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Figure 2. Procedure to generate flood event hydrographs at each grid cell using the EFAS/GloFAS historical re-analysis.
Flood Inundation Simulation
The LISFLOOD-FP 2D hydraulic inundation model (Bates & De Roo, 2000) was used to simulate flood inundation for EFAS and from version 4 for GloFAS (previously CA2D was used). As well as the flood event hydrographs generated in the previous step, LISFLOOD-FP required the following inputs:
- Topographic data for the floodplain: obtained from the MERIT-Hydro DEM (Yamazaki et al., 2019) at 3 arc second spatial resolution (~90 m)
- Floodplain roughness defined by the Manning's n parameter: these were defined from the Copernicus global landcover dataset version 3 at ~90 m resolution (Buchhorn et al., 2021), a lookup table was used to translate each landcover category into a Manning's n parameter value
- Channel width: obtained from the MERIT-Hydro dataset (Yamazaki et al., 2019) at 3 arc second spatial resolution (~90 m)
- Channel depth: no global dataset on channel depth was available, therefore 0 m channel depth was used. To compensate for this, the discharge associated with the 1.5 year return period, which represents the bankfull discharge, was subtracted from each of the flood event hydrographs so that they represented the floodplain flow only.
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 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) 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.
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 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.