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Selection of Design Floods in Southeast Asia Jian Liu *1)

The design flood criteria suitable for Southeast Asian nations are recommendedafter reviewing the design criteria and guideline of China, Japan, UK, USA and

ICOLD. The projects scale, the downstream hazard potential and dam typeshould be considered when the design floods are determined. PMF as a designflood is a suitable adoption for an embankment dam, but it is excessivelyconservative for a concrete dam. As case studies, the selection of design floodsfor the Kelai 2 Hydropower Project in Indonesia and the Namkok Hydropower Project in Myanmar are discussed.

IntroductionThe design floods for the dams and spillways in the Southeast Asian nations are determinedon the basis of the design standards of the countries outside the region, because they do not

have their own design criteria. In most cases, the design standards and guidelines of Australia,Canada, China, EU, Japan, Russia, USA and International Commission on Large Dams(ICOLD) are used without any modification, though the climate conditions in the monsoonregions are different from the countries whose criteria are used. Japan uses the maximumvalue between 200-year frequency flood and maximum experienced flood as the design floodfor concrete dams and 1.2 times the relevant value for the concrete dams for embankmentdams. All the other counties and organization mentioned early use the probable maximumflood (PMF) as the design floods for embankment dams. The design flood criteria of China,Japan and Russia for concrete dams are lower than the standards of the English-speakingnations that also use PMF as a design flood for concrete dams. Japan uses the lowest probableflood as a design flood in the world, but the dam failures in Japan due to low design floods

have not occurred until the present, and this means the low design flood would not absolutelyincrease the dam failure rate. Although there are a lot of arguments on PMF as a design floodregardless of dam type, for the embankment dam, PMF would be a better choice inconsideration of the structures weakness and high failure rate. However, the selection of PMFas the design flood for the concrete dams seems to be too conservative, taking the free boardeffects and high safety factors of concrete dams. The overall failure rate is considered asaround 1% and the annual failure risk for any dam is about 0.00001 on average. They have

been reducing with appearance and improvement of new investigation techniques, the widedissemination of knowledge on risks and increase of hydrological records. Actually, the damfailures mainly occurred before 1970, and most of them were small embankment dams builtin 1920-40s, when the hydrological data were limited and there was a lack of geological

knowledge. The failed concrete dams account for 3% of all damaged dams and almost noconcrete dams failed due to overtopping after 1970s (ICOLD, 1974; Lecornu, 1996).In practice, different countrys consultants often select different design floods for the same

project due to a lack of the unified standards. In most of the cases, PMF is chosen as a designflood because dam professionals do not want to be associated with dam failures which causedeath and property damage. This often results in unnecessary huge spillways and an increaseof project costs and duration. In order to find a suitable design flood criteria for the SoutheastAsian nations, most of which do not have their own dam design criteria, the design floodstandards of Great Britain, China, Japan, USA and ICOLD are reviewed and the selection of design floods for the Kelai 2 Hydropower Project in Indonesia and the Namkok Hydropower Project in Myanmar are discussed as case studies.

1) Department of River & Coastal Engineering, NEWJEC Inc.; e mail: liujn@osaka.newjec.co.jp

mailto:liujn@osaka.newjec.co.jpmailto:liujn@osaka.newjec.co.jp

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Design flood criteria in different countries and organizations

China China has 86000 dams with a total storage capacity of 560 billion m 3, and 90% of them areembankment dams. The large dams are up to 24136 in 1999, accounting for about 50% of the

