floods

floods Physically, a flood is defined as a high level of water which overtops either a natural or an artificial river bank, or rises above normal levels or sea defences in coastal environments. Unless such an event poses a threat to human life or property, however, it cannot properly be considered as an environmental hazard.

The physical causes of floods are highly variable. Figure 1 distinguishes between coastal and river floods. River floods can be caused by a number of atmospheric, seismic, or technological hazards and some scientists have argued that deforestation, particularly in large areas of the Himalayas, has increased the likelihood of flooding in recent years. The flood hazard may be exacerbated by various human activities, particularly urbanization (see hydrological cycle) and by the construction of flood defences. Urban areas, for example, increase flooding for four major reasons: (1) the creation of impermeable surfaces, which cause more surface runoff; (2) the presence of smooth surfaces, which increase flow velocities; (3) the constriction of natural channels by bridges and other structures, which can increase the depth of floods; and (4) increasing urbanization leading to the construction of urban storm drains, many of which cannot now cope with the increasing peak flows. In the UK alone, over 3000 sewer failures are reported annually. Many of these lead to the surging of water through the nearest point of escape in the sewer system.

Many scientists have argued that the construction of embankments and levees as flood defences provides a false sense of security for those people occupying surrounding low-lying land. In its natural state, the river floodplain provides an opportunity for the dissipation of energy and the attenuation of flood peaks. Construction of levees and embankments prevents the floodplain from performing this function and transfers the problem further downstream to areas which were not subject to flooding. Once started, there seems little option other than to continue constructing flood defences and levees. A dramatic example of this problem is to be found in the lower reaches of the Yellow River in China, near the city of Jinan. Levee building, once started, has continued over the past fifty years because of rapid sedimentation on the bed of the river has been too rapid for dredging to keep pace with it. The river bed currently stands between 6 and 10 m above the level of the surrounding floodplain. Once levees are breached, the severity of flooding and the longevity of the flood event is dramatically increased because water cannot return easily to the channel. This problem was graphically illustrated during the 1993 Mississippi floods.

Flooding is the most widespread of all geographical hazards. There were significantly more reports (1302) of flooding worldwide over the 25-year period 1968–92 than any other natural hazard with the exception of high winds (1494), yet in general floods cause less loss of life than earthquakes, drought and famine, and high winds (Fig. 2a). Although there are fewer mortalities, floods are responsible for a high incidence of injuries (Fig. 2b) and have a significant impact upon homelessness in comparison with other natural hazards (Fig. 2c). In most cases, floods are of limited geographical extent. In England and Wales and in Australia, for example, less than 2 per cent of the population live in flood-prone areas. In the United States, over 10 per cent of the population is at risk from flooding, particularly along the main floodplain of the Mississippi river. In contrast to the relatively small land areas subject to flooding in most countries of the world, the 1988 floods in Bangladesh inundated almost 50 per cent of the country to a depth of one metre. Nearly 1500 people died as a result of this flood alone.

While it is possible to generalize about the social impact and consequences of flooding by means of the figures given above, flooding is a complex subject because of the various causes of the flood hazard and human responses to it. Some particular areas are at greater risk of flooding than others. For example, populations occupying low-lying parts of active floodplains and estuaries, small basins subject to flash floods, areas below unsafe and inadequate dam structures, low-lying inland shorelines, and alluvial fan environments are all at high risk. In general, the nature of the flood hazard is different from other natural hazards (see droughts). With limited exceptions, floods tend to affect relatively small geographical areas and are short-lived, usually lasting only days to weeks. Population displacement is usually localized but structural damage to housing, communications, and power supplies is often severe: the estimated damage caused by flooding worldwide between 1988 and 1992 was in excess of US$8.5 billion. Damage is a function not only of the depth of water, particularly in river floods, but also of the velocity of the river and the amount of silt it carries: more severe damage is caused by high-velocity silt-laden rivers.

Despite increased global investment in flood control, flood losses continue to increase. This is true even when allowance is made for inflation. There would appear to be only two plausible explanations for this trend in recent years. First, that there has been an increase in the magnitude of floods caused by changes in climate. There is evidence to suggest that damaging floods in Eastern Australia, for example, have increased since 1945 in comparison with the previous hundred years or so. The second explanation, and one that by consensus seems more likely, is that the increased risk has been driven by greater floodplain occupancy putting more people at risk to the flood hazard.

The responses to flooding have evolved in what hydrologists have recently referred to as the structural era (1930s–60s), the unified floodplain management era (1960s–80s), and the post-flood mitigation era (1980s–). The structural era is a period in which reliance was almost totally placed in engineering solutions through the construction of reservoirs and levees and through channel improvements (usually widening and straightening). The unified floodplain management era is characterized by the use of a combination of mitigation measures which in part utilized structural approaches but is dominated by the use of non-structural approaches, which included: (1) improved flood warning, through the use of weather radar, rainfall and flood level telemetry, and computerized flood forecasting models; (2) land use planning, where high-value developments in flood-prone areas are controlled and regulated through the planning system; and (3) increased use of insurance to offset the costs of clean-up to national and local government. These measures have often proved to be less successful than anticipated; many of them rely on government-run rather than private schemes to be effective.

The post-flood mitigation era is one which is still emerging, but it includes measures taken in the post-flood period to effect better control or minimize risk in the future. A good example of such an approach is provided by an economic study of Soldiers Grove in the 1980s. Soldiers Grove is a small settlement located on the Kickapoo river in south-west Wisconsin, USA, which was severely damaged by a series of floods in the 1970s. Here, two post-flood solutions were considered; the construction of a flood storage reservoir with associated river channel levees; and the relocation of the urban population away from the flood-prone area. The cost of the two schemes was roughly equal and other benefits accrued as a result of relocation of the urban population.

While many of the schemes identified above may be applicable to small urban populations, it is not surprising that many of them are not appropriate in countries where flooding can cover a high percentage of the land area. In less developed countries, disaster aid, which includes technical assistance as well as disaster relief, is an essential factor in flood mitigation. After the 1988 floods in Bangladesh, the UN Development Programme commissioned a range of studies by overseas experts, who recommended an increased reliance on the construction of embankments along the major rivers. The scheme had an estimated cost of US $6 billion with an annual maintenance cost of US $600 million. On grounds of cost alone this scheme is unlikely to be implemented. Even then, the social costs have not been taken into account, for both fish farmers and jute farmers benefit from the spread of the monsoon floods in normal seasons. The proposed scheme would also be unable to cope with the most extreme floods and might therefore increase the hazard during these events.

Social scientists have argued that flood hazards impart a variable impact on people according to vulnerability patterns which are generated by the socio-economic system in which they live. This is seen as a function of class relations and the structures of domination within the society, which determine levels of ownership, the means of production, and the ability of individuals to respond to the flood hazard. In common with droughts, vulnerability to the flood hazard in less developed countries is essentially a function of strengths and weaknesses in the society. The homeless, women, and children appear to be those groups in society exposed to the greatest risk.

Ian D. L. Foster

Bibliography

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Penning-Rowsell, E. C.,, Parker, D. J.,, and and Harding, D. M. (1986) Floods and drainage. British Policies for hazard reduction, agricultural improvement and wetland conservation. Allen and Unwin, London.
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World Disasters Report (1994) International Federation of Red Cross and Red Crescent Societies, Geneva, Switzerland.