Spatio-temporal floodplain evolution over the Holocene under the influence of valley constrictions, lateral fan input, and reworking of glacial deposits and the implications for river restoration.

Post-glacial evolution of upland floodplains has been influenced by temporal changes in vegetation, sediment supply and hydrological regime. Channel-floodplain morphodynamics over the Holocene were conditioned by glacial deposits, lateral interaction with slope processes and fluvial sediment reworking, changes in flow and sediment supply regimes driven by climatic change, and more recently direct and indirect anthropogenic activities, e.g. deforestation, floodplain land use and channel modification. Current drives towards river re-naturalisation, often use floodplain topography as a guide to appraise such a planform state, however reconstruction of former channel state is often restricted to surface features visible on historic maps and aerial photographs. This research focuses upon the floodplain of the upper Swindale Beck, Lake District, UK, that was recently restored to a planform design based on the recent meander pattern visible in the floodplain topography. We investigate the potential of Ground Penetrating Radar (GPR) to reconstruct past channel pattern and evolution, and present findings from 40 intersecting GPR survey lines with a total length of over 3.2 km, covering >51000 m². A centre frequency of 100 MHz allowed reliable imaging of the stratigraphy above the glacial diamict and was supported by 19 soil cores for ground truthing. An outcrop of an andesitic sill just downstream the study site provides a local base level and several alluvial fans constrain the floodplain laterally but also provide sediment to the system. Upstream of the site Younger Dryas glaciation extended into Swindale and the deposited moraines present a source of sediment to Swindale Beck.

Analysis of GPR data revealed several stratigraphic units, including gravel/ cobble plains, small multi-channel systems, several levels of larger channels incised in diamict or fluvial deposits, layered channel fill and floodplain deposits. These were interpreted as braided systems, dynamic wandering planform and single-thread meandering systems with spatial transitions conditioned by tributaries and valley slope. GIS analysis of valley slope, channel gradient and local valley floor aspect allowed the interpretation of individual evolutionary stages of river and floodplain development at Swindale and provides links to processes in the wider environment such as the influence of alluvial fans and bedrock outcrops. Such information can be particularly valuable for restoration projects to aid design of channel dimensions, planform configuration, channel gradient, substrate characteristics and connection with tributaries. While restoration generally aims to resemble a more natural reference state, specific targets may seek to improve a particular set of functionalities (e.g. ecological, flood and sediment management, recreational) which should be resilient to the consequences of ongoing climatic changes and should be achieved sustainably (e.g. locally sourced gravel). Here, GPR-based floodplain analysis provides a non-invasive approach to understand possible evolutionary trajectories and to appraise a wider range of restoration options and sustainable resources.