Wind Erosion

There are two types of wind erosion
1)By Deflation, the wind picks up oarticles of fine grained sand and/or dust, moves and then deposites them elsewhere.
2)By Abrasion, the sandblasting like effect of windblown sand.

Onshore winds can lift sand from the beach and replenish the dunes behind the beach, but not today. Yet more sand is moving southwards along the beach.

Toxic Chemical Drums

Several drums of a toxic chemical, possibly a concentrated industrial cleaner. have washed ashore at beaches to the west of Venus Bay. The EPA warn that the drums should be left alone (and presumably reported), eleven people have already reported to hospitals after coming in contact with the drums.

The new beach profile

Taken at 4:25pm (approx 2 hours before "official" low tide) on 11/1/2009 atmospheric pressure was 1012 hPa

The low tides at the moment seem exceptionally low for three reasons

1. The 18.6 year maximum tide cycle (its related to the full moon)

2. High atmospheric pressure ( lowers the sea level)

3. the beach profile has change. (little of no summer berm developed and lowering of the beach generally)


The above photo shows how different the beach profile is this year. The erosion scars at the back of the beach are "healing", the front faces have mainly collapsing and are forming a more even rill, The dry sand is being carried by the wind, eroding a smooth slope in some places and depositing and riling down in others. In front of this in only a few places a narrow summer berm is developing. This is the area mainly above high tide where the beach can dry out and wind erosion take over as the main erosion agent. The wind flattens out this zone to almost horizontal (which is why it is called a berm) When it develops a distinct bump can be seen, where the gradient of the beach changes. In previous years this summer berm may have reach 10-30m wide by mid January. Now it is only present in a few places and seldom wider than 3m. The beach in front of that still have the steep gradient more characteristic of winter and the old beach profile has been clearly lowered (much sand eroded away). In front of that there are extensive flatter areas of sandbank being developed. This sand is frequently soft (unconsolidated) because it is being moved back and forth a lot with each tide). Extensive rutting, guttering and ripple pools are being developed and then wash away in the next tide or their position and form changing daily. The beach is reaching width of 100m to 150m from dune to water edge at low tide.

Understanding Longshore Drift

When Waves arrive at the beach at an angle to the shore line, sand will be pushed up the beach in that direction however in the backwash the sand will usually moved directly down the beach, perpendicular to the shore line. So the sand follows a zigzag pattern. The result is a net movement of sand along the beach. An enormous amount of sand can moved in this manner.

By looking at new sand bars you can determine the longshore drift. If the sand bar is attached to the beach at its northern end the longshore drift is to the south. Conversely if the sandbar is attached to the beach at its southern end the longshore drift is to the north.

Longshore drift can fluctuate from day to day, and may vary locally as the beach tries to readjust its shape to be in better equilibrium with the waves and currents.

The popular theory!

What is supposed to happen (in theory)

The most commonly applied explanation for erosion of sandy shores due to sea level rise is know as the Bruun Rule. It is a mathematical formula that relates the likely erosion to the amount of sea level rise, the width of the beach and the freeboard relative to still water level. The beach profile is supposed to be translated up and landward, with the eroded sediments deposited on the lower part of the profile. Putting this in the simplest terms the beach profile is predicted to move landwards and upwards with the rise in mean sea level.

Note: This diagram is not to scale and vertically exaggerated

The rule can be expressed mathematically as -
R= SL (hd+f)
where S is the amount of sea level rise
L is the active length of the beach profile
hd is the closure depth
f is the freeboard

This formula commonly predicts erosion encroachment of the coast R to be 50-100 times the magnitude of S the sea level rise. Many climate change evangelist have extrapolated these figures to predict extensive inundation around the world. At Venus Bay we may have the opportunity to put this rule to a test.

Whilst this Bruun Rule is widely discussed in relation to climate change, global warming, sea level rise and coastal erosion, but it is not without controversy, there are a couple of limitations. Firstly, the rule does not account for longshore interactions (sand drift and currents), and secondly, the rule assumes the wave regime is steady and hence the equilibrium profile remains the same.

The CSIRO's Sea Level Rise page gives a good overview of key issues here.

What is actually happening at the moment

The back of the beach is being eroded at several locations, as per the theory above but at the same time the lower sections of the beach are also being lowered (for example in this photo of beach one you can see exposed shell beds in front of an erosion scarp) this is producing a narrower beach at high tide but much wider at the lowest tides.

Perhaps longshore currents are drawing the sand down and along the coast?
Perhaps the swells pattern and currents have changed?
Perhaps there is a different explaination?