On 24 August, mini-tsunamis were observed along several stretches of Mallorca’s coastline. In Port d’Alcúdia, the water reached the shoreline and outdoor dining areas. Unusual changes in water levels were also reported from Port d’Andratx, Port de Sóller and Port Adriano.
The situation was more severe in Menorca. In the harbour of Ciutadella, the water level fluctuated by up to 1.44 metres on Monday. During the night of Friday 28 August, the bay of Santandria was also completely flooded. Water penetrated as far as the ground floor of a hotel and other premises there. The fire brigade and police pumped out the water. A warning remains in place for Menorca.
Mini-tsunami in Mallorca
Although the images from harbours and beach areas look dramatic, there have so far been no confirmed reports of damage to sailing or motor yachts in Mallorca or Menorca
What distinguishes a ‘Rissaga’ from a normal wave
‘Rissaga’ is the local term used in Menorca. ‘Meteotsunami’ is the scientific term. Whilst ‘mini-tsunami’ describes the phenomenon vividly, it can be misleading. It is not a single breaking wave, as is the case with surf.
Rather, the entire body of water oscillates. The water may initially drain out of a harbour at an unusually rapid rate. A few minutes later, it flows back in again. This movement can be repeated several times. It is not only the differences in water level that are crucial, but also the currents that arise as the water rapidly fills and drains.
A tectonic tsunami is typically triggered by a sudden movement of the seabed, such as during a seaquake. A meteorological tsunami, on the other hand, originates in the atmosphere. Rapid fluctuations in air pressure cause long waves to form on the sea’s surface.
A few hectopascals aren’t enough, but resonance is
It all starts with the inverse barometric effect: a change in air pressure of one hectopascal corresponds, statically speaking, to a change in water level of approximately one centimetre. A sudden change in pressure of just a few hectopascals therefore does not immediately cause a metre-high wave of cracks.
The key amplification occurs in several stages. A moving atmospheric pressure disturbance can repeatedly push a long water wave along if both are travelling at almost the same speed. This Proudman resonance works in a similar way to a swing that goes higher and higher with small but precisely timed nudges.
Such conditions have been particularly well studied for the Menorca Channel. A recent study in the Journal of Geophysical Research: Oceans states that, for optimum amplification, the propagation speeds of the atmospheric disturbance should be around 24 to 36 metres per second. This corresponds to around 86 to 130 kilometres per hour. At depths of around 70 to 120 metres, this corresponds to the speed of long water waves. Modelling suggests an amplification factor of approximately ten in the Menorca Channel.
This is followed by the second amplification point along the coast. If the long wave encounters a suitable bay or a long harbour basin, this can itself begin to resonate. The harbour then oscillates like a bath. In the worst-case scenario, what starts as a small fluctuation in sea level can develop into a much larger movement.
How a Rissaga is created
Why Ciutadella is particularly vulnerable
Ciutadella combines both effects. The long, narrow harbour channel is situated at the end of the Menorca Channel. The harbour basin has a natural period of approximately 10.5 minutes. If a long-period wave with a high energy content arrives within this time range, it is further amplified within the harbour.
The combination of Proudman resonance on the open sea and harbour resonance in Ciutadella is considered the key to major rift zones. A 50-year review published in 2026 A total of 191 meteotsunami events, each with a water level fluctuation of at least 60 centimetres, were recorded in Ciutadella between 1975 and 2025. The study also confirms that particularly extreme events involving water level fluctuations of over three to four metres were frequently associated with convective weather conditions and abrupt changes in atmospheric pressure.
Thus, two harbours situated close to one another may react completely differently to the same weather conditions. The atmosphere provides the initial impetus. Water depth, the orientation of the coastline, and the shape and natural period of a harbour all play a part in determining how strong the motion ultimately becomes.
This video explains the effect clearly
What poses a danger on board
In the Mediterranean, the normal astronomical tidal range is small. In Ciutadella, it is around 20 centimetres. A sudden fluctuation of one metre or more is therefore an exceptional situation for moorings, mooring lines and fenders.
If a yacht suddenly sinks, it may run aground or become lodged so low alongside the jetty that the mooring lines are subjected to unusual strain. As water rushes in, the water level and direction of flow change rapidly. Boats can be pushed against the quay, neighbouring yachts or jetties. In extreme circumstances, mooring lines may snap and boats may drift away.
During the severe Rissaga event of 15 June 2006, researchers in the port of Ciutadella modelled currents of up to approximately 1.8 metres per second, around 3.5 knots. Historical Rissaga events thus demonstrate the potential for damage.
Warnings help, but determining the exact figure remains difficult
Tidal waves are easier to detect today than they used to be. For Ciutadella, there are specific modelling approaches that combine weather data with the sea’s response. Nevertheless, the exact height remains difficult to predict. Even small differences in the direction, speed and temporal progression of a pressure wave can significantly alter the resonance.
Up to 1.20 metres had been forecast for Ciutadella on Monday, but the actual reading was 1.44 metres. The warning status is therefore more important for skippers than trying to deduce the exact water level at their own mooring from a regional forecast.
Proper preparation
For crews and those moored in harbour, this means: taking warnings seriously, checking the mooring lines and fenders, ensuring there is sufficient room to manoeuvre, and not leaving the boat unattended if there is any unusual water flow. Anyone in a particularly vulnerable harbour should follow local instructions from the harbour authorities, the marina and the weather service.
As regards preparations at the mooring, the same principles apply as in the event of a storm or exceptional flooding; however, they must be implemented with particular rigour in the case of a Rissaga. Long mooring lines and fenders allow for a quick change in water level. Additional lines, heavy-duty attachment points and abrasion protection distribute peak loads. In the event of swelling and significant movement, the following also help: stretchable mooring lines and Shock absorber. Fenders must be positioned in such a way that the ship’s side remains protected even when the ship’s position relative to the quay changes. The overview explains which types are suitable for this purpose Types of fenders on board.
Book recommendations
Seamanship It covers meteorology and tidal patterns as well as manoeuvres, safe boat handling and what to do in the event of an emergency. For those planning a long-distance voyage, the book is particularly useful in helping to link weather developments with practical preparations on board. Experienced skippers will also find a systematic refresher on mooring lines, harbour manoeuvres and emergency procedures should unusual conditions suddenly disrupt the normal routine on board.
Practical knowledge for skippers is designed more as a compact reference guide for use on board. It brings together information on weather, navigation, safe anchoring, mooring manoeuvres and emergency assistance in a concise format. It serves as a practical supplement to *Rissaga*: anyone wishing to check their mooring lines, berth and options for action on board following a warning will find the basic procedures explained concisely.
Have you experienced a Rissaga yourself, and how have the water level and current changed in your harbour?
Lars Bolle
Chief Editor Digital