Thailand

“Pipi Monyet”, Cara Bangkok Bebas Banjir

Arfi Bambani Amri, Denny Armandhanu

Sumber:   http://dunia.vivanews.com/ 30 OKTOBER 2010

Ribuan relawan di Bangkok melakukan operasi bersih-bersih (AP Photo/ Manish Swarup)

VIVAnews – Selain berhasil menekan kemacetan dengan pengembangan kereta bawah tanah, Bangkok, Ibukota Thailand, juga telah lama berhasil mengendalikan banjir. Bangkok telah berpengalaman puluhan tahun dalam menghadapi banjir yang menimpa daerahnya.

Warganya tidak lagi perlu takut akan akan adanya banjir parah, karena ibukota Thailand ini mempunyai sistem yang disebut “pipi monyet”.

Pipi monyet adalah sistem penampungan yang terdiri dari 21 wadah penampungan air hujan. Penampungan ini dapat menampung air hujan yang berlebih hingga 30 juta kubik. Lalu pada musim panas, air ini dapat digunakan untuk keperluan konsumsi warga Bangkok, termasuk diantaranya air minum dan air keran.

“Nama ini terinspirasi dari monyet yang biasanya makan berlebih. Kelebihan makanan ini disimpan di pipinya, sehingga pipinya menggembung. Ketika nanti dia merasa lapar, dia akan memakan makanan di pipinya tersebut,” ujar Gubernur Bangkok, Sukhumban Paribatra, yang ditemui usai konferensi pers gubernur dan walikota se-Asia Eropa pada pertemuan Asia Europe Meeting, Jumat, 29 Oktober 2010.

Sebenarnya Bangkok yang terletak satu meter di bawah permukaan laut rawan terkena banjir. Ditambah lagi jika terjadi hujan lebat, gelombang tinggi dari sungai Chao Praya akan meluap hingga ke pusat kota. “Jika hujan lebat datang, aliran air dari utara membanjiri daerah Bangkok,” ujar Paribatra.

Namun, berkat sistem yang telah dikembangkan puluhan tahun lalu oleh kerja keras Raja Thailand, Bhumibol Adulyadej, ketakutan itu sirna. Raja Thailand, kata Paribatra, memiliki pengetahuan dan ketertarikan yang besar terhadap sistem pengairan dan pengendalian banjir di Bangkok. “Beliau menginspirasi sistem ini sehingga dapat bekerja dengan baik,” ujar Paribatra.

Bangkok juga memiliki tanggul sepanjang 72 kilometer dan saluran air sepanjang 75 kilometer untuk mengalirkan air yang meluap dari sungai Chao Phraya. Hal ini, ujar Paribatra, adalah sistem yang telah dikembangkan selama puluhan tahun di Bangkok.

“Sistem pengendalian banjir ini mulai dikembangkan oleh Bangkok setelah kota ini didera banjir parah 27 tahun lalu. Kala itu Bangkok tenggelam selama hampir tiga bulan,” ujar Paribatra.

Kendati sistem penanggulanganh banjir Thailand yang canggih, Paribatra tidak dapat menjamin Bangkok tidak kebanjiran lagi. “Tapi setidaknya kami dapat memastikan banjir besar seperti 27 tahun lalu tidak akan terjadi lagi,” ujarnya.

Flood management in Chao Phraya River basin

Siripong Hungspreug, Wirat Khao-uppatum, Suwit Thanopanuwat1

Siripong Hungspreug is Director of Office of Budget Programming and Project Planning; Wirat Khao-uppathamis Director of office of Hydrology and Water Management; and Suwit Thanopanuwat is an expert on WaterResources Development Planning, all of Thailand’s Royal Irrigation Department.Sripong et al. Flood Management in Chao Phraya River BasinThe Chao Phraya Delta: Historical Development, Dynamics and Challenges of Thailand’ Rice Bowl 2

Abstract

The Chao Phraya Rivers and her tributaries, have played a major role in thenation’s history from the periods of Sukhothai to Ayutthaya and Bangkok at present.Their existence was critical to the growth of the Kingdom of Thailand and fostered thedevelopment of major cities, including the capital city. In Ayutthaya Period, Thai peoplewisely used a long inundation during flood time in the Delta as the natural barriersagainst her country’s enemy. The floodplains of these rivers also provide some of themost productive farmland and continually contribute to the economic growth of thecountry. The high level of investment in the infrastructure for flood reduction wereconstructed to minimize the annual risk. But naturally, in the Chao Phraya Delta, floodhas continuously been experienced every year due to hydrometeorological andgeographical condition.

Introduction

The flood of 1995 demonstrated that people and their properties in the floodplains of ChaoPhraya River basin were at risk. Excessive rainfall and related runoff inundated nearly15,000 sq.km. of floodplains and caused major damages to agricultural land andcommunities along the river. The inundation nearly exceeded 16 billion cu.m., dike breachingand over-topping occurred at almost every reach of the rivers. However, the extensiveinundation considerably relieved the urban areas located further downstream, like Bangkokand its satellites, from a serious disaster. Overall damages were extensive with about 72billion Baht and a large amount of unquantifiable impacts on the health and well-being of thepopulation. The Royal Irrigation Department thus initiated a formulation of an integratedstrategy plan of flood mitigation in the Chao Phraya River Basin in an inter-agencycooperation manner during December 1996 and August 1999. The plan was formulated withstructural and nonstructural measures covering the use of the floodplain by preservation ofpresent natural retarding effect ; assurance of safety level of major cities, enhancement ofsafety level in agricultural area, and an institutional arrangement for implementation ofmeasures.

1 Flood management in Chao Phraya river basin

1.1 Chao Phraya river basin

Fig.1 GENERAL MAP

The Chao Phraya River Basin as shown in Figure 1 is Thailand’s largest and most importantgeographical unit in terms of land and water resources development. It is located in thenorth and central regions of the country and occupies about 35 percent of the country’s totalarea. About 20 million people (30 percent of the population) reside in the basin in which morethan 70 percent are farmers. Rice is the main crop in both irrigated and rainfed areas of thebasin.

