probablemaximumprecipitationderivationinmalaysiareviewandcomparison

International Journal of Hydro-Climatic Engineering Assoc.Water and Enviro-Modeling Poon et.al.(pp.37 – 72) eISSN : 99990001

37

Probable Maximum Precipitation Derivation in Malaysia: Review and Comparison

*Hii Ching Poon1, Heng Hock Hwee2 1 Hydrologist in Department of Civil Engineering, Universiti Malaysia Sarawak. 2 Associate Professor in Faculty of Engineering and Science, Universiti Tunku Abdul Rahman, Malaysia *Corresponding Author Abstract The importance of PMP is always highlighted and emphasized in terms of public safety and hazard downstream of any major river regulating structures especially if it is located in the upland catchment of the populated town or city centers. As such the PMP convention is always a requirement as primary design dam/reservoir criteria when public safety is of concern. The objective of this review is to compare the Probable Maximum Precipitation (PMP) convention of SMHB/B&P vis-à-vis other consultants’ studies that have been carried out in Malaysia. The review primarily addresses the issues on the PMP derivation in Peninsular Malaysia and with minimum coverage of PMP issues in the Borneo states of Sarawak and Sabah. The short- and long-duration PMP values were applied in Peninsular Malaysia and Borneo states. The PMPs adopted by SMHB/B&P are further classified into two series, i.e. Coastal and Inland. Fourteen (14) reports and studies related to the derivation of PMP in Peninsular Malaysia by other consultants were discussed in detailed. Both PMPs adopted by SMHB/B&P and studies by other consultants are presented systematically in tandem in this review. It should be emphasized a priori that there is no single unified methodology or procedure in estimating PMP values in Malaysia, specifically for the case 24-hour duration PMP adopted in various studies throughout Peninsular Malaysia. Competent professionals and experts in this specialized field may even obtain different results because most these estimations required subjective judgments and empiricisms.

Keywords: Probable Maximum Precipitation (PMP), public safety, hazard.

Submitted: 27th June 2010 Published after corrections: 11th August,2010.

INTRODUCTION

The objective of this review is to

compare the Probable Maximum

Precipitation (PMP) convention of

SMHB/B&P vis-à-vis other

consultants’ studies that have been

carried out in Malaysia. For the past

40 years of the existence of SMHB in

the dam design/engineering

consultancy practices adopts and

inherits the conventional practices of


38

its former parent company in the

UK. The review primarily addresses

the issues on the PMP derivation in

Peninsular Malaysia and with

minimum coverage of PMP issues in

the Borneo states of Sarawak and

Sabah. An example that warranted

comparison is the PMP adopted for

the recent design and construction of

Selangor dam (CA=197 km2) at the

upper Sg. Selangor basin.

In the context of this review, the

terminology, PMP is defined by the

World Meteorological Organization

(WMO; 1986) 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”.

ADOPTED PMP CONVENTION

BY SMHB/B&P

SMHB Sdn. Bhd. (known as SMHB

hereinafter) and its predecessor,

Binnie and Partners (known as B&P

hereinafter), generally adopts both

the short- and long-durations PMP

values from their past numerous

dam design experiences and studies

in Singapore and Malaysia (Seletar,

Langat, Semenyih, etc). Specifically,

three (3) specific reports/studies

form the basis of PMP derivation at

that time. They were SSP/HH

(1979), B&P (1980), and SSP/SMEC

(1985) for project locations in

southern Johor and Singapore.

These PMP values have since being

adopted for all reservoir design

projects subsequently undertaken by

SMHB/B&P. Notable projects were

Seletar and Upper Pierce dams in

Singapore, Semenyih and Langat

and recently Tinggi dams in

Selangor, Linggiu dam in Johor,

Linggi dam in Negeri Sembilan,

Chereh dam in Pahang, Durian

Tunggal and Jus dams in Melaka,

etc.

The short- and long-duration PMP

values, by definition in this

presentation, divided arbitrarily at

the 24-hour duration. As such, PMP


39

values of duration less and more

than 24-hour are classified

accordingly as the short- and long-

duration PMP respectively.

However, a 6-hour duration is

sometimes used to demarcate the

short and long duration as being

adopted before by the Australian

Bureau of Meteorology (ABM).

The PMPs adopted by SMHB are

further classified into two series, i.e.

Coastal and Inland, while in essence

taking into consideration the vast

difference in meteorological and

geographical factors, as their names

implied at both peninsular coastal

and inland regions. For short

duration PMP series, the values

adopted as Coastal PMP are based

mostly on the Singapore rainstorm of

1978 and about 12% higher than the

inland PMP. After applying an

appropriate transposition factor, it is

transposed inland (specifically for

Semenyih reservoir design project),

this series is therefore known as

Inland PMP. Table 1 shows both

coastal and inland PMP derived and

adopted by SMHB/B&P for duration

ranges from 1- to 120-hour.

Table 1: Coastal and Inland PMP (Short- and Long-Duration) adopted by SMHB Duration (hour) Coastal PMP (mm) Inland PMP (mm) 1- 211/190.5# 188 3- 338 300 6- 440 391 12- 584 518 24- 777 692 Long Duration PMP (arbitrarily defined for this review) 48- 1356 908 72- 1593 1067 120- 2030 1360 # 19.5 mm or 7.5 in is originally quoted but 211mm is back calculated from 188 mm Short-Duration PMP The basis of the PMP derivation for

SMHB/B&P is based on actual

occurrence and records of maximum

rainfall in Malaysia and Singapore.

Some notable highest rainfall records

are (SMHB, 1992):


38

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 of 533 mm being recorded concurrently at two other stations on Singapore.

The December 1978 storm 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. 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, rainfalls of about 533 mm

were also recorded concurrently at

both Kim Chuan Road Sewerage

Works and Sembawang Agricultural

Research Station in Singapore.

Storm Maximization The storm depth was 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


38

point temperature for 12-hour at

1000 mb).

Based on an average of the recorded

values at Paya Lebar Airport and

Changi Airfield the 24-hour

persisting 1000 mb dew point

temperature at the beginning of

December 1978 storm was 23.8 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 = 777 mm. For PMP of

less than 24-hour duration, similar

methodology was applied to derive

1-, 3-, 6-, 12-hour duration PMPs.

Transposition Transposition is another major step

to be undertaken in any PMP

studies. Transposition of storms

from one location to others is

subjected to various important

contributing factors such as,

presence of topographic barrier,

elevation adjustment, distances from

the storm center, and etc. However

in SMHB/B&P practices, primarily

due to scarcity of the meteorological

data and uncertainties in the chosen


39

transposition technique, PMP values

at the coastal region (assuming that

Singapore is located in the coastal

region) might not be able to carry

out after all. As such, SMHB/B&P

adopts both short- and long-duration

PMP values without taking into

account the transposition factor for

PMP derivation in mostly coastal

region. It is though a conservative

measure, lest 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 SMHB/B&P

hydrological groups opined that by

adopting coastal PMP without

taking into account the transposition

effect was “unduly conservative”.

Therefore, some forms of technique

should be devised 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 been adopted for the

detailed design studies of both

Tinggi (formerly known as Buloh)

and Selangor dam in the Sg.

