Author Topic: il-Milghuba (play of the sea) - Malta's Seiche wave or surge waves  (Read 13358 times)

electrobleme

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    Milghuba - Maltese for play of the sea - Malta's Seiche wave or surge waves

    • Milghuba Malta's seiche wave or surge tides - Study of Seiche Oscillations
      • The Milghuba (Seiche) phenomenon in the Maltese Islands .pdf
        • Unusual wave hits coast: Milghuba Malta's seiche wave or surge tides
          • il-milghuba and Mt Etna activity in Sicily (1973)
            • La Rissaga , Menorca - roll cloud and La Rissaga linked?



              For a possible explanation of il-Milghuba (play of the sea) click here - Effective Cause?. Is it the air pressure or the sea or electrical activity in the atmosphere?



              Quote
              Study of Seiche Oscillations
              Physical Oceanography Unit - University of Malta

              A most remarkable feature in the sea level signals observed in Malta consists of a band of high frequency signals with periods ranging from several hours to as low as a few minutes. These non-tidal short period sea level fluctuations are an expression of a coastal seiche, known by local fishermen as the 'milghuba' . This phenomenon has now been observed to occur all along the northern coast of the Maltese archipelago and manifests itself with very short resonating periods of the order of 20 minutes in the adjacent coastal embayments. The phenomenology, generation and impact of the 'milghuba' on the coastal dynamics constitute the major focus of the study by DRAGO, 1999.

              Analysis shows that weak seiching is present uninterrupted and appears as a background 'noise' on the tidal records. During random sporadic events the seiche oscillations can however become greatly enhanced. The maximum range measured in Mellieha Bay reached 1.1m. The seiche is often big enough to mask completely the small astronomical tide.These large-amplitude sea level oscillations are accompanied by remarkable currents that are triggered by the sloshing water masses. These currents are comparable in size to those generated in coastal areas characterised by tidal forcing.

              Click for full article Milghuba Malta's seiche wave or surge tides - Study of Seiche Oscillations



              Quote
              The Milghuba phenomenon in the Maltese Islands
              Author - Aldo Drago
              Physical Oceanography Unit, University of Malta
              pdf of "The Milghuba (Seiche) phenomenon in the Maltese Islands"

              CONCLUSIONS (1)
              The ‘milghuba’ occurs throughout the year, but some seasonal characteristics are evident. The highest seiche amplitudes occur in the months March, May and September. The frequency of occurrence is smallest during summer. Seiches are more frequent in winter, but their amplitudes are generally smaller.

              During the summer months, in particular July and August, the seiche is very sharp, with a duration that can often be as short as a few cycles. The associated sea level fluctuations have a tsunami-like nature, starting with an abrupt and large impulse that subsequently decays after a few oscillations. Seiches during this time of year are particularly strong.

              Click for full article The Milghuba (Seiche) phenomenon in the Maltese Islands .pdf



              Quote
              UPDATED: Unusual wave action confirmed
              di-ve.com - Current Affairs -- 18 December 2008 -- 08:55CEST

              The head of the Physical Oceanography Unit, Aldo Drago, confirmed that unusual waves noted at Ghadira on Wednesday were the phenomenon known as the 'milghuba' or seiche. A seiche consists of a very rapid sequence of sea oscillations.

              The oscillations were recorded by the MedGLOSS station in Portomaso where sea level, sea temperature and atmospheric pressure are monitored in real-time by the Physical Oceanography Unit of the University of Malta. Dr Drago told www.di-ve.com that the seiche was a common phenomenon in Maltese coastal waters although it is usually much smaller in extent than that observed on Wednesday.

              The water rushed up the beach at Ghadira, almost as far as the road, went out again almost as far as the first row of buoys and then rushed back up again. This was all confirmed by the monitoring equipment at Portomaso, which showed that the sea level rose by over 0.5 metre.

              “Such strong sea level oscillations are very much like tsunami waves in behaviour, but their origin is completely unrelated to seismic activity,” he explained.

              Click for full article Unusual wave hits coast: Milghuba Malta's seiche wave or surge tides


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    « Last Edit: September 15, 2009, 07:39:41 by electrobleme »

    electrobleme

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    Milghuba - Study of Seiche Oscillations

    Quote
    Study of Seiche Oscillations
    Physical Oceanography Unit - University of Malta


    A most remarkable feature in the sea level signals observed in Malta consists of a band of high frequency signals with periods ranging from several hours to as low as a few minutes. These non-tidal short period sea level fluctuations are an expression of a coastal seiche, known by local fishermen as the 'milghuba' . This phenomenon has now been observed to occur all along the northern coast of the Maltese archipelago and manifests itself with very short resonating periods of the order of 20 minutes in the adjacent coastal embayments. The phenomenology, generation and impact of the 'milghuba' on the coastal dynamics constitute the major focus of the study by DRAGO, 1999.

