The FEBRUARY meeting of the Environmental Statistics Study Group will
be held on
10th February 2000
at 2pm at
RSS headquarters, Errol St, London.
The theme of the meeting is floods and gales and there are three
speakers
Dick Tabony, Met Office
The lowest possible temperature in the UK
Nick Cook, Bristol University
Extreme wind speeds for the design of structures
Duncan Reed, Institute of Hydrology
Regional Flood Frequency: What, why and how.
Abstracts
THE LOWEST POSSIBLE TEMPERATURE IN THE UK.
The nuclear industry is obliged to ensure that its equipment is
designed to withstand climatological extremes which have a
1:10,000 chance of occurring in a given year. Estimates of events with this
degree of rarity involve recognizing the existence of a physical
limit.
Annual extremes of temperature were compiled for 60 stations from 1914
to 1993. Stations on coasts and in the Scottish glens displayed an
approach to a lower limit but extreme value plots for stations in the
English midlands gave the appearance of being unbounded below.
A combination of physical reasoning and statistics were used to
explain this behaviour and a model was developed which estimated
return values of minimum temperature for any location in the UK.
Extreme wind speeds for design of structures
abstract ...
For many years, extreme wind speeds, and sometimes extreme
dynamic pressures, have been fitted to Fisher Tippett Type I distributions
to
derive values for use in engineering design. Recently, there has been
a movement towards adopting Generalised Extreme Value (Type III)
distributions, either directly or extrapolated from the Generalised
Pareto distribution to account for "observed" departures from Type I.
The Type III distribution predicts an upper limit to the values. With
wind speeds, the upper limit corresponds to values (around 50 - 55 m/s
for gusts) for which there is no physical limiting mechansim. Both
Type I and Type III are asymptotic distributions which apply in the
limit as the population from which the extremes are drawn approaches
infinity. The presentation will consider the rate of convergence of
the exact distribution of extremes to the asymptotic model and will
show that the annual population of independent wind speed events is
insufficient to achieve convergence. A method of pre-conditioning the
data for fastest convergence to Type I will be demonstrated that
produces results comparable to the Type III on un-conditioned data,
but without the implied upper limit. It is suggested that this
pre-conditioning method is generally applicable whenever the Weibull
distribution is a good model for the parent.
Regional Flood Frequency Analysis: What, Why and How?
Abstract
The Flood Estimation Handbook (FEH) presents new general
procedures for rainfall and flood frequency estimation throughout the
UK. The talk will attempt to:
+ introduce the FEH statistical procedure for (river) flood
frequency estimation,
+ discuss the philosophy behind use of regional frequency
analysis methods in hydrology,
+ illustrate the tools being provided to engineers and hydrologists.
In approaching flood frequency estimation from a statistical
perspective it is helpful to bear in mind two features. First, the
factors affecting the size of floods are many and varied. Second, the
rarity of events of interest in river flood design mean that the
estimation nearly always has to be undertaken from a position of data
insufficiency. Regional frequency analysis represents a compromise
between excessive reliance on a small sample of data at the study site
and undue pooling of data from sites that may not be representative of
flood behaviour at the study location.
Tea at 3.45pm
All welcome
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Dr. Marian Scott [log in to unmask]
Department of Statistics www.stats.gla.ac.uk/~marian
University of Glasgow Phone: +44(0) 141 330 5125
Glasgow G12 8QW Fax: +44(0) 141 330 4814
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