Bengoh Dam Reservoir

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CHAPTER 1
INTRODUCTION
1.1 BACKGROUND
The Bengoh Dam, located in the island of Borneo, Penrissen District, approximatedly 40
km south of Kuching, Sarawak. The dam is to increase reliable raw water supply to the
Batu Kitang Water Treatment Plant that supplies Kuching City at 1100 18’ E; 10 15’ N
(Ozgencil & Bruce, 2008).
It is located approximately 30 km upstream of existing Batu Kitang Water Treatment
Plant. The project was initiated by the State Government of Sarawak. The contractor is
Naim Cendera Sdn Bhd. (Ecosol, 2008).
Total Catchment area of Sungai Sarawak Basin constitutes about 1.98% of total
catchment area of the whole Sarawak.
Total Length of River within Sungai Sarawak Basin, which included Sungai Sarawak
Kiri and Sungai Sarawak Kanan, represents about 2.66% of total river lengths in
Sarawak.

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Figure 1.1: Bengoh Dam Project
1.1.1 Project Descriptions
The Project is designed to operate as a water reservoir dam. Some of the specific
functions of the Bengoh Dam are as follows:
 Storage of raw water for water supply;
 Release of raw water during droughts to provide sufficient raw water supply for
Batu Kitang Water Treament Plant, and to prevent back flow of saline water from
reaching the existing Batu Kitang intake point; and
 Flood Mitigation during wet seasons.

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1.1.2 Sub catchment areas
Bengoh Dam and it’s subcatchment areas :
Figure 1.2: Bengoh Dam and its surrounding catchment areas
Sungai Bengoh
Catchment Area
= 127 Km2
Sungai Sarawak Kiri
Catchment Area
= 700 Km2
Sungai Sarawak
Catchment Area
= 1,423 Km2

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Name of River Catchment Area (km2
) Mean Annual Flow (m3
/s)
Sg. Bengoh 127 km2
9
Sg. Semadang 28 km2
31
Sg. Sarawak Kiri 440 km2
45
Table 1.1: Sub catchments along Sungai Sarawak Kiri River
Catchment at the dam site represents about 20% of that at Batu Kitang or less than
10% of the 1,423 km2
area of the Sg. Sarawak basin as a whole at Kuching. Numerous
floods have occurred over the years, however the policy of flood management at bengoh
dam and its surrounding upstream zones are deemed local for its scale of impacts.
Downstream might demonstrate different characteristics. During January 2003, flood
marks indicates a level in excess of 15m above the gauge zero at the Bengoh stream flow
gauge and a flood rise (probably associated with backwater) of more than 11m at the
footbridge over Sg. Samadang at its confluence with Sg. Bengoh.
Presence of storage will likely reduce discharges along Sg. Sarawak Kiri River.
As demand increases, the incidence of drawdown at the end of the drier period of the year
(September-October) would itself increase therefore providing some flood absorption.
The reservoir will be kept full to minimize any risk of provision shall any influx
of water demand occurs in the future, enabling the dam to meet full water supply
demands. (KTA, 2003)

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Figure 1.3: Sungai Bengoh connected with Sungai Semadang to form
Sungai Sarawak Kiri downstream
The proposed damsite is located on the Sungai Bengoh 1.5 km upstream of the point at
which it joins the Sungai Semadang to form the Sungai Sarawak Kiri. The catchment area
of the Sungai Bengoh at the damsite is 127km2
which represents only about 7-8% of the
total catchment area of the Sungai Sarawak Kiri at Kuching. (Halcrow, 2009)
Sg. Bengoh
Sg.
Sarawak
Kiri Sg.
Semadang

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1.2 STATEMENT OF PROBLEM
Sungai Sarawak’s hydraulic model lacks of Bengoh dam, its reservoir and
Kampung Git’s upstream. Bengoh dam is currently under construction and once it is in
operations, the flow conditions will likely to transform; thus, an updated hydraulic model
of Sungai Sarawak is essential. Primary problem is to find out what would be the
flooding condition of Sungai Sarawak in a repetition of January 2009 Flood by inclusion
of Bengoh Dam.

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1.2.1 Water Level
Data from Kampung Git Gauging Station no. 1302428 was used in this study as it
has a reasonably complete record of water level till the very recent. Water levels were
taken from 9th January 2009 – 13th January 2009 for study. Rating curve as provided by
DID were used to calculate Inflow:
Q = 21.42 𝑥 (𝑊𝐿 − 1.35)1.45
(1.1)
Key sample data is recorded at Kampung Git, which is available in hourly basis. It
is deemed fit as it provides the best unbiased and consistent estimate until such time as
the flow data recorded at the Sg. Bengoh gauge are ratified and reliable “at-site”
hydrology becomes available. (KTA, 2003)
Hourly discharge flow times have consequently been estimated at the dam site
using the Kpg Git data alone. These were scaled by the respective catchment areas; which
implicitly assumes that the hydrological response of the entire catchment upstream of the
intakes is spatially homogeneous. That is, that the unit hydrological response (runoff/unit
area) is everywhere the same as that at Kpg Git and independent of any spatial variation
in physiography, vegetation or rainfall climate. (KTA, 2003)
Data available for Bengoh George Gauging Station no. 1202401, sited a few
hundred metres downstream of the proposed damsite has a reasonably complete daily
timeseries of water levels to the end of November 2002. However, the rating curve for
this station does not hold up to hydraulic scrunity. (KTA, 2003)

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1.2.2 January 2009 Flood at Bengoh Dam
I. Before Flood: (At Bengoh Dam Construction site)
Figure 1.4: A photo taken before January 2009 Flood event, line indicating the level of
flood water during 2009 Peak Flood Level, after flood, bridge and other machineries
were washed away by flood water
Peak Flood Level

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II. During Flood (At Bengoh Dam Construction site & Kampung Bengoh)
Figure 1.5: A photo taken during January 2009 Peak flood at Bengoh dam site
Figure 1.6: A photo taken at Kampong Bengoh after Peak Flood Level
Peak Flood Level

