Evaluation of Flooding in Sg Gita Catchment, Kuching

Abstract

This paper outlines a methodology of flow routing with inclusion of downstream river water level applied to a Sg Gita urbanized catchment beside Sg Sarawak in Kuching city, Sarawak, Malaysia. Evaluations are done by demonstrating the modelling of flooding scenarios using InfoWorks River Simulation (RS) that stresses on different aspects specific to Sg Gita's conditions, namely (1) impacts of high and low river water levels, (2) solely urban flooding and (3) the combination of the two. The outcomes indicate that the dynamics of downstream river water level influences the performance of the urban drainage that flowed into the river. Backwater is overriding the flows of urban drains. Therefore, the mentioned methodology is found superior than conventional methodology with only flow routing to represent the flow mechanism of urban catchment bounded by a downstream river.

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Journal of Applied Science & Process Engineering
Vol. 4, No. 1, 2017
* Corresponding author. Tel.: +6082-583207; fax: +6082-583410
E-mail address: ysmah@unimas.my
Manuscript History:
Received 28 January, 2017, Revised 26 April, 2017, Accepted 27 April, 2017, Published 28 April, 2017
ISSN: 2289-7771
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127
Evaluation of Flooding in Sg Gita Catchment, Kuching
D.Y.S. Maha,*, C.P. Hiib, C.Y. Ongb and Y. Puib
aHydro-Environmental Engineering Research & Development (HERD) Cluster, Department
of Civil Engineering, Faculty of Engineering, Universiti Malaysia Sarawak, 94300 Kota
Samarahan, Sarawak
bJurutera Adda Consulting Engineers, Bangunan USC, No 683, Lorong Song 1A, Off Jalan
Song, 93350 Kuching, Sarawak
Abstract
This paper outlines a methodology of flow routing with inclusion of downstream river water level
applied to a Sg Gita urbanized catchment beside Sg Sarawak in Kuching city, Sarawak, Malaysia.
Evaluations are done by demonstrating the modelling of flooding scenarios using InfoWorks River
Simulation (RS) that stresses on different aspects specific to Sg Gita’s conditions, namely (1) impacts
of high and low river water levels, (2) solely urban flooding and (3) the combination of the two. The
outcomes indicate that the dynamics of downstream river water level influences the performance of the
urban drainage that flowed into the river. Backwater is overriding the flows of urban drains.
Therefore, the mentioned methodology is found superior than conventional methodology with only
flow routing to represent the flow mechanism of urban catchment bounded by a downstream river.
Keywords: backwater, InfoWorks RS, river water level, stormwater, urban drainage
1. Introduction
Sg Gita catchment is located immediate upstream of Satok Bridge (seen on the left of Figure
1). The area is one of the highly flood prone area in Kuching city beside Sg Maong catchment at the
opposite bank [1],[2]. Its oldest settlement, Kpg Gita flanks a stretch of the northern bank of Sg
Sarawak. As such, it suffered repeating flooding as reported in the major flood events in 2003, 2004
and 2009 [3]; as well as the recent 2013, 2015 and 2016.
2. Rationale for flood investigation
Before any flood mitigation measures could be prescribed to Kpg Gita, the cause of flooding at
the area should be thoroughly scrutinized. This has become the intention of this paper to reconstruct a
historical flood event to provide insights to the occurrence of flood. Evidenced in Figure 1, Sg Gita
catchment has been heavily populated. Therefore, land drainage, in this case, the urban stormwater
drainage is significant [4]; at the same time, due to the closeness to Sg Sarawak, the hydrology and
hydraulics of the river also play a role.

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Figure 1. Sg Gita catchment.
To accommodate the said investigation, a computer simulation model is an appropriate tool to
help animate the flooding processes [5]. The subject of modelling is a drainage system; and this
drainage system of Sg Gita catchment has its outfalls pouring into Sg Sarawak. In addition, the river is
regulated by the Kuching Barrage [6]. It implies that Sg Sarawak is controlled at rather constant water
levels. During high tides, Kuching Barrage often closes its gates to stop tides from entering upstream
river; yet if high rainfall events happened to coincide with high tide, it may cause the river water
levels to soar to bank bursting levels [6]. Under such circumstances, the stormwater system would
cease to have free-flowing outfalls but influenced by high river water levels.
Conventional simulation of ground surface runoff along a waterway involves various methods
of flow routing. Solving the conservation of mass together with conservation of momentum allowing
these methods to carry water from one point to a downstream point [7]. Usually, an upstream
hydrograph is routed and a downstream hydrograph is computed as the result. However, downstream
fluctuating water levels could not be accounted for in these methods. Then, these methods would
represent poorly the field conditions of Kpg Gita.
InfoWorks RS, on another hand, simulates flow by defining both the upstream and
downstream boundary conditions. The dynamics of river water levels could be represented in the
latter. Although the name of the software suggests river, InfoWorks RS can model many forms of
waterway, including the urban drain. Therefore, InfoWorks RS is chosen for this modelling task.
Sg Sarawak
Satok
Bridge
Sg Gita
Kpg Gita
Sarawak
Kuching
City

