Hydropower Plants

Use of water power

Typical schemes and works

Barrages and Dams

Other components (spillways, inlet waterways, outlets, transport,..)

Intakes

Part I:

Main components of a Hydro Power Plants

damming works opere di sbarramento

ancillary works-dam outlet works (spillways and outlets)

opere complementari degli invasi (scarichi superficiali e di fondo);

water intakes opere di presa

Inlet waterways opere d’adduzione a monte delle condotte forzate;

Surge tanks vasche d’oscillazione;

penstock condotte forzate;

Power plant equipment: macchinari delle centrali:

Shut off valves/gates organi d’intercettazione

Pressure/velocity regulation valves

organi di regolazione della velocità e della pressione;

turbines turbine idrauliche

pumps pompe

Pump-turbines (pump-storage plants)

macchine idrauliche reversibili

Downstream outlet opere di scarico

•Dam•Water reservoir•Gate•Spillway•Surge tank•Pressure tunnel•Penstock•Water turbine•Draft tube•Tail race level•Power house

DAMS and WEIRS

dams or weirs: are barriers constructed across a water course for a safe retention and storage of water

For the International Commission on Large Dams (ICOLD), dams areconventionally considered (large) “dams” only the barriers having a height notless than 15 meters, regardless of the capacity of the reservoir they cause,and the height between 10 and 15 meters below special conditions ofsignificant lengths of the top, the storing capacity, flood discharge, difficulty offoundation, structural features.

The Italian Regulation for dam design, construction and operation of dams considers (large) “dams” the barriers not less than 10 meters high and those which result in maximum storage capacity of at least 100,000 cubic meters.

weirs, defined as barriers that create a backwater fully contained in theriverbed. Weirs are considered as (large) “dams” and subjected to damsregulations if the volume contained between the maximum backwater profileand low-water profile is higher than the limit indicated above (100000 meterscubes) or if the weir is higher than 10 meters. For barriers not subject to theaforementioned Regulation, the definition of regulatory rules to be applied isentrusted to the Office of Civil Engineers responsible for the territory.

DAMS and WEIRS

Sono definite dighe di ritenuta o traverse fluviali le opere di sbarramento realizzate in una sezione di un corso d’acqua

la Commissione Internazionale delle Grandi Dighe (ICOLD), sono convenzionalmenteconsiderate dighe solo le opere di sbarramento aventi un’altezza non inferiore a 15metri, qualunque sia la capacità dell’invaso che esse determinano, e quelle di altezzafra 10 e 15 metri sotto particolari condizioni di rilevanti lunghezze della sommità,capacità di invaso, portata di piena, difficoltà di fondazione, caratteristichestrutturali. Il Regolamento Italiano per la compilazione dei progetti, la costruzione el’esercizio degli sbarramenti di ritenuta considera dighe gli sbarramenti di altezzanon inferiore a 10 metri e quelli che determinano un invaso della capacità massimanon inferiore a 100000 metri cubi.

Le traverse fluviali, definite come sbarramenti che creano un rigurgito contenutonell’alveo del corso d’acqua, sono soggette alle norme citate se il volume contenutofra il profilo di rigurgito massimo ed il profilo di magra risulta superiore al limitetesté indicato (100000 metri cubi) oppure se la ritenuta è maggiore di 10 metri. Perle strutture non soggette al citato Regolamento, la definizione delle normeregolamentari da applicare è demandata all’Ufficio del Genio Civile competente per ilterritorio.

DAMS Classification

Masonry dam

Concrete dam

Earth dam

Rockfill dam

Timber dam, Steel dam, Composite dam,Combined concrete-cum-earth dam, ….

.

Based on material of Construction

La classificazione secondo Regolamento Italiano

1. sbarramenti murari;

2. sbarramenti in materiali sciolti;

3. sbarramenti di tipo vario;

4. traverse fluviali.

Rigid dams: A rigid dam is quite stiff. It is constructed of stiffmaterials such as concrete, masonry, steel and timber. These damsdeflect and deform very little when subjected to water pressureand other forces

Non-rigid dams: A non-rigid dam is relatively less stiff comparedto a rigid dam. The dams constructed of earth and rockfill are non-rigid dams. There are relatively large settlements and deformationsin a non-rigid dam.

