Convergent human and climate forcing of late-Holocene flooding in northwest England

Abstract

Concern is growing that climate change may amplify global flood risk but short hydrological data series hamper hazard assessment. Lake sediment reconstructions are capturing a fuller picture of rare, high-magnitude events but the UK has produced few lake palaeoflood records. We report the longest lake-derived flood reconstruction for the UK to date, a 1500-year record from Brotherswater, northwest England. Its catchment is well-suited physiographically to palaeoflood research, but its homogeneous, dark brown sediment matrix precludes visual identification of flood layers. Instead, an outlier detection routine applied to high-resolution particle size measurements showed a >90% match, in stratigraphic sequence, to measured high river flows. Our late-Holocene palaeoflood reconstruction reveals nine multi-decadal periods of more frequent flooding (CE 510-630, 890-960, 990-1080, 1470-1560, 1590-1620, 1650-1710, 1740-1770, 1830-1890 and 1920-2012), and these show a significant association with negative winter North Atlantic Oscillation (wNAO) phasing and some synchrony with solar minima. These flood-rich episodes also overlap with local and regional land-use intensification, which we propose has amplified the flood signal by creating a more efficient catchment sediment conveyor and more rapid hillslope-channel hydrological connectivity. Disentangling anthropogenic and climatic drivers is a challenge but anthropogenic landscape transformation should evidently not be underestimated in palaeoflood reconstructions. Our paper also demonstrates that flood histories can be extracted from the numerous lakes worldwide containing organic-rich, visually homogeneous sediments. This transformative evidence base should lead to more reliable assessments of flood frequency and risks to ecosystems and infrastructure.