Methodology for river flood modelling by the quasi two-dimensional approach

In many consulting applications, river flood modelling is performed by a one-dimensional full hydrodynamic modelling system. To model the floodplains with such a system, a "quasi two-dimensional approach" can be used. In this approach, the floodplains are modelled as a network of fictitious river branches and spills with the rivers. The river branches represent the topographical depressions (the floodplains, most often with drainage canals at the bed), while the spills represent topographical elevations (e.g. roads, railways) in between these depressions. Also the dikes or the river embankments in between the river and the floodplains are represented by spills. By taking the cross sections of the fictitious river-branches equal to those of the floodplains (cross sections perpendicular to the axis of the river), the storage of water in the floodplains (water levels, storage volumes and spatial extent) is described well. Also the discharges and the water velocities can be modelled for the floodplains if appropriate values are chosen for the friction coefficients in the fictitious river-branches. Copyright ASCE 2004.