Hydraulic Structures Chap 8 - 12 June 2009

5/19/2018 Hydraulic Structures Chap 8 - 12 June 2009

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Department of Civil Engineering

College of Engineering, UNITEN

Assoc Prof Ir Dr Lariyah Mohd Sidek

E-mail: [email protected]

Lecture NotesCEWB223Hydrology & Hydraulic Engineering
http://en.wikipedia.org/wiki/Image:Gelmersee04.jpghttp://en.wikipedia.org/wiki/Image:JhnDyDam2.jpghttp://en.wikipedia.org/wiki/Image:Hoover_dam.jpg


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Chapter 8

Hydraulic StructuresWeir

GatesSpillways

Dams

Department of Civil EngineeringCollege of Engineering, UNITEN


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What is Hydraulic Structures?

Hydraulic structures are used to regulate,measure and/or transport water in openchannels.

These structures are called cont ro lstructureswhen there is a fixedrelationship between the water-surface

elevation upstream or downstream of thestructure and the flow rate through thestructure.


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Type of Hydraulic Structures

Hydraulic structures can be group intothree categories:

(1) flow measuring structures, such as

weirs;

(2) regulation structures, such as gates;

and(3) discharge structures, such as

culverts.


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1. Spillwaycrest/bay

2. Pier

3. Spillway

4. Stilling Basin

5. Armored

Scour

Prevention

bed

6. Section

7. Power

Station


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WEIRS

Weirs are elevated structures in openchannels that are used to measure flowand/or control outflow elevations from

basins and channels. There are two types of weirs in common

use:

(1) sharp-crested weirs, and(2) broad-crested weirs.


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Sharp Crested Weir

Sharp-Crested Weirs, or thin-plate, weirs consistof a plastic or metal plate that is set vertically andacross the width of a channel.

The main types of sharp-crested weirs arerectangularand V-notch weirs.

In suppressed (uncontracted) weirs, therectangular opening spans the entire width of achannel;

In unsuppressed (contracted) weirs, therectangular opening spans only a portion of a

channel.


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Rectangular Weirs


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Flow over a Sharp-Crested Weir


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DERIVATION OF WEIR EQUATION

Assuming that the head loss is negligible along a streamline crossing Section 1,at elevation z1 and leaving Section 2 at elevation z2, then


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DERIVATION OF WEIR EQUATION

The estimated flow rate, , across Section 2 can be calculated by integrating the flow rates across element of area b dz2,where b is the width of the rectangular weir. Therefore


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Assumptions

Their weir equation was derived with the following theoreticaldiscrepancies:

(1) the pressure distribution in the water over the crest of the weir is notuniformly atmospheric;

(2) the water surface does not remain horizontal as the waterapproaches the weir; and

(3) viscous effect that cause a non-uniform velocity and a loss of energy

between Section 1 and Section 2 have been neglected. The error in the flow rate resulting from these theoretical discrepancies

is handled by a discharge coefficient, Cd, defined by the relation

Eq. 1


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It can shown by dimensional analysis that ( Franzini and Finnemore, 1997)

where Re is a Reynolds number,We is a Weber number and

Hw

is the height of the crest of the weir above the bottom of the channel .

Experiments have shown thatH/Hw

is the most important variable affectingCd, with We only important at low heads;

Re is usually sufficiently high that viscous effect can be neglected. An empirical formula forCd is (Rouse, 1946; Blevins, 1984)

Which is valid for H/Hw< 5, and is approximate up to H/Hw= 10. For H/Hw> 15, the discharge can becomputed from the critical flow equation by assuming yc= H (Chaudhry, 1993). It is convenient to

express the discharge formula,

where Cw is called the weir coefficient and is related to the discharge coefficient by

Eq. 3

Eq. 2


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Taking Cd = 0.62 in Equation 2 yields Cw =1.83, and Equation 1 becomes

which gives good result ifH/Hw < 0.4, which is within theusual operating range (Franzini and Finnemore, 1997 ).

Eq. 3.1


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Example 8.1 A weir is to be installed to measure flows in the

range of 0.5-1.0m3/s. If the maximum (total)depth of water that can be accommodated at theweir is 1 m and the width of the channel is 4 m,

determine the height of a suppressed weir thatshould be used to measure the flow rate.


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SOLUTION EXAMPLE 8.1

The flow over the weir is illustrated in Figure 2,where the height of the weir is Hw and the flowrate is Q. The height of the water over crest of the

weir, H, is given by

Assuming that H/Hw< 0.4, then Q is related to Hby Equation 3.1 where


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Taking b = 4m, and Q = 1 m3/s (the maximum flow rate willgive the maximum head, H) then

SOLUTION EXAMPLE 8.1

The height of the weir, Hw, is therefore given by

The initial assumption thatH/Hw< 0.4 is therefore validated and height ofthe weir should be 0.735 m.


