Alpine lakes like Lake Como cannot be said to be at risk from tsunamis. Firstly we are not prone, albeit not entirely immune, to the seismic shocks that typically create most tsunamis. Secondly a tsunami’s size and devastating power are derived partly from the time it takes for the waves to develop over oceanic distances. However we are susceptible to freak, or to use the more scientific term, anomalous waves. 

Evidence of erosion, landslides and fault lines can be seen around the lake as in this fault line on the mountain behind Lenno.

Research published in 2007 by Daniela Faletti and Luigina Vezzoli of the Universita dell’ Insubria has identified two occasions in the distant past when anomalous waves on Lake Como of up to 10 metres have caused extensive damage and undoubted loss of life within lakeside communities. Anomalous waves may not be as high or carry such initial destructive force as tsunamis but they can last longer in a lake due to the specific effects of reflection, resonance and oscillation. The eventual height of such a wave is also dependent on the specific topology of where it reaches land.

The northern tip of Bellagio looking over to Tremezzo. A major landslide on the underwater shelf between Bellagio and Tremezzo is said to have caused a devastating anomalous wave in the 500s.

These waves are most likely to have been caused by landslides involving massive falls of mountain detritus happening both above and below the water level. Geologists are able to identify such cataclysmic events by studying the layers of sediment on the lake floor. These extracts of sediment can reveal the past in a similar way to how growth rings on a tree trunk reveal past climatic conditions. Scientists have been able to identify some years in which more sediment than average had accumulated on the bottom of the lake. These deposits in a few instances were even greater than might be expected from a major flood. In these cases, the volume and depth of silt can only be explained by the displacement of large volumes of land mass, i.e. caused either directly or indirectly by a landslide.

A major landslide on the mountain of Le Grammont above where the Rhone enters Lake Leman is said to have triggered the movement of sediment that had built up at the river’s delta. This in turn is said to have caused the anomalous wave in 563 causing widespread damage along the length of the lake.

This phenomenon was first uncovered by scientists working at the University of Geneva who studied the sedimentary deposits on the bottom of Lake Leman (Lake Geneva). Local ancient history had always talked about a major disaster costing many lives and causing damage to lakeside communities back in the 6th century. Research verified that a major landslide must have taken place around that time at the eastern end where the Rhone enters the lake. First ideas were that a landslide at Tauredunum created a natural dam over the Rhone as it entered the lake. This dam was then said to have burst causing the huge anomalous wave which then travelled the length of the lake flooding communities along its way until it reached Geneva.  More recent researches by the University of Geneva have resulted in a new theory which claims a landslide back in 563 on the mountain called Le Grammont above Port-Valais caused a massive underwater shift in the sediment that had built up around the Rhone’s point of entry to the lake. It was the shift in this sediment which created the anomalous wave. The wave would then have travelled at about 70 km per hour to arrive 13 metres high in Lausanne 15 minutes later. 

The anomalous wave of 563 travelled east to west across the length of Lake Leman. The numbers in red denote the time in minutes it took to arrive. The numbers in yellow denote the height of the wave in metres.

For Lake Como, the two researchers from the Universita dell’Insubria identified two major incidents causing destructive anomalous waves. One has been dated to between 500 and 530CE. The other was sometime within the 12th century. These results followed examination of sediment taken from the deepest part of the lake – the area between Isola Comacina and Brienno. The older wave has been attributed to two underwater landslides occurring on the shelf that lies between Bellagio and Tremezzo. The cause of the 12th century incident is less clear but has been attributed to possibly the same sort of event occurring on Lake Leman or possibly due to an earthquake. In either case, the anomalous wave created would have amounted to 10 metres in height.

The statue of Pliny the Younger on the right-hand side of the main entrance to Como Cathedral

Both waves would have caused massive damage and loss of life. Some have suggested that the first incident in the 6th century was responsible for destroying the summer villa of Pliny the Younger. Pliny (born in Como in 61CE and brought up by his uncle Pliny the Elder) provided a description of his two Como villas in a letter to his friend Voconius Romanus. These villas were named Villa Commedia for the one on the lakeside and Villa Tragedia for the one on the mountainside. He stated that he was able to fish directly on the lake from within Villa Commedia. Paolo Giovio (1484 – 1552), Como’s famous historian, priest and art collector, claimed that Villa Commedia was on the shores of Lenno. In 1847 two Roman corinthian columns were recovered there from off the shoreline. However others have suggested the villa was on the Lecco leg of the lake in Lierna – a hypothesis supported by the discovery of extensive mosaics there in 1876. His mountain villa, Villa Tragedia, was most likely located in Bellagio where the Villa Serbelloni now stands. 