Rockfill dams are actually neither fully rigid nor fully nonrigid.These are sometimes classified as semi-rigid dams.

Based on Rigidity

DAMS Classification

Gravity dams

Buttress dams

Arch dams

Embankment dams

Earth dams

Rockfill dams

Others:

Steel dams

Timber dams

Based on on structural action

DAMS Classification

Concrete Dams

Gravity Dam

A gravity dam resists the water pressure and other forces due to its weight (or gravitational forces).

Usually made of cement concrete and straight in plan

are approx triangular in cross-section, with apex at the top.

In the past, the gravity dams were made of stone masonry,

Gravity dams can be subdivided in two subcategories:

Massive dams (horizontal section are full concrete)

Buttress dams (dighe “Marcello”, in horizontal section in the horizontal section alternate concrete elements alternate with empty spaces. Are a “light” version of gravity dams)

Gravity Dam, advantages:

gravity dams are quite strong, stable and durable.

are quite suitable across moderately wide valleys and gorges

having steep slopes where earth dams, if constructed, might slip.

can be constructed to very great heights, provided good rock foundations areavailable.

internal stress state is not strong concrete not partifcularly high-grade

are well adapted for use as an overflow spillway section. Earth dams cannotbe used as an overflow section. Even in earth dams, the overflow section isusually a gravity dam.

are specially suited to such areas where there is very heavy downpour. Theslopes of the earth dams might be washed away in such an area

Concrete Dams

Gravity Dam, advantages:

gravity maintenance cost of a gravity dam is very low.

does not fail suddenly. There is enough warning of the imminent failure and the valuable property and human life can be saved to some extent.

can be constructed during all types of climatic conditions.

sedimentation in the reservoir on the upstream of a gravity dam can be somewhat reduced by operation of deep-set sluices

Concrete Dams

Concrete Dams

Gravity Dam, disvantages:

gravity dams of great height can be constructed only on sound rock foundations. These cannot be constructed on weak or permeable foundations on which earth dams can be constructed.

initial cost of a gravity dam is usually more than that of an earth dam. At the sites where good earth is available for construction and funds are limited, earth dams are better.

usually take a longer time in construction than earth dams, especially when mechanised plants for batching, mixing and transporting concrete are not available.

require more skilled labour than that in earth dams.

subsequent raising is not possible in a gravity dam

diga a gravità massiccia (diga del Rio Fucino)

Concrete Dams

Concrete Dams

Drain wall

Sp

Arch Dam/Dighe a volta.

is curved in plan, with its convexity towards the upstream side.

transfers the water pressure and other forces mainly to the abutments by arch action (rocky abutments geology is important).

is quite suitable for narrow canyons with strong flanks which are capable of resisting the thrust produced by the arch action.

section is triangular and is comparatively thinner.

vertical plane.

are subjected to large stresses because of changes in

temperature shrinkage of concrete and yielding of abutments.

Concrete Dams

Arch Dam/Dighe a volta three subclasses can be identified:

Arch dams, with a single curvature, the arch effect

Double curvature dams (cupola): they are calculated as double curvature shells

Arch-gravity dam, large thickness wall, curved in horizontal plane, theresistance is partly due to curvature effect (arch) and partly to weight(cantilever effect)

Concrete Dams

Anche in questo caso, si possono considerare tre sottotipi e precisamente:

dighe ad arco, che vengono progettate e verificateammettendo che la resistenza alle azioni esterne sia dovutaunicamente all’effetto arco. Queste dighe sono costituite, diregola, da una superficie cilindrica con generatrici verticali omolto prossime alla verticale;

dighe a cupola, o ad arco a doppia curvatura, che sipossono considerare come derivate dal tipo precedente, pergraduale incurvamento delle rette generatrici. Questestrutture vengono calcolate come piastre a doppiacurvatura;

dighe ad arco – gravità, costituite da un muro a piantaarcuata e di forte spessore; il loro calcolo viene condottoammettendo che la resistenza sia dovuta in parte allacurvatura delle sezioni orizzontali (effetto arco) e in parte alpeso proprio (effetto mensola).

diga ad arco a doppia curvatura (diga dell’Ambiesta)

Concrete Dams

diga ad arco – gravità (diga del Fiastrone)

Concrete Dams

Concrete Dams

Arch Dams, advantages:

an arch dam requires less concrete as compared to a gravity dam as the section is thinner.

