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 se diment deposition resulting from out-of-bank flow in reach-scale natural co mpound channels. Simulation of this problem requires models capable of deal ing with the hydraulic and sediment transport effects of a dynamically movi ng inundation front. as well as a complex set of flow processes including m omentum exchange between main channel and floodplain, spillage of water acr oss meander loops and the impact of complex topography. Whilst the treatmen t of dynamic moving boundary problems is difficult, but attainable in finit e element hydraulic codes, the necessity to include dry areas within the mo del generates a number of problems that numerical solvers for fluvial sedim ent transport have, to date, failed to overcome. Accordingly, we develop a two-dimensional finite element approach that specifically accounts for sedi ment transport in domains undergoing wetting and drying. The hypothesis tha t at the reach-scale a two-dimensional depth averaged representation of flo w and suspended sediment is able to reproduce observed deposition patterns is then tested against average annual rates determined using (137)caesium a nalysis of floodplain sediments. Using reasonable parameterisation and no c alibration 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 tr ansport components of the model is undoubtedly required, the results provid e an initial assessment of reach scale process dominance for floodplain sys tems. (C) 2000 Elsevier Science B.V. All rights reserved.