As a rule the larger the sediment size the higher velocity of waves is required
to transport them.
What I'm about to do is an experiment to show the impact that
swash has on sediment. I've arranged four different objects from left to
right according to their weight. Now shortly I'm going to place them in the
water next to a stake and we're going to see what happens.
As you've just seen when the wave has come in the heavier objects closer to the
stick have stayed where they are, but the lighter objects have been either washed up
on the beach or even dragged backwards.
There is a maximum depth to which waves will cause any significant water movement.
We call this depth the wave base.
Thus below the wave base, waves have no
impact on the sediments below.
However when waves enter shallow water the usual
orbital motion of water becomes elliptical.
The waves slow down and become steeper and higher.
This is called shoaling.
When the waves become too high relative to the waters depth the waves break.
Since the water is shallow the breaking waves are able
to push the sediment up towards the beach.
This flow of water moving up a beach is called swash.
The returning flow of water down a beach is called backwash and also drags
sediment back with it. Diffraction occurs when waves bend around a
barrier such as an offshore island like Oaia Island off the coast of Muriwai.
Once waves pass the barrier they spread out in all directions rather than carrying on straight.
Refraction occurs in shallow water
when approaching ocean waves swing around headlands because the waves slow down in
the shallow water. As a result the refracting part of the wave may be forced to
break with the other part of the wave that's still traveling in deeper water towards the beach.
Refraction increases erosion at headlands
and deposition tends to take place in the lower energy bays and beaches.
For instance, on a small scale examine the waves approaching this rock at Maori Bay.
The waves pass the rock and curve around the rock
and deposit a small ridge of sand here, a bit like a miniature spit.
On a bigger scale waves bend around islands taking sediment with them.
In some instances you can walk to the island at low tide, like this one, Motukaraka Island here in Beachlands.
Eventually when enough sediment is built up, the split will fully connect with an
island and form a tombolo such as this one here at Cape Maria Van Diemen
in the far north of New Zealand.
The prevailing wind direction will cause the waves to approach the beach at an angle.
As a result the sediment that moves up the beach with swash will also
be moved at an angle. Likewise the backwash will move the sediment back down
the beach at an angle. The sediment will travel back and forth along the
beach in a zigzag like motion. We call this process longshore drift.
In the case of Muriwai, the prevailing wind direction is southwesterly, therefore
longshore drift will zigzag northwards.
When longshore drift encounters a river mouth or a change in direction of the
coastline, sediment can be deposited forming a spit.
For example this spit at Rawara Beach in Northland has formed in a southerly direction indicating that
the dominant longshore drift here is also southerly.
Sometimes longshore drift will cause the sand to deposit right across the river mouth forming a bar.
As a consequence many bars have lagoons and wetlands behind them.
Because of longshore drift one beaches loss is another beaches gain.
However when humans interfere with the natural processes by removing sediment
through either dredging sand for canal developments, marinas or sand mining, it
can deprive another beach of its naturally replenishing sand.
To mitigate this people have built groynes to catch the drifting sand.
The sand piling up behind this groyne at Mission Bay in Auckland
gives some idea of how much sand may be carried away by the prevailing longshore drift.
Waves can either build beaches up or erode them.
It is normal for beaches to have a summer and a winter profile.
Smaller waves have a stronger swash than backwash and build beaches up in the summer.
They are called constructive waves.
During winter there are frequent storms.
Waves are higher and more frequent.
These waves have a stronger backwash than swash and erode the beach.
Anything built on or near the beaches are at risk of being undermined and carried away by the waves.
For example Muriwai's lifesaving tower had to be moved by crane 100 meters back from where it used
to stand after waves scoured the dunes in 2008.
Many beaches have a berm which marks the edge of the swash zone, such as this one at Little Buckland's Beach.
However Muriwai lacks this feature due to the dominance of large waves washing such features away
Sometimes either erosion or deposition of a beach can become a dominant trend over a long period of time.
At Muriwai some parts of the beach have plenty of sand.
For example, at the river mouth of the Okiritoto stream there is a
long stretch of beach due to the continual replenishing of sediment from the stream.
However, at the southern end of Muriwai closest to the headland,
erosion has been threatening human development.
I'm standing here at the southern end of Muriwai Beach.
The first time I came here was actually in the early 1990s
when the frontal dunes we're not far from where I'm standing now.
But in the time since then, erosion has been so severe that it's been
threatening the carpark and the council had to decide to do something about it.
Previous local councils put rocks and gabion baskets onto the beach to try and protect it.
Over the years the local authorities have spent millions of dollars
fighting a losing battle against the erosion of the dunes, Consequently the
former Auckland Regional Council decided to relocate this car back further back from the beach.
Often human attempts to prevent erosion have ended up creating more erosion.
For instance the building of a seawall
such as this one here at Little Buckland's Beach.
When the waves hit the wall it is reflected.
The backwash reflected from the wall drags away sand from the beach.
Notice how along the same stretch of sea wall at low tide the
beach has been stripped off its sand.
At many Auckland beaches such as Mission
Bay, the council has had to truck in sand from elsewhere to replace the lost sand.
This seawall protecting the cliff between Mellon's Bay and Howick Beach
has itself become the victim to the forces of waves.
Given the failure of sea walls and other hard structures, new approaches have been taken to coastal management
Some people are advocating new approaches to coastal management.
For example, the creation of an artificial sand dune such as this one here at Maraetai Beach.