A study by the UIB and the IEO identifies the Balearic ports most vulnerable to ‘rissagas’

Researchers have drawn up a map of these sudden sea-level rises using data collected over nearly two decades.

A team of researchers from the University of the Balearic Islands (UIB) and the Spanish Institute of Oceanography (IEO) has led the first regional study to jointly analyse rissagas in the western Mediterranean.

Among the locations identified, in addition to Ciutadella, are Portocolom, Port de Sóller, Porto Cristo and Sant Antoni, according to a press release issued by the UIB.

The study identifies the most vulnerable ports and provides new insights into this phenomenon based on data collected over 17 years at 27 coastal stations in the Balearic Islands and along the north-western coast of the Peninsula.

A rissaga, also known in scientific terms as a meteotsunami, is a sudden oscillation in sea level, similar to a tsunami, but generated by atmospheric disturbances that cause abrupt pressure changes.

These changes produce long waves, different from those caused by wind, because they can extend for kilometres and move the entire water column rather than just the surface layer, as wind waves do.

Although their impact in open sea is small, resonance over the shallow continental shelf or inside ports can turn them into waves of more than one metre, capable of causing flooding and damage to port infrastructure.

The study shows that the impact of rissagas is “highly uneven” and depends largely on the shape and depth of ports.

The port of Ciutadella, already known as one of the world’s “critical points”, shares the spotlight with other locations that have so far been little studied, such as Vilanova i la Geltrú (Catalonia), Portocolom, Port de Sóller, Porto Cristo (Mallorca), and Sant Antoni (Ibiza).

In these places, “quite significant” sea-level oscillations of more than one metre have been recorded as a result of the combination of atmospheric disturbances and the natural resonance of the ports, although Ciutadella remains the place where they occur most frequently and with the greatest amplitude.

Larger ports, or those whose shapes are less favourable to amplification, such as Barcelona, Tarragona, Palma or Valencia, hardly show any notable storm surges.

The research also reveals that many rissagas do not occur in isolation. For example, up to 88 per cent of the episodes detected in L’Estartit (Girona) coincide in time with significant oscillations in other ports across the region.

According to the study, this simultaneity is due to the same meteorological pattern, common in spring and summer, which generates atmospheric waves capable of affecting much of the western Mediterranean.

However, in some places, such as the Bay of Alcúdia, rissagas are also recorded in winter, associated with severe weather situations such as cold fronts or deep storms.

The UIB stressed that the data on which this research is based come from an exceptionally dense network of 27 tide gauges.

In addition, one of the main innovations of the study is that, for the first time, minute-by-minute sea-level time series are available for the whole of the Balearic Islands and the eastern coast of the Peninsula, a “crucial” step in detecting and characterising rissagas.

This dataset includes the ten tide gauges of Puertos del Estado, active since 2006; six SOCIB sensors, operating since 2011; and the PortsIB tide gauge in Ciutadella, installed in 2014.

It also includes the Venom network, made up of nine low-cost sensors developed by the UIB and the IEO-CSIC and deployed from 2020 onwards, significantly increasing spatial coverage.

Lastly, the database is completed by the “valuable series” of Josep Pascual, an amateur meteorologist who has been collecting data in L’Estartit (Girona) since 1969 and who, since 2006, has also been recording at one-minute resolution.

Taken together, they highlighted, this body of data has made it possible to characterise the phenomenon for the first time at a regional scale and to study in detail the spatial and temporal variability of rissagas in the western Mediterranean.