Sunday, December 28, 2008

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.

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?

Sunday, December 14, 2008

Extreme Tides

Tides have a big influence on beach erosion, by lifting the water level and the waves against a new surface that is not in equilibrium with the wave action. Understanding the tides, is not as simple as looking at a tide chart.

Tides are primarily controled by the gravitation pull of the water in the oceans by the moon and sun (the gravitation gravitational from other planets does have a tiny little influence). This gravitational influence changes at any location because the earth rotates. There are two tides each day, Two high and two low tides. The moon rotation about the earth produces an additional monthly cycle and the earth rotates around the sun producing a yearly cycle, but there are many cycles influencing tide. The largest tides are often called "spring" tides, they occur twice each month with the new and full moon (when the moon and sun are aligned). Spring tides heave nothing to do with the season spring. These astronomical influences are largely predictable and can be used to produce tidal charts.

There are also some unpredictable causes of higher sea levels. When the air pressure is low the sea level will rise, around 1cm per hPa (hectoPascal). In addition this low probably also generates winds and a large low pressure can exert additional upward "suction" of the ocean surface. The wind blowing across the sea may also whip up swells (large ocean waves that can travel vast distance without loosing much energy). When these swells and the waves the create on the coast are larger than normal and extend over a longer period they can force water to bank up close to shore, this is known as wave setup. Strong winds can also lift waves further, this is know as wind setup. All or any of these phenomena are usually explained as a storm surge because they also produce nearby storms, but a storm is not necessary to the development of a "storm surge". When a spring tide corresponds with the conditions such as low atmospheric pressure, large swells and winds, tides can be extremely high compared with average.

The geometry of the sea floor close to the coast (bathometry) also has a large effect on how tides and these additional surge might be amplified (or cancelled).

But how high is extreme for Venus Bay?

And How often might that occur?

Saturday, December 6, 2008

What are the consequences of the extra erosion?

The simple answer is I'm not sure, that’s why this project was set up. Maybe with more observation the causes and consequences can be better understood. Here are a few likely short term issues

1) Because the lower part of the beach profile is flatter and lower -

1.1 Shell beds are exposed and partly eroded in many areas, so it is likely that the shell population will be reduced this season (eg the gatherers of pippis might find they are scarcer)

1.2 There may be more sandbar development early in the season (rather than mainly in late summer autumn). This could lead to significant changes and shifting of gutters and rips

1.3 Near shore current may be variable, even on a daily basis, as the sand shifts. The gutters may be deeper or even dissapera or rips suddenly become stronger (as more water empties from a broader beach) and potentially more dangerous.

2) Because the summer berm is limited or not developed at all -

2.1 Shore bird nesting habit could be significantly reduced (eg hooded plovers)

2.2 Less dry sand will be available to repair erosion and/or build new dunes

This pair of hooded plovers are attempting to nest south of beach one. One set of scratching (nest preparation) has already been washed away in early November. They now have now approximately 100m of sumer berm developing in their prefered nesting site, which is encouraging. It is roped with a sign at the northern end to try to discourage humans (and more particulalry their dogs) blundering across their nest. The Easters Hooded Plover are considered vunerable, because of low breeding success.

Help Save our hooded plovers