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GloFAS Rapid Risk Assessment

The rapid risk assessment product aims to quickly connect GloFAS streamflow forecasts to a library of global flood inundation extents and the population, land cover and urban areas which are exposed to the flood hazard. This procedure is summarised in the diagram below.

For each GloFAS grid cell with an upstream area greater than 5,000 km2, a flood inundation extent of the maximum return period exceeded by the ensemble mean over the next 30 days exceeds is defined if this maximum is equal or greater than the 10-year return period threshold (leftmost image in the diagram below). This flood extent is extracted from a catalogue (middle image in the diagram below) pre-generated by LISFLOOD-FP hydraulic model at the 0.00833 degree (~1 km) resolution for design flood events of 10, 20, 50, 100, 200 and 500 year return periods using the catalogue map whose return period is lower or equal to the GloFAS forecast severity. This is the Rapid Flood Mapping layer.

Using the flood inundation footprint of the Rapid Flood Mapping, total population and land covers within the flood inundation hazard (the rightmost image in the diagram below) are extracted and used to define the Rapid Impact Assessment as a risk matrix based on the maximum population affected and the timing of the flood peak.

Population data are taken from the Global Human Settlement Layer (GHSL: https://ghsl.jrc.ec.europa.eu/) and land cover data are taken from the European Space Agency - Climate Change Initiative (ESA-CCI: https://www.esa-landcover-cci.org/). The total population and land cover areas which are exposed to the flood hazard are aggregated onto administration regions which are sourced from GADM (https://gadm.org/) and NUTS in Europe (https://ec.europa.eu/eurostat/web/nuts/background).

GloFAS Rapid Risk Assessment

The GloFAS Rapid Risk Assessment procedure creates 2 products shown on the web interface:

  1. Rapid Flood Mapping
  2. Rapid Impact Assessment

1. Rapid Flood Mapping

Shows the 1km flood inundation footprint extracted from the static catalogue. This product is created during the second step of the rapid risk assessment procedure described above

Rapid Flood Mapping

2. Rapid Impact Assessment

  • Global administration regions are shaded according to an impact matrix (shown below) which combines the total population exposure to the flood hazard with the lead time of the flood event. Categories along the top of the matrix refer to the total number of people who live within the flood inundation footprint, if zero people live within the footprint then the rapid impact assessment product is not calculated in that administration region. The lead time refers to the earliest time to the peak GloFAS streamflow within a given administration region.
Impact Matrix

  • Each administration area can be queried by clicking it, a table will appear showing information such as: the total population exposed to flooding, the amount of each land cover which may be flooded, the names of urban areas within the flood footprint, the maximum exceedance probability of the 2, 5 and 20 year return period thresholds from the GloFAS streamflow forecasts.
Rapid Impact Assessment

Limitations

At present the information in the Rapid Impact Assessment and Rapid Flood Mapping layers are experimental. Care should be taken when interpreting the population affected as the procedure only considers the total number of people living within the flood inundation footprint, it does not consider their vulnerability nor their ability to evacuate. Therefore the affected population values can sometimes be higher than the actual number of people reported as being affected.

Only river basins with an upstream area greater than 5,000 km^2 are considered in this analysis. Flood impacts in smaller basins will not be shown in the rapid impact assessment layer but may still occur in reality.

The flood inundation catalogue is only available down to a 10 year return period. Flood events with a peak return period less than 10 years will not be shown in the rapid impact assessment nor the rapid flood mapping layer.

The procedure does not consider the impact of flood defences as this information is not available at a global scale. Where defences are available the resulting flood impact may be less than estimated in these products, therefore local knowledge should be applied if possible.

The impact products are generated only from the mean of the GloFAS ensemble streamflow forecasts and not the full ensemble. Future work aims to utilise the full ensemble so that a more comprehensive view of the impact can be presented.

No information regarding critical infrastructure, such as roads, hospitals, power generation etc, is included. Future work also aims to include this information.

References

The methodology behind the GloFAS Rapid Risk Assessment procedure has been adapted from that employed in the European Flood Awareness System (EFAS). Further publications on this method can be found:

Dottori, F., Kalas, M., Salamon, P., Bianchi, A., Alfieri, L., and Feyen, L., 2017: An operational procedure for rapid flood risk assessment in Europe, Nat. Hazards Earth Syst. Sci., 17, 1111-1126, https://doi.org/10.5194/nhess-17-1111-2017

Alfieri, L., Salamon, P., Bianchi, A., Neal, J., Bates, P.D., Feyen, L., 2014. Advances in pan-European flood hazard mapping, Hydrological Processes, 28 (18), 4928-4937, https://onlinelibrary.wiley.com/doi/abs/10.1002/hyp.9947