Universiti Malaysia Sarawak

COMPUTATION OF PROBABLE MAXIMUM PRECIPITATION FOR UPPER RAJANG RIVER BASIN, SARAWAK

Marina Patrick

Master of Engineering (Civil Engineering)

2014

UNIVERSITI MALAYSIA SARAWAK

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Final Year Project Report

Masters √ PhD

DECLARATION OF ORIGINAL WORK

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I, MARINA PATRICK, 14030078, FACULTY OF ENGINEERING hereby declare that the work entitled COMPUTATION OF PROBABLE MAXIMUM PRECIPITATION FOR UPPER RAJANG RIVER BASIN, SARAWAK is my original work. I have not copied from any other students’ work or from any other sources except where due reference or acknowledgement is made explicitly in the text, nor has any part been written for me by another person.

_______________________ ____________________________________________

MARINA PATRICK (14030078)

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I F.J PUTUHENA ) hereby certifies that the work entitled COMPUTATION OF PROBABLE MAXIMUM PRECIPITATION FOR UPPER RAJANG RIVER BASIN, SARAWAK was prepared by the above named student, and was submitted to the “FACULTY” as a partial fulfillment for the conferment of MASTER OF ENGINEERING (CIVIL ENGINEERING) and the aforementioned work, to the best of my knowledge, is the said student’s work.

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APPROVAL SHEET

This project report which entitled “Computation of Probable Maximum Precipitation for Upper Rajang River Basin, Sarawak’’ was prepared by Marina Patrick (14030078) is hereby read and approved by:

Prof. Dr. F.J Putuhena Date:

Project Supervisor

COMPUTATION OF PROBABLE MAXIMUM PRECIPITATION FOR UPPER RAJANG RIVER BASIN, SARAWAK

MARINA PATRICK

Master of Engineering (Civil Engineering)

2014

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To my beloved family

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ACKNOWLEDGEMENT

First and foremost, I would like to extend my sincerest gratitude towards my supervising lecturer, Prof. Dr F.J Putuhena. Attribute to his guidance and support throughout the duration of my study has contributed to this level of Master research.

I am grateful to acknowledge the contributions of the Hydrology and Water Resources Branch from the Department of Irrigation and Drainage, Sarawak for allowing me to gain access and collect necessary data and information in fulfilment of this thesis. I hereby would like to offer my gratitude towards their technical officers, Mr Naet anak Nyawem and Ms Jumaliah binti Sarkawi for their kind assistance.

I would also like to express note of appreciation to the Hydropower Development Department of Sarawak Energy Berhad for granting me to access their relevant reports and for providing the necessary documents to me as well. I hereby would like to extend my thanks towards the Project Director, Mr Brian Giles for his kind approval and Hydrologist, Ms Susie Nadya for her kind assistance.

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ABSTRACT

Developments of mega dam projects that are thriving in Sarawak particularly in the Upper Rajang River Basin may not only bring great prospects to this region, but could also cause irreversible destruction. Dam failures have always been associated with devastating floods. Correspond to maximum flood potential used for the safety of the dam spillway design is the Probable Maximum Precipitation (PMP).

Therefore, this study attempted to estimate and analyse the PMP for different rainfall stations within Upper Rajang River Basin using the statistical approach and prepare the spatial distribution for 1-day areal PMP. The amount of subjectivity in PMP estimations can be minimized, and consistent results for any location can be achieved with this research. Comparisons of the PMP estimates based on the statistical approach of Hershfield (1965) were analysed with the PMP values computed using the National Hydraulic Research Institute of Malaysia (NAHRIM) Technical Research Publication No. 1 (TRP 1: 2008) manual, and the Conventional method of the statistical approach. The results obtained by these methods were then compared with hydrological studies by Sarawak Electricity Supply Cooperation, SESCO (1983). It was found that PMP estimates by NAHRIM (2008) is conservative and may not be feasible; Hershfield method can produce reasonable PMP estimates and is valid for subsequent design calculations; Conventional method is comparable to the Hershfield method and have produced more conservative results by performing quick statistical analysis. When PMP estimates achieved from each method was validated with the estimates by SESCO (1983), the closest value was from the Hershfield method. All three methods discussed have proven to be useful for PMP estimations when practiced vigilantly. Hence, the use of statistical approach is deeming acceptable for computation of PMP estimates.

