Extreme Historical UK Floods and Maximum Flood Estimation

Historical flood events recorded as flood marks on bridges and houses or reported in newspapers and journals provide valuable information on the maximum size of floods which are likely to occur in the UK. For hydrological analysis peak water level must be converted to peak discharge, which is problematic even when the flood has been recorded at a flow gauging station. This paper describes some of the methods used to calculate the magnitude of extreme floods. Sixty-nine historical UK floods are plotted on a graph of discharge versus drainage area and compared with curves published in 1933 and 1960 which were used to estimate the Normal Maximum Flood required for dam spillway design before 1975. From that date the design standard became the Probable Maximum Flood. PMF is calculated for 17 sites where extreme floods have occurred. For six events the observed peak discharge exceeded PMF. Possible reasons for these findings are discussed.