how many 500-year-floods have occurred in europe and north america since 1934?

Question

...does nature now use a new calculator?

Answer 1

Weather anomalies are assigned a value called the Return Event, it's a measure of the frequency of the same event recurring in the same place. Arriving at a value is a long winded process involving pages of calculations, one of the most important factors is how often such an event has occurred in the past.

Taking Europe and North America as a whole is too vague, return events could be calculated for the entire regions, each individual state or country, even each town or city. Rather than looking at geographically defined areas the RE probability is calculated relative to the area that has been flooded.

Here in the UK we've just had two instances of extensive flooding, the first has a 400 year RE and the second has a 1000 year RE. If the same amount of rain had fallen in other parts of Europe or the US it would have been quite normal.

Calculating RE uses long term historical base periods and doesn't fully take into account the fact that global warming has affected the weather. If we assume there is no global warming then the probability of the UK having had the number of severe flooding events it has since 2000 works out to be a little under four billion to one; quite clearly the climate has changed so we need to update the methods of calculating RE's to take account of this (not as simple as it sounds).

Answer 2

Hi,
Could it be the method of computation and not the calculator that’s at fault? Seemingly (I had to do some research for this one) a 10 year flood has a discharge that has the probability of occurring once every 10 years. So a 10% probability each year while a 500 year flood has a 0.2% probability. The probability is calculated by looking at the historical record of streamflow for a given place. But the records at most gauging stations are no more than 50-70 years old, so the magnitudes of 100 year and 500 year floods are beyond the range of observed data. As well as this, what about the land-use changes that are bound to affect the flood plain? – Urbinization, road building, agricultural drainage, reservoirs all change the way water flows into the stream. So how accurate are the calculations? However going on those computations it seems that every flood season has exactly the same chance (one in 500) of producing a 500 year flood even in an area that experienced a 500 year flood the season before!
Polly

Answer 3

"There is no such thing in nature as a "500 year" flood, or "100 year" flood, for that matter. Those are man-made statistical values that are simplified to make more sense to the average joe."

Not true. The concept of a 100 year flood or a 500 year flood is solid science, widely used by professionals.

"100 year" flood plains are extensively calculated and widely used in land use planning and project planning, by every professional civil engineer involved with such projects. It's "standard practice". If an engineer said they were unscientific and shouldn't be used they'd be laughed at (and sued for malpractice). See (for example):

http://www.cceo.org/specialflood.htm

It's amazing how much stuff in this section is based on personal prejudice, rather than actual knowledge.

Answer 4

There is no such thing in nature as a "500 year" flood, or "100 year" flood, for that matter.

Those are man-made statistical values that are simplified to make more sense to the average joe.

Very simply put, 500 year corresponds to a calculated 0.2% probability, 100 year to a 1% probability. And statistical predictions, being the bullet-proof, carved-in-stone indicators of reality that WE ALL KNOW, provides the solid foundation upon which we can make all critical decisions [sarcasm].

However, for even the statistics-challenged individuals, they should probably realize that there are far more than 500 areas globally that are susceptible to major flooding, such that in any given year you can expect such an event somewhere.

Answer 5

Three.
Once in 1938 then again in 1954 and the most recent occured in 1978.

Answer 6

Ever wonder anyone could know how many 500yr floods occurred in North America since it was discovered just 500 years ago?

Answer 7

Yes, nature bought a new solar calculator.

Answer 8

Trick Question.

Answer 9

thats a pile of crap, so why dont you go to a public bathroom and take a nice crap