Mixing of Hillslope, River, and Alluvial Ground Waters in Lowland Floodplains

In this paper, we develop and apply a two‐dimensional flow and transport model to simulate the mixing of variously sourced water within a fluvial floodplain during one in‐bank and one out‐of‐bank flood event. The model is applied to two 120 m long transects through the floodplain of the River Severn, Shropshire, United Kingdom, which run to the river from the hillslope bordering the floodplain. For this site, hourly piezometric, river stage, and rainfall data are used to parameterize and validate the flow component of the model. Simulation of the movement of hypothetical conservative tracers driven by these hydrodynamic conditions is then undertaken to confirm and refine an evolving conceptual model of floodplain hydrology. This is based on the bank storage concept, and extended to include the impact of hillslope contributions to the floodplain/riparian zone and the impact of out‐of‐bank inundation on subsurface flow processes. The paper demonstrates that the role of the unsaturated zone has been underestimated in our previous investigations at this site and, in addition, that there is a potentially high residence time for solutes which remain in the unsaturated zone at the end of the flood event. In general, flood events in lowland plain systems are shown to be times of great hydrological and chemical activity. Lastly, the results confirm the complexity of interactions between surface and ground water flows in lowland plain settings and highlight the need for correct treatment of both dynamic boundary conditions and unsaturated zone processes.