Topic: Storm - overview

Topic type:

Tongaporutu Coastline - mega storm, Three Sisters

 

OVERVIEW

 

This section also acts as a Reference Section in that it will include every single date and every single place that I visited on the Tongaporutu coastline.  Importantly, it also includes every photograph that I took on the specific dates.  This is given by Puke Ariki Museum’s photo number.  For example, PHO2007-194, etc.

PLEASE NOTE:  The only photo that isn’t included under dates here is that of the MAP, PHO2008-361a.

Though each section is separate, they are in effect all family members.  Just as in real families, some members are more closely aligned than others.

Section Four on Cliffs, Section Five on Sea Caves and Section Six on Sea Stacks are closely aligned to each other.  As such they are grouped closely together.  The same also applies to Section Eight on Beaches and Section Nine on Flora, Fauna and Wash-ups.

This section will primarily consist of the date, the location(s) visited, the tide(s), the moon phase where known, then the weather and any other relevant information.

On 2 June 2011, I took a series of aerial photographs in sequence from Te Kawau Pa right down to White Cliffs. Also included were parts of this coastline that are normally inaccessible.  The photo numbers are:  PHO2011-2377-2415, 2418-2446.

 

TIDES

With regards to the tides, they are not straight forward.  The tide ranges mentioned here relate to the Tongaporutu coastline only.  Tidal ranges may differ at other locations.

At Tongaporutu, high low tides usually range between 0.8m and 1.4m.  They are opposed by low high tides that range between 3.0m and 2.5m.  (Specifically, L= 0.8m, H=3.0m and L=1.4m, H=2.5m).  Very low tides usually range from 0.0m to 0.3m.  They are opposed by very high tides that range from 3.9m to 3.6m.  (Specifically, L=0.0m, H=3.9m and L=0,3m, H=3.6m).  Mid-range low tides are from 0.4m to 0.7m.  They are opposed by mid-range high tides of between 3.1m to 3.5m.  (Specifically, L=0.4m, H=3.5m and L=0.7m, H=3.1m).

Very high and very low tides are usually referred to as ‘King tides’.  Generally speaking, these big tide extremes coincide with either a full or new moon.  However, this is dependent upon the time of the year and planet alignments.

During periods of very high tides and very low tides, the tides come in and go out more quickly than during periods of very low high tides and very high low tides.  This is because though they go out and come in, in the same amount of time as the other tides, they do so over a greater distance.

 

CLASSIFYING STORMS

My classification of storms is predicated on my observations of the Tongaporutu coastline.

I quickly learned that not all storms are born equal, but how do you differentiate one storm from another?  Generally speaking, the further out to sea the ‘wash zone’ extends, the more powerful, or energetic, the storm.  With regards to the Tongaporutu coastline in particular and I suspect New Zealand’s west coast in general, another indicator of a storm’s power is the amount of sea-foam it generates.  This makes it easier to quantify a storm’s true power because it washes up to your feet!

 

COMMON  STORMS  (Fronts, troughs and depressions)

These are your every day fronts and depressions that form the backbone of the more regular weather patterns.  Some can be quite feeble things, while others pack a real but generally short-lived wallop, like intense rainfall, thunder and lightning, wind downbursts or tornados.  Some can also be ‘straw breakers’.  These usually occur (but not always), within a few months after the coastline has been hammered by a much larger storm.  The susceptible parts of the coastline that didn’t fail at the time, are then ripe for one of these ordinary storms to finish the job.  Equally, a seemingly insignificant event can set up part of the coastline to fail during the next big storm.

 

ALPHA STORMS

These are extremely potent, usually large, individual storms.  I have only recorded two stand-alone alpha storms up at Tongaporutu, both in September.  (The others I have recorded were part of super-storm events).

I observed the first alpha storm on the 29th September 2003.  It coincided with a 3.7m high tide.  The wind was originally from the north-west.  It later changed to the south-west.  This storm destroyed the Little Sister and the Little Brother rock stacks.  It also had a distinctive ‘hum’.

