FIGURE 2Location of 25 major river basins in Thailand

FIGURE 3Location of main rivers and dams in Thailand

Flood forecasting and warning activities

Floods are Thailand's most disastrous natural phenomena. As with other countries in the SoutheastAsian region, the basic cause of severe flooding is found in the incidence of heavy rainfall. In manycases the most devastating flood-producing rainfall event is associated with cyclonic storms in theform of typhoons or depressions. The intensity of rainfall is high during such events and the storms aregeographically extensive. These twin factors mean that such storms are capable of producing extremeflood discharges in both large and small river basins.

Seeking a means to mitigate the damages, a mathematical flood forecasting model was first introducedto the Chao Phraya River Basin in 1975. In 1981, the flood forecasting system for other river basins

was established to a large extent, such as Nam Pong, Nam Chi, Nam Mun in the Northeast and MaeKhlong in the West.

Several government agencies and institutions are involved in developing and implementinghydrological forecasts. The Electricity Generating Authority of Thailand (EGAT) is responsible forreservoir regulation. The Royal Irrigation Department (RID) operates irrigation water utilities andflood control operations in cultivated areas, while the Meteorological Department (MED) preparesmeteorological forecasts. In addition, the Mekong River Commission (MRC) the Department ofEnergy Development and Promotion (DEDP), the Bangkok Metropolitan Administration (BMA) andthe Department of Local Administration (LAD) are involved in hydrological investigations for floodmitigation purposes.

Flood forecasting practice

Flooding is a natural hazard which affects national economic growth. During the past decade, Thailandencountered serious flooding problems in both cultivated and urban areas. The National Economic andSocial Development Board (NESDB) therefore has been seeking solutions to the problem. One suchsolution is seen in modern technology on flood forecasting. The first flood forecasting system wasestablished in the Chao Phraya River Basin as it is the major river contributing benefits to the country.

The Chao Phraya River Basin extends from the upper northern part of the country, consisting of aseries of parallel mountain ranges drained by torrential rivers, namely the Ping, Wang, Yom and Nan.There are four main tributaries flowing south, where they meet together to form the Chao PhrayaRiver. The Chao Phraya runs southward passing through the main cities and Bangkok, the capitalbefore draining into the Gulf of Thailand. The drainage area of the basin is about 162 000 km2 and hasa length from its source to the river mouth of about 1 000 km.

Normally, forecasts are made every five days. In addition, during flood-prone periods forecasts forshorter time periods (hourly or daily) are prepared.

Forecasts are disseminated to the appropriate government agencies. The purpose of the warnings is tostimulate the correct response from the forecast users; the real value of the forecasting system shouldbe assessed on this basis. For this reason, feedback from forecast users to the forecasting officeregarding the effectiveness of the forecast is important. Such information plays an important role inflood protection measures and water conservation in the reservoirs.

Flood forecasting and management in the Lower Mekong Basin in Thailand

General Background

The Mekong is the longest river in Southeast Asia and one of the largest rivers in the world. In termsof drainage area (795 000 km2), it ranks 21st, but in terms of runoff (475 000 MCM), it ranks 8th and12th in terms of length (4 200 km). The Mekong River originates on the Tibetan Plateau and flowsinto the South China Sea over a distance of 4 800 km.

The Lower Mekong Basin begins where the river leaves China and becomes the border between theLao PDR and Myanmar. It covers an area of more than 600 000 km2 which comprises the area drainedin four riparian countries - almost the whole of Lao PDR, Cambodia, one-third of Thailand (its entirenorth-eastern region and part of its northern region) and one-fifth of Viet Nam (its Central Highlandsand the Mekong Delta). It is inhabited by a population of some 50 million, representing approximatelyone-third of the total population of these countries.

Flood forecasting operations

Flood forecasting operations in the Lower Mekong Basin were initiated in 1970 by the MekongSecretariat. The objective of flood forecasting is to provide information on the future state of a river inorder to minimize the effects of flooding and provide warnings to those people who live in affectedareas. Since then, this operation has been repeated annually.

The Mekong Secretariat applied the Streamflow Synthesis and Reservoir Regulation (SSARR) modelfor forecasting on the upper and middle reaches of the Lower Mekong Basin, from Chiang Saen toPakse, during the July-October flood season. The real-time information (water level and rainfall data)includes data from five key hydrological and meteorological stations in Thailand; namely at ChiangSaen, Chiang Khan, Nong Khai, Nakhon Phanom and Mukdahan (Figure 4) and five key hydrologicaland meteorological stations in the Lao PDR: Luang Prabang, Vientiane, Thakhek, Savannakhet andPakse to be transmitted by radio or facsimile to the Secretariat daily at 1700 hours or, during peakperiods twice daily, at 1100 and 1700 hours. Normally, the forecast is issued five days in advance.

