STORM RAINFALL ANALYSIS FOR UPPER RAJANG RIVER … Rainfall Analysis for Upper Rajang... · STORM...

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


on 1st January, 1986

37


on 21st December, 1991

38


on 10th

October, 1992

38


on 1st June, 1993

39


on 13th

January, 1997

39


on 28th

February, 2001

40


on 12th

January, 2003

40


on 28th

January, 2004

41


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.

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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

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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.

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