Page - 4 - in Book of Full Papers - Symposium Hydro Engineering

Among these guidelines, Bulletin 137 provides information on reservoir- triggered seismicity (RTS), a hazard unique to large storage dams, which is often a key dam safety argument brought forward by opponents against new dams. For properly designed and constructed dams, RTS is not a new safety concern for dams. However, the publication with the greatest long-term impact on the seismic design and safety assessment of existing dams is Bulletin 148, which includes the concept of two earthquake levels for dams and safety-critical elements, i.e. the Operating Basis Earthquake (OBE) and the Safety Evaluation Earthquake (SEE). The safety-critical elements are the spillway and low level outlets, which are needed to control the reservoir level after the SEE or for lowering for repair works or for increasing the safety of a dam. This is new. Bulletin 120 complements Bulletin 148 as it includes conceptual features for the seismic design of dams, which are extremely important, as it is well known that it will be difficult to have a structure to perform well during an earthquake, when the basic seismic design concepts are not observed. Dams were the first structures designed against earthquakes, on a worldwide basis, starting in the 1930s. At that time, the ground shaking was the main seismic hazard and was represented by a seismic coefficient of typically 0.1, almost irrespective of the seismic hazard at the dam site, which was often unknown. The seismic analysis was done with the pseudostatic method, ignoring the dynamic characteristics of dams. Because of its simplicity, this method is still in use today, although it has become clear that this method is obsolete following the observations made during the 1971 San Fernando earthquake. The pseudostatic method is also not compatible with current seismic guidelines (Bulletin 148) and, therefore, this obsolete method shall no longer be used for the safety checks of large storage dams. One of the main tasks of the Committee is to promote good practice in dam engineering, which includes the dissemination of the international state-of-the-art. Using the pseudostatic concept, the seismic load case was very seldom the governing one. This has changed by using today’s rational concepts for seismic hazard analyses and dynamic analyses of dams. The earthquake load case has become the dominant one for most dams. Since the formation of the seismic committee, the magnitude 8 Wenchuan earthquake of May 12, 2008 that occurred in Sichuan province, China, was the most important earthquake for dam engineers as it damaged some 1580 dams. Most of them were small earth dams, but also some large dams were damaged. The main lesson from this earthquake was that the seismic hazard is a multi- hazard. Thousands of mass movements occurred in the mountainous epicentral region. Mass movements that can be triggered by strong earthquakes are often ignored or the hazard is assessed using criteria, which are different from those used for the dam body. Based on the past experience, it is obvious that dams are not inherently safe and can be damaged by strong earthquakes. The most vulnerable dams are those, which are poorly constructed and/or designed. Still a lot of work is required in order to ensure that all dams comply with modern seismic safety criteria, which is the main concern of the Committee.. 4