Selangor basin respectively. Table 2

shows the adopted PMP values by


40

SMHB for various durations, i.e. 1- to 24-hour.

Table 2: Comparison of PMP of Coastal and Inland PMP Values Duration (hour) Coastal PMP (mm) * Inland PMP (mm) # 1- 211 (190.5 mm or 7.5 in) & 188 3- 337 300 6- 439 391 12- 582 518 24- 777 692 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) 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 (1986). 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 1960’s and 1970’s. 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,

1999), 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


38

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, 1999).

Long-Duration PMP Long 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 totals at Air Tawau School

near Endau with maximum 5- and 7-

day totals 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).

Maximization 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.

However this is yet to be confirmed

as no confirmation or cross checking


38

with other reports and studies could

be made in this regard.

Transposition 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).

Table 3 shows the coastal and

inlands PMP adopted by

SMHB/B&P.

Table 3: Coastal and Inland PMPs (Long Duration) adopted by SMHB/B&P Duration (hour) Coastal PMP (mm) Inland PMP (mm) (1) (2) (3) 24-* 777 * 692 48-# 1356 # 908 72-# 1593 # 1067 120-# 2030 # 1360 col 2* 0.89; # col 2*0.67 REVIEW ON OTHER PMP STUDIES IN MALAYSIA Several reports and studies related to

the derivation of PMP in Peninsula

Malaysia, particularly in the State of

Selangor where seven (7) existing

dams/reservoirs are located are

relevant and they are presented as

below. The most recent study on the

issue of PMP in Selangor was a

paper presented in the 1-day

Specialty Seminar conducted by

JPS/HTC using Hershfield statistical

methodology (WMO, 1986). An

extended study on short duration

extreme rainfall is also being carried

out. The preliminary findings of this

study although without routine PMP

maximization is presented during

recent International Conference on

Urban Hydrology 2002. In addition,

a detailed hydrological study was

also carried out in the 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


38

NK/SMHB in earlier 2000. The

results obtained by this specific

study basically confirmed the

SSP/SMHB (1996) study on PMP

derivations in Kelantan.

A paper on estimation of PMP based

on proxy (radar satellite) data is also

reviewed in this presentation. This

paper was originally presented in a

workshop on satellite weather

forecasting in South America and

made available via world-wide-web

site http\\www.unesco.org.uy.

Comparison and references on PMP

values were also made on a much

small subcatchment of Sg.

Terengganu basin in the eastern

coastal region of the Peninsula

Malaysia. Due to these relevancies,

this particular paper was reviewed

as such.

SMHB/B&P PMP estimates for

various dam design projects

undertaken in Malaysia are based

largely on the findings and opinions

of SSP/HH (1979), B&P (1980), and

SSP/SMEC (1985) for short- and

long-duration event. Statistical

approach, i.e. Hershfield technique,

which is suitable for regions with

scarce hydrometric data (dew points,

wind records, etc), is sometimes

used for checking and verification. A

collaborated effort by JPS/HTC to

estimate 24-hour PMPs using this

technique based on statistical

technique in Selangor is also

presented.

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.

Langat Miles 24 Dam Stage II

Design, March 1976. Dams on Sg. Bekok and Sg.

Semberong, Detailed Investigation and Design: Hydrology, SSP and Howard Humphrey, October 1979.

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

Klang River Basin integrated Flood Mitigation Projects, Malaysia, Final Report, Kinhill Engineer Pty Ltd in


39

association with Ranhill Bersekutu Sdn. Bhd, November 1994.

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

Putrajaya: Padang Peras Reservoir Design Study, Angkasa-GHD, 1998.

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

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

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

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.

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

Gelami Dam Design Hydrological Study, JPS, 2002.

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

Feasibility Study on Water Resources Development for

Seremban and Port Dickson, SSP/SMEC, 1990.

Langat Dam Design 1976 The design storm (note: it was not

mentioned explicitly in the report

that design storm is PMP in present

day terminology but for the purpose

of reservoir design, one would

assume that PMP values were

implied in the dam design practices

in Malaysia) adopted in the design of

Langat dam was based on review of

all the maximum recorded rainfall of

both short- and long-durations in

both Singapore and Malaysia.

Notably for short duration storm of

less than 1 hour was based entirely

on an earlier B&P study on

envelopment of the highest recorded

rainfall for Singapore and east coast

Malaysia. The maximum 1-hour

rainfall by envelopment was 190.5

mm (or 7.50 in). This subsequently

is being adopted by SMHB/B&P in

most of their dam design

undertaking latter year in Malaysia.


40

Due to steepness of the Langat

reservoir catchement and relatively

smaller area, the shorter duration

PMP is far more important that those

of more than 24 hour. Comparison

between the available historical

storm records of both east and west

coasts indicated that the east coast

were generally higher that those of

the west coast (which come to no

surprise at all if one is familiar with

the weather and climate pattern of

Peninsular Malaysia). The highest

recorded rainfall was 754 mm at

Kota Baru JPS Store and 263 mm at

Tanjung Karang JPS Store. The

design report also included some

important historical rainfall event

experienced in Malaysia. For brevity

they are tabulated and summarized

in Table 4.

Selection of the design storm for

spillway was accomplished by

comparing the design storms

adopted in other studies such as

design of Seletar, Klang Gates, and

PMP at Batang Padang. In their

opinion, as the Langat reservoir is

less exposed to the northeast

monsoon than Seletar reservoir and

is at a higher elevation than Klang

Gates catchment, therefore, after

deliberation, the design storm of 432

mm was selected for 24-hour

duration. On the other hand, for

short intensity design storm, 190.5

mm was adopted for 1-hour

duration.

Table 4: Maximum Recorded Rainfall Prior to 1976 Station

Rainfall (mm) Remarks

Mersing 287 Segamat 260 Tangkak 267

7th January 1952 (B&P, 1959)

Sg. Sembrong 260 Mawai 437 Segamat 318 Labis 468 Pekan Nanas 257

10th December 1969 B&P (1970)


38

Kuantan (east) 327/ (868) MMS Mersing (east) 430 (679) MMS Subang Airport (west) 171 (293) MMS Ulu Langat Mile 22 (west) 166 (333) B&P (1971) Kepong 283 Serendah 268

From USBR 1954 Study

( ) 120-hour Klang River Basin Flood Mitigation, Kinhill/Ranhill, 1994 PMP study carried out by the dam

designer, United States of Bureau of

Reclamation (USBR) was reviewed

systematically for the Batu and

Klang Gates dams in tandem with

comparison with other studies in

Malaysia as shown in Table 5. PMP

for both dams are derived based on

the maximization of the storm

records of Air Tawar School and

Mersing during late 1970 to earlier

1971. These two stations recorded

the highest observed rainfall for long

durations, i.e. more than 1 day in

Malaysia.

