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.

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.

So this is summer …

What is the saying? “Beautiful one day, overcast the next” …Welcome to Venus Bay 2012

This summer so far has not been the typical hot blazing sun and deep blue cloudless skies. yet on the beach there are signs that it has been a much more typical summer than the past few years (at least for the beach profile)

The Erosion at the top of the beach seems to be very limited (last big events being in November) and the scarps are drying and collapsing leaving a steep wedge of sand rather a vertical cliff (except where marram grass clumps are hold up some areas (as shown above). The Average Height Tide Line is 10 to 20 meters seaward from these scarps, which has meant that the flatten out upper part of the beach (the summer berm) has been able to be strongly developed along the beach. This an area where dried out seaweed and other beach debris can accumulate. the sand dries out and is more susceptible to wind erosion so this part of the beach is flatter.  This is an important habitat for the shore nesting birds (like the hooded plovers). Hopefully they are enjoying this summer on the beach.

BTW its the Walk out on Point Smythe TODAY!

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.

Biggest Moon, maybe. Highest Tides not yet

There has been a lot of speculation and sensationalism on the net and news about the giant moon tonight. Ok it is as close to the earth as it gets and it is a full moon. BUT it is only fractionally closer and still a long long way away. If you can see the difference you may just be dreaming!

What about the giant tides, well we do get higher tides at around the full moon, but remember it is the combined affect of the sun and the moon not just the moon alone and that will be a little stronger in a few days (see tide chart above, compliments of willy weather). so look for extra high and low tides Wednesday and Thursday.

High Tide, Narrow & Closed Beach

2:54pm 1006hPa almost high tide, find the beach narrow and the strong swell and currents see the beach closed to swimmers.

No erosion

Low & Flat

Beach One at 6:48 pm 1009 hPa. A low low tide, a wide flat beach and exposed sandbars.

Does Venus Bay get King Tides?

With a lot of current focus on flooding and King Tides it is a good time to answer the question.

"Does Venus Bay get King Tides?"

It certainly does!

In fact the next ones are due now and over the weekend. We get higher tides twice each month corresponding roughly with the new moon and full moon (look outside tonight and you will see a full moon). These are also known as spring tides (despite the fact that they occut all year round) and also represent the times when the difference between high and low tides are at there maximum. So from tonight till monday will will have higher than average tides overnight and lower than average low tides in the late afternoon and evenings.

I found this graph on a great site called willy weather, which as well as putting weather details on the net for a lot of Australian locations, including focasts for Venus Bay (they are pretty much taken from the Bureau of Meterology's Pound Creek station), They also sell lots of things to do with weather and tide prediction, worth a look if you are interested in such things.

Wikipedia has a fairly comprehensive page on tides, if you are looking for more detail.

Remember atmospheric pressure helps raise and lower the sea level. So a high pressure system, as we are in right now (1012 hPa and rising) will lower the sea level and stop the higher tide reaching so high but helping the low tide go lower, compared with the predicted level in the chart above or any preprinted tide chart.

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.

Maxium Tidal Range for 19 years, over next three days

Over the next three days as we approach the full moon the tides at Venus bay are predicted to be at the highest and lowest with a 18.6 year cycle. Coresponding when the moon and sun being in direct mutual alignment and at there closest points to earth. This can occur on either a new moon or a full moon. (there is a reply in this forum that explains this in more detail)

Probably the most interesting place to make observations will be at low tide. When the recents movement of sand should form interesting ruts and pools.

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.

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?