CI-2013-0908105-1

FES Final Year Project Template

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LANGAT DAM SAFETY STUDY: OVERTOPPING PREVENTIONLEE CHONG YOUA project report submitted in partial fulfilment of the

requirements for the award of the degree of

Bachelor (Hons.) of Civil Engineering

Faculty of Engineering and Science

Universiti Tunku Abdul Rahman

April 2013DECLARATION

I hereby declare that this project report is based on my original work except for citations and quotations which have been duly acknowledged. I also declare that it has not been previously and concurrently submitted for any other degree or award at UTAR or other institutions.Signature:_________________________

Name:____LEE CHONG YOU____

ID No.:_______0908105___________

Date :____ 5 April 2013 ______APPROVAL FOR SUBMISSIONI certify that this project report entitled LANGAT DAM SAFETY STUDY: OVERTOPPING PREVENTION was prepared by LEE CHONG YOU has met the required standard for submission in partial fulfilment of the requirements for the award of Bachelor of Engineering (Hons.) Civil Engineering at Universiti Tunku Abdul Rahman.Approved by,

Signature: _________________________

Supervisor: Ir. Pan Wang FookDate : _________________________

The copyright of this report belongs to the author under the terms of the copyright Act 1987 as qualified by Intellectual Property Policy of University Tunku Abdul Rahman. Due acknowledgement shall always be made of the use of any material contained in, or derived from, this report. 2013, Lee Chong You. All right reserved.Specially dedicated to

my beloved grandmother, mother and father

ACKNOWLEDGEMENTS

I would like to thank everyone who had contributed to the successful completion of this project. I would like to express my gratitude to my lectures, research supervisor and advisor of Universiti Tunku Abdul Rahman for their invaluable advice, guidance and enormous patience throughout the development of the research.

In addition, I would also like to express my gratitude to my loving parent for their love and greatest support to me during my toughest time. Besides, I would like to express my thousand thanks to my friends who had helped and given me encouragement.LANGAT DAM SAFETY STUDY: OVERTOPPING PREVENTIONABSTRACT

A hydrological dam safety assessment was carried out for Langat dam (CA= 41 km2) by evaluating the performance of the bellmouth spillway in light of an extreme meteorological event of the PMP/PMF magnitude. It is important that the flood rise does not exceed or overtop the embankment dam crest level.

Langat dam (CA= 41 km2) is a small catchment regulating water supply embankment dam that supply raw waters to Langat Mile 10 Water Treatment Plant (WTP) downstream on the main stem of Sg. Langat. It is one of part of a parallel reservoir operation in Sg. Semenyih basin. This study adopts inland type of PMPs as derived previously by SMHB (2012). A catchment routing procedure is used to translate the PMPs to PMFs for 1- to 120-hour duration. The results of the PMPs/PMFs are comparable to the Creager type of catchment area-PMP relationship of various dams in Malaysia.

A conventional reservoir routing procedure by modified Puls technique is then carried out for all PMP/PMF durations, i.e. 1- to 120-hour. In general, the flood rises for all durations are marginally lower than the ECL, +223.8 m msl. It is therefore concluded that Langat dam (CA= 41 km2) with its ample surcharge capacity is safe from the onslaught of a PMP/PMF event. However, the provision of wave run-up, normally an additional 1 m or so free board is no longer available. Therefore it is recommended that a parapet wall of 1.0 m in height can be installed along the dam crest on the water edge could be of help to mitigate simultaneous PMP/PMF event with higher wave run-up. TABLE OF CONTENTS

iiDECLARATION

iiiAPPROVAL FOR SUBMISSION

viACKNOWLEDGEMENTS

viiABSTRACT

viiiTABLE OF CONTENTS

xLIST OF TABLES

xiiLIST OF FIGURES

xvLIST OF SYMBOLS / ABBREVIATIONS

CHAPTER

171INTRODUCTION

171.1Background

191.2Description of the Project

241.3Objectives

252LITERATURE REVIEW

252.1Problem Statement

292.2Tasks and Assignments

303METHODOLOGY

303.1Methodology of Hydrological Dam Safety Assessment

333.2Pmp Review and Study

343.2.1Rationale of PMP

373.2.1.1Hydrometeorological Approach

453.2.1.2Hershfield Technique

47Data Requirement for Hershfield Type PMP

513.2.2Adopted Pmp Convention By Smhb/B&P

743.2.3Areal Reduction Factor (ARF)

753.3Probable Maximum Flood

753.3.1Introduction

783.3.2Hp 11 Hydrological Procedure (Taylor And Toh, 1976)

803.3.3Rorb Win Model Description

863.3.4Comparison of PMFs

893.4Reservoir Routing

903.4.1Basic of Reservoir Routing Equation

923.4.2Modified Puls Or Storage Indication Routing Method

943.4.3Spillway Configuration

954RESULTS AND DISCUSSIONS

954.1PMP/PMF Catchment Routing

994.2Reservoir Routing

1085CONCLUSION

1085.1Conclusion

109REFERENCES

112APPENDICES

LIST OF TABLES

TABLETITLEPAGE

25Table 2.1: Partial List of Dam Failure and Main Reasons

41Table 3.2: 5-Day Depth Area Curve/Table for 1986 Storm in East Coast Peninsular Malaysia November 1986

42Table 3.3: 5-Day Depth Area Curve/Table for 1986 Storm in East Coast Peninsular Malaysia November 1986

51Table 3.4: Coastal and Inland PMP (Short- and Long-Duration) adopted by SMHB

54Table 3.5: Recorded Rainfall (NAHRIM 2008)

57Table 3.6: Comparison of PMP of Coastal and Inland PMP Values

59Table 3.7: Coastal and Inland PMPs (Long-Duration) adopted by SMHB

60Table 3.8: World Highest Precipitation

67Table 3.9: Temporal Storm Pattern: Fraction

68Table 3.10: Temporal Storm Pattern: Inland PMPs for various Durations

69Table 3.11: Temporal Storm Pattern: Fraction

87Table 3.12: Creager Type Curve: Catchment Area Versus Peak PMPs/PMFs For Various Dams in Malaysia

97Table 4.1: Creager Type Curve: Catchment Area Versus Peak PMPs/PMFs For Various Dams in Malaysia

98Table 4.2: Results of PMP/PMF Reservoir Routing: 1- to 120-hour Duration

LIST OF FIGURES

FIGURETITLEPAGE

19Figure 1.1: Sungai Langat Basin with Semenyih and Langat Dams 1/5



20Figure 1.4: Dams : Sungai Langat Basin with Semenyih and Langat Dams 4/5


22Figure 1.6: Morning Glory or Bell mouth Spillway of Langat dam

28Figure 2.1: PMP/PMF Routing and Reservoir Routing Flow Diagram in Dam Safety Assessment Undertaking

39Figure 3.1: 1-Day Depth Duration Area DAD Curve for Storms in East Coast Peninsular Malaysia after maximization and transposition

40Figure 3.2: 5-Day Depth Area Curve for 1986 Storm in East Coast Peninsular Malaysia

41Figure 3.3: 5-Day Depth Area Curve for 1986 Storm in East Coast Peninsular Malaysia

48Figure 3.4: Selangor PMP: 1-day (Desa, Noriah, and Rakhecha, 2001)

49Figure 3.5: Johor PMP: 1-day (Desa And Rakhecha, 2007)

61Figure 3.6: World Highest Precipitation: Depth Vs Duration

63Figure 3.7: Peninsular Malaysia PMP: 1-day

70Figure 3.8: Temporal Storm Pattern (west coast 3 hours): HP No: 1 (1982)





72Figure 3.13: Temporal Storm Pattern: Bell Shape Curve 3-, 12-, and 24- Hour

73Figure 3.14 Areal Reduction Factor: ARF (NWS, USA)

80Figure 3.15: Representation of sub catchment in RORB model

83Figure 3.16: Regionalized k-catchment area relationship

86Figure 3.17: PMF-Catchment Area Creager Curve: Malaysia Dam Inflows

88Figure 3.18: Discharge Hydrograph Routing Effects

92Figure 3.19: Reservoir Storage Routing Indicator

95Figure 4.1: PMP/PMF Catchment Routing: 1- to 120-hour

96Figure 4.2: Creager Type Catchment Area-PMF Relationship

99Figure 4.3: Langat Dam/Reservoir Routing: 1 hour duration








LIST OF SYMBOLS / ABBREVIATIONS

Xt

Rainfall for return period t, mm

Sn

Standard deviation of a maximum annual seriesXm

PMP values for any duration, mmKm

Frequency factor attributed to Hershfield, normally 15 is adoptedA

catchment area, km2I

Inflow, m3/s

Q

Outflow/ discharge, m3/s

S

Storage, m3S1

Storage in the reservoir at time step number t, m3S2

storage in the reservoir at time step number t + 1, m3m

Fitted parameterk

RORB calibrated parameter, ND

BCM

Billion Cubic Meter

CA

Catchment area, km2DAD

Depth Area Duration

ECL

Embankment Crest Level, m

FSL

Full Service Level, m

HTC

Humid Tropic Centre

ICUH

International Conference of Urban Hydrology

IDF

inflow design flood

JPS

Jabatan Pengairan dan Saliran

MMS

Modular Modelling System

MSMA Manual Saliran Mesra Alam

MCM

Million Cubic Meter

NWRS

National Water Resources Strategy

NWS

National Weather Service

ODEs

Ordinary Differentia Equations

PMF

Probable maximum flood

SDF

Spillway design flood

SDW

Serangoon Sewage Disposal Work

ABM/BOM Australia Bureau of Meteorology

ACE

area capacity elevation

ARF

Areal Reduction Factor

B&P

Binnie and Partners

1 INTRODUCTION

1.1 BackgroundA dam is defined as an artificial barrier together with appurtenant works constructed for the purpose of holding water or any other liquid material. Dam is normally located in the upper part/portion of a watershed that draining waters to the outlet of interest. It blocks almost the entire width of the river cross section by placing a monolithic and heavy man-made structure at this site. They can be built from many different materials, including earth, rock, tailings from mining or milling, concrete, masonry, steel, timber, miscellaneous materials (such as plastic or rubber) and any combination of these materials.In Malaysia, the most common type of dam are embankment earth fill dam, their construction is principally from required excavation using the available materials from the construction. Earth fill dams typically have a water-impermeable clay core, and a water cut-off wall from their base to bedrock to prevent underground seepage. During construction, the stream or river will be diverted either through the dam-site by means of a conduit, or around it by means of a tunnel. Normally a earth fill dam will built with some supplementary structures as spillways for discharging water from behind the dam. If sufficient spillway capacity is not provided, an earth fill dam may be damaged or even destroyed by the erosive water flowing over its crest. Unless special precautions are taken, such dams are also subject to serious damage or even failure, due to water seepage.