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V-Notch Weir A V-Notch weir is a sharp-crested weir that has a V-shaped opening

instead of a rectangular-shaped opening.

These weirs, also called triangular weirs, are typically used instead ofrectangular weirs tend to be less accurate.

V-notch weirs are usually limited to flows of 0.28 m3/s (10cfs) or less.The basic theory of V-notch weirs is the same as the rectangular weirs,where the theoretical flow rate over the weir, , is given


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V-Notch Weir Where b is the width of the V-notch weir at elevation z2

and is given by

Eq. 4


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Discharge Coefficient in V-Notch Weirs

Figure 1


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V-Notch Weir

The vertex angles used in V-notch weirs are usually between 10

and90.

Values ofCd for variety of notch angles,, and heads, Hare plotted inFigure 1.

The minimum discharge coefficient correspondent to a notch angle of

90, and minimum value of C for all angles is 0.581. According to Potter and Wiggert (1991) and white (1994), using Cd =

0.58 for engineering calculations is usually acceptable, provided that20 < < 100 andH> (2 in.). For H < 50mm, both viscous andsurface-tension effects may be important and a recommended value

of Cd is given by (White, 1994)

Eq. 5


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Example 8.2 A V-notch weir is to be used to measure channel flows in

the range 0.1 to 0.2 m3/s. What is the maximum head ofwater on the weir for a vertex angle of 45?

Solution:

The maximum head of water results from the maximumflow, so Q = 0.2 m3/s will be used to calculate themaximum head. The relationship between the head andflow rate is given by Equation 4., which can be put in theform


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Example 8.3

The discharge coefficient as a function ofHfor = 45 is given in Figure 1, andsome iteration is necessary to find H. These iterations are summarized in the

following table

Therefore, the maximum depth expected at the V-notch weir is 2.16ft = 0.66m.


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Gates Gates are used to regulate the flow in open channels.

They are designed for either over flow or underflow operation, with

overflow operation appropriate for channels in which there is asignificant amount of floating debris.

Two common types of gates are vertical gates and radial (Tainter)

gates.

Vertical gates are supported by vertical guides with roller wheels, and

large hydrostatic forces usually induce significant frictional resistanceto raising and lowering the gates.

A radial (Tainter) gate consists of an arc-shaped face plate supported

by radial arms that are attached to a central horizontal shaft that

transmit the hydrostatic force to the supporting structure. Since the

vector of the resultant hydrostatic force passes through the axis of the

horizontal shaft, only the weight of the gate need to be lifted to openthe gate.

Tainter gates are economical to install and are widely used in both

underflow and overflow applications. Structural design guidelines for

several types of gates can be found in Sehgal (1996).


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Gates


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Gates

Applying the energy equation to both vertical and Tainter gates, yields

Where Section 1 and 2 are upstream of the gate, respectively and energylosses are neglected. In terms of the flow rate, Q, leads to


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Gates

The depth of flow downstream of the gate, y2, is less than the gate opening, yg,since the streamlines of the flow contract as they move past the gate. Denoting the

ratio of the downstream depth, y2, to the gate opening, yg, by the coefficient ofcontraction, Cc, where


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Gates

The form of the discharge equation expresses the discharge in terms of anoffice-flow velocity,

times the flow area through the gate, byg, times a discharge coefficient, Cd, to account for deviationsfrom the orifice-flow.

The discharge coefficient depends on the amount of the flow contraction as measured by Cc and yg/y1.

In the case of a vertical sluice gate, it has been found that (Chadwick and Morfett, 1993)

Whenever 0 < yg/E1< 0.5 where E1 is the specific energy of the flow upstream ofthe

Tainter Gate

0 < yg/E1< 0.5

Vertical Sluice Gate


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Gate

In cases where the discharge through the gate opening is supercritical

and the depth of flow downstream of the gate exceeds the conjugatedepth of the gate opening, there is the possibility that the outflow will besubmerged and the discharge equation will not be applicable.

An approximate analysis of the submerged flow condition assumes that allhead losses occur in the flow downstream of the gate, between Section 2

and 3, in which case the energy equation can be written as


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Gate Where y is the depth of flow immediately downstream of

the gate. Between Sections 2 and 3, the momentumequation can be written as

And flow rate, Q, can be estimated by simultaneous solution of

Equations above where y1

and y3

are usually known and y2

is

estimated by Ccy

g


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Example 8.3

Water is ponded behind a vertical gate toa height of 4 m in a rectangular channel ofwidth 7 m. Calculate the gate opening that

will release 40m3/s through the gate. Howwould this discharge be affected by adownstream flow depth of 3.5 m?
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Spillway?