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3. Modelling approach
InfoWorks RS generally involves the following three (3) steps (see Figure 2). First, the source
of water should be dealt with. As rainwater enters the urban environment, the geography of the
location is playing an important role. Therefore, ground surface information is necessary. InfoWorks
RS requires the information in the form of Ground Model.
Once landed, running water travels on the ground surface. The mechanism of running water
could be simulated provided that the drainage network of its dimensions, invert levels and directions
of flow are available. The necessary information has been collected by field surveys. The data are
treated as inputs to the InfoWorks RS environment.
Figure 2. Modelling approach.
3.1. Rainfall
A historical event is selected to produce a reliable analysis. A most recent extreme storm event
which occurred on 27th February 2016 is selected for the purpose. The storm had the meteorological
station in Kuching Airport to record a total rainfall of 141.5 mm spanning over thirteen hours. Table 1
shows the hourly rainfall on 27th February 2016 recorded around Kuching and Figure 3 indicates the
isohyetal map of the storm event. Note that Sg Gita catchment (encircled in the said figure) is close to
the storm eye. Not surprisingly, Sg Gita catchment was flood stricken after the storm.
Step 1: Rainfall
Step 2: Ground Surface
Step 3: Flow Mechanism

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Table 1. Hourly rainfall on 27th Feb 2016 around Kuching [8]
No Station Name 0:00 1:00 2:00 3:00 4:00 5:00 6:00 7:00 8:00 9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 19:00 20:00 21:00 22:00 23:00
1 Bako Causeway 0 0 0 0 1 15.5 36.5 43.5 79.5 29 7.5 13 0.5 1.5 0 0 0 0 0 0 0 0 0 0
2 Batu Kawa Bridge 0 0 0 0 33 27 17 516 6.5 6 3 1.5 0 0 0 0 0 0 0 0 0 0 0
3 Kampung Nelayan 0 0 0.5 0 .5 5 23 41 .5 6 2.5 3 1.5 2 2.5 2 0.5 6 0 0 0 1 0 0 0 0 0 0 0 0
4 Kuching Airport 0 0 0.5 2.5 24.5 21.5 42.5 24 6 7 5 4.5 3 0 .5 0 0 0 0 0 0 0 0 0 0
5 Kuching City South 0 0 0.5 3.5 3.5 15.5 82 85.5 5 1.5 27 32.5 2 2.5 0 0 0 0 0 0 0 0 0 0 0 0
6 Kuching Seberkas 0 0 1. 5 6 8 12 43 6 7.5 42 14.5 27 2 6.5 1 .5 0 0 0 0 0 0 0 0 0 0 0
7 Kuching Third Mille 0 0 1 7 13 11 55 67 41 16 27 21 0.5 0 0 0 0 0 0 0 0 0 0 0
8 Malihah 0 0 0 2 20 65.5 40 9.5 14 .5 8 12 4 2.5 0 0 0 1 0 0 0 0 0 0 0
9 Maong Tengah Kiri 0 0 3 .5 8.5 9.5 4 59 46 37 13 18 30.5 1.5 0 0 0 0 0 0 0 0 0 0 0 .5
10 Mid Sungai Kuap 0 0 0.5 5 26 26 15 50 5.5 10.5 2 7.5 23 2 0 0 0 0 0 0 0 0 0 0 0
11 Sebubut 0 0 2 16 3 4 9.5 48 31.5 12 717 7 .5 4 0 0 0 0 0 0 0 0 0 0 0
12 Semariang Fisheries 0 0 0.5 0 0.5 3.5 45.5 25.5 30.5 30.5 23 3 0.5 0 0 0 0 0 0 0 0 0 0 0 .5
13 Siol JPS 0 0 0.5 3.5 2 2.5 44 46 5 6.5 35.5 10 5.5 0 0 0 0 0 0 0 0 0 0 1.5 0.5
14 Sungai Tengah 0 0 0 12 5.5 37.5 22 8 4 4 6.5 3 1 .5 0 0 0 0 0 0 0 0 0 0 0
Hourly Rainfall on 27.02.201 6
Figure 3. Isohyetal map of 27th Feb 2016 storm event.[8]