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1.3 OBJECTIVE
The objective is to study flooding scenarios along Sungai Sarawak River by
incorporating computerized river modeling tools. Sungai Sarawak river has recorded
worst flooding histories, for example during the January 2009 Flood Event.
The fluctuations of water levels in Sungai Sarawak Kiri River due to percolations,
loggings, evaporations, tidals at downstreams, rainfall events or combination of all the
stated will in turn affect the water level of Sungai Sarawak. Therefore, it is deemed useful
to model Sungai Sarawak Kiri upstream up to Bengoh Dam and it’s reservoir.
Specific objective were the application of Infoworks River System (RS) Version 9.0,
for mapping the inclusion of Bengoh Dam to differentiate the repetition of 2009 Flood
Event without it. The two scenarios are:
 January 2009 Flood Event with inclusion of Bengoh Dam
 January 2009 Flood Event without the inclusion of Bengoh Dam
Input parameters are essential for this application. Thus, all hydrological input, such
as January 2009 Flood Hydrograph are calculated using rating curve taken from DID
with the water level taken between 12.00am 9th
January 2009 until 11.00pm 13th
January
2009.
Flood mapping is partial of general supporting system for flood mitigation purposes.
It foresees the damages and costs that might be inflicted by floods.

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CHAPTER 2
LITERATURE REVIEW
2.1 FLOOD
Flood is a result of runoff from rainfall that exceeds the soil’s absorptive capacity and
the flow capacity of rivers, streams, and coastal areas. Thus, causes watercourse to
overflow its banks into bordering lands. Statistically, streams will equal or exceed the
mean annual flood once every 2.33 years (Leopold et al., 1964).

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2.2 FLOOD PLAINS
Figure 2.1: Floodplain
Topographically, Flood plains are flat regions of valley’s floor located on adjacent
to river channel. Geomorphologically, floodplains are built of unconsolidated
depositional material derived from sediments deposited by the river that flows through it
and hydrologically, it is a landform covered by water during floods when the river
overflows its banks. A combination of these characteristics comprises the essential
criteria for defining the floodplain (Schmudde, 1968).

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2.3 FLOODPLAIN HYDROLOGIC AND HYDRAULIC ANALYSIS
Figure 2.2: Diagramatic cross section of a river valley showing the relationship of flood
levels and flood plains.
Coverage of floodplains is normally associated with the flood frequency. A “100-
year flood” illustrates a 1% probability of a Probable Maximum Flood. The same concept
applies to “100-year floodplain”. Figure 2.1 shows the frequency in terms of flood levels
and floodplains. This concept does not represents that the flood will occur only once in a
hundred year but rather, there’s a 1% probability of such flood occurring in any given
year. Floodplains are mappable using Infoworks River System (RS), hence the boundary
of the 100-year flood are normally drawn for simulation of floodplain mitigation
programs to identify areas where the risk of flooding is significant.

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Other statistical frequency of flood events may be chosen depending on the
degree of risk that is selected for evaluation. In example, simulation of 2-year, 10-year
and 50-year flood has been conducted by previous study for Sungai Sarawak Kiri
(Norliza, 2009).
However, a climate, materials that makes up the banks of the stream, and channel
slopes plays a huge factor in the frequency of inundations.

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2.4 RAINFALL RUNOFF
2.4.1 Rainfall Intensity
Figure 2.3: Simplified diagram of the hydrological cycle (adapted from Ward, 1975)
Groundwater is derived from rainwater that has infiltrated into the soil and
drained beyond the rooting zone in excess of both the quantity needed for the crop or the
vegetation and the water-storage capacity of the soil (Chapman & Finkel, 1991).

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Rainfall that percolates beyond the lower limit of the rooting zone towards the
groundwater highly depended on the amount of water used for transpiration by
vegetations. In Tropical climate in Sarawak, the high water used by forest is due to its
generally greater transpiration rate and the deep roots that enable the vegetations to
absorb water from greater depths. Thus, forest vegetation in general increases the rainfall
and evaporation while it absorbs moisture and lessens runoff. (Chapman & Finkel, 1991).
However, changes in land use around the Kampung regions also affected the
quality of water transpiration and consequently change the quantity reaching the
groundwater. Poor water management practices along the Kampung regions also
contributed to the increase in proportion of rainfall lost as runoff, thus reducing base
flows and increases peak flows and the incidence of flooding.
Deforestation or logging practices due to the construction of dam reservoir will
eventually reduces the vegetation and the forest’s absorption capacity, thus increasing
runoff, hence, affecting the river channel both upstream and downstream from the dam
and reservoir. Evaporation increases as a result of expanded surface area of the reservoir,
and this process tends to degrade the water quality. The reservoir acts as a sediment trap
and the channel below the dam will regrade itself to accommodate the changes in
sediment load, as shown in Figure 2.4. The water, now with little sediment, scours the
downstream channel (Harper & Row, 1972)

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Figure 2.4: Schematic profile and cross section of a river showing both upstream and
downstream effects of a dam and reservoir.
Bengoh dam may also increases the ground water recharge. Water table level
surrounding the dam site may be raised and induction of ground water discharge into
adjacent channels might occur. Shall a catastrophic dam failure occurs, the rapid loss of
water from bengoh dam’s reservoir will introduce an instantaneously severe and dramatic
change downstream.
In summary, it is essential that the study recognize that changes brought on by
construction of Bengoh dam will affect the floodplain in multitude of approaches. Thus,
available data like that of 9th
-13th
January 2009 Flood event were used to evaluate, assess
and to foresees potential problems related to river hydraulics and floodplain dynamics.
Then, mitigation measures can be identified to avoid or minimize these hazards and
probably incorporated into the formulation of Bengoh Dam opearation rules.

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It is deemed that hydraulic structures are meant to function and manage the natural
flow of water, by means of diversion, restriction, or by stopping or any other methods
towards the original flow of water in a channel. The types of these structures can be
broadly categorized into two groups, the first would be one’s that flow takes place under
pressure through a definitely fixed cross section, in a manner somewhat analogous to pipe
flow, in example, flows through orifices, nozzles, short pipes, sluiceways or under gates.
The other group occurs through an initially undetermined cross section, as in open
channels such as flow over weirs, spillways, chutes, and drop structures and through
culverts and sewers (Simon & Korom, 1997).
Bengoh dam, however are deemed fit to be categorized as the second group
hydraulic structure.

