STORM RAINFALL ANALYSIS FOR UPPER RAJANG RIVER
CATCHMENT
Mohammad Ishamudin b Ismail
Bachelor of Engineering (B. Eng)
(Civil Engineering)
2006
STORM RAINFALL ANALYSIS FOR UPPER RAJANG RIVER
CATCHMENT
MOHAMMAD ISHAMUDIN B ISMAIL
This project is submitted in partial fulfillment of
the requirements for the degree of Bachelor of Engineering (B. Eng)
(Civil Engineering)
Faculty of Engineering
UNIVERSITI MALAYSIA SARAWAK
2006
i
For my beloved Family
ii
Acknowledgment
I would like to first and foremost thank my parents for their continual and
unwavering support throughout this study. I would also like to express my gratitude to
my supervisor, Assoc. Prof. F. J. Putuhena, for his guidance and mentoring. My
heartfelt thanks also go out to the Director Of DID Sarawak, Deputy Director of DID
Sarawak, Mr Thomas Lau, whose patience and willingness to help which I can not do
without, and employees of DID Sarawak for their assistance in carrying out this study.
Not to forget, the staff of JKR Sarawak for their assistance and help. Finally, I would
like to thank everybody else who are directly or indirectly involved in contributing to
the success of this study.
iii
Abstract
There are numerous examples throughout history where settlements, and even entire
civilizations were damaged due to rampaging floods which were caused by heavy
rains. Rainwater, in small amounts, are necessary as a water source, yet can cause
flash floods and landslides in large amounts. Therefore, proper planning need to be
carried out to avert such catastrophes, which include preparing adequate drainage,
stormwater management and flood control projects. Sarawak, with its many and wide
areas of catchments, is able to make use of rainfall through generation of
hydroelectric power. The Upper Rajang catchment, being one of the largest river
basins, has great potential in this respect. Usage of Department of Irrigation (DID)
Sarawak supplied temporal profiles and IDF curves do not yield satisfying results
when used in calculations in designing a Design Storm for large hydraulic structures,
such as a barrage or a dam, therefore, a study was warranted in order to overcome this
problem. The storm characteristics such as the temporal and spatial profiles are
necessary in order to produce a Design Storm. This study utilizes a threshold in order
to select a sufficient number of storms that is gathered from the historical data. The
spatial profiles are generated from the daily rainfall records, whereas a temporal
profile study was carried out using the hourly data, each collected from automatic and
manual recording apparatus. A relationship between storm depth and area covered for
high recorded storms was established. The study also points out differences observed
between the temporal profiles available from historical data, and those provided by
the DID guidelines.
iv
Abstrak
Kesan-kesan buruk air boleh diperhatikan melalui sejarah, di mana
penempatan malahan tamadun dirosakkan oleh banjir yang disebabkan oleh hujan
berlebihan. Air hujan yang sedikit sesuai dijadikan sumber air tawar, namun sekiranya
dalam jumlah yang terlalu besar, boleh menagkibatkan banjir kilat. Oleh itu,
perancangan yang rapi harus diambil bagi mengelakkan kesan negatif oleh banjir,
termasuklah pengairan, pengurusan air hujan dan projek pengawalan banjir. Sarawak
mempunyai kawasan tadahan air yang besar dan banyak, adalah berpotensi dari segi
penjanaan tenaga hidroelektrik. Kawasan tadahan Rajang Atas merupakan salah
sebuah kawasan tadahan yang terbesar, mempunyai potensi yang cerah sebagai
sumber penjanaan tenaga hidroelektrik. Penggunaan “IDF curve” dan profail masa
yang disediakan oleh Jabatan Pengairan Sarawak tidak memenuhi keperluan bagi
menyediakan “Design Storm”, oleh itu sebuah kajian perlu bagi mengatasi kekangan
ini. Kajian ini menggunakan nilai had bagi membolehkan jumlah ribut yang didapati
daripada data tahunan adalah mencukupi. Profail masa dihasilkan daripada data setiap
jam, manakala profail kawasan dihasilkan daripada data hujan harian. Suatu hubungan
di antara kedalaman hujan dengan kawasan yang diliputi telah disediakan. Kajian ini
juga menunjukkan perbezaan di antara profail daripada data yang didapati daripada
data hujan sebenar berbanding profail yang disediakan oleh JPS.