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3.2. Ground surface
Floodplain survey by means of UAV drone [9] had enabled the computation of Digital
Elevation Model (DEM) for Sg Gita catchment. Presented in Figure 4 is the mentioned DEM with
elevations ranging from -1 to 9 m MSL. It can be said that the low laying areas below 5 m MSL in the
study area are well captured.
A look into the DEM reveals that the northern part (Matang) is of higher elevations (7 – 9 m
MSL). Kpg Gita is generally situated on 5 – 6 m MSL in the eastern part. However, towards the
riverbank, the ground is descending. Around the vicinity of the mouth of Sg Gita (Kpg Gita Lama,
Kpg Gita Laut and Kpg Gita Tengah), the elevations are the lowest <4 m MSL.
Figure 4. DEM of Sg Gita catchment.
With the assistance of ground model, the exact location of the drains could be identified. Then
cross sections are created at the exact survey spots. Demonstrated in Figure 5, the width of a drain
could be represented in the Cross Chainage (m) column; while the depth of the drain could be
represented in Z (m) column. X (m) and Y (m) columns are the coordinate of the spot. These represent
Matang
Kpg Gita Lama
Kpg Gita Tengah
Kpg Gita Laut

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the depression on ground surface from which the water would travel into and flow off following the
land form.
Figure 5. Cross-sectional profile in InfoWorks RS.
3.3. Flow mechanism
Recall the previous sections, (Step 1) hourly rainfall are recorded in Table 1; (Step 2) surveyed
ground information allows creation of “virtual” drains in InfoWorks environment (see the dotted circle
in Figure 5). Next (Step 3), how much water to the drain depends on the sub-catchments.
3.3.1. Delineation of sub-catchments
Out of field surveys, the existing drainage network is surveyed of its direction of flow,
dimension and network layout. Sg Gita catchment is found to have major drainage / trunk drains on
both sides of Jalan Pinang Jawa. Here, they are referred to as Pinang Jawa Left Drain and Pinang
Jawa Right Drain. About half of the residential houses have their minor drains flowed to the
mentioned trunk drains, and eventually emptied to Sg Sarawak.
The overall drainage catchment is further sub-divided into eleven (11) sub-catchments by
tracing their connectedness and outlets. Each of the sub-catchment is a stand-alone network. Referring
to Figure 6 below, sub-catchments A1, A2, A3 and A4 are drained to Pinang Jawa Right Drain; while
sub-catchment A5, A6 and A8 to Pinang Jawa Left Drain. Sub-catchment A2 is again sub-divided to
five (5) smaller catchments due to the fact that the outlets of this catchment, namely Bunga Rampai
Right Drain and Bunga Rampai Left Drain are found to be flooded several times previously.
Therefore, a much in-depth analysis is carried out for sub-catchment A2.
Sub-catchments A7 and A11 are drained through Bunga Kenanga-Bunga Tiong Drain and
discharged directly to Sg Gita. Sub-catchments A9 flow to Taman Mawar Right Drain, while A10
flow to Taman Mawar Left Drain, in which both drains join at the end of drains before being
discharged to Sg Sarawak. Similarly, these two sub-catchments are known to be flood prone and
therefore they are further sub-divided to accommodate analysis of its drainage system. The area,

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length of overland flow, time of concentration and runoff coefficient for each sub-catchment are
tabulated in Table 2.
3.3.2. Computation of flows
HEC-HMS is utilised to compute the flow from each sub-catchment. The loss method and
transform method applied in this modelling is SCS Curve Number and SCS Unit Hydrograph
respectively. The parameters adopted are shown in Table 3 and the computed flows are checked so
that the peak discharges are fit with those obtained from Rational Method. Examples of the flow
hydrographs for Sg Gita sub-catchments are presented in Figure 7.
Figure 6. Delineation of Sg Gita sub-catchments.
A1
A5
A4
A3
A2
A6
A7
A9
A8
A5
A10
A11
Pinang Jawa Left Drain
Pinang Jawa_Right Drain
A2-2
A2-1
A2-3
A2-4
A9-1
A9-3
A9-2
A9-4
A9-5
A10-1
A10-2
A10-3
Taman Mawar
A10-4
A2-5
Sg. Sarawak

