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CHAPTER 3
METHODOLOGY
3.1
Introduction
Implementation of the Flood Mitigation Project has in general been based on
increasing the conveyance and hydraulic efficiency of the tributary and main river
channels. This has been achieved by straightening the alignment of the rivers, formalizing
the channel cross-section and ultimately concrete lining the channel sides where it was
fully improved. The design has been based on conveying flood discharge through the city
center. Increased hydraulic efficiency increases the celerity of the flood waves and has
the effect of increasing peak discharges downstream through the city. The effect has been
to efficiently convey increasing amounts of floodwater into the city center where the
design capacity is breached. This effect has been compounded by a mismatch between
the flood conveyance of newer upstream channels compared to the existing channels
through the city center, exacerbated by various infrastructure and buildings reducing the
available conveyance area of the river channels.
In general the land and main drainage channels are steep in the northeastern and
eastern areas of the river basin. Poor land development practice has resulted in very high
sediment loads from developing areas resulting in sediment deposition and reduced flood
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conveyance through the flatter (lower longitudinal gradient) river channels through the
main city area.
The previous and on-going cycle of upstream development and construction of
more efficient drainage infrastructure and upstream river channels combined with
constraints on the main river channels had therefore increased the risk of flooding in the
city center.
Another component of the overall flooding problem is inappropriate development
in flood-affected areas. Implementation of structural flood mitigation works can only be
effective in controlling and mitigating flooding up to the design flood event and structural
works should NOT be relied on as the only measure to be implemented. Ultimately, the
infrastructure will be not effective. A larger than design flood will occur or the design
will be negated by upstream developments. Flooding may also occur in smaller flood
events due to failure of a component such as a drainage gate or a section of floodwall.
3.2
The study on the Flood Mitigation of the Klang River Basin 1989’ by JICA
The JICA Study performed flood analysis and modeling with the following stated aims:
-
“to investigate the retardation effects for flood peak discharge by existing
dams, and
-
to formulate probable flood discharge distribution under present condition
and with flood control works proposed by this (JICA) study.”
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The study includes the following steps:
i.
construction of a river system model based on conceptual non-linear
storages to convert rainfall excess to runoff
ii.
calibration of the storage parameters in the river system model
iii.
frequency analysis of annual maximum catchment rainfalls for 1961-1985
(computed using Thiessen Polygons) to estimate design catchment
rainfalls for Average Recurrence Intervals up to 200 years
iv.
calculations of rainfall excess by use of the runoff coefficients
v.
computation of surface hydrographs at various locations within the river
system by applying the design rainfall to the calibrated river system model
vi.
addition of base flow (taken as the monthly flow for the wet season) to
produce complete flood hydrographs
The adopted runoff coefficients for the catchment above Sulaiman Bridge were
0.36 for 1985 conditions and 0.39 for conditions forecast for 2005 (increase of 8%). The
equivalent values for the catchment from Sulaiman Bridge to the mouth were 0.36 and
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0.40 (increase of 11%). These coefficients were based on coefficients for various
combinations of land use, slope and soils.
The river system model was calibrated on the data from 3 floods (1974, 1981 and
1982) and was calibrated only at the gauging locations of Batu River at Sentul and
Gombak River at Jalan Tun Razak. The model was not calibrated to the Sulaiman Bridge
gauge.
The land use conditions adopted for the design flood were the 2005 conditions.
The model was also adjusted to Batu Dam and the raising of Klang Gates Dam, (both
implemented after the calibration floods). An optional flood diversion channel from
Gombak River to Batu River to be used in conjunction with a retention pond was also
investigated.
The design discharges for the case with Batu Dam and the raised Klang Gates
Dam (but without diversion channel from Gombak to Batu) are presented in Table 3.1.
Table 3.1: Design Discharge (m3/s) from JICA Model (with 2 dams)
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RIVER
LOCATION
CATCHMENT
AREA
Batu
100 YEAR
50 YEAR
10 YEAR
(km2)
Inflow to Batu Dam
50.2
100
80
35
Outflow from Batu Dam
50.2
55
50
35
Downstream of Jinjang
97.2
155
135
75
147.2
270
230
125
Upstream of Batu confluence
122.0
240
190
95
Upstream of Klang
269.2
500
415
220
Inflow to Klang Gates Dam
76.7
145
110
50
Outflow from Klang Gates
76.7
130
120
70
153.8
295
255
145
At Sulaiman Bridge
458.2
795
665
360
At Puchong Drop
712
1130
920
485
At Klang Town
1288.4
1595
1250
625
confluence
Downstream of Gombak
confluence
Gombak
confluence
Klang
Dam
Downstream of Ampang
confluence
The study is considered to have adopted an appropriate approach to the estimation of
design floods within the Basin. However, the level of reliability of the seeding floods is
considered to be limited by several factors:
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i.
there was no assessment of the quality of rainfall and discharge data
ii.
the river system model was calibrated at only two locations, with
calibration on only one flood at one station (Gombak River at Jalan Tun
Razak) and three floods at the other (Batu River at Sentul);
iii.
the model tended to consistently underestimate flood discharges for Batu
River at Sentul, but overestimated flood discharges and the length of the
recession for the one calibration flood for Gombak River at Jalan Tun
Razak; and
iv.
as described above, the impact of urbanization and channel works was
included in the model, however the computation of these effects as
presented is not consistent with the description in the text and there is no
other documentation of the basis for changes to the model.
