The Weekly Reflektion 51/2021

We at Reflekt wish all our subscribers, and other business connections a very happy holiday period, and keep yourselves safe in these trying times. We look forward to seeing you next year.

Vajont Dam seen today from both above and below the dam

We often establish a risk picture at the start of a project which we use to conclude on the viability of reaching our goal. However sometimes changes occur that may undermine the assumptions. Are you looking for signals that the assumptions in your risk picture should be updated?that lead to answers that we want to hear. 

On the 9th of October 1963 a huge wave overtopped the VajontDam 60 miles north of Venice, in Italy killing an estimated 2000 people in the valley below the dam. The wave was caused by a landslide of ca. 260 million m3 from the Monte Toc mountainside. The dam was designed for 169 million m3 of water. Construction started in 1957 and fractures and shifts in the surrounding mountainsides, resulted in 3 reports concluding that the side of Monte Toc would likely collapse if filling was completed as planned. In 1960 during filling of the dam minor landslides and earth movements were noticed. These were reported by journalists, but the newspapers were sued by the government for ‘undermining the social order’. The government and the dam owners drive to complete the dam overshadowed concerns on the dam safety.

In November 1960 with the water level at 190m of the total planned of 262m an 800,000 m2 landslide collapsed into the lake which resulted in a stop in the filling and a reduction in the water level by 50m. In April/May 1962 filling started again, and with the water at a level of 215m, 5 earthquakes were reported by the people in surrounding villages. Tests indicated that a landslide into the dam would generate a 20m high wave, so the agreed maximum filling level was lowered to 25m below the top of the dam. The owners were authorised to continue filling the dam as it was assessed that they could react to any incidents and control the water level to prevent the water overflowing the dam. Slides, shakes and ground movements were continuously reported by the surrounding population as it was filled.

In September 1963 the entire side of Monte Toc slid 22 cm, and the water level was decreased by 20m to give an additional safety margin. In October the mountainside was still moving, up to 1 m per day, trees were seen falling and rocks were rolling into the lake. It was assessed that a major landslide was imminent and engineers from the dam project gathered on the top of the dam to observe the event. A 2km long landslide filled the reservoir in less than one minute with the resulting 250m high wave destroying villages on the opposite side of the reservoir to the landslide and sending 50 million m3 of water over the top of the dam into the valley below, taking the watching engineers with it.

The town of Longarone and other centres below the dam weredestroyed and of the ca. 2000 victims, only 1500 bodies were recovered. The immediate response from the government was that the incident was ‘an unexpected and unavoidable natural event’. The geological investigation concluded that the limestone formations were interbedded with clay layers giving weak shear zones. Today the dam is a dry basin open to visitors as shown in the photograph.

Focusing on the final goal and ignoring signals along the way that the original risk picture has changed can have catastrophic consequences. In your activities are you looking for signalsthat the risk picture has changed?

Reflekt AS