A dam/reservoir can serves many beneficial purposes such as providing water for Irrigation, Hydro-power, Water-supply, Flood Control, Navigation, Fishing and Recreation. Dams may be built to meet the one of the above purposes or they may be constructed fulfilling more than one. For dam that serves more than one purpose is called Multipurpose Dam. Even through dams and reservoirs serve a number of different functions, but most of the dams in Malaysia are use for water supply purpose. For a water supply purpose dam, in normal day, water will stored in reservoir until the time when water supply is needed, the reservoir operator will releases waters through series of outlets/valves to the downstream for augmenting low flow regime in the river. The released waters are then diverted for beneficial uses further downstream. The reservoir schemes are operated based on three modes of operation, they are namely, (1) direct supply, (2) regulating reservoir, and (3) pump storage scheme.Another major use of dams is power generation as hydroelectric. Hydropower development contributes roughly 10% of the global energy sector. It is dubbed mostly as renewable and yet cleans and low carbon emission technology with non consumptive usage of precious water resources. There are three (3) types of reservoir operations for power generation, namely, (1) run-of-river, (2) reservoir storage, and (3) peaking power mode of operation. Virtually no or minimum storage created by the low level/elevation intake structure is required for run-of-river type of power production. The energy is only attributed to the magnitude of runoff that passing through the turbine chamber at the intake. A low weir or head intake is sufficed for this type of simple configuration. To increase the head in the power equation, the waters could be diverted further downstream to the power or turbine house. This type of arrangement is suitable for steep topography and mountainous terrain.

Similar to water supply with option of reservoir storage facility, the flow fluctuation is moderated by the storage reservoir at the upper catchment of a watershed. Not only excess waters during high flow can be reserved for latter days power generation but it is also providing adequate hydraulic head for the same purpose.1.2 Description of the ProjectThe Sungai Langat drains a catchment of some 1,240 km2 at the stream flow gauging station at Dengkil (downstream of its confluence with the Sungai Semenyih) and around 1,815 km2 at the mouth of the estuary.

The Sg. Langat Basin forms the southern boundary of the State of Selangor and partially intrudes into the neighbouring State of Negeri Sembilan. The upper catchment comprises generally rugged mountain terrain with multiple land use classification. The lower catchment of the Sungai Langat basin is low lying, swampy land with some disused mining land.

The Langat dam was one of the first major water supply reservoir schemes in the state of Selangor and subsequently Kuala Lumpur when the capital was upgraded to federal territory status. The dam was commissioned in 1979 and in the vicinity on another tributary of Sungai Langat, the Semenyih dam in 1984 located in the upper reaches of the catchments, they serve as regulating reservoirs to augment flows at the intakes of the downstream treatment works during periods of low flow. Their primary purpose is therefore water supply. An augmentation scheme for the Sg. Semenyih was recently undertaken to provide an additional raw water supply (70 Mld) at the existing Semenyih WTP from a series of nearby abandoned mining ponds. The scheme is mainly for emergency use, but can also augment low flows at the Semenyih WTP.

To improve operating flexibility of the two major WTPs in the basin, that is Langat Mile 10 and Semenyih WTPs, an interconnected raw water pumping transfer from the Sg. Semenyih to the Sungai Lui , an upstream tributary of the Sungai Langat, was also constructed. Figures 1.3 to 1.7 shows the location, birdeye view and schematic of the dam body.

Figure 1.1: Sungai Langat Basin with Semenyih and Langat Dams 1/5Location Map [Source: Lembaga Urusan Air Sungai Selangor (LUAS)]

Figure 1.2: Sungai Langat Basin with Semenyih and Langat Dams 2/5Contour map

Figure 1.3: Sungai Langat Basin with Semenyih and Langat Dams 3/5 (source: www.syabas.com.my)

Figure 1.4: Dams : Sungai Langat Basin with Semenyih and Langat Dams 4/5Schematic diagram of Langat Dam (Source: www.syabas.com.my )

Figure 1.5: Sungai Langat Basin with Semenyih and Langat Dams 5/5

Google earth mapLangat dam/reservoir (CA= 41 km2) is located in the upper catchment of the Sg. Langat basin at river mile 24. The reservoir drains a catchment area of about 41 km2 on the eastern slopes of the central mountain range. The reliable yield based on conjunctive operation of the reservoir and intake downstream at Mile 10 (with a combined catchment area of 295 km2) was estimated in the design report (Binnie dan Rakan; 1976) to be 387 Mld. This allowed for a compensation release at the Mile 10 intake of 90 Mld. The gross reservoir storage is 35.4 million m3, of which 1.3 million m3 is allocated to dead storage. The dam is about 61 m high with its full supply level (FSL) at +221.0 m msl. The surcharge volume above +221.0 m msl is ample with the dam embankment crest level (ECL) at +223.8 m msl. The dam is equipped with outflow structures, such as water outflow pipe by drawing water at different elevation of the intake tower. The sluicing valve is operated by Larnar butterfly valve. The outlet serves to releases water from the reservoir in time of need to augment low flow at the Mile 10 water supply intake.

The spillway is located on the left bank of the dam abutment with a fairly large diameter bell mouth or morning glory spillway, i.e. 27.4 m in diameter. During an extreme meteorological event, the torrential flow of PMP/PMF magnitude will be evacuated via the bell mouth spillway so that the danger of dam overtopping is avoided at all cost. PMP/PMF convention is therefore used to design the spillway capacity. The morning glory or bell mouth spillway of Langat dam is shown in Figure 1.8.

Figure 1.6: Morning Glory or Bell mouth Spillway of Langat dam( Source: self taken pictures )

1.3 ObjectivesThe objectives of this study are:

a) Assessment of the hydrological safety of existing Langat reservoir/dam in the light of a meteorological extremity, such as occurrence of a PMP/PMF event.

b) Recommendation of appropriate and cost effective remedial and mitigation measures, if any, in case of inadequacy in the hydrological infrastructure assessment carried as per objective (a).

It is utmost important to ensure the dam/reservoir structure, especially earthfill and rockfill type could withstand the onslaught of torrential storm which in a way results in exceptionally high inflows into the reservoir water body.1 LITERATURE REVIEW1.1 Problem StatementDam safety is a vital important issue and must be kept in focus at all times as any failure of dam can lead to high hazard potential. Dam safety must be given high priority during the process while planning, design, construction, operation and even during maintenance. Safety assessment of the dam/reservoir structures are one of post construction operation and maintenance (O&M) undertaking that are being carried out regularly to ensure that the reservoir structure are always functioning well. Although dams are constructed according to careful survey, design, and construction stages, there are still many cases of serious dam accidents have occurred in the world. Therefore safety assessments during the post construction operation and maintenance are very important.

Dam safety assessments that carried out according to the O&M protocol are focus on few issues including natural catastrophes such as earthquake event, heavy precipitation events that lead to eventual high floods, evidences of animal burrowing of the dam/reservoir structure, in the case of earth and rock fill dam structures, etc. An inspection program seeks to identify the current status of the dam/reservoir structures in light of calamity that might impair the structural integrity of the structure itself. An inspection checklist gives the full safe/health status to the dam/reservoir scheme for continuous operation in many years to come until the next due inspection, which in the Malaysias Dam Inspection Guideline, calls for 5-year interval for any major dam structures. Dam failures can happened due to varies factors such as overtopping which caused by water spilling over the top of a dam, structural failure of materials used in dam construction, cracking caused by movements like the natural settling of a dam, internal erosion and also inadequate maintenance after built. The results of the dam failure could be far reaching and jeopardizing the structural integrity of the dam/reservoir. Table 2.1 shows some dam failure cases and the main reasons of failure.

