Seismic Risk Assessment of Dam Portfolio


To assist the goal of long-term risk reduction for dams in an owner’s portfolio, Slate Geotechnical Consultants worked with the owner to develop a consistent approach for evaluating the seismic response of 80 embankment dams. The owner is developing risk indices based on simplified slope stability procedures that will be used to rank dams to prioritize funding for future seismic retrofit projects. Slate developed an exhaustive database of dams in the owner’s portfolio and gathered data pertinent to the seismic response of those dams.

Existing seismic slope stability analyses in the portfolio were performed by a number of practitioners over more than 40 years and encompass a range of analysis methodologies, software, and strength property approaches. Inconsistencies in the approaches of previous seismic slope stability analyses indicated there were varying levels of conservatism in the results and thus could not be used in the risk indices. A consistent approach for the development of seismic slope stability analyses of embankment dams was developed and analyses performed for the portfolio of dams.

Conservatism built into the previous slope stability models were removed. If the conservatism was not removed then calculated yield accelerations would be too small, and the estimated risk of an uncontrolled rapid release of the reservoir would be too high. For the client, it was more useful to know the mean risk and an estimate of the epistemic uncertainty of that risk, than to have a single, conservative number. This is especially true when assessing an entire portfolio.

Seismic slope stability results included histograms of the post-earthquake factor of safety and yield coefficient. Assessment and commentary of the impact of parameters such as dam age, freeboard as a fraction of height, dam height were also provided by Slate.