FLOOD RELATED PROBLEMS ASSOCIATED WI TH_„CYCLONI C STORMS Rishi Raj* 1. Profile The tropical Cyclone is the primary agency that cause river flooding and storm surges, in the cyclone prone islands of the southern Pacific Ocean. Majority of the cyclones form from tropical depressions around the 10 decree latitude, gathers intensity as it moves towards the southern latitudes. Their movement is not consistent or uniform but haphazard depending on the surrounding meteorological conditions, and obstructions in its path. Fiji experiences some fifteen cyclones every decade on the average, each one causes severe flooding and destruction to infrastructure, private property, agriculture, industry and serious erosion and coastline damage. The numerous small, widely scattered lowlying islands are particularly prone to storm surge. The threat of rising sea level and increased tropical cyclone activity due to global warming resulting from increased atmospheric green house gases magnify the problem. Island nations of the Pacific with relatively small subsistence economies face problems in development with the recurrence o f ' disaster and massive losses associated with it. For example Cyclone Bebe in 1972 affected Tuvalu, causing severe damage in the island of Funafuti and several casualities. While in Fiji the same cyclone killed 19 people caused about US$23 million of damage, destroyed 12,000 homes, 90% of the crops in the affected area and rendered 120,000 (approximately 20% of Fiji’s population) people dependent on fZood rations for six months. A large proportion of the damage and loss was caused by floods. Floods of cyclone Bebe remain in the memory of the community in Fiji as one of the biggest in History. It was in the order of 1:20 year flood for Rewa River (Raj 1986), and possibly 1:50 years for others rivers. Similarly Cyclone Oscar in 1983 and cyclone Eric and Nigel in 1985 caused total estimated damage of US$123 million (10% of Fiji’s 1984 GNP). Flooding storm surge and wave damage constituted a substantial portion of the damage. Thus the immediate and short term set back to development caused by a single cyclone can be quite considerable but the true long term economic costs would be far greater than the assessed damage. Agriculture and Tourism are the two major economic activities of the island nation of the Pacific and both are highly vunerable to wind, flood and storm surge damage. River plains, valleys and coastal flats have more productive soils and hence most agricultural activities are concentrated there. ♦Chief Hydrologist, Min. Infrastructure & Public Utilites. Recently however, cultivation has extended to the steeper slopes because of the demand for land. The sizeable subsistence as well as the cash economy suffer from cyclonic storms and result in aggravation of disaster, due to the decline in vegetable, fruit and other ftTood crops production as well as in export earnings. (The 1990 cyclone "Ofa" that severely affected Western Samoa is a relatively fresh example). Due to aesthetic consideration the bulk of capital investment in the tourism relaled ltospjLality industries are concentrated near storm surge prone sea shores or small low lying islands. The value of these locations as tourist attractions is matched by their high vunerability to storm surge and wave damage. 2. CYCLONIC STORMS AND STORM SURGES Associated with cyclones are extreme rainfall events with very high intensities that cause disasterous floods in mainly smaller catchments. The cyclone Wally storm of 1980 produced rainfall totalling 700mm in 24 hours in the southern part of Viti Levu. The 1986 floods was caused by rainfall exceeding 500mm in 24 hours. A sample of rainfall is given in Figure 1. These severe storms acting in concert with cyclonic winds not only cause river and stream flooding but destructive storm surges as well. Maximum damage and disruption is concentrated in the river plains, valleys and coastal flats where bulk of the economic activity with its associated infrastructure is located. 2.1 Damage Causing Factors Of Cyclonic Storms It is the intensity of the storm or rainfall that determines the magnitude of floods while the windspeeds and the topography of the surrounding ocean floor determine the magnitude of the storm surge. However when both act in concert the extent of flooding is significantly increased. Figure 2 is an attempt to summarise the damage causing factors of cyclonic storms. 3.0 Factors Aggravating Floods Invariably a large proportion of losses and damage is the result of inadvertent human activity. Man has over the years exploited natural resources for his comforts and profits, upsetting a delicate natural environmental balance. 3.1 Land Use The hydrology of disasterous floods in tropical cyclone (and monsoon) countries are changing rapidly, mainly as a consequence of high rate and economic development accompanied by growing urbanisation. Major changes in the use of land and water resources have adversely affected the environment, in particular the hydrological regime of river basins. Agricultural activities have gradually extended from the river valleys and coastal flats into the steeper slopes with a gradual increase in population and demand for land. This gradual expansion gently termed development has accelerated erosion, increased siltation and unfavourably upset the flow characteristics of rivers and streams. 