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LISFLOOD-FP simulations were carried out every 9 km (GloFAS) and 5 km (EFAS) along the channel network defined by the 3 arc second MERIT-Hydro DEM, for grid cells with an upstream area greater than 500 km2 (GloFAS) and 150 km2 (EFAS). At each simulation location, the size of the domain was set to be a minimum maximum of either 18 x 18 km or twice the maximum distance from the channel network to the edge of the floodplain defined from Nardi et al., 2019. However, in rivers with a very low slope (<10-6 m m/m) it was necessary to define a larger domain in order to account for backwater conditions, in this case a domain size of 100 x 100 km was set. These domain sizes were applied to the downstream and left and right bank directions relative to the location of the inflow.
Downstream boundary conditions were also defined in the simulation, which allowed water to exit the simulation domain. These were defined at the three boundaries located furthest away downstream, left and right bank boundaries from the location of the inflow flood event hydrograph. A free water surface slope boundary condition was applied, this was calculated as the average channel slope between the location of the inflow the channel elevation 45 km downstream, however if this slope was less than 1 x10-4 m/m, it was recomputed along a greater distance of 135 km.
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