Arch dams are more suited to narrow, V-shaped valley, having very steep slopes.

Uplift pressure is not an important factor in the design of an arch dam because the arch dam has less width and the reduction in weight due to uplift does not affect the stability.

An arch dam can be constructed on a relatively less strong foundation because a small part of load is transferred to base, whereas in a gravity dam full load is transferred to base.

Concrete Dams

Arch Dams, disvantages:

an arch dam requires good rock in the flanks (abutments) to resist the thrust. If the abutments yield, extra stresses develop which may cause failure.

The arch dam requires sophisticated formwork, more skilled labour and richer concrete.

The arch dam cannot be constructed in very cold climates because spalling of concrete occurs due to alternate freezing and thawing.

The arch dams are more prone to sabotage.

The speed of construction is relatively slow.

Concrete Dams

Buttress Dams

an arch dam requires less concrete as compared to a gravity dam as the section is thinner.

Arch dams are more suited to narrow, V-shaped valley, having very steep slopes.

Uplift pressure is not an important factor in the design of an arch dam because the arch dam has less width and the reduction in weight due to uplift does not affect the stability.

An arch dam can be constructed on a relatively less strong foundation because a small part of load is transferred to base, whereas in a gravity dam full load is transferred to base.

Buttress Dams,

Buttress dams are of three types:

– (i) Deck type,

– (ii) Multiple arch-type, and

– (iii) Massive-head type.

A deck type buttress dam consists of a sloping deck supported by buttresses.

Buttresses are triangular concrete walls which transmit the water pressure from the deck slab to the foundation.

Buttresses are compression members.

The deck is usually a reinforced concrete slab supported between the buttresses, which are usually equally spaced.

In a multiple-arch type buttress dam the deck slab is replaced by horizontal arches supported by buttresses. The arches are usually of small span and made of concrete.

In a massive-head type buttress dam, there is no deck slab. Instead of the deck, the upstream edges of the buttresses are flared to form massive heads which span the distance between the buttresses.

Concrete Dams

Dighe a speroni, con pareti di ritenuta a volte e solette sostenute da contrafforti che, come risultadalla loro stessa definizione, si suddividono in:

- dighe a volte multiple, in cui le singole volte (a generatrici talora verticali, ma più spessoinclinate) si impostano su di una serie di contrafforti, che a loro volta resistonoper gravità;

- dighe a lastroni, analoghe al tipo precedente, salvo la sostituzione delle volte con superfici piane, che lavorano come lastre appoggiate su due speroni contigui.

Sbarramenti murari

diga a speroni (diga di Vinchiana)

Concrete Dams

diga a speroni (diga di Vinchiana)

Concrete Dams

Concrete Dams

Buttress Dams, advantages:

buttress dams require less concrete than gravity dams.

Uplift/ice pressure is generally not a major factor

can be constructed on relatively weaker foundations.

Power house and water treatment plants, etc. can be housed between buttresses.

vertical component of the water pressure on deck prevents the dam against overturning and sliding failures.

can be designed to accommodate moderate movements of foundations without serious damages.

Heat dissipation is better in buttress dams.

back of the deck and the foundation between buttresses are accessible for inspection.

can be easily raised subsequently by extending buttresses and deck

Concrete Dams

Buttress Dams, disvantages:

buttress dams require costlier formwork, reinforcement and more skilled labour. Consequently, the overall cost of construction may be more than that of a gravity dam.

Buttress dams are more susceptible to damage and sabotage.

Buttress dams cannot be constructed in very cold climates because of spalling of concrete.

Because the upstream deck slab is thin, its deterioration may have very serious effect on the stability.

Enbankment damsRockfill and Earth dams

Enbankment dams, an earth dam is made of earth (or soil) and/or stones and resists the forces exerted upon it mainly due to shear strength of the soil.

are usually built in wide valleys having flat slopes at flanks (abutments).

can be homogeneous when the height of the dam is not great.

are of zoned sections, with an impervious zone (called core) in the middle and relatively pervious zones (called shells or shoulders) enclosing the impervious zone on both sides. Nowadays majority of dams constructed are of this type.