large dams in the world (Pan & He, 2000). The large dam failure rate is estimated at 0.1% andalmost all are embankment dam due to piping, overtopping, design and operation mistakesand bad construction quality, but the damage is very serious. The most catastrophic damfailures occurred at the Banqiao dam and Shimantan dam in the Huai River basin on August 9,1975 in the Zhumadian Prefecture of Henan province in central China and the recent collapseis the Gouhou dam in August 1993 in Gonghe County of Hainan Tibet AutonomousPrefecture, Qinghai province in Northwest China. In August 1975, a typhoon passed throughthe whole of the region south of the Yellow River, and this led to a set of storms, whichdropped 1005mm of water in 24hr in the Huai River basin. After the storms, the 118mBanqiao Dam on the Ru River and the Shimantan dam on the Hong River collapsed due toovertopping as did other 60 small dams in the Huai River basin. Eleven million people wereseverely affected. The death toll estimates for these failures varied widely. Approximately26000 deaths occurred from drowning in the immediate aftermath of the dam collapses. Therewere as many as 230000 deaths if those who died of consequent health epidemics and famineare included. Because China did not establish its criteria, the two dams were all built in 1950saccording to the former Soviet Union criteria without any modification and the check designflood was designated as a 1000-year flood for the Banqiao dam, which was estimated at 530mm rainfall over a three day period, and 500-year flood for the Shimantan dam, which wasestimated at 480 mm rainfall over a three day period. The Banqiao Dam was originallydesigned to pass about 1742 m 3/s through its sluice gates and a spillway. The storage capacitywas set at 492 million m 3 with 375 million m 3 of the capacity reserved for flood storage. TheShimantan Dam had a capacity of 94.4 million m 3 with 70.4 million m 3 for flood storage. Itcan be found that the rainfalls used for calculating the check design floods of the two dams,which were determined on the basis of the maximum daily rainfall of 320mm are much lessthan the precipitation of 1005mm recorded in 24 hours in August 1975 (Dai, 1998). After thecatastrophe, a survey of historical floods has been emphasized, and PMF or 10000-year floodhas been using the check design flood for large embankment dams, and sometimes used for concrete dams until 1990. Since 1975, no dam has failed by overtopping because thedischarge capacities of all dam were checked and the new discharging structures for the damswith small releasing capacities were designed and constructed according to the revisedChinese criteria in which PMF was first used. The 71m high Gouhou concrete-faced rockfilldam (CFRD) was built in 1989 and collapsed by piping due to bad construction quality and

design mistakes on August 27, 1993. The dam failure, which is the first CFRD breach in theworld, caused 242 people dead, 330 injured, 2,932 houses collapsed and 90 ha. farmlandinundated. The property loss is up to US$22.7 million. The dam failure investigation indicatedthat the water level at the reservoir at the collapse (El. 3277.25) was 3.75m lower than thedam crest (El.3281) and the flood was smaller than the design flood with 500-year return

period and much smaller than the check design flood with 10000-year return period (Li et al.,1999). This dam failure resulted in that the CFRD guideline was heightened into the designcode and the construction specification on CFRD was developed.The Chinese design flood criteria were developed in 1964, and first revised in 1978 after thecatastrophe of the Banqiao and Shimantan dam failures and secondarily revised in 1990. Thedesign floods for dams and other relevant structures are determined by the project rank,

structure class and dam type. The water conservancy and hydropower projects in China areclassified into five different ranks in accordance with their scales, benefits and importance in

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national economy. The criteria for classifying project functions as specified in Chinese designcodes are listed in Table 1 (CEC, 2000). The various hydraulic structures of a project arefurther divided into five classes based on the rank of the project in which they work and their roles and importance in the project (Table 2). For example, the dam of a 1 GW hydropower

project that is classified as Rank 1 will be designated as a Class 1 structure, while a bridge on

an access road for the same project will be designated as a Class 3 structure, and thecofferdams for river diversion will be set as a Class 4 structure.

Table 1 Classification of Water Conservancy and Hydropower Projects in China

Flood prevention Water loggingcontrol IrrigationWater supply

Water power

Rank of project

Storagecapacity of reservoir (10 6 m3)

Cities &industrialregions

Farmland(10 3 ha)

Draining water logged area

(10 3 ha)

Irrigationarea

(10 3 ha)

Cities &mines

Installedcapacity(MW)

I > 1,000 Veryimportant > 333 > 133.3 > 100Very

important > 750

II 1,000

100Important 333 67 133.3 - 40 100 - 33.3 Important 750 - 250

III 100 10 Moderatelyimportant 67 20 40 - 10 33.3 - 3.3Moderatelyimportant 250 - 25

IV 10 - 1.0 Lessimportant 20 - 3.3 10 - 2.0 3.3 - 0.3Less

important 25 - 0.5

V < 1.0 < 3.3 < 2.0 < 0.3 < 0.5 Notes: 1. The storage capacity of reservoir means the storage of reservoir below check flood level.

