The importance of spill conceptualizations and head loss coefficients in a quasi two-dimensional approach for river inundation modelling

In a quasi two-dimensional approach, floodplains are modelled by means of a one-dimensional hydrodynamic modelling system through a network of fictitious river branches linked to the main river with spills. This paper analyses the effect of different spill model conceptualizations as well as the effect of selected head loss coefficients. A reduced area of the river Dender in Belgium was implemented in three different river modelling packages: Mike11, InfoWorks-RS and HEC-RAS. Simulations were conducted for three historical flood events and eight synthetic events. Evaluation and comparison was made for the maximum volume of water that passes through the banks to the flood branches, the peak discharges and the water levels in the floodplains. Varying the spill’s head loss coefficients shows variations less than 10 cm for the water level inside the floodplain, while the differences in results between the three modelling packages are as large as 25 cm.