In a recent interview, Prof ‘taking the Mickey’ Mann said this about floods in the US:
NOOR: So weve seen this historic flooding not only in Louisiana but more recently in West Virginia, even right here close to Baltimore in Ellicott City. Is it, are we seeing more and more of this and is this connected to climate change?
MANN: We often hear about events being characterized as thousand year events and what that means is just given the usual statistic of the weather, we wouldnt expect such an event to happen more than once in a thousand years. Meaning we probably wouldnt expect to see it during our lifetimes. And yet we are seeing a plethora of these thousand year events. Whether its the flooding events in South Carolina, in Arizona, in Texas as I said and of course this latest event in Louisiana and Alabama. We are seeing thousand year events far too often to be able to attribute them just to randomness. We are seeing the loading of the random weather dice by climate change.
Mann is disinforming the public here. 1000 year or 100 year flooding events measure the likelihood of that event occurring on a specific stretch of river, not in a selection of whichever random states happened to get a flood that year.
Wiki: In the United States, the 100-year flood provides the risk basis for flood insurance rates. Complete information on the National Flood Insurance Program is available here. A regulatory flood or base flood is routinely established for river reaches through a science-based rule making process targeted to a 100-year flood at the historical average recurrence interval.
Regarding ‘1000 year’ events. How long have we been keeping flood records in the US. It’s unlikely they predate Columbus’ discovery of the continent in 1493, so, not more than 500 years. How well established is the basis of characterising a flood as ‘once in 1000 years’ then? You can be sure there’s a model for it…
It’s also worth remembering that there’s around a 60%chance of seeing more than one ‘100 year flood’ in a specific location during a 100 year period anyway.
Wiki: A common misunderstanding exists that a 100-year flood is likely to occur only once in a 100-year period. In fact, there is approximately a 63.4% chance of one or more 100-year floods occurring in any 100-year period. On the Danube River at Passau, Germany, the actual intervals between 100-year floods during 1501 to 2013 ranged from 37 to 192 years. The probability Pe that one or more floods occurring during any period will exceed a given flood threshold can be expressed, using the binomial distribution, as
where T is the threshold return period (e.g. 100-yr, 50-yr, 25-yr, and so forth), and n is the number of years in the period. The probability of exceedance Pe is also described as the natural, inherent, or hydrologic risk of failure.
There is of course the possibility that Mann simply doesn’t understand the stats, given his previous record. There again, if that’s so, he shouldn’t be professor of rubber ducks, let alone a science with such enormous cost implications for public policy.