In high mountains there are always massive landslides, as a result of which tidal waves often rush through valleys and destroy everything on their way. These disasters almost always happen suddenly and without warning. The population in the affected valleys is then surprised by such tidal waves, and it is not uncommon for numerous people to be killed. One of the worst of its kind disasters occurred in February of this year in the northern Indian state of Uttarakhand, when more than two hundred people were killed in a landslide tidal wave raging through several valleys of the Himalayas. A group of German and Indian geoscientists have now meticulously traced the events and devised a system that could warn of such catastrophes in good time in the future.

The chain of fatal events in the Himalayas began on the morning of February 7th at an altitude of more than 5,500 meters on the slope of the glaciated Ronti Mountain, which is part of the Nanda Devi National Park.

At around 10:21 a.m. local time, a massive mixture, consisting of around 20 percent glacier ice and 80 percent boulders, broke up there for an as yet unexplained cause.

It had a volume of about 27 million cubic meters and fell at great speed almost two kilometers down the steep slope.

Less than a minute later, the mass hit the valley of the Ronti Gad river with the force of a huge explosion.

Seismometers sound the alarm

Because of the turbulence during the fall and the friction with the ground, most of the ice had melted when it fell. On impact, many of the large boulders also broke into smaller fragments, which, together with the melted ice, rushed down the narrow valley in a tidal wave at a speed of about 90 kilometers per hour. In less than ten minutes, the wave reached the Rishiganga hydropower plant, 15 kilometers from the point of impact. The facility was completely destroyed. 35 workers were swept away by the floods.

Even more people died at the Tapovan Vishnugad hydropower plant, which is under construction, about 15 kilometers further downstream.

At least 140 people died there in the almost 20 meter high tidal wave.

Almost forty minutes after the rock fall, the tidal wave, still 16 meters high, reached the district town of Joshimath, where several mountain rivers merge to form the Alaknanda, one of the important tributaries in the upper reaches of the Ganges.

At Joshimath, the wave destroyed an important bridge.

In the flatter valley of the Alaknanda, the tidal wave slowed down and lost more and more of its energy.

Nevertheless, almost 50 kilometers downstream from Joshimath, a height of over three meters was measured.

A few years ago, seismologists from the Indian National Institute for Geophysical Research (NGRI) set up a network of 76 modern earthquake stations in the state of Uttarakhand in Hyderabad. The network was mainly used to monitor seismic activity on the southern flank of the Himalayas. On February 7th, the seismometers did not come to rest. First of all, the measuring station closest to the Nanda Devi National Park registered ground vibrations caused by the falling rock. The impact of the rock and ice masses on the valley floor was then measured like a strong earthquake. Then the measuring devices of the Uttarakhand network recorded the ground movements of the tidal wave that raced through the mountain valleys.

The research group around Kristen Cook from the Geosearch Center in Potsdam has now evaluated the seismic recordings together with employees of the NGRI. As the scientists write in the journal Science, in this way they were able to understand in detail the individual phases of the chain of events leading to the disaster. Both the rock fall and the impact and finally the raging tidal wave produced characteristic, clearly distinguishable seismic signals.

Cook and her colleagues argue that these signals could serve as the basis for an early warning system for future floods. With such an automatic system, the construction workers at the Tapovan power plant could have been warned perhaps ten minutes before the wave would arrive, and the residents of Joshimath possibly even half an hour in advance.

However, the proposed warning system requires a dense network of seismic stations, which are not found everywhere on the long southern flank of the Himalayas in India, which is threatened by rock falls. However, the highest mountains in the world are not the only regions where such natural disasters can occur. In October 1963, in the border area of ​​the Italian regions of Veneto and Friuli, more than 2000 people died when a massive landslide fell from Monte Toc into the Vajont reservoir. The resulting tidal wave sloshed over the narrow dam wall, raced down into the valley at great speed and swept several villages with it.

In contrast to the Himalayas, however, there is a dense network of seismic stations in the Alps whose measurement data could be used for early warning.

However, such a system has yet to be developed.

The study now published is a first step in this direction.