Case Study: Mass Movement (SL IB Geography)

Vargas mass-movement facts

• Location – Vargas state, Venezuela
• Date – 15–16th December 1999
• Cause – Rainfall 40–50% above the usual average
• Events – Rainfall triggered flows of soil and debris
• Type of mass movement – Fast-moving debris flow

Hazard event

• Thousands of debris flows moved rapidly down the steep-sided mountains and narrow canyons
• The debris flow included boulders up to 10 metres in diameter
• In some places, the deposits created by the debris flow were several meters thick
• The debris flow speed was estimated at between 3 and 14.5 metres per second

Impacts

• Rain caused many mudslides, landslides and debris flows across the region
• There were between 10,000–50,000 deaths (many people were never found, and whole families were buried by the mudslides or swept out to sea)
• Over 150,000 people were made homeless
• Towns including Cerro Grande and Carmen de Uria were completely buried or swept away
• Over 70% of the population in Vargas state were affected
• The debris flow and mudslides destroyed many squatter settlements 
• Bridges and roads were destroyed
• The seaport at Maiquet was affected, leading to hazardous material leaking from containers
• Crops were destroyed
• Economic damage was estimated at US$3.5 billion
• Communication systems were destroyed 
• Supplies of food and water were affected for months
• Looting occurred across the region, meaning martial law had to be implemented for over a year

Factors affecting vulnerability

• The debris flows killed thousands of people as a result of a range of factors:
  • High population density in the coastal areas
  • Disorganised urban growth 
  • Poor quality buildings – many of the areas affected were squatter settlements
  • Corruption amongst government and public officials, which allowed homes to be built in vulnerable areas
  • In 1999, the government stopped collecting rainfall information:
    • This data was used to maintain bridges, reservoirs and other infrastructure
  • Lack of warning – no evacuation orders were issued
  • The government ignored a report from the Civil Defense Agency that urged them to declare a state of emergency 12 hours before the main debris flows

Ponzano facts

• Location – Ponzano, Italy
• Date – February 2017
• Cause – Combined effect of earthquakes and snowmelt leading to saturated soil and intense rainfall (81mm in four days)
• Type of mass movement – Slow-moving landslide

Hazard event

• The rate of landslide movement averaged one metre per day for two weeks
• Ponzano village in the north-east of Italy, about 30km north of Venice

Impacts

• An estimated 7 million m³ of material moved
• Over 100 people evacuated from 35 houses
• Collapse of several buildings
• Agricultural land around the village becoming unsafe to cultivate

Factors affecting vulnerability

• Low population density
• The slow movement of the landslide made evacuation easy
• The landslide was monitored and tracked by the Civil Protection Department
• Emergency services supported people to recover property from evacuated buildings
• Psychologists were provided to support people's mental health