Risk evaluation for dam break

A client asked us to perform a risk evaluation for dam break methodological review. As a result we reviewed risk guidelines regarding the public living or working in the inundation zone of a dam. Ultimately, the goal is to help decide tolerable risk vs. relocation of potentially impacted homes or livelihoods.

As a matter of fact, we think this is an important subject and have thus decided to publish it.

• In Section 1 we review the procedure leading to risk-informed decision making for inundation areas. Furthermore,
• in Section 2,3 there is a review of individual and societal risk guidelines for old and new dams from various sources. Sources include the Hydro industry and of course USACE.

As you will see there are various tolerance thresholds from around the world. Indeed, risk tolerance is generally jurisdictional (by country, region, etc.). As a result tolerance threshold are not universally valid .

1.Procedure to define individual and societal risk in dam break analyses

 The key point in this type of studies is to determine the likelihood of fatalities and their number on an annualized base (Annualized Life Loss -ALL-) .

To perform these evaluations, it is necessary to:

• Check the assumptions made in the inundation study. Scenarios, rheology, etc.
• Evaluate the likelihood of the inundation scenario. USACE uses the term Annual Probability of Failure (APF)). We would use ORE2_Tailings to define the APF.
• Check the quality of the topography (DEM) used to perform the inundation study.  Indeed, experience has proven details may be very important. Actually, a state of the art study should include erosion/deposition evaluation along the channel during the flood. Finally, it is necessary to:
• transform that study into a probabilistic evaluation , or, in other words, to define the probability of a certain depth and velocity of flow. This may require a few additional runs of the flow model, to be performed by the hydraulics engineers based on data sets we will supply.
• Use a “flow vs. lethality” function to locate endangered population and determine the ALL. The figure below is copied from our book on Tailings Dams, Chapter 12, Consequences. This graph was originally proposed by Smith at al., 2014.

The final step is to use the above to deliver the likelihood and number of fatalities from the break. Indeed, these constitute a “dot” in the individual and societal tolerance graphs (examples further down in this text).

 At that moment, risk informed decision-making can be performed regarding mitigation, system changes, etc.

2.Review of individual and societal risk guidelines for old and new dams

DAM SAFETY MANAGEMENT: Operational phase of the dam life cycle ICOLD 2017. See fig. B5, B6 below.

Interim Tolerable Risk Guidelines for US Army Corps of Engineers Dams (USACE) policy for the estimated Annualized Loss of Life (ALL) is:

• ALL > 0.01 lives/year: Risk in this range is unacceptable except in exceptional circumstances. Indeed it is reason for urgent actions to reduce risk. Background: Reclamation (2003) states, “… there is justification for taking immediate action to reduce risk.” This is represented by the upper minus 1 sloping line in Figure 3 (below).
• ALL between 0.01 and 0.001 lives/year: Risk in this range is unacceptable except in exceptional circumstances and is reason for mitigation of risk. Background: Reclamation (2003) states, “… there is justification for taking action to reduce risk.” This is represented by the area between the two minus 1 sloping lines in Figure 3,
• ALL < 0.001 lives/year. Risk in this range may be considered tolerable provided the other guidelines are met. The background comes from Reclamation (2003). Indeed, it states: “The justification to implement risk reduction actions or conduct additional studies diminishes as estimated risks become smaller than 0.001 lives/year.
  • Risk reduction action costs,
  • uncertainties in the risk estimates,
  • scope of consequences,
  • operational and finally
  • other water resources management issues

       play an increased role in decision making. One should consider   reasonable and prudent actions for implementation when the risk is in this range.” Finally,

• 5a, 5b also below come from the same reference. They display individual and societal risk guidelines for new and major modifications under section 216.

3.International review of risk acceptance criteria for dams.

A study from LI et al., 2015 shows a comparison of tolerances across different institution following table 1 and figure 3.

4.Closing remarks

One should not perform Dam break studies  deterministically . Indeed, one-magic-number results do not deliver enough to allow risk-informed decision-making. Futhermore, one number solutions may be too prudent or significantly underestimating reality.

As a result, state of the art modeling should include erosion/deposition evaluation along the flood channel.

Contact us to know more.

Tagged with: Annual Probability of Failure, dam break, dam break analyses, individual risk, Risk evaluation, societal risk

Category: Consequences, Hazard, Probabilities, Risk analysis, Risk management