dams introduction

CVE 471 Water Resources Engineering 1/101

VE 471 WATER RESOURCES ENGINEERING

DAMSDAMS


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Overview

Classification of DamsParts of DamsPlanning of DamsConstruction of DamsConcrete Gravity DamsArch DamsCross-sectional Layout Design of DamsLocal Scour at the Downstream of DamsDam Safety and Rehabilitation


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Overview

Classification of DamsParts of DamsPlanning of DamsConstruction of DamsConcrete Gravity DamsArch DamsButtress DamsEmbankment (Fill Dams)Cross-sectional Layout Design of DamsLocal Scour at the Downstream of DamsDam Safety and Rehabilitation


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Classification of Dams

A dam is an impervious barrier built across a watercourse to store water for several purposes:

water supply,creating head (energy generation),forming a lake,sediment control,flood control,recharging of groundwater, etc.

There are disadvantages of dams as well:imbalance of ecosystem,decrease amount of downstream water,reduction in the fertility of farmlands, etc.

Therefore, detailed survey should be carried out to ensure that the relative weights of advantages over disadvantages are higher.


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Dams can be classified into a number of different categories depending upon the purpose of classifications.A classification based on the type and materials of construction:

Gravity DamsConcrete gravity damsPrestressed concrete gravity damsRoller compacted concrete (RCC) gravity dams

Arch DamsConstant-angle arch damsConstant-center arch damsVariable-angel, variable-cemter arch dams

Buttress DamsFlat-slab buttress damsMultiple-arch buttress dams

Embankment (Fill) Dams


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Gravity DamsConcrete gravity damsPre-stressed concrete gravity damsRoller compacted concrete (RCC) gravity dams

Karun Dam, Iran

http://en.wikipedia.org/wiki/Dam

Shasta Dam, California, USA


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Arch DamsConstant-angle arch damsConstant-center arch damsVariable-angel arch damsVariable-center arch dams

Gordon Dam, Tasmaniahttp://en.wikipedia.org/wiki/Dam

Monticello Dam, California, USA


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Buttress DamsUsed mainly in wide valleys, it consists of an impermeable wall, which is shored up by a series of buttresses to transmit the thrust of the water to the foundation.

Flat-slab buttress damsMultiple-arch buttress dams


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Buttress DamsFlat-slab buttress dams

Lake Tahoe Dam, California, USA


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Buttress DamsMultiple-arch buttress dams

Bartlett Dam , Phoenix, Arizona, USA


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Embankment (Fill) DamsEarth-fill dams

Simple embankmentZoned embankmentDiaphragm type embankment

Upstream of Ataturk Dam, Turkey

Embankment (Fill) DamsRock-fill dams

Impermeable-faceImpermeable-earth core

Downstream of Ataturk Dam, Turkey


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A classifications based on purpose, such as

storagediversionflood controlhydropower generation

A classification based on hydraulic design such as

overflow dams,non-overflow dams Gilboa Dam, New York State, USA

http://en.wikipedia.org/wiki/Dam


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A classification based on dam height:According to the International Commission on Large Dams (ICOLD):

Large Dam if height > 15 mLarge Dam if 10 m < height < 15 m

reservoir storage > 106 m3

crest length > 500 mHigh Dam height > 50 mSmall Dam height < 10 m

Distribution of dam heights in Turkey as of 2002.


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Overview



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Parts of Dams

A dam is composed of the following structural componentsBody forms the main part of a dam as an impervious barrier.Reservoir is the artificial lake behind a dam body.Spillway is that part of a dam to evacuate the flood wave from the reservoir.Water intake is a facility to withdraw water from a reservoir.Outlet facilities are those appurtenances to withdraw water from the reservoir to meet the demands or to discharge the excess water in the reservoir to the downstream during high flows.

sluiceways,penstocks,diversion tunnels,bottom outlets, andwater intake structures

Others: Hydropower station, site installations, roads, ship locks, fish passages, etc.


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Overview



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Planning of Dams

There are commonly three steps in the planning and design:reconnaissance survey,feasibility study, andplanning study.