2. The irrigation and waterlogged areas refer to design areas.3. The rank of tide prevention projects may be defined referring to the stipulations for flood

prevention. Where disasters of tide are very serious, the rank may be raised properly.4. The importance of water supply works shall be defined in accordance with their scale, economic

and social benefits.

Table 2 Classification of hydraulic structures in ChinaGrade of permanent structuresRank of projects

Main structures Less important oneGrade of temporary

structuresI 1 3 4II 2 3 4III 3 4 5IV 4 5 5V 5 5 -

Notes: 1) Permanent structures are the structures used for operation of the project, and are divided into twocategories in accordance with their importance: Main structures that will cause a catastrophe indownstream areas in case of failure or seriously damage the function of project, such as dams, sluices,

pump station and hydropower houses. Less important structures that will not cause a catastrophe indownstream areas in case of failure and not cause serious influence to project benefits, such asretaining walls, diversion walls, and bank-protection works.

2) The temporary structures are the structures used during construction, such as diversion structures,cofferdams etc.

3) For projects of rank II to V and temporary structures, the grade of their structures may be raised or lowered in the following situations through evaluation:a) The location of projects is of vital importance and failure of structures may cause a serious

catastrophe. The grade of the structures may be raised by one grade. b) Where the engineering geological conditions of the hydraulic structure are very complicated, or

new type of structures are used. The grade of the structures may be raised by one grade.c) The grade of temporary hydraulic structures, if their failure will cause serious catastrophe or

influence seriously the construction program the grade may be raised by one or two.

d) For the projects which will not cause considerable influence after failure, the grade of their structures may be lowered properly through evaluation.

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The hydraulic structures of different classes are designed to fulfill different flood handlingrequirements, load conditions, and so on. Two different conditions are usually considered,namely: the normal operating condition (design condition) and the extraordinary operatingcondition (check condition). For permanent structures defined as the ones used for operationof the project, the design flood (normal condition) for different classes of structure is

recommended as shown in Table 3. For structures where failure would cause heavy loss of lifeand property, PMF should be considered as the extraordinary operation condition (check condition) for embankment dams, and 10000-year flood should be adopted for concrete dams(MWREP, 1978; MWR & MOE, 1990). For less important structures, the floods to beconsidered as extraordinary operation conditions are recommended as shown in Table 4. Theextraordinary operating condition (check condition) is a provision for extremely unfavorableconditions that might occur only very rarely. In some cases the safety factor of the dam can

be reduced. For example, the height of freeboard can be reduced by 0.2 m to 0.8 m for someclasses of structure. The design flood and check design flood criteria for powerhouses andnon-damming structures are shown in Table 5, and it can be found that the values are lower than those for damming structures because the powerhouses and non-damming structures

have less damage in the event of failure. For the powerhouse that is a portion of the dammingstructure such as the Gezhouba dam on the Yangtze River, the design flood and check designflood are selected by the standards for the dam. The design floods for temporary structuresshould be adopted in accordance with Table 6, based on the characteristics of the project,diversion scheme, construction period, utilization requirement, inundation effects andhydrological conditions of the river. The design floods and check design floods of somerepresentative projects in China are shown in Table7 (Cheng, 1989).

Table 3 Design flood criteria for permanent structuresClass 1 2 3 4 5

Return period of flood 500 100 50 30 20

Table 4 Check design flood criteria for permanent structuresClass 1 2 3 4 5

Embankment dams 10,000 or PMF 2,000 1,000 500 200Concrete dams, etc. 5,000 1,000 500 200 100

Return period

Notes: 1) The standards of powerhouse and irrigation structures (Classes 4 and 5) may be lowered according toactual situations.

2) For Class 1 embankment dam, PMF should be considered if its failure will cause catastrophe indownstream area, and for Classes 2 to 4 embankment dams, the check design floods may be raised byone grade.

3) For concrete dams that cause seriously damage in case of overtopping, 10000-yr flood may beadopted as check design flood after examination and approval by competent authorities.

4) For low water head structures and the structures that do not cause seriously damage, check designflood may be lowered by one grade after examination and approval by competent authorities.