Table of Contents
No Item Page
Dedication i
Acknowledgement ii
Abstract iii
Abstrak iv
1.0 Introduction
1.1 Introduction and Objectives 1
1.1.1 Introduction 1
1.1.2 Objectives 2
1.2 Background (Overview) of Storm Rainfall 3
1.2.1 Rainfall Definition 3
1.2.2 Storm Characteristics and Design Storm 3
1.2.3 Storm Event 4
1.2.4 Effects of Storm Rainfall 4
1.3 Sarawak and Upper Rajang River Basin Conditions 5
1.3.1 Climate and Physical Conditions 5
1.3.2 River Basins in Sarawak 6
1.3.3 Rainfall Stations at Upper Rajang River Basin 7
2.0 Literature Review 8
2.1 Forms of Precipitation 8
2.2 Types of Precipitation
2.2.1 Convective Precipitation 9
2.2.2 Orographic Precipitation 10
2.2.3 Cyclonic Precipitation 11
2.2.4 Thunder Storms 12
2.3 Rainfall Measurement
2.4 Presentation of Precipitation 15
2.4.1 Moving Average Curve 15
2.4.2 Mass Curve 15
2.4.3 Rainfall Hyetograph 16
2.4.4 Intensity-Duration-Frequency Curve 16
2.5 Rainfall Analysis 18
2.5.1 Spatial Analysis 18
2.5.1.1 Station Average Method 18
2.5.1.2 Thiessen Polygon Method 19
2.5.1.3 Isohyetal Method 20
2.5.1.4 Grid Point Method 21
2.5.2 Temporal Rainfall Analysis 22
2.6 Previous Storm Studies in Sarawak 25
2.7 Review About Rainfall Data Collection in Upper Rajang Basin 26
2.7.1 Rainfall Stations in Upper Rajang Basin 26
2.7.2 Manually Read Recording Stations in Upper Rajang 28
Basin
2.7.3 Automatic Recording Rainfall Stations in 28
Upper Rajang Basin
3.0 Methodology
3.1 Review the Rainfall Data in Upper Rajang Catchment 29
3.2 Study the Storm Characteristics 30
3.3 Discuss the Storm Characteristics 29
4.0 Data and Analysis
4.1 Review the Rainfall Data Collection in Upper Rajang Basin 31
4.1.1 Locations of Selected Rainfall Recording Stations 31
4.1.2 Maximum Rainfall From Stations in Upper Rajang Basin 33
From 1983 to 2004
4.2 Spatial Distribution from Selected Storms 35
4.2.1 Spatial Storm Profiles 35
4.2.2 Depth – Area Curve Derived From Historical Data 41
4.3 Temporal Storm Profile from the Highest Daily Storm Recorded 42
4.3.1 Temporal Data Analysis 43
4.3.2 Comparison of Actual Rainfall in Upper Rajang versus DID 44
Temporal Patterns
5.0 Discussion 45
6.0 Conclusion and Recommendation
6.1 Conclusion 47
6.2 Recommendation 48
References 49
Appendix 51
List of Tables
Table No Description Page
Table 1 Temporal Patterns recommended by DID for
Peninsula Malaysia 23
Table 2 Rainfall Stations in Upper Rajang Basin 28
Table 3 Locations of Rainfall Stations in Upper Rajang Basin 33
Table 4 Rainfall Data for Years 1983 to 1989 35
List of Figures
Figure No Figure Description Page
Figure 1 Location of Sarawak in Malaysia 5
Figure 2 River Basins in Sarawak 6
Figure 3 Locations of Upper Rajang Basin Rainfall
Stations
7
Figure 4 Components of convective precipitation 9
Figure 5 Components of orographic precipitation 10
Figure 6 Components of cyclonic precipitation 12
Figure 7 Tipping Rain Bucket Gage 14
Figure 8 Standard Rain Gage 14
Figure 9 IDF Curve of A Rainfall Station in Sarawak 17
Figure 10 Development of Thiessen Polygon to determine
average areal rainfall
20
Figure 11 Isohyet of Sarawak joining areas of equal annual
rainfall depth
21
Figure 12 Temporal Patterns developed for 12 periods 24
Figure 13 Temporal Patterns developed for 8 periods 24
Figure 14 Ishoyet Map produced from Study by Ministry of
Agriculture ,1983
25
Figure 15 Location of Stations in the Upper Rajang Basin 33
Figure 16 Rainfall Distribution in the Upper Rajang Basin
on 24th
August, 1985
37
Figure 17 Rainfall Distribution in the Upper Rajang Basin
on 1st January, 1986
37
Figure 18 Rainfall Distribution in the Upper Rajang Basin
on 21st December, 1991
38
Figure 19 Rainfall Distribution in the Upper Rajang Basin
on 10th
October, 1992
38
Figure 20 Rainfall Distribution in the Upper Rajang Basin
on 1st June, 1993
