Show Case Flood

Floods affect more people worldwide than any other natural hazard and the global expected annual flood loss in the built environment has been estimated to more than US$ 100 billion (GAR, 20151). Flood damages are expected to increase substantially due to a combination of socio-economic and climatic changes (Dottori et al., 20182).

Floods are cross-compartment phenomena

Flood events have a significant impact on landscape evolution, soil properties and matter cycles of terrestrial systems. They have the potential to massively mobilize fertile top soils, are crucial for bedload transport in rivers and are driving the morphological dynamics of running waters at the river reach and catchment scales. The propagation of extreme precipitation events through the catchment and the river system to the coastal zone is an important driver for the structure and functioning of terrestrial and coastal systems. For instance, floods affect the nutrient and health status of lakes and coastal environments by a pulse-like mobilization of nutrients, carbon and harmful substances from soils and a significant decrease in the retention time of these substances in the tributary waters, reducing their degradation and sedimentation rate prior to entering the coastal zones.

Risk reduction requires to understand the generation and impact of floods

Managing floods and reducing flood risk require to understand the generation and impacts of floods across compartments, and how they are affected by natural and human developments. This understanding can only be achieved by integrating data on physical, biogeochemical, engineering, and human processes.

Extreme floods are not just large versions of frequent events

Extreme floods that lead to severe socio-economic and ecological consequences are typically shaped by mechanisms which are different from those mechanisms that shape more frequent events. For example, the 2002 flood in Central Europe led to streamflow values at some catchments which were 2 or 3 time larger than what had been observed in the preceding decades. This surprising behaviour was caused by the combination of extreme rainfall with a highly nonlinear, threshold-like catchment response. Further, the relationship between the hydrological situation, e.g. flood peaks, and the resulting damage is highly nonlinear and volatile. For instance, in Cologne, the Rhine flood in January 1995 caused only half the damage compared to the flood in December 1993, although the water level was even a few cm higher in 1995. This was explained by improved early warning, precaution and response of affected people, and additional mobile flood defences. Hence, we need to understand how the different natural and human factors play out and shape exceptional events. The Flood Event Explorer we are developing in Digital Earth will contribute to improve this understanding.

1 UNISDR. (2015). GAR2015 - Global Assessment Report on Disaster Risk Reduction.

2 Dottori, F., Szewczyk, W., Ciscar, J.-C., Zhao, F., Alfieri, L., Hirabayashi, Y., Bianchi, A., Mongelli, I., Frieler, K., Betts, R. & Feyen, Luc. (2018). Increased human and economic losses from river flooding with anthropogenic warming. Nature Climate Change. 8. 10.1038/s41558-018-0257-z.