Table 5 Design flood and Check design flood criteria for powerhouse and non-damming structuresClass 1 2 3 4 5Design flood 100 50 30 20 10Check design flood 1,000 500 200 100 50

Return period

Table 6 Design flood criteria for temporary structuresType of structure 2 3 4 5 ClassEmbankment >50 50-30 30-20 20-10

Concrete and Masonry >20 20-10 10-5 5-3

Return

period

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Table 7 Design flood and check design floods of representative hydropower projects in ChinaProjects Drainage

AreaMax. recorded flood Max. Historic flood Design flood &

probability (P)Check flood &

probability (P)km2 m3/s Date m 3/s Date m 3/s P (%) m 3/s P (%)

Three Gorges 1,000,000 71,100 1896 110,000 1870 98,800 0.1 120,000 0.01Gezhouba 1,000,000 71,100 1896 110,000 1870 86,000 110,000 1.0

Sanmenxia 688,399 22,000 1933 36,000 1843 36,000 0.1 Xiaolangdi 694,155 22,000 1933 36,000 1843 40,000 0.1 52,300 0.01Liujiaxia 181,766 5,640 1946 7,500 1940 8,860 0.1 10,800 0.01Dahua 112,200 18,700 1968 21,800 1872 23,200 1.0 31,000 0.1Danjiangkou 95,200 50,000 1935 61,000 1583 64,900 0.1 82,300 0.01Wujiangxi 83,800 30,500 1935 41,700 1766 57,700 0.1 69,300 0.01Fengman 42,500 19,600 1953 15,300 1856 28,000 0.1 36,200 0.01Panjiakou 33,700 18,800 1962 24,400 1883 40,400 0.1 63,000 PMFWujiangdu 27,800 11,400 1964 15,000 18?? 19,200 0.2 24,400 0.02Beishan 19,000 11,800 1960 17,600 0.2 24,200 0.02Fengtan 17,500 16,900 1963 17,900 1927 20,400 0.1 26,600 0.01Xinanjiang 10,422 20,000 1942 22,900 1682 27,600 0.1 41,280 0.01Fengshuba 5,150 7,660 1935 11,100 0.1 13,200 0.02

Niululin 1,236 4,420 1950 8,600 1.0 11,400 0.1

JapanIn Japan, the inflow design floods for dams are stipulated in the Structural Standards for River Protective Facilities (Cabinet Order), which was drawn up on the basis of River Law.According to the standards, when the dam is constructed or reconstructed, the inflow designfloods for a concrete dam must be taken on the largest value among the following threedischarges: (1) 200-year flood at the damsite; (2) maximum experienced flood discharge atthe damsite and (3) maximum flood discharge that can be expected at the damsite based on

the maximum experienced flood discharge in the basins with similar hydrological conditionsor climate. For an embankment dam, the design flood should be specified to be 1.2 times of the relevant values for a concrete dam (JICE, 2000). The return period of the design flood for an embankment dam is actually equivalent to 1000 years or more.

United KingdomIn the United Kingdom, dam safety is entrusted to individual members of a statutory panel of engineers determined by the government to be qualified to design and inspect impoundments.The panel engineer is personally responsible for the safety of the dam he is hired to supervise,and no mandatory standards are imposed by the government. The design floods are generallydetermined by the guidelines published by the Institution of Civil Engineers, London in 1978.

Depending on the categorization (e.g. Categories A, B, C and D), the relevant design floodsare selected from PMF, 0.5PMF, 0.3PMF, 0.2PMF and/or the flood with a recurrence intervalof 10000, 1000, 150 years respectively (Table 8).

USAIn most cases, the design floods in USA are PMF. In case of adoption of an inflow designflood less than PMF, the owner, agency, or organization in charge of construction of the

project would be responsible for a dam break. Therefore, most engineers do not want anylevel of risk. The US Army Corps of Engineers (USACE) recommended that the design floodsshould be adopted by the height of a dam and storage impounded and, also, by hazard

potentials in the downstream areas in the event of a failure of the dam. The US Bureau of

Reclamation (USBR) uses the inflow design flood, which is PMF in most instances, as thedesign flood. The design flood criteria of USACE and USBR are shown in Tables 9 and 10

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(Committee on Safety Criteria for Dams, 1985).

