FLOOD RELATED PROBLEMS ASSOCIATED WI TH_„CYCLONI C STORMS

Rishi Raj*

1. Profile

The 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. 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 . 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 in 1972 affected , causing severe damage in the island of 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 in 1983 and 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 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 . 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 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. The ability to brace against disasterous floods and storm surges is dependant on the level of preparedness. Fiji places high priority on preparedness while considerable attention is focused on relief and rehabilitation.

Alerts, warnings and forecasts play a significant role in a community that has experienced or suffered recurrent disasters. Alerts are issued for any impending severe weather. The Regional Cyclone Forecasting Centre based at Nadi Airport posses the necessary equipment, technology and expertise to monitor severe weather and track cyclones in the region. Their satellite communication system and cyclone tracking equipment is highly sophisticated, although their weather surveillance radar (25 years old) can be improved.

4.1 Flood Forecasting

Flood foreacsting is the real-time estimation of stage, discharge, time of occurance and duration of flooding, especially of peak discharge at a specific point, resulting from precipitation, (and or snow melt) (WMO 1974) Considerable effort and resources have been used to establish flood forecasting services in major river basins on a global scale, however the small islands of the south Pacific with small catchment and steep slopes have not been subjected to investigations.

Fiji is probably the only country in this region apart from Australia and New Zealand that has flood forecasting capabilities, and that too is limited to only one catchment, the Rewa. The Rewa catchment covers approximately one third of the island of Viti Levu. A lead time of some six hours provides for any loss/damage prevention measures that can be taken. This flood forecasting system involves the acquisition of water levels from five locations upstream from the Rewa Delta. The water level is transmitted by radio and communicated via intermediate repeaters to a micro processor which does rapid analysis and makes forecasts. It has capabilities of triggering an alarm if necessary. The system was acquired from ’DSIR’ New Zealand and has operated satisfactorily.

All forecasts are disseminated through the Emergency Services Committee, an effective organisation with wide responsibilities estbalished over a decade ago.

4.1.1 Flash Floods

Flash floods are particularly difficult to forecast as the time of occurance of peak discharge is very close to the occurance of heavy rainfall. A significant impetus to research in flash flood forecasting was given during the Typhoon Operational Experiment (TOPEX) for the northern Hemisphere. The hydrological component of TOPEX was implemented on the basis of three water years starting in March 1981.

The objective was to reduce the risk of loss of life and damage from typhoon- related floods by improving the forecasting and warning capabilities of countries. Six countries designated nine river basins whose flood forecasting systems were evaluated during the three years (WMO 1985).

Although the problems may be similar research and technology is sadly lacking in the tropical cyclone area of the Pacific, largely because of lack of resources and possibly influence.

One of the parameters required for input to forecast flash floods is quantitative rainfall. None of the smaller countries of the South Pacific have the capability to make quantitative rainfall forecasts (QPF). Fiji’s weather surveillance radar which is 25 years old does not provide estimates of rainfall either. It is able to monitor cyclones approaching from a small western sector because it is impracticable to locate in a more advantageous location.

4.1.2 Storm Surge Forecasting

Storm surges are caused by cyclones moving onto or close to land. Coastal flats are susceptible to storm surge flooding. Cyclone movement is the main parameter used to forecast storm surge. The acccuracy of storm surge forecast is dependant on the accuracy of the forecast of the cyclone movement and intensity. Current •eye-trm’e cyclone forecasting techniques yield large errors (average of 200km in 24 hours of forecast of cyclone centre locations) the accuracy of storm surge forecasting techniques require high capacity computers, currently not within reach for Fiji and most small island countries. Qualitative storm surge warnings, however, accompany wind warnings in Fiji.

4.2 Shortcomings & Defeciencies Apparent

Flood forecasting system for the Rewa is possibly the best possible under the circumstances and complexities, however increasingly disasterous flash floods in smaller catchments where the rate of development, deforestation and urbanisation have proceeded unchecked need a flood flood forecasting system different from the conventional system. Quantitative rainfall forecast (QPF) would be a reasonable parameter to use for forecasting flash floods but Fiji Using real time (telemetered) rainfall data for flash flood forecasting may be effective for some catchments but for others where the time of concentration is only few(2-3) hours the forecast could be ineffective for any defensive action. A study of the possibility and effectiveness of such a system has not been undertaken.

