chap7

CHAPTER SEVEN

THE HUMAN IMPACT

In this chapter - Engineering control schemes - friend or foe? | Failure of engineering structures | Earthquakes triggered by human activities | Subsidence caused by withdrawal of underground liquids | Landslide triggered by construction of waste tips | Flood risk altered by land-use changes | Nuclear developments and risks | Air pollution - acid rain | Greenhouse gases, atmospheric warming and sea level rise

Hazards represent the point of contact between humans and the environment. Many hazards are natural, because the processes which cause them are natural ones. But human activities can alter hazard threats in various ways, including modifying risk by engineering control schemes and creating new hazards through ill-conceived environmental management or use of technology. Some examples are given below to illustrate the range of possibilities, but the list is far from exhaustive.

Engineering control schemes; friend or foe?

Most forms of adjustment to natural hazards are adopted to reduce hazard impact or losses. Ironically, they are sometimes liable to make things worse.

For example, the construction of embankments for flood control along rivers encourages building on floodplains. This will cause higher losses when the control structure gets overtopped by a larger flood. Hurricane Agnes, which struck the eastern United States early in 1972, caused extensive river flooding, and much of the damage was concentrated in the 40 percent of the floodplains which had embankments which were overtopped.

Similarly, an increase in erosion commonly occurs downdrift of coastal defence structures (such as groynes and sea-walls), due to a reduction in natural sediment supply by longshore drift. Here, reduction in hazard in the controlled zone causes an increase in hazard beyond it. Many coastal areas in southern England are exposed to such threats.

Failure of engineering structures

Major threats are posed by the prospect of failure or collapse of engineering structures like sea defence walls, river retaining walls and dams. Such structures are designed to withstand an event of a specified size (see chapter 3), and should an event of greater magnitude occur then the structure is likely to fail.

The most devastating dam disaster in history occurred in Italy on 9th October 1963. Vaiont Dam was completed in 1960, and - at 265 m - was then the world's second highest dam, with a storage capacity of 150 million m³. The disaster happened fast. At 10.41 am hillslope failure produced a slide of over 240 million m³ of material (mostly rock), which filled much of the reservoir within a minute. This causes a massive wave of water (up to 100 m high) to rush over the dam and down the valley. The wall of water, up to 70 m high, sweept away everything in its way for many kilometres downstream. By 10.55 the flood had passed, and around 2,600 people had died.

Earthquakes triggered by human activities

Earthquakes can be triggered off by imposing considerable pressures on previously stable land surfaces, such as through the weight of water stored behind large reservoirs. In 1935, for example, the Colorado River was dammed by the Hoover Dam to form Lake Mead. As the lake filled, and the underlying rocks adjusted to the load of over 40 km³ of water, a series of long-dormant faults in the area were reactivated, resulting in over 6,000 minor earthquakes within the first ten years. Over 10,000 events had been recorded up to 1973, about 10 percent of which were strong enough to be felt by residents, although none caused damage.

Other earthquakes have been triggered by the disposal of toxic waste water in Denver, Colorado. The water was contaminated during production of materials for chemical warfare and industrial use at Rocky Mountain Arsenal, and the plan was to dispose of it in a special well, over 3,500 m deep, dug into geologically stable bedrock. Disposal began in March 1962, and soon afterwards a series of minor earthquakes were detected in the previously stable area. None produced real damage, but they did give rise for concern. Between March 1962 and November 1965 over 700 minor earthquakes were detected around the site. When more water was disposed of down the well, earthquakes were more common (the water was lubricating long-dormant deep faults). Geologists argued that the fluid injection into underlying fractured Pre-Cambrian gneiss posed unacceptable hazard risk, and so disposal ceased in late 1965. The well was filled in during February 1966.

Subsidence caused by withdrawal of underground liquids

Hazards are also created when land subsidence is caused by extraction of underground resources, such as the withdrawal of oil and groundwater. Exploitation of the Wilmington Oil Field near Los Angeles began in 1938, and by 1941 subsidence of 0.4 m had been noted. By 1962 subsidence had spread over a wide area (Figure 12), and reached 8 m in the centre of the oval depression. Horizontal movements of over 3 m associated with the subsidence produced extensive damage to wharfs, pipelines, buildings, streets and bridges in the area. Costly repairs, extensive dyking and large-scale surface filling were required, bringing the estimated total costs of repair to about $150 million (mid-60s prices).

