Revised
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 Hazardous environments
Factors affecting the perception of risk
9.2 Hazardous environments resulting from mass movements Causes of mass movements
Revised
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).
Mass movement is any large-scale movement of the Earth’s surface that is not accompanied by a moving agent.
Table 9.3 Factors contributing to increasing shear stress and decreasing shear resistance Factors contributing to increased shear stress
Examples
Removal of lateral support through undercutting or slope steepening
Erosion by rivers and glaciers, wave action, faulting, previous rock falls or slides
Removal of underlying support
Undercutting by rivers and waves, subsurface solution, loss of strength by exposure of sediments
Loading of slope
Weight of water, vegetation, accumulation of debris
Lateral pressure
Water in cracks, freezing in cracks, swelling, pressure release
Transient stresses
Earthquakes, movement of trees in wind
Factors contributing to reduced shear strength
Examples
Weathering effects
Disintegration of granular rocks, hydration of clay minerals, solution of cementing minerals in rock or soil
Changes in pore-water pressure
Saturation, softening of material
Changes of structure
Creation of fissures in clays, remoulding of sands and clays
Organic effects
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.
Paper 2 Advanced Physical Geography Options
133
