The Dam That Created a 250-Meter Wave | Vajont Dam 1963

On the night of October 9, 1963, roughly 260 million cubic meters of rock broke loose from the side of Monte Toc and slid into the reservoir behind the Vajont Dam in northern Italy. The landslide displaced the water so violently that it sent a wave over 250 meters above the reservoir’s surface. The wall of water cleared the top of the dam and crashed into the valley below.

In less than four minutes, nearly 2,000 people were dead.

An Engineering Marvel with a Fatal Flaw

The Vajont Dam was a double-arch concrete dam built in the narrow Vajont gorge in the Italian Alps. At 262 meters tall, it was one of the tallest dams in the world at the time. Construction began in 1957 and was completed in 1959. It was designed to generate hydroelectric power for a rapidly industrializing postwar Italy.

The dam itself was an engineering achievement. The arch design was elegant and structurally sound. The problem was never the dam. It was the mountain next to it.

Monte Toc, the steep slope on the southern side of the reservoir, was geologically unstable. Engineers and geologists knew this. As early as 1960, studies identified the risk of a large landslide into the reservoir. Small slides had already occurred during early filling tests, and the slopes showed clear signs of movement.

SADE, the energy company behind the project, pressed forward anyway. The reservoir was too valuable. The investment too large.

Warning Signs Ignored

Between 1960 and 1963, the signs of disaster became impossible to miss. The southern slope of Monte Toc was moving. Instruments recorded the creep. Animals on the mountainside grew restless and abandoned their usual grazing areas. Cracks opened in the ground above the reservoir. Small landslides sent waves across the water.

In 1960, a slide of about 700,000 cubic meters fell into the reservoir, generating a two-meter wave. Engineers responded by lowering the water level. They did not stop filling the reservoir permanently. Instead, they ran tests, raising and lowering the water level repeatedly, trying to find a safe operating range.

Residents in the villages below the dam, Longarone, Pirago, Villanova, Rivalta, and others, grew alarmed. Some wrote letters to authorities. A local journalist, Tina Merlin, published warnings in her newspaper about the instability of the mountain and the danger to the towns downstream. She was sued for spreading alarm. She was acquitted, but her warnings went unheeded.

By late September 1963, the slope was moving at rates of up to 20 centimeters per day. Engineers knew a large slide was coming. They tried to lower the reservoir level, but it was too late.

The Night of October 9

At 10:39 p.m., the entire southern face of Monte Toc gave way. An estimated 260 million cubic meters of rock, roughly twice the volume of the reservoir itself, plunged into the water at speeds of up to 110 kilometers per hour.

The impact generated a series of massive waves. One wave surged 250 meters up the opposite slope of the valley. Another, estimated at over 50 million cubic meters of water, cleared the top of the dam and roared down the gorge toward the Piave River valley below.

The dam held. It was never breached. The wave simply went over it.

The wall of water hit the town of Longarone at roughly 70 kilometers per hour. The town was erased. Buildings, roads, bridges, and nearly every person in it were swept away in seconds. The smaller villages of Pirago, Villanova, Rivalta, and Faè were destroyed as well. Upstream of the dam, the village of Erto was also damaged by the wave action within the reservoir.

By morning, the valley looked like a moonscape. Where Longarone had stood, there was mud, rock, and silence.

The Toll

The official death toll stands at 1,917 people, though some estimates place it higher. Longarone alone lost over 1,450 of its roughly 1,700 residents. Entire families were wiped out. Many bodies were never recovered.

Rescue and recovery efforts were slow. The roads into the valley were destroyed. Survivors from the surrounding areas who arrived at first light found almost nothing left to save.

Accountability

In the years that followed, a lengthy legal battle played out. In 1968, an Italian court found several SADE engineers and officials guilty of negligence. The sentences were light. Most were reduced on appeal. No one served significant prison time.

The court established what the scientific community had been saying for years: the disaster was foreseeable and preventable. The geological instability of Monte Toc was documented. The risk of a catastrophic landslide was known. The decision to continue filling the reservoir despite clear evidence of danger was a choice made for profit.

The dam still stands today in the gorge, silent and unused. The landslide scar on Monte Toc is still visible. Longarone was rebuilt in the years that followed, but the community that existed before October 9, 1963 was gone forever.

The Lesson

The Vajont disaster is studied in engineering and geology programs around the world as a case study in what happens when economic interests override scientific evidence. The dam did not fail. The engineering was sound. The geology was ignored.

Nearly 2,000 people died not because of a natural disaster, but because the people in charge of a reservoir knew the mountain was unstable and chose to keep filling it anyway. The warnings were there. The data was there. The decision to proceed was made with full knowledge of the risk.

The Vajont Dam stands as a monument to a preventable catastrophe. The tallest dam in the valley, with no water behind it, and an entire town buried beneath it.