In reconnaissance surveys, the alternatives, which seem infeasible without performing intensive study, are eliminated.Feasibility Study:

Estimation of water demandDetermination of water potentialOptimal plansDetermination of dam site

TopographyGeologic informationFoundation conditionsFlood hazard


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Planning of Dams

Feasibility Study:Determination of dam site (cont’d)

Spillway location and possibilityClimateDiversion facilitiesSediment problemWater qualityTransportation facilitiesRight of way cost

Determination of type of damsProject design

Hydrologic designHydraulic designStructural design


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Planning of Dams

Planning Study:Topographic surveysFoundation studiesDetails on materials and constructional facilitiesHydrologic studyReservoir operation study


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Overview



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Construction of Dams

Details of dam construction are beyond the scope of this course. The principal steps to be followed during the construction of any type of dam briefly:

Evaluation of time schedule and required equipment. Diversion of river flowFoundation treatment

Evaluation of Time Schedule and Required Equipment. Items to be considered:

the characteristics of dam sitethe approximate quantities of workthe preservation of construction equipment and materialsdiversion facilities and urgency of work


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Diversion of River FlowDiversion of the river flow is may be accomplished in one of the following ways1. Water is diverted through a side tunnel or channel.

(Applicable for low flow depths ~1.5 m)

Diversion by side tunnel or channel


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Diversion of River Flow (cont’d)Typical cross-section of earth cofferdams

f: free board f=0.2(1+h)h: flow depth (meters)G=z/5 + 3 (meters)

Cofferdams should be constructed during the low flow season.For fill type dams, embankment cofferdam may be kept in place as part of the embankment (e.g. Keban Dam and Ataturk Dam).For concrete dams, embankment cofferdam should be demolished after the dam has been constructed.

Earth cofferdam on impervious foundation Earth cofferdam on pervious foundation


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Hoover Dam, USA

Diversion of River Flow (cont’d)


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Hoover Dam Overflow Tunnels (spillways), USA



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Hoover Dam Overflow Tunnels (spillways), USA



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Construction of DamsHoover Dam Overflow Tunnels (spillways), USA



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Two-stage diversion

Diversion of River Flow (cont’d)2. Water is discharged through the construction, which takes place in two

stages.This type of diversion is normally practiced in wider valleys.


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Two-stage diversion



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A cofferdam on the Ohio River, Illinois, USA, built for the purpose of constructing the lock and dam.



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Diversion of River Flow (cont’d)Selection of a proper diversion scheme is based on the joint consideration of

hydrologic characteristics of river flow,type of dam and its height,availability of materials,characteristics of spilling arrangements.

The optimum design is based on cost minimization.The cost analysis is carried out for various sizes of diversion tunnels or channels to determine the corresponding total costs.The optimum tunnel diameter or bottom width of a lined trapezoidal channel is then determined according to the minimum total cost of the facility.


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Foundation TreatmentFoundation treatment for dams is essential

to achieve less deformation under high loads,to decrease permeability and seepage,to increase shearing strength, andto satisfy slope stability for the side hills.

Highly porous foundation material causes excessive seepage, uplift and considerable settlement.Such problems can be improved by a grouting operation.In this operation, the grout mix is injected under pressure to decrease the porosity, and hence to solidify the formations underlying the dam and reservoir.


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Overview



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Gravity Dams

Gravity dams are satisfactorily adopted for narrow valleys having stiff geological formations.Their own weight resists the forces exerted upon them.They must have sufficient weight against overturning tendency about the toe.The base width of gravity dams must be large enough to prevent sliding.These types of dams are susceptible to settlement, overturning, sliding and severe earthquake shocks.


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Gravity Dams

Concrete Gravity DamsConcrete gravity dams area built of mainly plain concrete to take compressive stresses.

Shasta Dam, California, USA


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Gravity Dams

Concrete Gravity Dams (cont’d)Concrete gravity dams have lower maintenance and operation costscompared to the other types of dams.In the design of these structures, the following criteria should be satisfied:

Dimensions of the dam are chosen such that only compressive stresses develop under all loading conditions.

