Beach by Beach Erosion Panormas

Beach One

Beach Two

Beach Three

Beach Four

Beach Five

Significant Erosion Event, Particularly at Beach One

I have been looking for the effects of sea level rise, and in particular related erosion along the coast, for some time now. What I have observed was this erosion is not as simple as many sea level rise models might suggest. In particular I had not noticed so much erosion at the pathways leading down to the beach. This suggested the ocean was not advancing a fast as many fear,  There was erosion, but it would be just one of the 5 “official” paths at a time that would show erosion at the base of the path. This along with other observations suggested to me that the erosion was mainly localised at any given time. This year has seen more significant events but they still seem somewhat localised.

At the moment there is a very big “step” at the bottom of the beach one ramp (about 1.2m), where recent high tides have washed deep back into the dunes and also the revegetation fencing and drainage pit. Again there is ample evidence to suggest that the amount of erosion is more localised, rather than generally along the beach. Although this years king tides (around the time of full moons) and strong swells have lead to more erosion scarps along much of the coast than over the past couple of years.

The super moon term has caught the public’s imagination, but I strongly doubt it alone is responsible for a super tide. The moon was however a full moon and it occurred as the moon is closest to the earth, so there will be a contribution. The real culprits are combination of three additional things. Strong swells associated with a significant storm front last Tuesday (often this is referred to as a storm surge) The low air pressure associated with the storm which crossed the coast at Venus Bay just on sunset. Finally the presence of deep gutter (rip area) just south of beach one ramp (see photo below) which focused the wave action at a small semi-circular area in front of the ramp.

Focusing on Andersen Inlet erosion risks

I guess there has been a lot of discussion in this blog about changes on the surf beach at Venus Bay but in reality the most significant erosion is going on all along Andersen Inlet.The deeper channels (think of them as a submerged meandering but tidal streams) are snaking back and forth changing position within the inlet progressively widening the inlet and eroding its banks (particularly at Inverloch, on the north eastern side, and around doyle’s road, on the western side. Sand is progressively  moving deeper into the inlet and this hinders the development of mangroves, which are really the only way to slow down eroding the shores. The open “beaches” allow waves to undercut the peat layer of the salt marshes and once compromised like this the salt marshes quickly wash away. Trees offer only slightly more resistance but they soon also succumb to the erosion.

Kings Tides and beach profile

At the same time as last weeks, transit of venus there was also a full moon and a very low atmospheric depression out to the east over the Tasman Sea.. Perfect conditions for a really high tide. I did check the beach on the 7th. and could only see the old erosion face from last year and the regrowth on the sand screen below them. There was a slight notch in several location that looked as if it could be more than a few days old but it was not very extensive (these are labeled in white in the above photo). I revisited the same areas south of beach one today and can see a fresh cut (only 5 to 20cm tall) but extending most of the way along the beach (labeled in yellow). Also the beach below the cut does seem to be “steeper”. So I suspect there has been some significant erosion/sand movement along the beach in the past week. So the picture that the erosion events are most likely to occur with the king tides (full moon) and atmospheric lows does have a little more circumstantial support.

Sand in movement

Panoramic photo taken on Tuesday at Beach 1 showing some erosion occurring to the fore-dune. Notice the water pooling in the bottom right.

This next photo is facing SE at the same site and the undulating effect from the pooling water extends to the horizon.  Truly a dynamic environment!

Adventures in Environmental Science

The Rounded Noon Flower Disphyma crassifolium subsp. clavellatum, sometimes called Pigface, an endemic species of our coastline are coming into flower now.





I would also like to post a link to a film I made called Adventures in Environmental Science which is about some of the activities of the Venus Bay Observation Project.


I am hoping that those who do view the film would find it to be educational and entertaining, but most of all I hope it inspires others to be brave and become involved and add their contribution to the Venus Bay Observation Project.

Does Marram Grass Encourage Deeper Erosion?