Figure 12. Land subsidence at long Beach, California, due to oil extraction between 1936 and 1962. The map shows lines of equal subsidence (in metres) round the centre of the subsidence depression.

Source: after Poland and Davis (1969)

Landslide triggered by construction of waste tips

Further hazards can be created by constructing unstable spoil heaps. The tragic hillslope failure in Aberfan, in the coal-mining area of South Wales, occurred on the morning of 21st October 1966. It was triggered off by seepage of water from spring lines which reduced the internal cohesion of a 67 m high solid heap of colliery waste. The resulting slope failure caused a rapid debris flow of over 100,000 m³ of material which engulfed part of the town of Aberfan, where 144 died (116 children and 5 teachers were entombed in Pantglas Infants School). Since 1969 the colliery tip complex has been entirely removed and other potentially dangerous tips in the valley have been lowered, regraded and landscaped to prevent the occurrence of similar events (although there is still some concern about stability of waste tips in Welsh valleys).

Flood risk altered by land-use changes

One of the most widespread ways in which human activities modify hazard risks is through land-use change. Such changes are designed to enhance human welfare by increasing agricultural production, improving land quality or enabling urban and industrial development. But unforeseen changes often follow in the magnitude and frequency of natural environmental processes, such as river flooding downstream. Forest clearance promotes increased runoff and soil erosion on steep slopes, and many parts of the Himalayas are now paying the price for extensive forest clearance for fuel wood. Urban development introduces impermeable surfaces and artificial drains which promote higher peak discharges and faster rates of hydrograph rise, so that towns and villages downstream from major urban areas are often flooded more regularly and more deeply than before the urban expansion.

Nuclear developments and risks

The development of means of exploiting nuclear explosions for either peaceful purposes (nuclear power) or for nuclear war (nuclear weapons) has produced a series of entirely new hazard possibilities within the last fifty years.

Fallout of radioactive material from nuclear weapons testing has been detected in many parts of the world since the 1960s, highlighting the mobility and persistence of this particular hazard. Some nuclear scientists have speculated what might happen following (even limited) nuclear warfare, predicting a nuclear winter of sustained darkness and cold, with highly radioactive conditions throughout the environment and extreme shortage of food and usable water. The prospects are not rosy!

The risks posed by nuclear power are both more widespread and more current. There are many opportunities for contaminating air, water and land with radioactive material at all stages in the nuclear fuel cycle (Figure 13) - by accidents during storage, transport and processing as well as within the reactors of nuclear power stations (like Chernobyl; see Figure 2).

Figure 13. The nuclear fuel cycle

Source: Park (1989)

Air pollution; acid rain

Air pollution has created a range of serious hazards, mainly as side-effects of industrial activity. Air pollution poses special problems as a type of hazard because it generally builds up over a long period, atmospheric mixing processes make it extremely mobile, and many pollutants in the atmosphere are persistent, toxic and potentially harmful to people, plants and animals.

Acid rain is created when sulphur dioxide (SO₂) and nitrogen oxides (NO_x) gasses (from power station and factory chimneys, and from vehicle exhausts) interact with other chemicals and sunlight in the atmosphere and are washed back to earth either in rainfall (wet deposition) or as aerosols or gasses (dry deposition). Damage attributed to acid rain - such as forest dieback, declining fish populations in lakes and streams, massive fish deaths in some rivers, decay of stone monuments and buildings - has been reported downwind from most industrial areas of the world, although Scandinavia, parts of Western Europe, and eastern Canada and the United States appear to be suffering more than other areas.

Greenhouse gases, atmospheric warming and sea level rise

Without doubt the most serious global hazard today is the prospect of atmospheric warming caused by air pollution by so-called greenhouse gases. These are trace gases - like carbon dioxide (CO₂), nitrogen oxide (NO), chlorofluorocarbons (CFCs) and tropospheric ozone (O₃) - which alter the heating rates in the atmosphere by allowing incoming solar energy to pass through but trapping the heat emitted back by the earth's surface.

Some scientists have predicted that if emissions of greenhouse gases continue at present rates, the earth's temperatures will increase on average by between 1.5^oC and 2.5^oC (some pessimists put it as high as 4.5^oC) by the year 2050. Any rise of this level would far exceed natural temperature changes over the last 8,000 years, which have been in the order of 1^oC.

