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New Estimates of 24-Hour Probable Maximum Precipitation (PMP) for the British Isles

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.