Dam safety | References (continued)


[8] EN 1998 (Eurocode 8), “Design of Structures for Earthquake Resistance”, Part 1, “General rules - seismic actions and rules for buildings”, 2004; Part 2, “Bridge”, 2005


[9] P. Bisch et al. (2011) “Eurocode 8: Seismic Design of Buildings Worked Examples”, Workshop EC 8: Seismic Design of Buildings, Lisbon, 10-11 Feb. 2011


[10] D. Dragojevic, R. Salic and Z. Milutinovic (2013), Analysis of Exponent K Based on “Share” Project Data and Its Implications on Importance Factors of EN 1998-1”, Research Square, DOI: https://doi. org/10.21203/rs.3.rs-746902/v1


[11] L. Stempniewski & G. Maltidis (2012), “Seismic actions for waterways engineering” (in German), Conference Proceedings of Eurocodes for Waterways Engineering, 8-9. 10.2012 in Karlsruhe, Germany, pp 65-71


[12] Z. Wang & L. Ormsbee (2005), “Comparison Between Probabilistic Seismic Hazard Analysis and Flood Frequency Analysis”, EOS, Transactions, American Geophysical Union, Vol. 86, No. 5, pp 45, 51-52


(2) Interpolation and extrapolation In principle, interpolation of an expected PGA for lower return periods from known higher return periods using the formula is generally acceptable. In contrast, extrapolation from lower to higher return periods should be approached with caution. In this regard, Stempniewski & Maltidis11


found that equation (3)


can provide satisfactory extrapolation results up to a 1400-year return period earthquake based on a 475- year event. For return periods exceeding 1400 years, they recommend conducting a site-specific seismic hazard assessment. The likely cause of errors at high return periods is that regional tectonic and geological conditions are not considered in the formula. After all, the formula cannot present the full complexity of seismology, and it is impossible to use a formula to represent all earthquakes. Therefore, the formula should be regarded as an approximation. From a mathematical standpoint, inaccuracies in known PGA values at low return periods can be accumulated and magnified during extrapolation, potentially causing large discrepancies that render the extrapolated PGA for higher return periods unreliable or even unusable. For the above reasons, it is not recommended to extrapolate the PGA at very high return periods (e.g. over 1500 to 2500 years) based on low return period events (e.g. 145 or 475 years). In practice, some people extrapolate the PGA for SEE based on the PGA for OBE. This approach is flawed for two main reasons:


● First, as discussed, extrapolation from low return periods introduces significant error.


● Second, OBE in principle represents a serviceability limit state and is an economic criterion to ensure the dam against economic losses from damage or loss of service, which is relevant primarily to the dam owner, while SEE pertains to dam safety as a safety criterion and represents an ultimate limit state. Therefore, using an economic index (OBE) to extrapolate a safety index (SEE) is fundamentally incorrect.


(3). Reference earthquake parameters As well-known, site-specific SHA is typically established for large and important projects during the construction design phase (and occasionally during tender design). For small projects or during the feasibility study phase, SHA is rarely conducted. In such cases, country’s seismic hazard maps are often used to determine reference earthquake parameters. However, caution is needed, especially when extrapolation PGAs for high return periods, because significant errors may be hidden in the extrapolation of PGA to high return periods. The country’s seismic hazard maps generally do not account for local conditions at dam sites. Typically, these maps are intended for buildings, which are often founded on soils and designed for a DBE with a return period of 475 years. The average value of propagation velocity of shear (S) waves in the top 30 m (VS30 soil profile used in such maps generally VS30 m/s (Ground Class C and D8


) of ≤750 ). In contrast, most dams,


especially concrete dams, are founded on sound rocks with VS30


1000 m/s (Ground Class A and B8 ).


As a result, PGAs from country’s seismic hazard maps developed for buildings on soil may be higher than


28 | January 2026 | www.waterpowermagazine.com


It is worthwhile mentioning that the prescribed service life of river diversion works varies by projects. The recommendations above are based on a service life of 3 to 5 years. If the actual service life is shorter or longer, the seismic design criteria should be adjusted accordingly.


those determined via SHA for dams on sound rock foundation, potentially leading to overestimation of the PGA and conservative design. In this regard, country’s seismic hazard maps should be carefully reviewed when used for dam design on rock foundation.


(4). Challenges in fitting multiple PGA vales A practical problem is that it is difficult to fit all data points with a single equation (Eq. 3) when more than three PGA values are provided in an SHA. This limitation should be acknowledged during the analysis.


5.3 Example of interpolation Frequently, SHA provides the earthquake parameters for OBE, DBE and SEE for a dam project. However, the earthquake parameter for the design of cofferdams and other temporary structures, typically associated with a 50-year (or 25-year) return period, is not provided. In such cases, interpolation can be performed to determine the PGA for cofferdam design based on available PGA values, such as that of the DBE. For example,


Alternatively, using a seismic exponent k=3.0, we obtain PGA50


=0.084g. In this example, the discrepancy using


the two seismic exponents is minor. According to the previously proposed threshold, the seismic action can be omitted in the design of the cofferdam.


6. Conclusions and recommendations Construction earthquakes should be properly


addressed in the design of temporary works, such as cofferdams, in dam projects – not only for economic reasons but, more importantly, for public safety. For the seismic design of river diversion facilities, the following are recommended:


● The design for cofferdams and other temporary works shall consider earthquake actions and the potential consequences of failure.


● A peak ground acceleration (PGA) of 0.10g is recommended as the threshold for initiating seismic load consideration. Cofferdams designed and constructed using modern engineering practices can typically withstand earthquakes up to this level without significant damage.


● The hydrological design criteria for river diversion works may also serve as the basis for seismic design. In particular, earthquakes with return periods of 25 years for concrete dams and 50 years for embankment dams are recommended. These criteria are considered logical and reasonable.


● Risk analysis should be an integral part of the design process for temporary structures. In our opinion, a probability of exceedance of 20% for concrete dam construction and 10% for embankment dam construction is prudent and thus acceptable.


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