Table 8 Reservoir flood and wave standards by dam category, United Kingdom (ICE, 1978)Dam design flood inflowCategory Initial reservoir

condition General standard Minimum standard if rare overtopping is

tolerable

Concurrent wind speed andminimum wave surchargeallowance

A SpillingLong-termaverage dailyinflow

PMF 0.5PMF or 10000-yr flood (take larger)

B Just full (i.e.no spill)

0.5PMF or 10000-yr flood(take larger)

0.3PMF or 1000-yr flood (take larger)

Winter: maximum hourly windonce in 10yr Summer: average annualmaximum hourly windWave surcharge allowance notless than 0.6m

C Just full (i.e.no spill)

0.3PMF or 1000-yr flood(take larger)

0.2PMF or 150-yr flood (take larger)

Average annual maximumhourly windWave surcharge allowance notless than 0.4m

D SpillingLong-termaverage dailyinflow

0.2PMF or 150-yr flood

Not applicable Average annual maximumhourly windWave surcharge allowance notless than 0.3m

Notes: 1) Category A= reservoirs where a breach will endanger lives in a community; Category B= reservoirswhere a breach (i) may endanger lives not in a community (ii) will results in extensive damage; Category C=reservoirs where a breach will pose negligible risk to life and cause limited damage; and Category D= specialcases where no loss of life can be foreseen as a results of a breach and very limited additional flood damagewill be caused. 2) For the reservoirs with Categories B and C, alternative standards of the dam design floodinflow, if economic study is warranted, are the flood with probability that minimizes spillway plus damagecosts, inflow not to be less than minimum standard but may exceed general standard. 3) Where reservoir control

procedure requires and discharge capacities permit, operation at or below specified levels defined throughoutthe year that may be adopted providing they are specified in the certificates or reports for the dam. Where a

proportion of PMF is specified, it is intended that the PMF hydrograph should be computed and then allordinates be multiplied by 0.5, 0.3, or 0.2 as indicated.

Table 9 Design flood criteria of the US Army Corps of EngineersSize classification

Category Reservoir capacity (hm 3) Height of dam (m)Small From 0.62 to 1.23 From 7.6 to 12.2Intermediate From 1.23 to 61.5 From 12.2 to 30.5Large 61.5 30.5

Hazard potential classificationCategory Loss of life (extent of development) Economic lossLow None expected (no permanent structures for

human habitation)Minimal (undeveloped to occasionalstructures or agriculture)

Significant Few (no urban developments and no more thana small number of inhabitable structures)

Appreciate (notable agriculture, industry or structures)

High More than few Excessive (extensive community, industryor agriculture)

Recommended safety standardsHazard Size Safety standardLow Small

IntermediateLarge

50-yr to 100yr frequency100-yr to 1/2 PMF1/2 PMF to PMF

Significant SmallIntermediateLarge

100-yr to 1/2 PMF1/2 PMF to PMFPMF

High SmallIntermediateLarge

1/2 PMF to PMFPMFPMF

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Table 10 Design flood criteria of US Bureau of ReclamationHazard Size Safety standardHigh Small

IntermediateLarge

PMFPMFPMF

Notes: Design floods for dams with significant and low hazard potential are not specifically defined.

ICOLDAs a general rule, ICOLD recommends that design floods shall be PMF (Table 11) and thecapacity of gated spillways shall be sufficient to discharge the full design flood without takinginto account the dampening effect resulting from flood routing through the reservoir. But for the ungated spillway of a flood control dam, the retention effects may be considered whencalculating the design outflow flood (ICOLD, 1984; ICOLD, 1992).

