Using geophysical subsurface data for the reconstruction of valley-scale spatio-temporal floodplain evolution: Implications for upland river restoration

Abstract

The use of analogues of previous river styles is highly significant for successful river restoration, yet some existing techniques available to assist practitioners are still not widely applied. We explore the use of Ground Penetrating Radar (GPR), to explore past river styles in an upland river valley in the UK, and explore the potential of the approach to reconstruct former channel pattern. 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 restoration 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, which was recently restored to a planform design based on the recent meander pattern visible in floodplain topography. We show the potential of GPR to reconstruct a wider array of past channel pattern and evolution at a site characterised by largely aggradational conditions and consistent sediment supply from glacial deposits at the valley head. Analysis of GPR data from 40 intersecting GPR survey lines revealed several stratigraphic units, including gravel braidplains, berms, chutes and bars, several levels of larger channels and their layered fill as well as backwater deposits. These were interpreted as braided systems, dynamic wandering planform and single-thread meandering systems with spatial transitions conditioned by tributaries and valley slope. Optically Stimulated Luminescence (OSL) dates in combination with 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 over at least the last millennium and provides links to processes in the wider environment including the role of alluvial fans in supplying sediment and forcing channel migration. 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.