Assessing the impact of hydrodynamics on large-scale flood wave propagation – a case study for the Amazon Basin
Authors: Hoch Jannis M. , Haag Arjen V. , van Dam Arthur , Winsemius Hessel C. , van Beek Ludovicus P. H. , Bierkens Mark F. P. 
Published: August 2016

Abstract. Large-scale flood events often show spatial correlation in neighbouring basins, and thus can affect adjacent basins simultaneously, as well as result in superposition of different flood peaks. Consequently, such flood events need to be addressed with large-scale modelling approaches to capture these processes. Many approaches currently in place are based on either a hydrologic or a hydrodynamic model. However, the resulting lack of interaction between hydrology and hydrodynamics processes, by for instance implementing groundwater infiltration on inundated floodplains, can hamper modelled inundation and discharge results where such interactions are important. In this study, the global hydrologic model PCR-GLOBWB at 30’ spatial resolution was one-directionally and spatially coupled with the hydrodynamic model Delft3D Flexible Mesh (FM) for the Amazon River Basin at a grid-by-grid basis and at daily time step. The use of a flexible unstructured mesh allows for fine-scale representation of channels and floodplains, while preserving a coarser spatial resolution for less flood-prone areas, thus not unnecessarily increasing computational effort. In addition, we assessed the difference between a 1D-channel/2D-floodplain and a 2D schematization in Delft3D FM. Validating modelled discharge results shows that coupling PCR-GLOBWB to a hydrodynamic routing scheme generally increases model performance compared to using a hydrodynamic and hydrologic model only for all validation parameters applied. Closer examination shows that the 1D/2D schematization outperforms 2D for r2 and RMSE whilst having lower KGE. We also found that the 1D/2D set-ups have the significant advantage of a better representation of smaller streams throughout the model domain. Implementing 1D channels is therefore particularly of advantage for large-scale inundation models as they are often built upon remotely sensed surface elevation data which often enclose a strong vertical bias, hampering downstream connectivity. Another advantage for large-scale application is the 25 % lower clock wall time of the 1D/2D set-up compared to 2D only. Since only a one-directional coupling approach was tested, and therefore important feedback processes are not incorporated, simulated discharge for both coupled set-ups is generally overpredicted. Hence, it will be the subsequent step to extend it to a two-directional coupling scheme to obtain a closed feedback loop between hydrologic and hydrodynamic processes. As spatial resolution is an influential factor, future studies will also have to focus on the impact of re-fining spatial resolution of the hydrologic input and the flexible mesh used for hydrodynamic computations. The current findings demonstrating the potential of one-directionally and spatially coupled models to obtain improved discharge estimates form an important step towards a large-scale inundation model with a full dynamic coupling between hydrology and hydrodynamics.