Latest updates in global flood modelling: channel bifurcation and global river width database

Dai Yamazaki, University of Tokyo, Institute of Industrial Science, Bunkyo-ku, Japan, Shinjiro Kanae, Tokyo Institute of Technology, Department of Civil and Environmental Engineering, Tokyo, Japan, Yukiko Hirabayashi, The Shibaura Institute of Technology, Tokyo, Japan, Fiachra O'Loughlin, University College Dublin, School of Civil Engineering, Dublin, Ireland, Mark Adam Trigg, University of Bristol, School of Geography, Bristol, United Kingdom and Paul D Bates, University of Bristol, School of Geographical Sciences, Bristol, United Kingdom
Abstract:
Global flood modelling is a relatively new framework in earth system studies, and there still exist many rooms for improving model physics. A typical grid size of global models (generally >5 km) is coarser than the scale of the topography of river channels and floodplains, therefore flood dynamics in global flood models is represented by sub-grid parameterization. Here, we introduce two latest updates in flood dynamics parameterization, i.e. channel bifurcation scheme and global river width database.

The upstream-downstream relationship of model grids is prescribed (i.e. parameterized) by a river network map, where each grid has been assumed to have only one downstream grid. We abandoned this “only one downstream” assumption, and succeeded to represent channel bifurcation in a global flood model. The new bifurcation scheme was tested in the Mekong River, and showed the importance of channel bifurcation in mega-delta hydrodynamics.

Channel cross-sectional shape has been parameterized using an empirical equation of discharge (or drainage area), and it is a major source of uncertainties in global flood modelling. We recently developed a fully-automated algorithm to calculate river width from satellite water mask. By applying this algorithm to SRTM Water Body Data, the Global Width Database for Large Rivers (GWD-LR) was constructed. The difference between the satellite-based width and empirically-estimated width is very large, suggesting the difficulty of river width parameterization by an empirical equation.

Improvement in flood dynamics parameterization reduces uncertainties in global flood simulations. This enables advanced validation/calibration of global flood models, such as direct comparison against satellite altimeters. A future strategy for advanced model validation/calibration will be mentioned in the conference presentation.

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