A regional frequency analysis of United Kingdom extreme rainfall from 1961 to 2000

Abstract

Multi‐day rainfall events are an important cause of recent severe flooding in the UK, and any change in the magnitude of such events may have severe impacts upon urban structures such as dams, urban drainage systems and flood defences and cause failures to occur. Regional pooling of 1‐, 2‐, 5‐ and 10‐day annual maxima for 1961 to 2000 from 204 sites across the UK is used in a standard regional frequency analysis to produce generalized extreme value growth curves for long return‐period rainfall events for each of nine defined climatological regions. Temporal changes in 1‐, 2‐, 5‐ and 10‐day annual maxima are examined with L‐moments using both a 10 year moving window and the fixed decades of 1961–70, 1971–80, 1981–90 and 1991–2000. A bootstrap technique is then used to assess uncertainty in the fitted decadal growth curves and to identify significant trends in both distribution parameters and quantile estimates. There has been a two‐part change in extreme rainfall event occurrence across the UK from 1961 to 2000. Little change is observed at 1 and 2 days duration, but significant decadal‐level changes are seen in 5‐ and 10‐day events in many regions. In the south of the UK, growth curves have flattened and 5‐ and 10‐day annual maxima have decreased during the 1990s. However, in the north, the 10‐day growth curve has steepened and annual maxima have risen during the 1990s. This is particularly evident in Scotland. The 50 year event in Scotland during 1961–90 has become an 8‐year, 11‐year and 25‐year event in the East, South and North Scotland pooling regions respectively during the 1990s. In northern England the average recurrence interval has also halved. This may have severe implications for design and planning practices in flood control. Copyright © 2003 Royal Meteorological Society