The dam must be safe against overturning, shear and sliding.


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Gravity Dams

Concrete Gravity Dams (cont’d) In the construction of concrete gravity dams special care is required for the problems due to shrinkage and expansion.

Formation of the body of the concrete gravity dam


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Gravity Dams

Concrete Gravity Dams (cont’d)Forces Acting on Gravity DamsThe weight:

Wc= dead load

Hydrostatic forces:

Uplift Force:

Free body diagram. Forces acting on a concrete gravity dam

φ: uplift reduction coefficientMoment arm of Fu=B(2h1+3h2) / 3(h1+h2)

Actual uplift pressures are determined by pressure gauges installed at the bottom of the dam.


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Gravity Dams

Concrete Gravity Dams (cont’d)Forces Acting on Gravity DamsSediment Force:

Ice Load (Fi):


γs: submerged specific weight of soilKa: active earth pressure coefficient according to the

Rankine theory.Ka = (1-sinθ)/(1+sinθ)


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Gravity Dams

Concrete Gravity Dams (cont’d)Forces Acting on Gravity DamsEarthquake Force:

Dynamic Force in the reservoir induced by earthquake

Dynamic Force acting on a spillways


k: earthquake coefficient

obtained using momentum equation

cd kWF =

21726.0 hCkFw γ=

′−=

9017.0 θC

uQF ∆=Σ ρ


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Gravity Dams

Concrete Gravity Dams (cont’d)Forces Acting on Gravity Dams

Wave Force may be considered for wide and long reservoirs.

Temperature Loads may be severe during construction because of hydration reactions



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Gravity Dams

Concrete Gravity Dams (cont’d)Stability Criteria

Stability analyses are performed for various loading conditionsThe structure must prove its safety and stability under all loading conditions.Since the probability of occurrence of extreme events is relatively small, the joint probability of the independent extreme events is negligible.In other word, the probability that two extreme events occur at the same time is relatively very low.Therefore, combination of extreme events are not considered in the stability criteria.

Floods (spring and summer) Ice load (winter).No need to consider these two forces at the same time.


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Gravity Dams


Usual LoadingHydrostatic force (normal operating level)Uplift forceTemperature stress (normal temperature)Dead loadsIce loadsSilt load

Unusual LoadingHydrostatic force (reservoir full)Uplift force Stress produced by minimum temperature at full levelDead loadsSilt load

Extreme (severe) LoadingForces in Usual Loading and earthquake forces


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Gravity Dams


The ability of a dam to resist the applied loads is measured by some safety factors.To offset the uncertainties in the loads, safety criteria are chosen sufficiently beyond the static equilibrium condition.Recommended safety factors: (USBR, 1976 and 1987)

However, since each dam site has unique features, different safety factors may be derived considering the local condition.

F.S0: Safety factor against overturning.F.Ss: Safety factor against sliding.F.Sss: Safety factor against shear and sliding.


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Gravity Dams


The factor of safety against overturning:

The factor of safety against sliding:

∑∑=

00.

MM

SF r

∑∑=HVf

SF s.

where ΣMr: total resisting moment about the toe.ΣM0: total overturning moment about the toe.

where f: coefficient of friction between any two planesΣV: vectorial summation of vertical forces.ΣH: vectorial summation of horizontal forces.


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Gravity Dams


The factor of safety against sliding and shear:

∑∑ +

=HcAVf

SF ss.

∑∑ +

=HrAVf

SF sss

τ. (in the dam)

where A: Area of the shear plane,τs: shear strength of concreter: factor to express max allowable average shear stress r=0.33, 0.50, and 1.0 for usual, unusual, and extreme loading, respectively.f: coefficient of friction between any two planes

ΣV: vectorial summation of vertical forces.ΣH: vectorial summation of horizontal forces.

