When 1-in-1000 is no longer 1-in-1000

I started an interesting post over at Climate Progress, which made me think about the following question before I even finished that post to read the punch line:

What happens when a 1000-year-flood occurs? Well, to answer that question, first we need to answer the question of what a 1000-year-flood is. Simply put, such a flood is the statistical probability of such a flood happening once over a given period of time. In other words, every year, there is a 1-in-1000 chance that such a flood will happen, however, that kind of flooding can happen more than once during a millennium (or not at all). To that extent, calling these events 1000-year-floods (or 1000-year-storms, or the like) is a bit of a misnomer. However, what happens when the chance of deluge increases -- one expected consequence of climate change?

If we are going to enter into a period of greater rainfall intensity -- and, consequently, more flash-flooding -- then the number of expected large floods should also increase. If we continue to term floods as 1000-year-floods (or 300-year-floods, or any other X-year-floods), then shouldn't they change to meet the new understandings of flooding? I mean, if a 1000-year-flood for a particular stretch of river is currently listed as a discharge of 10,000 cfs, that means that every year, there is a 1/1,000 chance that a discharge of 10,000 cfs will be reached. However, let's assume that climate change will change the hydrological impacts for that region, increasing the intensity of flooding such that a 10,000 cfs discharge has a 1/700 chance each year. That will mean that the same 10,000 cfs discharge will now be a 700-year-flood.

What does that mean for planning? I mean, 1000-year-floods are such rare occurrences that they are unlikely to be planned for due to a variety of reasons (including prohibitive cost for construction and maintenance). However, what of the smaller floodings that are important thresholds for planning? If current 200-year-floods (which are usually cited as the minimum required limit for planning) will occur more often, then that means that future 200-year-floods will have a higher discharge than currently. That means that existing minimum-required designs will be less capable of protecting against a range of statistically likely flood scenarios. Let's assume that a current 200-year-flood occurs at a discharge of 1,500 cfs, and city flood-water management systems are developed to protect against a likelihood of flooding of 1/200. However, if the future 200-year-flood occurs at a discharge of 1,700 cfs (and the 1,500 cfs discharge were a 150-year-flood), then the existing infrastructure will only be able to protect against a likelihood of flooding of 1/150. The laws would have to change to set flooding to a lower level of safety (the old 200-year-flood discharge level), retrofits will have to be undertaken, or insurance measures would need to be undertaken by those living in the zones affected by all floodings below the new 200-year-flood levels.

Just another way in which future planning and laws will be affected by climate change.

UPDATE: the CP blog post only recounts the increased number of recent deluges in the UK, US, and Australia, but not the implications of what it means for planning.