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ABSTRAK

Perkembangan projek empangan mega yang berkembang maju di Sarawak terutamanya di Lembangan Sungai Hulu Rajang, bukan sahaja boleh membawa prospek yang besar ke rantau ini, tetapi juga boleh menyebabkan kemusnahan yang tidak dapat dibaikpulihkan. Kerosakan empangan sentiasa dikaitkan dengan kejadian banjir. Bersesuaian dengan potensi banjir maksimum yang digunakan untuk keselamatan reka bentuk alur limpah empangan adalah Kebarangkalian Hujan Maksimum (PMP). Oleh itu, kajian ini bertujuan untuk menganggar dan menganalisis PMP untuk stesen tadahan air hujan yang terdapat di Lembangan Sungai Hulu Rajang, dengan menggunakan pendekatan statistik dan menyediakan areal taburan hujan untuk durasi 1-hari. Jumlah subjektiviti dalam anggaran PMP dapat dikurangkan, dan hasil yang konsisten untuk mana-mana lokasi boleh dicapai melalui kajian ini. Perbandingan anggaran PMP berdasarkan pendekatan statistik Hershfield (1965) telah dianalisis dengan nilai-nilai PMP yang diperoleh secara manual dari Institut Penyelidikan Hidraulik Kebangsaan Malaysia (NAHRIM) Penerbitan Penyelidikan Teknikal No. 1 (TRP 1: 2008), dan kaedah Konvensional pendekatan statistik. Keputusan yang diperoleh melalui kaedah ini kemudiannya dibandingkan dengan kajian hidrologi oleh Perbadanan Pembekalan Letrik Sarawak ((SESCO), 1983). Hasil daripada kajian ini, telah didapati bahawa anggaran PMP dengan menggunakan manual NAHRIM (2008) adalah konservatif dan kemungkinan tidak sesuai; kaedah Hershfield pula menghasilkan anggaran PMP yang munasabah dan sah untuk pengiraan reka bentuk yang selanjutnya; kaedah Konvensional setara dengan kaedah Hershfield telah menghasilkan keputusan yang lebih konservatif dengan melakukan analisis statistik yang ringkas. Apabila anggaran PMP yang dicapai daripada setiap kaedah telah dibandingkan dengan anggaran SESCO (1983), nilai yang paling hampir adalah daripada kaedah Hershfield. Ketiga-tiga kaedah yang dibincangkan telah terbukti berguna untuk anggaran PMP apabila diamalkan dengan betul. Oleh itu, penggunaan pendekatan statistik boleh diterima untuk membuat pengiraan anggaran PMP.

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TABLE OF CONTENT

CONTENTS PAGE

ACKNOWLEDGEMENT iii

ABSTRACT iv

ABSTRAK v

TABLE OF CONTENT vi

LIST OF TABLE ix

LIST OF FIGURE xii

LIST OF ABBREVIATION xv

CHAPTER 1 INTRODUCTION

1.1 Background 1

1.2 Objectives and Scope of Study 5

1.2.1 Objectives 5

1.2.2 Scope of Study 6

1.3 Report Outline 7

CHAPTER 2 LITERATURE REVIEW

2.1 Overview of Probable Maximum Precipitation 8 2.2 Definition of Probable Maximum Precipitation 9

2.2.1 Conceptual Definition 9

2.2.2 Operational Definition 10

2.3 Methods of Estimating Probable Maximum 11

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Precipitation

2.4 Estimation of Probable Maximum Precipitation 14 2.4.1 Depth-Area-Duration Curves 15 2.4.2 Standard Isohyetal Pattern 17 2.4.3 Orientation Adjustment Factor 18

2.4.4 Critical Storm Area 19

2.4.5 Isohyetal Area Factor 22

2.5 Probable Maximum Precipitation Statistical Estimates

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CHAPTER 3 METHODOLOGY

3.1 Study Area 50

3.2 The Selected Approach 53

3.3 Collection of Hydrological Data 55

3.4 Checking of Data Consistency 58

3.5 Statistical Parameters 60

3.6 Development of Frequency Factor Envelope Curve

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3.7 Derivation of Point Probable Maximum Precipitation

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3.8 Mapping of Probable Maximum Precipitation Isohyets

