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Rissaga..... please can someone help explain this phenomena.

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Offline steveT

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« on: February 28, 2008, 00:11 AM »
In the early 90s I read in a local paper in Palma de Mallorca that the sea had risen about a metre and caused flood damage in in Soller, and then receeded. I was fascinated by this. Some years later I met some one who was asleep on a yatch in a mediterrannean harbour ( I think it was in the Balearics), only to wake up and find no sea in the harbour and the yatch on its side. I was off Ibiza on a boat, asking a captain about this type of phenomena a few years ago and he said that where we were the year before, suddenly the sea level dropped and where we were was not passable.

This phenomena is known as Rissaga and I understand it happens usually around Spring, when the Med is cold and warm air arrives from Africa.....I've read that you then get a cold stable lower air mass with an unstable upper air mass......then I think what happens is you get openings in the cold air mass, which then allow strong up and down drafts. These then increase or decrease air pressure ..... which in turn can change local sea levels significantly. These openings are long and have regional effects. Again I think these phenomena last a few minutes or much longer ....... I'm not really sure on my facts and what I've said could be wrong ...... I'm looking for someone to explain ..... as what I've read on the net hasn't fully helped......as there appears to be alot more to the phenomena than what I've just highlighted....ie you get amplification of the effect in inlets and by waves.

Thanks

SteveT

Offline Technopat

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« Reply #1 on: February 28, 2008, 00:38 AM »
Greetings steveT.
You mention that you've read some stuff on internet - did you visit the  Sp. Wikipedia, from which I got the following:
Quote
La última gran rissaga en Ciutadella de Menorca, la más importante en 20 años, se produjo el 15 de junio de 2006, con oscilaciones de hasta 4 metros que provocaron desperfectos en numerosas embarcaciones. Para que se produzca una rissaga de estas características, como ya sucedió también en esta misma localidad el 21 de junio de 1984, es necesario que a las condiciones en las que éstas ocurren normalmente, se sume una tormenta con fuertes vientos y cambios bruscos de la presión atmosférica. En las rissagas más comunes, las oscilaciones del nivel del mar son de 60 a 120 cm.

Se suelen dar unos 50 partes de avisos meteorológicos al año.

with a link to the English Wikipedia for Meteotsunami

The first external link at the bottom of the Sp. page is quite interesting and gives the castellano translation as resaca - which of course I only knew of as meaning 3:

Quote
DICCIONARIO DE LA LENGUA ESPAÑOLA - Vigésima segunda edición

resaca.

(De resacar).

1. f. Movimiento en retroceso de las olas después que han llegado a la orilla.

2. f. Limo o residuos que el mar o los ríos dejan en la orilla después de la crecida.

3. f. Malestar que padece al despertar quien ha bebido alcohol en exceso.

Real Academia Española © Todos los derechos reservados

Hope that's useful,
Technopat
PS.
My initial reaction was that someone had been spinning you a yarn, 'cos I've never seen the double-s in a Spanish word before - maybe our correspondents in Catalunya can inform us as to the existence or not of it in catalán/mallorquín - and it was too similar to the risa to be true.

P*S.
Looking forward to reading Caesar's take on this ...
« Last Edit: February 28, 2008, 00:41 AM by Technopat »
Technopat's disclaimer: If this posting seems over the top and/or gets your goat (Sp. anyone?), please accept my apologies and don't take it personally - it's just my instinctive tendency to put my foot in it whenever/wherever possible. See also:
http://www.iberianatureforum.com/index.php/topic,266

Offline nick

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« Reply #2 on: February 28, 2008, 13:04 PM »
I remember the the 2006 event being covered on the Catalan weather forecast (afterwards that is). I didn't know the name rissaga.

More here
http://www.proteccioncivil.org/vademecum/vdm004.htm#0405b

English translations are meteorological tsunamis or as TP says meteotsunami.

"Ciutadella Harbour (Menorca Island, Spain) is one of the places where such
meteorological tsunamis are quite common. They are known by the local name
of ‘rissaga’ (Ramis and Jansà, 1983; Monserrat et al., 1991). Rissaga waves with
heights more than 3–4 m, causing severe damage to the coastal area, ships and
harbour constructions, have been observed in Ciutadella several times. Similar
oscillations also occur in some other harbours and inlets of the Balearic Islands
and the Mediterranean coast of the Iberian Peninsula, although they are typically
not so strong as in Ciutadella."

http://www.pac.dfo-mpo.gc.ca/sci/osap/projects/tsunami/documents/NH_1998.pdf
« Last Edit: February 28, 2008, 13:06 PM by nick »
Nick
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Offline steveT

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« Reply #3 on: February 28, 2008, 22:44 PM »
Tp and Nick,

Thanks for the links ....I'm getting a better picture but there are still a few things I don't fully understand .....like on one web site the 2006 event in Ciutadella is linked to a sudden but short pressure change .....I saw the air  presssure graph ......but I don't see how this caused the emptying and filling of the large inlet ...... or if this just part of the event. There seems to be several factors all working together.

