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.

First Feathers

There where a lots of black feathers on the beach today, but no mutton bird carcasses. This possibly marks the return of the migrating shearwater (mutton birds). Let’s hope not as many perish this year.

Having fun …BUT

 
These very noisy mechanized wave runners are obviously have fun, albeit in a high risk way BUT they certainly didn’t impress the other beach one users. The fishermen were muttering about yoboos, the pipi gathers were yabbering about the strong smell of the spilt fuel and the tradition board riders shaking their heads in despair, What I notice is their wasn’t a single bird to be seen anywhere close by. Yet up at beach five there were plenty.
 
Is this a case of NIMBY over-reaction or is jet skying one of those recreations that really does needs to be controlled in national parks?

Venus Bay Inspired Art

The Last Pipi Harvest at Venus Bay

Mixed Media on Canvas
102 by 102cm
By Norm Hanson 2012
This is a story of anthropogenic extinction, as written into future stone. The ochre coloured sandstone [1] was at one time a thriving beach, brimming with life and specifically a prolific population of Donax Deltiodes, also known as Pipis or Surf clams. However the Pipis were so yummy and they were harvested in such numbers that fewer and fewer shells reached maturity each year. Then suddenly in one season, the entire population failed to reproduce. Worse the rich food chain of the beautiful beach collapsed, with their demise, and the once thriving beach became a pale version of itself. Only a few brave worm burrows [2] could be found. Then all signs of life disappeared [3].

This story need not come true.


The work was created first as a mould of the main units in the sand south of Beach one. This included leaving some of the worm burrow/air holes undisturbed. broken Pipis and other shells where collected nearby and scattered in the mould. The mould was filled with expanding polyurethane foam and cover with the canvas, burried and left to harden for an hour or so. The foam on canvas mould was next painted with several coats of sand, gesso and latex to create an absorbent sandstone like finish. Finally this was “stained” and painted with various oxides. It represents a future geological rock face, perhaps a few million years into our future. This rock face runs close to the bedding planes with the oldest layers, the ochre sandstone on the lower left, closest to you. You are looking "up" in time.

This work will be on display at the Mount View Art Show, 14th - 17th June 2012

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!

Alone at last!

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.

The Hooded Plovers Web of Life

I found this wonderful illustration on the Culture Victoria website under stories/ Land & Ecology.

The first ring has all sorts of tiny crabs and critters that Hoodies like to eat. The second ring shows some of the birds that share the beaches with Hoodies - some that stay there all year round and others that fly long distances to spend summer on the beach in Australia. Most birds share the crabs and critters with the Hoodies, but others like to eat the Hoodies, and other birds!
These creatures are linked together because they need each other to survive. Of course, humans are part of this web of life, and we have an affect on how it works.

More details on the CV website

Recreational Pressures

Whilst the rest of the nation celebrates Australian Day, maybe the beach at Venus Bay is getting just a little more stressed each year. Certainly the number of “pipi tourists” peaks on public holidays like Australia Day. There are strict guidelines for pipi collection and hopefully we can avoid the fate of Port Stephens in NSW.

This year has also seen the common sighting of jet skis on the beach. Whilst these particular craft were used sensibly and away from swimmers, others where spotted “hooning” around over summer. It is worth remembering it not just beach goers, the fish and birds will also be distributed.

Once upon a time (just a few years ago) I seldom saw any rubbish on the Venus Bay beaches. Other than typical ocean flotsam and jetsam. Now a short look at the accumulations above the summer berm will show a surprisingly lot of litter, particularly plastic bottle tops, disposable bottles and plastic bags.

All of these stresses can be easily minimized by us, the beach users, so that Venus Bay can remain the wonderful stretch of beach it is today,

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.

Beach-Nesting Birds Workshop & Beach Walk

Learn about Birds Australia’s Beach-nesting Birds project and how to safely monitor nesting
birds. All welcome!

Location: Venus Bay Surf Life Saving Club (Surf Drive)

Date: Saturday 6th November
Indoor presentation: 11:00 am -12:00 pm
Break for lunch 12:00 - 1:00pm
[Bring along your lunch; tea/coffee/juice and cake will be provided]

Beach walk*: 1:00pm departing Number one beach (end of Surf Drive), heading east along the beach. We will walk at least 1km and depending on your fitness, there is the option join us on a longer walk.



* For those going on the beach walk, please be prepared for the weather and bring along wet weather gear or sun protection as required. If you have binoculars, please bring these along. You might want to bring along a bottle of water to keep hydrated.

Drifting North Again

This panorama was taken at the beach 5 car park and shows how the sandbars are attached to the beach at the southern end (see this photo full size). You can also see the ruts developing into rip channels also point to a return to genrally northerly longshore drify

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.

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?

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.