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COMPUTATION OF PROBABLE MAXIMUM PRECIPITATION FOR
UPPER RAJANG RIVER BASIN, SARAWAK
Marina Patrick
Master of Engineering
(Civil Engineering)
2014
UNIVERSITI MALAYSIA SARAWAK
Grade:
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Final Year Project Report
Masters √
PhD
DECLARATION OF ORIGINAL WORK
This declaration is made on the day of . Student’s Declaration:
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)
Supervisor’s Declaration:
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.
Received for examination by: Date: _______________
F.J PUTUHENA
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research was done)*
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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
ii
To my beloved family
iii
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.
iv
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.
v
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.
vi
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
vii
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
25
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
61
3.7 Derivation of Point Probable Maximum
Precipitation
62
3.8 Mapping of Probable Maximum Precipitation
Isohyets
64
3.9 Comparison of Probable Maximum Precipitation
Estimations
65
CHAPTER 4 RESULTS, ANALYSIS AND DISCUSSION
4.1 Homogeneity of Hydrological Data Series 66
viii
4.2 Statistical Estimates using the Hershfield
Method
67
4.3 Statistical Estimates using the NAHRIM (2008)
Manual
71
4.4 Statistical Estimates using the Conventional
Method
74
4.5 Probable Maximum Precipitation Isohyetal
Maps
75
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
ix
LIST OF TABLE
CONTENTS PAGE
Table 2.1 Values of Average Catchment Rainfall over Point Rainfall
Estimate
39
Table 3.1 Details of the Rainfall Stations in the Upper Rajang River
Basin According to Grid Coordinate System
52
Table 3.2 Details of the Rainfall Stations in the Upper Rajang
River Basin Sarawak
57
Table 4.1 Level of Significance Ratio of Data Series for Individual
Rainfall Stations
66
Table 4.2 Frequency Factor, (Km) and Mean One-Day Maximum
Rainfall, n
68
Table 4.3 New Frequency Factor, (Km) and Point PMP for One-Day
Duration using the Hershfield (1965) method
70
Table 4.4 Frequency Distribution of Frequency Factor, (Km) values
for One-Day Duration
71
Table 4.5 New Frequency Factor, (Km) and Point PMP for One-Day
Duration using the NAHRIM (2008) manual
73
Table 4.6 Frequency Distribution of Frequency Factor, (Km) values
for One-Day Duration
74
Table 4.7 New Frequency Factor, (Km) and Point PMP for One-Day 75
x
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
87
Table 4.10 Derived PMP values for Various Durations for Upper
Rajang River Basin
92
Table 4.11 Comparison of the Average PMP over Area values for 24-
hour Duration
93
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
118
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, (Km)
of Long Singut Rainfall Station
125
Table B-2 One-Day Maximum Rainfall and Frequency Factor, (Km)
of Long Sambop Rainfall Station
126
Table B-3 One-Day Maximum Rainfall and Frequency Factor, (Km)
of Long Luar Rainfall Station
127
xi
Table B-4 One-Day Maximum Rainfall and Frequency Factor, (Km)
of Long Lidam Rainfall Station
128
Table B-5 One-Day Maximum Rainfall and Frequency Factor, (Km)
of Long Busang Rainfall Station
129
Table B-6 One-Day Maximum Rainfall and Frequency Factor, (Km)
of Long Jawe Rainfall Station
130
Table B-7 One-Day Maximum Rainfall and Frequency Factor, (Km)
of Entawau Rainfall Station
131
Table B-8 One-Day Maximum Rainfall and Frequency Factor, (Km)
of Belaga Rainfall Station
132
Table D-1 World’s Greatest Observed Rain Gauge Depths (as at year
1965)
135
xii
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
4
Figure 2.1 Nomograph of Km as a Function of Rainfall Duration and
Mean of Annual Series
27
Figure 2.2 Adjustments of Mean of Annual Series for Maximum
Observed Rainfall
29
Figure 2.3 Adjustments of Standard Deviation of Annual Series
for Maximum Observed Rainfall
30
Figure 2.4 Adjustments of Mean and Standard Deviation of Annual
Series for Length of Record
32
Figure 2.5 Adjustments of Fixed-Interval Precipitation Amounts for
Number of Observational Units within the Interval
34
Figure 2.6a) Isohyetal Pattern Centred over Basin as would be the
Case for Storm- Centred Depth-Area Curves
37
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
37
Figure 2.7 Depth-Area, or Area-Reduction, Curves for Western 38
xiii
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
52
Figure 3.3 Flowchart of the Procedures using the Statistical
Approach
54
Figure 4.1 Plot of Frequency Factor, (Km) against Mean One-Day
Maximum Rainfall using the Hershfield (1965) method
69
Figure 4.2 Plot of Frequency Factor, (Km) against Mean One-Day
Maximum Rainfall using the NAHRIM (2008) manual
72
Figure 4.3a) Isohyetal Pattern of PMP for One-day Duration using the
Hershfield (1965) method
77
Figure 4.3b) Isohyetal Pattern of PMP for One-day Duration using the
NAHRIM (2008) manual
78
Figure 4.3c) Isohyetal Pattern of PMP for One-day Duration using the
Conventional Method
79
Figure 4.4a) Isohyetal Pattern of PMP for One-day Duration using the
Hershfield (1965) Method layered on Generalised Map
80
Figure 4.4b) Isohyetal Pattern of PMP for One-day Duration using the
NAHRIM (2008) manual layered on Generalised Map
81
Figure 4.4c) Isohyetal Pattern of PMP for One-day Duration using the
NAHRIM (2008) manual layered on Generalised Map
82
Figure 4.5 Generalised PMP Isohyets for One-day Storm in East
Malaysia
85
xiv
Figure 4.6 Comparison of Point PMP values for One-Day Duration
using Various Calculation Methods
89
Figure 4.7 Comparison of the Highest Point PMP values for One-
Day Duration from Various Calculation Methods
91
Figure C-1 Location Map of Upper Rajang Catchment (Dam Site
Catchment Areas)
133
Figure C-2 Location Map of Upper Rajang Catchment
(Hydrometeorological Network of Upper Rajang River
Basin)
134
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 105th
Meridian
137
Figure D-3 Analysis of Isohyetal Orientations for Selected Major
Storms, adopted as Recommended Orientation for PMP,
within ± 40º
138
Figure D-4 Nomograph for Determining Isohyet Precipitation Values
from the PMP Estimate for a Given Storm Area
139
Figure D-5 Depth-Area-Duration Envelope Curves 140
xv
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
1
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.
3
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))
~ .. ''''';'':
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.
