9.2 Hazardous environments resulting from mass movements

At an individual level there are three important influences on an individual’s response to risk. For example: l experience – the more experience of environmental hazards the greater the adjustment to the hazard l material well-being – those who are better off have more choice l personality – is the person a leader or a follower, a risk-taker or riskminimiser?

Ultimately there are three choices: do nothing and accept the hazard; adjust to the situation of living in a hazardous environment; leave the area. It is the adjustment to the hazard that we are interested in. The level of adjustment will depend, in part, upon assessing the risks caused by the hazard. This includes: l identification of the hazards l estimation of the risk (probability) of the environmental hazard l evaluation of the cost (loss) caused by the environmental hazard

Now test yourself

7 What are the factors that influence an individual’s response to risk? 8 What choices do people have with regard to living in areas at risk of natural hazards?

Answers on p.219

Tested

9.2 Hazardous environments resulting from mass movements

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 and pore pressure. Increases in shear stress and/or decreases in shear resistance trigger mass movement (Table 9.3). Table 9.3 Factors contributing to increasing shear stress and decreasing shear resistance

Revised

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

Factors contributing to increased shear stress

Removal of lateral support through undercutting or slope steepening Removal of underlying support

Loading of slope Lateral pressure Transient stresses

Factors contributing to reduced shear strength

Weathering effects

Changes in pore-water pressure Changes of structure Organic effects

Examples

Erosion by rivers and glaciers, wave action, faulting, previous rock falls or slides

Undercutting by rivers and waves, subsurface solution, loss of strength by exposure of sediments Weight of water, vegetation, accumulation of debris Water in cracks, freezing in cracks, swelling, pressure release Earthquakes, movement of trees in wind

Examples

Disintegration of granular rocks, hydration of clay minerals, solution of cementing minerals in rock or soil Saturation, softening of material Creation of fissures in clays, remoulding of sands and clays Burrowing of animals, decay of roots

Human activities can increase the risk of mass movements, for example by: l increasing the slope angle by cutting through high ground – slope instability increases with slope angle l placing extra weight on a slope (e.g. new buildings); this adds to the stress on a slope

Expert tip

Shear stress refers to the forces trying to pull a mass downslope, while shear resistance is the internal resistance of a slope.

l

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

There have been various attempts to manage the hazard of mass movements. Methods to combat mass movements are largely labour intensive and include: l building restraining structures such as walls, piles, buttresses and gabions –these can hold back minor landslides l excavating and filling steep slopes to produce gentler ones – this can reduce the impact of gravity on a slope l draining slopes to reduce the build-up of water – this decreases pore-water pressure in the soil l watershed management, for example afforestation and agroforestry (‘farming the forest’) – this increases interception and reduces overland flow

Case study The Italian mudslides of 1998

Now test yourself

9 How can human activity increase the risk of mass movements?

Answer on p.219

Tested

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 the two preceeding 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 fastflowing rivers.

The disaster was only partially natural; much of it was down to human error: l The River Sarno’s bed had been cemented over.

l

l

l

l

The clay soils of the surrounding mountains had been rendered dangerously loose by forest fires and deforestation. Houses had been built on hillsides identified as landslide zones. Over 20% of the 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 79ad. 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.

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 hazard: l The first step involves clearing the land for development. 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. l In Sicily up to 20,000 holiday homes have been built on beaches, cliffs and wetlands in defiance of planning regulations. l In Italy over 200,000 houses have been built without permission, and many are without proper drainage or foundations. l Many stand close to riverbeds that seem empty and remain empty until storms occur.

Now test yourself

10 What are the natural reasons why northern Italy is at risk from mudslides? 11 What human factors have increased the risk of mudslides in the region?

Answers on p.219

Tested

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. There is also a relationship between the number of avalanches and altitude. For example in the Swiss Alps most occur between 2000m and 2500m and there is reduced occurrence both higher up and lower down.

Revised

Fences

Rakes

Wedge Accumulation zone Starting zone Walls Avalanche gully

Track zone Runout zone Avalanche shed

Afforestation Direct protection Deflector Retardant mounds

Figure 9.2 Avalanche protection schemes

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. Moreover, they occurred in an area that was thought to be fairly safe. In addition, precautionary measures had been taken, such as an enormous avalanche wall to defend the village of Taconnaz. However, 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 killed 11 people and destroyed 18 chalets (Figure 9.3). 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 (–7°C), 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 had experienced up to 2m of snow in just 3 days. Ongoing avalanche monitoring meant that villagers and tourists in the ‘safe’ zone thought that they were safe. 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.

Avalanches are more likely when: • slopes are steeper than 30° • a lot of new snow falls over a short period • winds lead to drifts • old snow melts and refreezes, encouraging new snow to slide off. At Montroc, heavy snow fell on Monday and Tuesday, but melting and refreezing of old snow was thought not to be responsible.

Les Houches Le Praz Chamonix Le Tour Montroc Bec Rouge Les Chosalets Argentière

La Flégerè Mont Blanc Mgne de Peclerey Les Grandes

Montets Tuesday 9 February, 2.35pm (1.35pm GMT): a tidal wave of snow 150 m wide and 6 m high crashes down into the valley at 60 mph and buries much of the village of Montroc

3

10 people killed, 11 rescued unhurt, five with minor injuries, one seriously injured. Two believed missing

4

Snow storms on Tuesday night prevent rescues by helicopter during the first vital hours. The snow is packed so high that only mechanical diggers can hack their way through to the chalets

6

The force of the avalanche is so great that it sweeps through Montroc and travels 40 m uphill to smash into the village of Le Tour. The avalanche carries some of the chalets as far as 400 m (a quarter of a mile)

5

Sheer weight of new snow causes massive avalanche

2

Over six feet of snow falls in a few days on the Chamonix valley

1 Argentière Montroc Le Tour

Chamonix

Taconnaz

Mt Blanc 0 km 3 I TA LY F RANC E SWITZERLAND