Mass Movement Case Studies

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Vargas Case Study

Date: 15 December 1995

Location: Vargas state of Venezuela

Reason: Torrential rains and flash floods triggered soil instability and flow of debris.

The neighborhood of Los Corales was buried under 3 metres of mud and a high percentage of homes were simply swept away. Whole towns like Cerro Grande and Carmen de Uria completely disapeared - 10% of the population of Vargas perished during this event.

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Background knowledge

Population density

  • The alluvial fans that are built as deposits from floods provide the only extensive flat surfaces along the mountainous coastline of North central Venezuela.
  • Thus, there is high population density which increases risk to life and property from flash flood and debris flow events.


  • In a 52 hour span during the 14th, 15th and 16th of December 1999, 91.1cm of rain fell - average rain for a year
  • Oversteepend hillslopes failed during the rainstorm, sending landslides of soil into the channels - supplying sediment to flash floods and debris flows
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  • Heavy rains fell betwen 14th & 16th of December - 91.1cm
  • Runoff entered chanels and rushed towards the sea, picking up and depositing sediment on its way
  • The rains also triggered thousands of shallow landslides that stripped soil and rock off of the landscape - water liquefied these landslides into debris flows
  • Debris flows moved rapidly - deadly - 3.3 to 14.5 metres per second
  • Debris flows incised new channels to depths of several metres into every alluvial fan delta on the vargas state coastline
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Caraballeda fan

  • The Caraballeda fan was one of the most hard-hit areas
  • Heavily-urbanised - many high-rise buildings and multistory houses
  • Debris flows overwhelmed concrete lined channel and flows overtopped the banks wherever the channel changed direction
  • Geologists estimate the deposit volume to be at least 1.8 million cubic metres
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  • The disaster caused estimated damges of $1.75 to $3.5 billion
  • The death toll was considered to be between 10,000 and 30,000 - no reliable census data - especially for shanty towns and small communities which were completely wiped out.
  • Only some 1,000 bodies were recovered
  • More than 8,000 homes and 700 apartment buildings were destroyed in Vargas
  • This displaced 75,000 people
  • Over 70% of the population of Vargas was affected by the tragedy
  • Public services, like water, electricity, phone lines, and land transportation completely disappeared in some places
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  • No supplies of water or food for months in some areas
  • Most of the population had to be evacuated - more than 100,000
  • Looting and sacking sprouted up everywhere
  • This forced the military to implement martial law for more than one year
  • Venezuelans who were not affected told to 'adopt a family'
  • Children that were feared to be orphaned were given shelter in La Casona, the Presidential residence in Caracas
  • Famous baseball shortstop Omar Vizquel helped raise more than $500,000
  • US sent helicopters and troops - arrived two days after the disaster
  • 450 marines and naval engineers
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Yungay Case Study

Date: May 31st 1970

Location: Yungay is a town in the Ancash Region in North central Peru, South America

Reason: An undersea earthquake struck 22 miles off the coast of Casma on the Pacific Ocean, where the Nazca Plate is being subducted by the South American Plate. The shock affected the Peruvian regions of Ancash and La Libertad. It had a magnitude of 7.9

This caused the northern wall of Mount Huascaran to become destabilised, causing a rock, ice and snow avalanche - burying the town of Yungay

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The Landslide

  • The landslide started as a sliding mass of glacial ice and rock about 3,000 feet wide and one mile long.
  • It advanced 11 miles to the village of Yungay at an avergae speed of 280 to 335km per hour
  • By the time it reacher Yungay, it is estimated to have consisted of about 80 million cubic metres of water, mud and rocks
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Casualties and Damage

  • The reported death toll from the earthquake and avalanche totalled 74,194 people - although some estimates place it as high as 80,000
  • Roughly 25,600 went missing abd 143,331 were injured
  • Over 1,000,000 people were left homeless and anither 3,000,000 were affected
  • In Yungay alone, 20,000 people died from the landslide - only 400 survived
  • Survivors included people who were at higih points of the town - such as the cemetery - as well as 300 children attending a circus at the local stadium
  • The earthquake affected an area of about 83,000km - an area larger than Belgium and The Netherlands combined
  • Nearly all infrastructure was lost
  • Economic losses surpassed billions of US dollars
  • In Chimbote, about 80% - 90% of buildings were destroyed
  • The Pan-American highway was also damaged
  • The Peruvian gov has forbidden the excavation of Yungay, declaring it a national cemetery.
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Nevada Del Ruiz Case Study