Every afternoon at 1700 hours the consolidated forecast is transmitted to the National MekongCommittee in each riparian country.

Flood warningsThe Department of Energy Development and Promotion (DEDP) is the Thailand National MekongCommittee-related agency directly responsible for hydrological investigations, and flood warningsregarding the Mekong mainstream flooding in Thailand.

The DEDP prepares warnings and disseminates them to the appropriate government agencies, if theforecast indicates that water levels of the forecasting stations will reach critical stage. The DEDPresponds as follows:

• by sending warnings to the Thai Government Public Relations Department for radiobroadcasts which inform residents in affected areas regarding the developing situation, inorder to help minimize damage to property as well as the loss of lives; and

• by sending warnings to provincial governors in flooded areas, especially Nong Khai, NakhonPhanom and Mukdahan provinces, for further action.

Disaster management in Thailand

The Minister of Interior is the National Civil Defense authority for Thailand. According to the CivilDefense Act, 1979, disaster management covers both natural and man-made disasters. Thailand'sDisaster Management System is divided into three levels of responsibility with authority as follows:

Policy and strategic responseDisaster policy and strategy is conducted in cooperation with government policy and the NationalEconomic and Social Development Plan as follows:

• The National Safety Committee of Thailand (NSCT) is responsible for policy and strategyresponse in relation to emergencies classified as accidents.

• The Civil Defense Committee (CDC) is responsible for policy and strategy regardingdisasters, rear security and damage control.

• The National Security Council (NSC) is responsible for policy and strategy in relation tonational security, i.e. warfare.

FIGURE 4Map of Mekong tributaries and five key stations in Thailand

Policy implementationThe Civil Defense Secretariat is responsible for implementation of the policies which the saidcommittees have assigned by making the National Civil Defense Plan as the Disaster Master Plan forinvolved government agencies. This includes making guidelines in planning for operational units andby dictating practical procedures for each type of disaster, such as flood, storm, fire and drought.

Disaster mitigationAccording to the Civil Defense Act of 1979, local authorities are responsible for creating preparednessfor disaster, whereas mitigation in each administrative area remains the responsibility of thepreparation unit. However, after budget constraints and reduction of government equipment costs inlocal administrative areas, Thailand has improved its disaster mitigation system on a regional basis bysetting up units at three operational levels: Emergency Units, Main Units and Support Units. They areresponsible for designated areas in Bangkok and the provinces.

Natural disasters in thailand

Risk assessment

Thailand is fortunate that it does not experience significant natural disasters as in many other countriesin the region. However, there are some natural hazards which have a potential to create greatdestruction such as floods, typhoons and storm surges, and strong winds.

As indicated in Table 1, there is both a high probability for flooding and a high likelihood of risk.However, although typhoons and storm surges occur less frequently their rate of risk is higher thanthat of floodingTABLE 1Risk assessment

Type Frequency ofoccurrence

Rate ofrisk Risk area

Floods High High Countrywide, especially northern,central and southern regions

Typhoons and stormsurges Low Moderate Southern region

Strong winds High Low Countrywide

Table 2 presents details on losses experienced in the years 1990–1995. Economic costs are given inThai baht(25 Thai Baht = 1 US dollar)

TABLE 2Disaster situations in 1990–1995 because of floods

Year No. of Provinces No. of deaths No. of injuries Loss (million Baht)1990 58 50 19 6,6521991 66 43 26 2,6211992 66 16 - 5,2411993 11 47 254 2,1821994 74 59 12 6,0441995 73 442 - 11,859

Conclusion

Thailand has established the Office of the National Water Resources Committee to manage waterresources in its major river basins as part of a master plan for water utilization, water conservation,water quality control, flood protection and water law.

Review of flooding and flood management: Viet Nam country statement

Background

Flooding is a major problem which severely affects all the Mekong River riparian countries. With thelowest areas of the Mekong Basin, Viet Nam's land is flatter, flooding is more frequent - and the areasflooded are larger.

The network of inter linking rivers is complex. At Phnom Penh the Mekong River meets the Tonle Sapand then branches into the Mekong (Tien) and Bassac (Hau) rivers before entering Viet Nam. The tworivers themselves branch into many tributaries with natural streams and a dense network of man-madecanals. Moreover, the area has other important rivers such as the Vain Co Dong and the Vam Co Tay(see Figure 1).