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3.9 Comparison of Probable Maximum Precipitation Estimations

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CHAPTER 4 RESULTS, ANALYSIS AND DISCUSSION

4.1 Homogeneity of Hydrological Data Series 66

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4.2 Statistical Estimates using the Hershfield Method

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4.3 Statistical Estimates using the NAHRIM (2008) Manual

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4.4 Statistical Estimates using the Conventional Method

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4.5 Probable Maximum Precipitation Isohyetal Maps

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4.6 Analysis of Probable Maximum Precipitation 86 4.7 Evaluation of Statistical Analysis 93

4.8 Discussion 99

CHAPTER 5 CONCLUSION AND RECOMMENDATIONS

5.1 Conclusion 104

5.2 Recommendations 109

REFERENCES 111

APPENDIX A 117

APPENDIX B 125

APPENDIX C 133

APPENDIX D 135

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LIST OF TABLE

CONTENTS PAGE

Table 2.1 Values of Average Catchment Rainfall over Point Rainfall Estimate

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Table 3.1 Details of the Rainfall Stations in the Upper Rajang River Basin According to Grid Coordinate System

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Table 3.2 Details of the Rainfall Stations in the Upper Rajang River Basin Sarawak

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Table 4.1 Level of Significance Ratio of Data Series for Individual Rainfall Stations

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Table 4.2 Frequency Factor, (K_m) and Mean One-Day Maximum Rainfall, _n

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Table 4.3 New Frequency Factor, (Km) and Point PMP for One-Day Duration using the Hershfield (1965) method

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Table 4.4 Frequency Distribution of Frequency Factor, (Km) values for One-Day Duration

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Table 4.5 New Frequency Factor, (K_m) and Point PMP for One-Day Duration using the NAHRIM (2008) manual

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Table 4.6 Frequency Distribution of Frequency Factor, (K_m) values for One-Day Duration

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Table 4.7 New Frequency Factor, (Km) and Point PMP for One-Day 75

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Duration using the Conventional method

Table 4.8 Point Indicators for Station Name 76

Table 4.9 Summary of Point PMP for One-Day Duration from Various Calculation Methods

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Table 4.10 Derived PMP values for Various Durations for Upper Rajang River Basin

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Table 4.11 Comparison of the Average PMP over Area values for 24- hour Duration

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Table 4.12 Comparison of Derived PMP values for 24-hour Duration 100 Table A-1 Mann-Kendall Rank Test for Long Singut Rainfall Station 117 Table A-2 Mann-Kendall Rank Test for Long Sambop Rainfall

Station

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Table A-3 Mann-Kendall Rank Test for Long Luar Rainfall Station 119 Table A-4 Mann-Kendall Rank Test for Long Lidam Rainfall Station 120 Table A-5 Mann-Kendall Rank Test for Long Busang Rainfall Station 121 Table A-6 Mann-Kendall Rank Test for Long Jawe Rainfall Station 122 Table A-7 Mann-Kendall Rank Test for Entawau Rainfall Station 123 Table A-8 Mann-Kendall Rank Test for Belaga Rainfall Station 124 Table B-1 One-Day Maximum Rainfall and Frequency Factor, (K_m)

of Long Singut Rainfall Station

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Table B-2 One-Day Maximum Rainfall and Frequency Factor, (K_m) of Long Sambop Rainfall Station

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Table B-3 One-Day Maximum Rainfall and Frequency Factor, (K_m) of Long Luar Rainfall Station

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Table B-4 One-Day Maximum Rainfall and Frequency Factor, (K_m) of Long Lidam Rainfall Station

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Table B-5 One-Day Maximum Rainfall and Frequency Factor, (K_m) of Long Busang Rainfall Station

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Table B-6 One-Day Maximum Rainfall and Frequency Factor, (Km) of Long Jawe Rainfall Station

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Table B-7 One-Day Maximum Rainfall and Frequency Factor, (Km) of Entawau Rainfall Station

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Table B-8 One-Day Maximum Rainfall and Frequency Factor, (Km) of Belaga Rainfall Station

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Table D-1 World’s Greatest Observed Rain Gauge Depths (as at year 1965)

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LIST OF FIGURE

CONTENTS PAGE

Figure 1.1 Locality Map of Rajang River Basin 3

Figure 1.2 Locations of Existing and Proposed HEP Dams in Sarawak

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Figure 2.1 Nomograph of K_m as a Function of Rainfall Duration and Mean of Annual Series

