922 was that a channel was scoured between the two creeks and the Barron River developed a new path to the sea that was 7 km shorter than the old channel.  Much of the sand carried by the river followed the new path to the sea and Machans Beach lost much of its sand supply.  Adding to the issue, the old mouth of the Barron also shifted and the sand that was coming out was channeled out into deeper water.  Sand miners were also dredging very large amounts of sand out of the river channel.  It is an open question whether the mulit-million dollar seawall now under construction would have been needed if some controls had been placed on inappropriate land clearing 100 years ago and sand extraction from the bed of the Barron River was banned earlier.  A full description of the root causes of the beach erosion issue was written up in a Mulgrave Shire Council report, dated 1986.

Machans Beach Esplanade is very narrow, so another way had to be found to access the seawall
Construction of the seawall began at the northern end near Barr Creek at the beginning of May 2014.  A causeway was constructed about 15 m out from the existing seawall.  This method of construction avoided having to use the narrow esplanade and having to remove all the trees that were perched at the top of the old seawall. Seventy tonne trucks would reverse down the causeway and dump stone to the side of the wall so that a large excavator (30 tonne?) could individually pick them up and place them at the end of the causeway to extend the causeway. The size of the largest rock approaches the size of some of the smallest cars and it surprising that the excavator could handle them so easily.

Tipping out rocks
A larger rock barely fits in the bucket
Reversing along the wall to the excavator
Mirrors are handy for this job
By November 2014 the seawall stretched more than half way along the Esplanade and the causeway was in excess of 1 km long.  Trucks would drive forwards along the causeway and turn on little ramps that ran from the causeway to the Esplanade.  These ramps are also where the excavator is parked when the tide is high.  The trucks then reverse the final hundred metres or so the the waiting excavator.

The causeway is now so long, that the excavator has to wait for loads of stone to arrive
Before placing rock, mud is dredged from the tip of the causeway and placed on the side
A bucket full of mud - the mud was plastic so not much turbidity resulted
Dumping large rock with the excavator poised to catch the truck should it start to roll over
View along the top of the partially finished new seawall, showing soil being laid where the footpath will be
The seawall is made up in layers, which small stone (~150 mm) being formed into bank, which is then covered with a layer of geotextile.  Geotextile will add strength to the seawall as it has a very high tensile strength but its main purpose may be to stop fine material being eroded out of earth behind the wall by the movement of tides and waves.  Loss of fine material can create deep holes in the ground.  On top of the white geotextile is a layer of smaller rock, each of the smaller stones would be the size of the bucket on a builders wheelbarrow, which is roughly the size of the stone in parts of the original seawall.  On top of the smaller stone, truly large stones are being places with may having a long dimension of close to 3 metres.

Layered construction of seawall
The project will continue well into 2015 and I will keep you posted.

1 comment:

  1. In many construction projects,excavator and excavator attachments are very helpful. They are a must in a my project.