TITLE:
New Estimates of 24-Hour Probable Maximum Precipitation (PMP) for the British Isles
AUTHORS:
Colin Clark, James Dent
KEYWORDS:
Probable Maximum Precipitation, Moisture Maximisation, Modified Gumbel Reduced Variate
JOURNAL NAME:
Journal of Geoscience and Environment Protection,
Vol.9 No.7,
July
29,
2021
ABSTRACT: Estimates of PMP are needed in
order to estimate the spillway design flood for dams which must be capable of safely
passing the probable maximum flood (PMF). For over forty years the standard estimates
of the Flood Studies Report (FSR) have almost always been used. However, since then
several studies have shown that these values are too low, and new estimates for
South West England and then Britain were published. This paper extends these studies
to include the whole of the British Isles. The study uses 6570 station years’ data
in a new method of frequency analysis that identifies part of the data which represents
the long term situation. Further analyses to support the results of this approach
have been produced from the in situ maximisation of 12 historic storms using World
Meteorological Organisation methods. The results are broadly consistent with those
obtained from frequency analysis. Values of 24 hour PMP in Britain range from 600
mm in upland areas of the Lake District, to 400 mm in parts of East Anglia. This
range of values is present in Wales and parts of South West England. For Ireland
the highest values are over the upland areas, such as in the Wicklow Mountains (550
mm) and SW Ireland (600 mm), while in the lowlands values around 350 mm can be expected.
The question of uncertainty in estimating PMP is considered, most of which is due
to the temporal and spatial shortcomings of the data. The implications for the probable
maximum flood (PMF) and spillway design flood mean that widespread reassessment
of dam safety should be carried out at once.