39
Figure 21 Rainfall Distribution in the Upper Rajang Basin
on 13th
January, 1997
39
Figure 22 Rainfall Distribution in the Upper Rajang Basin
on 28th
February, 2001
40
Figure 23 Rainfall Distribution in the Upper Rajang Basin
on 12th
January, 2003
40
Figure 24 Rainfall Distribution in the Upper Rajang Basin
on 28th
January, 2004
41
Figure 25 Rainfall Distribution in the Upper Rajang Basin
on 26th
November, 2004
41
Figure 26 Depth-area curve developed from rainfall data 42
Figure 27 Accumulated Rain Total in 24 Hour Period for
Upper Rajang Basin
43
Figure 28 Rainfall Depth in 24 Hour Period for Upper
Rajang Basin
43
Figure 29 Average Rainfall of Upper Rajang Basin on 12th
January, 2003
44
Figure 30 Rainfall at Long Sambop vs DID temporal pattern
of 360 minutes
45
Figure 31 Rainfall at Long Sambop vs DID Temporal
Pattern of 180 minutes
45
Notation
IDF - Intensity-Duration-Frequency Curve
ARI - Average Recurrence Interval
DID - Drainage and Irrigation Department
MMS - Malaysian Meteorological Service
mm - millimeter
mm/hr - millimeter per hour
PMP - Probable Maximum Precipitation
in - inch
km - kilometer
1
Chapter 1
Introduction
1.1.1 Introduction
Storm characteristics vary from each storm to the next, which means that the
effects on catchment areas and ultimately river flows are different. Therefore, a study
on the characteristics of the aforementioned storms are required to yield a Design
Storm – a synthetically derived (i.e. derived from calculations and not recorded actual
storms) rainfall event with the characteristics such as annual probability of
exceedance or recurrence interval, rainfall distribution, rainfall depth, computational
time interval, spatial distribution, and temporal distribution. The exceedance
probability is selected based on the particular application, in this case for the un-
gauged river. The rainfall depth of a specified exceedance frequency and rainfall
duration is determined from an IDF (Intensity-Duration-Frequency) relationship.
The Design Storms calculated would then be used to determine a Design
Flood, a particularly important aspect in the un-gauged rivers. Rainfall data is used to
derive the flood frequency; however if recorded data is unavailable, the annual
maximum daily rainfall is used. The rainfall data is recorded by the Department of
Irrigation and Drainage Department (DID) Sarawak, and from the data collected,
there is no information on the average characteristics of the annual maximum rainfall.
The storm characteristics -area coverage, duration and profile- are then needed to be
determined.
2
1.1.2 Objectives
The objective of this study therefore consists of:
a. To review the rainfall data collected in the Upper Rajang Catchment area.
b. To study the storm characteristics (Duration, Temporal Profile and Area
Coverage) from historical data, with the emphasis on Area Coverage
c. To discuss the storm characteristics from historical data for further
application in producing Design Storm.
In the interest of creating a more conventional Design Storm, the storms studied will
be limited to the daily rainfall recorded from 0800 hrs to 0800 hrs (+8 GMT) with the
exceedance of a threshold amount (e.g. 400 mm/day)
3
1.2 Background (General Overview) of Storm Rainfall
1.2.1 Rainfall Definition
Rainfall is a form of precipitation, together with snow, sleet, hail and mist.
The main source of precipitation is the sea, with evaporation taking place on the
surface. The water vapour is absorbed in the air streams moving across the sea’s
surface. As the water laden air keeps the water vapour absorbed, it will then come
back down as precipitation after cooled to below dew-point temperature – known as
either convective precipitation, orographic precipitation or cyclonic and frontal
precipitation.