(at foundation level)


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Gravity Dams


The contact stress between the foundation and the dam or the internal stress in the dam body must be compressive:

IMc

AV±= ∑σ

where σ : vertical normal base pressureA: Area of the shear plane,M: net moment about the centerline of the base (M = ΣV.e)e: eccentricity ( )c: B/2I : Moment of inertia (B3/12)

ΣV: vectorial summation of vertical forces.

xB −2/

Normal stress Bending or flexural stress

Base pressure distribution


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Gravity Dams



In order to maintain compressive stresses in the dam or at the foundation level, the minimum pressure σmin ≥ 0.This can be achieved with a certain range of eccentricity.

σmin ≥ 0 can be achieved if e ≤ B/6Full reservoir σmax at the downstream faceEmpty reservoir σmax at the upstream face

06112/

2/3min ≥

−

Σ=

××Σ−

Σ=

Be

BV

BBeV

AVσ

Base pressure distribution

IMc

AV±

Σ=σ for a unit width


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Gravity Dams



Tension along the upstream face of a gravity dam is possible under reservoir operating conditions.

z = 1.0 (if there is no drainage in the dam body)z = 0.4 (if drains are used)P: hydrostatic pressure at the level under consideration


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Gravity Dams


Concrete gravity dams have varying thickness.Hence the inclined compressive stresses parallel to the face of the dam need to be computed.

For a concrete gravity dam with slopes of 1V:mH at the upstream face and 1V:nH at the down stream face, the major principle compressive stresses, σiu (parallel to the upstream face) and σid (parallel to the downstream face) are obtained from the static equilibrium of forces in the vertical direction as: (ΣFy=0)

where σu and σd vertical normal compressive stresses and pu and pd hydrostatic pressures at the upstream and downstream faces, respectively.


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Gravity Dams


Internal horizontal and vertical shear stresses at the upstream and downstream faces are obtained by equating the total moment to zero as (ΣMA=0, ΣMB=0):

where τhu, τhd, τvu, and τvd are the horizontal and vertical internal shear stressesat the upstream and downstream faces, respectively.


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Gravity Dams


The maximum compressive stress, σmax ,must be smaller than a certain fraction of the compressive strength of concrete, σc, and foundation material, σf.

Safety criteria for concrete gravity damsUnconfined compressive strength, σf

for foundation materials


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Gravity Dams


Excessive care must be taken during the filling of the reservoir.Initially 1/3 of the dam height may be filled first.After waiting for several weeks and assuring that the dam is safe, further filling is performed.Since safety levels change with respect to upstream water depth,gravity dams must be analyzed for various operating levels and empty reservoir cases, separately.For the empty reservoir case, the overturning tendency must be checked with respect to the toe and heel, separately.The stability against sliding may be improved by providing a cut off wall in the foundation at the upstream side.


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Gravity Dams

Prestressed Concrete Gravity Dams In a prestress concrete dam, forces are applied to the dam before the reservoir is filled in order to counter undesirable stress that would develop in the absence of the prestressing forces.For prestressing, either small-diameter high-tensile wires or high-tensile steel bars can be used.

Roller Compacted Concrete (RCC) Gravity Dams RCC dam is constructed using cement, water, fine and course aggregates, and fly ash which are mixed in certain proportions to have a no-slump, rather dry composition.Construction is based on the compaction of this mixture by heavystatic or vibrating rollers.Construction period of RCC dams is shorter than that of conventional concrete gravity dams.


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Overview



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Arch Dams

Arch dams are thin concrete structures.Gokcekaya, Oymapinar, Karakaya, Gezende, and Berke dams in Turkey.

Gokcekaya Dam Berke Dam Karakaya Dam


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Arch Dams

Arch dams: Oymapinar Dam, Turkey


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Arch Dams

Hoover Dam, USA


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Arch Dams

Arch dams are thin concrete structures.Stability of an arch dam is based on its self weight and its ability to transmit most of the imposed water loads into the valley walls.At the sites of arch dams, the side formations and foundations should be very stiff to resist the applied load.For effective arching action, the radius of the arch should be as small as possible.They are formed by concrete blocks having base dimensions of approximately 15 m by 15 m and height of 1.5 mReinforcement is not generally required in thick arch dams because it increases the cost drastically. Arch dams have normally higher structural safety than conventional gravity dams.


