Tuesday, May 24, 2011

Probability - A Reflection from Today's Lectures

Flood defense projects are built with a safety buffer based on past probabilities. Past flooding averages are used to determine the probability of having flooding events of various strengths. A 1 in 10 year flood occurs more often and is lower in strength than a flood that is said to occur 1 in 1000 years. Both numbers, though, represent a probability and do not actually say that only one of each type of flood will happen in the time frame.

I remember in my Water Resources class doing exercises to find the rain events associated with 1 in 10 or 1 in 100 year events, as these are the common time frames used for US design. However, in today's presentation at Deltares (the Dutch institute for national and international water, soil, and subsurface issues), Nathalie Asselman showed maps of the Netherlands using up to a 1 in 10,000 year flood probability. At first glance, that looks like any structure using that design parameter will have a slim chance of failure.

This is, and is not, entirely true.

The Dutch accept that climate change is occuring and know that they must do something to protect their land and people. Also, climate change is causing probability calculations based on historical data to be obsolete. As I learned today:

Probability is dead*.
*Disclaimer: This does not mean everyone can skip Statistics from now on...

This statement really means that we can no longer trust the historical record to predict a future of extremes. It looks like for now, the Dutch are using extreme design factors (like the 1 in 10,000 year flood) to try and prepare. In the meantime, they are also collecting new data to better examine what the future might hold. Although still a guess, it will be a more defined guess... and the safety factors built-in will ensure that the Netherlands will be prepared for, but not immune to, climate change.

(...some fun pictures/videos to come later.)


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