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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.

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1. Medium-range flood forecast: every time a flood event greater than the 10 year return period is forecasted in EFAS or GloFASCEMS-Flood, the magnitude return period of the forecasted peak maximum discharge (based on the ECMWF-ENS ensemble forecast median for the full forecasting period) is compared against the local flood protection levels. 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 for the following steps of the procedureand form the protected scenario.
2. Rapid flood mapping: for each EFAS/ GloFAS river section CEMS-Flood river 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 100 by 100 m and is 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).
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.

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.