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Arch Dams

Types of Arch DamsArch dams are classified according to geometric characteristics of the valley where they are adopted. Arch dams are classified according to the location of the center and magnitude of the central angle

Constant-center (variable angle) arch dams are suitable for medium-high dams in U-shape valleys. They have single curvature in plan with vertical upstream face.


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Arch Dams

Types of Arch DamsVariable-center (constant angle) arch dams are suitable for V-shape valleys.

Radius of the arc reduces with respect to depth.So arching action is more pronounced at low depths.Since these types of dams are normally thinner than constant-center dams, they are more elastic and safer. Variable-center (constant angle) arch dams


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Arch Dams

Types of Arch DamsVariable-center (variable angle) arch dams are composed of the combination of two types described above.

Load distribution in vertical direction governs the cross-sectional shape of the dam.This type has a pronounced double curvature They utilized the concrete strength more compared the other types resulting in thinner and more efficient structure.However, tensile stresses may develop in the dam body.

Variable-center (variable angle) arch dams

Gokcekaya Dam


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Arch Dams

Types of Arch DamsVariable-center (variable angle) arch dams

Cross-section of Gokcekaya Dam

Gokcekaya Dam


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Arch Dams

Design of Arch DamsStructural design of an arch dam requires the determination of load distribution in the dam body using the trial load method and applications of the theory of elasticity and the theory of shells.Structural design is beyond the scope of this course.Simplified design:

The determination of the thickness at any elevation of an arch dam whose crest elevation has already been determined in the hydrologic design step.

In the arch dams, the total load is shared by arch and cantilever actions and transmitted to the sides and foundation, respectively.Therefore, the base width of arch dams is usually much narrower than that of concrete gravity dams having almost the same height.Hence, the effect of uplift pressure can be ignored.


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Arch Dams

Design of Arch DamsHowever, effect of temperature stresses should be checked to ensure that they are smaller than tensile strength.Near the crest of the dam, most of the loads taken by arches and transmitted to the side abutments.Near the bottom of the dam, cantilevers take most of the load and transmit to the foundation.

Gokcekaya Dam


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Arch Dams

Design of Arch DamsIn the following analysis, the water thrust induced by hydrostatic pressure is assumed to be taken by arch action only and transmitted to the sides.

The differential force acting on a differential element having a central angle of dΦ is

dFv= P r dΦ

The vertical component of this force is

dF'v= P r dΦ sinΦ

Free-body diagram for arch dam analysis


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Arch Dams

Design of Arch DamsIntegration of this force along the arc length gives the total horizontal force, Hh.


where h: the height of the arch rib relative to the reservoir surfacer: the radius of archθa: the central angle

2sin2

22cos

2cos2sin2

2

22

aah hrhrdhrH

a

θγθππγφφγ

π

θπ=

−−

−== ∫

−


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Arch Dams

Design of Arch DamsThe equilibrium of forces in y-direction involves


where R: the reaction offered by the sides against the transmission of water thrust.

As observed from the R = γhr, the reaction at the sides is directly proportional to the arc radius at a given height. Therefore, narrow valleys having stiff geological formations and small r-values are suitable for arch dams.

hrR

Rhr

RR

RH

aa

ay

yh

γ

θθγ

θ

=

=

=

=

2sin2

2sin2

2sin

2where

Therefore


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Arch Dams

Design of Arch DamsIf the thickness of the arch rib, t, is relatively small as compared with r, there is small difference between the average and maximum compressive stresses in the rib and σ≈R/t. The required thickness of the rib is then

where σall: the allowable working stress for concrete in compression.

all

hrtσγ

= (the thickness varies linearly with depth.)


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Arch Dams

Design of Arch DamsThe volume of concrete per unit height of a single arch rib across a canyon of width of Ba is

V=Ltwhere L is the arch length which is equal to rθa (θa in radians).

