Modelling suspended sediment deposition on a fluvial floodplain using a two-dimensional dynamic finite element model

In this paper we outline a new numerical model for predicting floodplain sediment deposition resulting from out-of-bank flow in reach-scale natural compound channels. Simulation of this problem requires models capable of dealing with the hydraulic and sediment transport effects of a dynamically moving inundation front, as well as a complex set of flow processes including momentum exchange between main channel and floodplain, spillage of water across meander loops and the impact of complex topography. Whilst the treatment of dynamic moving boundary problems is difficult, but attainable in finite element hydraulic codes, the necessity to include dry areas within the model generates a number of problems that numerical solvers for fluvial sediment transport have, to date, failed to overcome. Accordingly, we develop a two-dimensional finite element approach that specifically accounts for sediment transport in domains undergoing wetting and drying. The hypothesis that at the reach-scale a two-dimensional depth averaged representation of flow and suspended sediment is able to reproduce observed deposition patterns is then tested against average annual rates determined using 137caesium analysis of floodplain sediments. Using reasonable parameterisation and no calibration of the sediment transport component, the developed model is able to replicate an encouraging amount of the observed spatial variability in this data set. Whilst further testing of both the hydraulic and sediment transport components of the model is undoubtedly required, the results provide an initial assessment of reach scale process dominance for floodplain systems.