Is Your Dam Vulnerable to Seismic Hazards? Don’t Wait to Find Out.

By Nick Oettle, Senior Geotechnical Engineer

Seismic hazards exist in most of the United States. There is high likelihood of strong shaking in the western United States and potentially significant shaking in the central and eastern portions of the country. And, while the scientific understanding of earthquakes and how dams respond to them has evolved significantly over the past few decades, the seismic energy released by earthquakes remains a major risk for dams.

Since most dams in the United States were built more than 60 years ago, and prior to our modern understanding of earthquakes, many dams, perhaps in the tens of thousands, were unknowingly built with seismic vulnerabilities. But not all dams face the same risk levels, and we have assessment tools that can help dam owners’ efforts to quantify their risks.

A dam’s seismic performance can be assessed in many ways, including traditional deterministic approaches, increasingly popular semi-quantitative risk approaches, and fully quantitative risk approaches. In this blog, I’ll review these methods and the advantages they can provide.

The Traditional Approach

The traditional deterministic approach reviews the known seismic sources that may affect the dam. While most state regulators require this approach, the agency that licenses hydropower projects – the Federal Energy Regulatory Commission (FERC) – has recently adopted a risk-based approach. Many dam owners not subject to FERC regulation have voluntarily adopted risk-based approaches recently to reduce their agency’s liability, improve their operational reliability, and prioritize projects when funds are limited.

A consultant evaluating seismic risk may start with traditional deterministic assessments. Although this type of assessment does not directly quantify risk, when combined with experienced judgment, this step can typically identify the major seismic concerns for a dam.

Often, this kind of assessment is adequate for a dam IF regulatory requirements are met, a deterministic analysis shows a lack of seismic vulnerability, and sound judgment about the consequence level of the dam is applied.

Semi-Quantitative Risk Approaches

Often, the next step in seismic risk evaluation is to perform a semi-quantitative risk analysis (SQRA). This step has become extremely popular due to FERC’s new Part 12D Comprehensive Assessment process.

SQRA’s are advantageous because they can approximately quantify (typically to an order-of-magnitude) dozens of potential failure modes (PFMs) for a dam in a relatively efficient process. The process combines a dam’s existing data with readily available probabilistic seismic hazard data. One of the downsides of SQRAs is that they largely rely on expert opinions to judge and extrapolate risk from limited information. However, this step can be adequate for low-risk PFMs. The SQRA methodology is also a valuable tool to prioritize work especially for owners that have a portfolio of dams.

Fully Quantitative Risk Approaches

For more complex situations where further precision is required and supported by adequate information, quantitative risk analysis should be on the table to further support management decisions. This approach conducts detailed assessments of the dam’s response at multiple loading levels and combines those results with seismic loading probabilities and an analysis of the consequences of the failure mode.

For seismic issues that can be clearly identified as high risk without a detailed analysis, a full quantitative risk analysis can aid in identifying suitable temporary and permanent mitigation measures and prioritizing projects. Many consultants believe this to be the best, most detailed approach available and the right one for dam owners considering a major seismic retrofit project.

How Owners Can Use Risk Assessments

Dam owners use these three assessments to evaluate their exposure to various consequences. Liability for downstream damage, reputational loss, loss of use of the reservoir, and the challenge of reconstructing the dam are some of the potential consequences from earthquakes. Because no dam owner wants this to happen to their dams, dam owners are using risk assessments to better understand their exposure and to sometimes take action to reduce their risk if they choose to.

Often, risk studies identify that the earthquake risk of a dam is very low. This can help assure owners that their risk is appropriately low, along with regulators, governing boards, stakeholders, and the public.

When more significant seismic risks are identified, dam owners are typically able to decide what they want to do about the risk. The risk analysis itself may identify cost-effective ways to reduce the risk, which could include operational modifications or capital improvements.

How owners decide the acceptable level of risk is often up to them. Although there is no universally recognized way to identify risk acceptability, some general guidelines are commonly used for dams. These often depend on the potential downstream consequences and the direct consequences to the owner. Certain regulators may also mandate minimum levels of risk acceptability.

Although the most common risk assessment application is to an owner’s portfolio of existing dams, proposed new dams, or proposed dam modifications can also be assessed for risk. This can be an especially smart risk application because no dam owner wants to invest significant time and resources into a capital improvement project, only to find out later that their risk is still higher than they wanted. Sometimes small modifications to a proposed project can result in significantly increased risk reduction. Our team has worked on several such projects recently.

Many dam industry regulators are rapidly transitioning to a risk-based system. This means that forward thinking dam owners will want to determine if their dams – especially currently envisioned capital improvement projects – will likely meet future dam regulations. Getting a head start to understand what regulators may require in the future will help owners with their capital and operational planning.

In conclusion, many dam owners want to understand their seismic risk and stay ahead of managing it. Partnering with experts on the evaluation, analysis, and the approaches available is crucial to properly quantify risks and make appropriate decisions in the short and long term.

Contact me if you have questions regarding seismic assessments of your dams and want to discuss options.