Putting the English Flooding of 2019–2021 in the Context of Antecedent Conditions

Abstract

England experienced a sequence of extreme flood events between June 2019 and April 2021. To understand the severity and likelihood of the events, a set of over 300 flow and river level stations was investigated for key events (identified by Environment Agency Area Teams), focusing on frequency analysis of peak flow, peak level and cumulative flow volume. In addition, groundwater, soil moisture and seasonal total rainfall were analysed to understand the antecedent conditions affecting the impacts of the rainfall experienced. While the period contained some of the wettest months on record, there were few extreme short-duration rainfall events. Record-breaking flows and river levels were seen across the country, in part due to the extreme antecedent conditions where many parts of England had record groundwater levels and soil moisture content preceding the events. A kernel density approach was used to identify statistically significant clusters of events over the study period (compared with a Poisson process) and found that most stations in northern and western England experienced a cluster during the study period. Urbanisation was investigated as a possible driver of these trends, but urban increase was not seen to be a significant driver.