Quite a good example of wave setup - Cue Graham?

https://twitter.com/severeweatherEU/...06437466337280

--
Norman Lynagh
Tideswell, Derbyshire
303m a.s.l.
https://peakdistrictweather.org
Twitter: @TideswellWeathr

On Saturday, March 24, 2018 at 12:11:02 PM UTC, Norman Lynagh wrote:
Quite a good example of wave setup - Cue Graham?

https://twitter.com/severeweatherEU/...06437466337280

--
Norman Lynagh
Tideswell, Derbyshire
303m a.s.l.
https://peakdistrictweather.org
Twitter: @TideswellWeathr

It certainly is!

It can be very dangerous when it occurs in benign weather, especially with the tide out allowing the sea to surge right up the flat area of beach.

I remember a lovely quiet summers day about 10 years back now. There were occasional sets of really big waves. The lifeguards closed the beach at Sennen, but had a nightmare job keeping people from the waters edge at low tide.. You could paddle in harmless waves in a ft of water one minute, then suddenly you'd be out of your depth even between the waves and the sea would surge half way to the high tide mark.

If anybody had drowned it would of course have been a freak wave!

Graham
Penzance

snip

Norman Lynagh wrote:


On 16th Dec 2016 there was a westerly swell with a period of 17-18
seconds propagating eastwards through the English Channel. At the
western end of the English Channel the significant height was 15-18
feet, decreasing eastwards. These were fairly unusual conditions,
though not unprecedented.

Perhaps the most noteworthy event of this nature in the English
Channel occurred on 13th Feb 1979 when a long-period westerly swell
resulted in waves overtopping Chesil Bank, the crest of which is
about 12 metres above high water level. This occurred when a light
easterly wind was blowing. A Google search will probably throw up
references to this event.

Apologies for replying to my own posting but I have some additional
relevant comments.

When these events occur it is often the case that only a relatively
short stretch of coastline experiences any significant events. Almost
certainly, this is due to the way the wave energy is focussed by the
bathymetry as a result of refraction.

Waves begin to 'feel' the bottom when the waterdepth falls below 50% of
the deepwater crest-to-crest wavelength. This wavelength (in metres) is
calculated as

wavelength = 1.56 x the square of the wave period

Therefore, for a wave period of 18 seconds, the deep water
crest-to-crest wavelength is about 500 metres. This is dependent solely
on the wave period. The wave height does not come into it. Therefore,
waves with a period of 18 seconds will begin to 'feel' the bottom when
the water depth drops below 250 metres. The shallower the water, the
greater the effects of refraction, resulting in focussing of wave
energy.

The whole of the English Channel has waterdepths much shallower than
250 metres. Therefore, waves with a period of 18 seconds will be
'feeling' the bottom throughout their propagation up the Channel. Every
event will be different. Slight changes in wave direction, wave period
and wave height may make significant differences to the effects. Also,
the water depths will vary depending on the state of the tide. There is
therefore an almost infinite variability which results in any one
stretch of coastline likely to receive significant effects only once in
a blue moon. The fact that only one stretch of coastline is affected
significantly by any one event is not an anomaly. It's just physics.


--
Norman Lynagh
Tideswell, Derbyshire
303m a.s.l.
https://peakdistrictweather.org
Twitter: @TideswellWeathr

SNIP
When these events occur it is often the case that only a relatively
short stretch of coastline experiences any significant events. Almost
certainly, this is due to the way the wave energy is focussed by the
bathymetry as a result of refraction.

Slight changes in wave direction, wave period
and wave height may make significant differences to the effects. Also,
the water depths will vary depending on the state of the tide. There is
therefore an almost infinite variability which results in any one
stretch of coastline likely to receive significant effects only once in
a blue moon. The fact that only one stretch of coastline is affected
significantly by any one event is not an anomaly. It's just physics.


--
Norman Lynagh


So true.

On paper a number of recent storms have produced similar wave periods & heights & therefore power, on the north Cornish coast. March 2008, Jan 2014, Feb 2016, most recently 3rd Jan 2018 being examples. However, the areas where there has been large amounts of erosion have varied a lot. 2014 was particularly interesting for affecting some small areas normally fairly immune, such as near the harbour at Newquay normally protected from the swell direction by Pentire. Yet some other beaches facing straight into it were relatively unscathed.

It is often quite baffling as to why sand has suddenly vanished from a particular beach, when others nearby and very similar in most respects, are totally unscathed. It's been a topic of conversation between myself on John Chappell at Land's End weather on a few occasions. Certainly, at Sennen, simply the position of low water sandbars can affect which bits higher up the beach (which is 1mile long including the Gwenver end) are stripped of sand in a big swell. They are very effective in focusing long period swells. The next storm the sandbars are in different positions.

I would have thought the energy from a ships wake a few miles away would be absolutely minimal, after all where is all the energy coming from? Though it can be put to good use https://www.youtube.com/watch?v=rTY8mrwhOTQ

Graham
Penzance - A stunning sunny day down here

On 25/03/2018 19:40, Norman Lynagh wrote:

snip

Norman Lynagh wrote:


On 16th Dec 2016 there was a westerly swell with a period of 17-18
seconds propagating eastwards through the English Channel. At the
western end of the English Channel the significant height was 15-18
feet, decreasing eastwards. These were fairly unusual conditions,
though not unprecedented.

Perhaps the most noteworthy event of this nature in the English
Channel occurred on 13th Feb 1979 when a long-period westerly swell
resulted in waves overtopping Chesil Bank, the crest of which is
about 12 metres above high water level. This occurred when a light
easterly wind was blowing. A Google search will probably throw up
references to this event.

Apologies for replying to my own posting but I have some additional
relevant comments.

When these events occur it is often the case that only a relatively
short stretch of coastline experiences any significant events. Almost
certainly, this is due to the way the wave energy is focussed by the
bathymetry as a result of refraction.

Waves begin to 'feel' the bottom when the waterdepth falls below 50% of
the deepwater crest-to-crest wavelength. This wavelength (in metres) is
calculated as

wavelength = 1.56 x the square of the wave period

Therefore, for a wave period of 18 seconds, the deep water
crest-to-crest wavelength is about 500 metres. This is dependent solely
on the wave period. The wave height does not come into it. Therefore,
waves with a period of 18 seconds will begin to 'feel' the bottom when
the water depth drops below 250 metres. The shallower the water, the
greater the effects of refraction, resulting in focussing of wave
energy.

The whole of the English Channel has waterdepths much shallower than
250 metres. Therefore, waves with a period of 18 seconds will be
'feeling' the bottom throughout their propagation up the Channel. Every
event will be different. Slight changes in wave direction, wave period
and wave height may make significant differences to the effects. Also,
the water depths will vary depending on the state of the tide. There is
therefore an almost infinite variability which results in any one
stretch of coastline likely to receive significant effects only once in
a blue moon. The fact that only one stretch of coastline is affected
significantly by any one event is not an anomaly. It's just physics.



I tend to focus on storm surges as that is what affects us , up a river
off Southampton Water sheltered by the IoW, forgetting about other
mechanisms. It was only 2 months ago that I walked along the top of
Hurst Spit. A massive structure raised by the sea and can be taken away
by the sea . Even those massive getting on for room-sized blocks of
granite imported from Norway ISTR, could move like lego bricks given the
right/wrong sea state.

Anyone following this thread, and can get to Southampton, I arrange
science talks and the next one is this sort of topic
http://www.diverse.ip3.co.uk/scicaf.htm