3. Why does dam related emergencies occur?

There are different types of hazards possible for large storage dams (a comprehensive overview can be found in the final report published by the World Commission on Dams in 2000):

  • Earthquakes
  • Landslides
  • Floods
  • Aging of the dam and of its components 

It is necessary to distinguish between the following main types of dam-related hazards:

Natural– strong earthquakes, excess precipitation or snow melt, which cause water overtopping, or landslide occurrence, which can damage the system due to rocks falling or sliding into the reservoir.

Man-made – errors in use of dams (excessive release of water from spillway and sudden fluctuations in release of water from the operation of the power plant); terrorism; mistakes made during design or construction stages; deficiencies in building materials; aging of installations and dam materials; water quality issues in the reservoir (water-borne diseases; deterioration of water quality due to organic matter, release of hazardous substances, waste from aquaculture, etc.) affecting people, fishery and others..

Natural hazards occur due to natural large-scale processes (like earthquakes, which occur due to the movement of enormous tectonic plates) and it is impossible (in the case of earthquakes) or very difficult (in the case of landslides and floods) to prevent them. 
The hazards from the natural environment are site-specific, whereas man-made hazards mainly depend on people’s behaviour and actions and are usually not site-specific. There are, moreover, a number of project-specific hazards which depend on the dam type, design, construction, shape and height, foundation conditions, seepage control, age, reservoir volume, water depth etc.

The result of such hazardous events could be dam failure and the generation of a flood wave (the latter is mainly due to either destruction of the dam or simply the overtopping of water). A further distinction needs to be made between the technical hazards expected in a new project and hazards affecting an existing project.