    Analysis shows that weak seiching is present uninterrupted and appears as a background 'noise' on the tidal records. During random sporadic events the seiche oscillations can however become greatly enhanced. The maximum range measured in Mellieha Bay reached 1.1m. The seiche is often big enough to mask completely the small astronomical tide.These large-amplitude sea level oscillations are accompanied by remarkable currents that are triggered by the sloshing water masses. These currents are comparable in size to those generated in coastal areas characterised by tidal forcing.
    Time series of sea level measurements in Mellieha Bay showing seiching events

    While these seiche induced currents can be an important means for the flushing of coastal inlets and harbours, they can on the other hand be dangerous to navigation.

    Similar seiche oscillations have been reported in other parts of the world ocean, with the most recent studies being those conducted in Puerto Rico and the Philippines (Giese et al., 1982), in Nagasaki on the southern coast of Japan (Hibiya & Kajiura, 1982), at Capetown in South Africa (Shillington, 1984), at the Balearic Islands (Monserrat et al., 1991), and in the South Kuril Islands (Djumagaliev & Rabinovich, 1993). They are reported to cause severe damage to coastal areas, boats and port constructions. Due to their close relation with the behaviour of tsunami, research on seiches is considered to be important in disaster mitigation studies (Taku et al., 1992).

    Reference to similar sea level variations (known as the 'Marrubbio') on the southern coast of Sicily is found in the Italian 'Portolano' for ship navigation. Their occurrence is reported to be most frequent in May or June in association to southeasterly winds, and their crest-to-trough amplitudes reach as high as 1.5m. Literature on the 'Marrubbio' is however very scarce and the only relevant publication is by Colucci & Michelato (1976). It is interesting to note that one of the first scientific studies on seiches by Sir George Airy (1878) refers precisely to the Grand Harbour in Malta. Since then the seiche phenomenon in Malta remained unstudied until the work conducted recently by the PO-Unit, which indeed constitutes the first study dealing with these high frequency sea level oscillations in the Central Mediterranean.

    The large amplitude seiches in the Maltese Islands are mainly of an atmospheric origin. The simultaneous measurements of sea level and atmospheric pressure reveal the presence of pressure fluctuations of the order of a few millibars in coincidence with the occurrence of seiche events. These pressure signals are believed to be the surface expression of atmospheric wave disturbances that propagate in the lower troposphere as internal gravity waves. Their interaction with the open sea surface triggers the long period waves that subsequently force the seiches in the coastal areas.
    « Last Edit: June 25, 2009, 02:47:50 by electrobleme »

    electrobleme

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    The Milghuba (Seiche) phenomenon in the Maltese Islands
    « Reply #2 on: June 25, 2009, 05:38:27 »
    The Milghuba phenomenon in the Maltese Islands
    Author - Aldo Drago
    Physical Oceanography Unit, University of Malta

    pdf of "The Milghuba (Seiche) phenomenon in the Maltese Islands"


    CONCLUSIONS (1)
    The ‘milghuba’ occurs throughout the year, but some seasonal characteristics are evident. The highest seiche amplitudes occur in the months March, May and September. The frequency of occurrence is smallest during summer. Seiches are more frequent in winter, but their amplitudes are generally smaller.

    During the summer months, in particular July and August, the seiche is very sharp, with a duration that can often be as short as a few cycles. The associated sea level fluctuations have a tsunami-like nature, starting with an abrupt and large impulse that subsequently decays after a few oscillations. Seiches during this time of year are particularly strong.


    CONCLUSIONS (2)
    The occurrence of large seiches (the 'milghuba' waves) in the northwestern coast
    of Malta are to a large extent caused by the presence of atmospheric pressure
    disturbances

    Although a definite correlation between time variations of atmospheric activity
    and long wave intensity is testified, there is an unexpected lack of DIRECT
    spectral coherence between the atmospheric and the long period sea waves

    The amplitude of the sea level oscillations tends to be higher in relation to the
    strength of atmospheric disturbances which generate them, but this is not always
    the case

    The spectral energy content of the pressure disturbance strongly dictates the
    nature of its interaction with the sea surface. The additional energy is mainly in
    the frequency band 0.1 - 2cph and decreases rapidly with frequency. The air-sea
    coupling does not appear to be related to any particular single frequency but to a
    range of frequencies


    CONCLUSIONS (3)
    The large amplitude sea level oscillations observed on the northern
    coast of Malta in the long wave frequency band contain substantial
    energy in the range of frequencies 0.2 - 2cph. The lower frequency
    signals are associated to longitudinal, latitudinal and mixed stationary
    modes that develop on the highly irregular shaped continental shelf.
    It is inferred that the observed waves are not only ones that cross the
    shelf from the deep sea, but that comparable energy occurs in the form
    of trapped waves associated to bathymetric features on the shelf.