Table 2.1: Partial List of Dam Failure and Main Reasons

Source : Centre for the Assessment of Natural Hazards and Proactive Planning

Overtopping is the most common reason causing dam failure among the others and often led to serious hazard. Overtopping can occur when the water level in the reservoir rises rapidly and most of the time without prior and/or even with a short time warning. The examples are flash floods, heavy storm/rainfall, a landslide occurs in the upper catchment of the dam/reservoir that send torrential water wave toward the dam embankment at the downstream. Overtopping can also happen when the spillway is too small or becomes blocked. If the amount of water coming into the reservoir is greater than the amount that the spillway was designed for, or if the spillway becomes blocked, the floodwater might start to overtop the dam crest.Failure of dam by overtopping show up that the importance to have an accurate assessment of their safety feature such as emergency action can be planned and implemented ahead of probable catastrophic events. One of these measures is the hydrological inspection and evaluation that plays a role in the overall inspection program to estimate the overtopping probability of the dam during PMP(probable precipitation) scenario. Amongst the dam structural appurtenances, spillway capacity is one of the most significant factors that affect the ability of a dam to pass the maximum flood. The performance of a spillway on the onset is crucial to the dam/reservoir structure itself. Overtopping over the dam crest occurs if the reservoir cannot adequately and sufficiently attenuate the inflows into the reservoir in case of an extreme PMP/PMF event. Therefore, assessment and appraisal are carried out on the existing spillway capacity in light of a PMP/PMF event during the hydrological inspection.Overtopping criteria is rather important if the dam is of embankment fill type, such as conventional earthfill and rockfill dams (not so extent, if the dam structure is concrete buttress dam). Potential risk of damages, mainly erosion of the downstream face of the dam is higher than ever if the torrential inflows are allowed to over spill the crest. The high velocity flow, in the magnitude from 15 to 30 m/s could induce cavitation (implosive bubble formation) on the downstream surface of the embankment dam. This negative pressure is created as a result of high velocity, according to Bernoulli equation, induces suction forces that might rip apart or erode the surface of the dam. With time, the dam structure could be rendered unsafe and running into the risk of collapse. A vivid tunnel spillway accident during high flows in Glenn Canyon dam in 1986 has demonstrated the vulnerability of the spillway against the onslaught of cavitation as the flood waters pass through it at an exceptionally high velocity. The design parameter, PMPs used in the earlier design of dam/reservoir structures are subjects of review during each inspection interval. Unfortunately, during the earlier design of the dam/reservoir structure, the hydrological information at the dam/reservoir sites or in the vicinity might not be adequate, thus prevented a thorough and comprehensive assessment at the time of detail design stage. This is especially true for the dam/reservoir structures that were been designed and built in the earlier 1950s where the hydrometric collection and sampling program are generally lacking and inadequate at the earlier years of Independence. As such it is a pressing need for a comprehensive assessment of the hydrological safety criteria of the dam/reservoir structures in the light of climatological/meteorological extremities. An illustrative example of the inadequacy of the hydrological assessment is the PMPs that are adopted in the dam/reservoir design. The PMPs are mostly inconsistent as they are subjects to the knowledge and experiences accumulated by individual designer, i.e. consulting engineers/specialists experiences. Even for the same dam, most if not all of the time, the PMPs are reported and derived differently by different engineering specialists. In summary, the hydrological assessment of the dam safety is an essential part of the dam inspection program, in light of a new and updated set of PMP using concurrent and latest observed hydrological information. It is therefore utmost important to carry out this special task of the hydrological investigation.1.2 Tasks and AssignmentsTasks to fulfill the objectives of this Study are as follows:

Derivation of PMPs by reviewing various studies and works that were carried out earlier in Malaysia, notably, major flood mitigation studies carried out in Kelantan (SSP/SMHB, 1997), Interstate Raw Water Transfer from Pahang to Selangor (NK/SMHB, 2000), SMHB/RB/JPZ (NWRS, 2000), NAHRIM (2008) and other scholastic researches/reports by various institutions of higher learning in Malaysia.

PMP to PMF routing using one of the acceptable PMP/PMF rainfall runoff routing techniques, i.e. a modified version of synthetic unit hydrograph approach of HP 11 (Taylor and Toh, 1976), and

Conventional reservoir (modified Puls) routing procedure to estimate the flood rise during a PMP/PMF event. The reservoir helps to moderate the humongous inflows by attenuating the peak discharge of an inflow hydrograph. Figure 2.1 shows the step-by-step approach in assessing hydrological aspect of dam safety program.

Figure 2.1: PMP/PMF Routing and Reservoir Routing Flow Diagram in Dam Safety Assessment Undertaking (source: www.noaa.gov)1 METHODOLOGY1.1 Methodology of Hydrological Dam Safety AssessmentFor this project report, the methodology that used for assessing the hydrological dam safety primarily focuses on the review of the spillway capacity and dam overtopping likelihood. The assessment basically involved few steps which are:

1. Derivation of PMPs at the project/study site,

2. Translation of PMPs to PMFs/SDFs using a catchment rainfall runoff or response function model and

3. A conventional reservoir routing technique to estimate the flood rise over the dams full supply level (FSL).

The derivation of PMPs in this study is carried out mostly by reviewing the available past studies and findings in Malaysia. The prevailing PMP convention is duly reviewed and adopted as appropriate. Catchment response and convolution lumped parameter model is used to translate PMPs to PMFs for various durations. Finally, the derived PMFs are then appropriately routed through a lumped parameter reservoir. The final results of this exercise/undertaking are to ensure that the dam is not overtopped passing its embankment crest level (ECL).

Probable Maximum Precipitation (PMP), by definition, according to WMO (1986, 2009) is the greatest depth (amount) of precipitation, for a given storm duration, that is theoretically possible for a particular area and geographic location. PMP is generally derived based on mostly observed maximum rainfall records with the provision of storm maximization and transposition technique in tandem. Many floods calculation that are typically evaluated in dam engineering include frequency based storms (e.g. 2-year or average flow through a 500-year or higher flood) and the Probable Maximum Flood (PMF), which is developed based mostly on the occurrence of Probable Maximum Precipitation (PMP).Spillway Design Flood (SDF) or Inflow Design Flood (IDF) is another term that is important in dam/reservoir design. Dams/reservoir structures are mainly designed or required to safely pass the typically Spillway Design Flood (SDF) or Inflow Design Flood (IDF). In this regard, it is often used interchangeably with the term PMF. The magnitude of this type of extreme flows typically range from the 100-year flood in the past to the contemporary adoption of PMF. The selection of a SDF/IDF/PMF is normally dependent on the classification of hazard category of the dam structure and the potential for loss of life or property damage that would result from a dam failure during a given magnitude of flood. In the past, without or due to limited knowledge and understanding on the hydrological aspect of dam/reservoir design, the dams are designed based mostly on the observed floods and past empirical experiences. With the current design philosophy after acquiring many design experiences worldwide, PMFs are mostly selected for the purpose of dam/reservoir structure design to ensure the safety of the downstream riparian users in time of dam breach or failure.

Hydrologic analysis for estimating the SDF/IDF/PMF for dams includes:

Delineating the watershed or drainage boundary contributing to the dam

Developing theoretical precipitation amounts and distribution over the storm duration

Estimating infiltration to compute runoff volume

Computing runoff distribution based upon a synthetic hydrograph theory

Routing of the inflow through the impounding water body (lake, reservoir, etc.)In addition to estimating the SDF/IDF/PMF, hydrology for dam projects could include evaluation of flood protection provided by the structure. Also, many dams provide water supply and the hydrologic analysis for these structures could extend to drought hydrology for sizing reservoirs and defining releases to address environmental concerns.

The tasks of hydrologic analysis to be carried out in the dam safety assessment can range from simplified equations and methods to relatively complex computer/mathematical models, including commonly used models developed by the US Army Corps of Engineers (USACE) and Natural Resources Conservation Service (NRCS). Currently, these computer models are being interfacing with Geographic Information Systems (GIS). This has also helped engineers and scientists to develop watershed parameters more quickly and accurately.

A thorough knowledge of hydrologic analysis for dams/reservoir should be developed as well as to understand of the hydrologic cycle and flood events and their interactions and also should be proficient in reading and interpreting topographic maps and conversant in computer models. In addition, an understanding of working GIS knowledge is now considered an essential part of dam assessment assignment. And therefore, a working knowledge of the GIS system is also highly recommended.1.2 Pmp Review and StudyPMP review and study is one of the main task in this report, this segment of the study aims to summarize the Probable Maximum Precipitation (PMP) convention used in the dam/reservoir design in Malaysia. These conventions were mostly adopted by local dam engineering specialists in their respective undertakings in order to design a sustainable dam/reservoir.To design a sustainable dam/reservoir and ensure the safety of the downstream community of a dam/reservoir, dams are always designed based on probable maximum precipitations (PMPs) that can possibly occur during the design life of a reservoir/dam scheme. PMP data is utmost important especially while derivation of the Spillway Design Flood (SDF). The review and assessment of PMPs is imperative in light with the proliferation of reservoir and dam development activities in Malaysia. Besides, as nowadays there are a lots of telemetric stations were installed at various strategic locations along the river, therefore there is ample long-term hydrometric information (i.e. rainfall) is made available. This important and vast database of records could be of use for a thorough PMP assessment which can greatly improve the accuracy of the result obtained.The understanding of the PMPs is vital and critical to the development of a reservoir/dam scheme. This undertaking in general, is suitable for review and assessment of the PMPs for flood mitigation feasibility study.1.2.1 Rationale of PMPThe importance of PMP/PMF in dam/reservoir project is its vast and broad implication to the public safety and disaster prevention downstream of a major water retaining and regulating structure, such as reservoirs etc. These gigantic structures should be able to withstand in terms of structural integrity, the torrential forces of extreme storm event so that disastrous failure such as dam breaching, especially if it is located in the upland or headwater region or catchment of the populated town or urban habitat could be avoided.The assessment of the Probable Maximum Precipitation (PMP) is important in order to derivate the hydrological calculations and hydrological estimation, for example Probable Maximum Flood (PMF), which in turn is used to design the reservoir outflow structures, such as spillways, bottom outlets etc. The PMFs derived from the PMPs is the design flood inflow into the reservoirs where no risk of failure of dam structure should be allowed to occur.PMP data within a region is possible to obtain or estimated through meteorological methods and historical records. The historical observed and sampled data consists of point precipitation amounts measured at rain gages throughout the region being studied, or a region with very similar meteorological and topographical characteristics. Review and appraisal of the past PMPs segmental studies of various dam/reservoir designers in Malaysia is crucial to derive the design parameters for subsequent engineering undertaking. Specifically, methodologies adopted by one of the major dam designers in Malaysia, SMHB (formerly Binnie & Partners, B&P, UK). The consulting form have been active and had have carried out major consultancy works in dam and reservoir design and supervision in Malaysia for over the past 40 years. The consultant is also the principal engineering consultant for Semenyih dam in the earlier 1980s.