The flood season extends from June to November, with 80% of the annual flow volume occuringduring this period. There are two types of flooding which affect the area: one being floods from theupstream Mekong Basin, while the other one is tidal water from the Bien Dong Sea (South China Sea)known as tidal flooding.

Le Thi Tam Thien, Deputy Chief, Hydro-Met, Forecasting & Services Division, Southern Centre forHydro Meteorological Forecasting, Ho Chi Minh City, Viet Nam

Flooding occurs annually in varying degrees and frequency, from June to November. The early floodoccurs during June and July, while the main flood occurs from late August to October and causesflooding in the area with different levels. Every year, more than 1 million ha are flooded, with depthsof up to 3.5 m of water, and durations of more than three months.

Apart from flooding in the rainy season, there is flooding which occurs in the coastal areas during thedry season. This is caused by tidal waters from the Bien Dong Sea. Such salt water flooding causesdamage to fruit trees and freshwater products by increased salinity.

While flooding causes damage, it also brings benefits.

In recent years, the Government of Viet Nam has given much attention to the above mentionedproblems. Many reports were submitted to the Government to find ways to reduce the damagingimpact and to make the most of positive effect of flood for the area.

FIGURE 1Location of the Mekong Delta

View of floodingFlooding situation

Mekong Delta flooding occurs as a consequence of three different effects, often in combination withone another:

• high discharge from the Mekong River coming from Cambodia;• heavy rainfall in the Delta affected by monsoon and typhoons; and• high tides in the Bien Dong Sea.

In general, the early flooding is mainly a result of the high discharges by river from Cambodia, whichmay cause inundation at some locations upstream, while main flooding is caused by all the factors thatcause inundation over large areas.

The areas known as the Plain of Reeds and the Long Xuyen Quadrangle are especially flood prone. Inprevious years, these areas have experienced flooding every year and severe flooding two or threetimes a decade. There is a close correlation between flooding and more widespread inundation.

In recent years, with the area's boom in economic development, human activity has changed thecorrelation. Severe inundation now occurs even in the years of moderate flooding on the mainstream.Severe floods were experienced in 1961, 1966, 1978, 1984, 1991, 1994, 1995, and 1996. (see Table 1and Figure 2). The 1996 flood was the most severe. At that time all three effects mentioned werepresent: high water levels on the Mekong River, high tides in the Bien Dong Sea, and heavy rainfall inthe area.

TABLE 1Annual maximum water level at different stations

Year Tanchau Chaudoc Xuanto Triton Mochoa Kienbinh Tanhiep DiffTC-MH1 1984 4.80 4.40 2.46 1.91 1.63 2.342 1991 4.64 4.27 3.99 2.74 2.48 1.90 1.54 2.163 1994 4.53 4.23 4.00 2.93 2.59 2.08 1.71 1.944 1995 4.30 3.91 3.64 2.76 2.46 1.97 1.54 1.845 1996 4.86 4.54 4.22 3.08 2.79 2.219 1.90 2.076 1997 4.18 3.79 3.68 2.75 2.29 1.87 1.55 1.89

Usually, the water level at the main stream of the Mekong Delta begins rising in June or July. At thattime if the water level at Tan Chau station (upstream) is over 2.20 m, there is early flooding causingsevere damage to both summer and fall crops.

FIGURE 2Annual maximum water level at Tan Chau

Heavy rainfall often occurs in the Mekong Basin beginning in August. Water levels rise quickly andreach their peak between the end of August and October. Flood waters in the field rise and recede laterthan floods in the main stream. An example is shown in Figure 3.

FIGURE 3Daily maximum water levels at Tan Chau Station (m)

Currently it is difficult to estimate the flooding base from water levels at Tan Chau. The differencebetween the water levels at Tan Chau, Moc Hoa and the field stations has tended to become smalleryear by year (see Figures 4 and 5). In 1995 the maximum water level at Tan Chau was 4.30 m, 12 cmhigher than the mean, but water levels at Moc Hoa were 73 cm higher than the mean.

DamageLoss of life has been increasing in recent years - 672 in 1994, 379 in 1995, 410 in 1996 and 607 in1997 (see Table 2). More than half a million homes were damaged by the floods of 1994, while some839 000 homes were carried away by floods in 1996.

