Rapid laminated clastic alluviation associated with increased Little Ice Age flooding co-driven by climate variability and historic land-use in the middle Severn catchment, UK

Abstract

The analysis of exceptionally well-preserved visible clastic laminations in deep alluvial sediments at Kempsey, Worcestershire (UK), allows a new highresolution analysis of late-Holocene flood-history in the largest UK catchment, as well as local human response. At the sample site over 4.5m of sandy-silt overbank-alluvium accumulated on the floodplain and optically stimulated luminescence (OSL) dating of the upper 2.25m demonstrates accretion from the late 14th century CE onwards. Sub-centimetre to centimetre resolution multi-proxy sediment analysis (loss on ignition, magnetic susceptibility, particle size, ITRAX and portable XRF) demonstrate clear variations in depositional history over the last millennium due to channel stability. Between c. 1380 and 1550CE overbank sedimentation was driven by lower energy flood events, with negligible effect from climatic conditions during the Spörer Minimum (1460–1550CE). After c. 1550CE the magnitude of flooding events increased and by c. 1610CE, the start of the visible sub-centimetre laminations, the accumulation rate regularly exceeded 3mm year⁻¹, which increased to 4.5mm year⁻¹ between c. 1690 and 1710CE, and 3 and 3.5mm year⁻¹ between c. 1790 and 1840CE before alluviation was altered by an embankment. The greatest extent of coarse overbank deposition and increased accumulation rate occur concurrently with periods of climatic instability associated with the Maunder (1645–1715CE) and Dalton (1790–1820CE) Minima, the periods of largest historical floods and during the intensification of arable cultivation across the middle Severn catchment. This data correlates well with other sites in the catchment suggesting that these are basin-wide forcing-responses. We also present evidence that this catchment-wide hydro-geomorphological history had local effects in shifting the geographical focus of an important settlement away from its historic floodplain edge location – which can be viewed as an adaptation to the flood risk.