The second alpha storm occurred on the 19th September 2005.  It coincided with a 3.9m high tide, the biggest tide of the year and took out a large chunk of the cliff face on the landward side of Cathedral Cave.  The wind was originally from the west, north-west.  It later changed to the west, south-west.  My barometer at home recorded a barometric pressure of 964 hPa.  This is the lowest pressure I have ever personally recorded.  Generally speaking, the lower the barometric pressure, the higher the sea level, the greater the storm surge.

Weather bombs also fall into this category.  Some, like the weather bomb that devastated south Taranaki and other areas on 3 March 2012, can reach hurricane force.  This particularly destructive weather event was equal in power to a Category two hurricane.

 

SUPER-STORM EVENTS  (Multiple storms that occur in quick succession to each other).

I have recorded two super-storm events at Tongaporutu.  Both were preceded by a period of wet weather that set things up to fail.

A super-storm is where, say, three separate storms run so close together that they in effect become a single event.  This is because the time between each storm is insufficient to allow any meaningful recovery.  Thus each successive storm increases  the damage inflicted by its predecessor.  This is an important point.  It’s not the size of each individual storm that marks a super-storm;  it’s the cumulative effect of all three individual storms together.  Another important point is that the likelihood of at least one of the storms coinciding with a high tide exceeding 3 metres is all but guaranteed.

The first super-storm event I recorded was in February 2004.

STORM ONE was observed on 15.2.2004.  High tide was 2.7m.  There was steady, persistent rain with no wind at all.  The wind, when it did arrive up at Tongaporutu (after I’d left), blasted in from the south-east.  In fact, the contrast couldn’t have been more stark.  Up at Tongaporutu it had been so calm that I’d been able to stand on the cliff-top photographing in the pouring rain with my umbrella up.  However, when I entered the Uruti Valley on the drive home, I slammed into a wall of wind that had left smashed down trees in its wake.  This storm, which was also an alpha storm, went on to devastate the southern half of the North Island.

STORM TWO was observed on 22.2.2004.  High tide was 3.7m.  The wind was a stiff westerly.  This storm was notable for its huge swells.  These subsequently  generated giant waves that smashed into the cliffs.  I also recorded that the cacophony of noise down on the beach was absolutely deafening.

STORM THREE was observed on 29.2.2004.  High tide was 2.5m.  The wind was originally from the north, north-east, an unusual quarter for Taranaki.  In its dying stages it changed to the south-west.  This storm, like Storm One, was also an alpha storm.  It was the combination of a deep depression that had combined with the remains of Tropical Cyclone Ivy.  This particular storm was notable for its extreme rain event.  The rainfall was so heavy and intense that it literally melted the Whitecliffs Walkway road.  It also caused rock cliff collapses and gargantuan soil bleeds from the predominantly soil cliffs of Beach One.  In fact the runoff was so massive that it stained the Tasman Sea a reddish brown for some considerable distance up the coast.

Apparently these storms were made more severe due to the higher than normal sea temperatures of the Tasman Sea.  Added to this, February is not generally renowned for its storm activity.

The second super-storm event I recorded was in July 2008.

STORM ONE was observed on 20.7.2008.  High tide was 3.2m.  The wind was from the north-west, later changing to the west.  This storm delivered raging seas, howling winds and horizontal rain.  It also destroyed the Pilot Point arch, the Pilot Point cave system on the northern side of the arch and a partial cave/cliff collapse on the southern side of the arch.  This is what I call a ‘ripple effect’.  I discovered later that this storm had also most likely caused the major cave collapse at the Twin Arches cave system.

I now quote from my diary entry.  “Down on the Three Sisters Beach the vigorous depression had monstered the beach and the coastline, from what I could see of it.  It is the worst destruction I have seen and quite surprising as I have documented more potent storms.”  I concluded that this was an alpha storm.

STORM TWO was observed on 24.7.2008.  High tide was 3.1m.  This storm, the worst of which roared through yesterday, was an alpha storm, like two of the three storms in the February 2004 super-storm event.  It packed north-westerly winds, heavy rain and generated up to 7 metre swells.  The seas were such that they destroyed the sand/land bridge that had connected the Three Sisters Beach dune to Mammoth Rock (Pa Tangata).  This had been in place for at least 50 years.

