Quantitative Risk Assessment of an Embankment Dam
When our confidential client turned to GEI to design a new embankment dam, we took safety one step further by incorporating a seismic Quantitative Risk Assessment (QRA) into the design process.
Evaluating risk is an emerging trend in the dam industry – one that can help dam owners better understand their exposure to risk from earthquakes, floods, and other natural disasters. Risk assessments evaluate the probability of a dam failure, or failure of a dam component, and what the consequences of that failure might be. There are several ways to evaluate risk, including semi-quantitative risk assessments and quantitative risk assessments. A quantitative risk assessment is the most comprehensive and involves numerically estimating the probabilities of individual events occurring. By conducting a QRA on our dam design, we helped our client understand seismic risks to their new dam and enabled them to make informed decisions on how to address those risks.
The first step to analyze seismic risk is to understand the earthquakes that can occur in the area and to understand what type of ground motion those earthquakes are likely to produce. This involves using the available science on large earthquakes that have historically occurred and developing an understanding of the regional tectonic setting. For this project, our risk assessment experts selected 30 earthquake time histories and rotated and linearly scaled the time histories to the hazard spectra at three Annual Probabilities of Exceedance (AEP). We carefully selected ground motions so their scaled peak ground velocity, Arias intensity, cumulative absolute velocity, and significant duration would be evenly distributed within their expected plus-and minus-two standard deviation range at each AEP. We completed the analyses for two different controlling magnitudes from nearby faults to fully integrate the risk assessment with the aspects of the seismic hazard at site.
With the developed ground motions, GEI performed advanced nonlinear seismic deformation analyses with FLAC for three dam cross sections. This process enabled us to assess how the dam would likely respond to each of the 30 earthquake time histories we developed. The results of the FLAC modeling informed how we developed seismic deformation fragility models for the dam. These models measure how resilient the dam is to earthquakes and what is needed to evaluate the seismic risk.
While GEI’s original dam design was shown to be very low risk, by conducting a risk assessment, we were able to make informed, minor adjustments to the dam design to ensure the new dam can better withstand risks such as earthquakes. Today, our client is moving forward with its dam replacement with the confidence that GEI’s design provides a balance between risk, cost, and efficiency for a dam that will provide a reliable, safe source of clean water for the community.
The new embankment dam is scheduled for completion in the next few years.