Flood forecasting problemsInaccurate flood forecasting can be attributed to six groups of causes:

• Poor flood forecast models• Human resource problem• Staffing problem for data collection in the riparian countries• Slow and vulnerable communications• Poor data collection management• Insufficient rainfall and weather data

FIGURE 4Yearly maximum water level (m), compared

Flood managementRapid economic development has brought increased settlement in the area, increased development ofinfrastructure such as the system of roads, embankments, dikes around villages, etc., which hasadversely affected the capacity of the drainage system. This situation makes rural flooding deeper andlonger in duration. This is shown for the years 1978 and 1996 in Figures 6 and 7.

FIGURE 5Difference in water level, Tan Chau, Mochoa (m)

Most important now is how to manage floods to minimise damage and to attempt to derivesome positive benefit from the flood for that population which has accepted to live their livesin an environment of endemic flood conditions.

Two basic approaches to flood management have been suggested by Vietnamese scientists: structuraland non-structural measures.

TABLE 2Total damage of severe flooding 1978–1997

Unit 1978 1984 1991 1994 1995 1996 1997

1 Human loss(children) Person 87 105 158 407 265 199 180 250 160 607

2 Damagedhouse Family 57 005 41 835 185 127 593 107 26 326 42 150 99 238

3 House carriedaway by flood Family 9 005 8 219 12 350 2 799 28 240 839 686 74 368

4 Householdsmigrated Family 245 500 10 744 15 600 20 125 11 431 39 378

5Rice areasentirelydamaged

ha 307 100 175 626 171 898 26 868 11 101 60 368 19 758

6 Rice areas withyield decrease ha 113 600 111 879 88 873 202 186 62 399 132 309 251 341

7 Estimateddamage

thousandmillionVND

1 261 1 247 883 2 284 700 2 673 6 966

* The data of 1997 include the damage caused by flooding and by typhoons.

Non-structural measures• Upgrade the quality of hydrometeorological measurement and data collection and

transmission basinwide by implementation of the hydrometeorological network and developcooperation between riparian countries.

• Strengthen the capability of flood forecasting, as well as the meteorological forecasting andwarning network, especially in the medium and long term. This includes the estimate of stage,discharge, time of occurrence, and the duration of floods and may involve action to protectproperty from flood damage and/or evacuation planning. In some situations, flood forecastingand warning can be the most important non-structural measure in the management of flood-liable lands and in reducing flood losses. The value of flood forecasting and warning to themanagement of flood-liable lands is direct and can prevent i loss of life and material damageto property.

• Provide public information and education, covering development of needed technicalinformation and public education programmes, especially for officials and planners who havethe main task of interpretation and application. This is essential in an effective flood lossprevention programme.

• Inform people living in the flood path so that they may adjust themselves.• Diversify activities to get higher income before, during and after the flood by applying

integrated farming systems (crop, livestock and fish) or sloping agricultural techniques.• Improve land use management including adjusting cropping patterns by selecting shorter

duration crop varieties and earlier crop establishment to avoid predictable floods.• Modify catchment conditions such as on-site storage for use in the dry season.• Increase flood proofing of buildings. Some damages from flooding may be minimised by

appropriate measures to prevent floodwaters from entering properties.

• Intensify flood fighting, which has been generally defined as taking precautionary measuresagainst disaster at times of flooding or surge water.

FIGURE 6Flood zone in the Mekong Delta 1978

Structural measures• Levees and flood walls: the principal purpose of levees and flood walls is to protect only the

area immediately behind them against a pre-determined flood height in early floods for whichthey were designed. These areas may include settlements or limited crop areas.

• Channel modifications, which include straightening, deepening or widening of the channel,removing vegetation or debris, lining the channel, raising or enlarging bridges and culvertswhich restrict flows, and removing barriers which interfere which flows.

• By-pass flood-ways, including those natural (existing) watercourses and man-made channels.New construction must take note of such conditions.

• Building infrastructure and drainage systems to improve drainage of water originating as run-off from the protected area behind levees of flood walls.

FIGURE 7Flood zone in the Mekong Delta 1996

ConclusionsYear by year, the severity of flooding damage in the delta becomes higher and higher, resulting inhuman loss and very serious and costly damage to infrastructure damage.Apart from the harm as mentioned, there are positive effects of flood which are important. The floodsmainly benefit future rice production, increase soil fertility, wash out soil acidity, and decreasedamage by rats while increasing rice yields.The policy of the Government of Viet Nam is to accept flooding as a normal and natural phenomenonand to accept to live with it. This requires an urgent re-orientation of flood management in the delta.At present, some of the above mentioned measures of flood management which are financed by theGovernment are being tested.In the opinion of flood forecasters, our hydrometeorological activities are most necessary for floodmanagement in Viet Nam.