Pervasive and long-term forcing of Holocene river instability and flooding in Great Britain by centennial-scale climate change

Abstract

This paper presents the first probability-based record of flooding in Europe that spans the entire Holocene. An analysis of 506 14C dated fluvial units collected across the whole of Great Britain provides a novel and robust methodology for improving flood risk assessment by geographically and temporally extending the record of extreme flood events. Sixteen episodes of increased flooding occurrence are identified, 12 of which (at c. 11 160, 5730, 4840, 4520, 3540, 2730, 2550, 2280, 1950, 1290, 660, 570, cal. BP) are recorded in most regions, whereas four phases (at c. 6820, 5540, 1650, 860 cal. BP) affected some parts of Great Britain more than others. In all regions large variations in flood frequency and magnitude occurred before forest clearance and indicate an underlying climatic control. During the mid-Holocene there is evidence for a hydroclimatic 'system switch', which in terms of catchment hydrology in Great Britain was marked by two sudden increases in both the frequency and severity of floods at c. 5000 cal. BP and, most notably, at c. 3000 cal. BP. The marked non-stationary behaviour of the British flood series at these times reflects a regional hydrologic response to large-scale ocean-atmospheric circulation changes superimposed on a pattern reflecting both long-term land-use change and the preferential preservation of later fluvial units. Centennial-scale variations in the occurrence of extreme flood events in Great Britain appear to be a sensitive indicator of past and present climate change in the North Atlantic region.