Table 11 Design flood guidelines of ICOLD

Hazard Size Safety standardHigh Small

IntermediateLarge

-PMFPMF

Significant SmallIntermediateLarge

-PMFPMF

Case study

Namkok hydropower project in Myanmar The Namkok hydropower project on the Namkok River, a tributary of the Mekong River islocated in the famous Golden Triangular Area in Myanmar. The damsite is at Hwai Sai Lonevillage, Shan state near the border between Myanmar and Thailand. In order to reduce povertyin the area and promote access to electricity, the Myanmar government and the consortiumthat consists of the corporations from Thailand and Japan signed an agreement on thedevelopment of the Namkok project by BOT in 1990s. This project is also the first BOThydropower project in Myanmar. The Developer is responsible for the projects investigation,design, construction and operation. The consultants from Japan, United Kingdom andThailand carried out the feasibility study on this project. The main parameters related to thedesign flood are shown in Table 12, and PMP and the probable flood discharges for differentreturn periods are listed in Table 13. In order to select a suitable design flood for the project,the Engineer reviewed the criteria of Japan, China, UK and USACE because Myanmar andThailand did not have their own design flood criteria. The Engineer recommended the10000-year flood or PMF of 1550m 3/s be the design flood in consideration of the specialconditions of the project (e.g. bad security situation in the project area and minimization of the influence of the dam failure to the downstream area). The PMF of 1550m 3/s was obtained

by the Tank model on the basis of the measured flood data and the probable maximum precipitation (PMP) in the reservoir area. The parameters of the Tank model were verified bythe measured discharges at the Ban Tha Ton gauging station and the recorded rainfalls for about 30 years. The figure of 1550m 3/s is smaller than the PMF of 3000m 3/s, obtained by theCreager Curve in Thailand during the pre-feasibility study. The larger PMF was finally

determined as the design flood by the Developer from the safest point of view. Moreover, if the criteria of China or UK were adopted, the relevant design flood would be 980m 3/s or

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1200m 3/s, as shown in Table 14 (NEWJEC et al., 1998). This indicates different criteria andcalculated methods for PMF would give different design floods and during the selection of adesign flood, a lot of people prefer to take larger figures to avoid any risk.

Table 12 Main characteristics of Namkok Hydropower ProjectItem DescriptionRiver Namkok River Damsite Hwai Sai Lone, Shan state, Myanmar Catchment area at damsite 2953km 2 Annual average rainfall 1572mmAverage discharge at damsite 68.5m 3/s

Nearest discharge station Ban Tha Ton, Chang Rai, Thailand (CA=2980km 2)Inflow design flood (PMF) 3000m 3/sUngated spillway capacity(120m wide 5m high)

2700m 3/s

Reservoir storage capacity 355 million m 3 (active); 730million m 3 (gross)Type of dam RCCLength of dam crest 538mElevation of dam crest El. 535m (Non-overflow portion)

El. 530m (Overflow portion)Height of dam 63mFlood water level (at PMF) El. 534.99mHigh water level El. 530mLow water level El. 518mInstalled capacity 55MW

Table 13 Probable basin rainfalls and floods at Namkok damsiteReturn period (yr) 3-day basin Rainfall

(mm)Discharge by recorded data(m3/s)

Discharge calculated by Tank Model (m 3/s)

50 161 689 770

100 168 755 804200 175 820 839500 185 906 8841000 192 971 91410000 216 1187 1023PMF 328 (PMP) - 1522Maximum record 551 on Aug.25, 1972PMF during pre-feasibility study obtained by Creager Curve in Thailand 3000

Table 14 Determination of a design flood for Namkok projectCriteria Return period Discharge (m 3/s) RemarksJapan 200yr 820

China 1000yr 980 Check floodUK 0.5PMF/10000yr 1200 Recommended by theEngineer

USACE PMF 1550Design flood 3000 Determined by the

Developer

Kelai 2 Hydroelectric Power Project The Kelai 2 hydroelectric Power Project on the Kelai River is located in East Kalimantan

province, Indonesia. The average annual rainfall over the project area of 4478km 2 is about2800mm, and the average annual discharge at the damsite is 259m 3/s. The height of theembankment dam is 65m and the gross reservoir storage capacity is 1175 million m 3. Themain parameters of the Kelai 2 project are shown in Table 15, and the PMP and PMF arelisted in Table 16. Because there are not design flood criteria in Indonesia, the PMF calculated

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on the PMP with a 3-day duration was selected as the design flood for the spillway accordingto the criteria of USBR and USACE, taking the Monsoon climate and weakness of theembankment structure into account (JICA, 1998). It should be noted that the peak value of thePMF was obtained from the ECAFE Curve developed by the United Nations in1970s for Southeast Asia, because the peak discharges at the gauging stations were not recorded.