There are signs that the changes have already started. For example, average world temperatures have risen by 0.5^oC over the last century; global temperatures are now higher than at any previous time during the period of instrumental records; and the four hottest years of the past century - up to 1990 - all occurred during the 1980s (1980, 1981, 1983 and 1987).

Some startling scenarios have suggested what might happen if global warming were to proceed on the scale envisaged.

(a) Sea level rise

As temperatures rise the oceans warm, and the water within them expands. Glaciers and possibly polar ice caps would also melt. The combined effect would be a rise in sea level; the best estimates are a rise of between 24 and 34 cms over the next 60 years (it is currently rising at about 2.4 mm a year).

About one third of the world's population lives within 60 km of a coastline. Many low-lying countries - like the Netherlands, Bangladesh and Egypt (where a sea level rise of 90 cm would flood up to 15 percent of the arable land, and force 8 million people to move) - would be threatened by rising sea levels, as would low-lying cities (like New Orleans, Miami and Shanghai) and delta areas of the Ganges and the Yangtse (which house millions). Low-lying oceanic islands (like the Maldives in the Indian Ocean, and Kiribata, Tuvalu and the Marshall Islands in the Pacific) would soon be under water and uninhabitable. Coastal areas around Britain, like the Thames Estuary and East Anglia, would be at risk; and the Thames Barrier could lose its effectiveness in preventing London from flooding.

The costs of coping with sea level rise would be huge, to relocate millions of displaced people, build dykes and sea walls, and so on.

(b) more common extreme weather events

Climatic shifts triggered by the global warming would bring a rise in the frequency and intensity of floods, droughts, typhoons, tornadoes and hurricanes to many areas (by changing the distribution so that hitherto unaffected areas will suffer). This would further aggravate the losses and hardships caused by sea-level rise.

World climate would change in a number of ways. For example, cold seasons would become shorter and warm ones would become longer; northern latitudes would have wetter autumns and winters, and drier springs and summers; there would be more rainfall in the tropics; sub-tropical areas could become drier.

These would create serious risks for agriculture, especially because some important food-producing areas (like much of North America) could experience more heatwaves and droughts and shortage of irrigation water. There would also be the threat of lost food production in developing countries through low yields and greater losses( because of increased flooding, drought, erosion and desertification). Mass famine in some areas is not inconceivable.

(d) mass extinctions of wildlife

Wildlife would also be at risk from mass extinctions caused by rapid environmental change and lack of refuges. This could seriously affect habitats like the tropical rain forests, which contain over half (perhaps up to 90 percent) of the world's species. Sea level rise would also threaten wildlife dependent on habitats like salt marshes, coral reefs.

Sources and solutions

Simple solutions to the problem are difficult to find because the greenhouse gases come from various sources and the contributory factors are all inter-woven.

Most of the CO₂ is produced when fossil fuels are used, although tropical deforestation also important (there is a two-fold impact; dead and burning trees add carbon to the atmosphere, but they also mean a loss of the natural pollution filter which removes CO₂ from the atmosphere). NO is naturally created by microbial activity in soils, but it is increased by the use of nitrogen-based fertilizers, and the burning of timber, crop residues and fossil fuels. Methane comes mainly from cattle and termites; world cattle population has doubled in the last 40 years, and termites thrive on grasslands (created by the clearance of forests for pasture). CFCs come entirely from industrial activities - they are widely used in refrigerators, air conditioners and fire extinguishers; as aerosol propellants and solvents; and as foam blowing agents for plastics. Ozone is formed when nitrogen oxides (from burning fossil fuels) and hydrocarbons (in car exhausts) interact in the presence of sunlight.

So, what are the possible solutions? Conservationists stress the following;

use less energy
encourage greater energy efficiency (such as more building insulation, and energy-efficient manufacturing processes)
use more nuclear energy (although this poses its own set of risks and hazards as we have seen above)
encourage more afforestation and prevent further deforestation
reduce vehicle emissions (such as by tougher emission standards, enforcement of speed limits and greater use of public transport and bicycles)
reduce CFC emissions, by producing cheap alternatives and banning the use of CFCs (the Montreal Protocol aims to phase out production of the most damaging CFCs by the year 2000)
encourage farmers to use less fertilizer and cut down extensive biomass burning (as in stubble burning, and forest and woodland fires).

Many of these would require radical changes in people's attitudes, values and behaviour. Time alone will tell whether (both individually and collectively) we take the risk of global warming seriously enough to act without delay; the stakes are high!