A geochemical record of flooding on the upper River Severn, UK, during the last 3750 years

Projected future changes in climate call for more definite information on the magnitude–frequency relationships of major floods than can be obtained from short instrumental or historical records alone. Floodplains represent a sedimentary archive of unrecorded flood events, which can potentially be used to extend existing flood records by centuries or millennia. In this study a c. 3750-year flood record was produced using ln(Zr/Rb) profiles from two 4-m-deep cores through floodplain silts at the Roundabout in the upper Severn catchment, UK. Sediment geochemical profiles were obtained using high resolution (500 μm) XRF core scanning. Ages were assigned to the flood record using 14C dating and contaminant Pb profiles. Comparison of sediment ln(Zr/Rb) with sediment grain size showed that ln(Zr/Rb) increases with increasing grain size and indicates that it can be used as a grain size proxy at this site. Within the ln(Zr/Rb) flood record, two floods that occurred at c. 235 and c. 10 cal. BC were probably the largest that have occurred at the site during the past 3750 years while the historical flood of A.D. 1795 appears to be the largest since c. AD 200. The record shows that, during the past c. 3750 years, multicentennial periods characterized by the occurrence of high magnitude floods (return periods > c. 30 years) have alternated with periods of similar length without such floods. These periods correspond to large-scale variations in hydroclimate recorded in the North Atlantic region. However, the incidence of the highest magnitude floods appears to be unaffected by changes in catchment land use. This study provides information about flood magnitude–frequency relationships and their controls at a local level that could be used to help catchment managers in assessing future flood risk at a time of rapidly changing climate.