A2 CAMBRIDGE GEOGRAPHY: HAZARDOUS ENVIRONMENTS - HAZARDOUS ENVIRONMENTS RESULTING FROM MASS MOVEMENTS. It contain case studies: Italian Mudslides 1998, New Zealand Landslip 1979, European Avalanches 1999.

1. A2 GEOGRAPHY
HAZARDOUS ENVIRONMENTS
9.2 HAZARDOUS
ENVIRONMENTS RESULTING
FROM MASS MOVEMENTS
CASE STUDIES
ITALIAN MUDSLIDES 1998
NEW ZEALAND LANDSLIP 1979
EUROPEAN AVALANCHES 1999

2. CAUSES OF MASS MOVEMENTS
Mass movements are a common natural event in unstable,
steep areas. They can lead to loss of life, disruption of
transport and communications, and damage to property and
infrastructure.
The most important factors that determine movement are :
• Gravity
• Slope angle
• Pore pressure
Increases in shear stress and/or decreases in shear resistance
trigger mass movements. Shear stress refers to the forces
trying to pull a mass downslope, while shear resistance is the
internal resistance of a slope.

3. MASS MOVEMENT
Is any large-scale movement of the Earth’s surface that is not
accompanied by a moving agent.

4. HUMAN ACTIVITIES
Human activities can increase the risk of mass movements,
for example by:
Increasing the slope angle by cutting through high ground
– slope instability increases with slope angle.
Placing extra weight on a slope (like in the case of new
buildings), this adds to the stress of a slope.
Removing vegetation – roots bind the soil together and
interception by leaves reduces rainfall compaction
Exposing rock joints and bedding planes, which can
increase the speed of weathering.

5. MANAGING THE HAZARD
OF MASS MOVEMENTS
There have been various attempts to manage the hazard of mass
movements. Methods to combat mass movements are largely
labour intensive and include:
Building restraining structures such as walls, piles, buttresses
and gabions – these can hold back minor landslides;
Excavating and filling steep slopes to produce gentler ones –
this can reduce the impact of gravity on a slope;
Draining slopes to reduce the build-up of water – this decreases
pore-water pressure in the soil;
Watershed management, for example afforestation and
agroforestry (farming the forest) – increasing interception.

6. CASE STUDY
THE ITALIAN MUDSLIDES OF 1998

7. THE ITALIAN MUDSLIDES OF 1998
In May 1998 mudslides swept through towns and villages in
Campania, Italy killing nearly 300 people.
Worst affected was Sarno, a town of 35.000 people.
Up to a year’s rainfall had fallen in 2 preceding weeks.
Campania is Italy’s most vulnerable region. Since 1892,
scientists have recorded over 1170 serious landslides in
Campania and Calabria.
Geologically the area is unstable.
It has active volcanoes such as Vesuvius, many mountains and
scores of fast-flowing rivers.

9. HUMAN ERROR
The disaster was only partially natural; much of it was down to
human error:
• The River Sarno’s bed had been cemented over.
• The clay soils of the surrounding mountains had been built on
hillsides identified as landslide zones.
• Over 20% of houses in Sarno were built without permission.
• Most were built over a 2-metre thick layer of lava formed by
the eruption of Vesuvius in 79 AD. Heavy rain can make it
liquid and up to 900 million tonnes of land are washed away
in this way every year.
• Hence, much of the region’s fragility is due to mass
construction, poor infrastructure and poor planning.

11. HUMAN IMPACTS
It is likely that the landslides of Northern Italy will be
mirrored by landslides in the Mediterranean region as it
becomes more developed.
All across southern Europe human impacts have combined to
increase the mass movement hazards.
First step involves clearing the land for developments.
The easiest way for this is a forest fire.
Large numbers of fires are started deliberately by developers
to ensure that the areas that they target lose their natural
beauty. One of the side effects of the fire is to loosen the
underlying soil.

12. 1979 ABBOTSFORD LANDSLIP
NEW ZEALAND
On 8 August 1979, a major landslip occurred in the Dunedin,
New Zealand suburb of Abbotsford.
It was the largest landslide in a built-up area in New Zealand's
history, resulting in the destruction of 69 houses – around one
sixth of the suburb – but no fatalities.
Much of the northeastern end of Abbotsford's residential area
was built on unstable ground.
Schist bedrock is covered with a thick layer of mudstone, with
a top coating of sand and clay-rich Cenozoic alluvial soil.
This type of surface becomes slick during moderate rainfall.

13. Final failure began at
about 9 pm on 8th
August. It lasted about
30 mins, during which
time a large block
moved forward by
about 50 m, leaving a
graben structure
behind that was about
16 m deep. This is very
clear in the image
below. The slide was a
deep-seated
translational block
slide that covered an
area of about 18
hectares. It was about
800 m, 400 m long and
up to 40 m deep. The
average movement
was about × 400 m, up
to 40 m thick).