Being the main source of rivers, rainfall influences all stormwater studies and
designs. Therefore, in order to be able to prepare satisfactory drainage and stormwater
projects, a good understanding of rainfall processes and appreciation of the
importance of the rainfall design data is required.
1.2.2 Storm Characteristics and Design Storm
Rain storms differ in form, with precipitation varying from 0 mm to over 700
mm per day, and duration as well. A light storm can deliver 20 mm of rain, whereas
an intense storm can deliver the same amount in just an hour. Therefore, the runoff
characteristics will also change with the storm characteristics.
Storm events can be divided into 2; actual storms and design storms. An actual
storm event is a series of rainfall measured over time at rain gages, and rainfall
analysis is based on these records.
A design storm, on the other hand, is not an actual measured storm- it may
have never occurred nor may it ever occur- but is a rainfall hyetograph with pre-
4
selected characteristics. However, most design storms have the average characteristics
of previous recorded storms to be able to better predict the average characteristics of
future storms. Most, if not all hydrologic designs are based on the design storm
approach.
1.2.3 Storm Event
Return period (also known as frequencies) are assigned to rainfall events on
the basis of a number of parameters, such as total volume, average intensity, peak
intensity, duration or inter-event time. The rainfall series must first be separated into a
series of discrete, independent events according to the parameters. After this is done,
the rainfall series may be ranked by volume or any parameter, and a conventional
frequency analysis may then be performed.
For ease of computation, a statistical measure is usually employed to separate
independent storm events. A Minimum Inter-event Time (MIT) is defined so that
rainfall pulses separated by a time less than this value is considered as part of the
same event.
1.2.4 Effects of Storm Rainfall
Problems result when rainfall occurs at extreme volumes or rates. High rainfall
rates of rainfall on small urban watersheds may overcome the existing water drainage
facilities and cause floods. Besides that, high rainfall rates can also cause severe
damage to crops. Finally, high rainfall rates can also contribute to increased surface
runoff into rivers, which in turn may cause to rivers to overflow and flood low-lying
areas.
5
1.3 Sarawak and Upper Rajang River Basin Conditions
1.3.1 Climate and Physical Conditions
Sarawak, the largest state in Malaysia, is blessed with a tropical climate and is
outside volcanic, tornado and severe drought belts (See Figure 1: Location of Sarawak
in Malaysia). Most towns in Sarawak are located on coastal alluvium; however, most
of Sarawak consist of limestone which are found in the interiors. Sarawak, as is the
rest of Malaysia, is warm and humid throughout the year, which can be seen through
the average annual rainfall of more than 2500 mm. Most storms occurring in Sarawak
is generated by the Northeast Monsoon, which occurs from November to March
(Malaysia Meteorogical Service). The Southwest Monsoon, which happens in May to
September, on the other hand, carries very little rain to Sarawak.
Figure 1: Location of Sarawak in Malaysia
6
1.3.2 River Basins in Sarawak State
In order to facilitate and encourage rapid development, Sarawak has been divided into
21 river basins namely, Kayan, Sg Sarawak, Samarahan, Sadong, Lupar, Krian,
Saribas, Oya, Mukah, Balingian, Tatau, Kemena, Similajau, Suai, Sibuti, Niah,
Baram, Limbang, Lawas, Trusan and Upper and Lower Rajang river basins. The
location of the river basins are shown in Figure 2: River Basins in Sarawak.
Figure 2: River Basins in Sarawak
7
1.3.3 Rainfall Stations at Upper Rajang Basin
Due to the large size of the Rajang river basin, the authorities decided to
divide the catchment area into two, namely the Upper and Lower Rajang Basins. Both
basins cover a total area of 47880 square kilometers.
The Upper Rajang Basin is located in the Seventh Division (now known as
Kapit). The main river of the catchment is Batang Rajang, which has a combined
length of 760 kilometers and is the longest river in Sarawak. The Upper Rajang Basin
has 29 rainfall stations. The locations of the stations can be seen in Figure 3: Location
of Upper Rajang Basin Rainfall Stations.
Figure 3: Locations of Rajang Basin Rainfall Stations
8
Chapter 2
Literature Review
2.1 Forms of Precipitation
Atmospheric moisture in the form of vapour originates from large bodies of
water, namely oceans and seas. Water vapour evaporated are absorbed in the air
streams moving across the surface, and come back down to earth due to condensation.