Inserting the values of L and t into the equation above

aallrhV θ

σγ 2=


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Arch Dams

Design of Arch DamsThe optimum central angle θa for a minimum volume of arch rib can be determined as 133º34‘ by differentiating V with respect to θa and equating the result to zero.This is the reason why a constant-angle arch dam can be design to require less concrete than a constant-center dam.In practice, the central angles of arch dams vary from 100º to 140º.However, the formwork of a constant-angle dam is more difficult.


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Arch Dams

Design of Arch DamsThe optimum central angle θa for a minimum volume of arch rib can be determined as 133º34‘ by differentiating V with respect to θa and equating the result to zero.This is the reason why a constant-angle arch dam can be design to require less concrete than a constant-center dam.In practice, the central angles of arch dams vary from 100º to 140º.However, the formwork of a constant-angle dam is more difficult.


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Overview



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Buttress Dams

A buttress dam consists of a sloping slab.Depending on the orientation of slab, a buttress dam may be classified as

flat-slab buttress dammultiple-arch buttress dam

Elmali Dam construction, Istanbul, 1941

Elmali Dam

A typical buttress dam.


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Buttress Dams

Flat-slab buttress dams

Lake Tahoe Dam, California, USA


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Buttress Dams

Multiple-arch buttress dams

Bartlett Dam , Phoenix, Arizona, USA


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Buttress Dams

Some advantages of buttress dams over conventional gravity dams:They can be constructed on foundations having smaller bearing capacity then required for gravity dams.Since they have thinner slabs, possibility of development of vertical cracks is less.Problems encountered during the setting of concrete are reduced.Unless a mat foundation is used, uplift forces are negligibly smallbecause of hollow spaces provided between the buttresses.Ice pressures are also small as the ice sheet slides up the inclined slab.

Main disadvantage of buttress dams:May have comparable costs, because of increased formwork and reinforcement .

There is only one buttress dam inTurkey (Elmali 2 Dam).

Elmali 2 Dam


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Overview



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They composed of fill of suitable earth materials at the dam site.Coarse-grained soils (gravel and coarse sand)

relatively pervious,easily compacted,resistant to moisture,

Clay is considered as a core material (impermeable)unstable when saturated (expands due to wetting, hard to compact)

Therefore, clay mixed with sand and fine gravel is used as a core.Core must be compacted in thinner layers with fairly accurate moisture control.Compacted asphalt may also be used as an economical core material in case of loose foundations.Asphalt can absorb earthquake shocks effectively.


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Embankment dams are usually safer against deformations and settlements.Embankment dams

Earth-fill damsRock-fill dams

(More than 50% of the total material is of rock.)

Earth-fill dams in TurkeySeyhan DamDemirkopru DamCubuk 2 DamBayindir Dam

Rock-fill dams in TurkeyKeban DamAtaturk DamHasan Ugurlu Dam

Hasan Ugurlu Dam


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Body volume of embankment dams is relatively greater than the other types of dams.Normally cheaper than the other types where there is enough fill material in the close vicinity.Fill dams comprise more than 70% of the dams in the world and 90% in Turkey.

Keban Dam


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Earth-fill DamsConstruction:

Placement of selected material on layers of 50 cm thick and compaction.Non-organic and non-plastic soils are needed.The embankment soil is usually irrigated at the borrow area.Piezometers can be placed in the embankment at various depths during the construction to measure the pore water pressure.

A typical earth-fill dam is constructed in a multi-layer formation.


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Earth-fill DamsA typical earth-fill dam is constructed in a multi-layer formation.

Earth dam on pervious foundation


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Earth-fill DamsSeepage through an earth-fill dam.

The flow rate, ∆q, between two flow lines can be expressed using the Darcy law as

∆∆

∆==∆LhDKKAiq

K: the hydraulic conductivityi : the hydraulic gradient∆h: head loss (h/N) N : number of equipotential drops

′=NKhNq

The total flow rate, q

N’: the number of stream tubes


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Earth-fill DamsDrainage systems in an earth-fill dam.

Chimney drains, in the embankment as well as enlarged toe drainsare effective in controlling the seepage through the dam.


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Rock-fill DamsHaving relatively high pore spaceCan be adopted to weaker foundations where a gravity dam cannot be constructed.

