Water table fluctuations within the floodplain of the River Severn, England

Abstract In contrast to extensive research on hydrological processes operating in headwater basins, there has been relatively little attention paid to the hydrological processes that occur on the floodplains of lowland rivers. This dearth of information is all the more surprising given current interest in the use of floodplains as buffer zones between farmland and the riverine environment. In the previous paper (Water Resour. Res. 36 (2000) 2517), Bates et al., reported preliminary results from a field site on the floodplain of the River Severn in Shropshire, UK, a large lowland river by British standards. Piezometric data suggested that during out-of-bank conditions a reverse groundwater ridge develops in the floodplain subsurface and results in strong groundwater flux velocities directed towards the base of hillslopes adjoining the floodplain. Bates et al. showed that the impact of such ridges was to switch off hillslope inputs to the riparian zone; they hypothesised that this occurred when surface inundation approached the back of the floodplain. In this paper, we provide a more detailed analysis of the events considered by Bates et al. and extend the analysis to the more common in-bank flood condition. In total, five events are considered (two out-of-bank and three in-bank); these were chosen to represent a wide range of event magnitudes, antecedent conditions and local patterns of rainfall and runoff. The analysis demonstrates that the reverse groundwater ridging process identified by Bates et al. also occurs during in-bank events. Hillslope inputs to the floodplain are also ‘switched off’ in these events if the flood stage is high. In smaller floods, water continues to move from slope to floodplain, although coupling between slope and channel is only re-established later in the recession. We conclude that, contrary to the conclusions of Bates et al., this process switching is not necessarily dependent on surface inundation approaching the back of the floodplain. Whilst the paper broadly confirms the operation of the simple reverse groundwater ridging process described by Bates et al., it shows that antecedent conditions, local rainfall and runoff, and flood stage all act to complicate this basic pattern. Lastly, we consider the implications for catchment water quality of the newly identified processes.