The water vapour comes back down in a number of forms, namely rain, snow, drizzle,
glaze, sleet, hail and dew. Fog and frost, despite involving water vapour, is not
considered as precipitation since the water moisture is suspended in the atmosphere
and not coming down to earth. Rain happens when the precipitation reaches the earth
surface in the form of droplets of water. Sizes of raindrops range from 0.5 to 6 mm,
while larger sizes break up while traveling through the air. Rain can be classified
according to the intensity; heavy (up to 2.5mm/h), moderate (2.5 mm/h to 7.5 mm/h),
and heavy (over 7.5 mm/h). Snow is rain which has been frozen into ice in the
atmosphere and occurs in countries of temperate climate due to the low temperatures
during winter.
2.2 Types of Precipitation
In order for water to condense, the water molecules first need to cling to nuclei
in the atmosphere – in the current age of rapid industrialization, there is no lack of
potential nuclei comprising of dust and carbon particles released during combustion.
Adiabatic cooling through lifting facilitates condensation in the atmosphere.
9
Precipitation can be classified according to the lifting factors, namely i) convective ii)
orographic iii) cyclonic and iv) thunderstorms.
2.2.1 Convective Precipitation
Convective precipitation, also known as thermodynamic precipitation,
happens due to lifting of water vapour from the surface into the atmosphere. This
lifting is due to the water vapour itself lifting after absorbing a certain amount of heat,
rather than carried directly by the wind (such as in orographic precepitation). As the
water vapour is carried ever higher, it is cooled to below the dew point, thereby
releasing latent heat (approximately 539 cal/g) as the water vapour undergoes
condensation. This heat in turn causes further heating of the air, making the air mass
move upwards. This starts a chain reaction, causing more and more water vapour to
be carried up into the atmosphere. When the amount of condensation reaches a certain
point, it will come back down as rain. This form of precipitation is mainly found in
the equatorial zone, since it requires large amounts of water vapour in the air, and is
of extremely limited aerial extent. Finally, normally convective precipitation is
accompanied by lightning and thunder while falling at a high rate. Please see Figure 3
for a visual representation of the process.
Figure 4: Components of convective precipitation
10
2.2.2 Orographic Precipitation
Orographic precipitation is also known as stratiform, relief or dynamic
precipitation. Like convective precipitation, it is initiated by the lifting of water
vapour from the surface to the atmosphere. However, it differs in that it does not
require instability of the water vapour (i.e. the water vapour does not need to absorb
heat to rise by itself) and relies mainly on the other two factors, namely moisture and
lift. Anabatic wind coming from large bodies of water such as the seas and oceans,
encounter hills, ranges and other mountainous terrain at the sea side. Therefore, the
wind would rise as it traverses the geography. The rising action of the moisture-laden
wind causes the water vapour being carried to become cooler, undergoing
condensation and forming clouds. As more water vapour gets collected by the cloud,
it will reach a limiting point, and would then come back down as rain. This
phenomenon would normally see that one side of hills and mountains experiencing
rain while the other side experiences none. Finally, unlike convective precipitation,
orographic precipitation is not accompanied by thunder and lightning, and falls at a
gentler rate. (Please to Figure 5)
Figure 5: Components of orographic precipitation
11
2.2.3 Cyclonic Precipitation
Cyclones, infamous due to their destructive capabilities, have humble
beginnings. It results from having two fronts (warm and cold) colliding into each
other. These two fronts are in turn created by unequal heating of the earth’s surface.
On the basis of the nature of the meeting of fronts, cyclonic precipitation can
be further classified into two; frontal and non frontal. Frontal precipitation is when
warm air is pushed above the cold air at the contact point. Non-frontal precipitation,
on the other hand, occurs when the air is lifted through the covergence of the inflow
into a low pressure area. This form of precipitation creates large scale tropical
cyclones, with a very low pressure in the middle of the cyclone (which is known as
the eye), and creates havoc due to high wind speeds surrounding the eye. The cyclone
derives its energy from sea vapour, therefore it will slowly dissipate at it crosses over
land. Curiously, cyclones in the northern hemisphere move in anti-clockwise
direction, whereas cyclones in the southern hemisphere move in the opposite
direction. Cyclones bring large amounts of rain, typically lasting from a few hours to
a few days. The components of cylconic precipitation can be seen in Figure 6.