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Figure 2.5: Schematic diagram illustrating relationship between rainfall, infiltration and
runoff (Linsley et al, 1958)
Water reaching ground surface infiltrates into the soil until it reaches a stage where the
rate of rainfall intensity exceeds the infiltration capacity of the soil. Therefore, surface
puddles, ditches, and other depressions are filled, after which runoff is generated. The
process of runoff generation continues as long as the rainfall intensity exceeds the actual
filtration capacity of the soil but it stops as soon as the rate of rainfall drops below the
actual rate of infiltration. (Chapman & Finkel, 1991)

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2.4.2 Catchment Factors
Aside from rainfall intensity, numerous catchment factors have direct bearing on the
occurrence and volume of runoff:
I. Soil Type
Figure 2.6: Infiltration capacity curves for different soil types

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The average size of raindrops increases with the intensity of rainstorm. In a high
intensity storm the kinematic energy of raindrops is considerable when hitting the soil
surface. This causes a breakdown of soil aggregates as well as soil dispersion with the
consequence of driving fine soil particles into the upper soil pores. This results in
clogging of the pores, formation of a thin but dense and compacted layer at the suface
which higly reduces the infiltration capacity. (Chapman & Finkel, 1991)
The subcatchment geology of Bengoh dam site is dominated by sandstones as
opposed to broad mix of sandstones, limestones and igneous formations further
downstream. The vegetation is almost entirely natural forest, the slopes are steep and
rainfall argued lower than for the larger drainage area at Kpg Git (KTA, 2003).
Infiltration capacities of Bengoh dam downstream (down till Kampung Git) region
will be much affected as the geological made up are mainly sandstones.

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II. Vegetation
The amount of rain lost to interception storage on the foliage depends on the kind of
vegetation and its growth stage. A more significant effect of the vegetation has on the
infiltration capacity of the soil; a dense vegetation cover shields the soil from raindrop
impact and reduces the crusting effect as described earlier. The root system as well as
organic matter in the soil increases the soil porosity thus allowing more water to
infiltrate. Vegetation also retards the surface flow particularly on gentle slopes, giving the
water more time to infiltrate and to evaporate.
For Bengoh dam’s downstream, as it’s vegetation are largely made up of forest, it yields
less runoff than bare ground.

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III. Slope and catchment area
Figure 2.7: Runoff efficiency as a function of catchment size (Ben Asher 1988)
According to investigations on experimental runoff plots (Sharma et al, 1986) have
shown that steep slope plots yield more runoff than those with gentle slopes. In addition,
it was observed that the quantity of runoff decreased with increasing slope length. For
bengoh dam’s downstream, the slope length decreases; thus, runoff will increase.

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IV. Runoff coefficients
Runoff [mm] = K x Rainfall depth [mm]
K = % according to figure 2.7 (2.1)
In rural catchments where no or only small parts of the area are impervious, the
coefficient K, which describes the percentage of runoff resulting from a rainstorm, is
however not a constant factor. Instead its value is highly variable and depends on the
above described catchment-specific factors and on the rainstorm characteristics.
Mays (1996) indicate that rainfall intensity and duration drives the rainfall-runoff
process. This is followed by the catchment characteristics that translate the rainfall input
into an output hydrograph at the outlet of the basin. The magnitude and time distribution
of both rainfall and runoff is needed for most floodplain studies. One of the simplest
rainfall-runoff formulas is as follows:

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2.4.3 Modified Rational Method
Qp = CsCiA (2.2)
Where;
Qp = peak flow (cfs or m3
/s)
Cs = channel Storage Coefficient (dimensionless)
C = runoff coefficient representing a ratio of runoff to rainfall (dimensionless)
I = average rainfall intensity for a duration equal to the time of concentration,
for selected return period, (mm/h)
A = catchment area (ha or acres)
Large catchments usually require a consideration of the entire hydrograph because timing
and storage issues become important. (Jenny, K.A.K, 2006)

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2.4.4 TIME AREA METHOD
qi = Ii.A1 + Ii-1.A2+…..+I1.Ai (2.3)
where
qi = the flow hydrograph ordinates (m3/s)
Ii = excess rainfall hyeograph ordinates (mm/hr)
Ai = time-area histogram ordinates (ha)
i = number of isochrones area contributing to the outlet
Time Area Method routes rainfall excess hyetograph with a time-area diagram
representing the progressive area contributions within a catchment in set time increments.
Pervious and impervious surfaces within the catchment are generated into separate
hydrographs. These are then combined to estimate outflows from individual sub-
catchments. (Jenny, K.A.K, 2006).

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2.5 HYDRODYNAMIC ROUTING
Unsteady flow equations enable simulation to be done on a wide range of flow
conditions and channel characteristics. Generally, hydraulic models are physically based
since only a single parameter was used to estimate or calibrate. Roughness coefficients
are estimable accurately from waterway inspection, which makes the hydraulic methods
rather applicable to an ungauged condition. Methodologies are based on Saint-Venant
equations of one-dimensional flow
The Wallingford Software’s InfoWorks River Simulation (RS) Version 9.0 is two
dimensional model used for prediction of discharge and water level for a wide range of
rivers, reservoirs and complex floodplains under both steady and unsteady conditions. It
also computes flow depths and discharges using a method of the Saint-Venant equations,
together with the proper boundary conditions, in mathematical terms as non-linear
hyperbolic partial differential equations.

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2.5.1 SAINT-VENANT EQUATIONS
The equations that describe 1-d unsteady flow in open channels, the Saint-Venant
equations, consist of the continuity equation, Equation (2.2), and the momentum
equation, Equation (2.3), The solution of these equations defines the propagation of a
floodwave with respect to distance along the channel and time.
q
t
y
B
x
y
VB
x
V
A 








(2.4)
t
V
gx
V
g
V
x
y
SS of









1
(2.5)
Where;
A = cross-sectional flow area
V = average velocity of water
x = distance along channel
B = water surface width
y = depth of water
t = time
q = lateral inflow per unit length of channel
Sf = friction slope
So = channel bed slope
g = gravitational acceleration

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For Continuity equation:
x
V
A


= prism storage
x
y
VB


= wedge storage
t
y
B


= rate of rise
q = lateral inflow per unit length
For momentum equation:
Sf = friction slope (frictional forces)
So = bed slope (gravitational effects)
x
y


= pressure differential
x
V
g
V


= convective acceleration
t
V
g 
1
= local acceleration

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The Saint-Venant equations operate under following assumptions:
I. The flow is one-dimensional with depth and velocity varying only in the
longitudinal direction of the conveyance. This implies that the velocity is constant
and the water surface is horizontal across any section perpendicular to the
longitudinal axis.
II. There is gradually varied flow along the channel so that hydrostatic pressure
prevails and vertical acceleration can be neglected.
III. The longitudinal axis of the channel is approximated as a straight line.
IV. The bottom slope of the channel is small and the bed is fixed, resulting in
negligible effects of scour and deposition.
V. Resistance coefficients for steady uniform turbulent flow are applicable, allowing
for a use of Manning’s equation to described resistance effects.
VI. The fluid is incompressible and of constant density throughout the flow.