14. THE NON-SEISMIC EVENT
Landslides of this type of material have been relatively
widespread within the Greater Dunedin area throughout both
recent prehistory and historical times.
The land was also sloping, and quarrying and the construction
of the nearby Dunedin Southern Motorway during the 1960s
and early 1970s may have further affected the land's stability.
The event was non-seismic, the increased rainfall over the
previous decade was the trigger and the slope gave way due to
the clay seams that formed parallel to bedding.
The clay seams formed due to flexural slip.

15. On the left side of the
image (the east) lies a
quarry from which
about 300,000 cubic
metres of sand had
been removed. Running
across the centre right
of the image is a large
array of cracks that
were opening up as the
landslide moved. These
cracks clearly extend
into the suburb,
although they become
less easy to discern in
this area. In fact
cracking was first
noted in some of the
houses in 1972 or even
before. Through time
the cracks grew until in
1978 they defined the
rear scarp of the
landslide as seen above.
The cracks caused
several water main
breakages in late 1978
and early 1979.

16. So why did the slope fail? Well, the first key factor is that the site was susceptible to failure under
natural conditions. The materials were dipping in the same direction as the slope and were weak and
susceptible to sliding. There were ancient landslide deposits on the site that point to previous
instabilities, well before humans could have played a major role. Second, the removal of the sand
from the quarry removed support from the slope, making it far more likely to fail. The slide was
probably triggered by increased water levels in the slope.

17. AVALANCHES
Avalanches are mass movements of snow and ice. Average
speeds are 40-60km/h, but speeds of up to 200km/h have been
recorded in Japan.
Loose avalanches, comprising fresh snow, usually occur soon
after a snowfall. By contrast, slab avalanches occur at a later
date, when the snow has developed some cohesion.
They are usually much larger than loose avalanches and cause
more destruction.
They are often started by a sudden rise in temperature, which
causes melting. This lubricates the slab and makes it unstable.
Many of the avalanches occur in spring when the snowpack is
large and temperatures are rising.

18. NUMBER OF AVALANCHES AND ALTITUDE
There is a relationship between the number of avalanches and
altitude.
In the Swiss Alps most occur between 2000m and 2500m and
there is reduced occurrence both higher up and lower down.
Although avalanches cannot be prevented, it is possible to
reduce their impact – various methods are:
Afforestation;
Active techniques as direct protection;
Deflectors;
Retardant mounds.

19. An avalanche snow bridge near
a ski-resort in Vorarlberg (A).

20. AFFORESTATION
Afforestation is considered to be very important for any
avalanche protection work because trees can anchor the snow
and prevent the avalanche from ever happening.
Trees can be planted, increasing stability of the slope and
helping to reduce the damage further down the valley.

21. Preservation and protection of forests
in avalanche starting zones is one of the oldest and
most widely used avalanche mitigation measures.
The Swiss recognized the value of forests for
avalanche protection and enacted legislation in
1876 preventing forest removal above villages in
avalanche areas.

22. ACTIVE TECHNIQUES
Some of the active techniques are:
- disrupting weak layers in the snow pack,
- increasing the uniformity of the snow pack, and
- lessening the amount of snow available in snow pack for
entrainment in an avalanche.
This can be accomplished either by triggering smaller less
hazardous avalanches, or by directly influencing the structure
of the layering of the snow pack.

23. Avalanche blasting in the French ski resort of Tignes (3,600 m)

24. CASE STUDY
THE EUROPEAN AVALANCHES OF 1999
The avalanches that killed 18 people in the Alps in February
1999 were the worst in the area for nearly 100 years.
They occurred in an area that was thought to be fairly safe.
Precautionary measures had been taken, such as a huge
avalanche wall to defend the village of Taconnaz (F).
The villages of Montroc and Le Tour, located at the head of the
Chamonix Valley, had no such defences.
The avalanche that swept through the Chamonix Valley, had no
such defences.

26. CHAMONIX VALLEY AVALANCHE
The avalanche that swept through the Chamonix Valley killed
11 people and destroyed 18 chalets. It was about 150m wide,
6m high and travelled at a speed of up to 90km/h.
Rescue work was hampered by the low temperatures (-7C)
which caused the snow to compact, and made digging almost
impossible.
Nothing could have been done to prevent the avalanche and
avalanche warnings had been given the day before, as the
region ad experienced up to 2m of snow in just 3 days.
Ongoing avalanche monitoring meant that villagers and
tourists in the safe zone through that they were safe.

28. MONTROC/LE TOUR AVALANCHES
Buildings in Montroc were classified as being almost
completely free of danger.
By contrast, in the avalanche danger zones no new buildings
had been developed for many decades.
Meteorologists have suggested that disruption of weather
patterns resulting from global warming will lead to
increased snow falls in the Alps, which will be heavier and
later in the season.
This would mean that the conventional wisdom regarding
avalanche safe zones would need to be re-evaluated.

30. AVALANCHES MORE LIKELY WHEN...
Slopes are steeper than 30 degrees.
A lot of new snow fall over a short period.
Winds lead to drifts.
Old snow melts and refreezes, encouraging new snow to
slide off.