River Itchen Flood Alleviation Scheme Geoarchaeological Monitoring of Ground Investigation Works

A programme of geoarchaeological monitoring of GI works associated with the River Itchen Flood Alleviation Scheme (RIFAS; the 'Scheme') was undertaken, followed by a programme of geoarchaeological deposit modelling integrating the results of the monitoring with BGS archive borehole data and geotechnical data from previous phases of GI works (Garner SOU 1671). The results have provided a baseline dataset to inform on the Quaternary deposits present in the Site, and the scope and requirement for any further work that may be required to evaluate their geoarchaeological potential. The sequence at the Site comprises Pleistocene Fluvial Sands and Gravels of two river terraces, likely of middle Devensian and Late Devensian date, and a sequence of Holocene alluvial sediments which in places contain peat and organic alluvium, capped by modern made ground. The Fluvial Sands and Gravels are coarse-grained deposits formed within a cold-climate, braided river. The earlier, higher terrace evident within the Scheme, with a basal elevation of c. -2 to -4 m OD, is likely equivalent to the Broadland's Farm terrace of the River Test, dated to between MIS 4-3 and of Early to Middle Devensian date. The lower terrace, closer to the modern course of the River Itchen and with a basal elevation of c. -5 to -9 m OD, is likely to be of Late Devensian (MIS 2; c. 17-11.7 Kya) date. These deposits are of moderate geoarchaeological potential to contain reworked Middle or Upper Palaeolithic archaeology, with their upper surfaces (where undisturbed) of high potential to contain archaeology of Upper or Late Upper Palaeolithic date. A sequence of minerogenic alluvial sediments was widespread within the Scheme overlying the lower of the gravel terraces at elevations between c. -9 and 1 m OD. The sediments likely accumulated primarily under the influence of rising sea levels during the Holocene, forming in intertidal mudflats or salt marsh on the floodplain of the River Itchen. These deposits are of low geoarchaeological potential but may mask organic-rich units and features (e.g. palaeochannels) of high geoarchaeological potential. Peat and organic alluvial sediments were present locally within the Scheme, forming sub-units within the Holocene alluvial sequence. These sediments would have formed in low energy environments such as sedge fen, reed swamp or alder carr. While undated, peat deposits at similar elevations within the lower valley of the River Itchen have been radiocarbon to the Mesolithic to early Neolithic. Peat deposits are of high geoarchaeological potential to preserve material suitable for radiocarbon dating, palaeoenvironmental analysis and may contain in situ archaeological artefacts or features. The impacts from the proposed Scheme are currently unknown. If deposits of moderate or high geoarchaeological potential are impacted by final design, purposive geoarchaeological boreholes are recommended as a suitable mitigation measure. These boreholes would target the peat, organic alluvium and Pleistocene fluvial sands and gravels of the 'upper' terrace. The boreholes would offer an opportunity to obtain core samples suitable for palaeoenvironmental assessment and scientific dating. The Pleistocene fluvial sands and gravels within the Scheme are too deep to evaluate by conventional methods (e.g. test pitting) and boreholes would enable evaluation of these deposits, including their potential to contain fine-grained units suitable for scientific dating and palaeoenvironmental assessment.