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Figure 2.2 Adjustments of Mean of Annual Series for Maximum Observed Rainfall

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Figure 2.3 Adjustments of Standard Deviation of Annual Series for Maximum Observed Rainfall

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Figure 2.4 Adjustments of Mean and Standard Deviation of Annual Series for Length of Record

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Figure 2.5 Adjustments of Fixed-Interval Precipitation Amounts for Number of Observational Units within the Interval

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Figure 2.6a) Isohyetal Pattern Centred over Basin as would be the Case for Storm- Centred Depth-Area Curves

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Figure 2.6b) Two Possible Occurrences of Isohyetal Patterns over a Geographically Fixed Area as would be the case in Development of Curves for a Geographically Fixed Area

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Figure 2.7 Depth-Area, or Area-Reduction, Curves for Western 38

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United States

Figure 2.8 Maximum Depth-Duration Curve 41

Figure 3.1 Locations of the Selected Rainfall Stations 51 Figure 3.2 Locations of the Rainfall Stations in the Upper Rajang

River Basin According to Grid Coordinate System

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Figure 3.3 Flowchart of the Procedures using the Statistical Approach

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Figure 4.1 Plot of Frequency Factor, (K_m) against Mean One-Day Maximum Rainfall using the Hershfield (1965) method

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Figure 4.2 Plot of Frequency Factor, (Km) against Mean One-Day Maximum Rainfall using the NAHRIM (2008) manual

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Figure 4.3a) Isohyetal Pattern of PMP for One-day Duration using the Hershfield (1965) method

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Figure 4.3b) Isohyetal Pattern of PMP for One-day Duration using the NAHRIM (2008) manual

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Figure 4.3c) Isohyetal Pattern of PMP for One-day Duration using the Conventional Method

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Figure 4.4a) Isohyetal Pattern of PMP for One-day Duration using the Hershfield (1965) Method layered on Generalised Map

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Figure 4.4b) Isohyetal Pattern of PMP for One-day Duration using the NAHRIM (2008) manual layered on Generalised Map

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Figure 4.4c) Isohyetal Pattern of PMP for One-day Duration using the NAHRIM (2008) manual layered on Generalised Map

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Figure 4.5 Generalised PMP Isohyets for One-day Storm in East Malaysia

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Figure 4.6 Comparison of Point PMP values for One-Day Duration using Various Calculation Methods

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Figure 4.7 Comparison of the Highest Point PMP values for One- Day Duration from Various Calculation Methods

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Figure C-1 Location Map of Upper Rajang Catchment (Dam Site Catchment Areas)

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Figure C-2 Location Map of Upper Rajang Catchment (Hydrometeorological Network of Upper Rajang River Basin)

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Figure D-1 World’s Greatest Observed Rain Gauge Depths 136 Figure D-2 Standard Isohyetal Pattern Recommended for Spatial

Distribution of PMP East of the 105^th Meridian

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Figure D-3 Analysis of Isohyetal Orientations for Selected Major Storms, adopted as Recommended Orientation for PMP, within ± 40º

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Figure D-4 Nomograph for Determining Isohyet Precipitation Values from the PMP Estimate for a Given Storm Area

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Figure D-5 Depth-Area-Duration Envelope Curves 140

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LIST OF ABBREVIATION

ARF - Areal Reduction Factor DAD - Depth-Area-Duration

DID - Department of Irrigation and Drainage ESRI - Environmental Systems Research Institute GIS - Geographic Information Systems

HEP - Hydroelectric Power HMR - Hydrometerological Report MMS - Malaysia Meteorological Services MPP - Maximum Possible Precipitation

NAHRIM - National Hydraulic Research Institute of Malaysia NOAA - National Oceanic and Atmospheric Administration PMF - Probable Maximum Flood

PMP - Probable Maximum Precipitation PMS - Probable Maximum Storm

SESCO - Sarawak Electricity Supply Corporation SHSB - Sarawak Hidro Sendirian Berhad SMT - Storm Maximisation and Transposition SIWRS - Sarawak Integrated Water Resources WMO - World Meteorological Organization

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CHAPTER 1

INTRODUCTION

1.1 Background

As the country is rich of water resources due to the seasonal monsoon precipitation, the needs for developing of proper water resources is important in Malaysia. It is essential to ensure sufficient supply of potable and industrial water and provide irrigation systems for food production as well as for hydro power generation.