Table 5: Comparison of PMP for Batu, Klang Gates, and Inland Dams

Duration (hour)

Batu Dam PMP (mm) *

Klang Gates Dam PMP (mm)*

Inland Dam PMP (mm) ARF applicable

12 391 375 474 18 460 449 N/A 24 528 517 643 36 687 675 N/A 48 846 834 863 72 1164 1153 1067

* Data were originally obtained from USBR, 1984 and reproduced in the Kinhill/RHB (1994) report If assuming equal

opportunities/probabilities of PMP

could also be occurred in the

Semenyih or Selangor basins and

considering both dams are located in

the same interior region of the State

of Selangor, the PMP values

estimated by USBR in the design of

both Batu and Klang Gates dams

therefore can be transposed to other

dam sites within the same

geographical regions as well. As


38

shown in table above, they are

founded to be somewhat lower vis-à-

vis SMHB/B&P’s for shorter

durations, albeit are compared

reasonably well as far as the long-

duration PMP, 48- and 72-hour is

concerned.

Kelantan River Flood Study, SSP/SMHB, 1999 The derivation PMP using hydro-

meteorological technique was

required for the design of the

proposed Kemubu (on Sg. Galas)

and Lebir (on Sg. Lebir) dams on

each of the major tributaries in the

upper catchment of the southern Sg.

Kelantan basin. Due to their relative

larger sizes of the dam catchment,

only long-term duration PMP, i.e.

more than or equal to 1-day are of

importance and was subsequently

derived for this study. By leaving

the short duration PMP aside

essentially circumventing the

problems encountered on the

scarcity of pluviograph or automatic

recorder data on short-duration

rainstorm in the whole Sg. Kelantan

basin.

Extensive search of the JPS extreme

rainfall database indicated 7 major

storms (Jan, 1967; Dec 1970, 1971,

1972, 1982, 1983, and 1986) have

occurred in the east coast regions i.e.

the states of Kelantan and

Terengganu. The maximization

factors for various storms were

estimated based on precipitable

water ratio of a reference extreme

dew point temperature at 27 oC. The

12 hours persisting dew point

temperature ranges from 22.0 oC to

24.6 oC. Correspondingly, the

maximization factors range from the

lowest 1.23 (Dec 1972) to the highest

1.55 (Dec 1970).

Explanations on the rationale of

transposition factor are presented in

the preceding section 2.1 or in the

reference, SSP/SMHB (1999).

Comparison of mainly long duration

PMP is tabulated in Table 6.

Columns 2 and 3 values are obtained

after taking into consideration the


39

transposition factors, 0.70 and 0.85

for Kemubu and Lebir dams sites

respectively.

Table 6: Comparison of PMP of Kemubu, Lebir, and Selangor Dams

Duration (hour)

Kemubu PMP (mm)

Lebir Dam PMP (mm)

Selangor Dam PMP (mm)

ARF applicable (1) (2) (3) (4) 24 301 468 643 48 651 910 863 72 840 1190 1067 96 980 1350 N/A 120 1100 1490 1360

Col 2 and 3 values are obtained after taking into consideration the transposition factors

Putrajaya: Perang Besar Reservoir, Angkasa-GHD, 1998 The PMP used in the main dam

design in the new administrative

center of the Government of

Malaysia, Putra Jaya, was based on

procedures outlined in the Bulletin

53, 1994 of the Australian Bureau of

Meteorology (ABM), Australia. This

procedure, as quoted ad verbum,”

…is based on the Depth-Area-Duration

(DAD) method, which is used

extensively in Australia and the United

States. Its basic assumption is that

PMP is produced by large stationary

thunderstorms positioned over the

catchment. The method was developed

on the basis of data colleted principally

in the US, and assumed that the basis of

data are similar throughout the world”,

and thus it is implicitly in a way

considered applicable in Malaysia.

The results of the short- and long-

term duration PMP values adopted

in the design of Perang Besar dam

and compare with the Inland PMP of

SMHB/B&P are tabulated and

summarized in Table 7.

Table 7: Comparison of Prang Besar PMP and Inland PMP Duration (hour)

PMP (mm) Angkasa-GHD

PMP (mm) SMHB in Sg. Selangor dam

% Difference

0.5 250 N/A N/A 1 N/A 188 N/A 3 610 300 +103% 24 1100 692 +84% 72 1400 1067 +31%


38

Comparing with the PMP values

adopted by SMHB in the design of

Sg. Selangor dam, it was obviously

shown that the PMP values in

Angkasa-GHD design was higher

than SMHB’s. The difference is

considerably prominent for the

shorter duration PMP, i.e. for 3-hour

and lower duration by interpolation

to a certain extent. In this regard, the

Angkasa-GHD derived PMP is

almost doubled that of SMHB.

However for longer durations, says

for both 24- and 72-hours, the

difference somewhat reduces from

+103% to +84% and +31%

respectively.

The validity of adopting ABM

(derived from adjusted storm data of

both USA and Australia) PMP

estimation in Malaysia remains

controversial as obvious differences

in meteorological condition is self-

evident in the first place. From the

past studies, many arguments were

traded with regard to the adoption

of ABM’s PMP procedure in

Peninsular Malaysia. Specifically,

SMHB (1994) pointed out that “ (it) is

in their (SMEC/ABM) judgements

and professional opinions, it is valid

to transpose generalized storm data

from similar tropical regions of the

United States and northern Australia

to Peninsular Malaysia”.

Radar and Storm Model for PMP

Estimation, Hardaker and Collier, 1999

The purpose of this paper is to

estimate the PMP in the tropics, an

example quoted in the paper was a

subcatchment of Sg. Terengganu,

Sg. Petong catchment (CA=120 km2),

using meteorological parameterized

storm model of Austin et al (as

quoted in the presentation, 1995) of

UK Meteorological Office. In

essence, most of the input data are

obtained from the Doppler weather

radar. Therefore, in a way that this

proxy and indirect methodology

offers an obvious advantage vis-à-vis

the conventional maximization and

transposition methods that requires

bulk of measured rainfall records,


38

primarily to construct a Depth-Area-

Duration (DAD) curves for the

largest storms ever recorded in the

project area or at its vicinity.

The preliminary results of the point

PMP for 3-, 6-, 24-, 72-, and 120-hour

duration were estimated. The

authors concluded that the estimated

PMP using weather data were of the

same order of magnitude with the

past studies (quoted from Mr. David

Mcdonald, Chief Hydrologist of

Binnie and Partners of UK). The

authors however in their final

remarks cautioned that the

promising results obtained in their

study were of preliminary nature so

the adoption of such methodology

was still pending and warranting a

further in-depth investigation. For

comparison purpose, the results are

compared with the SMHB PMP

values as listed in Table 8.

Table 8: Comparison of PMPs of Hardaker & Colier and SMHB Coastal PMP Duration (hour)

PMP (mm) Hardaker and Collier 1999

PMP (mm) SMHB Coastal PMP

% Difference

3 484 338 +43% 6 593 440 +35% 24 818 777 +5% 72 924 1593 -42% 120 1029 2030 -49%

The variation between these two

methodologies is rather significant,

ranging from +43% to –49% of the

PMP values adopted by SMHB.

Specifically for long duration PMP

values, the difference is significant

by almost half of SMHB’s. The

exception was the 24-hour PMP that

differed only by 5%. It should be

noted that most of the east coastal

extreme rainfall only occurred

during monsoon months of

November to Earlier January. This

tropical storm surge, as it is

preferably termed by the

meteorological experts, is the results

of a regional cold front that is

originated from the northern

hemisphere. This Siberian cold front

reaches the equator sometimes at the


38

end of the calendar year. The surge

most likely lasts for several days. On

the other hand, the storm cell model

calculated in this study is based on

localized storm mechanisms and to

somewhat resemblance of a

convective storm process. Therefore,

it might be possible that this is the

reason for which lower PMP values

were calculated for longer duration

of more than 24 hours.