    SOME PUBLICATIONS

    DRAGO A. & FERRARO S. (1994). Oscillazioni del livello del mare nel Porto di Malta, Proceedings of the XI Congress of the Italian Association of Limnology and Oceanography.

    DRAGO A. (1999). A study on the sea level variations and the ‘Milghuba’ phenomenon in the coastal waters of the Maltese Islands, Ph.D Thesis, School of Ocean and Earth Science, University of Southampton.

    DRAGO A. (2001). Sea level variations in Mellieha Bay, Malta, Rapp. Comm. Int. Mer Medit., 36, p95.

    DRAGO A. & BOXALL S.R., (2002), The use of the wavelet transform on hydro-meteorological data, in Physics and Chemistry of the Earth, no.27, pp.1387-1399.

    DRAGO A., (2007),
    Numerical modeling of coastal seiches in Malta, Physics and Chemistry of the Earth, Vol 33/3-4 pp 260-275


    electrobleme

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    UPDATED: Unusual wave hits coast
    di-ve.com - Current Affairs - 17 December 2008 - 15:45CEST

    The seawater in Ghadira rose suddenly on Wednesday at 0800h, sweeping up the bay and inundating the beach and concessions there.
    One eye witness said that water swept up the bay – almost as far as the road – and did not subside for about 10 minutes.

    Another person told www.di-ve.com that the water came up the beach, then went out nearly as far as the first set of buoys before rushing back again.

    The phenomenon that occurred was likely to be a ‘seiche’, according to Dr Aldo Drago from the Physical Oceanography Unit at the University of Malta. He said that the seiche is common – and well known to fishermen - but it would be rare to have one as large as this. The wave is caused by atmospheric condition and starts in the open sea but its effect is magnified when it reaches shallow waters.

    Dr. Drago said that the strongest oscillation was at 6am.

    Seismic activity was ruled out by the Seismic Monitoring and Research Unit at the University of Malta.

    The surge could also have been responsible for seawater flooding across the road in Msida.






    ------------------------------------------------------------------------------------------------------------------------------------


    UPDATED: Unusual wave action confirmed
    di-ve.com - Current Affairs -- 18 December 2008 -- 08:55CEST

    The head of the Physical Oceanography Unit, Aldo Drago, confirmed that unusual waves noted at Ghadira on Wednesday were the phenomenon known as the 'milghuba' or seiche.

     A seiche consists of a very rapid sequence of sea oscillations.

    The oscillations were recorded by the MedGLOSS station in Portomaso where sea level, sea temperature and atmospheric pressure are monitored in real-time by the Physical Oceanography Unit of the University of Malta.

    Dr Drago told www.di-ve.com that the seiche was a common phenomenon in Maltese coastal waters although it is usually much smaller in extent than that observed on Wednesday.

    The water rushed up the beach at Ghadira, almost as far as the road, went out again almost as far as the first row of buoys and then rushed back up again. This was all confirmed by the monitoring equipment at Portomaso, which showed that the sea level rose by over 0.5 metre.

    “Such strong sea level oscillations are very much like tsunami waves in behaviour, but their origin is completely unrelated to seismic activity,” he explained.

    “They are triggered by particular atmospheric conditions and start as a much smaller wave in the open sea; as the wave approaches shallow waters it is greatly magnified and sets embayments and harbour waters in resonance with sea oscillations of the order of several tens of centimeters in amplitude and a period of a few minutes.

    “Due to the shape and configuration of Mellieha Bay, these seiches are known to be quite intense in this area.”

    Dr Drago said that the strongest oscillation occurred at 0600h, but was subsequently followed by a “train” of other oscillations in sea level.

    Daily updated charts from the MedGLOSS station can be followed online by linking to http://www.capemalta.net/pounit/levmalta.html

    The Met Office director, Charles Galdies said that the event could have also been triggered by an astromonical phenomenon.

    "Last week we had the closest approach of the moon which could have led to an oscillation in the eastern Mediterranean basin," Dr Galdies confirmed.





    electrobleme

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    il-milghuba, rumbling noise and Mt Etna activity - 9th July 1973
    « Reply #4 on: August 05, 2009, 21:15:32 »
    il-milghuba and Mt Etna activity in Sicily

    Quote
    Such an occurrance took place on the night of July 9, 1973. Fishermen and residents in the Salina Bay area (Malta) reported a marine disturbance, which old fishermen called "il-milghuba", and which was alleged to have occurred a few years previously. At about 3 am, the sea level went down by couple of feet. A short while later, the sea rose a couple of feet above the normal level, before settling down to its original level. Some boats, especially those anchored in shallow water, were seen resting on the seabed. When the sea rose again, a "rumbling noise" was heard by several people in the area, and the resulting wave covered up normally dry land up to 400 feet inshore. Mt. Etna in Sicily was reported to be very active a few days before.
    Earthquakes: Malta and il-milghuba