Enbankment damsRockfill and Earth dams

Masonry dams/dighe in muratura a secco, made with stones of differentsize, placed and displaced by hand;

Enbankment dams are made mainly from natural materials, can besubdivided in two classes:

earthfill dams/dighe in terra, consisting of mixtures of natural soils, incase added special materials (bentonite clay, sand etc.) can be added,emplaced and compacted with special treatments and methods, and

rockfill dams/dighe in pietrame alla rinfusa o a scogliera, builtfrom stone in bulk and then compacted;

All three of these types of barrier present special problems both hydraulic and static,so that, while always being the overall effect of the earth weight to determine thestability conditions, their dimensioning has to be done by following special safetycriteria.

Enbankment damsRockfill and Earth dams

dighe in muratura a secco, costituite da pietrame di varia pezzatura disposto e sistemato a mano;

dighe in pietrame alla rinfusa o a scogliera, costituite da pietrame gettato alla rinfusa e successivamente costipato;

dighe in terra, formate da miscele di terre naturali, eventualmente addizionate di materiali speciali (argille bentonitiche,sabbie ecc. ), poste in opera e costipate con particolari cure e modalità.

Tutti e tre questi tipi di sbarramento presentano particolari problemi sia idraulici che statici, così che, pur essendo sempre l’effetto globale del peso proprioa determinarne le condizioni di stabilità, il loro dimensionamento deve essere fatto seguendo particolari criteri di sicurezza.

Enbankment dams

Enbankment Dams are made of earth (or soil) and/or stones and resists the forces exerted upon it mainly due to shear strength of the soil.

Embankment dams can be classified in broad terms as being earthfill or rockfill dams:

homogeneous earthfill dams: An embankment may be categorized as an earthfill

dam if compacted soils account for over 50% of the placed volume of material. An earthfilldam is constructed primarily of selected engineering soils compacted uniformly andintensively in relatively thin layers and at a controlled moisture content. Outline sections ofsome common variants of the earthfill embankment are illustrated in Fig. 1.2.

rockfill dams, permeable earth or stones, zoned, with false core sealing. The designation

‘rockfill embankment’ is appropriate where over 50% of the fill material may be classified asrockfill, i.e. coarse-grained frictional material

The division between the two embankment variants is not absolute, many dams utilizing fill materials of both types within appropriately designated internal zones. The conceptual relationship between earthfill and rockfill materials as employed in embankment dams is illustrated in Figure (from Novack, IV edition):

Enbankment dams

Enbankment Dams

are usually built in wide valleys having flat slopes at flanks (abutments).

can be homogeneous when the height of the dam is not great.

can be of zoned sections, with an impervious zone (called core) in the middle and relatively pervious zones (called shells or shoulders) enclosing the impervious zone on both sides. Nowadays majority of dams constructed are of this type.

Enbankment damsEnbankment Dams

a cross-section (or slice) through an embankment dam shows that it is shaped like a bank, or hill. Most embankment dams have a central section, called the core, made from an impermeable material to stop water passing through the dam. Clayey soils, concrete or asphaltic concrete can be used for the core.

Rockfill dams are permeable. They can have a core or an impermeable cover on the upstream face. Materials used for the cover include reinforced concrete and asphaltic concrete.

Enbankment dams Earth Dams

Earth dams are constructed with soil or earth. In earthed dams resistance is given mainly by shear strength of soil in addition to self weigth.

Preferred wherein the valley is very wide and other types of dams are not suitable

Can be built in any kind of foundations. They can be built on hard rock or softer soils, as they do not exert too much pressure on their foundations.

Are usually cheaper than gravity dams if suitable earth or soil in abundabt quantity is easily available near the site.

Enbankment dams-Earth dams

diga in terra (diga di Maria al Lago), planimetria e sezione tipo

Enbankment dams-Earth dams

diga di Lago Verde (Val d’Ultimo)

Enbankment dams-upstream face steel shield

Enbankment dams-upstream face steel shield

diga di Zoccolo (val d’Ultimo)

Enbankment damsupstream face asphaltic concrete

Enbankment damsupstream face asphaltic concrete

diga di Conza della Campania

Enbankment damsCentral core

Enbankment dams

Earth Dams advantages

homogeneous are usually cheaper than gravity dams if suitable earth for construction is available near the site.

can be constructed on almost all types of foundations, provided suitable measures of foundation treatment and seepage control are taken.

can be constructed in a relatively short period.

skilled labour is not required in construction of an earth dam.

can be raised subsequently.

are aesthetically more pleasing than gravity dams.

are more earthquake-resistant than gravity dams.