Table 15 Salient features of Kelai 2 projectItem DescriptionRiver Kelai River Damsite Muaralasan, East Kalimantan province, IndonesiaCatchment area at damsite 4478km 2 Annual average rainfall 2800mmAverage discharge at damsite 259m 3/s

Nearest discharge station Lesan Dayak (CA=4542km2)Inflow design flood (PMF) 6200m 3/sGated spillway capacity(four bays, each 12m wide 12m high)

4400m 3/s

Reservoir storage capacity 1175 million m 3 (gross); 567million m 3 (active)Type of dam Zoned rockfill dam will center coreLength of dam crest 55.5mElevation of dam crest El. 90mHeight of dam 65mFlood water level (at PMF) El. 87.8mHigh water level El. 85.0mLow water level El. 72.5mInstalled capacity 115MW

Table 16 Probable basin rainfalls and floods at Kelai 2 project siteReturn period (yr) 1-day basin rainfall (mm) Daily discharge (m 3/s) Remarks50 80 1574100 87 1715200 93 1835500 101 19961000 108 214010000 129 2578 (peak=3083)PMF (1day) 228 (PMP) 4641 (peak=5550)PMF (2day) 274 (PMP) 5158 (peak=6169)PMF (3day) 291 (PMP) 5217 (peak=6200) Design flood

ConclusionsThe design flood criteria and guidelines of China, Japan, UK, USA and ICOLD are reviewed

and almost all counties and organizations except Japan have similar design flood criteria for embankment dams. The criteria for concrete dams are different and the criteria of Japan andChina are generally are not so stringent as compared with those of UK, USA and ICOLD inwhich the dam type is not taken as a classification factor. In spite of the failure rate of concrete dams being much less than that of embankment dams, the criteria of China and Japanin which the dams are classified into concrete and embankment dams seem to be morereasonable. From a safety point of view, PMF as a design flood is a suitable adoption for anembankment dam, but it is excessively conservative for a concrete dam. From a conservative

point of view, the US criteria and ICOLD guidelines for embankment dams would be suitablefor determination of the design floods in the Southeast Asian Nations. For concrete dams, theChinese and UK standards would be better from an economic, hydrological, geographical and

technical point of view. The Chinese criteria are especially recommended in consideration of Southeast Asia and South China having similar meteorological and geological conditions.

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The selection of design floods for the Kelai 2 Hydropower Project in Indonesia and the Namkok Hydropower Project in Myanmar are discussed, and PMF was adopted as the designfloods for the two projects. For the Namkok project, PMF is too conservative and should bereevaluated during the detail design phase. However, it is a suitable choice for the Kelai 2

project, though the value of PMF should be checked after the measured peak discharges are

available.

Acknowledgements The author is grateful to Professor Ellis of Sonoda Womens University for reviewing this

paper.

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National Academy Press.Dai Q. 1998. The River Dragon Has Come! M.E. Sharpe, New York.Department of Standardization, China Electricity Council (CEC). 2000. GB50199-1994Unified design standard for reliability of hydraulic engineering structure. The Standardscompilation of water power in China, China electric Power Press. pp.1-33.International Commission of Large Dams (ICOLD). 1947. Lessons from dam incidents.ICOLD. 1987. Dam safety guidelines. Bulletin 59. pp.45-49.ICOLD. 1992. Selection of design flood. Bulletin 82.Japan International cooperation Agency (JICA). 1998. Feasibility study on the developmentof Kelai 2 Hydro Electric Power Project, final report (Vol.2: Main report).Japan Institute of Construction Engineering (JICE). 2000. Description of the Structuralstandards for river protective facilities. Sankaido (in Japanese)Lecornu J.1996. Dam safety in 1996, from engineers duty to risk management. ICOLD website, http://www.icold-cigb.org/hydropower.htm Li J., Li L. and Shen J. 1999. Breach Process, Failure mechanism and lessons of Gouhouconcrete face gravel dam. Proceeding of International Conference on Dam Safety andMonitoring. pp.503-508.Ministry of Water Resources and Electric Power (MWREP). 1978. Grade classification anddesign standard for water conservancy and hydropower projects (mountain and hilly regions),SDJ12-78 (in Chinese).Ministry of Water Resources (MWR) and Ministry of Energy (MOE). 1990. Supplementarystipulation on grade classification and design standard for water conservancy and hydropower

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