3.1.1 Agriculture Expansion and Cultivation As agriculture/activity expand on to hillslopes to cater for an increase in demand for food crops, the sediment load in rivers and streams increase, silting the channels resulting in increased incidence and magnitude of floods. A sediment yield of some 8,300 tonnes/sq.km/year for the Waimanu river (Fiji) gives an erosion rate of 7.8mm/year and a soil loss rate of 113kg/ha/year (Hassan 1986). Such high erosion rates magnify the problem of river flooding, by reducing the capacity of rivers. 3.1.2 Deforestation Agricultural expansion accompany deforestation which in turn increases the rate of runoff. Further indescrimate logging of forests together with its activities of road construction on hillslopes have destabilized slopes, causing landslides and thus triggering further destruction of vegetation. Sedimentation and erosion are possibly the major factors that enhance the magnitude of floods in Fiji. Dredging to deepen the waterway is in fact only a short term or immediate measure attempting to retain the flood water in the channels. The channels are most likely to silt up again in the next few if not the immediate next storm. 3.1.3 Urbanisation Access, availability of freshwater and suitable soil for subsistence crops have dictated locations for settlements. These have gradually expanded into urban centres. With an exception of only very few all cities and towns in Fiji are located either on river banks or seashores which are vunerable to floods and storm surge. The economic justification of locating industries close to or within the urban centres is matched by the potential risk of floods, storm surges and wind damage. This justification may not apply as urban boundaries are extended. The increase in damage and losses to infrastructure , housing and private property in recent years can be attributed to the gradual expansion in urban boundaries and reclamation of lowlying tidal land for development. The removal of mangroves from seashores and mudflats have inadvertently increased the damaging effects of storm surges and high tides, while restrictions in river channels have aggravated the problems of river floods. 3.2 Land Use Studies & Applied Research Very few countries have undertaken any studies on the effects of land use on the hydrological regimes of different basins. Systematic and intensive studies to investigate the effects of changing land use on flood characteristics have been undertaken only in Japan. One such study of the flood hydrology of the Sakujii river basin (48sq.km) during its 20 years of urbanisation (1958-1977) has shown that the 1977 peak had increased by up to 2.5 to 3.0 times that in 1958. This increase corresponds to an increase in (urban) residential area from 33% to 52% of the catchment (Uehara 1987) see Figure 3. For small island nations in the Pacific such studies have not been undertaken however, the results of studies undertaken in Japan which experiences (anticyclones) typhoons may be used with some degree.confidence by islands with physiographic characteristics that is similar to Japan. Figure 4 illustrates the effects of urbanisation on a 66.7 sq.km basin the Thurumi River Basin in Japan. 3L3 Storm Surge Studies The extent of damage and loss associated with storm surges are not adequately recorded in most small countries largely because the immediately needs are pressing. Island nations with limited resources find higher priorities for immediate socio-economic development projects. The more scientific long term data collection for future investigations and development takes on lower priority or even neglected. Most island nations surrounded with coral reefs and irregular seabed are fortunate in having the effects of storm surges reduced by the reefs. Fiji islands are no exception. Further the mangroves on the foreshore act as a buffer and dampen the effect of surges. The detrimental effects of clearing or removing mangroves, as regards to storm-surges, have not been sufficiently understood or studied. 3j_3^1 Cyclonic Winds And River Flooding Gale and hurricane force winds against the direction of flow of rivers tend to push the water upstream causing widespread flooding. This is particularly strong when such river opens up into bays, that create a "funnelling effect". The Nadi river is an example where the wind is chanelled along the river by the semicircular bay, and flood waters together with storm surge aggravate the floods in Nadi and further upstream. 4. Civil Defense - Flood Mitigation Strategies Nature may not be tamed sufficiently to eliminate all losses but there is a distinct possibility of reducing losses to a minimum by bracing against impending disaster.