As mentioned previously, the PMP derivation is needed for calculation of PMF where both the structural safety and integrity of the dam structure as well as the impact of the torrential flood flows on the safety and hazard mitigation the downstream riparian users, i.e. populated cities or suburban centres.

It is the intention that the extreme flood flow could safely pass the major structures, spillway and/or other emergency outlet structures, without damaging impact on their structural integrity in the event of an extreme event or occurrence of PMP magnitude.

In Malaysia, the PMP values adopted by specialist dam designer engineering consultant, SMHB and Binnie and Partners (B&P) group (SMHB/B&P) have been the primary sources and references for other dam and reservoir designers as well.

Besides methodology espoused by SMHB/B&P, there are a few further major undertakings on PMP estimation since 1990, i.e. in the Pahang Selangor Water Transfer project (NK/SMHB, 2000), National Water Resources Study (SMHB/RB/JPZ, 2000), and an ongoing review undertaken by NAHRIM (2008).

This review primarily addresses on the issues and techniques of PMP derivation in Peninsular Malaysia and with no coverage or citation for the Borneo states of Sarawak and Sabah. In the context of this report, comparison and review are then made as appropriate with respect to the Semenyih reservoir in the original PMP values for detailed reservoir design.

In the context of this study, the terminology, PMP is defined by the World Meteorological Organization (WMO 332, 1986 and WMO 1045, 2009) as the greatest depth of precipitation for a given duration meteorologically possible for a given basin of a particular time of year, with no allowance made for long-term trends. Utility of the PMP to generate the PMF is the industrial standard for dam design worldwide. The development of PMPs for a given basin or catchment can be complicated, lest time consuming, and requires the expertise input of professional hydro-meteorologists. However at the operational level, as recommended by WMO 332 and 1045 (1986; 2009) guideline, PMPs can be estimated by both (a) hydrometeorological and (b) statistical approaches.

The latter is a useful mean for making quick and ballpark estimates and where other pertinent and comprehensive meteorological data, such as dew point, and wind records, are lacking to warrant a sophisticated and thorough a hydro-meteorological type of analysis.

Other than derivation of PMPs based on the prevailing WMO 332 and 1045 (1986; 2009) manual/guideline, some dam/reservoir projects in Malaysia also adopted the Australia Bureau of Meteorology (ABM) on both short and long duration PMP derivation (Bulletin 53; ABM, 2003).

The techniques recommended the estimation of PMPs based on the climatologic homogeneous zones using several derived depth area duration (DAD) curves while taking into consideration of the topographic and geographic features. These techniques are sometimes adopted by Malaysias engineering consultants based on the premise that the derived PMPs from Australia database and condition could be transposed to humid tropics region, such as Malaysia as well. As such PMPs estimates espoused by ABM are generally higher than other techniques adopted in Malaysia.

As mentioned earlier, two (2) general techniques as described in WMO 332 (1986) are currently being used in Malaysia for derivation of PMPs for reservoir/dam projects. They are

Hydrometeorological approach by storm maximization and transposition

Hershfield and its variants (Desa et al, 2001; Desa and Rekhecha, 2007); Statistical/frequency point analysis approach; mainly for checking and verification purpose.

Bulletin 53 (1996, 2003 new addendum) of Australia Bureau of Meteorology (ABM) is also been used by some dam designers mainly on PMPs derivation and checking and confirmation.

Brief introduction on these two techniques are presented as follows: 1.2.1.1 Hydrometeorological ApproachThe basis of the PMP derivation using the hydrometeorological approach is mostly based on vast database of historical observed records of maximum rainfall/precipitation in a specific region. For example, the derivation of PMPs for Malaysia may not only be based on observed rainfall records in Malaysia per se, but might also take into consideration regional occurrences of extreme storm events such as in Indonesia, Thailand or Singapore etc.

Alternatively, as mentioned earlier for dam engineering projects in Malaysia, the PMP procedures developed by the Australias Bureau of Meteorology (BOM), which are originally derived based on exclusively observed records in Australia, are being adopted by Malaysian Designers from time to time.

The prerequisite of a PMP study using hydrometeorological approach is the availability of storm records for various durations and other meteorological measurements, such as relative humidity, wind direction, etc. these information in fact is not available in most of the countries due to expensive outlay and investment in acquisition of such records.

The first methodological step of the procedure is to extract the highest rainfall records from the database. These records could be acquired with a modest fee from the JPS TIDEDA database system and MMS principal rainfall station records. Other organizations such as large scale oil palm and rubber estate and plantation do have their own rainfall monitoring and sampling programs in place. Their database might be traced back to the earlier years of burgeoning plantation activities. However, the information is rather difficult to come by on time. Most of the hydrological assessments carried out in Malaysia lest along the organized PMP/PMF studies do not have the privilege of acquiring such database from various private entities.

In terms of observed rainfall records, the eastern coastal regions of the peninsula (Kelantan, Terengganu, Pahang and Eastern Johor) are aggravated by the annual Northeast Monsoon (from November and extending to January next year). The rainfall records in this region therefore always provide a good and reliable source of highest rainfall records for meaningful PMP analysis. The brief procedure of PMP derivation at the operational level is presented in the following subsection.

The maximum rainfall records for durations starting from 1 day to 5 days are tabulated and plotted spatially on a regional map. Contours/isohyet or equal magnitude of rainfall storm depth are then delineated as appropriate. Each associated influential areas are attributed to a particular rain storm depth are then duly prepared. This type of plot is termed as PMP depth-area duration curves (DAD).

Normally, depth area curve for duration of more than 1-day could be easily available in Malaysia as majority of rainfall stations are of non-recording type where the rainfall depths are measured within a day, i.e. gauging and reading manually at 8.00 am daily. Three (3) critical processes are required before the derived PMPs could be adopted.

Envelopment of depth area duration curves: to encompass the highest value

Maximization of precipitable water by determining the state of saturation of the atmospheric waters at the project site: probable extreme meteorological condition

Transposition of the derived PMPs to project site by ratios of mean annual rainfall, maximum rainfall depths for the same duration/periods, etc,

Envelopment or the maximization process is then carried out manually and literally to envelop a highest single curve that encompasses all possible highest values for that specific duration. An example of envelopment is shown in Figure 3.1 for 1-day rainfall records in the east coast of Peninsula Malaysia (courtesy of NK/SMHB [2000] Study).

On the other hand, Figure 3.2 and figure 3.3 shows the plot of rainfall depth-area map for 5-day total maximum observed rainfall for some 24 recording and non-recording type of rainfall stations in the vicinity of Kuala Terengganu during the months of November to early December in 1986. The enveloping catchment areas are then delineated for each rainfall depth interval in the form of depth-area table as shown in Tables 3.2 and 3.3.

Another key step in the derivation of PMP is to maximize the storm of the individual duration by a factor that relies on both (a) the maximum dew point temperature and (b) elevation of the project site relative to the mean seal level (MSL). The former factor generally reflects the increase in precipitable water as a result of higher dew point temperature. Coefficients ranging from 1.5 to 3.0 are not uncommon and they are then factored to the derived PMP value for specific duration. The maximum 24 hour average dew point temperature is normally adopted to represent the maximum precipitable water scenario.

Transposition is also an essential step is to be taken into consideration by transposing the PMP from a region where they are being derived to the project sites of interest which is always located some distances landward in Malaysia where most of the dams are located.

