Naturalistic and technical aspects of the landslide and the vajont dam
The Vajont Stream Valley connects the wide valley of the River Piave with the Friuli plain through Val Cellina. It is a narrow valley, characterized by steep and precipitous slopes; on both the right bank and the left bank of the river, there are the villages of Erto and Casso and the hamlets of Le Spesse, S. Martino, Prada, Liron, Pineda.
At the end of the fifties and the beginning of sixties, a few hundred meters upstream of the confluence with the River Piave, the stream Vajont was blocked, with hydroelectric purposes, by a dam. This plant was never officially operative because a huge landslide partially filled the tank causing a huge wave and nearly 2,000 victims.
Geology and geomorphology
The Vajont landslide is the latest and most tragic chapter in the long and complex history of the life of a mountain, during which rocks were formed, they were raised and then eroded; in this story you can also find some of the causes that have provoke the landslide.
The formation of rocks
165 million years ago, the area of Mount Toc and that of the Vajont valley were part of an underwater slope extended that served as a link between a shallow sea area and a deep sea one ( 700-1000 m).
Due to frequent landslides the materials became homogeneous limestone rock formations, then thin layers of limestone alternated repeatedly with levels of clay and carbonate mud.
The landslide of Mount Toc involved precisely the thick and uneven alternation of limestone and clay; the latter, moreover, during the movement of the landslide have acted as a lubricant material facilitating the sliding.
The lifting of the rocks
About 30 million years ago the limestone rock formations and alternations of limestones and clays were folded, fractured and subsequently lifted, due to the events that gave rise to the Alps. The detachment area thus coincided precisely with the slope that goes up from the valley to the Mount Toc’s peak.
The erosion of the rocks
Over the past two million years, the weather and the action of the glaciers shaped the slopes of the Vajont valley. More substantial changes to the morphology of the valley were impressed by man and nature, between 1957 and 1963: meadows, gorges and cultivation present on the steep slopes of the valley were buried by the artificial lake, under about 150 million meters cubes of water; then the landslide of Mount Toc turned into a hill what previously used to be a part of the valley.
The landslide that detached from the northern slopes of Mount Toc had huge dimensions: with a front of more than two kilometers, a width of at least 500 meters and a height of about 250, it transported downstream over 270 million m3 of rocks and debris. This mass, if it were removed by 100 trucks, would decrease of 1 mm per day: at such pace, it would take 7 centuries to remove it all!
The landslide had, in addition to the enormous size, even a high speed: advancing at about 100 km / h, this was the acceleration that, within few seconds, climbed up the opposite side for more than a hundred meters, barring the valley and changing it permanently.
At the time of the disaster, the height of water in the vicinity of the dam was equal to 240 m and the tank contained little more than a third of the total of the reservoir. The impact force of the landslide mass created two waves that went one toward the mountain, wiping away out the villages along the shores of the lake and the other toward the valley. The latter overcame the artificial barrier rising above it until it reached the lower houses of the village of Casso, located 240 m above the dam; then it was channeled into the narrow gorge of the Vajont, gaining more and more speed and energy;at the exit of the gorge, the mass of water, 70 meters high and with a speed of about 96 km / hour, poured into the Piave valley razing to the ground the village of Longarone and some nearby villages.
The victims of this tragic disaster, which occurred in less than 5 minutes, were 1909.
Already at the beginning of the century some private companies had realized the possibility of a widespread exploitation of the waters of the River Piave as a basin to produce electricity. Between the 30s and 60s were formulated and gradually perfected various projects leading to the creation of a series of dams, reservoirs and hydroelectric stations in more places along the course of the River Piave and its main tributaries. It also started the construction of a complex and great network of forced ducts that, by connecting the various reservoirs, allowed to exploit the same water several times. In this logic of rational and widespread use of the Piave basin, the Vajont dam took on a key role: indeed it received the waters from all the tanks located in the upper valley of the River Piave, which were then conveyed into the artificial lake Val Gallina, cargo tank above the Soverzene hydroelectric station. Just in relation to its important role the Vajont dam was redesigned and enlarged, becoming the “Grand Vajont” project.
In 1957, the Venetian company SADE introduced the final project and started the work that was completed in 1959. The dam, an arched building 264.6 m high, was the largest of its kind and the second ever in the world. Since 1960 began the dam testing with the filling of the reservoir. Already from the first reservoir was clear the general instability of the lake and above all of the left bank: the slope was indeed affected by signs of movements such as tilted trees, cracks in the ground and cracks on the houses’ walls; on November 4th , 1960 a landslide detached from the mount and slid into the lake while, at the top, appeared a long fracture in the form of M, which caused then the the landslide detachment of the 9th of October 1963. Some engineers formulated two hypotheses depending on the type of movement : the first one predicted a subsequent landslide of medium size, the other a single, large sliding; but no one expected the speed and violence with which the event occurred.
- HEIGHT: 264,6 m
- MAXIMUM AMOUNT OF THE RESERVOIR: 722,5 m asl
- MINIMUM AMOUNT OF THE RESERVOIR: 462 asl
- MAXIMUM RESERVOIR: 725,5 m asl
- TOTAL RESERVOIR: 168.715.000 m³
- PROFIT RESERVOIR : 150 million m³