Here is the text of Pliny’s letter to Romanus in which he suggests that he could almost fish from his bed when at Villa Commedia:

I am pleased to find by your letter that you are engaged in building; for I may now defend my own conduct by your example. I am myself employed in the same sort of work; and since I have you, who shall deny I have reason on my side? Our situations too are not dissimilar; your buildings are carried on upon the sea-coast, mine are rising upon the side of the Larian lake. I have several villas upon the borders of this lake, but there are two particularly in which, as I take most delight, so they give me most employment. They are both situated like those at Baiae:[135] one of them stands upon a rock, and overlooks the lake; the other actually touches it. The first, supported as it were by the lofty buskin,[136] I call my tragic; the other, as resting upon the humble rock, my comic villa. Each has its own peculiar charm, recommending it to its possessor so much more on account of this very difference. The former commands a wider, the latter enjoys a nearer view of the lake. One, by a gentle curve, embraces a little bay; the other, being built upon a greater height, forms two. Here you have a strait walk extending itself along the banks of the lake; there, a spacious terrace that falls by a gentle descent towards it. The former does not feel the force of the waves; the latter breaks them; from that you see the fishing-vessels; from this you may fish yourself, and throw your line out of your room, and almost from your bed, as from off a boat. It is the beauties therefore these agreeable villas possess that tempt me to add to them those which are wanting.—But I need not assign a reason to you; who, undoubtedly, will think it a sufficient one that I follow your example. Farewell.

The location of the lake surrounded by tall mountains is at the heart of Como’s beauty and magnificence. Yet it is an environment that must be treated with respect since this combination forms a single ecological and geological system whereby changes to one element may impact the other – and those changes need to be constantly monitored to avoid the sort of calamity that befell the communities in the Prealpi Carniche between the regions of Friuli and the Veneto.

The dam completed in 1959 across the Vajont on the borders of Friuli and the Veneto was a marvel of Italian engineering but the resulting lake destabilised the surrounding mountains with tragic results.

In 1948 it was decided to build a dam across the Vajont stream where it cut a deep gorge below Monte Toc. The purpose was to provide for hydroelectric power. The dam that was built was (and still is) a great example of Italian engineering. A deep lake formed behind its solid concrete curtain closing off the mountain valley. In 1960 a landslide on the left bank of the lake caused a 10 metre high wave which the dam resisted and contained. But at 22.39 on 9th October 1963, a great chunk of mountain separated from Monte Toc on the right bank of the lake. The landslide was 2 kilometres long, 150 metres high and dislodged around 260 million cubic metres of material which travelled down the mountainside at between 75 to 90 km/hour, arriving at the lake within a mere 20 seconds. The result was a massive wave raising the level of the lake from 700 metres above sea level to 930. At least 25 million cubic metres of water went over the top of the dam destroying all in its path downstream. The town of Longarone suffered 1450 dead with a total mortality in the area rising to 1900. The dam itself survived but not enough attention had been paid to the geological structure of Monte Toc and how it could have been destabilised by the creation of a lake at its feet. The engineering was first class but the lack of attention to detail, the refusal to pay attention to local knowledge and insufficient monitoring resulted in a frightful tragedy. 

9th October 1963, 260 million cubic metres of mountain fell off Monte Toc into the lake below. 25 million cubic metres of water went over the top of the dam causing over 1900 deaths in the valley below. The dam survived and still stands as a monument to the dangers of ignoring environmental factors.

While we can have no idea of how many died as a result of those early anomalous waves on Lake Como, we can be thankful that nothing on the scale of Vajont in 1963 has happened here. However we have had a full history of floods over the last 200 years and also a very tragic loss of life following extreme weather and landslides along the course of the River Adda before it enters Lake Como along the Valtellina. Written accounts of floods improved once local newspapers like ‘La Provincia’ were established in the mid nineteenth century. There are reports of significant flooding in Como on 6th October 1868 and also on 11th September 1888, a year apparently when the weather was particularly cold and wet with snowfall visible even at the height of summer along the mountain tops surrounding the lake.

