Climatological risk: Droughts

Abstract

Flood disasters affect a large number of people across the world every year with severe social and economic impacts. Floods can come from rivers bursting their banks, from storm surge in the ocean, from groundwater rising and from surface water runoff in our cities. In this subchapter, we focus on riverine (fluvial) floods, which are largely driven by the duration and intensity of rainfall and the sudden melt of snow during spring. Their occurrence also depends on the characteristics of the river catchment (topography, geology, soil type, landscape, land use), as well as on the way the river and its catchment are managed. In most parts of the world flood records are short and it is difficult to determine how often and with what severity a flood might be expected at a given location. Flood events also exhibit natural variability on decadal scales, and flood frequency and severity is also changing across the world in response to long-term changes in climate. Global data and tools are crucial to assist flood risk assessment. Regional scale flood hazard mapping has been improved with the use of satellite data and flood models to map flood inundation pathways. Risk mapping can be derived when factors such as exposure and vulnerability are also taken into account. These maps are essential for a wide range of decision challenges, from land use planning and floodplain management to disaster response planning. All layers of information have associated uncertainties which must be communicated to users. Different floods are predictable with different time ranges. For example, flash floods driven by convective rainfall are notoriously challenging to predict in terms of location and sufficient lead time to produce effective early warnings, whereas slower developing floods on large rivers can be predicted several days ahead with the use of probabilistic flood forecasting systems. The increased efforts on data collection and the setup of early warning systems have enabled improvements in our ability to provide effective information on the likelihood and severity of upcoming flooding events, with more accuracy on the intensity and extent of the affected areas. There are several current approaches to monitoring, modelling and predicting floods. The Copernicus Emergency Management Services for floods, the European (EFAS) and the global (GloFAS) flood awareness systems, are used as illustration. Our best strategy for flood management today is learning to live with flooding and preparing ourselves to be better adapted for flood risks tomorrow. Focus on developing resilience and improving preparedness go alongside improved planning, land use, urban design, natural flood management and better defence and other risk management strategies.

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