What looked to people like a “mini tsunami”, which yesterday washed over low-lying areas along the east coast and flooded roads and quays, is actually a relatively common phenomenon known as seiching.

A large volume of seawater flooded the Msida parking area near the Workers’ Monument while, in Marsaxlokk, an eyewitness said the sea withdrew “as much as a metre” before surging back, submerging the quay and washing away small items such as shoes.

A St Julian’s restaurateur compared it to a “mini tsunami”, adding that “the tide went up and down really drastically within minutes”.

I would describe yesterday’s seiching as moderate seiching

Boats moored in Mellieħa bumped into each other while swimmers were somewhat stunned by the rise in sea level which from knee height suddenly reached up to their chests.

Seiching is, in fact, a relatively common phenomenon usually known under local names such as milgħuba in Maltese, marrubio in Italian and rissaga in Spanish.

Aldo Drago, professor at the International Ocean Institute of the University of Malta, explained that seiching is about “atmospheric gravity waves which produce open sea oscillations”.

“These oscillations are eventually reinforced by resonance when they reach coastal embayments and harbours, leading to a larger-amplitude seiching event.”

Any enclosed or partially enclosed body of water, such as a bay or harbour, has a number of natural frequencies at which it will resonate, depending on the dimensions of the body. The water can be set into a wave motion at one of these frequencies – for example, by wind or by seismic activity – resulting in standing waves that move vertically, but not horizontally.

Seiching happens all year round, Prof. Drago explained, but is often unnoticeable because the waves are usually of low amplitude. “I would describe yesterday’s seiching as moderate seiching. One of the largest seiching events recorded on the Maltese Islands was, I believe, in 1996 where the oscillations were of some 1.2 metres.”

Seiching has nothing to do with global warming or tides. Neither does it herald storms or bad weather.