1. The majority of impounding reservoirs are formed as a result of

the Construction of a dam

2. By its very nature, the stream flow which supplies a reservoir is

variable

3. It follows that there will be times when the reservoir is full and

the stream flow exceeds the demand.

4. The excess water must therefore be discharged safely from

the reservoirs.

5. In many cases to allow the water simply to overtop the dam

would result in a failure of structure.

6. For this reason carefully designed overflow passages known

as spillways are in corporate as part of the dam design


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2. Pier

3. Spillway

4. Stilling Basin

5. ArmoredScour

Prevention

bed

6. Section

7. Power

Station


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Spillway?

7. The spillways capacity must be sufficient to accommodate the

largest Flood discharge The probably maximum flood or 1 in

10000 years Flood) likely occur in the life of the dam.

8. Basically spillway is an open channel with large slopes that

allows the excess water to flow over it at super critical

velocities.

9. The ideal longitudinal profile of an overflow spillway should

flow along the same curve as the underside of the tree-falling

water nappe to minimize the pressure on the spillways surface.

10.However, caution must be exercised to avoid any negative

pressure on the surface

11.Negative pressure is caused by separation of the high-speed

flow from the spillway surface, resulting in a ponding action


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Types of Spillway

Chute Spillway


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Types of Spillway

Ogee Spillway

Without Spillway Weir


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Types of Spillway

Bell Mouth/Morning Glory Spillway


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Types of Spillway

Service Spillway


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Types of Spillway

Auxiliary Spillway


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Types of Spillway

Emergency Spillway


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Stilling Basin

Stilling basin is a structure

Without Spillway Weir


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Stilling Basin

Stilling Basin


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Stilling Basin

Baffle Piers



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Dams

Definition of DamsAdvantages and Disadvantages of

Dams

Classification of DamsTypes of Dams


College of Engineering, UNITEN


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Dams

h t i ?
http://en.wikipedia.org/wiki/Image:Hoover_dam.jpghttp://en.wikipedia.org/wiki/Image:Gelmersee04.jpghttp://en.wikipedia.org/wiki/Image:JhnDyDam2.jpghttp://en.wikipedia.org/wiki/Image:Scrivener_Dam_Canberra-01JAC.jpg


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What is a Dam?

A dam is a structure built acrossa stream, river or estuary to

retain water.

Dams are made from a varietyof materials such as rock, steel

and wood.


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2. Pier

3. Spillway

4. Stilling Basin

5. ArmoredScour

Prevention

bed

6. Section

7. PowerStation

Ad t f D


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Advantages of Dam

Irrigation

Water Supply

Flood Control

Hydroelectric

Recreation

Navigation

Di d t f D
http://simscience.org/cracks/intermediate/dams1.htmlhttp://simscience.org/cracks/intermediate/dams1.htmlhttp://simscience.org/cracks/intermediate/dams1.htmlhttp://simscience.org/cracks/intermediate/dams1.htmlhttp://simscience.org/cracks/intermediate/dams1.html


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Disadvantages of Dam

Dams detract from natural settings, ruin nature's workDams have inundated the spawning grounds of fishDams have inhibited the seasonal migration of fishDams have endangered some species of fishDams may have inundated the potential forarchaeological findingsReservoirs can foster diseases if not properlymaintained

Reservoir water can evaporate significantlySome researchers believe that reservoirs can causeearthquakes

Cl ifi ti f D


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Classification of Dams

Storage DamDetention DamDiversion Dam

Coffer DamDebris Dam

Classification based on function

T i l St D


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Typical Storage Dam

Srinagarind Dam

Vajiralongkorn Dam

T i l St D


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Typical Storage Dam

Maeklong Dam

Tha Thung Na Dam

Cl ifi ti f D


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Classification based on hydraulic design

Classification based on material of construction

Overflow Dam/Overfall Dam

Non-Overflow Dam

Rigid DamNon Rigid Dam

Cl ifi ti f D


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Classification based on structural behavior

Gravity Dam

Arch DamButtress DamEmbankment Dam

G it D


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Gravity Dam

Gravity dams are dams which resistthe horizontal thrust of the water

entirely by their own weight.

Concrete gravity dams are typically

used to block streams throughnarrow gorges.

G it D
http://simscience.org/cracks/intermediate/big_grav.html


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Gravity Dam

Cross Section Plain View

Arch Dam


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Arch Dam

An arch dam is a curved damwhich is dependent upon archaction for its strength.

Arch dams are thinner andtherefore require less material thanany other type of dam.

Arch dams are good for sitesthat are narrow and have strongabutments.

Arch Dam
http://simscience.org/cracks/intermediate/big_arch.html


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Arch Dam


Buttress Dam


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Buttress Dam

Buttress dams are dams in which theface is held up by a series of

supports.

