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

Saturday, 3 January 2015

Fighting the Sea with Sand

Machans Beach holds back the sea with a wall of giant rocks but adjacent Holloways Beach relies on a barrier of sand.  Trouble with rock seawalls is that once you get them, the beach usually does not return as seawalls create conditions unfriendly to deposition of sand.  So at Holloways Beach, sand plays the leading role in the fight to protect the private and public space from the sea.  Holloways Beach is again at the point of needing more sand so this is a timely post.

trees falling into sea
San Remo Beach (north end of Holloways Beach) chewed up by trade winds, Dec 2014
Cyclones can damage the beach but this is fairly rare.  The basic cause of the recurring erosion is lack of sand supply due to previous sand mining activity in the Barron River and changes to the location of the the Barron River mouth, which resulted in sand being delivered to deep water instead of the beaches.  A combination of high spring tides and rough weather caused by persistent tradewinds can also do as much damage over several days as a typical cyclone and have caused most of the erosion this year.

trees beach erosion scarp
Holloways Beach in 2009
Sand has been pumped from the Richters Creek Mouth at the far northern end of Holloways Beach to the southern end of the Beach at least twice in the thirty years I have been associated with this area.  A few years ago another method of moving sand was tried, trucking the sand along the beach.  As for the pro's and con's of trucking sand compared with pumping sand, I am not totally sure but here are my best guesses:

  • Trucks run along the intertidal part of the beach so do not damage any private or public property whereas a pipeline that is installed for up to six months needs to be placed above the tidal zone.
  • Trucks can place sand anywhere on the beach whereas pipelines are less flexible in this regard;
  • If the sand is dirty, some of the dirt can be removed when the sand is loaded into truck whereas pumped sand includes the dirt (silt), however the silt is usually washed out at the receiving end leaving the beach clean but creating a turbidity plume in coastal waters which is undesirable; and
  • In areas where the beach is narrow and the watertable is high, it might be difficult to get the pumped sand to stay on the beach rather than being washed into the sea by the stream of water that moves the sand.

The response of the operator was that trucking sand had been selected for environmental reasons.  In all approximately 14 000 cubic metres of sand were moved up to a distance of 2 km.

repairing beach erosion
Truck hauling sand along the beach
Using beach to move sand
End loader back blading the beach (dragging the blade) to flatten the beach for the trucks
Fighting coastal erosion
Later in the afternoon, the tide came in and washed over the track
Initially the sand is dredged up by the same sort of dredge that would be used for sand pumping.  The sand is dropped into a bunded area which captures the silt-laden water running from the pumped sand.  Bunds are low earth or sand walls.  In this case, the bunded area was on a broad unvegetated part of the beach and much of the muddy pumped water was able to be filtered as it sank into the beach sand.  The area was also quite large so any sediment suspended in the water had time to settle once the water slowed down.
Pumping sand for beach repair
Dredge in mouth of Richters Creek
A small excavator would periodically heap the pumped sand into large heaps to allow the sand to drain and would also load the trucks.  The trucks were large articulated six wheel drives.  Despite looking like the ultimate all terrain vehicle, these trucks have one serious limitation.  They need a track that has very little crossfall and cannot drive along a beach that slopes at the normal angle. Their high centre of gravity and the fact that the back of the truck can rotate independently of the front creates means that the trucks could roll over relatively easily on sloping ground.  The slope of the beach was resolved by using an end loader to maintain a flat track near the high waterline.

Sediment pond and sand recovery
Bunded area/sediment pond with excavator for retrieving sand from the end of the pipeline
Stockpile of gray sand - see colour of sand on adjacent beach
silt present in coastal sand
Sediment captured within bunded area, went hard with time and
the area was buried in sand when the site was cleaned up.
The new sand was coarse and gray having been stained by black organic material which is present below the surface of the underwater sand reserves.  The coarse gray sand was not very pleasant and a few months had to pass to give sun and sea time to rework the sand and return a  pleasant texture and colour.

So was the trucking of sand successful?  I think so.  The beach has maintained its position for six years.  Without the sand, it is likely that the seawall constructed to protect homes in Hibiscus Lane would have been exposed and wave action and currents would have changed to make Holloways Beach hostile to natural beach maintenance processes.

The downside of restoring Holloways Beach with sand was the removal of sand from the mouth of Richters Creek.  A large sand spit had formed a hundred metres out from the mouth and possibly was on the verge of becoming a vegetated sand ridge.  Removal of sand from the mouth of the creek lead to the collapse of this sand spit.  Only six years later is the spit beginning to reform.  Sometimes I wonder if the spit building up at the mouth of Richters Creek would have caused the Holloways Beach to prograde as the spit changed the shape of the coast.  As waves crash directly into the sand spit there is no long shore drift and this area of beach becomes a natural sand trap.

Natural sand harbour, Holloways Beach
The sand spit that was lost due to the dredging, Holloways Beach in background