Average annual rainfall in the basin ranges from 1,000 to 1,400 mm. The climate isdominated by the Southwest monsoon, which occurs between May and October. About 90percent of annual rainfall occurs during this period, causing heavy floods. The scarcity of rainbetween November and April makes agricultural conditions unfavourable. On average, thetotal volume of available water is estimated at 31,300 million cu.m. per year.

1.2 Characteristics of the basin’s upper and lower reaches

The river basin can be characterized geographically into upper and lower basins. The upperbasin is mountainous, with 40 percent forest cover and 41 percent cultivated land.Traditionally, agriculture has been practiced in the river valleys. Shifting cultivation hascaused soil degradation and erosion in some areas and has changed the hydrologicalregime. The lower basin (the river’s delta) is the floodplain and is well suited to ricecultivation. After irrigation water became available in the 1970s, farmers in the lower basinswitched from growing floating rice to cultivating higher-yielding varieties.

Among the basin’s major infrastructure, the Chao Phraya barrage irrigates an area of about1.2 million ha in the lower part of the river basin to increase wet season rice production. Themultipurpose Bhumibol and Sirikit dams are located upstream of the barrage. Theirconstruction enabled provision of water to 400,000 ha of dry-season cropping in the lowerpart of the river basin. Hydropower from these dams has become a major source of electricalenergy for Thailand.

Flood protection is important in the lower part of the river basin because of the risk of largescaledamage to public and private property. Dikes were constructed downstream of theChao Phraya barrage to prevent the inundation of cultivated land from small flood. Residential area were also protected by dikes and polder system at a higher safety level,together with a number of pumping stations, especially in the downstream-most of the river,Bangkok and vicinities.

The Chao Phraya River is the principal source of water for domestic and industrial uses inthe basin. The major user is the Bangkok Metropolitan Water Work Authority (MWWA), withan annual requirement of about 1,100 million m3. This amount has been supplemented with groundwater. The MWWA intends to terminate the use of groundwater and replace it with transferring water from the Mea Klong basin. Presently, less than 10 percent of water use in Bangkok comes from groundwater.

2 Floodplain management

From history of the country, the capital of Thailand in each era is always located in thefloodplain of Chao Phraya River Basin in the bank of the river and its tributary. Thai people inthe former time were accustomed with river flood and developed the way of living in this vastfloodplain and had wisely used a long inundation during flood time in the Delta as the naturalweapons against her country’s enemy. Until recently, from 1970, when a modern waterresources development scheme were constructed and significantly improve the livingcondition, enhance economic activities through better regulation of water and reduction offlood in the delta.

Though efforts have been made to mitigate flood damage in the Chao Phraya River Basinthrough the construction of dams, reservoirs, dikes and pump stations, flooding problem stillpersists due to the increase of flood discharge as a result of deforestation, expansion offarmlands and urban areas, etc., in line with the economic growth.

The flood damage potential is increasing due to rapid urbanization and land development indownstream areas, particularly, the Bangkok metropolitan area and other municipalitiesalong the Chao Phraya River. A disastrous flood occurred in October 1995, resulting in theextensive damage to properties and loss of human lives.

2.1 Causes of floods

The causes of floods, in general, may come from two main sources: nature and humanintervention, as follows:

2.1.1 Natural causes

The main natural causes are overbank flow of the rivers, heavy rainfalls and tides.

Overbank flow : Floods in Thailand are also generally caused by overflow from the rivers,which results in widespread flooding. During the peak flood in 1995, the flow in the ChaoPhraya River passing through Bangkok metropolis to the Gulf of Thailand was much higherthan the capacity of the Chao Phraya River and caused severe flooding in the Chao PhrayaDelta and Bangkok metropolis.

Heavy rainfall : Heavy local rainfall is usually the main cause of inland floods, as it oftenexceeds the drainage capacity of the local areas or streams. For example, several tropicalcyclones passed through Thailand and caused heavy rains in 1995, including the depressionstorm “Lois”.

Influence of tides : Tidal fluctuation at the river mouth has often affected the drainage ofriver floods into the Gulf of Thailand. This effect prolongs the period of flooding, especially inthe coastal provinces of the Chao Phraya River basin, Samut Prakan, Bangkok metropolisand Samut Sakhon.

 

2.1.2 Man-made causes

 

The most common man-made causes in Thailand are deforestation, uncoordinated urbandevelopment, over-abstraction of groundwater, and destruction of flood embankments.

Deforestation : This is the most significant man-made cause that increases flood peak fromrainfall and reduces the lag time between rainfall and run-off. In a deforested area, surfacerun-off and peak flood discharge tend to be higher, since there are no trees to obstruct theflow. Moreover, the rapid run-off will increase erosion of soil surface particles, resulting inhigher turbidity and more serious sedimentation. This results in reducing function of the riverand water sources.

Uncoordinated development : In urban development, most of the surface areas arecovered with houses, roads or paved surface having lower water absorption and rainfalltends to convert almost immediately into run-off flowing into the drainage system. Thisphenomenon is in contrast to that in rural areas, where rainfall can be retained by vegetationcover and absorbed by soil. Many kind of development in delta area have related to creatinghigher flood risk for example protection of urban and high-value farm will reduce space offlood inundation accordingly such as orchards, aquaculture, etc. Construction of roads andrailways will also obstruct flow especially for inland flow. Housing construction in public areasalong river or canal banks is another example of action that reduces the stream cross-sectionand thus its flow capacity. Uncontrolled dumping of sewage and garbage may obstruct theflow and cause siltation in the drainage streams. As a consequence, uncoordinateddevelopment in many parts of the country has resulted in decrease in drainage efficiencyboth inland and river courses.

Destruction of flood embankments : There have been cases in which inhabitants living inareas outside the protection of flood embankments destroyed those embankments in thehope of reducing the flood-water level in their areas. Protection of these embankments wasdifficult, although there are government agencies responsible for the maintenance andmonitoring of the embankments. These events resulted in abrupt flooding of the protectedresidential areas.Over-abstraction of groundwater : Pumping of groundwater is one of the main causes forland subsidence, which has resulted in deeper flooding and longer waterlogging. Thegovernment agencies concerned are trying to limit the pumping of groundwater and this efforthas been emphasized in the Bangkok metropolitan Area.

