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Flooding in Bristol

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The effects of a changing climate affect us all. Pictures taken by hydrologist Jim Freer on 21 November 2012 at Whiteladies Road in Bristol.

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Converting probabilities between time-intervals

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This is the first in an irregular sequence of snippets about some of the slightly more technical aspects of uncertainty and risk assessment.  If you have a slightly more technical question, then please email me and I will try to answer it with a snippet. Suppose that an event has a probability of 0.015 (or 1.5%) of happening at least once in the next five years. Then the probability of the event happening at least once in the next year is 0.015 / 5 = 0.003 (or 0.3%), and the probability of it happening at least once in the next 20 years is 0.015 * 4 = 0.06 (or 6%). Here is the rule for scaling probabilities to different time intervals: if both probabilities (the original one and the new one) are no larger than 0.1 (or 10%), then simply multiply the original probability by the ratio of the new time-interval to the original time-interval, to find the new probability. This rule is an approximation which breaks down if either of the probabilities is greater than 0.1. For exa...
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1-in-200 year events

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You often read or hear references to the ‘1-in-200 year event’, or ‘200-year event’, or ‘event with a return period of 200 years’. Other popular horizons are 1-in-30 years and 1-in-10,000 years. This term applies to hazards which can occur over a range of magnitudes, like volcanic eruptions, earthquakes, tsunamis, space weather, and various hydro-meteorological hazards like floods, storms, hot or cold spells, and droughts. ‘1-in-200 years’ refers to a particular magnitude. In floods this might be represented as a contour on a map, showing an area that is inundated. If this contour is labelled as ‘1-in-200 years’ this means that the current rate of floods at least as large as this is 1/200 /yr, or 0.005 /yr. So if your house is inside the contour, there is currently a 0.005 (0.5%) chance of being flooded in the next year, and a 0.025 (2.5%) chance of being flooded in the next five years. The general definition is this: ‘1-in-200 year magnitude is x’ = ‘the current rate for eve...
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A dirty relationship

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I went to see Cabot Institute Artist in Residence Neville Gabie ’s Archiving Oil installation in the Basement Stores of Geology last night (16 May 2013).   It’s pretty cool to be down in the depths of the Wills Memorial Building at the University of Bristol and I can safely assure you I saw no ghosts.   I started off by going into a lift and as the doors opened into the basement, there was an eerie darkness with a bright light emanating from a creepy corner.   A man dressed in white was in front of me and he was pouring a sticky black substance into buckets.   A distinctly thick, gloopy and dirty sound filled my ears.   I promise you it wasn’t a ghost but the image in front of me was quite harrowing.  We use oil in everything we do and here was oil in its bare nakedness – black, shiny, thick, dirty.   I stopped and stared for a while, mesmerised by the horribleness of the clean white background being splatted with this dirty substance.   ...
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