An artist’s impression of Pliny the Younger’s Villa Commedia located by Paolo Giovio as being built on the lakeside in Lenno. This impression fails to reflect Pliny’s own description of the villa being directly on the lakefront.

Tragedy did however strike our province much more recently when heavy rains provoked landslides with many victims along the length of the Valtellina. On 17th July 1987 a series of summer storms fell on the Valtellina. The streams were already full but the volume of water was added to by snow and ice separated from the mountain tops by the force of the storms. The ground could not absorb any more water and the mountainsides could not hold the additional weight of the sodden soil. On the next day, the 18th, a terrifying landslide fell on the town of Tartano killing 19 people sheltering in one of the hotels.  The floods that followed were worse around Morbegno close to where  the Adda enters Lake Como by the Pian di Spagna. The floods even reached Como with Piazza Cavour and surrounding streets covered in water up as far as the Duomo. 

The Pian di Spagna, an alluvial plain created by sediment brought down the Valtellina by the River Adda as it enters Lake Como in the north,

However the landslide above Tartano on the 18th was just a prelude to further tragedy that befell the Valtellina 10 days later. Most of the communities in the plain of the River Adda from Bormio to Morbegno had been evacuated. Geologists  had also noted instability in the Val Pola and the mountains around Monte Zandila,  The area there had also mostly been evacuated except for seven workmen seeking to repair the damage to the main road up the valley to Bormio – a road normally used by up to eighty trucks a day transporting Levissima mineral water down the valley from the spring in Cepina.

Monte Zandila in the Val Pola where 40 million cubic metres of mountainside fell down into the valley bottom on 28th July 1987 to form a natural dam further threatening the Valtellina with further flooding.

They were already at work when at 07.23  a very loud blast sounding like a whiplash was heard as far away as Bormio. Thirty seconds later 40 million cubic metres of mountainside had dislodged itself and tumbled down into the valley. The scale of this landslide was immense. The split in the mountain appeared in just 8 seconds and the subsequent fall was over in the next 23 seconds. The rock and detritus fell a total of 1250 metres hitting the valley floor at between 275 to 390 kilometers an hour. The seismic shock caused by the landslide was registered as 3.9 on the Richter scale. The debris fell into the small lake of Morignone creating a wave ninety five metres high which still stood at between 15 to 20 metres high after travelling for 1.5 kilometres. All seven workmen were killed alongside twenty eight victims from Aquilone. That town had not been evacuated since no-one had expected such a devastating landslide. In fact those victims did not die due to the direct impact of the landslide but due to the displacement of air caused by it – a phenomenon also recorded at Valjont but normally associated with atomic explosions. 

Piazza Cavour, Como flooded following the rain and landslides in the Valtellina on 17th and 18th July 1987

If the inhabitants of the Valtellina had not suffered enough throughout that fateful summer, their drama was set to continue since that mass of 40 million cubic metres of rock, soil and wood had formed a dam across the entire valley floor which caused a build up of water behind it. This dam threatened to give way at any time and threatened a further wave of damage. A commission was established to design and execute a solution which would drain the newly created lake and redirect the Adda along its previous course. Here again, Italian engineering rose to the challenge and managed to secure the area from further threat by the end of August.

The lake formed by the landslide in the Val Pola which formed a dam across the floor of the valley.

Ours is a truly beautiful area but the history of anomalous waves in the distant past and the more recent history of tragedy arising from exceptional weather does mean we have to continue to monitor the stability of lake and mountain. While Italian engineering has, from Roman days, excelled in its achievements, we need to ensure that sufficient time and attention is paid to the maintenance and monitoring of any plants likely to impact the environment and the delicate ecological system formed out of lake and mountain. That challenge is made even more demanding faced with the obvious effects of climate change and in the way annual  rainfall distribution has changed over recent years. Some also have suggested that human intervention through building in mountainous areas may have exacerbated the Val Pola landslide.