National Water Resources Study, SMHB/RHB/ZAABA, 2000 Hydrometeorological technique

(maximization and transposition

approaches) was used to generate

generalized isohyetal PMP maps of

various durations (3-, 6-, 12-, 24-, 72-,

120-, and 168-hour) for Peninsula

Malaysia. In this approach the

maximum rainfall recorded for

various durations (mostly acquired

from PKM database) are maximized

in situ using index of both prevailing

dew point temperature during storm

and maximum persistent dew point

temperature. Seven (7) isopleth

maps for all durations mentioned

above are then prepared

accordingly. Cautions are taken into

account when generating the

isopleth as a differentiation should

be made on the influence of

Northeast Monsoon in the east coast.

For brevity, the following table

presents the “point” PMP values for

all durations extracted directly from

the isopleth maps at the Sg. Selangor

dam site. Comparison with the

SMHB’s PMP (both Coastal and

Inland) values is shown in Table 9.

Table 9: Comparison of PMPs of NWRS 2000-2050 and SMHB Duration (hour)

PMP (mm) NWRS 2000-2050 Approx

*

PMP (mm) SMHB Coastal

PMP (mm) SMHB Inland

% Difference

3 430 338 300 (261) +65% 6 590 440 391 (352) +68% 12 680 582 518 (474) +43% 24 820 777 692 (643) +28% 72 1300 1067 (1035) +26% 120 1500 2030 1360 (1360) +10% 168 1650 N/A N/A

* choosing/interpolating to nearest contour as point value ( ) in col 3 indicates after applying the ARF


37

After reviewing the long duration

rainfall records, the investigators

concluded that any storm coming

from the northeast monsoon in the

upper region of the east coast of

Peninsular Malaysia, i.e. Terengganu

and Kelantan could also affect

rainfall on the west coast as

evidenced in the 1971 storm.

Likewise, a storm closer to Johor

Bahru and Kuantan in the lower

region of the east coast can also

affect the west coast of the

Peninsular Malaysia at the other side

of the Banjaran Titiwangsa (main

range).

Desa, Noriah, & Rakhecha. 2000 The Regional Humid Tropics and

Water Resources Centre for

Southeast Asia and the Pacific (HTC)

of JPS undertook the commendable

task of deriving 24-hour PMPs for

the State of Selangor by making use

of the relatively long-term rainfall

records in the JPS TIDEDA Database

System. This study is essentially

based on Hershfield technique using

some 32 rainfall stations of varying

years of records ranging from 34 to

62 years of long-term records to

derive the 24-hour PMP for the state

of Selangor. Subsequently an

isohyetal 24-hour PMP was

presented. This study carried out by

JPS/HTC opines that that the

recommended frequency factor

(Km=15) by Hershfield (1965) was

higher for Malaysia condition

(WMO, 1986). A review of such

nature i.e. in search of a common

basis of Km frequency factor is in

line with the recommendation of

WMO (1986). Therefore an entirely

new set of frequency factors was

subsequently derived using

observed one-day highest rainfall of

various station records and vital

statistics such as mean and standard

deviation. The new Km values are

ranging from low 2.0 to as high as

8.4. Three (3) point PMP values are

of importance for comparison

purpose due to their proximities to

the dam site and the upper Sg.

Selangor basin. They are Ldg Escot,

Hospital Kuala Kubu Baru, and Ldg


38

Batang Kali. The calculated 24-hour

PMP values are presented in Table

10.

Table 10: Comparison of PMP (SMHB and JPS/HTC) Station Name Station No. 24-hour PMP

using Km = 8.7 (mm)

24-hour PMP using Km = 15

(mm) (1) (2) (3) (4) Ldg Escot 3615001 378 607 Hospital KKB 3516023 367 540 Ldg Batang Kali 3416025 347 508

It is obviously indicated that the

PMP value using Hershfield

technique is some 50% lower than

the PMP series (if choosing Km

equal 8.7) adopted by SMHB in the

Sg. Selangor dam design, i.e. 24-hour

inland PMP of 692 mm. However if

strictly following the

recommendation outlined in the

WMO (1973; 1986) manual on the

Km factor and no adjustment made

to the mean and standard deviation

of the maximum storm annual series,

the results presented in column 4 of

the table above are consistent and

comparable to the SMHB PMP value

vis-à-vis those of using a lower Km

value.

The statistical approach for any dam

design assignments is always

subjected to controversies,

uncertainties and criticisms under

the pretext that PMPs derived in this

way is only representative of a

“point” PMP value. Furthermore the

length of records that is used to

derive the statistic oftentimes is

rather short to be statistically

meaningful (Hersfield suggested a

minimum of 20-year records by

Hershield, as quoted in WMO

[1986]). As a result, some significant

rainfall event might be missed out

entirely due to non-recording or

error in instrumentation.

In addition, another important point

that is subjected to further argument

is the conversion from point PMP to

areal PMP, which is usually

accomplished by applying a


38

reduction coefficient (Areal

Reduction Factors; ARFs) based on

both the size of the catchment and

the chosen duration. For longer

durations and smaller catchment

areas, the reduction factor is

approaching unity. The ARF curves

are mainly derived by U.S. National

Weather Services (NWS) and

presumably using mostly storm

information in the continental USA.

Thus the validity and subsequently

suitability of such ARF factors

application in Malaysia is again

doubtful. Although there was a

study undertaken by the JPS in the

mid 1980’s, however the results were

inconclusive as “the ARFs derived in

this study were based on very short

records and therefore should be used

with caution” (Ong and Liam, 1986).

Furthermore, the author also

explained the fact that the ARF is

considered rather low for tropical

climatic condition such as in

Malaysia where the storms are

mostly of Blitz type thunderstorm

and short duration nature. This

probably explained low ARF factors

vis-à-vis the US NWS’s.

Interstate Raw Water Transfer, NK/SMHB 2000 This recent study is basically an

independent review and appraisal

on the substantial bulk of works

carried out previously in the

Kelantan Flood Mitigation Study

(SSP/SMHB, 1999) before adopting

PMP values in the design of both

Kelau (Catchment Area =331 km2)

and Telemong (Catchment Area

=360 km2) dams in the interior State

of Pahang. Eight major storms

mostly in the east coast regions of

Kelantan and Terengganu during

Northeast monsoon were identified

for the preparation of Depth-Area-

Duration curves (DAD). Each

individual DAD curve was prepared

using about 20 to 30 station records

in the vicinity of the storm epicenter.

Both auto and manual recorded

rainfall values were utilized though

in general, the auto-record data is

scarce in the regions


39

notwithstanding with only relatively

short period of records.

Long-duration PMP i.e.24- to 48 hour

was then derived after appropriate

adjustment for ‘unrestricted’ values.

The PMP values were then

enveloped, maximized and

transposed to the inland dam sites

that are closer to each other.

Similarly, short-term PMP for less

than 24-hour was also derived albeit

using different approach for reason

explained earlier on the scarcity of

auto-recording rainfall stations in the

east coast region as a whole.

