Water pouring out of a dam spillway very fast at sunset.

Understanding Risk-Informed Decision-Making for Dams and Levees

Understanding Risk-Informed Decision-Making for Dams and Levees

August 16, 2023
Stacy Vorster, PE, Elizabeth Landowski, PE

Part one of a two-part series, this blog lays the groundwork for performing a risk analysis workshop by explaining the risk assessment process.

Across the U.S., communities depend on more than 91,000 dams and more than 7,000 levee systems for water, power, recreation, and flood control. The failure of even one can result in catastrophic property and environmental damage, personal injury, or death, making dam and levee reliability critically important.

Federal guidelines require some owners to perform risk assessments to evaluate the safety of their dams and levees. Still, the process benefits all owners, not just those required by law. Risk-informed decision-making (RIDM) provides a systematic approach to identifying and understanding the risks of dams, levees, and related facilities. It enables owners to prioritize safety management actions. By understanding the background and different risk analysis types, dam and levee owners can use the risk process to make informed decisions to safeguard their assets and the communities they serve.

What Is Risk-Informed Decision-Making?

RIDM is a method of evaluating dams and levees that identifies risks and prioritizes ways to mitigate them. Risk estimates inform decisions for the infrastructure by determining vulnerabilities that might not be diagnosed through standards-based evaluation.

Background of Risk Assessments for Dams and Levees

Risk assessment for dams and levees is a relatively recent practice, stemming from several dam failures in the 1970s and the passing of the Reclamation Safety of Dams Act of 1978. Federal agencies began investigating dam safety nationwide, forming the first dam risk cadre in 1998 through the U.S. Bureau of Reclamation. Hurricane Katrina accelerated the inclusion of levees in the risk assessment process, culminating in the establishment of the Risk Management Center by the U.S. Army Corps of Engineers in 2009.

Dam and levee owners benefit from understanding the risks and adopting this RIDM framework. Proactive owners implement these practices, even when they’re not regulated.

Risk Assessment Goals

To achieve the best outcome for your risk assessment, begin with the end in mind. The goal is to understand better the project, its features, and potential failure modes (PFM). Risk assessment challenges the conventional approach of relying solely on health and safety factors, acknowledging that no design can guarantee zero risk of failure.

Performing risk assessments offers several advantages for dam and levee owners, including:

  1. Mitigating risks from the start: Early hazard identification and analysis allows for implementing proactive risk reduction measures.
  2. Understanding the facility: Risk assessments provide a comprehensive understanding of the dam or levee, its features, and its PFMs.
  3. Informing decisions: Risk-informed decisions support operating procedures and prioritize investigations and risk reduction measures.
  4. Justifying expenditures: The risk assessment results help justify improvement investments and practices to stakeholders.
  5. Effective communication: Risk assessment results are non-technical and can be leveraged to communicate risk effectively to the public and other stakeholders.

Risk Management Framework

Understanding the risk management framework (RMF) is crucial. Risk measures the probability and severity of adverse effects on life, property, or the environment and is determined by the hazard, performance, and consequence factors. Risk analysis estimates the risk, while risk assessment evaluates the significance of the risk and the need for risk reduction measures. The risk management process applies policies and procedures to assess, mitigate, and monitor risk.

Risk is a measure of the probability and severity of adverse effects on life, property, or the environment, determined by the hazard, performance, and consequence factors.

Beware of Cognitive Biases

Our brains develop mental shortcuts called heuristics to solve problems and make quick and efficient judgments. While they can be beneficial and help us to navigate the world around us, they also can lead to cognitive biases and cloud judgment.

For example, in the wake of a publicized dam failure caused by flooding, dam owners might be vigilant of PFMs such as overtopping, seepage, or structural failures. But equal or greater risk may exist from cyberattacks, operator error, or earthquakes. It is essential to consider all PFMs and not default to those fresh in our minds.

While we cannot eliminate biases, we can be aware of them and minimize their effect on our perceptions and judgments while analyzing risks and PFMs. We can rely on facts rather than judgment and incorporate diverse views and experiences from multiple experts. To prevent prior analyses from inadvertently biasing current assessments, we must brainstorm PFMs with open minds.

Risk Analysis for Dams and Levees

Risk analyses are performed at multiple levels, ranging from qualitative to quantitative approaches. In this blog, we describe the different levels based on the Federal Energy Regulatory Commission’s (FERC) nomenclature. Still, the general framework and scope are similar for dams and levees falling under other regulatory agencies.

