Flood impact forecasts are especially important in the preparedness phase, to support the planning and allocation of rescue assets, and to get a first estimate of the forecasted flood event’s potential socio-economic consequences.
In CEMS-Flood, the flood impact forecasts are based on three components: 1) medium-range/ 30 days 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 forecast period (30 days in GloFAS, 10 days in EFAS) is computed in each grid cell (shown by the coarse cells in figure above). 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 cell identified in step 1, flood prone areas are delineated, using a catalogue of higher resolution flood inundation maps. The obtained event-based inundation map has a spatial resolution of 3 arc seconds (approximately 90 m) (up to GloFAS version 3, the resolution was 30 arc seconds (approximately 1 km), and up to EFAS version 4 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 inundation maps catalogue is generated using the LISFLOOD-FP hydraulic model (CA2D hydraulic model for GloFAS until version 3).
- Rapid impact assessment: the event-based 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 result is summarised on administration units sourced from NUTS in Europe and GADM in the rest of the world.
The EFAS and GloFAS flood impact forecasts are provided to the users as a set of dedicated layers described in EFAS Rapid Flood Mapping and Rapid Impact Assessment and GloFAS Rapid Flood Mapping and Rapid Impact Assessment.
Rapid Flood Mapping
This procedure creates an estimate of the potentially inundated area by using a lookup between the GloFAS/EFAS grid cells and the high resolution flood maps generated for different return period scenarios. For each EFAS/GloFAS grid cell where the maximum median forecasted discharge is expected to exceed the 10 year return period, a lookup table is used to find the closest matching high resolution flood map. The forecasted return period in the GloFAS/EFAS cell is matched to the return period scenarios for which flood maps have been generated (10, 20, 50, 75, 100, 200, 500 years), this is done by rounding down to the nearest scenario, for example if a return period of 17 years was forecast, the flood map for the 10 year return period scenario would be selected.
Once all the high resolution flood maps have been selected, they are mosaicked into a single gridded layer, where multiple flood maps overlap the maximum depth value is selected. The mosaicked layer is then converted into a binary flood mask, all depth values >=0.099m are set to 'flooded', all other values are set to 'not flooded'. This masked layer is shown on the EFAS/GloFAS webviewers as the 'Rapid Flood Mapping' layer.
An example of the Rapid Flood Mapping layer in the northern Philippines.
Rapid Impact Assessment
The Rapid Flood Mapping layer, created in the procedure above, is overlaid with exposure information regarding population, landcover and critical infrastructure, to estimate the possible impacts related to a forecasted flood event.
The following exposure information is used:
- Population: Global Human Settlement Layer (GHSL- Population (GHS-POP; Schiavina et al., 2023) v.2023A for the 2020 epoch at 3 arc second spatial resolution using the WGS84 projection. Download link
- Landcover: Copernicus global land cover version 3 (Buchhort et al., 2020)
- The original landcover classes were summarised into 6 categories:
- Artificial surfaces
- Agriculture
- Forest and semi-natural
- Coastal
- Water
- Bare surfaces
- The original landcover classes were summarised into 6 categories:
- Critical infrastructure: Hospitals, education, airport and power generation facilities, downloaded from OpenStreetMap. Note, for GloFAS version4version 4.0 these critical infrastructure data are not currently available, this will be resolved in a subsequent update
- Cities: Downloaded from OpenStreetMap. Note, for GloFAS version 4.0 this dataset is not currently available, this will be resolved in a subsequent update
For each of these exposure types, they are overlaid with the flood inundation extent predicted by the Rapid Flood Mapping layer to identify the population, landcover and critical infrastructure which lies within the flooded area. These datasets which have been masked by the flood extent are then summarised over administration regions defined by the NUTS (in Europe) and GADM (outside Europe) datasets, typically administration region level 1 or 2 units are used. This procedure is done for both the protected and unprotected Rapid Flood Mapping scenarios, which respectively consider and do not consider the potential role of flood defences. These summarised results are presented in a table which appears in a pop-out window when an administration region, highlighted in the Rapid Impact Assessment layer, is clicked. The table shows the total population, the total area of each landcover type and the total number of assets for each infrastructure type within the forecasted flood extent within that administration region, there is one column each for the results from the protected and unprotected Rapid Flood Mapping scenarios.
Exposure information table which is shown in the pop-out window when the Rapid Impact Assessment layer is clicked. It summarises within an administration region the population, cities, critical infrastructure and landcover types which lie within the flood extent forecasted in the Rapid Flood Mapping layer.