STORM THREE was observed on 3.8.2008.  High tide was 3.4m.  The wind was from the north-west.  It was cold and showery.  This last storm was the most devastating.  Not because it was bigger than its predecessors, but because of the cumulative effect of the other two storms which were much more powerful than this more ordinary one.

Relative to the Tongaporutu coastline, I have found super-storms to be the most destructive storm events of all.

 

MEGA-STORM

A mega-storm is the T. Rex of all individual storms.  They are the chart-toppers.  Hurricanes, typhoons, cyclones and the massive super-cell thunderstorms that occur in America’s Tornado Alley, all fall into this category.

The first and only mega-storm that I have observed was on the 18th September 2010.  (Note the month, September again).  At its peak, this storm, which originated in the Southern Ocean, was the biggest storm on the planet.  Its barometric pressure was a jaw-dropping 950 hPa.

One of the things that saved the Taranaki and Tongaporutu coastlines from catastrophic damage was that this storm coincided with a relatively low 2.8m high tide.  Another thing in our favour was that we only copped the top of this massive storm.  Despite our distance, conditions up at Tongaporutu were still absolutely hideous.  Southland, being closer to the storm proper, lost three million newborn lambs to the freezing blast, while Invercargill lost the roof to its sports stadium.

A STORM’S ENERGY

Relative to the Tongaporutu coastline, each storm has three main energy sources.  They are:  Wind, sea and rain.  At different stages of a storm’s life-cycle, one energy source will be at a high end.  For example, it can be teeming down with rain, but the sea can be calm with no wind.  This is often linked to a storm’s location, the direction its travelling and its rotation, (anti-clockwise in the southern hemisphere).  For example, if a storm is passing down the east coast, then Gisborne could be deluged with rain, while Taranaki on the west coast, could be hammered by south-easterly gales.

Different parts of the Tongaporutu coastline, (as do other coastlines), have their own natural frequencies.  These form a vibration signature which is a combination of the sum of its parts.  Another factor is the Tongaporutu River.  Which side of the estuary does it empty into the sea from?  The Pilot Point side (north) or the Three Sisters Beach side (south).  Then there is the volume of water and speed of flow at any given time.

If a storm’s dynamic forces produce a vibration signature that reinforces any of the coastlines, then you have a super-position of forces and catastrophic damage can occur.  This damage tends to be quite specific.  A prime example of this was the alpha storm of 29.9.2003.  This storm destroyed the Little Sister and the Middle Brother rock stacks, but left the cliffs and other rock stacks relatively untouched.

Equally, if a storm’s vibration signature cancels out that of most of the coastline’s structure, then you still have a super-position of forces, but one where little or no damage is done.  This ‘super-positioning’ explains why some storms do tremendous damage, either overall, or just affect specific parts of the coastline, while others, seemingly just as powerful, do very little damage.

SURFACE DIMENSIONS

Then there is the cliff face surface area to consider.  Generally speaking, the more three-dimensional the surface area, the more subject it is to fracturing, cave forming and carving off.  The more two-dimensional the surface area, the less subject it is to fracturing, cave forming and carving off.

The same principle applies to the bays, inlets and coves.  The more sharply indented/curved they are, the more vulnerable they are to wave reflection induced cliff/cave collapses and erosion.  Relatively straight run cliffs are less susceptible to wave reflection and as such are more stable.  For example, the cliffs in the middle part of Rapanui South Beach are relatively straight and more stable compared to the more highly indented cliffs on the Three Sisters Beach which are extremely unstable.

Perhaps this dimensionality can be extended to the weather.  For example, the more energetic the weather system, (cyclone/low pressure), the more three dimensional it is.  Conversely, the calmer the weather system, (anti-cyclone/high pressure), the more two-dimensional it is.

Weather systems and coastlines are subject to fractal scaling.  Further, fractals are associated with Chaos Theory.  Think butterfly wings and hurricanes.  Small changes can have large effects down the track.  And large changes can have enormous effects ...

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