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In accordance to these assumptions, formal statements of the conservation of water
volume (mass) and conservation of water momentum can be developed. The conservation
of volume (mass) principle relates to flows and changes in the quantity of water stored in
the channels and reservoirs. No forces of any kind are considered in the conservation of
mass. Forces, momentum fluxes, and the momentum of water in storage are related in the
conservation of momentum principle. The factors involved in this equation are:
I. Gravity force on the water in the channel,
II. Friction force on the wetted perimeter of the channel,
III. Pressure force on the boundaries,
IV. Wind force on the water surface, and
V. Inertia of the water.
Some of these factors can be omitted to simplify the unsteady-flow computations. If
all these factors are included in the analysis, the equations are referred to as the complete,
full, dynamic, Saint-Venant, or shallow-water equations. If the inertia of the water is
ignored, the zero-inertia form of the motion equation is obtained. If, in addition, the
variations of pressure force along the channel are ignored because they are thought to be
small, the kinematic form of the motion equation is obtained. Reservoir routing also is a
form of unsteady-flow analysis in which the motion equation is simplified to a relation
between water-surface elevation and the flow.

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2.5.2 LEVEL POOL METHOD
2.5.2.1 Probable maximum precipitation
The probable maximum precipitation is the greatest depth of precipitation for a given
duration meteorologically possible for a given size storm area at a particular time
duration at one location, with no allowance made for long-term climatic trends. PMP are
normally revised and determined by estimation from previous data. In this study, the
PMP will be acquire from Bengoh Dam’s assessment study by KTA (Sarawak) Sdn. Bhd.
2.5.2.2 Probable maximum flood
Probable maximum flood (PMF) is defined as the greatest flood to be expected with an
assumption that all factors that would produce the heaviest rainfall and maximum runoff
were in coincidence. However, existing PMF shall always be evaluated and updated in
accordance to revised PMP and updated flood routing criteria. Revised PMF shall be
determined once Bengoh dam were built. PMF is considered in routing as Bengoh dam’s
reservoir retains a huge volume of water. PMF data were taken from assessment study as
well.

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2.5.3 Hydrology and water resources data
2.5.3.1 Available Data and Data Review
Hydrological analysis in the vicinity of the dam has been carried out in accord to
available data on rainfall, stream flow and evaporation. Data are acquired from DID.
The information has been interpolated and rationalized to estimate the data for the
low and peak discharges, evaporation rate and maximum precipitation.
From the new assessment study on reservoir area-capacity, data indicates that at a
full supply level of 80m LSD, the reservoir surface area and the gross storage volume
would be 8.8 km2
and 144.1 Mm3
respectively (KTA, 2003).

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Figure 2.8: Storage area and storage capacity data (KTA, 2003)

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2.5.4 The estimation of the Probable Maximum Flood at Bengoh.
2.5.4.1 Probable Maximum Precipitation.
In accordance to World Meteorological Organisation (WMO) procedures, the
feasibility study Valued 954 mm for a 24-hour Probable Maximum Precipitation. A 24-
hours rainfall event has been considered due to the size of Bengoh Reservoir catchment
area of just 127 km2
, thus the critical storm duration has been identified at less than 24-
hours.
A generated hydrograph and storm profile adopted to distribute the 954 mm
rainfall mentioned is illustrated in Figure 2.
Figure 2.9: PMF Flood Hydrograph and 24 hours PMP Storm Profile (Adopted from
KTA, 2003).

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2.5.4.2 The estimated Probable Maximum Flood
Probable Maximum Flood value of 2420 m3
/s determined in the Feasibility Study
has been confirmed based on the PMP value using DID and WMO procedure. (KTA,
2003)
In earlier years, there were scopes for drawdown of stored water with provision
for flood storage. It is deemed unnecessary then as benefits of such would not be much
remarkable as the dam catchment is only about 20% of Sg. Sarawak Kiri and a fraction of
the whole Sg. Sarawak Basin. Abolishment of such leads to analysis of the needs of the
dam to be full at the beginning of the 1:50 year low flow sequence to meet the projected
2030 demand. At full storage, the dam is deemed able to reduce peak analysis from its
catchment (KTA, 2003).
Figure 2.10: Determination of the Assured Yield for a Given Volume of Active Storage

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2.5.5 Flood Routing
Effects of a detention basin on a given flood can be evaluated by routing the flood
hydrograph through the basin. Parameters dedicated for flood routing includes the inflow,
hydrograph, initial conditions and reservoir characteristics. Outflow hydrograph are then
produced. Stage-storage and stage-discharge (outflow) relation normally represents the
reservoir characteristics. The stage indicates the elevation of water surface in the
reservoir.
The storage represents the volume of water in the reservoir. Thus, storage is
directly proportional to the stage, relationship are non-linear but in accordance to several
aspects such as the shape and size of the reservoir. The stage-discharge relationship,
however, were governed by the hydraulics of the outlet structures. A relationship between
the storage and the outflow rate are then developed using the stage-storage and stage-
discharge relationships. Initial condition required is at the level where water level in the
reservoir at the time the incoming flood reaches the detention basin.
The change of volume of water in storage in the pond is described by the equation
𝐼 − 𝑄 =
𝑑𝑆
𝑑𝑡
(2.6)
Where I = inflow rate
Q = outflow rate
S = storage volume
t = time