Other high technology instruments including radar and satellites for weather surveillance are, at present, out of reach for most of the small nations in the Pacific.

5.0 Mitigation and Long Term Preventive Measures

The recurance of disaster despite reasonably successful preparedness and rehabilitation measures stresses the need for stronger mitigation measures as a complimentary strategy. Mitigation measures , however, pose a complex of consideration ranging from economic /justification to social acceptance and the political will for implimentation of such measures. In resource limited developing countries the immediate and visible socio- economic development priorities are paramount.

Further, no strong argument for mitigation can be made without first carrying out a comprehensive hazard assessment and risk mapping which is the common denominator on which all other considerations must rest. Such comprehensive risk evaluation have not been carried out in Fiji for any type of disaster. Regional development planning requires a comprehensive disaster risk as input to assist in systematic mitigation efforts.

Rather complex social and cultural sensitivities are bound to confront the formulation of mitigation strategies and their implimentation. In particular any legislative measures adopted and consequent regulatory steps (eg. relating to land use, flood zoning, land acquisition and cultivation techniques or limitations) are most likely to pose difficulties.

The utilization of Natural resources and indiscriminate development of agriculture, industry and infrastructure are recognised as being responsible for environmental problems 0 1 1 the global scale and the Pacific region is no exception. The inadequacy and absolence of legislation on conservation and environment is recognised by most small nations as is the need for new legislation to cover all aspects of environmental management.

Π.1 Development Planning & Deficiencies

The constraints natural disasters pose to development is well recognised. In agricultural sector which is the most susceptible, greater geographical spread of food crops in particular is a broad strategy, the rational being that few single disaster events, if any, affect the whole country. Drainage construction and maintenance work, dredging and watershed management are likely to reduce the damage and losses, however, no cost-effective analysis is available and the side effects may be detrimental to the maintenance of healthy sustainable environment. Dredging is by any standards an expensive exercise.

In Fiji an environmental Planning Unit has been established within the Directorate of Town and Country Planning to provide an impetus to the implementations of aspirations on environmental management envisaged. There has been limited disaster - specific inputs that have gone into these efforts, having being based on no hazard assessments or risk mapping.

Building codes are being established to incoperate high wind conditions based on analysis of historical cyclonic wind data. Such codes for buildings in flood prone areas does not exist. Arbitrarily prescribed threshold heights above mean sea level for buildings in flood prone urban areas may not be adequate.

Regardless of Legislation, the insurance industry has played a significant regulatory role by raising premium or refusing insurance for buildings constructed below certain height above mean sea level for specified areas.

One problem that became very evident during the last decade or so is the increased dependence by communities on external assistance, following generous government assistance following several severe cyclones and storms. This has resulted in the depradation of coping methods and self reliance (Campbell 1984)

6. OPTIONS FOR POLICY FORMULATION

Preparedness measures have achieved a high degree of success in Fiji in reducing lives lost but it has not been possible to contain damage to immovable property and the set back to economic development brought by disaster. Affordable and viable prevention measures seems an obvious option to explore.

Some of the more useful options that deserve consideration are:-

(a) Analysis and description of hazards and consequent risk involved using historical data. Where no data is available a simulation exercise will suffice. Computer simulation of various scenarios for different coastal areas and cyclone intensities, approach directions etc. using available historical data to compliment the first estimates of risk.

(b) Based on the analysis of risk analysis, and in light of the social and cultural sensitivities, investigate where this has not been done already, of the economic viability and feasibility of measures such as flood plain management, land development policies flood plain zoning, designation of floodway and encroachment lines, subdivision regulations as well as flood control measures of embankments, seawalls for storm surge protection, detention reservoirs, river channel improvement and watershed management.

(c) Preparation and adoption of Legislation, consistent with criteria for protection of the environment, to cover all aspects related to flooding and storm surge, namely preparation of risk micro zoning maps, control of industrial and residential development in flood prone or high risk areas, building codes providing for minimum safety standards, soil and plant measures for erosion control, engineering measures for the management and control of rivers, canals and waterways, public health measures, etc.

