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?

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?

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

A steeper beach profile

If you look at the beach profile at the moment it shows both erosion at top of the beach (backshore) and also in the areas exposed at low tide (foreshore)

This is not quite what the theory suggests might happen with erosion due to rising sea level. When the top of the beach erodes the lower section should be filled with the sand eroded, in other worlds the whole beach profile should lift. This is described in what is know as the brunn rule. However the beach at venus bay often has this form in winter

This photo was taken at beach one on 2/11/2008 at 11:17 am

If you notice new erosion scarps at the top of the beach (or at the base of the dunes). Take a photo, note the location and time and uploaded it. Just into the photos on Flickr (with the tag VBOP) and/or make a blog post here about it.

At low tide take photos of what is exposed.

So here is a summary list of what you may be able to include include

1) Take a photo, if you can. of the erosion
2) Record the place, data and time you took it

Any of the following information is likely to be helpful, so record that as well if you can

3) Look up a tide table and record the last high tide
4) Note the air pressure, and also if it is rising or falling
5) Describe the weather, and wind
6) Describe the waves (and swell)

Undertanding some names for parts of the beach

Before getting too involved with recording changes in the beach it is a good idea to become familiar with a few of the technical terms used to describe parts of the beach (or maybe more technically the coastal zones)

Note: this diagram is not to scale and is also vertically exagerated

There are two commonly refered to lines along the beach. The first is the coast line and it is the line that appears on maps. It marks the foot of cliffs or dunes, and it is usually marked by a distinct change of slope. This is the most important area to measure and observe coastal erosion (often called coastal retreat). The second is the shore line, which is very hard to see and define. It is conventionally described as the conceptual intersection between the high watermark and the shore (beach) and is usually only loosely indicated on navigation charts or nautical maps. This line is really difficult to define in practice beacuse the high water line (MHWL, mean high waterline) is a fleeting event and as we will see changes over time.



The Beach (you may see it called shore in american texts) actually extends past the shoreline out to the low tide point (MLWL, Mean Low waterline). After that the sandy slope and water above it is know at the Littoral Zone. This extends out to the closure depth, the conceptual depth beyond which there is no longshore or cross shore transportation of sand. Usually the breaker zone extends out towards to this depth.

What is happening to the beach?

The beach along Venus Bay has been eroding significantly this year, with many high scarps left around beaches 1, 2 & 3, So what is going on? Superficially it looks like it might be evidence of sea level rise, perhaps strong evidence of global warning. However if that were the case the beach should be higher and narrower, whereas the beach is wider than normal for this time of year and lower.



The erosion has been occurring at specific location associated with specific high tide events. The king tides (higher spring tides) and some storm tides (mainly due to strong onshore winds & swells rather than storms) have been responsible for specific stretches of erosion rather that a continuous event along the beach. At the same time the beach profile has deepened exposing shell beds in many locations and is clearly lower at most locations,



I suspect that many of these changes started last spring with the king tides, when a few erosion scarps occurred in only a couple of places along the beach, At the same time the normal change of the profile from its winter shape to summer shape did not occur along much of the beach at all last summer. The winter berm, (the beach’s winter shape) has a distinct parabolic shape with a steeper gradient at the back of the beach. The same winter profile appears to be continuing this year. The summer berm, (the beach’s summer shape) has a flat upper dry section at the back of the beach and long flat foreshore with extensive development of sandbars. This flatter profile in the tidal zone now seems to be occurring at many places but under an obvious winter berm at the top of the beach.



I have been trying to work out the source of all the sand that builds the venus bay spit and direction of sand drift for a few years now. I have also noticed that last year the dominant sand drift direction has been fluctuating locally from a general northern drift over previous years. My current thoughts are that the beach is not washing away, the sand is just being moved up and down the coast but that the beach profile has been rotated down a little, I suspect that the shape of the coast is trying to adjust to a subtle change in the equilibrium of the forces that shape it, currents, swells, tides, storms and climate generally.



This blog is a place for everyone to recorded their observations.