Global Warming Causes

Flood Hazard Mitigation

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The historical record of tropical cyclones in the South Pacific indicates that storms rarely develop close to the main island archipelagoes lying south of 10°S. Instead, nascent tropical disturbances tend to form farther north and then approach on southerly tracks over 2 or 3 days as they mature. This crucial delay should prove invaluable for alerting vulnerable populations

Fig. 9.15. Maximum flood heights in several rivers on Vanua Levu island in Fiji, generated by Tropical Cyclone Ami in January 2003. Comparison is made with other major cyclone-induced floods. Source: Hydrology Division of the Fiji Public Works Department.

Fig. 9.16. Severe inundation of Labasa town on the northern coast of Vanua Levu island in Fiji, resulting from the combined effects of inundation by the sea (storm surge) and flooding of the Labasa and Qawa rivers. Photos courtesy of the Fiji Meteorological Service.

inhabiting low-lying coastal areas and valley bottoms, in advance of expected floods. Unfortunately, dispersed rural communities are often insufficiently prepared to cope with flood hazards. All too commonly, either people ignore warnings issued by national meteorological offices because they are unwilling to leave their homes, farms and property, or others who are better educated in the dangers do not have adequate mobility to escape to higher ground. As inundation waters rise, many families become trapped in their houses, panic and then try to wade to dry ground sometimes with fatal consequences.

In the future, the magnitude of hydrological hazards for the high Pacific islands may increase if ocean warming leads to more intense tropical cyclones (Kostaschuk et al. 2001). Based on this scenario, improved disaster-reduction programmes are needed in all island nations to avoid escalating loss of life. A survey by the World Bank (2000) suggested a range of options for moderating the flood impact of cyclones. These are grouped according to the following categories.

9.2.5.1 Flood Control

There are several engineering measures for controlling river floods, including both structural and 'soft' techniques. River diversion channels, retarding basins, cut-off channels and retention dams are structural controls, whereas raising embankments, river channel widening and riverbed excavation are soft engineering measures (JICA 1997). Although costly to build, retention dams may be one of the most beneficial options for Pacific Islands where suitable topography exists for their construction, because dams can be used for water-resource development as well as for controlling floodwaters (Terry 2002).

9.2.5.2 Catchment Management

Improving catchment management involves the combination of several activities, such as reforestation, soil conservation, regulating land development and protecting natural wetlands. These measures help to improve the natural water-retention function of drainage basins and thereby maintain existing flow capacities of rivers by avoiding excessive silting up of channels.

9.2.5.3 Mitigation

It is possible to reduce the potential damage caused by floods by restricting the urbanisation and settlement of low-lying areas and by promoting the use of flood-proof house design where necessary. The resilience of social infrastructure can also be increased through community-education programmes to raise awareness of tropical cyclone characteristics and behaviour. This needs to take place in conjunction with the introduction of better communication systems within relevant government institutions, so that the public may be more effectively warned about impending flood hazards, both before and during tropical cyclone events.