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For a finite time period, t, Eq. (2.6) can be written in finite difference form and
rearranged as
(𝐼1 + 𝐼2) +
2𝑆1
∆𝑡
− 𝑄1 =
2𝑆2
∆𝑡
+ 𝑄2 .
(2.7)
Where I1 = inflow rate at start of the time period
I2 = inflow rate at end of the time period
t = duration of the time period
S1 = storage at beginning of the time period
S2 = storage at end of the time period
Q1 = outflow rate at the beginning of the time period
Q2 = outflow rate at the end of the time period
The unknowns in Eq. (2.7) are Q2 and S2. Using the storage-discharge relationship of the
pond along with Eq. (2.7), we can determine Q2 and S2. However, in many cases the
storage-discharge relationship is not in equation form, and a semi graphical procedure is
needed (Mays, L.W, 2004)

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2.5.6 Spillway of a reservoir
In avoidance of reservoir from overtopping and damage due to flood water, A
spillway is designed to cater for the release of excessive storage from the reservoir to the
downstream region. Series of gated and ungated spillway are available. For gated
spillways, mechanical structures are designed to control the operation of release of water
while ungated spillways release water when water level rises above the spillway crest.
Bengoh Dam is designed with an ungated spillway release facilities. However, its
reservoir drawdown shafts are controlled mechanically in accordance to water level in the
reservoir. Its culverts are gated, and easily sealed whenever deemed necessary.
Series of spillways are illustrated as figures below:

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2.5.6.1 Free Overfall (Straight Drop) Spillways
A free overfall, or straight drop, spillway is one in which the flow drops freely
from the crest (Bureau of Reclamation, 1987)
Figure 2.11: Overfall Spillway (Novak, P. et al, 1997).

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2.5.6.2 Ogee (Overflow) Spillways
The ogee spillway has a control weir that is ogee-shaped (S-shaped) in profile
(Bureau of Reclamation, 1987). The ogee shape at the crest prevents the formation of air
void under the flow sheet thus achieving the an almost maximum discharge efficiency.
Figure 2.12: Crest of an Ogee Spillway (Nalluri & Featherstone, 2001).

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2.5.6.3 Side Channel Spillways
Control weir is located alongside and approximately parallel to the upper portion of the
spillway discharge channel (Bureau of Reclamation, 1987). Discharges flows into the
bottom of the channel and discharges will be conveyed by the channel.
Figure 2.13: Side Channel Spillway (Novak et al, 1997)

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2.5.6.4 Chute (Open Channel or Through) Spillways
A spillway whose discharge is conveyed from the reservoir to the downstream
river level through an inclined channel, placed either along a dam abutment or through a
saddle, might be called a chute, open channel, or through spillway (Bureau of
Reclamation, 1987). The flow will be smooth, fast and drops sharply between the two
levels.
Figure 2.14: Chute Spillway (Bureau of Reclamation, 1987)

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2.5.6.5 Drop Inlet (Shaft or Morning Glory) Spillways
A drop inlet or shaft spillway is one in which the water enters over a horizontal
lip, drops through a vertical or sloping shaft, and then flows to the downstream river
channel through a horizontal or nearly horizontal conduit or tunnel (Bureau of
Reclamation, 1987)
Figure 2.15: Drop Inlet Spillway (Novak et al, 1997)

45
2.5.6.6 Siphon Spillways
Siphon spillways are closed conduits in the form of an inverted U with an inlet, short
upper leg, throat (control section), lower leg, and outlet (Novak et al, 1997)
Figure 2.16: Siphon Spillway, Spelga Dam, UK (Potskitt & Elsawy, 1976).

46
2.7 Discharge over a crest
The Bengoh dam uses uncontrolled ogee type of crest. The selection of such type
of crest because the ogee shaped crest is designed for neutral pressure on the curved
section of the spillway crest (KTA, 2004).
The discharge over the ogee crest as given in (Bureau of Reclamation, 1987) is
𝑄 = 𝐶𝐿𝐻1.5 (2.8)
Where Q = discharge
C = discharge coefficient
L = effective length of crest
H = actual head being considered on the crest

47
The discharge coefficient, C is influenced by the depth of approach, heads
difference from design head, upstream face slope, downstream apron and downstream
submergence. The effective length of crest is also influenced by pier and abutment. This
is because the present of piers and abutments causes side contractions of the overflow
which make the effective length is less than the actual length. The equation for effective
crest length is
𝐿 = 𝐿′
− 2(𝑁𝐾𝑝 + 𝐾𝑎 )𝐻𝑒 (2.9)
Where L = effective length of crest
L’ = net length of crest
N = number of piers
Kp = pier contraction coefficient
Ka = abutment contraction coefficient
He = actual head on crest

48
2.6 APPLICATION OF GEOGRAPHICAL INFORMATION SYSTEM (GIS)
A geographic information system (GIS) integrates hardware, software, and data
for capturing, managing, analyzing, and displaying all forms of geographically referenced
information (ESRI, 2008).
The GIS is used for watershed delineation, runoff estimation, hydraulic modeling
and floodplain mapping. Regardless of the definition, GIS record observations or
measurement that can be thought of as features, activities, or events. A feature is a term
from cartography that refers to an item or piece of information placed on a map. Point
features have a location such as a rain gage or a benchmark while line features have
several locations strung along the line in sequence such as river and stream. Area
features such as watershed or floodplain boundaries which consist of lines that form loop
or polygon. Human activities can often be described with geographical patterns and
distribution. Population map, census map and urban infrastructure maps such as sewer
and water distribution networks are examples that show these patterns (Bedient and
Hubber, 2002). According to Mitchell, event implies something that occurs at a point in
time and can be mapped over time.

