How has flood flashiness evolved across the UK?

Floods are one of the most common natural hazards in the UK. Significant changes in flood behaviour in the UK have been observed in previous research, including notable shifts in flood frequency, magnitude, and timing. Flood hazards are increasing in some regions, with high-magnitude floods occurring over shorter durations (i.e., becoming ‘flashier’). These flashier floods develop rapidly and tend to have high peak flows, leaving less time for flood warnings and emergency response, which can potentially lead to more severe impacts. However, studies on flood flashiness are limited. Our study aims to investigate changes in flood flashiness across the UK using an event-based approach. A total of 158,682 individual flood events were identified across 354 selected UK catchments between 1980 and 2019. Two flashiness metrics, the flashiness index (FI) and rate of change (RoC), were employed to quantify the rapidity of river flow changes during flood events, with FI measuring flow variability and RoC assessing the rate of flow increase. Statistical methods (e.g., Mann-Kendall test and Theil-Sen estimator) were applied to detect trends in flashiness across all the study catchments. Spatial patterns of flashiness changes were examined on both long-term and seasonal scales. In addition, correlation analysis was performed to explore potential relationships between flashiness patterns and catchment attributes (e.g., catchment area, slope, longest drainage path, and the degree of flood attenuation due to lakes and reservoirs). Preliminary results indicate that increases in flood flashiness were regionally focused in some cases, but also spatially heterogeneous in many others. Flood events mostly occur in winter. accompanied by significant trends in flashiness changes that diverge in direction regionally, with northeast Scotland experiencing significant increases, whereas most other regions showing significant decreases. The two metrics, which capture different aspects of flashiness, correlate with various catchment attributes (e.g., FI decreases with increasing catchment area). Moreover, rainfall patterns, pre-event soil moisture conditions, and urbanization are likely key factors influencing the observed geographical and temporal variations. Further exploration is required to understand how these factors influence the observed patterns and how their interactions contribute to changes in flashiness. These insights are expected to inform the development of more effective flood management strategies.