Figure 3.1: 1-Day Depth Duration Area DAD Curve for Storms in East Coast Peninsular Malaysia after maximization and transpositionSource: NK/SMHB, 2000

Figure 3.2: 5-Day Depth Area Curve for 1986 Storm in East Coast Peninsular Malaysia (November 1986 27th November to 1st December 1986, near Kuala Terengganu )Source: SMHB/SSP, 1999, Sg. Kelantan Flood Mitigation

Figure 3.3: 5-Day Depth Area Curve for 1986 Storm in East Coast Peninsular Malaysia

Table 3.2: 5-Day Depth Area Curve/Table for 1986 Storm in East Coast Peninsular Malaysia November 1986

Station NoTotal 5-day Rainfall mm

50310621408

5033069898

51020401014

5130065315

5230041849

52300421149

52320651224

5518035377

4923001341

451603135

462004591

53220441041

53280441234

5320038446

5520001203

5522047672

47340791037

52210471344

50290341376

58240801385

5824079877

49300381372

55240021100

5722057743

Source: SMHB/SSP, 1999 Sg. Kelantan Flood MitigationTable 3.3: 5-Day Depth Area Curve/Table for 1986 Storm in East Coast Peninsular Malaysia November 198627th November to 1st December 1986, near Kuala TerengganuRainfall depth mmNext rainfall mmSurface area km2Difference in area km2Rainfall volume MCMAccumulated volume MCMAverage rainfall mm

1400

1300

1200

1100

1000

900

7801450

1400

1300

1200

1100

1000

900202

1146

1879

3084

4871

5711

6104202

944

733

1205

1787

840

393288

1274

916

1386

1876

798

330288

1562

2479

3864

5741

6539

68691425

1363

1319

1253

1179

1145

1125

Rainfall Time 24 hour 48 hour 72 hour 96 hour 120 hour

mmArea km23320171713

33%53%70%87%100%

1425

1363

1319

1253

1179

1145

1125202

1146

1879

3084

4871

5711

6104470

450

435

413

389

378

371755

723

699

664

625

607

596998

954

923

877

825

801

7881240

1186

1148

1090

1025

996

9791425

1363

1319

1253

1179

1145

1125

Generally the transposition of a storm from the east coastal regions of Peninsular Malaysia will result in reduction in precipitation if it traverses across over a topographic barrier such as at the main range of the Peninsular Malaysia, i.e. Banjaran Titiwangsa, which is located in the middle of peninsula. A barrier of such altitude normally blocks the storm surge during prevailing northeastern monsoon but on occasion, the monsoonal torrents do spill over to the west coast. Events on 1971 Kuala Lumpur flood and some major flooding events in the northern states of Kedah and Perlis were the remnant of the monsoon during the months from October to January next year. For this reason, to transpose maximum rainfall to the west coastal region of Peninsula Malaysia is normally not adopted in most of the dam design in the west coastal region. However it is rather common to use PMPs of the east coastal region as a check and reference.

A review of the available topographic mapping generally would indicate the extent and influences of the topographic feature and land form pertaining to the transposition of PMPs. Sometimes a coefficient essentially based on the ratio of the elevation between project sites and the region where PMPs have been derived could then be adjusted as deemed necessary.

Another technique noteworthy of comparison is the long-term or event based maximum storm records of the region where PMPs have been derived and the project sites, if available. It is however of high certainty that the transposition of PMPs derived in the coastal region to project sites inland without adjustment based on topographic and observed maximum rainfall depth is unduly conservative.

The derivation of the hydrometerological approach depends strongly on the availability of the rainstorm records. For a longer duration of more than 24 hour or 1 day, records are mostly available in Malaysia. However for less than 24 hour duration, observed records are generally lacking due to expensive capital as well as operating costs of acquiring recording type of rainfall depth. Due to scarcity of the short duration rainfall records, it is sometimes reasonable to adopt short duration PMP procedure published by ABM in Australia.

Several dam (outlet structures and spillways) designs in Malaysia have been based on this premise and principle of PMP transposition from Australia to Malaysia, i.e. Kenyir, Pergau, Kelinchi, Kinta, Ahning, Prang Besar (Putrajaya), Batu Hampar, Bengoh, Jelai, and dam design projects mostly associated with Australian consultants.

1.2.1.2 Hershfield TechniqueThe procedure by Hershfield (WMO, 1986; 2009) is a statistically based methodology relying on the theoretical background of frequency or probability analysis of occurrence or non-occurrence of events of specific level of severity. It resembles a general frequency factor equation (Chow et al., 1988) as shown below.

The frequency factor varies with different type of probability distribution commonly used in the hydrologic frequency analysis.

Similarly, in the Hershfield analogy, if the maximum observed rainfall () and the frequency factor () are substituted for and respectively, then a frequency factor type of equation is formed.

Both and factors represent the arithmetical mean and the number of standard deviations deviated from the sample or annual maximum series respectively. Hershfield found out that varies depending on the number of extreme data that could be obtained from the sample and to a certain extent the magnitude of maximum rain storm recorded by the station.

Hershfield recommended a frequency factor of 15 as the universal maximum value to be adopted after analyzing thoroughly a vast maximum rainfall records database worldwide. Since its advent in the earlier 1960s, subsequent revisions, modifications, and improvements have also been carried out (Koutsoyiannis, 1999), and locally in Malaysia, Desa and Rakhecha (2007, 2009). The commonly adopted frequency factor originally proposed by Hershfield is found to be overly conservative and modification should be made to suit local climatic and meteorological variability. Therefore, some of these adjustments made in subsequent studies take into account on the data quantity, such as the length of the records, maximum records rainfall, etc.

APPENDIX A shows the adjustment factors as proposed in WMO 332 and 1045 (1986, 2009).

The Hershfield form of the PMP equation is presented as follows:

Adjustment of and for Maximum Observed EventOutliers are often found to have occurred at some time during much shorter period of record, say, 30 years. They may have an appreciable effect on the mean and standard deviation of the annual maximum series. However, the magnitude of this effect is less prominent for long records as compared to short records. In short, PMP varies with the presence of outliers. Hershfield (WMO, 1986, 2009) recommended that adjustments for both and with lengths of annual maximum series. These factors could be referred in WMO 332 (1986, 2009) publication on estimation of PMPs.

Data Requirement for Hershfield Type PMP

Data requirement for Hershdield type of PMP estimation procedure is simple and straightforward. First of all, long-term annual maxima series for various durations are obtained from digital hydrometric library. In Malaysia they are acquired mostly from JPS TIDEDA system database using PEXTREME/PMOVE built-in routine.

The short duration annual maxima series (arbitrarily defined as less than 24 hour storm duration) could also be extracted from these long-term database of automatic (or known as recording) rainfall stations where the rainfall depth register is of small time step resolution.

On the other hand, long duration series (i.e. equal or greater than 1 day) are obtained mostly from non-recording stations where daily observed rainfall depths of each station are duly recorded daily.

Derivation of PMP Using Hershfield Procedure

Hershfield procedure is carried out conveniently using an Excel spreadsheet setup. The step-by-step procedure is described as follows:

Annual maxima storm series of varying durations, i.e. from 15-minutes to 6-day are summarized in a table format;

The mean () and standard deviation (), i.e. first and second moments of the sample of the annual maxima series for each duration are then computed respectively;

The PMPs for each individual duration are then calculated using Hershfield equation (these PMP values are Unadjusted PMPs);

The adjusted values of and are then estimated from Figures 4.2 and 4.3 of WMO 332 (1986) to the mean and standard deviation of the annual maxima series after excluding the maximum observed records in the series; and

The PMPs for each individual duration are then repeated using adjusted and (results as Adjusted PMPs).

Due to the tediousness and elaborated data requirement of hydrometeorological approach, sometimes, the Hershfield procedure is carried out (as historical rainfall records are relatively easier to acquire) in most feasibility and preliminary design studies. It is sometimes also been used for cross checking purpose vis--vis other published PMPs of other studies.

One of the advantages of the Hershfield procedure, inter alia, is the considerably less time input is needed to derive PMPs vis--vis the hydro-meteorological approach. Above all, it is also easier to understand (as it is a form of probability or frequency analysis commonly used in the statistical science and hydrology, Chow et al, 1988) and is practical and robust. However, a major drawback of this simple approach is its point values of PMP estimation and thus suitable area-reduction factors (ARF) are applicable for adjusting the point values to transpose to various sizes of basin area.

Alternatively, various type of regionalization techniques, such as cluster analysis, index rainfall, and geostatistical approach of Kriging, etc.

A relatively simpler approach is none other than an areal wide isohyetal PMP maps that could also be produced with adequate point rainfall stations as shown in the contour map for rainfall in the states of Selangor and Johor (see Figures 3.4 and 3.5).

For comparison purpose, it is considered adequate and sufficient to provide such a check in line with the good practices of the hydrological design for a sizable project where failure of the engineering structures will have detrimental effects on the downstream riparian users.

Figure 3.4: Selangor PMP: 1-day (Desa, Noriah, and Rakhecha, 2001)

Figure 3.5: Johor PMP: 1-day (Desa And Rakhecha, 2007)1.2.2 Adopted PMP Convention by SMHB/B&PAs one of the premier dam designers, SMHB (and its predecessor, Binnie and Partners) generally adopts a uniform set of PMP values derived from their past numerous dam design experiences in Singapore and Malaysia (i.e. Seletar, Upper Pierce, Langat, Semenyih, Terip, Linggiu; Durian Tunggal before 1990; Selangor, Tinggi, , Chereh, Teriang, Jus after 1990).

Specifically, three (3) specific reports/studies formed the basis of PMP derivation at that time before 1990. They were (1) SSP/HH (1979), (2) B&P (1980), and (3) SSP/SMEC (1985) for project locations in southern Johor and Singapore. These PMP values have since being adopted for almost all reservoir design projects undertaken by SMHB/B&P.

Notable major dam project undertakings were Seletar and Upper Pierce dams in Singapore, Semenyih, Langat, Tinggi, and Selangor dams in Selango, Linggiu and Juasseh dams in Johor, Terip and Teriang dams in Negri Sembilan, Chereh dam in Pahang, Durian Tunggal and Jus dams in Melaka, Gerugu dam in Sarikei, Sarawak, etc.