50
2.7 GIS-BASED HYDROLOGIC AND HYDRAULIC MODEL
With the combination of advanced ISIS Flow simulation engine with GIS
functuality and database storage within one application, inforworks RS is able to
tansform source data into hydraulic models. In fact, Infoworks RS enables the modeling
of open channels, floodplains, embankments and hydraulic structures. Rainfall-runoff
simulation is available using both event based and conceptual hydrological methods.
Cross sectional profiles and elevation data relating to the river floodplain are the
key data required for modeling rivers. Profile data are represented in a series of x, y and z
values whereby z represents the elevation. For Bengoh Dam site, a LIDAR scanned
elevations were incorporated into the GIS to get the exact digital elevation of the dam’s
vicinity. The preparation of a digital elevation of the floodplain is the clearest example of
the necessity for integrating GIS technology.
Efforts to simulate various complex rainfall patterns and heterogeneous
watersheds through the usage of well known numerical methods are known as
hydrological modeling. Stormflow and baseflow in upper portion of the Gwynns falls has
been examined using hydrologic simulation program [Fortran (HSPF)], thus a guide for
assessing the impact of watershed-scale development has been produced, which also
represents the hydrologic system behavior through a development gradient. (Brun &
Band, 1999)

51
Dynamic management of water systems and related infrastructures for the
industries, domestic supplies, fisheries, agriculture, and energy and water quality control
requires hydraulic modeling to identify and predict the amount of excess water that might
cause damages in the any future flood events, thus the related bodies will be able to
apprehend such problems before it happens. An application called DUFlow modeling in
the Netherlands was used for decision-making, adaptation of plans by stakeholders of
certain industries and predict consequences (Blind et. Al, 2000). HEC-RAS has been
developed by the U.S Corps of Engineers Hydrologic Engineering Centre was
intentionally made for calculating water surface profiles for steady, gradually varied flow
in natural and man-made channels (Jenny, K.A.K, 2006)

52
CHAPTER 3
METHODOLOGY
3.1 INTRODUCTION
Figure 3.1: Bengoh dam & its catchment (Ecosol, 2008)

53
Figure 3.2: Modeling approaches

54
3.2 HYDRAULIC MODEL INPUT PREPARATION
A TIN (Triangular Irregular Network) Ground Model were created using Esri
Arcview 3.1 and 3D Extension for model building in Infoworks RS (River System).
The hydraulic modeling of Bengoh Dam was carried out using Infoworks RS for
steady and unsteady state dynamic flow. The basic equation is based on St. Venant partial
differential equation which takes into accounts both the flow continuity and momentum.
Before any unsteady computation were carried out, an initial model’s base condition were
generated under steady state computation, which indicates the initial state of the river.
The unsteady flow model is especially suitable for river system that is tidally
affected, of which the flow direction could be upstream or downstream, and the water
profile.

55
A 1: 10 000 scaled key plan in AutoCAD by KTA Consultant of 10 m contour
intervals featuring the Bengoh Dam project area is used to create a digital GIS map, as
shown in Figure 3.2, using ESRI ArcView v3.1 software
Figure 3.3: Bengoh Dam Reservoir

56
By using ESRI ArcView 3D-Analyst Extension v1.0, The digital terrain model
(DTM) of the project area is constructed as a TIN. In order to create DTM as an input
into the Infoworks RS model, the digital map were first converted into TIN surface
model, as shown in figure 3.3.
Figure 3.4: DTM of Bengoh Reservoir

57
3.3 MODEL BUILDING IN INFORWORKS
Nodes (the lowest mid point in the river cross section) were established at
convenient points and cross section along the Reservoir channels to suit major
geographical landmarks (Bengoh Dam) and suitable segmentation of the flow paths.
Nodes can be linked between each other and can be further expanded.
An inflow is modeled as a boundary node, which consists of a Flow-time
boundary node and a stage-time boundary node.
In construction of the model, digital map is imported into Infoworks RS as a background
graphic. Nodes that are in the shape of the river that defines the lowest mid-point of the
channel bed was extracted from DTM and then imported into Infoworks RS as the
channel’s centre points.

58
Figure 3.5: Diagram shows a previously constructed model up to Kampung Git along
Sungai Sarawak Kiri (Mah, D.Y.S, 2009)
A previous model has been constructed from Kuching Barage till Siniawan for
Sungai Sarawak Kanan (F.J Putuhena et al, 2007). Another side of the previous model
has been constructed up till Kampung Git. (Mah, D.Y.S et al, 2009). Current objectives
require the construction of model starting from Kampung Git till the edge of Bengoh
Dam’s reservoir at a total of 3.25km distance.

59
Figure 3.6: Locations of Kpg. Git, Kpg. Bengoh, Bengoh Dam, Reservoir and
Boundary Nodes
Reservoir
Reservoir
Boundary
Node
Bengoh
Dam
Kpg. Git
Kpg.
Bengoh
Culvert
Boundary
Node

60
Distances between nodes and are taken from JKR Sarawak, it will then be
projected in infoworks RS long section function. Each node are provided ground levels
and indications of the channel cross sections.
Each of the nodes had a series of chainage and height coordinates applied to describe the
section, which gave ground level, width and depth of channel shown in figure 3.6.
Figure 3.7: Bengoh Dam Downstream node

61
3.4 ROUTING OF INFLOW
In this study, level pool method is used to establish the relationship between the
outflow discharges and the water depth. The flood routing will be done with a spillway
length at 77.5m as per engineering drawing’s length. The steps of flood routing were
explained as below (Mays, L.W, 2004) :
1. From the given stage-storage and stage-discharge relationship obtain a storage-
discharge (S versus Q) relationship.
2. Select a time increment, ∆t. Calculate the quantity 2S/∆t + Q as a function of Q.
3. For any time step computations calculate I1 + I2 from the inflow hydrograph, and
2S1/∆t – Q1 from either the initial conditions or previous time-step calculation.
4. Calculate 2S2/∆t + Q2 from Eq. (2.2).
5. Obtain Q2 from the graph developed in step 2. This will be the outflow rate at
time t2.
6. Calculate 2S2/∆t + Q2 by subtracting 2Q2 from 2S2/∆t + Q2 and go back to step 3.
7. Repeat the same procedure until the routing is completed.
The outflow obtained in the flood routing will be plotted with the inflow in the inflow
and outflow hydrograph for determination of spillway capacity.

62
Figure 3.8: Development of elevation-storage and elevation-storage function (Chow, V.T
et al, 1988).
Figure 3.9: Head Elevation-Storage Curve (Bureau of Reclamation, 1987).

63
Figure 3.10: Head Elevation-Discharge Curve (Bureau of reclamation, 1987).
Figure 3.11: Example of flood routing by pool level method (Chow, V.T et al, 1988).

