Case Study: Mass Movement (DP IB Geography)

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

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

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

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

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

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

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

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