Many populated areas are affected by a wide variety of disasters, such as earthquakes, landslides, tsunamis, flooding, volcanic eruptions, heavy rains, wildfires, etc. Many analyses of disasters take a single-mode approach, which treats disasters as being separate and independent. In many cases, however, the temporal and spatial distributions of these disasters overlap and there can exist interaction relationships between disaster types.
A combined disaster could be defined as a temporal and spatial coincidence of two or more at least medium-scale independent disasters whose consequences do not change in time, resulting in an impact greater than what we would obtain by considering separately the impacts of each disaster independently and summing these up 1. Figure 18.104.22.168 provides a pictorial representation for simultaneous occurring disasters.
A combined disaster could also be defined as the consecutive occurrence of one at least medium-scale disaster triggering one or more secondary disasters, thus forming a chain reaction (cascade/domino effect), which acts synergistically, and results in a greater catastrophe than what would be expected by a single-mode disaster. In such case we consider the status of the disaster to change in time.
In the evaluation of the aftermath of a combined disaster the approach should be differentiated between a situation where a primary disaster triggers secondary disaster(s) (e.g. a flood triggering a landslide) and a situation where that primary disaster increases the possibility of secondary disasters occurring. The occurrence of a given disaster may not only cause additional events via cascade or domino effects, such as earthquakes triggering tsunamis, or volcanic eruptions triggering earthquakes, but the initial event may also increase the vulnerability of the region to disasters in the future. An example of this would be a case of an earthquake, which would damage a flood defense structure like a dam.
There is also a direct relationship between the intensity or magnitude of the primary disaster and the intensity of the secondary disaster(s) which may amplify the total impact.
Table 22.214.171.124.1 summarizes qualitatively the main characteristics of combined disasters, as these pertain to the aforementioned definitions for the purposes of this report.
|Main characteristics||Disaster mode||Occurence||Scale of single disaster||Disaster status|
Does not change
Changes with time
Table 126.96.36.199.2 provides a summary of major combined disasters, which occurred in the 2005-2013 period with their respective impacts.
|Year||Disaster Name||Intensity (Richter)||Death Toll (persons approx.)||Affected Area (103km2)||Economic Losses (billion $)|
Hurricane Katrina in USA
1,836 pers. dead
Flood in Thailand
815 pers. dead
East Japan earthquake and tsunami
15,870 pers. dead
2,814 pers. missing