Natural hazards are caused by large-scale processes and it is very difficult to prevent hazardous consequences for dams. The only real method of such disaster risk reduction is to design the dam in such way that it can resist or reduce the negative impact on natural forces. In the case of an earthquake, this is achieved by making a detailed study of the construction area – active faults, their seismic potential, recurrence period of earthquakes, geotechnical properties of the site and by erecting the dam so that it will withstand seismic shaking without substantial damage.
In contrast to large-scale natural hazards, man-made hazards can be reduced by timely intervention.
For example, a good real-time monitoring network measuring dam and foundation deformations can signal that the strains in the system are close to the theoretically predicted critical values, estimated in the process of dam design. In this case the operating staff of the dam can decrease the filling/discharge rate to lessen the strain rate.
One important dam-related hazard caused by human activity is the phenomenon of reservoir-induced seismicity (RIS). There are two main models of RIS:
i. impact of direct loading of the earth’s crust by lake weight;
ii. increase in pore pressure of fluid in porous geological formation.
In the first case the load can release the normal stress on some types of tectonic faults, thus decreasing the friction resistance to tectonic stresses and facilitating the slip along the fault, i.e. initiating the earthquake.
In the second case the friction resistance of the fault decreases due to the increase in pore pressure. As the pressure of the water in the pores of the rocks increases, it “lubricates” the fault’s slip planes. This activates faults, which are already under tectonic strain, but are prevented from slipping by the friction of the rock surfaces. It is still not clear whether reservoir-induced seismicity can be predicted with sufficient accuracy, which is necessary for carrying out pro-active measures.
The main hazards and the corresponding protective measures are presented in the table below:
Comments on the table:
(A) The internal alert triggered by an unusual situation can be managed and controlled by the dam’s operating staff.
(B) A developing situation exists when the observed incident is clearly beginning to turn into a serious threat to the dam’s safety and the population in the downstream area. At this stage it is not yet known whether the situation can be brought under control.
(C) An imminent situation has developed when it has become clear that the progress of the incident or threat cannot be stopped, but its consequences can still be mitigated, such as the evacuation of the population in danger.