On Thu, 17 Jun 2004 22:10:26 +0100, Martin Wiseman
wrote:

Hi,



Actually, I think you will find that the fens are all coloured blue
because they really do have a =1% risk of flooding in any given year
(the EA's criteria). The big problem with the maps is that they don't
tell you how far above the 1% any given area is.

1% is an average of once per century, although I've heard somewhere
recently that we could start getting the once a century floods about
once every 20 years due to global warming, etc., etc.

Such a flood has a *return period* of 100 years and is often known as
the *100 year flood*. The calculation of flood return periods is
flawed , to say the least, and I know from experience how poor they
can be.
Without going into the details (although I can of anyone /really/
wants to know), the main problems a
1. most rivers have maybe 20 or 30 years of flow data so making
calculations about flows which may never have occurred in the dataset
is dubious.
2. calculations are very sensitive to the length of data available
3. the calculation assumes that not only would climate not change
during the 100 years (if calculating the 100 year flood), but also
that climate did not change during the period of the dataset. Even
without *global warming* this is a false premise.
4. Any flow with a return period of more than about 1.5 years will be
out of bank in most UK rivers. Such flows are often estimated rather
than directly measured anyway.

I would be very wary of making decisions such as buying a house based
on calculations such as these.

David
--
David Shepherd
Wymondham,
Norfolk