3.3
“Klang River Basin Integrated Flood Mitigation Project, Malaysia, 1994” by
Kinhill in association with Ranhill Bersekutu Sdn.Bhd, 1994.
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This was a Project Preparatory Technical Assistance (PPTA) commissioned by
the Asian Development Bank (ADB), comprising a feasibility study to integrate the three
ongoing flood mitigation projects in the Klang River Basin into one project for the
Government of Malaysia. The integrated three ongoing projects were to be packaged for
investment purposes and the overall objectives of the integrated project were to mitigate
major flooding problems in the Klang River Basin.
3.4
Klang River Flood Mitigation Project –Klang River Improvement Works
(from Klang Lama to Klang Town – Review Report (DNA September 1996)
This report includes a short review of the assumption, criteria and methodologies
adopted by the Klang River Basin Integrated Flood Mitigation Project, Malaysia
(KRBIFMPM) Kinhill, 1994. The review encompassed further analysis on the hydrology
and hydraulics of the whole Klang Valley basin. The review supported the overall
approach adopted in Kinhill (1994) and the catchment sub-division adopted for the
RORB rainfall-runoff model. However, the review proposed an alternate method for
apportioning catchment storage within sub-catchment, which in the KRBIFMPM model
was based only on reach length. The review proposed that the catchment storage should
be based on reach length divided by the square root of the average gradient in the reach.
The latter approach is recommended in the RORB manual for river gradients that vary
substantially. In the review, the percentage impervious for all catchment was varied from
that adopted in calibration.
(Note that runoff coefficient is considered to be the same as fraction impervious
in the review). For example, according to the DNA review Kinhill (1994) adopted 19%
impervious fraction for the catchment area upstream of Sulaiman Bridge. The DNA
review varied these figures based on topographical maps; aerial photographs and
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proposed future land use maps and adopted 15% impervious for the January 1971 flood,
32% for the 1988 flood and 34% for the “Future Case”. For the catchment from Sulaiman
Bridge to the river mouth, the Kinhill study adopted an average impervious fraction of
11% while the DNA review increased this in their model to 33%.
The DNA review also adjusted the parameters that represent the slope and form of
the channels within the basin. The review adopted react slope and form based on the data
available at the time of the review.
The RORB model was calibrated on the recorded floods for 1971, 1981 and 1988.
The plots presented in the report show that the RORB model agreed very well with the
recorded hydrograph for the May 1988 flood Sulaiman Bridge, and agreed reasonably
well with the January 1971 flood at the same location. The only hydrographs presented
for the Gombak and Batu Rivers are for the 1971 floods.
These show the RORB model rising much faster than the recorded hydrograph,
and having a much shorter recession than the recorded hydrograph. One possible
explanation for this is that the RORB model underestimates storage within the catchment
for very large, long duration floods although it adequately represents catchment storage
effects for smaller, shorter duration floods (such as the 1988 flood). However another
explanation might be that the storage within the catchment, particularly storage
associated with ex-mining ponds had reduced obviously from the year1971 to 1988.
3.5
Formulation of Master Plan and Proposed Flood Mitigation Plan
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Japan International Cooperation Agency (JICA) considered that flooding is caused by
both depression type monsoon storms and thunderstorms. JICA summarized the reasons
for flooding in the Klang River Basin as:
3.5.1 Increase in Run-Off
i.
Increase of run-off coefficient due to rapid urbanization
ii.
loss of ex-mining ponds as potential retention ponds due to refilling
housing developments
iii.
Improvement of trunk drainage or tributaries to an excessive scale
compared to the discharge capacity of downstream stretches; and
iv.
flowing down of floating logs and debris in the river channel during floods
3.5.2 Problems of Drainage Facilities
i.
Inadequate flow capacity of river channel or trunk drainage
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ii.
Existence of low lying areas
iii.
Insufficient clearance at bridge crossing; and
iv.
Tidal effect in the downstream reaches
The proposed flood mitigation plan included the following measures:
i.
Construction of the Batu Retention Pond
ii.
Construction of the 4.3 km long Gombak Diversion Channel
iii.
Use of the Natural Retarding Basin at Sungai Rasau
iv.
Channel Improvement to 94.7 km of the Klang, Gombak and Batu
Rivers;
v.
Construction of a pumping station in Kampung Baru ; and
vi.
Reconstruction of 10 bridges
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3.6
Proposed Klang River Components
As an outcome of the technical studies and feasibility study phases of the Project
Preparatory Technical Assistance (PPTA), seven sub-projects were identified for
inclusion in the Project that is briefly detailed below:
i.
Sub-project 1: Klang River Channel Improvement
ii.
sub-project 2: Klang River Levee Extensions
iii.
sub-project 3: Sediment Trapping
iv.
sub-project 4: Tributary River Channel Improvement
v.
sub-project 5: Flood Forecasting and Warning
vi.
sub-project 6: Integrated Catchment Management Pilot Programmed
vii.
sub-project 7: Solid Waste Management
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In general terms, the outcome of the PPTA can be considered as a consolidation of the
ongoing project into defined project packages and programs for future implementation.