Thanks

steveT

Offline Technopat

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« Reply #4 on: February 29, 2008, 01:39 AM »
Greetings steveT and All,
For want of a better explanation - Caesar's, for example - could this be a Subtle Sign that there is, iberianatureforumers' scepticism notwithstanding, an Intelligent Designer, who on finishing His/Her bath, pulls the plug ...?

Regs.,
Technopat
PS.
Either that or it has summat to do with pulleys and/or sluices ...
Technopat's disclaimer: If this posting seems over the top and/or gets your goat (Sp. anyone?), please accept my apologies and don't take it personally - it's just my instinctive tendency to put my foot in it whenever/wherever possible. See also:
http://www.iberianatureforum.com/index.php/topic,266

Offline Jill

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« Reply #5 on: February 29, 2008, 16:02 PM »
Caesar is bogged down with A level studies - so i'll have a go at answering this. Not that I really know the answer, but I don't think that anybody does.

The rissaga is basically a meteorological tsunami, as Nick has already said. It looks and acts exactly like a tsunami / tidal wave, but it doesn't have its origins in an earthquake; it's origins are meteorological.

As to why it happens in a place like Ciutadella - TP has hit the nail on the head with his bath-water scenario: too much water trying to move from one place to another too quickly, through too small an opening. The higher up the inlet you go the more dramatic the effect. At sea, a tsunami is effectively invisible; the wave only builds into a tall, rolling thing when the water is shallow.

I saw some photos of the 2006 rissaga. They showed boats afloat, then on their sides, and then afloat again. I think it only lasted for a minute or so. But it must have been a very scary minute...

I guess that if the weather watchers knew precisely what caused the meteo-tsunami they would warn people that one was due. As it is... they woffle about what causes it. Changes in air pressure certainly have a very dramatic effect on sea levels, so that a sudden change would obviously have a sudden effect. Again, the effect would be invisible at sea; it would only be manifest when the rise in sea-level was relative to the coast, and unless it was huge it would only be dramatic in a constricted area..

Jill

Offline jeyroger

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« Reply #6 on: October 09, 2008, 13:03 PM »
I was part of a research team that worked on this phenomenon about 1990 - the leader on the
Mallorquin side was Sebastia Monserrat from Universitat Illes Balears. As I understand it, the name
is Catalan, and the phenomenon is driven by atmospheric gravity waves depressing the sea surface
followed by amplification in the local topography such as Cituadella. Our University Meteorology
Department constructed three microbarometers, capable of resolving changes of about one microbar
(millionth of an atmosphere) in pressure, and we took them down to Mallorca by van via the port of
Barcelona (where the van was broken into and one piece of non-essential equipment was stolen!).
The microbarometers were installed at Porto Colom and Palma on Mallorca, with the third on Menorca,
and they were self-powered and recorded data in monthly stretches between downloads. As part of
the collaboration, and continuing technical development, I had to revisit Mallorca twice, and discovered
what a lovely place this is away from the coastal hotspots (who said science can't be fun?).
  For a detailed explanation of the meteorology see the Royal Met Soc references via Googled "rissaga"
and also some great pictures of a roll cloud that may be associated with the pressure wave during one
specific event.

Offline nick

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« Reply #7 on: October 09, 2008, 13:16 PM »
Hi Jey?

Interesting addition , and welcome to the forum

nick
Nick
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Offline Maria

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« Reply #8 on: October 09, 2008, 16:52 PM »
Interesting stuff.
You can find quite a bit on you tube as well.

Offline jeyroger

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« Reply #9 on: October 09, 2008, 17:49 PM »
Hello again...I re-read some of the above posts (postings?)
more carefully, and although I'm not a really a meteorologist,
more an environmental engineer, I'll defend them from the
accusation of 'woffle' (i.e. waffle):  predicting natural effects
is quite tricky because of the physics (see Chaos Theory etc)
and has serious implications where people or property might
have to be moved around, not least if many false alarms can
occur ('crying wolf', if you like) which might diminish the effect
of future warnings. The driver, and presumably the funding,
for the work we did came from the marine insurance companies
and it is worth noting that in Britain these guys actually use the
Met service a lot for statistics in setting premiums when they
insure public events like your local Agric Show or rock festival.
Accountants don't throw their money about, so this indicates
their confidence in the science, in a way. Regarding Rissaga,
I don't know what the outcome of the work was because my
part was done early on, but thanks to this forum I am now
going to chase it up and find out what the state of prediction
is. It is likely that the cost of maintaining or expanding the
network of sensors (it would ideally cover much of the Med, to
be really useful for prediction) was prohibitive, bearing in mind
that the Rissaga is not generally life-threatening, merely a bit
destructive on occasions. I may have some pics somewhere of
the aftermath of one event: upturned/smashed boats etc.
Or if anyone else can help?