Each risk analysis level follows the same framework: identify, describe, discuss, and evaluate factors that make events more or less likely to occur for each credible PFM, then estimate the likelihood of failure and consequence given that failure.

The level of detail and effort increases with the scale, allowing owners to tailor assessments to their needs. Time, resources, and cost increase as the analysis moves from qualitative to quantitative. The level of the analysis’s detail and rigor should depend on the confidence needed to support the decision-making.

The level of detail and effort required increases from the bottom of the pyramid to the top. Levels 1 and 2 (screening and periodic) require less time and cost, while Levels 3 and 4 (semi-quantitative and quantitative risk analyses) require more.

Level 1: Screening

The first level, screening, is a qualitative risk analysis to rank risk among dams and levees within an owner’s portfolio, providing a starting point for future efforts. Screening uses readily available information to inform an owner of the potential risks of the facility. Level 1 risk analyses require minimal effort and can be conducted by a small team quickly.

During Level 1, PFMs are identified to evaluate the risks for four categories: static, flood, seismic, and operational. The highest risk for each category determines the inventory risk score. Level 1 risk analyses are performed once for a facility, providing a starting point for future efforts and more detailed analyses.

It is important to note that screening level risk analyses provide insight into a dam’s risk compared to others within an inventory. They do not result in an understanding of tolerable risk and are not appropriate for use as the basis of decision-making.

Levels 2 and 3: Semi-Quantitative

Level 2 and Level 3 are semi-quantitative risk analyses (SQRA) that identify PFMs and provide risk estimates for credible PFMs. In a facilitated workshop, subject matter experts (SME) estimate the likelihood of failure and incremental consequences within an order of magnitude. Results are portrayed as an order of magnitude square on a risk matrix.

A risk matrix portrays risk estimates for likelihood and consequence if a potential failure mode occurs.

SQRAs require more effort than Level 1 risk analyses because more detail and effort are put into several aspects of the analysis.

First, there is more detail in identifying the PFMs. For example, a general PFM identified during Level 1, such as “internal erosion through the embankment,” should be enhanced to provide the location and specific internal erosion mechanism, such as “concentrated leak erosion at the embankment to foundation contact.”

Next, more effort is required to understand the hazards. Probabilistic loading data for flood and seismic hazards provide a starting point to estimate the likelihood of failure. Finally, participants need a better understanding of the consequences to quantify an order of magnitude risk of failure.

The scale of the effort varies greatly depending on the size of the facilities being evaluated, the number of PFMs, and specific issues with the project that need to be addressed. Unlike a Level 1 screening level risk analysis, an SQRA can be used to support dam safety decision-making.

Performing an SQRA gives the owner a better understanding of the baseline risk and helps determine if PFMs require additional analyses, remediation, or other dam safety management actions.

Level 2 and Level 3 risk analyses are both SQRAs. The key difference between them is that Level 2 risk analyses are performed periodically as part of FERC’s 10-year comprehensive assessment, the comprehensive review performed by the Bureau of Reclamation, or the U.S. Army Corps of Engineers’ periodic assessment.

For unregulated dam owners, this may be a periodic review of safety evaluated on a term determined by an adopted dam safety program. The Level 2 risk analysis is comprehensive and leverages the project review information being performed for the periodic assessment.

A Level 3 risk analysis is an SQRA not performed during a periodic review cycle and may be performed for a specific concern. It may focus on a subset of PFMs, such as a spillway or seismic risk analysis. A trained facilitator should lead both types of risk analyses.

Level 4: Quantitative

Level 4 is a quantitative risk analysis focused on specific PFMs that require additional scrutiny before implementing permanent risk reduction measures or deciding that the risks are tolerable. Level 4 may involve performing further engineering analyses and investigations to quantify and reduce uncertainty, which may have been estimated qualitatively for the other levels of risk analysis.

A Level 4 risk analysis breaks down the PFM failure progression into an event tree, with each event representing one node on the tree. SMEs estimate the likelihood at each node to determine a specific value rather than an order of magnitude. Effort varies significantly for a quantitative risk analysis depending on the PFMs and issues being evaluated.

Embracing RIDM practices empowers dam and levee owners to effectively protect lives, property, and the environment and allocate resources where needed most to reduce risk. The risk assessment process allows for strategic risk reduction measures and transparent communication, providing a safer future for communities and critical infrastructure.

Stay tuned for the second blog of this two-part series, which will dive into applying the risk assessment process to run a risk workshop.


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