The PMP in the context of SMHB/B&P lexicon is arbitrarily divided into both short- and long- storm duration. By definition, the dividing line is at the 24-hour duration. However, there is also exception where 6-hour duration is sometimes used to define the boundary between the short and long durations as being adopted by the Australian Bureau of Meteorology (ABM) in their dam design projects in Malaysia. However it should be borne in mind that the demarcation is simply adopted for convenience by various organizations.

The PMP values adopted by SMHB/B&P are further classified into two series, i.e. (1) Coastal and (2) Inland. This purportedly takes into consideration the vast difference in meteorological and geographical factors, as their names implied at both peninsular coastal, comprising of primarily east coastal region and inland, mainly interior part of the Peninsular regions. For short duration PMPs, the values adopted as Coastal PMP are based mostly on the Singapore rainstorm of 1978.

After adjustment for an appropriate transposition factor, it is transposed inland (specifically for Semenyih reservoir design project), this series is therefore known as Inland PMP. This series was used in most of the dam design projects undertaken by SMHB/B&P since then, including recent Tinggi and Selangor dams in Selangor, and Teriang dam which is now under construction, in Negri Sembilan. Table below shows both coastal and inland PMPs derived and adopted by SMHB/B&P for duration ranges from 1- to 120-hour.Table 3.4: Coastal and Inland PMP (Short- and Long-Duration) adopted by SMHBDuration (hour)Coastal PMP (mm)Inland PMP (mm)

1-211/190.5#188

3-338300

6-440391

12-584518

24-777692

Long Duration PMP (arbitrarily defined for this review)

48-1356908

72-15931067

120-20301360

# 19.5 mm or 7.5 in is originally quoted but 211mm is back calculated from 188 mm

Short-Duration PMPThe basis of the PMP derivation for SMHB/B&P was based on the premise of actual measured records of maximum rainfall both in Malaysia and Singapore. Some notable highest rainfall records are presented as below (as quoted from SMHB, 1992). These measured records formed the basis of short duration PMP series for dam/reservoir design (see Table 3.1: Recorded Maximum Rainfall, NAHRIM 2008).

Near Kuantan in late December 1926, with the bulk of the rainfall falling between 27th and 31st of December including 631 mm (24.85 inches) within one day at the Jeram Kuantan Estate;

In Singapore on 17th July 1941 when in a very intense but relatively short storm 65.6 mm was recorded in 30 minutes, 120 mm in 60 minutes and 147 mm in 2 hours;

In Singapore on 9th and 10th December 1969 when 478 mm was recorded in 24 hours;

Near Mersing between 29th December and 4th January 1971 when 541 mm occurred in a 24 hour period and a total of 1453 mm (1600 mm and1800 mm are being reported elsewhere) was measured in 120 hours; and

In southern Johor and Singapore on 2nd and 3rd of December 1978 when 537 mm was recorded in a 24-hour period at Serangoon Sewage Disposal Works with values been recorded concurrently at two other stations on Singapore.

The December 1978 storm in southern Johor and Singapore was primarily selected as basis for short duration PMP derivation because it was the most severe recorded storm in the southern region of the Peninsula Malaysia other than the highest recorded storm in Jeram, Pahang (unfortunately actual water depth was not accurately quantified). These short duration PMPs are derived by maximizing the 24-hour rainstorm during 2nd and 3rd December, 1978 in both southern Johor and Singapore.

A 24-hour record of rainfall (536.5 mm) was recorded at the Serangoon Sewage Disposal Works (SDW) station. In addition, observed rainfalls of about 533 mm were also recorded concurrently at both Kim Chuan Road Sewerage Works and Sembawang Agricultural Research Station in Singapore.

From screening the up-to-date observed records, there are no recent recorded extreme storm events that exceed the derived PMP values to warrant revision or review of the short term duration PMPs. Although in the recent NAHRIM (2008) study highest long-duration observed rainfalls were found in the state of Kelantan and Terenggnau (see Table 3.5). Nevertheless, the highest values were of single station to be rather representative of an areal wide storm event.

For convenience, in some design undertakings also adopt in to Australias ABM short duration PMP enveloping curves for humid tropics region (NK/SMHB, 2000). This is presumably based on the premises and assumptions that the observed maximum storm records in Australia could be transposed to regions in South East Asia as well. These values are higher than the SMHBs short duration PMPs.Table 3.5: Recorded Rainfall (NAHRIM 2008)

Storm Maximization of short duration storm

The storm depth is commonly maximized based on the ratio/index of maximum precipitable water in the air column to the actual precipitable water during the storm (as a function of maximum and persistent dew point temperature for 12-hour at 1000 mb).

Based on an average of the recorded values at Paya Lebar Airport and Changi Airfield/Airport the 24-hour persisting 1000 mb dew point temperature at the beginning of December 1978 storm was 23.8o C. At this dew point temperature the precipitable water in the atmosphere prior to the storm was estimated at 73.5 mm.

The period of dew point temperature was searched based on hourly records from 1955 to 1978 presumably from the historical database of the Paya Lebar Airport station and Changing Airfield.

Coupled with the consideration of the limiting influence of the maximum sea temperature, it was deduced that the estimated maximum 24-hour persisting dew point temperature could not be possibly more than 28oC (as originally presented in PUB, 1980). At this dew point temperature, the corresponding precipitable water was 106.8 mm.

The storm-maximizing factor is then calculated simply as an index of the ratio of the maximum precipitable water to the prevailing precipitable water content prior to the storm, i.e. 106.8/73.5= 1.45. Therefore the maximum rainfall that could possibly occur in 24 hours is 1.45 *536.5 mm, i.e. 777 mm. For PMP of less than 24-hour duration, similar methodology/technique was applied in terms of maximization to derive 1-, 3-, 6-, 12-hour duration PMPs. Transposition of short duration storm

Transposition is another ensuing major step in PMP studies. Transposition of storms from one location to others is subjected to various important contributing meteorological as well as geographical and topographical factors such as, presence of topographic barrier, elevation adjustment, distances from the storm center, and meteorological factors etc.

However in SMHB/B&P practices, primarily due to scarcity of the meteorological data and high uncertainties in the chosen transposition technique, estimation of PMP values at the coastal region (assuming that Singapore is located in the geographically similar coastal region) might not be able to carry out after all. As such, SMHB/B&P adopted both short- and long-duration PMP values without taking into account the transposition factor for PMP derivation in mostly coastal region. Though it is a conservative measure, but it should be acceptable in the absence of both concrete authoritative recommendations and limitations in understandings and knowledge of PMP derivation in Malaysia.

Nevertheless, for interior region, the consensus amongst the experts in various SMHB/B&P branch office hydrological groups opined that by adopting coastal PMP without taking into account the transposition effect was somehow unduly conservative. Therefore, some forms of downward adjustment should be made for such purpose. An example of the application of transposition technique was demonstrated in the derivation of PMP values for Semenyih dam.

In this particular study, the PMP derived from 1978 storm in Singapore was transposed to the Semenyih dam site. By taking into consideration the highest persistent dew point temperature in the State of Selangor (assuming PKM Petaling Jaya station is representative of the whole state of Selangor, at 26.7 oC; precipitable water 95 mm), the transposing factor was estimated as 0.89 (95/106.8=0.89). This amounting to some 11% reduction was primarily considered justifiable due to relatively lower historical persistent dew point temperature in the state of Selangor.

By making the same assumption that the PMP at the Semenyih dam site is having the same probability of occurrence in the northern catchment, the PMP series was also subsequently been adopted for the detailed design studies of both Tinggi (formerly known as Buloh) and Selangor dams in the Sg. Selangor basin respectively. Table 3.6 shows the adopted coastal and inland PMP values by SMHB for various durations, i.e. 1- to 24-hour.Table 3.6: Comparison of PMP of Coastal and Inland PMP ValuesDuration (hour)Coastal PMP (mm) *Inland PMP (mm) #

1-211 (190.5 mm or 7.5 in) &188

3-337300

6-439391

12-582518

24-777692

Col3 *transposition factor, 0.89; * based on Singapore 1978 storm; # Semenyih dam design; & in original text It should be reiterated that the adopted PMP values by SMHB though not strictly are based on WMO (1986, 2009) guideline per se, for which incorporating rather complex maximization and transposition approaches, nevertheless the general principles on maximization and transposition techniques are duly and consistently obeyed. In addition, the WMO approaches require some detailed meteorological as well as topographic information for which most of the times are unavailable.

This essentially precludes an elaborate and thorough PMP studies in line with the WMO 332, 1045 (1986, 2009). This is particularly true for the case in Malaysia, as most of these meteorological parameters and data are difficult to come by in the earlier years of 1960s and 1970s. These two series of PMP are adopted for designs and studies undertaken by SMHB subsequently.

Other transposition approaches for subsequent dam design assignment was also undertaken, such as those adopted in the recent Kelantan River Flood Mitigation Plan (SSP/SMHB, 1997), based mostly on the assumptions of reducing storm intensity and volumes when a storm travels in land from the coast to the dam catchments mostly located in the interior region of Kelantan. Analyses of the rainfall data immediately after the storm did support such observation. In this study, several transposition factors were derived, such as based on the ratios of annual rainfall, wet seasonal rainfall, maximum 5-day rainfall, and multiple regression approach of 5-day 100-year maximum rainfall.

These transposition factors were estimated ranges from as high as 0.92 to as low as 0.40. After diligent deduction, a medium value of 0.70 and 0.85 was finally selected for Kemubu and Lebir dams respectively (SSP/SMHB, 1997).Long-Duration PMPLong duration PMP adopted by SMHB/B&P is based on the maximization of the December 1970 and January 1971 storm records of Mersing and Air Tawar rainfall stations near Endau. These values have also been previously used earlier studies by SSP/SMEC (1985; as quoted in SMHB, 1985) flood studies on Sg. Batu Pahat basin.