Therefore it was not being able to

develop the DAD curves for shorter

duration from the amount of data

available.

In such case, recourses are then

made to adopt the short duration

PMP derivation based on procedure

developed by Australian Bureau of

Meteorology (ABM, 1985). This

technique is in turn, a procedure

based on the method of ‘adjusted

United States data’ as explained in

earlier subsection. Studies on the

maximum enveloping values

between these two continents

(although of relatively different

climatological characteristics

nevertheless lend support on the

adoption of the vast USA database in

Australia continent, (ABM, 1985)). It

was concluded that pattern and

nature of occurrence of short

duration storm are the same between

these two continents of almost the

same longitude. Based on this

similar argument, the short duration

PMP methodology developed by

joint US and Australian

meteorologists could be also used

elsewhere in the Southeast Asia

region. Essentially it assumes that

“the extreme rainfalls for short

durations and small area will be

produced by large, efficient, and

virtually stationary thunderstorms or

from part of a meso-scale or synoptic-

scale storm system with embedded

cumulus cells” (as quoted ad verbatim

from NK/SMHB, 2000; cf WMO,

1986).


40

As a check on the PMP values

derived from the hydro-

meteorological approach, Hersfield

technique was also used to ascertain

if the estimated PMP values are at

least in the same order of magnitude.

Nine (9) rainfall stations in the

vicinity the project site were used.

The results of 24-hour point PMP are

comparable, ranging from 410 mm to

highest 637 mm. These are

compared with the areal PMP at

both dam sites, i.e. 625 mm and 615

mm for Kelau and Telemong dam

sites respectively. Table 11 shows the

comparison of PMP values used in

the Interstate Water Transfer vis-à-

vis SSP3 projects.

Table 11: Comparison of PMP Derived from Interstate Water Transfer Project Duration (hour)

Kelau dam CA=331 km2

Telemong dam

CA=360 km2

Selangor Dam * CA=197 km2

ARF Adjusted Selangor

Dam 1 270 260 188 0.75 141 2 310 300 N/A 0.87 N/A 3 370 350 300 0.90 261 6 480 460 391 0.92 352 12 530 520 518 0.93 474 24 625 615 692 0.95 643 48 905 895 908 0.97 863 72 1050 1040 1067 N/A 1035 120 N/A N/A 1360 N/A 1360

N/A not available for duration more than 72-hour; * inland PMP

If conservatively assuming that the

PMP values for both Kelau and

Telemong dams can be transposed in

toto to the Selangor dam site without

adjustment, the PMP values for all

duration except for shorter duration,

are closely comparable. However it

should be borne in mind that, this is

nevertheless a conservative

approach by assuming that both

maximization and transposition

factors are the same for the dam

catchments at both sides of the

Banjaran Titiwangsa (main range of

the Peninsular Malaysia). This in

essence implies an equal

opportunity/probability of storm

occurrence in both the east and west

coasts notwithstanding the nature of

storm. Dominant storm in the west

coast region is often of the

convective nature, which is typified


38

by very intense and heavy

precipitation that last for a short

duration, i.e. 2 to 6 hours and

oftentimes occurs during the two

intermonsoon periods (from April to

May and October to November).

While in the east coast region of

Peninsular Malaysia and the coastal

region of Sabah and Sarawak, the

monsoon storms during November

to January are prevalent and

predominant and they are

widespread as well as could last for

a longer period, from several days to

a week. However it does not

preclude the occasional occurrence

of such long duration storm or surge

as preferably termed in the

meteorological communities in the

west coast.

Anecdotal observation of the

historical concurrent extreme storm

events in both east and west coasts

of the Peninsula Malaysia seemed to

prove otherwise (as evidenced by

less devastating flood consequences

and lesser rainfall). With the

exception of well-documented 1971

storm event in the west coast, hardly

any significant storms of the same

order of magnitude on a par with

1971 in latter years occurred in the

west coast. The consensus amount

the hydrologists within the

SMHB/B&P groups on the

transposition of east coast storm to

the sheltered inland or west coast (by

crossing the Banjaran Titiwangsa)

without appropriate transposition

factors are considered as “unduly

conservative” (SMHB, 1992). To

quote an example, the transposition

methods used in the Kelantan Flood

Mitigation Feasibility Study

(SSP/SMHB, 1999) were based on

ratios of 1-, 5-day total rainfall.

Short Duration Rainfalls in Selangor, Desa and Rakhecha, 2002 This study was undertaken by HTC

is an extended study on earlier 24-

hour PMP derivation in Selangor

and mainly focuses on the short

duration extreme rainfall. The

purpose of this study is to provide

pertinent information on the

characteristics of short duration


39

rainfalls for the design of small and

moderately sized hydraulic

structures in the urban areas. This is

implied that the study is more of

concern of low probability

occurrence of extreme rainfall, such

as in the range of return periods

from 2- to 100-year. This study in

essence, is a collation and

documentation of the highest auto

recorded rainfall event for short

duration in Selangor without further

analysis to derive PMPs. Therefore,

at this preliminary stage, it does not

take into consideration of converting

these observed rainfall records to

PMP by conventional maximization

technique using meteorological

parameters.

Short duration of auto-recording

rainfall data (from 1971 to 1999) of 13

recording rainfall stations in the state

of Selangor are compiled in the

respectively tables. The severity of

the extreme rainfall event is

quantified as percent chance of

occurrence (or represented by return

period in standard and conventional

hydrological practices).

In the records, the stations in the

state of Selangor received heavy falls

of rain of the order of 42 to 106 mm

in 0.25 hour, 60-183 mm in 0.50 hour,

73 to 365 mm in 1 hour, 114 to 522

mm in 6 hour, and 132 to 523 mm in

a 12-hour duration. In terms of

duration, the maximum rainfall

converged to about 6 hour, which is

evidently represented by some 90%

of the rainfall in the previous time

periods. In the opinion of the

authors, the prevailing extreme

rainfall events were mostly of short

duration nature.

In addition, it is also found out that a

majority of top 20 highest storm

events were mainly occurred during

the inter monsoon season from April

to May and October to November.

Table 12 shown below summarizes

the highest recorded rainfall for

different durations.


40

When comparing the highest rainfall

records vis-à-vis adopted PMP value

at Selangor dam, it should be in

mind highest recorded point

rainfalls in table above do not take

into account of conventional PMP

derivation technique of storm

maximization (irrelevant for

transposition in the context). The

implication of these findings is far

reaching in such a way that a

reassessment of the short duration

PMP rainfall in Selangor is most

likely warranted.

From Table 12, it could be seen

clearly that the first, second, and fifth

highest recorded rainfalls for 12-

hour duration are located some

distances from upland catchment

except for Sek Men Kg. Lui and Kg.

Kalong Tengah station. Comparison

of these observed upland basin

highest rainfall records vis-à-vis the

Selangor PMP is imperative to

ensure that the PMP convention of

SMHB/B&P is indeed adequate.