2.2 An overview of past floods and developments

2.2.1 Past flood damage

Floods continue to cause annual damage in the Chao Phraya River Basin, as summarized intable below. It may be noted that the flood damage was estimated based mainly on thevalue of property damage and did not include the cost of lost production and other forms ofeconomic loss. The trend towards higher flood damage reflects a number of factors,including the higher value of property and the ongoing development of property more vulnerable to floods.

2.2.2 Major change in the flooding regime

Changes in the flooding regime in the Chao Phraya River basin reflect not only changes ineach factor causing flood but also in the interaction among them. While riverine floodingcaused by the overbank flow of the main river discharge remains an important cause formajor flooding and flood damage, the coinciding of floodwaves from the tributaries, urbanfloods and high tides appear to have become more frequent and are thus an increasinglythreatening factor. In order to give an overall picture of the flooding regime in the lower ChaoPhraya River basin, the most important causes of flooding in the three most severe floodsduring the past 60 years are briefly summarized in following table.

As Chao Phraya delta is the highest economic growth area of the country, the high level ofinvestment in the infrastructure, with the extensive development of natural resources over theyears in the basin, have led to a complex change in the flooding regime. This change is alsoeffected by the complexity in water resources management. Apart from the construction ofvarious storage reservoirs in the upper part of the basin, the following measures have been implemented:

2.2.3 Summary of past development

Overbank flow protection scheme : The dykes were constructed along both sides of theChao Phraya River banks from Nakhon Sawan down in to Bangkok under the supervision ofthe Royal Irrigation Department. Nowadays, these dykes of 300 km length are used ashighways. The regulators were installed at the confluence of the tributaries to controlbackwater flow from the Chao Phraya River. However, the dykes are incomplete in somepart, such as on the western side of Bangkok for which the Bangsai-Bangkok Highway isbeing temporarily used as a part of the dykes.

Flood control schemes for major municipal areas : There are seven major municipalitiessituated along the Chao Phraya River: Nakhon Sawan, Chai Nat, Singha Buri, Ang Thong,Ayutthaya, Patum Thani and Nonthaburi. The Public Works Department (PWD) plans toprovide flood protection to these cities by polder systems, which consist of retaining walls,embankments, regulators and pumping stations.

Flood control schemes for Bangkok and vicinity : The master plan for flood protection inthis area was launched in 1984. The first scheme was finished in 1995. According to thatscheme, 1,500 km2 will be protected from floods. The scheme consists of several initiativesof His Majesty the King, such as a 74-km dyke, starting from the Rangsit Canal down towardthe sea at Samut Prakan, and 20 pumping stations along the Chao Phraya River with a totalof 30 million m3/day. The construction of regulators and drainage systems in Nonthaburi andSamut Prakan is part of the scheme. According to the work plan of the Bangkok MetropolitanAdministration (BMA), a permanent concrete floodwall with a total length of 80 km along the Chao Phraya River will be constructed within the next five years together with increasing thedrainage capacity up to 65 million m3/day.

Flood control schemes of agricultural area : The Royal Irrigation Department (RID) hasestablished a flood protection and drainage for the agricultural area in the lower ChaoPhraya basin. Dykes have been constructed along the Bang Pakong River in the east, ThaChin River in the west and also the Chao Phraya River, to provide flood protection to anagricultural area of 5,000 km2. More than 50 regulators and pumping station with a capacityof 50 million m3/day were constructed.

2.2.4 Evaluation of the current flood control situation

An evaluation of the current situation of flood control in the basin is summarized below:

The upstream reservoirs have markedly helped regulate the flow regime. For a flood of themagnitude of that of 1995, corresponding to a 25-year flood, the flood peak at NakhonSawan, will be reduced by 1,000 m3/s, or over 20 per cent of the natural flood. It may benoted that the capacity of these reservoirs in flow regulation is also important in reducing thevolume of the floodwaves travelling down to the Bangkok region during the critical period.

The development and protection of large areas in the lower Chao Phraya basin, especiallyfor agriculture and urban development, have greatly reduced the natural capacity of the basinin dispersing and reducing the floodwaves when passing through this area.

The confinement of the river to its main channel in Bangkok, although not complete, has ledto an increase in the flood-water level resulting from an increase in the outflow to the sea.This fact has consequently required an increase in the level of protection against the riverfloods.

3 The 1995 Flood

Fig.2 ESTIMATED INUNDATION MAP

The depression storm “Lois” in 1995 caused heavy rainfall in the north. Spillage of the SirikitDam and high discharges of the Nan, Yom and Chao Phraya rivers inundated large areas inthe Phrae, Sukhothai, Phitsanulok, Phichit and Nakhon Sawan provinces as well as all otherprovinces along the Cho Phraya River downstream of Nakhon Sawan as shown in Figure 2. Bangkok and vicinity suffered from the flood for more than two months. However, the citycore of Bangkok was saved by the flood protection system that had been constructed in1984. The flood in 1995 was considered a severe flooding event affecting the entire country.

The main causes of flooding are low flow capacities river channels. The present river channelcapacities are between 3,000 and 4,000 m3/s in the stretch near Nakhon Sawan, about 1,300m3/s in the upstream near Ayutthaya, about 2,900 m3/s in the downstream near Ayutthaya,and about 3,600 m3/s at Bangkok as shown in Figure 3. The flow capacity decreases towardthe downstream, this implies that spillage from the river channel gradually occurs in theupstream when a large scale flood occurs; hence, spillage do not concentrate in thedownstream. This situation contributes to alleviation of flood damages to Bangkok, while the spilled water is widely retained in the agricultural area in a manner of inundation for 2-3months. Huge inundation took place over 15,000 sq.km. Most of the floodwater wasgenerated in the Nan and Yom river basins. Firstly, the floodwater originated in the upperbasin then flowed down along the Nan and Yom rivers, repeating collection of tributaries’runoff water and spillage by overtopping and dike breaching. The total water volume whichswelled to 31 billion m3 at Nakhon Sawan then entered the delta areas and spilled over theriver banks to the adjoining flood plains caused inundation to nearly 16 billion m3. Thecontribution of local rainfall to the inundation in the Lower Central Plain, namely the Higherand Lower deltas, was not significant. The estimated flow capacity near Bangkok was about3,600 m3/s, or at its maximum capacity.