SSP/SMEC (1985) study used maximum rainfall records at Mersing Meteorological Station for their works but it was reported later by SMHB (1985) that recent investigation indicated several other rainfall stations recorded maximum rainfall in excess of those reported by SSP/SMEC (1985).

Notably, the highest total rainfalls were at Air Tawau School near Endau with maximum 5- and 7-day total of 1453 mm and 1632 mm respectively. Other rainfall stations nearby, JPT Setor in Endau, about 5 km from the Air Tawar School station also recorded higher rainfall, i.e. 1600 mm for 5-120 hour within a 7-day duration. This infers unrestricted total records from automatic station at JPS store in Endau, 5 km away from Air Tawar School. The 120-hour or 5-day PMP was 2030 mm if maximization factor (i.e. about 1.27) was taken into consideration (SMHB, 1986).

Table 3.7 shows the long duration PMPs adopted by SMHB in its respective dam design assignment.Table 3.7: Coastal and Inland PMPs (Long-Duration) adopted by SMHB

Duration (hour)Coastal PMP (mm)Inland PMP (mm)

(1)(2)(3)

Short Duration PMP

24-*777* 692

Long Duration PMP

48-#1356# 908

72-#1593# 1067

120-#2030# 1360

col 2* 0.89; # col 2*0.67

Maximization of long duration storm

For 120-hour storm, a maximization factor of 1.40 has been used. It is therefore assumed that storms between 24- to 120-hour duration would have adopted the same factor.

Transposition of long duration storm

For transposition, a factor of 0.67 was applied for PMP duration of more than 24 hour. This factor was adopted based on the USBR recommendation in their review of PMP estimates for the Batu, Gombak, and Klang Gates dams (as quoted in SMHB, 1994; USBR, 1984).3.2.6Review on Other PMP Studies in MalaysiaA fair and reliable source of PMP studies is from the past studies carried out by various engineering consultants in Malaysia. The results of PMPs needless to say, are also based on the consultants engineering intuitions and their relative accumulated experiences especially in reservoir/dam engineering design and construction projects.

These are good sources for reference and therefore, accorded with priority to review their findings as appropriate. A short summary of PMP estimates of various reservoir/dam projects in Malaysia is presented previously in Table 3.5.

Of particular interest to this study, PMP reports and studies in Peninsula Malaysia, particularly in the State of Selangor (of which seven (7) existing dams/reservoirs are located including Batu and Klang Gates dam under the jurisdiction of Kuala Lumpur are located) are also utmost relevant in this review.

In addition comparison to the world maximum value is also imperative. World Meteorological Organization (WMO) maintains a register of record of maximum precipitation throughout the world (see Table 3.8). Figure 3.6 shows the world maximum PMPs as varied with the storm duration.Table 3.8: World Highest Precipitation Source: WMO 1986

Figure 3.6: World Highest Precipitation: Depth Vs DurationIt is undoubtedly that during the course of dam and reservoir engineering design in Malaysia, PMP/PMF derivations for various projects are primarily hinged on the practices and conventions that are familiar to the engineering consultants.

Fragmentation and diversification of opinion are the norms as the PMPs derived respectively vary from consultants own experiences in their undertakings. Thus this appears to be subject of contention and judgement. For example, SMHB/B&P relies mostly on the storm records in the Southern Johor in their deliberation of PMP values for various dam/reservoir projects in Malaysia, whilst some other consultants rely mostly on the practices of ABM. Opinions and PMPs are thus greatly varied from one consultancy practices to the others. Recent NAHRIM (2009) shows that the maximum observed rainfalls in the east coastal region of Kelantan and Terengganu have surpassed the observed maximum in Mersing, Air Tawar in the southern state of Johor.

One of the first unified attempts/efforts made to present a comprehensive review and derivation of the PMP in Malaysia, was a paper presented in the 1-day Specialty Seminar by Jabatan Pengairan dan Saliran and Humid Tropic Center (JPS/HTC) using Hershfield statistical methodology (WMO 1986, 2009) and subsequent several publications was made by the same authors for PMPs in the states of Selangor and Johor.

Under the auspice of National Water Resources Study 2000-2050 (SMHB/RB/JPZ, 2000), a peninsular wide PMP study was carried out as part of the baseline or background design parameters for future dam/reservoir schemes in Malaysia. PMPs for various durations, starting from 1- to 120-hour were derived using storm maximization of point rainfall station throughout Peninsular Malaysia. Regionalization of the point rainfall stations was carried out by mapping the isohyetal line or contour ranging from 1- to 120 hour. Figure 3.7 shows a 24-hour PMP contour map for peninsular Malaysia.

Figure 3.7: Peninsular Malaysia PMP: 1-day (Al Mamun and Hashim, 2004; originally published in NWRS (2000))Other than local interests and efforts in PMP derivation, a paper on the estimation of PMP based on proxy (i.e. radar satellite) data was also available for review. In this paper, the results of PMPs derived in Malaysia were compared to the observed radar reading. This paper was originally presented in a workshop on satellite weather forecasting in Uruguay (http\\www.unesco.org.uy) and subsequently a full paper and results were published in Elseviers Journal of Hydrology. In this study, comparison and references on PMP values were also made on a small subcatchment of Sg. Terengganu basin in the eastern coastal region of the Peninsula Malaysia. Due its relevance to the PMP study, this particular paper was reviewed as appropriate and pertinent information is therefore excerpted for comparison purpose. SMHB/B&P PMP estimations for various dam design projects undertaken in Malaysia were based largely on the findings and opinions of SSP/HH (1979), B&P (1980), and subsequent review on Bekok dam by SSP/SMEC (1985) for both short- and long-duration events.

Alternatively, Statistical approach, i.e. Hershfield technique, suitable for regions with scarce hydrometric data (dew points, wind records, etc), is sometimes used for checking and verification as deemed appropriate. With sufficient and significant in length of the historical rainfall records, Hershfield technique can be performed readily. As a result, collaborated efforts by JPS/HTC to estimate 24-hour PMP based on statistical technique in Selangor and Johor were materialized.

In addition, a detailed hydrological study was also carried for the detailed design of the Perang Besar dam in the new Government Administrative Center of Putrajaya in southern Selangor. Besides, independent reviews on the PMP using hydro-meteorological or traditional approach was also undertaken in a hydrology study by Nippon-Koei/SMHB in earlier 2000. The results obtained by this specific study basically confirmed the earlier SSP/SMHB (1996) study on PMP derivations in the state of Kelantan using east coastal observed storms.

The following reports/studies are reviewed and the results of PMP derived and used in their respective reservoir or dam design projects are presented in the following subsections accordingly.1 Langat Miles 24 Dam Stage II Design, March 1976

2 Dams on Sg. Bekok and Sg. Semberong, Detailed Investigation and Design: Hydrology, SSP and Howard Humphrey, October 1979

3 Modifications to the Seletar and Upper Peirce Reservoirs to Provide Additional Storage, PUB, B&P, 1980

4 Klang River Basin integrated Flood Mitigation Projects, Malaysia, Final Report, Kinhill Engineer Pty Ltd in association with Ranhill Bersekutu Sdn Bhd, November 1994

5 Kelantan River Flood Mitigation Plan Feasibility Study, SSP/SMHB, 1999

6 Putrajaya: Perang Besar Reservoir Design Study, Angkasa-GHD, 1998

7 Radar and Storm Model-based Estimation of Probable Maximum Precipitation in the Tropics, P.J. Hardaker and C.G. Collier, 1999, www.unesco.org.uy

8 National Water Resources Study 2000-2050, Hydrology Chapter on PMP Derivation, SMHB/RHB/ZAABA, 2000

9 Pahang-Selangor Raw Water Transfer Project Engineering Services and Detailed Engineering Design: Hydrology, August 2000

10 Probable Maximum Precipitation for 24 Hours Duration over Southeast Asian Monsoon Region- Selangor Malaysia, Desa, Noriah, Rakhecha. Extreme of the Extreme Rainfall in Selangor, JPS/HTC Seminar, September 2000

11 Short Duration Extreme Rainfall in Selangor, Desa and Rakhecha, ICUH 2002. Proceeding 2002

12 Gelami Dam Design Hydrological Study, JPS, 2002

13 Sg. Kelinchi Dam Water Resources Study, SSP/MM, 2001

14 Feasibility Study on Water Resources Development for Seremban and Port Dickson, SSP/SMEC, 1990

APPENDIX B summaries some of the PMPs adopted by above mentioned studies.3.2.7Temporal Distribution of PMPTemporal pattern of the PMPs is needed for convoluting the inflow hydrographs to the reservoir. This can be accomplished by knowing the temporal or time distribution of the PMP pattern in a region. Normally to carry out the task of deriving temporal pattern of storm rainfall occurrence requires task of searching through the recorded rainfall database. The observed temporal distribution could therefore be used to represent the PMPs in a given watershed or basin. Fortunately in Malaysia, these temporal patterns are being well documented in the JPS standard engineering manual of practices. Several pattern or arrangement of rainfall is available as follows:

(1) Bell shape time distribution of PMPs for all durations is generally acceptable for this purpose in reservoir/dam design.