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Table 2. Information of Sg Gita sub-catchments
Sub-
Catchment
Area
(m2)
Length of
Overland
Flow (m)
Time of
Overland
Flow, td
(min)
Channel
Flow Time,
to (min)
Time of
Concentration, tc
(min)
Runoff
Coefficient, C
A1
44113
43.0
32.464
8.875
41.339
0.80
A2-1
72221
10.0
19.800
13.618
33.418
0.80
A2-2
132320
32.6
55.268
10.041
65.309
0.80
A2-3
6445
10.0
1.464
4.340
5.804
0.80
A2-4
20012
10.0
26.619
5.626
32.245
0.80
A2-5
28790
42.2
43.015
4.177
47.192
0.80
A3
38664
57.0
95.099
5.088
100.187
0.48
A4
306426
106.0
116.946
38.985
155.931
0.56
A5
24864
10.0
1.464
3.030
4.494
0.80
A6
14860
55.1
71.977
1.754
73.731
0.80
A7
162127
10.0
1.464
12.658
14.122
0.80
A8
28454
40.0
52.818
0.953
53.771
0.60
A9-1
17195
10.0
1.464
24.057
25.521
0.60
A9-2
25473
10.0
1.464
12.572
14.036
0.80
A9-3
7085
10.0
1.464
3.350
4.814
0.80
A9-4
5883
10.0
1.464
2.413
3.877
0.80
A9-5
56402
53.2
40.083
6.796
46.879
0.80
A10-1
15882
10.0
1.464
19.097
20.561
0.80
A10-2
6440
10.0
1.464
2.925
4.389
0.80
A10-3
6579
10.0
1.464
1.678
3.142
0.80
A10-4
133626
10.0
1.464
9.372
10.836
0.80
A11
10356
100.6
114.925
1.641
116.566
0.55
Table 3. Parameters for SCS hydrograph method
Sub-
Catchment
Area (km2)
Initial Abstraction
(mm)
Curve
Number
Impervious
(%)
Lag Time
(min)
A1
0.044130
4
94
80
41.30
A2-1
0.072210
4
96
90
33.40
A2-2
0.132320
4
94
90
65.30
A2-3
0.006445
4
93
70
5.80
A2-4
0.020012
4
96
90
32.25
A2-5
0.028790
4
96
90
47.00
A3
0.038664
4
89
50
100.20
A4
0.306426
4
86
50
155.93
A5
0.024864
4
96
90
4.49
A6
0.014860
4
94
80
73.73
A7
0.162167
4
96
90
14.12
A8
0.028454
4
92
60
53.74
A9-1
0.017195
4
96
90
25.52
A9-2
0.025473
4
96
90
14.04
A9-3
0.007085
4
96
99
4.81
A9-4
0.005883
4
97
99
4.81
A9-5
0.056402
4
94
80
46.88
A10-1
0.015882
4
96
90
20.56
A10-2
0.006440
4
96
90
4.84
A10-3
0.006579
4
96
90
3.14
A10-4
0.013626
4
94
80
10.84

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Figure 7. Flow hydrographs due to 27th February 2016 storm event.

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3.3.3. Drainage network
Generally, five (5) drainage networks are computed. In the schematic diagram in Figure 8, the
trunk drains of Pinang Jawa Left Drain and Pinang Jawa Right Drain are visible on the left. Bunga
Kenanga - Bunga Tiong Drain (in the middle) is created to cater for sub-catchment A7 that did not
drained to Pinang Jawa drains. Another two networks on the right are named Taman Mawar Right
Drain (for sub-catchment A9) and Taman Mawar Left Drain (for sub-catchment A10).
Figure 8. Drainage networks.
4. Investigative modelling
4.1. Control Scenario
All networks depicted in the previous figure receive waters from its sub-catchments at different
points of the urban drains. Eventually the waters flow to the end of the drains that may or may not be
obstructed by river water level of Sg Sarawak. Sg Sarawak is not modelled explicitly, but represented
as high/low river water levels at the end of drains.
First, as a control, flows within the drain are kept to minimum (1 m3/s) to compare with the later
modelling. Figure 9 below demonstrates the outcome of 1.8m MSL river water level (no bank burst at
this level). The model suggests under-sized drain at Location A that caused spill. While the Locations
B and C together with unnamed patches in the figure suggest low ground levels (lower than drain
water level) that the model interprets them as submergence. Keeping the same drain flows, Figure 10
shows the extent of flooding resulting from different river water levels. Source [10] had determined
that flood to cause river water level raised to 3m MSL as 2-year return period flood; and 5m MSL as
50-year return period flood. The figure shows that river water level >2m MSL would have inundated
Sg Gita catchment excessively.
4.2. Scenario of urban flooding
Continued from the previous scenario, the drain flows are changed to those of 27th February
2016 event derived in Section 3.3.2. The downstream river water level is set to 1.8m MSL so that a
comparison could be made with Figure 9. Extent of the urban flooding is depicted in Figure 11. It
indicates that inundation along Jalan Bunga Kenanga that conforms to the finding from interview
with residents. Flooding at Jalan Bunga Rose increases. The rest of the patches are similar to what
predicted in Figure 9 that the modellers consider them as outliner.

