...
In CEMS-Flood, the flood impact forecasts are based on three components: 1) medium-range flood forecasts, 2) event-based rapid flood mapping, and 3) impact assessment.
- Medium-range flood forecast: every time a flood event greater than the 10 year return period is forecasted in CEMS-Flood, the return period of the maximum discharge (based on the ECMWF-ENS ensemble forecast median and mean in GloFAS and EFAS respectively) over the entire forecast period (30 days in GloFAS, 10 days in EFAS) is computed in each grid cell (shown by the coarse cells in Fig. 1). These values do not consider the possible role of local flood defences, therefore they represent an unprotected scenario. A second scenario which considers flood defences is computed by comparing the forecasted return period values against estimated flood protection levels from FLOPROS (Scussolini et al., 2016). River grid cells where the protection levels are exceeded are selected and form the protected scenario.
- Rapid flood mapping: for each CEMS-Flood river section cell identified in step 1, flood prone areas are delineated, using a catalogue of higher resolution flood hazard inundation maps. The obtained event-based hazard inundation map has a spatial resolution of XXX 3 arc seconds (approximately 90 m) (up to GloFAS version 3, 0.000833 by 0.000833 degrees or approximately 90 by 90 m the resolution was 30 arc seconds (approximately 1 km), and up to EFAS version 4 XXX) and is the resolution was 100 m). This is produced for both the unprotected and protected scenarios, the former is shown on the “Rapid Flood Mapping” layer on the webviewer. The flood hazard inundation maps catalogue is generated using the LISFLOOD-FP hydraulic model (CA2D hydraulic model for GloFAS until version 3).
- Impact assessment: the event-based hazard maps inundation maps for both the unprotected and protected scenarios are combined with exposure information to assess regional impacts (shown on the “Rapid Impact Assessment” layer). Considered exposure includes population, critical infrastructure and land cover. The impact is calculated by combining the flood severity and probability with the exposed population. The The result is summarised on administration units sourced from NUTS in Europe and GADM in the rest of the world.
...
Figure 1. The three components to produce the GloFAS flood impact forecast products, adapted from Dottori et al., 2017.
Flood Inundation Map Generation Methodology
The flood inundation maps are created by using the GloFAS and EFAS re-analyses to generate flood event hydrographs for different return period events which are then used as inputs to a 2D hydraulic flood inundation model. More details about the methodology are given in Alfieri et al., 2014 and Dottori et al., 2017.
Flood Event Hydrograph Generation
Flood event hydrographs refer to the rise and fall of river discharge during an event. These were generated in every grid cell with an upstream area above a predefined threshold (150 km2 from EFAS version 5 and 500 km2 from GloFAS version 4) using historical river discharge data from the EFAS and GloFAS re-analyses. Firstly, in each grid cell a gumbel extreme value distribution was fitted to the maximum annual river discharge values, this was used to compute the river discharge associated with the 10, 20, 50, 75, 100, 200 and 500 year return periods. Next, a flood event hydrograph was generated for each of these return period scenarios, the hydrograph peak equalled the discharge associated with the return period, the rate of the hydrograph rise and fall was defined from the time of concentration (Tc) which was computed using the Giandotti 1934 method (see Grimaldi et al., 2012 for a description):
Tc = (4* SQRT(A) + 1.5L) / 0.8*SQRT(H)
A = upstream area (km2), L = length of the main river channel (km), H = difference between the mean basin elevation and the elevation at the grid cell being analysed (m)
These parameters for the Tc calculation were obtained from the MERIT-Hydro Digital Elevation Model (DEM) (Yamazaki et al., 2019) which has been resampled to the resolution of EFAS (1 arc minute from version 5) and GloFAS (3 arc minute from version 4).
...
The time between the start of the hydrograph and the peak was set to the maximum of either Tc or 5 days, this allowed sufficient time for the water to pass to the downstream end of the 2D hydraulic simulation domain. The time between the hydrograph peak and the end of the simulation was set to the maximum of 2*Tc or 10 days, the longer time for the falling limb was to allow sufficient time for the peak discharge to travel downstream within the simulation domain. The flood event hydrographs were used as inputs to a 2D hydraulic inundation model simulation, described in the next step.
Figure 2. Procedure to generate flood event hydrographs at each grid cell using the EFAS/GloFAS historical re-analysis.
...