(2) Patterns up to 6-hour time interval such as tabulated in the MSMA Manual (JPS, 2000)

(3) Patterns from 3 hour up to 24-hour obtained from JPS Hydrological Procedure No: 1 (Fadhillah et al, 1982)

Tables 3.9 and 3.10 show the fractions and inland PMP distributions for bell shaped temporal pattern from 3- to 48 hour, whereas Tables 3.11 show the PMP temporal fractions for 3- and 6-hour duration in MSMA (2000). Temporal pattern presented in the Hydrological Procedure No: 1 (Fadhillah et al, 1982) is shown in Figures 3.8 to 3.12.Experiences showed that amongst the tree temporal patterns as mentioned above, bell shape pattern is commonly associated with PMP/PMF undertakings. Most if not all dam/reservoir design projects in Malaysia are based on this form of distribution which convolutes the highest PMF for a given PMP distribution.Figure 3.13 shows the bell shape temporal pattern of the PMPs adopted in this study.Table 3.9: Temporal Storm Pattern: Fraction

Table 3.10: Temporal Storm Pattern: Inland PMPs for various Durations

Table 3.11: Temporal Storm Pattern: FractionJPS MSMA (2000) for 3- and 6-hour Duration

Example for 391 mm 6 hour storm

Figure 3.8: Temporal Storm Pattern (west coast 3 hours): HP No: 1 (1982)





Figure 3.13: Temporal Storm Pattern: Bell Shape Curve 3-, 12-, and 24- Hour

24 Hour @ 0.5 and 1 Hour Time Step Increment

1.2.3 Areal Reduction Factor (ARF)Areal Reduction Factor (ARF) is defined as Ratio of a mean areal rainfall for a given duration and given return period to a mean point rainfall for the same duration and same return period in the same area. Areal reduction factor (ARF) is a key quantity in the design that used to adjust the PMP values from point to areal or catchment wide domain. The adjusted PMP values will then used for generating PMF values. The main reason to adjusting the PMP values is to prevent hydrologic extremes or in overestimate of PMPs. This reduction accomplished by multiplying the point PMP to an appropriate reduction coefficient.

Figure 3.14 shows ARFs for duration ranging from 0.5- to 24 hour that are originated from US National Weather Services (NWS) and are also being widely adopted in Malaysia.

For a smaller reservoir catchment area, the reduction factors are negligible. They are ranging from 0.83 to 0.98 for durations starting from 0.5- to 24-hour. It is therefore proposed that no reduction factor will be taken into account in the derivation of PMFs.

Figure 3.14 Areal Reduction Factor: ARF (NWS, USA)1.3 Probable Maximum Flood1.3.1 IntroductionProbable Maximum Precipitation (PMP) is derived from observed maximum rainfall records with some stringent rule and provision of storm maximization and transposition procedure in tandem (WMO, 1986, 2009). Theoretically, the magnitude of precipitation (or rainfall) on par with the PMP event is perhaps never observed in the life time of a reservoir/dam structure. As discussed earlier, PMPs are basically reviewed from time to time with additional observed maximum records over the years. However, there is no guideline on the span or interval of such review should be taken place.

Based mostly on practical experiences in Malaysia, PMPs are derived based on the historical maximum rainfall records mostly in the east coastal regions of the Peninsular Malaysia. These states are from the northeastern corner of Kelantan, proceeding down to the southern tip of the Peninsular Malaysia, including the states of Terengganu, Pahang, and most of the eastern seaboard of the state of Johor. These particular regions experience the most heaviest downfalls during the monsoonal months from November to earlier February of next calendar years (known as Northeastern Monsoon, locally). The storms can run for a span of several days to a week with intermittent rainfall events of various intensities.

The observed records that were collected over the years, both recording and non recording maximum rainfalls alike. These records provide the basis of PMP derivation. Major undertakings were carried out in the Kelantan Flood Mitigation Project (SMHB/SSP, 1997) and Interstate Raw Water Transfer from Pahang to Selangor (NK/SMHB, 2000).

The PMPs as mentioned earlier, are seldom observed in real world or else the adoption of such PMP values in dam and reservoir design would be considered a gross underestimatation. It is generally according to the world wide practices in dam design, adopted for the design of the dam structure (in addition, this methodology is also applicable in the nuclear power facility design) out of concern for public safety in light of the occurrence of the exceptionally heavy rainfall events.

The outlet structures (sluice gates and spillways) of a dam shall be able to evacuate an extreme flood of PMP magnitude. To estimate the incoming flood into a reservoir, an appropriate technique is needed to translate the PMPs into probable maximum flood (PMF). In turn it is adopted for the spillway design of a dam.

The objective of this segment of the study is to address the mechanisms and procedures on the translation of the Probable Maximum Flood (PMF) using Probable Maximum Precipitation (PMP) derived and addressed earlier in the PMP issue. The technique is termed as convolution. The Probable Maximum Precipitation (PMP) is defined by the World Meteorological Organization (1986; 2009) as a quantity of precipitation that is close to the physical upper limit for a given duration that is meteorologically possible over a particular basin.

By the same definition, PMF is the flood that may be expected from the most severe combination of critical meteorological and hydrologic factors/conditions that are reasonably possible in a particular catchment basin/area (Chow et al, 1988). The Probable Maximum Flood (PMF) is derived from the Probable Maximum Precipitation (PMP) is the design flood inflow into reservoirs where involve in spillways design. So that, designing spillways using PMF is to avoid the overtopping of dams and avoid from failure of dam structure.

It is always necessary to determine the largest flood possible at a location when designing a dam for maximum reliability as well as safety. In the case where the risk of dam overtopping is deemed unacceptable, an estimate of the PMP is used to generate the Probable Maximum Flood (PMF) at the dam site. The translation of PMP to PMF is akin to a conventional rainfall runoff routing technique, has become the standard for the dam worldwide.

As mentioned earlier, rainfall runoff modelling approach has been the convention as well as industrial standard in assessing the quantitative flooding impacts of extreme storm events in a watershed/basin. The mathematical modelling tools are also being frequently used to assess the magnitude of flood flows and stages for a given probability of rain storm occurrence in a river basin or channel.

On further elaboration, they are also being routinely used to generate rainfall induced runoff hydrograph for special hydraulic structure design such as dams and outlet structures, etc.

The modelling approaches perform the task of translating and routing of PMPs into PMFs of various rainstorm durations. Out of many hydrological rainfall runoff techniques available worldwide and on occasion of those have been adopted from time to time, two approaches or models are the most commonly used in Malaysia. They are:

1. HP 11: Unit flood hydrograph type (Taylor and Toh, 1976)

2. RORB: Rainfall runoff model for basin scale (Laurensen and Mein, 1985)

Unit hydrograph UH approach similar to the technique espoused in HP 11 (1980) was also been used in the design study of some dams in the earlier 1960s when computer modeling tools are basically not available at that time. A unit hydrograph of specific duration was generated using an effective rain storm volume of 1 cm. Unit hydrographs were also developed for other storm durations up to 48 hour as well. The last step of the derivation using HP 11 (Taylor and Toh, 1976) technique was the convolution of the hydrograph.

The unit hydrograph for any duration could then be then used to convolute the PMFs for given PMPs value of specific duration by assumption of superposition of rainfall depth.1.3.2 Hp 11 Hydrological Procedure (Taylor And Toh, 1976)

HP 11 (Taylor and Toh, 1976) was a modified Snyder type of synthetic unit hydrograph approach that use for simple flood estimate procedure for a small basin(maximum catchment area up to 518 km2) in Peninsular Malaysia. This is therefore supposedly the upper limit of the catchment area draining the stream flow station used in the calibration exercise.

The advantages of this unit hydrograph based model is that it can be used to distribute runoff from storms of varying temporal pattern and also the fact that the calibrating parameters required are kept to minimum but generally includes essential topographic and geographic/geometric properties, such as the catchment slope, catchment area, reach lengths, etc.The performance of HP-11 (Taylor and Toh, 1976) was generally deemed satisfactory and acceptable in PMP/PMF translation as adequate understandings of the mechanisms of runoff generation mainly in Peninsular Malaysia. The output of a HP 11 (Taylor and Toh, 1976) is the 10 mm-unit hydrographs for various storm durations by the catchment geophysical characteristics, such as time of concentration, general catchment slope etc. Subsequently hydrographs are derived using convolutional approach.

This procedure is a variant of the time honored Clark Hydrograph technique (Subramanya, 1994) where in the unit hydrograph derivation for catchments without actual observed rainfall and runoff records are unfortunately unavailable. It is a shortcoming in Malaysia as well as worldwide that the data sets that is used for calibration is not readily available. Therefore recourses are then made to correlate geographic properties that are readily measured from topography map to the hydrograph. Compared to other much more sophisticated model, it does not seem to make much difference in the hydrographical variables.The steps for estimating the design flood hydrograph are as follows:

1. For a topographical map, compare the topography of the catchment with similar catchments studied in the investigation and select the appropriate hydrographical group. Compute L, Lc, A and S for the catchment.2. Calculate Lg for the catchment using equation below with n equal to 0.35, and C t values can obtained from appendix c table 1.Lg = Ct x (LLc/ S1/2)n3. Calculate the design storm for the catchment using D.I.D. hydrological procedure No.1 (Heiler 1973). The design storm should be calculated for a range of durations. Experience suggests that the critical duration giving highest peak discharge is often similar to the catchment lag time.

4. Calculate Q from equation below.Q = 0.33P

Q = P2/(P +6)

5. Calculate qp from the following equation, if the total hydrograph

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