Table 12: Highest Records Extreme Rainfalls for Various Durations in Selangor Duration (hour) No Name of the rainfall station ¼- ½- 1- 6- 12-

1 Pejabat Pos Manggis 42 60 73 119 154 2 Ibu Bekalan Enam Kaki 96 96 96 114 169 3 Pintu Kawalan P/S Teluk Gong (1) 91 183 365 522 523 4 Setor JPS Kajang 54 71 89 165 171 5 Puchong Drop (2) 55 61 91 126 316 6 Pusat Penyelidikan JPS Ampang 54 68 96 125 136 7 Sek Keb Kg Lui (4) 99 99 142 197 197 8 Rumah Pam JPS Paya Setia 64 68 112 163 169 9 Setor JPS Tanjung Karang (5) 74 74 87 174 175 10 Kg Kalong Tengah (3) 68 68 76 226 227 11 Loji Air Kuala Kubu Baru 106 106 106 154 154 12 Ibu Bekalan Sg. Bernam 62 85 104 147 165 13 Rumah Pam JPS Bagan Terap 101 102 102 132 132 Excerpt from Desa and Rakhecha (2002); ( ): ranking in terms of highest rainfall Table 13 shown below summarizes

highest records of rainfall stations in

the upper catchment in Selangor.

Setor JPS Kajang and Sek Keb Kg.

Lui are located in the Sg. Langat

basin, while the other two, Kg

Kalong Tengah and Loji Air Kuala

Kubu Baru stations are within the


38

upper Sg. Selangor catchment downstream of the Selangor dam.

Table 13: Comparison of Higher Rainfall Records in Upper Catchment in Selangor Duration (hour)

Setor JPS Kajang

Sek Keb Kg Lui

Kg Kalong Tengah

Loji Air Kuala Kubu

Baru

Selangor Dam CA=197 km2

¼ 54 99 68 106 ½ 71 99 68 106 1 89 142 76 106 188 2 N/A 3 300 6 165 197 226 154 391 12 171 197 227 154 518

The comparison in Table 13

apparently is not in anyway

unreasonable and indicative of

underestimation of the Selangor

PMP by SMHB. If the observed

highest values are maximized by

some coefficients, say, 50% higher

due to moisture saturation in the

upper planetary boundary (by

measuring the maximum persistent

dew point temperature and storm

dew point temperature) do not

exceed the PMP values except 1-hour

duration rainfall of Sek Keb. Kg. Lui

station. Only the highest rainfalls

recorded for almost all durations in

the downstream and near the coastal

region, such as Puchong Drop and

Tanjung Karang, are exceeding the

Selangor PMP. It is not known at

present the reasons of such highest

PMP values due to limitation on the

knowledge in the area of expertise of

meteorology. However, this is not in

anyway to ignore such highest

records in the lower catchment

where no major water detention

structures are most likely to be

located especially in the state of

Selangor. Nevertheless,

counterchecking with neighboring

stations is imperative to ascertain if

these values are not isolated event in

the vicinity.

Sg. Kelinchi Dam, SSP/MM, 2001 The PMP derivation was part of the

Kelinchi dam design study by

SSP/MM (2001). The Kelinchi

reservoir is located within the upper


38

catchment of Upper Muar reservoir.

The purpose of the reservoir is to

facilitate water transfer across the

mountain range (via tunnel) to the

Sg. Terip WTP. Besides, it is also

served as extra holding for raw

water via pumping from upper

Muar reservoir downstream. Table

14 is extracted from the design

report on PMP.

Table 14: Comparison of PMP at Upper Muar Dam and Kelinchi Dam Upper Muar Dam Duration (hour)

PMP (mm) Kelinchi Dam Duration (hour)

PMP (mm)

3.5 315 3 337 6.5 400 6 394 12.5 525 12 521 24.5 689 24 692 120 1800 Excerpt from SSP/MM 2001 Upon checking, the results of PMP

analysis are agreeable to SMHB,

although Hershfield’s methodology

was used by SSP/MM in their

assignments. In line with the

recommendation by Hershfield on

the derivation of Km factor,

SSP/MM (2001) slightly modified

the Km values in their computations.

This is accomplished mostly by the

reduction of Km with increased

annual maximum rainfall in the 2600

stations that were used in the

original derivation of the Hershfield

equation. The Km factor is lower for

short duration, i.e. 13 is adopted for

3- and 6-hour duration respectively.

It is however, a blanket adoption of

such adjustment might not be

warranted in the tropic region, such

as in Malaysia. This is due to the fact

almost 90% of the data selected by

Hershfield in the derivation of

statistically based equation were

from USA. As such, if no reduction is

made on the Km factors, the

calculated PMP could be higher than

those tabulated and summarized in

Table 14 (see column 4).

Unfortunately, at the time of this

writing, the mean and standard

deviation of the rainfall records are

not available for calculation using a

constant Km value, i.e. 15.


38

Gelami Dam Design JPS, 2002 The proposed Gelemi reservoir/dam

is a small scheme water supply for

the aquacultural research station in

the upper teriang basin in Negeri

Sembilan. The proposed dam site is

located on one of the tributary of Sg.

Teriang basin. The hydrology

division of the JPS carried out the

detailed PMP study as part of the

assignment for engineering

(spillway) design. The methodology

adopted in this study is of statistical

approach (Hershfield, 1965) but with

modification to the derivation of the

frequency factor, Km. This

modification is based on a research

note published in the Water

Resources Research (Koutsoyannis,

1999). Other than Km modification,

the estimation by JPS also made

appropriate conventional

adjustments for mean and standard

deviation of the maximum rainfall

annual series for individual

duration. For information, the Km

factor for most of the standard

derivation is 15 as this is the highest

enveloped value in the original

Hershfield analysis (WMO, 1986).

The modification is achieved by

recalculating the Km factors for each

duration. As a result, Km factors are

lower to as low as 10.4 for 0.25-hour

duration to as high as 17 for longer

duration, i.e. 120 hour. For

comparison, for a 24-hour duration

PMP, Km is adjusted slightly

upward to about 16 vis-à-vis 15, that

is normally in statistically based

PMP studies.

Due to its close proximity to Sg.

Semenyih basin, i.e. neighboring

basin, comparison of PMP vis-à-vis

those adopted in the Semenyih dam

design is considered reasonable.

Except for 3-hour duration, the

estimated PMP by JPS using

modified version of Hersfield

methodology is mostly lower than

SMHB, i.e. from 9% to as high as

30%. As mentioned above, the

designer adopted adjusted mean and

standard deviation in the calculation.

This adjustment essentially lowers

both the mean and standard


39

deviation of the maximum rainfall

annual series for reason of shorter

length of records vis-à-vis

recommendation in WMO (1986). If

assuming that no adjustments are

made, the estimated PMP for each

duration will then be slightly higher

than before. For such purpose, the

recalculated PMP values without

adjustments are shown in column 6

in Table 15.

