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. During large-scale flood events, EFAS impact forecasts can be used to pre-task the acquisition of satellite images for a possible activation of the Rapid Mapping component of the CEMS.


In EFAS, the flood impact forecasts are based on three components: 1) medium-range flood forecasts, 2) event-based rapid flood mapping, and 3) impact assessment.

 

The three components of the flood impact forecasts
The three components of the flood impact forecasts
  • Medium-range flood forecast: every time a flood event greater than the 10 year return period is forecasted in EFAS, the return period of the maximum discharge (based on the ECMWF-ENS EFAS ensemble forecast median for the full forecasting period) is computed in each grid cell.  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. River grid cells where the protection levels are exceeded are selected and form the protected scenario.
  • Rapid flood mapping: for each EFAS river section identified in step 1, flood prone areas are delineated, using a catalogue of flood inundation maps. The obtained event-based flood inundation map has a spatial resolution of 90 by 90 m and is shown on the “Rapid Flood Mapping” layer. The Rapid Flood Mapping layer shown on the EFAS web viewers is from the unprotected scenario, it does not consider the role of flood defences.
  • Impact assessment: the event-based flood inundation maps are combined with exposure information to assess regional impacts (shown on the “Rapid Impact Assessment” layer). Considered exposure includes population, infrastructure and land cover. The rapid impact assessment considers two scenarios, unprotected and protected

    • Unprotected scenario: exceeding 10 year return period.
    • Protected scenario: exceeding 10 year return period or protection level, where the protection level is based on the estimated design return period for flood defenses (e.g. dikes), assuming any flood event over the protection level, will cause flooding. The data on level of protection offered by flood defenses comes from the dataset FLOPROS and is available at administration region level. To get an overview on the flood protection levels used within the different areas of interest, have a look at the 'flood protection level' layer in the EFAS Map Viewer. The colour shading (yellow, orange, red) of each administration region shown in the "Rapid Impact Assessment" layer, is chosen using an impact matrix which summarises the lead time to the event and the total population potentially exposed, both of which come from the protected scenario. Grey shaded administration regions signify regions where 0 population is expected to live within the forecasted flood inundation area according to the protected scenario, however the unprotected scenario predicts >0 population living within the forecasted flood inundation area. 

     

A detailed description of the methodology and the data used to deliver EFAS Impact Forecasts in available from CEMS- Flood general information.