(d) Adoption of Legal and regulatory measures to reduce the effects of floods and storm surges must be strengthened. (e) Intensify research and technology transfer to produce crops resistant to hazards and intorduction of crops that could possibly take advantage of the seasonality of hazards, as well as reduce the vunerability of agricultural crops.

(f) Improvement of flood and storm surge monitoring and documentation and the establishment of a database for economic vunerability assessments and planning.

(g) Public Awareness education on the benefits and need for mitigation programmes and legislation.

The above is not exhaustive, and it is hoped that positive and firm steps will be taken in the near future possibly within the International Decade for Natural Disaster Reduction. References

Campbel1, J R 1984: Dealing with Disaster: Hurricane Response in Fiji, Government of Fiji, and PIDP, East West Centre, Honolulu.

ESCAP/WMO/LRCS, 1977: Guidelines for Disaster Prevention and Preparedness in Tropical Cyclone Areas.

HASSAN, M R 1986: Watershed Management Study Report FJ/86/004 UNDP/FAO Field Document 1.

Longworth M & Raj R Report on the Hydrological Aspects 1988 of Tropical Cyclone. Regional Association V (South West Pacific) Working Group on Hydrology (co­ rapporteurs )

SEHMI N S 1988: The Hydrology of Disasterous Floods in Asia - An Overview in Hydrology of Disasters, Proc, of Tech. Conference in Geneva Nov,. 1988.

Raj R, 1986: Report on the Flooding of Rewa River PWD report 1/86

Raj R 1987: Floods and Rainfall in . PWD report 2/87.

Uehara, S 1987: Prevention of Disasters Related to Rivers. Technology for Disaster Prevention, Vol II (Sept. 1987) p. 81 NRCDP, Japan,

WMO 1974: International Glossary of Hydrology WMO - No 385, Geneva, Switzerland.

WMO 1985: Activities under the Hydrological component of TOPEX Report No: TCP-20, WMO/TD No 37 Geneva, Switzerland. n b i FigLA/ZB 2.

CROP LOSSES SALINIZATION.OF. SOILS, WATER-SUPPLY iQOAST-AL INUNDATION 1 < DISEASE \ LOW ATMOSPHERIC DAMAGE TO BUILDJNGS, PRESSURE I 1 STORM SURGE ------ROOD'S ANO-OTHER •STATE O F LOCUM- T »P £S COASTAL INSTALLATIONS • LOCAL COASTAL MOM*HOLPG\ ! COASTAL EROSION

LOSS OF LAND CROP LOSSES

HEAVY SEAS . LOSS/DAMAGE TO SHIPPING HIGH WINDS FLYING DEBRIS

STRUCTURAL. DAMAGE CROP LOSSES HIGH WINDS S' ON LAND VEGETATION DAMAGE FALLING TREES ELECTROCUTION TRANSMISSION LINES LOSS OF COMMUNICATIONS DAMAGE LIVESTOCK LOSSES CROP LOSSES RACING FLOODWATERS FLOOD PLAIN DAMAGE TO BUILDINGS. ROADS CROP LOSSES FLOODING INUNDATION SILTING POTENTIAL LIVESTOCK LOSSES PONDING, STANDING DAMAGE TO BUILDINGS WATER, WATERLOGGED CROP LOSSES HEAVY RAINS SOIL'S WATER SUPPLY CONTAMINATION •LOCAL TOPOGRAPHY RIVER BANK EROSION. LOSSOF LAND •LOCAL SOILS AND GEOLOGY CROP LOSSES •LOCAL VEGETATION DAMAGE TO BUILDINGS. ROA-DS. BRIDGES

LANDSLIDES RIVER DIVERSION FLOODING

LOSS OF LAND CROP LOSSES

EROSION SOIL DEGRADATION CROP LOSSES (LONG TERM)

Damage-causing factors of hurricaiies in Fiji lo lw cags u i ubnzto, uji ie,J Uehara ( n a p Ja river, jii ku a h S urbanization, io due changes flow Flood

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Effect of urbanization on flood flows at Ochiaibashi (Japan) (Kinosiia et al 1986)

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