64
3.5 BOUNDARY CONDITION
All hydraulic models require initial conditions and boundary conditions to be
established before any commencement of routing. Initial conditions are simply stated as
the conditions established by specifying a base flow within the channel at the start of the
simulation. Boundary conditions are known relationship between discharge-time or stage-
time. Hydraulic routing computations require the specification of upstream, downstream,
and internal boundary condition.
At Bengoh Dam’s upstream, a flow-time boundary was set 16.3m3
/s, which is the
calculated average inflow as taken in January 2009. A stage-time boundary was set at
bengoh dam’s maximum level at 80m as it represents the dam structure.
At Bengoh Dam’s downstream, a flow-time boundary was set at bengoh dam’s
culvert, where the events of January 2009 floods inflows were inserted. A stage-time
boundary was set at Kuching’s Barage in accordance to recorded January 2009 flood
event stage data.

65
3.5.1 Reservoir Upstream
Figure 3.12: Average Inflow as taken from January 2009 at Reservoir’s edge Boundary
node
Figure 3.13: Reservoir’s Maximum Stage at Bengoh Dam’s spill

66
3.5.2 Reservoir Downstream
Figure 3.14: January 2009 Flood Event (PMF) at Bengoh’s dam culvert
Figure 3.15: January 2009 Flood Event (Routed PMP) at Bengoh’s dam culvert

67
Figure 3.16: January 2009 Flood Stage at Barrage Downstream
Figure 3.17: Inflow and Outflow hydrograph in comparison
0
100
200
300
400
500
600
700
800
0 20 40 60 80 100 120 140
Q (m3/s)
Time (hr)
Inflow (m3/s)
Outflow (m3/s)

68
3.6 MODEL RESULT
The result consists of flood area mapping, flood flow, flood stage hydrograph and
longitudinal section of flood water level.

69
CHAPTER 4
RESULTS AND DISCUSSION
4.1 RESULTS
The simulation for Sarawak River had been done by Infoworks RS Software. The
simulation was intended to show 2 scenarios of flows along Sungai Sarawak to identify
difference of flow characteristics with the inclusion of Bengoh Dam; the 2 scenarios are:
1. Effects of 100-year flood event without Bengoh Dam
2. Effects of 100-year flood event with Bengoh Dam

70
January 2009 Flood event is considered as an extreme case of 100-year recurrent-
interval flood event. Simulated results of such flood with and without bengoh dam enable
identifications of Bengoh dam’s ability to provide flood absorption. Results generated are
listed as table below:
Results Purpose
Flow Rate Flow Rate if flood in m3/s
Stage Also known as Flood water level, it tells the maximum or peak level of
water during the 5 days 100-year flood event
Volume Amount of water flows through the specific cross section throughout
the 5 days 100-year flood event.
Table 4.1: Characteristics of flows generated using Infoworks RS

71
4.1.1 Scenario without Bengoh Dam
In this study, an initial investigation shows that flood occurred from Kampung Git
onwards, all the way down to Kuching Barrage.
Figure 4.1: Peak Flood Map without Bengoh Dam at Sarawak River

72
4.1.2 Scenario with Bengoh Dam
In this study, a secondary investigation shows that flood occurred from Kampung Git
onwards, all the way down to Kuching Barrage. Water upstream were retained in the
reservoir.
Figure 4.2: Peak Flood Map with inclusion of Bengoh Dam at Sarawak River

73
Bengoh Dam downstream
Without Dam (Original January 2009 Flood hydrograph)
Figure 4.3: Boundary Condition at Bengoh dam’s culvert
With Dam (Routed hydrograph)
Figure 4.4: Boundary Condition at Bengoh dam’s culvert

74
4.2 FLOOD MAPPING, FLOOD FLOW AND STAGE HYDROGRAPHS
4.2.1 At S65 - Batu Kawa Bridge (Flood Map)
Figure 4.5: Flood water at Batu Kawa Bridge before inclusion of dam
Figure 4.6: Flood water at Batu Kawa Bridge with inclusion of Bengoh dam

75
4.2.2 At S65 - Batu Kawa Bridge (Stage & Flow)
Figure 4.7: Comparison of Flow rates, Q; before and after inclusion of
Bengoh dam at Batu Kawa bridge.
Figure 4.8: Comparison of water stage; before and after inclusion of
Bengoh dam at Batu Kawa bridge.
-1000
-500
0
500
1000
1500
0/1/1900 0:0014/5/1901 0:0026/9/1902 0:008/2/1904 0:00
Flow,Q(m3/s)
Hour
Flow Vs. Time
After Dam
Before Dam
-1
0
1
2
3
4
5
0 500 1000 1500
Stage(m)
Hour
Stage Vs. Time
After Dam
Before Dam

76
4.2.3 At S65 - Batu Kawa Bridge (Cross Section)
Figure 4.9: A cross-sectional view at Batu Kawa bridge before construction of dam
Figure 4.10: A cross-sectional view at Batu Kawa bridge after construction of dam

77
4.2.4 At S46 - Sungai Maong (Flood Map)
Figure 4.11: Flood water at sungai maong before inclusion of dam
Figure 4.12: Flood water at sungai maong with inclusion of bengoh dam

78
4.2.5 At S46 - Sungai Maong (Flow & Stage)
Figure 4.13: Comparison of flow rates, Q; before and after inclusion of
bengoh dam at sungai maong.
bengoh dam at sungai maong.
-3000
-2000
-1000
0
1000
2000
3000
0 500 1000 1500
Flow,Q(m3/s)
Hour
Flow Vs. Time
After Dam
Before Dam
-1
0
1
2
3
4
5
0 500 1000 1500
Stage(m)
Hour
Stage Vs. Time
After Dam
Before Dam

79
4.2.6 At S46 - Sungai Maong (Cross Section)
Figure 4.15: A Cross-sectional view at sungai maong before construction of dam
Figure 4.16: A Cross-sectional view at sungai maong after construction of dam

80
4.2.7 At S31 - Grand Margarita (Flood Map)
Figure 4.17: Flood water at Grand Margarita Kuching before inclusion of dam
Figure 4.18: Flood water at Grand Margarita Kuching with inclusion of
Bengoh dam

