Re-assessing the flood risk in Scotland

Abstract

This paper presents a review of changes in flood risk estimation on Scottish rivers resulting from re-analysis of flood records or from the application of new methods. The review arises at a time when flood damages have received recent prominence through the occurrence of a number of extreme floods in Scotland, and when the possible impacts of climate change on flood risk are receiving considerable attention. An analysis of the nine longest available peaks-over-threshold (POT) flood series for Scottish rivers reveals that, for thresholds yielding two events per year on average, annual POT frequencies on western rivers have increased in the 1980s/1990s to maximum recorded values, while in the east, values were highest in the 1950s/1960s. These results support the results of flood modelling work based on rainfall and temperature records from the 1870s, which indicate that, in western catchments, annual POT frequencies in the 1980s/1990s are unprecedented. No general trends in flood magnitude series were found, but an unexpected cluster of extreme floods is identified as having occurred since 1988, resulting in eight of Scotland's 16 largest gauged rivers producing their maximum recorded flows since then. These shifts are related to recent increases in the dominance of westerly airflows, share similarities with the results of climate change modelling, and collectively point to increases in flood risk in many parts of Scotland. The paper also reviews advances in flood risk estimation arising from the publication of the UK Flood Estimation Handbook, developments in the collection and use of historic flood estimation and the production of maps of 100-year flood areal extent. Finally the challenges in flood risk estimation posed by climate change are examined, particularly in relation to the assumption of stationarity.