Floods

Flood Categories, Causes, and Effects

Flood types are mainly associated with the natural feature and the physical process (dynamics) that originate the inundation phenomenon.

In particular, riverine and coastal floods are determined, respectively, by the excess water flow derived from fluvial channels and by the ocean surge that may occur, respectively, under severe hydrologic (e.g., rainfall) and climatic (e.g., strong winds, tropical cyclones) conditions. Those different and diverse factors may also combine, as in the case of estuarine floods, where the ocean and fluvial waters interact. Floods may be indirectly caused by another hazard, as in the case of a submarine earthquake (tsunami) or the collapse of a water retention structure (e.g., levee, dam).

A different categorization method pertains to the analysis of the flood dynamics that is mainly based on the duration and speed of the event, with slow floods usually driven by heavy rains and/or snowmelt on mid-large rivers and fast (flash) floods caused by convective precipitation (e.g., thunderstorm) in small river basins. The concentration of sediments that propagate with the flood wave is another factor, with mud and debris flows that mainly characterize alluvial fans and mountain areas.

The passage of the flood through the territory has often devastating direct effects connected to the physical damages to human properties (e.g., roads, bridges, buildings, agricultural fields, and industries) and casualties but also significant indirect effects such as business interruptions, the evacuation of affected people, water-driven diseases, and other hygienic issues. The negative environmental, economic, and social effects of floods are often temporary, but they may also last for months if an effective crisis management plan hasn't been developed and preliminarily implemented for tackling this natural disaster in both the pre-event and post-event period.

Flood Crisis Management

Floods are natural (extreme) events that cannot be avoided in most cases. Low-lying river valleys and coastal areas have always been the subject of recurrent inundations with overland river flows that in ancient times had also been the source of beneficial (fertilizing) effects, as in the well-known case of the Nile River flooding. Nevertheless, in modern times, urbanization and the increasing human activity and presence are contributing to an increase of the negative impact of floods, and the sustainable development of the territory cannot avoid considering the maximum inundation extent and dynamics as major factors affecting urban expansion and land use plans. Thus, even if the adverse impact of floods cannot be neglected, significant mitigation may be achieved by effective flood risk management that should include a preparedness, prevention, mitigation, monitoring, response, and emergency plan.

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