Cross-sections of typical rock-fill dams


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Rock-fill Dams (Ataturk Dam)Largest dam in TurkeyReservoir Volume: 48.7 x 109 m3

Installed capacity: 2400 MW Annual energy production: 8.9 x 109kWhIrrigated land: 874200 ha (with the completion of the project)

A cross-sections of the Ataturk Dam


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Rock-fill Dams (Ataturk Dam)

A cross-sections of the Ataturk Dam


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Overview



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Cross-sectional Layout Design of Dams

A suitable dam cross-section should be provided such that both safety and desired functionality concerning service requirement are attained.Sufficient crest width, tc must be provided.

a width of two lane traffic may be selected.For small embankment dams up to Hf=15 m.

tc=0.2Hf+3 For large embankment dams up to Hf=150 m.

tc=3.6 (Hf )1/3

where tc and Hf are in meter.


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By examining some existing muti-purpose concrete gravity dams throughout the world, Yanmaz et al. (1999) proposed the following regression equations to define the shape of a gravity dam.

H*=0.1075 Ht

tc=0.0475 Ht +2.392

where all variables are in meter


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United States Bureau of Reclamation (USBR) propose the followingformulas for cross-sectional layout of arch dams:

where Ba: the span width at the crestB0.15: the span width at 15% of the dam height above the baset0.45Ht: the dam thickness at 45% of the dam height above the base.

( )

bH

Ht

atb

atc

tt

HBBHt

BHt

t

t

95.0400

0012.0

2.101.0

45.0

3400/

15.0

=

=

+=

All the dimensions are in ft


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The crest elevation of a dam is to be determined such that there is almost no overtopping danger of the flood wave during the occurance of the design flood.

Greater freeboards are required for embankment dams since they are susceptible to erosion at the downstream face due to overtopping from their crest. The required side slopes of concrete gravity dams are determined from stability analyses.The maximum downstream slope of gravity dams is 45°.Side slopes of embankment dams are determined on the basis of seepage and slope stability analyses.

Freeboards on flood levels for concrete and embankment dams


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Overview



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Local Scour at the Downstream of Dams

Excessive kinetic energy of the flowing water at the downstream of outlet works (spillways, sluiceways etc.) should be dissipated in order to prevent the erosion of the streambed and the banks below the dam.

Excessive scours at the downstream of Keban Dam have resulted in serious foundation stability problems (depth of approx 30 m).

Local scour at the downstream of the dam and sluice gates


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Local Scour at the Downstream of Dams

Some of the scour prediction equations are given in the table.Scour prediction equations for the downstream of dams. ds: the maximum depth of scour hole in m.

b: the thickness of the jet in m.

Φ: the side inclination for the scour hole,

Fr: Jet Froude number.

U: the velocity of the jet in m/s

∆=(γs- γ)/γ, γs: : specific weight of sediment in kN/m3

γ : specific weight of water in kN/m3.

Wf: Fall velocity in m/s

q: unit discharge in m3/s/m

Hg: gross head in m

h: tailwater depth in m

D50: median size of bed material in m.


3. DAMS3. DAMS

Overview



3. DAMS3. DAMS

Dam Safety and Rehabilitation

Excessive care must be taken in planning, design, and construction stages of a dam.Major causes for a dam break:

Inadequate spillway capacity,Improper construction of any type of dam,Insufficient compaction of embankment dams or compaction with undesirable water content,Improper protective measures,Excessive settlements, etc

Continuous inspection and monitoring are required to assess the safety level of the dam throughout the lifetime.


3. DAMS3. DAMS

Dam Safety and Rehabilitation

Upon periodic inspection, the following deficiencies may be observed that are indicators of problems:

Large horizontal and vertical movements of crest,Tilting of the roadway along the crest,Deformation of embankment slope,Higher than usual pore water pressure in embankment dams,Unusual seepage at the toe or edges of an embankment dam,Seepage flows with not decreasing with low flow conditions,Turbit outflow through the embankment,Tilting of the spillway crestIncreased leakage into inspection galleries in concrete dams, etc.

Documents

dams introduction