Buttress dams can take manyforms - the face may be flat orcurved.

Buttress Dam


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Buttress Dam


Embankment Dam


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Embankment Dam

Embankment dams are massivedams made of earth or rock.

They rely on their weight toresist the flow of water.

Embankment Dam


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Embankment Dam


Types of Dam


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Types of Dam

Types of Dam


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Types of Dam

Factors governing selection

of types of dam

A Narrow V-Shaped Valley : Arch DamA Narrow or Moderately with U-ShapedValley : Gravity/Buttress DamA Wide Valley : Embankment Dam

Topography-Valley Shape

Types of Dam


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Types of Dam


of types of dam

Solid Rock Foundation : All typesGravel and Coarse Sand Foundation :Embankment/Concrete Gravity Dam(H15 m)

Silt and Fine Sand Foundation :Embankment/Gravity Dam (H8 m)Non-Uniform Foundation : -

Geology and Foundation Condition

Types of Dam


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Types of Dam


of types of dam

Climate conditionsAvailability of construction materials

Spillway size and locationEnvironmental considerationsEarthquake zoneOverall cost

General considerations

Hydropower Plant


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Hydropower Plant

InsideHydropower plant

Hydropower Plant


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Hydropower Plant

Hydropower Plant


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Hydropower Plant

Generator

Turbine

Hydropower Energy Calculation


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P

E

= n. .Q.H

T

------------------ 9.1)

PE = Power (kW)

n = Efficiency (%)

= Specific Weight of Water (9.81 kN/m3)

Q = Water Discharge (cms)

HT = Head (m)

E = PE.T ------------------(9.2)

E = Energy (kWh)

T = Time (h)

Hydropower Energy Calculation

Three Gorges Dam


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Three Gorges Dam

Three Gorges Dam


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Three Gorges Dam

Type: Concrete Gravity DamCost: Official cost $25bn - actualcost believed to be much higherWork began: 1993Due for completion: 2009Power generation: 26 turbines onleft and right sides of dam. Sixunderground turbines planned for2010Power capacity: 18,000megawattsReservoir: 660km long,submerging 632 sq km of land.

When fully flooded, water will be175m above sea levelNavigation: Two-way lock systembecame operational in 2004.One-step ship elevator due toopen in 2009.

Three Gorges Dam


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Three Gorges Dam

Three Gorges Dam


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Three Gorges Dam

Shipping Locks

Shipping Locks

CASE STUDY : The Malaysia Mega Project Bakun


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CASE STUDY : The Malaysia Mega Project BakunHydroelectric Project (Bakun HEP) in Rejang River

Basin, Sarawak.

The Bakun dam is a 205-meter-high Concrete Face Rockfill Dam

(CFRD), with a length of crest of 740 meters, a base width of 560meters and a crest width of 12 meters. It can generate 2400MW(max). This makes it one of the highest rockfill dams in the worldafter China. It will flood 69,640 hectares of land, an area as size ofSingapore.

Diversion Inlet

Diversion Outlet Auxiliary Cofferdam

Reservoir


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Engineering contribution to Bakun HEP

This project is fully contributed by latest technology with theknowledge and experience of engineers.

Mechanical Engineers turbine design water flow (penstock) Control gate

Electrical Power Engineers transformer Generator Power line

Civil Engineers Designing Mixture of concrete The strength of tunnel


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Positive Impact to the project

Regulation of river flow

Generates economy

Mitigation of floods

Reduces fossil fuel consumption

Reducing CO2 emissions

Carbon Dioxide Emission and Contribution To Global Warming

Source Environmental & Natural

Resource Implications

Carbon Dioxide

Emission(g/mj)1Relative Ratio of Contribution to

Global Warming

Hydro Clean and renewable 7.1 1

Natural Gas Moderately clean and depletable 171-188 24-26

Fuel Oil Polluting and depletable 204 29

Coal Polluting and depletable 831-1938 117-273


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Negative Impact to the project

Highly cost more than RM 11.36 billions (1994)

Destroying the rainforest affecting 105 species of animals protectedunder Malaysian wildlife legislation and innumerable plant species.

Resettlement - All the 15 affected communities will be resettled inareas near the Belaga River, about 30 kilometres from the Bakun damsite

Reservoir water quality - Decay of submerged forests can upset theecological balance of the area, leading to loss of water quality


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Pergau Hydroelectric Scheme

Type of Dam: Earthfill

Y f C l ti 1996


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Kuala Yong Dam

Year of Completion: 1996

Crest Length: 750m

Height of Dam: 75m

Photo courtesy of En. Mohd Khanil

Taib


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Power Intake

After Reservoir Filling


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Documents

Hydraulic Structures Chap 8 - 12 June 2009