Besides the natural causes, it is pointed out the several causes of human interventionactivities such as land use in flood risk areas, development of upstream, operation of flowcontrol facilities such as dams, and coordination among agencies concerned on floodmanagement are with the increase in flood damage.

3.1 Flood damage condition

When floods strike, people try to protect their assets or mitigate flood damage by takingcountermeasures. Since inundation comes up slowly, people can determine when the floodcomes and have enough time to take countermeasures. Inundation level comes up severalcentimeters per day, and it took 20 to 30 days from the beginning of the inundation to reachthe highest level in the case of Ayutthaya in 1995. The flood information is frequentlybroadcast on radio and TV. People make efforts to move their assets such as furniture andtelevision to the second floor or an upper place like a shelf or desk in most cases. Shopowners construct protection walls made of cement.

Large factories are located around municipalities or sanitary cities in general. Most of thesefactories have their own dikes for protection against floods. They usually have large amountsof assets such as facilities, machinery, relatively protected against floods. However, oncethese are submerged, the damage amount becomes very large.

People have developed the wisdom of providing flood countermeasures through theirexperiences. However, they cannot prevent or mitigate big floods and, actually, muchdamage has been inflicted.

Fig.3 PRESENT FLOODING SITUATION

It is difficult to estimate the overall flood damage. Theoretically, it is divided into twocategories, tangible damage and intangible damage. The latter, which includes negativepsychological impact such as fear, depression and health, etc., cannot quantify in monetaryterms. Only tangible damage is further considered into two categories: direct damage andindirect damage. Direct damage is measurable and often referred to as the damage. Indirectdamage is not physical but subsequent negative effect on economic activities. Sales loss, forinstance, is a typical indirect damage due to business suspension in a shop forced to closeby inundation. Direct damage also includes loss of tourism and expenses for diseases andso on. Under the foregoing circumstances, estimation of flood damage in the 1995 floodamounts to about 72 billion baht. It was also estimated the future condition (20 year) when land use will be changed toward more urbanization, the magnitude of 1995 flood will increasedamage to 164 billion baht, as demonstrated in Figure 3.

3.2 Royal flood management initiatives and ongoing flood mitigationproject by agencies concerned

His Majesty the King has shown great concern for flooding in regions outside Bangkok bysuggesting various methodologies to suit local conditions and to conform with the availabilityof government officials and budgetary constraints.

The low-lying flat terrain of Bangkok causes floodwaters to drain from the area slowly Manycanals have a small gradient while others are silted or filled up. Weeds and other items blockthe flow of water. There are some of the factors why Bangkok and the surrounding area havebeen subject to heavy flooding for many years. His Majesty has devised a flood managementsystem for Bangkok which he calls Kaem Ling (Monkey’s Cheeks). The concept is totemporary storing water in some places at high tide and drain out at low tide.

Together with the Royal-initiated Projects, the agencies responsible for flood mitigation anddrainage works have made serious efforts under the following projects:

• Heightening of flood barrier at Bangkok Metropolitan Area by BMA
• Provision of polder system together with the improvement of drainage system by PWD
• River improvement and drainage system improvement by RID
• Loop-cut at Bangkok Port and construction of multipurpose dams by RID
• Monkey’s cheeks projects on the coastal area both east and west of Chao Phrayaestuary by BMA and RID

Through these projects, the flood protection level in major urban areas is, in general,expected to increase. However, protection works in the upstream sometimes bring aboutadverse influences to the downstream: i.e., the protection level in Bangkok is expected todecrease due to the construction of polder in Pathum Thani and Nonthaburi. For theagricultural areas it is expected that the present low protection level against floods ismaintained in the future.

4 Formulation of Master Plan

4.1 Basic concept for the formulation of master plan

• According to the past flood occurrence, flood conditions in the Chao Phraya River Basinis featured with the existence of extensive inundation areas, which play an important roleto retain flood discharge flowing into the Chao Phraya River, resulting in the mitigation offlood damage in the downstream. In this connection, the flood mitigation plan is formulated putting emphasis on preservation of the natural retarding effect. The conceptis a global one for flood mitigation through the provision of nonstructural measures andalso corresponds to the monkey cheek concept.
• On the other hand, the Chao Phraya River basin, especially the Chao Phraya Delta,tends to be developed continuously in the future, even in such natural retarding area,which results in the decrease of retarding effect causing the increase of flood dischargeto the downstream. To minimize the influence due to decrease of retarding effect,suitable measures for comprehensive flood mitigation including structural andnonstructural measures are introduced.
• Flood mitigation measures are also classified according to the flood, i.e., basin-wide floodor local inland water. The Master Plan is formulated by putting emphasis on floodmitigation measures for basin-wide flood. As for local inland water, the conceivablemeasures for drainage system improvement is examined for agricultural areas as well asthe prioritization for implementation of improvement works.
• The flood prone areas in the basin are mainly composed of urban and agricultural areas.Among them, the urban areas will take priority for flood protection because their socialand economic impacts are considered much higher than the agricultural areas. Urbanareas like Bangkok, Ayutthaya, Nakhon Sawan and others are to be protected byeffective measures against bigger scale floods.
• The agricultural areas play an important role as retarding area during big scale floods,and this role should be preserved. On the other hand, for small-scale floods, the presentflood damage condition should be improved, providing suitable measures within theallowable extent that will not cause adverse influence to the other areas.
• Flood damage conditions are influenced by nonstructural such as control of land andgroundwater extraction, and such influence would finally affect the effectiveness ofstructural measures. In this connection, the selection of an optimum measure is madethrough a comparative study on alternatives, considering the most effective combinationof structural and nonstructural measures.
• Although the purpose of the Master Plan is flood mitigation, it is also important toconsider the shortage of municipal water supply in the dry season. The multipurpose useof the proposed flood mitigation measures is thus examined, especially for irrigation andmunicipal water supply.

