For reasons that will be obvious, no names are given in what follows. The numbers, however, remain roughly as first identified.
–Researchers estimated the annual probability of a major stretch of island levees failing ranged between 4% to 24% due to a slope failure. (Slope instability in this scenario would be caused by flooding behind the levee as well as high water levels on its water side.)
Our estimates were considerably higher than the official one, in large part because the research project relied on validated methodologies for accommodating uncertainties.
–We presented the findings to the island’s management board. Their first and really only question was whether our estimates would be revealed to the island’s insurers.
–We had a “hotwash” afterwards to figure out their–how to put it?–lukewarm response:
- Didn’t they understand the upper range, 24% per annum, implied a levee breach nigh inevitable with respect to our scenario? Or to put the question to our side, in what ways did the 24% per annum estimate fall far, far short of being a failure probability of 1.0?
- But if as high as 24% per annum, why hadn’t there been a levee breach over the many decades since the last major one there?
- What about the islands nearby? Assuming even a few of these had a similar upper range, why weren’t levee failures happening more often there?
- The 4% – 24% range was with respect to annual levee failure due to slope instability only. If you add in all the levee failure modes possible (e.g., due to seepage rather than overtopping and flooding), the combined probability of levee failure would have to be higher. (But then again, what are the conditions under which the more ways there are to fail, the more likely failure is?)
- You could say one reason why levee failure there hadn’t happened–yet–was because it had been long enough. That is: a long enough period to observe levee breaches so as to form the distribution from which the 24% could be established empirically. But these levees, and worse ones nearby, had been in place for decades and decades. The burden of proof, in other words, was on us, the team of levee experts, to explain why this wasn’t “long enough” or what that long-enough might actually look like.
- The levee stretch in question could be “failing to fail.” It might be that this stretch had not undergone events that loaded it to capacity and worse. (But that again: How much worse would the conditions have to be in our expert view? Just what is a probability of failing to fail?)
- Or to put all this differently, was this levee stretch on that island more diverse and more resilient (say, in the way biodiverse ecosystems are said to be more resilient) than current methods capture but which islanders better understood and manage?
–But our most significant observation was the one none of us saw need to voice: How could we accuse the management board and islanders of being short-sighted or worse, with so much else going on challenging us, the team, to make sense of such estimates?
for Robert Chambers