The more I look the more I see evidence that recent erosion is strongest (ie leaving the tallest erosion scarp) in areas where the dunes are heavily covered by marram grass. Compared with areas where only runners of coastal spinifex, appear to leave a partially covered and “less well protected” dune front. These two photos where taken from the same location and the difference is very clear.

The photo on the right is looking slightly north to an area of heavy marram grass on the dune, and two recent erosion events. Further the beach infront of this is still showing its steeper curves winter shape. Compare this with the photo on the right, just taken straight-on here only a few runners of the coastal spinifex have regrown in front of previous erosion, but the slope is already flatter and the beach infront of this point has already started to develop its summer shape. This is often termed the summer berm. The beach above the high water mark has started to dry out and the curve of the upper becah is being flattened by wind erosion, leaving the characteristic wind blown ripple marks, with extra seaweed & other heavier debris to collect sand with wind gutters around the base and often a wind shadow of dry sand. It is the development of this summer berm that provides the best home for beach nesting birds, a rest place for animals and migrating bird (like penguins, terns and seals).

For a long time I have assumed that the taller erosions scarps are associated with deeper gutters out in the sand bars, and I still see that there will be a distinct gutter or channel in the vicinity of fresh erosion but the relationship with marram grass on the foredunes does seem to be becoming more compelling.

So  the big question is -
Does marram grass encourage deeper erosion of the dunes?

Storms & More Erosion

The past few weeks has seen more erosion along the Venus Bay beach in many locations. The profile of the upper beach is also steeper (more the classic winter berm) in front of the recent erosion. The erosion scarps are up to 3m high, probably the highest this year.

These are classic features of low pressure (storm surge) corresponding with higher tides and strong on-shore swells. You’ll see the sand is still there, as a wider beach and sand bars at low tide.

Two Generations of Erosion

At many location along the Venus Bay beaches, you can see two generations of erosion on the one dune face. The upper erosion scarp (a geological term for the small vertical face, escarpment, created when waves washed away some sand)  has itself been partly eroded. As the sand face dries out it becomes unstable and collapses along wind picks up other dry sand and piles it infront of the face. Re-grown of saltbush grasses help stabile the sand collecting infron of the dune. Eventually the scarp itself will dissapear and the dune just have a steep front face. The recent erosion can be seen at the base as a new vertical face, with a areas where blocks of unstable sand have already tumbled from the sand faces above.

REMEMBER: These sand faces are remain very unstable, even if they sometimes look dry and stable.Even just a single square meter of wet sand will weigh close to 2 tonnes enough to fatally bury an adult. So children (even grown up ones) must be discouraged from playing on such erosion scarps

Still Eroding

 

Having been away for a couple of what I have come to know as the most active erosion months (August and September, presumably because of the strong swells and storms are common in these months). I was surprised that maybe there had not been as much erosion as I was expecting. Still quiet a lot of the beach shows an erosion scarp of 1 to 2m. I was also surprised to see so much seaweed on the beach (perhaps the swells have also disturbed the kelp forest off shore).

This location (just south of the beach one ramp) also shows clearly something I had noted before. The fenced “rehabilitation” area, has mainly coastal spinifex, where as the eroded area to the south is dominated by marram grass. Perhaps the flatter slope angle of the dunes with coastal spinifex lets the occasional high waves run up their surface rather than starting an erosion notch.

Bigger swells and more dune erosion

I was keen to have a look at the beach and see how the big swells of last week, affected the beach. Swells bought to our coast by really big storms and heavy weather in the southern oceans. The surfers where disappointed because the winds where not off shore, to lift the waves, but the strong on-shore winds made matters worse for erosion, that push the water up against the coast even more and clearly helped the swells and waves they produced reached right up to the dunes. Even though we not quiet to the full month.

The result is a lot of new erosion, not everywhere but obvious all along the beach.

Taking on the winter shape…already

The beach is loosing the distinct flatter area of dry sand from the upper beach zone and a continuous sloping curve, typical of the winter shape (or winter berm) has quickly developed for the beach profile. The reason is clearly the strong onshore winds, strong swells and powerful waves crashing onshore. The highest tides are however not reaching the scarps from previous erosion.

