Safety |
Earthquake prediction and dam safety
Dr Martin Wieland, Chairman of ICOLD’s Committee on Seismic Aspects of Dam Design, reflects on the seismic safety of dams
IN THE EARLY 1970S, when I first got involved in the dynamic analysis and safety assessment of concrete dams and was working in the Laboratory of Hydraulics, Hydrology and Glaciology at the Swiss Federal Institute of Technology in Zurich, Switzerland, some dam engineers believed or hoped that by means of earthquake prediction or early warning it would be possible to ensure the seismic safety of dams. Most of the Swiss dams are concrete dams, which have an average age of about 75 years and were designed against earthquakes using the pseudo-static analysis method with a horizontal seismic coefficient of 0.1, ie a method which is considered obsolete today. Thus by timely earthquake prediction, it would have been possible to lower the reservoir and the people living downstream of a dam would have been safe. However, it turned out that this was a pipe dream and is still unattainable today. Hardly any progress has been made in the last 50 years in the prediction of strong earthquakes, which includes the prediction of their magnitude, place and time. If earthquake prediction is possible and the warning time is sufficiently long, then the reservoir could be lowered before a strong earthquake occurs.
About 20 years ago, two earthquake early warning
Figure 1: Overtopping of the 262m high Vajont arch dam in Italy caused by rockslide into the reservoir in 1963: Slightly damaged dam crest with visitor pathway (maximum depth of water wave overtopping the dam crest: ca. 100m) (left) and scarp of 260Mm3
rockslide at the left
bank of the reservoir (right) Photos taken by author on 18.9.2017
systems were installed for the Ignalina nuclear power plant in Lithuania and the city of Istanbul in Turkey. These systems included seismic sensors installed at different locations at a distance of 30km around the power plant site, on several islands in the Marmara Sea in the vicinity of Istanbul and provided theoretical warning times of less than ten seconds. This required real-time monitoring and processing of the seismic
data. It is obvious that within this short period of time no actions could be taken at large storage dams except for alarming the people living downstream of a dam. Although alarms would not save the lives of people unable to escape from buildings shaken by earthquakes, they could save them from a flood wave caused by the failure of a dam, which should be standard practice today. Therefore, the only option is to design and build dams that must be strong enough to withstand the strongest ground motion expected at a dam site, which is the current seismic safety concept of ICOLD for high-consequence failure of water storage dams.
Ambiguous
It is well known that the concept of strongest ground motion is somewhat ambiguous as there are large uncertainties involved in its estimate. Thus, it is recommended to review the seismic design and safety criteria periodically and in the case of important changes in the seismic hazard, the risk classification of dams or the seismic safety criteria, it is necessary to carry out a re-assessment of the seismic safety of dams. As well-designed, well-built and well- maintained dams have a very long lifespan such a safety assessment may be carried out repeatedly at intervals of say 20 to 40 years. Moreover, for estimating the seismic hazard both deterministic worst-case earthquake scenarios and probabilistic seismic hazard analysis methods should be used. If both types of analysis are carried out the most unfavourable ground motion parameters should be used as the input for safety analyses. As seismic hazard
24 | March 2022 |
www.waterpowermagazine.com
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