81
4.2.8 At S31 - Grand Margarita (Flow & Stage)
Figure 4.19: Comparison of Flow rates, Q; before and after inclusion of
bengoh dam at Grand Margarita Kuching.
bengoh dam at Grand Margarita Kuching.
-5000
-4000
-3000
-2000
-1000
0
1000
2000
3000
4000
0 500 1000 1500
Flow,Q(m3/s)
Hour
Flow Vs. Time
After Dam
Before Dam
-2
-1
0
1
2
3
4
5
0 500 1000 1500
Stage(m)
Hour
Stage Vs. Time
After Dam
Before Dam

82
4.2.9 At S31 - Grand Margarita (Cross Section)
Figure 4.21: A Cross-sectional view at Grand Margarita Before construction of dam
Figure 4.22: A Cross-sectional view at Grand Margarita after construction of dam

83
4.3 LONGITUDINAL SECTION
Bengoh Dam’s upstream (After Dam)
Figure 4.23: A Long section view from reservoir’s edge until bengoh’s dam culvert.
Bengoh dam’s downstream (After Dam)
Figure 4.24: A long section view from bengoh dam’s culvert until barrage with the
inclusion of Bengoh dam

84
4.4 DISCUSSIONS
Comparing the Overview before and after the inclusion of Bengoh Dam,
we can obviously tell the differences; Bengoh Dam brought in a substantial
amount of water volume to the reservoir for storage.
Manning’s n roughness coefficient depends on channel material, surface
irregularities, variation in shape and size of cross section, vegetation and flow
conditions.
Infoworks RS isn’t made for modeling dam structures, thus, a cross
section at Bengoh dam’s spillway were edited to be at 80m height. It is advisable
that the use of other relevant software for modeling of Bengoh dam be carried out
instead.

85
At S65 – Batu Kawah Bridge
Parameters Without Inclusion
of Bengoh Dam
With Inclusion of
Bengoh Dam
Flow Rate Q (m3/s) 1174.094 807.5633
Volume of water (Mm3) 247.37 224.25
Maximum Stage (m) 4.249853 4.071358
Table 4.2: Characteristics of flow at S65 (Batu Kawah Bridge)
In accord of the data recorded before and after inclusion of Bengoh Dam, we can
relate that the flow rate has decreased by 366.53m3
/s at peak.
The volume of water that passes through Batu Kawah Bridge are predicted to be
reduced by a total of 23.12Mm3 within the 5-days flood event period after the
construction of Bengoh Dam.
The Flood water level was predicted to reduce by 0.18m depth.

86
At S46 – Sungai Maong
Parameters Before Bengoh
Dam
After Bengoh Dam
Flow Rate Q (m3/s) 2408.379 2233.303
Maximum Stage (m) 4.148217 3.984594
Table 4.3: Characteristics of flow at S46 (Sungai Maong)
tell that the flow rate decreased by 175.08m3/s at peak.
reduced by a total of 27.85 Mm3 within the 5-days flood event period after the

87
S31 – Grand Margarita Hotel
Parameters Before Bengoh
Dam
After Bengoh Dam
Flow Rate Q (m3/s) 2927.948 2756.686
Maximum Stage (m) 4.269028 4.149092
Figure 4.4: Characteristics of flow at S31 (Grand Margarita Hotel)
tell that the flow rate decreased by 171.26m3/s at peak.
reduced by a total of 29.11Mm3 within the 5-days flood event period after the

88
CHAPTER 5
CONCLUSIONS & RECOMMENDATIONS
5.1 CONCLUSION
This study has been conducted to review the ability of Bengoh Dam to absorp flood
with the impacts of an extreme case of a 100-years recurrent-interval flood on Bengoh
Dam downstream.
For modeling of the Bengoh Dam Reservoir and downstream, three engineering
softwares were used, namely Wallingford InfoWorks River Simulation (RS) version 9.0
and ESRI ArcView version 3.1 cum its 3D-Analyst extension and AutoCAD 9.1. For
flood routing, Microsoft Excel spreadsheet software was used.
A TIN surface model had been developed for the base condition using ESRI
ArcView for upstream of Kampung Git towards Bengoh Reservoir. This is followed by
development of Second TIN surface model, by incorporating the proposed Bengoh Dam
into the model. Bengoh Dam represents a structure that retains water for its reservoir.
Subsequently, the DTM was processed using ESRI ArcView to produce the ground
model as an input for InfoWorks RS model.

89
The simulated results of January 2009 flood (100-year return period) obtained
showed that the Bengoh Dam are able to retain significant portion of waters from
Sarawak River Kiri. Results taken from Batu Kawah Bridge showed an average water
level reduction of only about 2.16% which reflects the ability of Bengoh Dam to absorb
flood, although not significant.
The inclusion of Bengoh dam shows it retains a substantial volume of water, at the
same time it decreased the flow rate and decreases the total volume of water that drain
downstream, reducing flood along the channel. It appears that Bengoh Dam works out
beneficial towards populations/inhabitants along Sungai Sarawak considering that a 100
year flood can cause massive amount of damages to people living in that region.
Therefore providing some flood absorption to Sungai Sarawak.
Bengoh dam are able to reduce the flood level by only 12cm-18cm, this is because
the catchment area are too small; thus, it can be concluded that Bengoh dam’s Primary
objective is mainly to secure water resources for water supply, minimal flood absorption
do comes in handy during flood events.

90
5.2 RECOMMENDATIONS
Based on the simulation of Bengoh dam, the following comments are
recommended for further studies:
1. Study for Water supply and flood operation rules during low, normal and peak
flow to meet the rising water demand in Kuching
2. Study for controlled gated spillway can be conducted to check the flood pattern
downstream of the dam.
3. The analysis should be done with other flood routing methods with inclusion of
rainfall data and abstractions such as consideration of the culvert gates were
always unlocked to compare with the results which have been obtained.

91
Based on simulation analysis, the following comments are recommended for
further studies:
1. Use different software like Dazztech’s FLOW-3D which enables simulation of
dam and spillways, hydraulic jumps and structural, which is more comprehensive
for a dam’s case study.
2. Erosion on banks might occur due to sudden influx of water each time water was
released from the culverts. Study of Erosion rate might be beneficial for Bengoh
Dam.
3. It is crucial that the dam’s spillway and culvert’s gates undergoes regular
inspection and maintenance in order to ensure that the forces due to water flows
will not damage the dam structures. Study on Dam break might be essential.
4. Further study can be carried out regarding this matter to examine the condition
when dam break occurs.

Bengoh Dam Reservoir

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