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In the case of Oroville dam, the population exposed to the potential dam failure has increased significantly, normalization of deviance increased the probability of failure, so is it possible that Oroville Dam risks became unmanageable?
When looking at risk mitigation the old saying is that you can either lower the probability or lower the consequences. That statement requires some explanation: what mitigation alone can do is exclusively to reduce the probability of failure ( in the case below this will not change the blue area). In order to reduce consequences (i.e. the damages within the blue area) one has to change the system. In the case of Oroville Dam the probability of failure (of any part of the system) can be reduced by reinforcing, mitigating, etc., BUT reducing the consequences may require, for example, moving population and infrastructures away from the blue zone.
We are regularly asked to advise our clients if a risk is manageable or not, so we thought we could share our thoughts and analysis process to answer this question.
The question is actually more complex and should sound like this: Which risks are:
At inception of the dam, the state of California provided specs for the design of the dam in the form of the FoS which indirectly defines a likelihood of failure, but in those times the relationship was not established (as it is rarely done today, BTW). The dam was therefore considered safe, without asking any further questions. Today, if a probability of failure is evaluated and consequences of a failure estimated, the risk can be compared to a selected tolerance threshold (regional, national, etc.). NB: the idea is not to allow for nefarious accidents to occur, but to evaluate if risks are, for example, higher than those normally accepted by a society.
The reply to the questions above is possible using appropriate risk analysis techniques.
Mitigations aiming at reducing the likelihood of failure or increasing the overall FoS are probably inconceivable, from a financial point of view, beyond the repair works that have been undertaken (which do not influence the overall system, but bring it back to a “point in history”). Even their feasibility is uncertain, given the size of the damaged or damageable infrastructure.
Note that, in the aftermath of a major catastrophe, especially the nuclear ones ( such as Chernobyl or Fukushima), people are evacuated and the evacuation can last a few years or “at perpetuity”. Such a move, which is a “strategic shift”, is very hard to justify as a preventative measure and certainly any authority that would impose it would not remain popular during next elections! Small scale cases of forceful evacuations have occurred around the world and are well documented, including legal actions against the authorities/decision-makers.
So, in the case of Oroville Dam it seems that the only other possible strategic shift could be to permanently reduce the stored volume of water.
Did Oroville Dam risks became unmanageable? Well, we have not performed an analysis, but it seems that the state of California did and the number of available options are rather limited. The selection of alternatives requires an attentive comparative risk assessment as provided by ORE and cradle to grave service/maintenance risk informed cost analysis such as provided by CDA-ESM.