4.2 Measures of the Master Plan

The Master Plan is formulated with 2018 as the target year. In general, several measuresconsisting of structural and nonstructural ones are considered to cope with the floodingproblems, as shown in Figure 4. Specific measures as follows are required to deal with theabove-said issues in the Chao Phraya river basin.

4.2.1 Preservation of present natural retarding effect and minimization of Increase offlood damage

To maintain the present natural retarding effect and to minimize the increase of flooddamage, nonstructural measures, especially land use control and guidance, are essential.For the realization of land use control and guidance, flood risk maps as shown in Figure 5 are provided, so that all agencies concerned can prepare the development plan based onthese maps considering the influence of development. Also, people who are going todevelopment the land are forewarned through publication of the flood risk maps.

4.2.2 Assurance of safety level against flood at Bangkok and urban areas

To assure the safety level of urban areas against floods, nonstructural measures such as themodification of reservoir operation rule, flood forecasting, flood fighting and land use controland guidance are considered, while ring levee with drainage system improvement is appliedas the structural measure.

To assure the safety level of a 100-year return period at Bangkok, the following alternativesare proposed in combination with the ring levee provided by PWD, as shown in Figure 6.

• Alternative 1, Partial Protection of Pathum Thani and Nonthaburi
• Alternative 2-1, Heightening of Flood Barrier
• Alternative 2-2, Diversion Channel

4.2.3 Enhancement of Safety Level against Flood in Agricultural Areas

To enhance the safety level against flood in agricultural areas, nonstructural measures suchas the modification of reservoir operation rule, flood forecasting, flood fighting and land usecontrol and guidance, etc., are proposed. On the other hand, the following structuralmeasures are proposed for flood mitigation in agricultural areas: (a) river improvement; and(b) distribution and drainage systems improvement.

The protection level of agricultural areas in the downstream of Chai Nat could be enhancedto a 10-year return period by a combination of the above measures.

4.2.4 Institutional Arrangement for Implementation of Measures

In principle, the existing agencies concerned will handle these measures under their ownresponsibilities. To smoothly implement these measures, however, it is necessary to set up anew organization, the River Basin Committee, as the coordination agency among theagencies concerned, because such an organization does not exist at present in the Thaigovernment.

4.3 Economic evaluation of the Master Plan

The cost, benefit and economic viability of the measures proposed in the three (3)alternatives are evaluated in monetary term as shown below.

4.3.1  Comparison of Alternative Measures

The alternative measures have individual advantages and disadvantages. The major issuesare mainly the following:

(1) Alternative 1 (Partial Protection of Pathum Thani and Nonthaburi)

From the technical point of view, this is slightly better than “do nothing” since it limits theprotection area and is not effective to mitigate flood damage in a wide area. From theeconomical and environmental points of view, the construction of this alternative may nothave serious issues. However, from the social point of view, inhabitants will not accept thedelineation of protected and not protected areas within the same administrative area.

(2) Alternative 2-1 (Heightening of Flood Barrier at Bangkok)

From the technical point of view, this is a measure to absorb the adverse influence ofincrease of flood discharge at Bangkok due to protection of Pathum Thani and Nonthaburi.This alternative may enhance the protection level in the upstream area to a certain extent.From the economical point of view, this alternative may not have serious issues. However,from the social and environmental points of view, it will cause serious issues due to theconstruction of a ‘high’ barrier between inside and outside of the riparian area. The barriermay affect daily activities of so many people concerned, who are using infrastructures andfacilities provided for business activities.

(3) Alternative 2-2 (Construction of Diversion Channel)

From the technical point of view, this is an only alternative that can provide opportunity forenhancement of protection level in upstream area, while satisfactory protect downstreamarea. From the economical and social points of view, this alternative will require an enormous investment as well as a number of house evacuation and land acquisition. On theother hand, the diversion channel can be multi-purposely used as large-scale infrastructurefor regional development such as transportation, land and town planning, etc.

4.3.2 Selection of alternative measures

Though the Master Plan of 1999 had proposed the three alternatives, it had not selected orsuggested the most suitable one. It is recommended that all concerned in the nation isnecessary to use this study results for further discussions in order to select the mostacceptable alternatives. During January to October 2000, Crown Property Bureausupported a technical program through Office of National Water Resources Committee byorganizing an inter-agency working group for synthesizing the frame work of integrated watermanagement program in Chao Phraya River. The working group on flood resulted inselection of alternative 2-2 (construction of Diversion Channel) as the most suitablealternative for further implementation. The main reasons are; it has a basin-wide effectivenot only benefit Bangkok but also agricultural land upstream; and it improves hydrauliccondition of lower delta in sustainable manner.

The project will bring about many intangible benefits such as the stabilization of people’sliving condition, decrease of waterborne diseases, increase of work opportunities, and so on.Among the alternatives, the diversion channel can be used for water resources developmentpurposes.

4.3.3 Environmental consideration

The Forest area was reduced from 166,000 km2 in 1942 to 106,000 km2 in 1983 and to92,000 km2 in 1995. Such deforestation influences flood occurrence through the reduction ofwater conservation effect, resulting in the increase of peak flood discharge. Wetlands in lowlyingarea also effected by siltation and encroachment for farmland. The loss of wetland andupland cover and modification of the landscape throughout the basin over the last 50 yearssignificantly increased runoff. However, in condition of large scale flood, the heavy and longduration of rainfall over the upper part of basin creates saturated condition on forest andland, produce high rate of runoff. Although upland watershed treatment and restoration ofupland and low-lying wetland can possibly reduce flood stages in more frequent floods (5years or less) , it is questionable whether they would have significantly altered the 1995conditions.

5 Conclusion

The Master Plan of integrated flood damages mitigation in the Chao Phraya River Basin hasbeen formulated in accordance with the “Monkey Cheeks” concept for preservation of thepresent retarding effect and, also, with the introduction of suitable flood mitigation measures.To realize the Master Plan, several projects have been selected for urgent implementation. Itis concluded that flood mitigation in the context of the Master Plan is essential fordevelopment of the basin and the country as whole, and the implementation of selected projects is the most effective means to achieve the safety level of 100-yr for urban areas and10-25 yr for agriculture land.

