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Framework for a system-wide dam risk
reduction program in northern California
Jennifer L Donahue
Geosyntec Consultants, San Francisco, CA, USA
Kathryn E. Wooddell
Sr. Seismologist Pacific Gas and Electric Company, San Francisco, CA, USA
ABSTRACT
A key issue for seismic risk studies is the large epistemic uncertainties in the estimated risk values. To quantify and
reduce these risks, PG&E is developing a long-term seismic risk program for its hydroelectric system including over 170
dams in California. The program objective is to rank all dams in the portfolio in terms of risk while simultaneously
quantifying the variability and relative impact of the risk parameters for each dam. The risk ranking will help prioritize
mitigation efforts as part of a long-term risk reduction program; and the framework is comprised two parts, a PSHA and
Risk Analysis.
For the PSHA, the seismic source characterization (SSC) model includes 160 crustal fault sources and the Cascadia
megathrust. For unmapped faults, areal source zones are used. Crustal faults in the SSC model are characterized
using a logic tree approach with 7 nodes describing uncertainty in: source geometry, magnitude probability density
functions, fault slip rate, and time-dependent recurrence. The GMC logic tree structure has four nodes for each
attenuation type: the ground motion prediction equation (GMPE) for median spectral acceleration, the additional
epistemic uncertainty capturing the statistical uncertainty from limited empirical data and regional ground-motion
variations, and the ground motion uncertainty (sigma) model, and the upper tail of the ground-motion distribution.
Seismic sources for GMC are separated into three groups: shallow crustal earthquakes in the transpressional region,
shallow crustal earthquakes in the extensional region, and subduction earthquakes.
Given the SSC and GMC input models, rapid, simultaneous computation of seismic hazard for all dams is accomplished
utilizing Amazon Web Services cloud computing, and “Tornado plots” showing the sensitivity of the ground motion at the
10-3 and 10-4 annual probability of failure hazard levels are created. For the risk analysis, the PSHA hazard curves are
convolved with the fragility curves. The fragility curves for dams are not well established and work continues to modify
these models, improving on the current simplistic approaches.
Ultimately this study will quantify the uncertainty and relative importance of each input parameter in terms of it’s
contribution to the risk at each dam, and the parameters will be ranked in terms of their overall impact to the PG&E
portfolio. The ranked list will be used to prioritize future seismic research that will lead to the greatest reduction in
uncertainty in seismic risk at the utility’s dams. This paper summarizes the framework and lessons learned as we
proceed through the project.