Table 15: Comparison of PMPs of JPS (2002) and SMHB Inland Series Duration (hour)

Modified Km factor

(JPS, 2002)

PMP (mm) JPS (2002)

PMP (mm) SMHB Inland

% Difference PMP (mm) without Xn and

Sn adjustment JPS (2002)

((1) (2) (3) (4) (5) (6) 3 12.394 315 300 (261) 5.00% 412 6 13.645 356 391 (352) -8.95% 461 12 14.958 460 518 (474) -11.20% 540 24 15.947 598 692 (643) -13.58% 664 72 16.679 774 1067 (1035) -27.46% 1069 120 16.974 941 1360 (1360) -30.81% 1312

( ) in col 3 indicates after applying the ARF Kelinchi Dam Feasibility Study, SSP/SMEC, 1990 Short duration PMPs are calculated

at each dam site using the Australia

Bureau of Meteorology (ABM)

generalized approach. As

previously compared with other

studies, the ABM estimates are

seemed to be rather high PMPs

value. The estimated PMPs are also

adjusted by the elevation/moisture

inflow barrier and dew

point/maximum atmospheric

moisture factors. The adjustment

procedures could be further referred

in the ABM bulletin for short

duration PMP estimation (recent

amendment of PMP was made in

1996 and issued as Bulletin 53). The

estimated PMPs by ABM are also

tabulated in the Table 16. For

comparison, the Selangor PMP for

similar duration periods is also

written.

Table 16: Short Duration PMP for Various Dam Sites in Negeri Sembilan Duration (Hour)

Kelinchi dam

Sikamat dam

Teriang dam

Gelami dam 1 (mm)

Gelami dam 2

Selangor (mm)


38

(mm) (mm) (mm) (mm) 1 349 352 342 342 352 188 2 627 634 611 611 634 3 836 848 816 816 848 300 Calculation based on ABM short duration methodology

Jemelan Reservoir, JPZ, 1998 The project encompasses a small

reservoir/dam and appurtenances

on Sg. Jemelan, Jerebu, Negeri

Sembilan. The reservoir drains about

7.2 km2 of Teriang Forested Reserve.

The PMP is estimated using

statistical approach, presummedly

Hershfield’s methodology. Table 17

shows the PMP calculated up to 24

hour.

Table 17: PMP in Jelebu, Negeri Sembilan. Relationship of PMP Selangor Dam Rainfall duration

(hour) PMP (%) (mm) (mm) 3 67% 322.40 300 6 83% 399.39 391 12 94% 452.32 518 1-day 100% 481.19 692 Excerpted from JPZ (1998) CONCLUSIONS Both PMPs adopted by SMHB/B&P

and studies by other consultants are

presented systematically in tandem in

this review.

It should be emphasized a priori that

there is no single unified

methodology or procedure in

estimating PMP values in Malaysia

as can be evidently seen in Table 18,

specifically for the case 24-hour

duration PMP adopted in various

studies throughout Peninsular

Malaysia. Competent professionals

and experts in this specialized field

may even obtain different results

because most these estimations

required subjective judgments and

empiricisms. This is further

concurred by WMO (1986),

“Procedure for estimating PMP cannot

be standardized as they vary with

amount and quality of data available,


38

basin size, and location, basin and

regional topography, storm type and

climate…”.

Two series of PMP values, namely

Coastal and Inland PMPs are

basically adopted by B&P/SMHB in

their dam design projects

undertaken throughout the year.

The Inland PMP values were

adopted in several dam design

studies, such as Linggi, Semenyih,

Tinggi, and Selangor dams, as it is

most suitable to represent the

prevailing weather pattern in the

interior west coastal basin.

The long duration PMP, which

derived based on several recorded

extreme events (about 8 of them) in

the Northeast Monsoon in the east

coast are basically consistent in the

same order of magnitude for 3

independent studies, i.e. SSP/HH

(1979), SSP/SMHB (1999), and

NK/SMHB (2000).

Table 18: Selected PMP Estimate for Dams in Peninsular Malaysia PMP Duration Dam Catchment

area (km2) 3-hour (mm) 6-hour (mm) 24-hour(mm) 72-hour(mm) Sources/Remarks

Ayer Hitam 5.8 230 259 366 (B&P, 1976) Langat 41 191 (1-h) 432 && (B&P, 1976) Kenyir 1600 470 1280 (ABM and SMEC, 1976) Layang 31 490 840 (T&T, 1977) Semberong 130 312 541 817 (SSP and HH, 1979) Semberong 130 203 352 531 (Grout, 1980) Semberong 130/123 297 697 1412 (ABM and SMEC, 1984) Bekok 350 291 524 791 (SSP and HH, 1979) Bekok 350 207 372 562 (Grout, 1980)) Bekok 350/349 297 687 1412 (ABM and SMEC, 1984) MacRitchie 7.7 675 (PUB, 1979) Seletar 10.8 338 440 777 (B&P, 1980) Semenyih 58 300 390 692 (B&P (1983) Tinggi (Buluh) 40 300 390 692 (SMHB, 1990) Selangor 192 300 390 692 (SMHB, 1999) Ahning 120 244 348 436 (JK and SMEC, 1982) Ahning 120 740 1160 1720 (ABM, 1984) Ahning 120 323 634 806 (B&P, 1986) Machap 30 234 396 577 (SSP and HH, 1979) Malut 3.4 440 (1-h) 600 (2-h) 740 (3-h) (JK et. at., 1984) Malut 3.4 N/A N/A N/A N/A **(SMHB, 1994) Batu 50 337 470 820 (USBR, 1984) Klang Gates 77 330 460 812 (USBR, 1984) Gombak 88 329 458 806 (USBR, 1984) Tembat 101 510 660 1010 1660 (SMEC, 1985) Linggiu 208 723 1541 (SMHB, 1985)


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Terengganu 2 44 640 840 1140 1330 (ABM and SMEC, 1988) Kuala Yong 91 600 780 1080 1270 (ABM and SMEC, 1988) Paya Peda 77 610 850 1655 (ACE, 1995) Noak 15.8 250 350 560 720 (SMHB, 1984) Puah 410 420 560 470 ? 1150 (SMEC, 1983) Upper Muar 148 295 390 685 1412 (SMHB, 1993) Beris 116 204 385 536 1015 (ACE, 1994) Timah Tasoh 191 402 560 1090 (ACE, 1996) Padang Saga 12 330 577 695 (ACE, 1987) Padang Saga 12 466 577 695 (JPZ, 1997) Aur 98.3 542 756 1428 (JPZ, 1998) Bukit Merah 480 525 630 (MACE, 1987) Bukit Merah 480 707 1408 (JPZ, 1997) Congok 17.6 777 2030 * (SMHB, 1988) Lebam 18.9 777 2030 * (SMHB, 1991) Telemong 360 350 460 615 1040 # (NK and SMHB, 2000) Kelau 331 370 480 625 1050 # (NK and SMHB, 2000) Lebir 1465 * (SSP and SMHB, 1999) Kemubu 1090 * (SSP and SMHB, 1999) Babagon 30 320 575 (7-h) 700 (KKPK and SMHB, 1992) Gelami Lami 22.2 315 356 598 774 (JPS, 2002) Kelinchi (SSP and MM, 2001) Kelinchi 836 (SSP and SMEC, 1990) Sikamat 848 (SSP and SMEC, 1990) Gelami 1&2 816/848 (SSP and SMEC, 1990) Jemalan 322 399 481 (1-d) (JPZ, 1998) * 120-hourduration; # 72 hour; && not equated to PMP in B&P (1976) report; PMP values without areal adjustment: Excerpted from SMHB, 1992; SMHB, 1994; checked with SMHB/RHB/ZAABA, 2000.

