A quasi-2D unsteady flow and sediment transport model suitable for the simulation of large lowland river systems,including their floodplains,is presented.The water flow and sediment equations are discretised using an interconnected irregular cells scheme,in which different simplifications of the 1D de Saint Venant equations are used to define the discharge laws between cells.Spatially-distributed transport and deposition of fine sediments throughout the river-floodplain system are simulated.The model is applied over a 208-km reach of the Parana River between the cities of Diamante and Ramallo(Argentina) comprising a river-floodplain area of 8100 km~2.After calibration and validation,the model is applied to predict water and sediment dynamics during synthetically generated extraordinary floods of100,1000,and 10,000 years return period.The potential impact of a 56-km long road embankment constructed across the entire floodplain is simulated and compared to model results without the embankment.The embankment results in increases in upstream water levels,inundation extent,flow duration,and sediment deposition. A quasi-2D unsteady flow and sediment transport model suitable for the simulation of large lowland river systems, including their floodplains, is presented. The water flow and sediment equations are discretized using an interconnected irregular cells scheme, in which the respective simplifications of the 1D de Saint Venant equations are used to define the discharge laws between cells. Spatially-distributed transport and deposition of fine sediments throughout the river-floodplain system are simulated. The model is applied over a 208-km reach of the Parana River between the cities of Diamante and Ramallo (Argentina) comprises a river-floodplain area of ​​8100 km ~ 2.After calibration and validation, the model is applied to predict water and sediment dynamics during synthetically generated extraordinary floods of 100, 1000, and 10,000 years return period. potential impact of a 56-km long road embankment constructed across the entire floodplain is simulated and compared to model results without the emban kment. the embankment results in increases in upstream water levels, inundation extent, flow duration, and sediment deposition.