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.

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.

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.

Another light show in the sky

On the morning of the 13th August there will be an aligment of Venus Saturn and Mars, and a crescent moon in the eastern sky. Unfortunately it will occur a little after sun rises and not break on the eastern horizon till almost 8am. What you might see with the naked eye is a thin cresent moon and a bright star (venus) nearby The famous Perseid meteor shower should then becoming to its maximum at roughly the same time on the northern horizon, or slightly east of north. A great place to view this is from the bike track to Lower Tarwin on the rise at the start of the track near the intersection of Jupiter boulevard and Lees Road.

View Viewing perseids meteor shower in a larger map
Look across the adjacent field at the horizon to the right of the Pine trees close to Lees road, just before dawn, for the best chance of viewing meteor trails.

The moon, sun and planetary alignment in conjunction with the current lower air pressure (stormy weather) is likely to produce a very high tide and lead to significant new erosion at the top of the ocean beach profile.

There is a great article on viewing the alignment & meteor shower by Dr Tony Philips of Nasa, but remember his times and date relate to the Northern Hemisphere.

Venus & the Crescent Moon

Venus is visible as a very bright start at and just after sunset in the west and yesterday it was very close to the crescent moon.

A great photo opputunity!

By the way Venus Bay is not named after the planet Venus. It was changed to Venus Bay by Francis Peron on one of Baudain's Charts, and is thought to have been named after George Bass's ship the copper sheathed brig Venus. In appreciations of Bass's kindness in providing the distressed French expedition with fresh food.

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.

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?