Abstract
The report is the outcome of a 3-year investigation of "regional flood and
storm hazard over reservoired catchments" funded by the Department of the
Environment's reservoir safety committee. It describes an analysis of spatial
dependence in annual maximum 1, 2, 4 and 8-day rainfalls in the UK
A procedure is presented for assessing the annual collective risk of an extreme
rainfall being experienced at one (or more) of a network of critical sites. The
concept of an equivalent number of independent sites is introduced: the
number is shown to be strongly influenced by the number of sites and their
spatial density, with lesser effects attributable to region, rainfall duration and
return period.
A corollary of spatial dependence is that, when an extreme rainfall event
occurs, it may lead to design exceedances occurring simultaneously at a
number of sites. The report draws conclusions for the perception of reservoir
flood risk in the UK. Attention is also drawn to marked regional variations
in point rainfall growth curves that are not fully represented in existing design
procedures.
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