On the other hand, for short

duration PMP, due to the primarily

scarcity of automatic recorded

information available in Malaysia,

the agreements amongst various

consultants that undertook PMP

studies in Malaysia cannot be

reached amicably. ABM who

establishes itself in Malaysia as the

principal specialist on PMPs in the

Kenyir (Terengganu) and Pergau

(Kelantan) dams design in the east

coast and several other reservoir

design in both Sabah and Sarawak

(Batang Ai, Gerugu, Kelolong, and

Babagon), derives its short duration

PMP of less than 6-hour duration

based on adjusted storm records

occurring in Australia and USA

(ABM Bulletin 51; now revised

version Bulletin 53, 1994 with

amendment issued in 1996). In their

contentions, it is valid to transpose

generalized storm data from similar

tropical regions of the USA and

Australia to Peninsular as well as

Borneo Malaysia. To start with, it

however remains doubtful if such a

generalized methodology could be


38

used in its entirety in Malaysia,

which rarely experiences such

extreme weather conditions (such as

typhoons, hurricanes, and

tornadoes) as in the USA and/or

perhaps Australia. The short

duration PMP derived by ABM is

always higher vis-à-vis

B&P/SMHB’s conventions, which is

based mostly on maximized

observed records of short duration

rainstorm in Singapore and southern

Peninsular Malaysia (mostly the1978

storm).

As a check, comparison was also

made with the statistical approach,

i.e. Hershfield technique for 24-hour

PMP in the state of Selangor. It

concludes and shows consistency in

PMP values derived by both totally

different approaches albeit the

Hershfield technique is mostly in the

lower end.

Appendix A: List of Abbreviations ABM Australia Bureau of Meteorology

Angkasa-GHD Angkasa-GHD Sdn. Bhd.

ARF Areal Reduction Factor

B&P Binnie and Partners Pty. Ltd.

CA Catchment Area

DAD Depth-Area-Duration

HH Howard Humphrey

HTC The Regional Humid Tropics and Water Resources Centre For Southeast Asian and the Pacific

JPS Jabatan Pengairan dan Saliran (Department of Irrigation and Drainage, Malaysia)

JPZ JuruPerunding Zaaba Sdn. Bhd.

Kinhill Kinhill Engineer Pty. Ltd.

km Hershfield’s Equation Frequency Factor

km2 square kilometer

MM Mott MacDonald

NK Nippon Koei


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PKM Perkhidmatan Kajicuaca Malaysia (Malaysia Meteorological Service)

PMP Proable Maximum Precipitation.

PUB Public Utility Board

Ranhill Ranhill Bersekutu sdn. Bhd.

SDW Sewage Disposal Works

SMEC Snowy Mountain Engineering Consultant

SMHB Syed Mohammad, Hooi and Binnie Sdn. Bhd.

SSP Setia Sepakat Sdn. Bhd.

TIDEDA Time Dependant Data Program

USBR United States of Bureau of Reclamation

US NWS United State National Weather Services

WMO World Meteorological Organization

References Australian Bureau of Meteorology (ABM), 1985. The Estimation of Probable Maximum Precipitation in Australia for Short Durations and Small Areas. Bulletin 51, Australian Government Publishing Service, Canberra (replaced by Bulletin 53, 1994; with amendment in 1996).

Angkasa-GHD, 1998. Putrajaya: Perang Besar Reservoir Design Study, Appendix D: Flood Hydrology for Main Dam, Final Design Report.

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

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

Desa M, M.N., and P.R. Rakhecha, 2002. Short Duration Extreme Rainfalls in Selangor, Malaysia. In International Conference on Urban Hydrology for the 21st Century, Kuala Lumpur, Malaysia, October 2002.

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

Hershfield, D.M., 1961. Estimating the Probable maximum precipitation. Proc. ASCE, Journal of Hydraulic Division, vol 87, 99-106.


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Hershfield D.M., 1965. Method for Estimating Probable Maximum Precipitation. Journal of American Waterworks Association, vol 57, 965-972.

JPS, 2002. Projek Empangan dan Kolam, Gelimi Lemi, Jelebu, Negeri Sembilan, Hydrology Report, Unit Hidrologi Gunaan, Jabatan Pengairan dan Saliran (JPS) Malaysia.

Jurutera Perunding Zaaba, 1998. Detailed Design of Small Dam and Related Works at Mardi Station, Jelebu, NS. Volume 2. Draft Final Report Appendices. Jabatan Pengairan dan Saliran Malaysia.

Kinhill and Ranhill, 1994. Klang River Basin integrated Flood Mitigation Projects, Malaysia, Final Report.

Koutsoyiannis, D. 1999. Probabilistic view of Hershfield’s method for estimating probable maximum precipitation. Water Resources Research Vol 35 No. 4, pp 1313-1322.

NK and SMHB, 2000. Pahang-Selangor Raw Water Transfer Project Engineering Services and Detailed Engineering Design: Hydrology, Final Report, JBA, JKR Malaysia.

National Research Council, 1985. Safety of Dams: Flood and Earthquake Criteria, National Academic Press, Washington DC, USA.

National Research Council, 1994. Estimating Bounds on Extreme Precipitation Events: A Brief Assessment, National Academic Press, Washington DC, USA.

Ong, C.Y. and W.L. Liam. 1986. Variation of Rainfall with Area in Peninsular Malaysia. Water Resources Publication No. 17, Jabatan Pengairan dan Saliran, Kementerian Pertanian Malaysia.

SMHB, 1985. Feasibility and Preliminary Engineering Studies for the Joint Development of the Johor River Water Resources, Technical Report Vo. 4 on Hydrology, Malaysia/Singapore Joint Technical Subcommittee for the Joint Development of the Johor Water Resources, Malaysia.

SMHB, 1992. Study in Comprehensive Water Resources Planning and Development in the State of Pahang, Economic Planning Unit, Government of Malaysia.

SMHB, 1994. Study in Comprehensive Water Resources Planning and Development in the State of Johor, Government of Johor, Malaysia.

SMHB, RHB and ZAABA, 2000. National Water Resources Study 2000-2050, Hydrology Chapter, Final Report, EPU, Government of Malaysia.

SSP and HH, 1979. Dams on Sg. Bekok and Sg. Semberong, Detailed Investigation and Design: Hydrology, JPS Malaysia.


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SSP and SMHB, 1999. Kelantan River Flood Mitigation Plan Feasibility Study, SSP/SMHB, Jabatan Pengairan dan Saliran, Malaysia.

SSP and MM, 2001. Sg. Kelinchi Water Resources Study. Final Report, Jabatan Bekalan Air Negeri Sembilan, Negeri Sembilan, D.K.

SSP and SMEC, 1990. Feasibility Study for Water Resources Development for Domestic and Industrial Uses for the District of Seremban and Port Dickson. Volume 1: Technical and Financial Report, Final Report, Jabatan Kerja Raya, Malaysia.

WMO, 1973. Manual for Estimation of Probable Maximum Precipitation, first edition, (WMO- No 332), World Meteorological Organization Operational Hydrology Report No. 1.

WMO, 1986. Manual for Estimation of Probable Maximum Precipitation, second edition, (WMO- No 332), World Meteorological Organization Operational Hydrology Report No. 1.

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