Enbankment dams

Earth Dams disvantages

are not suitable for narrow gorges with steep slopes.

cannot be designed as an overflow section. A spillway has to be located away from the dam.

cannot be constructed in regions with heavy downpour, as the slopes might be washed away.

maintenance cost of an earth dam is quite high. It requires constant supervision.

sluices cannot be provided in a high earth dam to remove slit.

fails suddenly without any sign of imminent failure. A sudden failure causes havoc and untold miseries..

Enbankment dams-Rockfill dams

Rockfill Dams

a rockfill dam is built of rock fragments and boulders of large size.

An impervious membrane (cement concrete or asphaltic concrete or earth core) is placed on the rockfill on the upstream side to reduce the seepage through the dam.

A dry rubble cushion is placed between the rockfill and the membrane for the distribution of water load and for providing a support to the membrane.

side slopes of rockfill are usually kept equal to the angle of repose of rock (1.4:1 or 1.3:1).

Rockfill dams are quite economical when a large quantity of rock is easily available near the site.

Enbankment dams-Rockfill dams

Rockfill dams

Rockfill Dams advantages

rockfill dams have almost the same advantages and disadvantages over gravity dams as discussed for earth dams. Particular advantages and disadvantages over earth dams.

are quite inexpensive if rock fragments are easily available.

can be constructed quite rapidly.

can better withstand the shocks due to earthquake than earth dams.

can be constructed even in adverse climates

Rockfill dams

Rockfill Dams disvantages

rockfill dams require more strong foundations than earth dams.

Rockfill dams require heavy machines for transporting, dumping and compacting rocks

Site selection for a dam

Rockfill Dams disvantages

A dam is a huge structure requiring a lot of funds.

Extreme care shall be taken while selecting the site of a dam.

A wrong decision may lead to excessive cost and difficulties in construction and maintenance.

Various factors should be considered when selecting the site of a dam.

Site selection for a dam

Topography

Suitable Foundation

Good Site for reservoir –(i) Large storage capacity

(ii) Shape of reservoir basin

(iii) Watertightness of the reservoir

(iv) Good hydrological conditions

(v) Deep reservoir

(vi) Small submerged area

(vii) Low silt inflow

(viii) No objectionable minerals

Spillway site

Availability of materials

Accessibility

Healthy surroundings

Minimum overall cost

Other considerations

Selection of type of dam

selection of the most suitable type of dam for a particular site requires a lot of judgment and experience.

It is only in exceptional cases that the most suitable type is obvious.

Preliminary designs and estimates are usually required for several types of dams before making the final selection on economic basis.

The salient features of different types of dams discussed in the preceding sections should be kept in mind while selecting the type of dam.

Various factors govern the selection of type of dam

Selection of type of dam

Topography and valley shape

Geology and foundation conditions

Availability of construction materials

Overall cost

Spillway size and location

Earthquake hazards

Climatic conditions

Diversion problems

Environmental considerations

Roadway

Length and height of dam

Life of dam

Miscellaneous considerations

Selection of type of dam

The optimum type of dam for a specific site is determined by estimates of cost and construction programme for all design solutions which are technically valid. Where site circumstances are such that viable alternatives exist it is important that options are kept open, assessing the implications of each with respect to resources, programme and cost, until a preferred solution is apparent. It may also be necessary to take account of less tangible socio-political and environmental considerations in the determination of that solution.

FOUR CONSIDERATIONS OF CARDINAL IMPORTANCE ARE DETAILED BELOW.

1. Hydraulic gradient: the nominal value of hydraulic gradient, i, for seepage under, around or through a dam varies by at least one order of magnitude according to type.

2. Foundation stress: nominal stresses transmitted to the foundation vary greatly with dam type.

3. Foundation deformability: certain types of dams are better able to accommodate appreciable foundation deformation and/or settlement without serious damage.

4. Foundation excavation: economic considerations dictate that the excavation volume and foundation preparation should be minimized

Selection of type of dam

Novak (Ivth edition)

Coffer dams

source: “ ELEMENTS OF CIVIL ENGINEERING AND ENGINEERING MECHANICS” Di R. V. RAIKAR