Date: November 13th 1985

Location: Armero, located 48km from the Nevado del Ruiz volcano - 169km away from Colmbia's capital Bogota

A prominent farming town, it was responsible for one-fifth of Colombia's rice production

Built on top of an aluvial fan - the town had been destroyed by a volcanic eruptino in 1595 and by mudflows in 1845

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Preparation and attempted evacuation

In September 1985, as earthquakes and phreatic eruption rocked the area, local official began planning for an evacuation.

In october, a hazard map was finalized for the area around Nevado del Ruiz

The map was poorly distributed to the people at high risk from Ruiz - many srvivors had never heard of it, even though several of the country''s major newspapers featured versions of the map.

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  • At 9:09pm on Novermber 13th, 1985, Nevado del Ruiz ejected dacitic tephra more than 30km into the atmosphere
  • The total mass of the erupted material was 35 million metric tonnes
  • The eruption created two pyroclastic flows that melted summit glaciers and snow, generating four thick lahars that raced down river valleys on the volcano's flanks
  • The lahars ran down the volcano's sides at an average speed of 60km per hour - dislodging rock and destroying vegetation
  • The lahars follwed the six river valleys leading from the volcano, where they grew to almost four times their original volume
  • Max width of 50m
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  • More than 25,000 people died - buried in mud, vegetation, debris etc.
  • The loss of life was exacerbated by the lack of an accurate timeframe for the eruption and the unwillingness of local authorities tot ake costly preventative measures without clear signs of imminent danger
  • The eruption occured at the height of guerrilla warfare in Bogota, and so the government and army were occupied at the time of the eruption
  • Only a few buildings and structures remained standing
  • The damages were assessed at $6billion  - approx one-fifth of Colombia's 1985 GNP
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Relief Efforts

  • The eruption occurred at the same time as the 1985 Mexico City earthquake, limiting the amount of supplies that could be sent to each of the disasters
  • Makeshift triage statinos were established in Lerida, Guayabal and Mariquita
  • The remaining victims wer directed to Ibague's hospitl
  • The US gov spent over $1 million in aid - another initial donation of $25,000 to Columbian disaster assistance institutions
  • The US sent additional aircraft and supplies, including 500 tents, 2,250 blankets and several tent repair kits
  • 24 other nations contributed to the rescue and assistance of survivors
  • Ecuador supplied a mobile hospital, and Iceland's Red Cross sent $4,650
  • Japan $1.25 million
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Aberfan Case Study

Date: 21st October 1966

Location: Welsh village of Aberfan, near Merthyr Tydfil

Reason: Caused by a build-up of water in the accumulated rock and shale, hich suddenly started to slide downhill in the form of slurry

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For 50 years up to 1966, millions of cubic metres of excavated mining debris fron the National Coal Board's Merthyr Vale Colliery were deposited on th side of Mynydd Merthyr, directly above the village of Aberfan.

Huge piles of loose rock mining spoil had been built up over a layer of highly porous sandstone that contained numerous underground springs.

Concerns over area had been largely ignored by the NCB's area management

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After several days of heavy rain, a subsidence of about 3-6 metres occured on the upper flank of colliery waste tip.

At 9:15am, more than 150,000 cubic metres of water satirated debris broke away and flowed downhill at high sped.

The tipping gang saw the landslide start but were unable to raise alarm because their telephone cable had been repeatedly stolen - inquiry established that a telephone conversation would not have stolen any lives.

40,000 cubic metres of debris smashed into the village in a slurry 12 metres deep.

The slide destroyed a farm and 22 terraced houses along Moy Road and slammed into the northern side of Pantglas Junior School - 116 kids died + 28 adults killed

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Parents/untrained volunteers hindered the efforts of trained professionals.

A few children were pulled out alive in the first 2 hours, but no survivors were found after 11am

Next day, 2,000 emergency services workers and volunteers were on the scene

Nearly a week before all the bodies were recovered

Totak death toll was 144

Whole generation wiped out and Aberfan was never rebuilt

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