Offline Jadra

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« Reply #10 on: March 17, 2009, 12:20 PM »
Late answer  ???
Ok… it goes something like this:

1.   There is a pronounced air pressure disturbance (sudden air pressure rise or drop) in the atmosphere. This disturbance has a large tendency, e.g. change of 3 hPa in 10 min. And this disturbance is not too big… several tens of km (nothing compared to pressure lows or highs!).

2.   Now when an air pressure disturbance like this one, travels above the OPEN SEA (far from bays, harbours, coasts…), it generates sea waves. Where the pressure is higher, air presses the sea harder, and the sea level gets lower. Where the pressure is lower, air presses the see less, and the sea level is higher. It’s called an inverse barometric effect and in normal cases you should have a change of sea level of 1 cm for 1 hPa of air pressure change. However, sometimes this air pressure disturbance is travelling with a speed of long ocean waves. Speed of long ocean waves is a function of sea depth: v=sqrt(g*h), where g is gravity acceleration and h is sea depth. When this happens, when the speed of air pressure disturbance is equal to the speed of  sea waves, then this sea waves became much higher than prescribed 1 cm/1 hPa. This way, the waves can reach heights up to 50 cm… still not to much. But, keep in mind, this are not everyday sea waves! If an air pressure disturbance is several km long, then this sea waves are also several km long. You can not really notice them by watching. And as I said, they are not too high.

3.   Eventually, this long open sea waves hit a bay, for example Ciutadella. Here is another thing you should know. Bays act as resonators. Water in them oscillates up and down. So, for example in Ciutadella, sea level goes up for about 5 min and then it goes down for about 5 min. This happens most of the time. But this sea level changes are very small, perhaps few centimetres, and they are also hard to notice. But, what happens if those long waves coming from the sea have a same period as Ciutadella oscillations? You guess! Resonance! So, 5 minutes between ridge (high point) and trough  (low point) of open sea wave, 5 min between high and low sea level in Ciutadella. Enough for resonance. Oscillations in Ciutadella become violent and lot of damage can be done!

As for prediction of an event, there is an effort at the moment to build an efficient warning system for Ciutadella. This system should be based on air pressure and sea level measurements, on northern coast of Mallorca, and on a buoy between Menorca and Mallorca. Then, when a dangerous air pressure disturbance is observed (along with some sea level oscillations) a warning can be issued for Ciutadella.

Offline Technopat

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« Reply #11 on: September 10, 2009, 04:05 AM »
Greetings Jeyroger, Jadra and All,
Thanks for your contributions here. Missed 'em first time round, but I always usually get there in the end.

Just to tone down the high level of this 'ere thread, I just want to mention that I've literally just got off the plane after spending ten days near Ciutadella (trip report in pipeline) and had the opportunity of chatting with a couple of people (laypersons) who had direct experience of the 2006 rissaga - one of whom had a yacht which was a total write-off as a result. The other person, who works right on the quayside, told me that small rissagas were extremely common - on a twice-monthly basis - in June, July and August and go mostly unnoticed. She also mentioned that the early warning system from Mallorca was now functioning. Apparently UIB is now looking into tectonic plates in the region as a major cause.

For those of you wishing to catch up on the literature and what's left of this summer's opportunities for poolside reading:
http://www.imedea.uib.es/natural/goi/oceanography/html/research/Projects/WEB_rissagues/Rissagues/publicaciones_cast.htm#

Assuming that Nordic fjords are not affected by such phenomena, would it be 'cos a) they're much deeper than the port of Ciutadella, b) temperatures there are much lower, or c) not many people live there and there is therefore no destruction to property and any effects go unnoticed?

The Menorca tourist board translates rissaga as tidal bore, but from what little I understand of this, it would seem to be the opposite, as the effect seems to be more the sudden drop of the water level rather than a rise followed by a drop. Or are we back to the chicken and the egg?

But as Jill pointed out, pretty scary if you're sitting there having a nice cuppa and admiring the view. No laughing matter, despite the apparent etymology.

Regards,
Technopat

PS.
Jeyroger, if you think your visits to the off-the-beaten-tracks of Mallorca were well worth all those years of studying to be a scientist, wait till you get the chance to visit Menorca. Or better still, think up some research project you can do there 'cos it's an amazing place...
Technopat's disclaimer: If this posting seems over the top and/or gets your goat (Sp. anyone?), please accept my apologies and don't take it personally - it's just my instinctive tendency to put my foot in it whenever/wherever possible. See also:
http://www.iberianatureforum.com/index.php/topic,266