In previous years these changes have more typically occur in April/May.

Why is the summer berm returning?

Undeveloped ocean beaches like Venus Bay typically have a different shape in summer and winter. Normally in Australia these are called the Summer Berm and Winter Berm. At the moment you can see both shapes and we haven't seen a true wide summer berm develop along the beach for several years (possibly 2006/2007 was the last period).

So What is a Summer Berm?

The term Berm has a few different meanings, It can be a mound of earth set up to protect low lying areas behind it (a levy bank). It the USA it often is the name given to a beach or flat river bank or even the edge of road. Any sub level surface will be called a berm but the flat/horizontal part will be called a Bench , the change of slope called the Berm Crest and the sloping front face the Berm Slope.

During summer when the waves and swells are generally calmer the top part of the beach start to dry out and wind action starts to level out the beach changing its gradient and making it flatter. A surprising lot of sand can be moved bu the wind and often extra sand is "stolen" from lower wetter parts of the beach and builts up this top part of the beach. The first sign most of us will notice is a soft sand pile built up at the bottom of the ramp or steps down to the beach.

In winter, storms and pounding winter swells bringing a lot more wave energy to lift and redistribute the sand both on the beach and below the waterline. The shape is now a single gentle curve getting steeper up the beach. Whilst the winds can be stronger in these times the sand is generally very wet so less wind erosion in involved. The winter shape has dominated changes in the Venus Bay beach over the past few years.

I am beginning to suspect the return of the sumer berm is related to the general change to a northwards longshore drift.

It will be interesting to see if the hooded plovers and other shore birds have a more succesful breeding season with a wider upper beach and less risk of nests being washed away.

What is a Spit?

The geomorhpic definition of Spit is a sandy bar projecting from the mainland into open water. Spits are formed by deposition of sediments moved by longshore drift. (here is a wikipedia link on spits, if you want more details)

So the Big question; Is Point Symthe a Spit?

Well it is widely described as a spit in tourist literature guide books, Often as Victroia's largest spit. However there may be a few problems calling it a true spit

1) The Andersen inlet estuary is no longer "open water" and there is a very strong tidal inflow and outflow cutting across the longshore drift

2) Observations in this project suggest that the longshore drift does change directions and has been larger going southwards for the past few years

3) It doesn't have the characteristic hooked end.

Yet physically it is a very large continuous section of sand and the present situation may represent a very late stage of spit development. Where by the entrance to Andersen inlet may close over with a continous sand bar from Point Symthe to Inverloch.

However as Andersen inlet gradually fills with sediment (not sure if anyone is accurately measuring this) The effects of the tidal flows (it is the same amount of tidal change but in a progressively shallower inlet) mean that there is now considerable erosion cause along the main chanels giving outlet to the sea. These infact are behaving like a meandering streams thrashing back and forth across the inlet floor and heavily eroding the coast & beaches but just at very specific locations (where the chanels curve most).

Who will win? Longshore drift or the tidal flow from Andersen Inlet.

Why are Moon Phases Important?

I've included a phases in the moon applet over on the right because the position of the moon, more particularly the alignment of the sun, moon &earth, has a big influence of the magnitude of our tides. We see these tides as a roughly twice daily rise & fall of the sea level. The so called spring tides, are not just limited to spring (or autumn), they occur roughly twice a month (as the graphic above shows) at either the new moon or the full moon. Other times of the month see lesser tidal range because the gravitational pulls are not aligned. You can get the predicted tide times from the Bureau of Meterology Website (Venus Bay tides are roughly the same time as for the Port Phillip Heads)

Venus Bay does get higher tides in spring and autumn that have a significant extra height due to strong ocean swells and waves. Sometimes these highs can be forced higher by a storm surge or strong on-shore winds.

So the new moon this weekend make it a candidate to see more erosion, possibly higher than last month.

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.

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?

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)

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.