It is necessary to note that floods are natural phenomena; mankind tries to overcome theseproblems by numerous measures and most of the time forgets the fact that water needsspace. Once an area has been protected from flooding, it will automatically exacerbate theflood problem in another area. Basinwide planning is necessary for effective floodmanagement, and changes in land use and expansion of the community are the greathindrances. A good land-use planning procedure and good enforcement of water-relatedlaws are a must. Floods can be prevented, controlled or avoided only to a certain degree, Ithas to be realized that at present all kinds of measures applied for the benefits of floodcontrol can be effective only at a certain magnitude of flood.

The 1995 flood was a significant hydrometeorological event. In some areas it represented anunusual event, however, it was just another of the many that have been seen before and willbe seen again. The flood magnitude of 1995 is less than 1942 resulted partly fromconstruction of reservoirs during 1960’s and 1970’s. For Over 30 years the people in deltahas learned that effective flood plain management can reduce vulnerability to damages andcreate a balance among natural and human uses of flood plains and their related watershedsto meet both social and environmental goals. Damages in 1995 was estimated at 72 billionBaht and expected to increase to 164 billion Baht in future land use. The country can nolonger afford such costs, economical and socially. The Master Plan and Follow-up activityprovide a solution for flood plain management in this delta. The government, at all levels, allbusinesses and all citizens must take the actions required to do so by sharing responsibility andaccountability.

References

Crown Property Bureau. 2000. Framework for Integrated Water Management and Development in Chao Phraya RiverBasin. Report prepared for seminar during 4-5 August 2000. (Thai version)

CTI Engineering International Co., Ltd. and INA corporation. 1999. The Study on Intrgrated Plan For Flood Mitigationin Chao Phraya River Basin. Japan International Cooperation Agency (JICA); Royal Irrigation Department, Thailand.

Economic and Social Commission for Asia and the Pacific. 1999. Regional cooperation in the Twenty-first Century onFlood Control and Management in Asia and the Pacific. United Nations. New York

Report of the Interagency Floodplain Management Review Committee to the Administration Floodplain ManagementTask Force. 1994. Sharing the Challenge : Floodplain Management into the 21st century. Washington, DC.

Royal Development Project Board; Department of Technical and Economic Cooperation; United Nations DevelopmentProgramme. 1997. Concepts and Theories of His Majesty the King on Development. Bangkok. Thailand.

Vadhanaphuti, Boonyok; Klaikayai, Thanom; Thanopanuwat, Suwit and Hungspreug, Natha.1991. Water ResourcesPlanning and Management of Thailand’s Chao Phraya River Basin. World Bank Technical Paper No.175.

Integrated Flood Mitigation Management in the Lower Chao Phraya River Basin

Dr.Somkiat1 Prajamwong and Dr. Pornsak2 Suppataratarn

1: Senior Expert on Civil Engineering (Project Planning), Royal Irrigation Department

2: Managing Director, ASDECON Corporation Co.,ltd.

1. State of Flood Problem in Lower Chao Phraya River Basin

1.1 Physical Features

The Chao Phraya river basin in the upper part consists of the Ping, Wang, Yom and Nan tributaries. The central part basin receives water from the Sakae Krang river while the Tha Chin river takes water out and drain directly to the sea. The Pasak river joins the lower part of the basin, The river network of the lower Chao Phraya river basin is shown in Figure 1.

Figure 1 River network of the Lower Chao Phraya river basin

1.2 Inundation Areas

Generally, the lower Chao Phraya river basin is the low lying plain area which has ground elevation ranging from 0 to 20 meters above mean sea level. The Chao Phraya river is the main channel which conveys runoff from its tributaries from the upper part and discharges to the sea in the south. During the rainy season, water in the channel is often spill over the bank or embankment to the plain due to mild channel slope and tidal influence. Flooding areas during the last 30 years shown in Figure 2 have changed significantly due to implementation flood protection system.

Figure 2 Severe Flood and Inundation area in the Chao Phraya river basin

1.3 Flooding Conditions

Climate of the Chao Phraya river basin is mainly influence by monsoon. Low pressure depression creates tropical storm and result in the heavy rain during May to October. Occasionally, inflow runoff exceeds the upstream reservoir storage capacity and discharging to downstream that resulting in flood event and overflow in the end of August to December. This also causes long term flood in the lowland areas.

Based on the historical hydrological data, the average discharge of the Chao Phraya river at Nakhon Sawan province is found to be 2,500 cms while the discharge downstream of the Chao Phraya barrage in Chainat province, 100 kilometer downstream, is about 2,320 cms.

The discharge amount at Nakhon Sawan province is the key indicator station for the flood management action. Communities downstream start to be threatened if the discharge amount exceeds the average discharge. However, the damage is limited if the amount is less than 3,000 cms with proper management of the Chao Phraya barrage. The flood risk increase significantly if the discharge at this station is exceed 3,000 cms, flood water in the channel spill over both banks of the Chao Phraya river. Many cities of the lower Chao Phraya river namely Singburi, Angthong, Ayutthaya Pathum Thani, Nonthaburi and Bangkok provinces are partly flooded. Socio-economic of these city are crucial effected if the discharge is more than 4,000 cms as in the years of 1995, 2006.

1.4 Causes of Flood

Flooding in the lower Chao Phraya river basin is caused by the follows:

1. Topography of the basin. The Chao Phraya river has relatively steep slope basin in the upper part and mild slope in lower part. Such topography makes the flood flows rapidly in upstream and slow down in lower areas. Moreover, the influence of tide makes the flood situations become worse as it overflows to the both river banks where economic area is located.
2. High discharge from the upstream e.g. flooded in the years 1983, 1995, 2002 and 2006.
3. ocal heavy rainfall e.g. flooded in the years 1983, 1996 and 2002.
4. Reduction of retention area along both bank of the river and Change of flow pattern. The cause is mainly due to:
   • Reduction of the agriculture area due to rapid encroachment of the urbanization and industrialization which have their own protection system
   • Flood protection schemes for agricultural area are implemented to reduce the damage to the agricultural product
5. Land subsidence due to soft clay at delta area and over groundwater pumping. This lower down the flood embankment level and cause the difficulty for the local drainage thus increase flooding risk.
6. The global warming. This create tendency of higher fluctuating of rainfall. In addition, sea level trends to be higher which effect to community and economics area in low land of Chao Phraya river. In general, the risk of flooding will increase in the future.