The construction of dams and storage reservoirs has been used for centuries to collect and store runoff water for the needs of the people. In the earlier days, the design of large water resources projects such as dams and storage reservoirs were based on the analysis of major recorded storms within that region. Nonetheless, questions arise whether these records of heavy rainfalls will continuously supersede or whether there is existence of physical limit to these rainfall records. Hence, the concept of probable maximum storm or also known as probable maximum precipitation (PMP) was introduced (NAHRIM, 2008).

For a river basin, the PMP refers to the amount of rainfall depth that is close to the physical upper limit for a given duration over a particular drainage area. The estimates of PMP are needed to calculate the resulting probable maximum flood

2

(PMF) hydrograph which is the design flood for spillways of large dams without considering any risk of failure. PMF is put into considerations in the design stage to prevent the potential danger and damage that may occur due to breaching of the dam wall by overtopping (Rakhecha & Singh, 2009).

For PMP estimation practises in Malaysia, there is non-uniformity in the methods adopted throughout the country. Major studies of water resources projects such as dams were carried out by various agencies, thus the calculated PMP values differentiates from different studies. Most studies were conducted by maximising the largest recorded storm in the region and by transposing to the site area. There were also studies based on the Hershfield statistical approach considering the frequency factor of 15, which are the highest value in the world and not a reliable value for Malaysian climatic region (NAHRIM, 2008).

Adoption of statistical approach is useful for analysing the PMP estimates when other meteorological data such as the dew point temperature records are unavailable. However these are point estimates, and the conversion of point PMP to areal PMP were conducted by applying the areal reduction factor (ARF) based on both the size of the catchment and chosen duration. The application of ARF factors in Malaysia is yet to be investigated and it is known to be high in tropical climate condition such as in Malaysia.

Realising the importance of the PMP rainfall in the dam design work, an attempt has been made in this research to estimate PMP for 1-day duration for various rainfall stations in Sarawak. The Upper Rajang River Basin is one of the major river basins in Sarawak and the Rajang River located in this basin is the longest river in Malaysia. Figure 1.1 showed the locality map of Rajang River Basin.

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It is known that one of the tributaries of the Rajang River is a site to the largest hydro power plant in Malaysia, referred as the Bakun Hydro Electric Dam. In the upstream of the Rajang River, there are four phases of hydroelectric power (HEP) project within the river basin as shown in Figure 1.2. The Murum Hydroelectric Project is the second phase, located 70km (43miles) from the constructed Bakun HEP downstream and is currently under operation since the year 2013. The remaining Pelagus and Belaga HEP projects are currently undergoing planning stage to date (SIWRS, Sarawak 2008).

Figure 1.1 Locality Map of Rajang River Basin (Source: DID Sarawak, (2009) as cited by Lau, (2011))

4

Figure 1.2 Locations of Existing and Proposed HEP Dams in Sarawak (Source: SIWRS, Sarawak (2008))

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KALIMANTAN

-- . _. ^,--,- - _{- -} - _{' - - -}

5

1.2 Objectives and Scope of Study

The aim of this study is to estimate and analyse the PMP for Upper Rajang River Basin for different rainfall stations within that region using the statistical method and prepare the spatial distribution of 1-day areal PMP. With this research, the amount of subjectivity in PMP estimations can be minimized, hence more uniform practices and consistent results for any location can be obtained. The objectives and the scope of works for this research are further discussed as follows:

1.2.1 Objectives

i) Study will examine and analyse the yearly maximum 1-day precipitation records of twenty to thirty years (20-30 years) from selected rainfall stations located in Upper Rajang River Basin.

ii) Based on the PMP estimates, a generalised map (isohyetal map) will be prepared and presenting the spatial distribution of 1-day areal PMP in the study area.

iii) Comparisons of the computed PMP values based on the selected approach will be analysed with the PMP values computed using the approaches in the National Hydraulic Research Institute of Malaysia (NAHRIM) Technical Research Publication No. 1 (TRP 1: 2008) manual, and recent studies by Hydro Electric Power’s Consultants in Sarawak.