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Figure 9. Modelling of 1.8m MSL river water level (no bank burst).
Figure 10. Impacts of river water level to Sg Gita catchment.

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However, the model has not predicted any flooding in Jalan Bunga Rampai, in which the
residents maintained that flash flood often occur at the juncture. It suggests that should in case of
flooding at that location, it could be due to other reason.
Figure 11. Urban flash flood.
4.3. Scenario of 27th February 2016 event
This scenario represents the drain flows of 27th February 2016 event collided with the actual
river water level during the event reaching 3m MSL. The resulted flood map for Sg Gita catchment is
presented in Figure 12. However, it should be noted that the model are synthetic by nature and could
not be calibrated. No record of the actual drain flow is available. By practice, the relatively constant
shapes of the urban drains enable acceptable flow estimation. Yet, the model could be verified with
field flood depth obtained through interview with the residents. Four (4) locations are identified and
the comparisons are tabulated in Table 4. The model is found to be able to reconstruct reasonably well
of the flooding event.
It should be noted that the flood extent in the figure below has a major portion of it produced by
high river water level (3m MSL) (as in Figure 10). It can be deduced that the flooding was majorly of
river flooding from adjacent Sg Sarawak that had filled up much of the capacity of the drains. This
explains the flooding at Jalan Bunga Rampai.
The intense rain storm had worsened the low laying areas further. One infamous photo depicted
in the local newspapers as well as in neighbouring Brunei and Singapore (inset below) shows a
frowning pak cik in the thigh-deep flood water (about a meter) in Jalan Pinang Jawa. Our model
could get the same flood depth in the same location.
Jalan Bunga Kenanga
Jalan Bunga Rampai
Outliner
Outliner
Outliner
Jalan Bunga Rose

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Figure 12. Flood mapping of 27th February 2016 event.
Table 4. Verification of 27th February 2016 event
Location
Computed Flood Depth
(mm)
Field Flood Depth
(mm)
Remarks
Jalan Bunga Rampai
200 - 300
200
Matched
Jalan Kenanga
100 - 200
300
Close
Taman Mawar
100 - 500
300
Close
Pinang Jawa End
1000
1000
Matched
How the flood water over-spilled the drains is not clear in the flood map. Therefore, long
section profiles would be a better medium to enlighten the matter. Table 5 demonstrates four drains,
from which they reinstate that high river water level has caused backwater in most drains. Pinang
Jawa Drains are overwhelmed and an upgrade of its capacity is suggested.
5. Conclusion
A reconstruction of historical extreme event in Sg Gita catchment has allowed investigation into
the causes of flooding in the area. It allows what-if scenarios that could not be done in real life.
Ground surface runoff alone could cause small impact to Sg Gita catchment. River water level on the
other hand, is found to be the culprit of extensive flooding. With this model, it shows flow routing
with inclusion of downstream river water level explain well the flooding of an urban catchment with
lined urban drainage and bounded by a downstream river. Conventional urban drainage model with
only flow routing is found inadequate to represent the said catchment condition.

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Table 5. Long section profiles with identified problems
Drain
Cause
Indication of Model Direction of Flow
Pinang
Jawa Left
Drain
Over spill
Backwater
Pinang
Jawa Right
Drain
High overland
flow & backwater
Bunga
Rampai
Right Drain
Backwater in
Pinang Jawa
Right Drain to
cause congestion
of overland flow
within Sub-
catchment A2
Bunga
Kenanga -
Bunga
Tiong
These drains are
found with
backwater effects
from Sg Sarawak
Taman Mawar Left Drain
Taman
Mawar
Right Drain
Taman
Mawar Left
Drain
Acknowledgement
The authors are thankful to the Department of Irrigation and Drainage (DID) Sarawak for their
supports.

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