Flooding in lower Chao Phraya river basin is significantly affecting the socio-economic condition of the Thai nation. Past directed flood damage recorded only for public properties and agriculture product are shown in Table 1 as follows:

Figure 3 Flood Flow Pattern in 2006

2. The Flood solution in the Chao Phraya area.

Along the river bank of the lower Chao Phraya river basin are situated by many large and important communities of 9 provinces including Bangkok Metropolitan region. The total population of these provinces is about 16.2 million with GPP of 3.7328 Trillion Baht amounted to 44 % of Thai GDP. It is realized as the important economics area for the country which is threat by the flooding. The existing measures could mitigate only the flood of low return period of occurrence, more measures are urgently implemented to mitigate high flood peak.

2.1. Flood mitigation measures

The proposed flood mitigation scheme includes both structure measure and non structure measure. The most economical measures are to save life and protect the high value properties or the communities according to the set priority. These could be done through the control of the flood peak discharges/volume so that it does not exceed the capabilities of the flood control facilities and measures in order of the set priority locations. The proposed structures table summarized below:

The Chao Phraya river basin post a large area of low lying area and many canals inter connected. Use of these as the retention pond are very supportive by His Majesty King Bhumibol Adulyadej which so called “Monkey Cheek Principle”. The projects are under development and implementation including:

Lowland as a large retention storage: The Chao Phraya river plain have many low lying area which generally are rice cropping area. Some are natural floodplain area and do not cultivated in the rainy season. Some are in the irrigation area and protected from the flood up to specific degree. These lowland areas are developing and integrated into a part of  flood control  facilities to reduce flood risk for the communities and alleviate flood damage in the basin as  a whole. Water in these low lying areas generated from local rainfall are discharged to river as far as flow in the river is not overbank. The vacant storage will prepare the low lying land to accommodate next storm and at the same time serve as the retention pond to cut the flood peak

in the river if it threat to flood the communities.

Canals or ponds as the Monkey Cheek: Monkey Cheeks use the canals or ponds in and nearby the community areas as facilities. Water levels in canals or ponds are lower down as much as possible and whenever it is possible to free the storage for next local rainfall or accommodate the surrounding runoff. In general, the free storage could accommodate local rainfall up to 7 days.

At present, the Royal Irrigation Department (RID) has developed 8 water storages, marshes, and ponds in the Chao Phraya River Basin areas with total capacity of 70 mcm while Bangkok metropolitan administration (BMA) has developed 21 marshes in the city with total storages of 12.7 mcm. In addition, Monkey Cheeks of the canal network are jointly developed by RID, BMA and public work and town planning department under the initiation of His Majesty King. The project, located in the southern part Bangkok in the west, use Klong Sanam Chai and Maha Chai canals and nearby canals as the storage. Although, the project could help solve the flood  effectively for the rainfall volume up to 60 mm and limited area, however it will be served as a pilot project for other development Monkey Cheek using the canal system.

2.2. Lowland Management

Definition

In this paper, Lowland is defined as a large low lying or agricultural area flooded frequently and can effectively lower down flood peak level in river nearby community area.

Concept

Lowland is usually the rice field agricultural area. High value properties or communities in the lowland shall be protected at least up to the flood storage level. Lowland shall be developed so that it could hold large amount of water and can be managed to divert in peak flood water from the river and discharge out the storage water within specific time period. Effectiveness of the lowland depends very much on the operation management strategy. In general, the Lowland development and operation shall take into account of the followings:

1. Water level in the lowland shall keep minimum during the rainy season to have more storage for flood peak water. Water in the lowland shall drain out to the river through the gates in general, pump shall be used only when the incoming flood peak threat to damage the protected communities.
2. In general, lowland can be used to store flood volume only once in a year, therefore diversion of excess flood peak to designated lowland shall commence when the river water level almost reaches its highest peak. Accurate forecast of peak flood is crucial for the flood operation management.
3. Diversion of river flood peak to the lowlands shall start from the one located at the most upstream downward to downstream lowland.
4. Land use in the Lowland shall be reserved and controlled as cultivation land, urbanization should not allow to minimize damages in the Lowland.
5. Flood management criteria for the use of Lowland shall be set up along with decision support system. These include flood forecasting and warning system for the whole basin and Lowland area.
6. Fair and fast damage compensation method and process due to the diversion of water into the lowland shall be agreed by the stakeholders.

Potential Lowland area in the Lower Chao Phraya River basin

Study of potential use of 13 Lowland areas in the Chao Phraya river basin as shown in Figure 4 are can be summarized as follows:

• 5 Lowland Areas in the Upper Chao Phraya River Basin. The 5 Lowland areas with 835 sq.km. can accommodate flood volume of 1,161 mcm. Benefit can be claimed from the communities in Phitsanulok and Pichit provinces which located along the Yom and Nan Rivers. The said flood volume amount could effectively lower down flood level from 0.2 to 1.2 meters and help alleviate flood with return period less than 5 year.
• 8 Lowland Areas in the Lower Chao Phraya River Basin. The 8 Lowland areas with 925 sq.km. can accommodate flood volume of 1,738 mcm. Benefit can be claimed from the communities in provinces located along the lower Chao Phraya river bank which include Signburi, Angthong, Ayuthaya, Phratum Thani, Nonthaburi and Bangkok metropolitan. These group of Lowlands could help alleviate flood for a specific range of discharges, released from the Chao Phraya Dam, amounted from 3,000 to 4,200 cms. which corresponding to the flood of 5.5 to 20 year return period. The effectively lower down of flood level is from 0.2 to 1.0 meters.

Structural Components for the Chao Phraya Lowland

Water management in flood retarding areas will be conducted by keeping the retention area to be in normal condition without inundation and releasing water to the area when flood and river water level comes up to the crisis. This will help reduce the water level in the streams, damage in economic area and community area as well as in the flood retarding areas itself.

