An updated national-scale assessment of trends in UK peak river flow data: how robust are observed increases in flooding?

J Hannaford,N Mastrantonas,S Turner,G Vesuviano

doi:10.2166/nh.2021.156

Abstract

Abstract A cluster of recent floods in the UK has prompted significant interest in the question of whether floods are becoming more frequent or severe over time. Many trend assessments have addressed this in recent decades, typically concluding that there is evidence for positive trends in flood magnitude at the national scale. However, trend testing is a contentious area, and the resilience of such conclusions must be tested rigorously. Here, we provide a comprehensive assessment of flood magnitude trends using the UK national flood dataset (NRFA Peak Flows). Importantly, we assess trends using this full dataset as well as a subset of near-natural catchments with high-quality flood data. While headline conclusions are useful for advancing national flood-risk policy, for on-the-ground flood-risk estimation it is important to unpack these local changes to determine how climate-driven trends compare with those from the wider dataset that are subject to a wide range of human disturbances and data limitations. We also examine the sensitivity of reported trends to changes in study time window using a ‘multitemporal’ analysis. We find that the headline claim of increased flooding generally holds up regionally to nationally, although we show a much more complicated picture of spatio-temporal variability. While some reported trends, such as increased flooding in northern and western Britain, appear to be robust, trends in other regions are more mixed spatially and temporally – for example, trends in recent decades are not necessarily representative of longer-term change, and within regions (e.g. in southeast England) increasing and decreasing trends can be found in close proximity. While headline conclusions are useful for advancing national flood-risk policy, for flood-risk estimation it is important to unpack these local changes, and the results and methodological toolkit provided here could provide such supporting information to practitioners.

Highlights

In early 2020, the UK experienced one of the most severe nationally significant flood events of recent decades (Parry et al ; Sefton et al in press)
Comparisons between UKBN2 and the full set can be drawn in the sense of spatial trend patterns (Figure 2)
Similarities exist, for example, the positive trends apparent in areas of northern and western Britain that are present in both datasets to some extent

Summary

Introduction

In early 2020, the UK experienced one of the most severe nationally significant flood events of recent decades (Parry et al ; Sefton et al in press). These floods came only 3 months after devastating – and record-breaking – flooding in northern and central England (Muchan et al ). In the winter of 2013–2014, sustained flooding affected large areas of the UK and caused severe impacts in southern England. While this event was less exceptional in terms of flood peaks, the duration and geographical extent were remarkable (Muchan et al )

Methods

Results

Discussion

Conclusion