The environmental aspect of a dam is paramount in dam design. Recent questions raised by environmental movements require review of these issues.

Dam designers can be considered to be environmental engineers, as environmental issues are raised throughout the construction and operation of a dam. Perhaps the most widely acceptable part of dam design is related to the need to meet environmental parameters. Nevertheless, dam design proceeds in conditions of great uncertainty. It should be recognised that throughout the years of the project natural phenomena are not always evaluated to the highest standards, and the response, in the process of design, is continuous assessment — perhaps described as ‘learn as you go’.

This continuous struggle of dam engineers is often a dilemma, as further information does not always become available and experience must be used as a cautious complement.

Environmentalists, even those who were previously dam engineers, demand reviews of earlier hydro developments to the extent sometimes of demolition (for example Glen Canyon in the US). But it must be acknowledged that, despite the cost, this opposition can, in the long term, improve the development process.

Environmental concerns have been examined often, and their complexities simplified. One example is the matrix of the international-commission-on-large-dams (ICOLD) which classifies concerns into 14 groups, and into three domains — environmental, social and economical. While one element of concern may delay a project, several elements in parallel can halt the development process or delay it so far that it is a de facto cancellation. The Sardar Sarovar project in India raises concerns in 25 of these elements, for example, while for the Three Gorges project in China 12 elements are noted.


One of the most important environmental elements concerns relocation of the local population.The engineering aspects of relocation have two important elements:

•Efficiency, expressed as the number of people affected per MW of power generated. The largest known to the author is Rengali, India, with 1333 oustees per MW.

•Density of the initial population, which varies greatly. The largest known is in Nepal, where the Marsayangdi plant affects 5000 people per km2.

The efficiency and density are associated directly with a more complex environmental issue, the land lost to reservoirs. This loss is of global significance, representing as it does 2.4-3.35% of the total land surface worldwide. Correlating efficiency and density to land-use ratio, expressed by the ratio of power generated to the

land surface covered by the reservoir, supports classification of these three environmental concerns as shown in the panel below.

For some power plants (58%) the three environmental parameters allow the environmental effects to be classified using the ranking from the table. Of that number, 32% are multipurpose dams used also for irrigation and flood control, while 68% are hydro developments focused on power generation.

The environmental parameters can be used in preliminary screening of new sites. However, for design and financial decisions more complex elements must also be incorporated. Comprehensive environmental parameters have been suggested by, for example, the World Conservation Union.

It should be noted that relocation of the local population is only one of the major environmental concerns to be considered; for other specifics, however, quantitative data are rarely available. (With exceptions: data are available from British Columbia in Canada on forestry impact, following the flooding of 2667km2 in 42 reservoirs). Realistic ecosystem measurements should be collected as a basis to enhance environmental design, and assist in the sustainable development of hydro projects.

To resolve environmental concerns, systematic monitoring and recording of environmental impacts are prerequisite. This data must be available to the developers considering new sites. Systematic collation and provision of this data would best be managed by a specialised international body, and this may be a useful result of the deliberations of the World Commission on Dams. This systematic monitoring and recording could become part of the ongoing structure of the Global Terrestrial Observing System, an initiative being pursued by the UN Environment Programme.

The genesis of the World Commission on Dams is well-timed to help in creating better tools for sustainable development. Financial constraints on the modern power industry will remain, but priority must be given to resolving the environmental concerns raised by dam development.

The complexity of relocation is well illustrated by India

India has 43 important dam developments, representing 18% of the 242 dams worldwide that require resettlement.
These dams have required resettlement much greater than the average when considered worldwide. The reason is the dam’s purpose. In India many dams are multipurpose (46% compared with a world average of 29%) or mainly for irrigation (44% compared with a world average of 12%). This means that the settlements affected by the dams have been generally larger, and the population density greater, than is the case for dam projects whose main or only purpose is power generation. In India the average density of resettled population is 1050/km2 — more than four times the world average, which is 224/km2.