Monday, December 15, 2008

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?

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