To  develop the selected 13 area to be use for Lowland, some structural components shall be improved or constructed as below:

Figure 4  13 Potential Lowland areas in Chao Phraya River Basin

Benefit

Benefit of the project is claimed from reduction community damages due to lowering of peak water levels along the river and communities protection system located in the Lowland area. Addition benefit come from reduction of unexpected flooding in the lowland while negative benefit is from the direct damage to the agricultural product in the Lowland.

Prospect and Impact

Socio-economic and environmental impact study have been carried out in 2 pilot project areas : 1) Chum Saeng – Kao Lieo Lowland in Nakhon Sawan Province and 2) Thung Phu Khao Thong – Bang Pahan Lowland in Ayutthaya and Ang Thong Provinces. The result reveal that Stake holder especially the people living in the Lowland willing to proceed and implement of Lowland projects with the following reasons:

• Compensation of damage in Lowland due to divert in flood water will be paid reasonably to the market price and quickly.
• People living in the Lowland gain the benefit from the flood protection and drainage system infrastructures development.
• People have the opportunity to participate in the project management.

However, people live in the Lowland worry in some aspects as follows:

• Retention level in the area will be higher than the specified.
• Retention period will be last too long.
• Compensation will not be paid.

Therefore, people have the following recommendations:

• Royal Irrigation Department and concerned agencies should built up the confidence to people by disseminate the information, news and facts periodically and continuously.
• Stake holder should participate in the project implementation every step from the project preparation, survey, detailed design, construction and project management.

Key Success Factors

Significant risks and factors effecting to the success of flood retarding area development are:

• The uncontrolled land use development in lowland as well as the effect of climate change on the hydrological and meteorological design conditions.
• Government policies and sanction facilitating to Lowland area development, such as the issue of regulations/laws/Acts and enforcement for damage compensation fund and land use plan of the Lowland.
• The concreteness and stability of management organizations
• Decision support systems and database used for management
• Procedure of public participation in every stage of development to draw out the recommendation from the people in the areas to adjust the development in compliance with their requirement until their acceptance is achieved.

2.3 Water Management  in the East of the Lower Chao Phraya River Basin

Physical Features:

The area of the East of Lower Chao Phraya irrigation, as shown in Figure 5, is  approximately 5,200 sq.km. It covers 7 provinces surrounded by the Chao Phraya river, the Pasak river, the Nakhon Nayok river, the Bang Pakong river, Gulf of Thailand and the King dike. Other features are as follows:

Figure 5 Flood barriers and canal network in the Eastern Lower Chao Phraya River Irrigation Area.

• On the North (the Pasak River), there is the Pasak Jolasid Dam located upstream. The dam can regulate to control maximum discharge of 800 cms. A 36 km-long dike is also equipped along the Pasak river. There is also the Raphiphat canal receiving water from the Pasak river of 120 cms at maximum before distributing it to 17 irrigation canals through the East of Lower Chao Phraya irrigation subprojects lining from the north to the south.
• On the West (Chao Phraya River and King dike), a 120 km-long dike is constructed to protect spillage of flood water into the irrigation area and to protect the inflow of water from the irrigation area to the city. Along the dike, regulators are constructed at the canal crossing. Pumps are equipped at some specific gates which can drain water to the Chao Phraya River with the maximum discharge capacity of 203 cms or 14.9 MCM per day.
• In the East (Nakhon Nayok and Bang Pakong rivers), there is a 145 km-long dike equipped with regulators and pumped which can drain water with the maximum discharge of 169 cms or 10.6 MCM per day.
• In the Lower part (Gulf of Thailand), there is a 39 km-long dike protecting sea water intrusion and high flood levels in the canals. There are also regulators equipped with pumps with the maximum discharge of 345 cms or 23.9 MCM per day. Moreover, there is the Suvarnabhumi drainage canal which can drain water out of the area of Suvarnabhumi Airport with the maximum discharge of 100 cms through the pump.
• For drainage within the east of the Lower Chao Phraya irrigation area, there are many canals lining along the north to the south and the east to the west with total length of 300 km. There are also regulators and pumps which pump water out along the east to the west with the maximum discharge of 156 cms.

Hydrological Features

Flooding in the east of Lower Chao Phraya irrigation area is due mainly from the local heavy storm, spillage from the high river discharge, and high tide. It is found that average annual rainfall in is 1,290 mm and more than 88.64% is in the rainy season (May-October). The area is also threaten from the overbank flow of high discharge from the both the Pasak river and the Chao Phraya river during August to October. While high tide is usually in the period of October to March. The most critical period is in October when all factors combine.

Strategic Water Operation Management

To alleviate flooding in the east of Lower Chao Phraya Irrigation area, water management shall be operated in follow strategies:

1. Water stored in the irrigation area shall divert to the Nakhon Nayok and Bang Pakong rivers as much as possible using the existing east-west canals. At the same time water levels in the both rivers shall be manipulated correspondingly by regulating discharge released of the Khun Dan Prakanchon Dam. Internal flow direction can be forced using existing regulators and pumping stations installed at the internal drainage canals.
2. Gates shall be operated to drain water out with optimum use of tide fluctuation while internal drainage shall be regulated correspondingly.
3. Monitoring stations shall be defined for flood warning and decision on  management operation.
4. Water level in the internal canals and ponds shall lower down to free the storage for the incoming storm of 200 mm of rainfall. These could be done by empty 50% of storage in the existing 112 MCM pond and lower down water levels in the canals and low lying area by 0.2 meter.
5. Water levels in various canals and flood volume shall be forecasted using the existing 48 rain gauge stations and tide prediction. Results shall be used to managing water in the irrigation area.
6. Since the project area covers several sub irrigation projects which are under the responsibility of various agencies, it is recommended to the set up joint working group consist of member of each agency. The working group shall work together to share and synchronize the operations.

3. Conclusion

Use of the low lying area as the flood retarding using is one of the measures on flood mitigation in the Chao Phraya Basin. The objective is to lower flood peak levels in the river or the local heavy rainfall. However, its’ effectiveness is limited to some specific design return period. To handle the flood with higher period of occurrence, other flood control measures shall be included.