26

Loading

27

28

Primary loads 1.   Water load. This is a hydrostatic distribution of pressure with horizontal

resultant force P1. (Note that a vertical component of load will also exist

in the case of an upstream face batter, and that equivalent tailwater loads

may operate on the downstream face.)

2.   Self-weight load. This is determined with respect to an appropriate unit

weight for the material. For simple elastic analysis the resultant, P2, is

considered to operate through the centroid of the section.

3.   Seepage loads. Equilibrium seepage patterns will develop within and

under a dam, e.g. in pores and discontinuities, with resultant vertical

loads identified as internal and external uplift, P3 and P4, respectively.

(Note that the seepage process will generate porewater pressures in

pervious materials, and is considered in this light as a derivative of the

water load for the embankment dam)

29

Secondary loads 1.   Sediment load. Accumulated silt etc. generates a horizontal thrust,

considered as an equivalent additional hydrostatic load with horizontal

resultant P5.

2.   Hydrodynamic wave load. This is a transient and random local load,

generated by wave action against the dam (not normally significant).

3.   Ice load. Ice thrust from thermal effects and wind drag, may develop in

more extreme climatic conditions (not normally significant).

4.   Thermal load (concrete dams). This is an internal load generated by

temperature differentials associated with changes in ambient conditions

and with cement hydration and cooling.

5.   Interactive effects. These are internal, arising from differential

deformations of dam and foundation attributable to local variations in

foundation stiffness and other factors, e.g. tectonic

6.   Abutment hydrostatic load. This is an internal seepage load in the

abutment rock mass (It is of particular concern to arch or cupola dams.)

30

Exceptional loads 1.   Seismic load. Oscillatory horizontal and vertical inertia loads are

generated with respect to the dam and the retained water by seismic

disturbance. For the dam they are shown symbolically to act through the

section centroid. For the water inertia forces the simplified equivalent

static thrust, P8 (slide 27), is shown

2.   Tectonic effects. Saturation, or disturbance following deep excavation in

rock, may generate loading as a result of slow tectonic movements.

31

Predimensioning of a gravity dam

h S

P

Sp

S = 12!wh

2

h/3

P = 12!dbh Sp =

12µ!whb

Water load

Self-weight load Uplift load

b/3

32

To calculate the unknown width b impose that the section of the foundation, supposedly rectangular with unit deep, is subject only to compression, it follows that the reaction of the soil must be applied within the middle third; as limit condition it can passes in the limit of the core. central core of inertia, is defined as the area in which the load must fall for the homogeneous type of stress, for which the section is the whole or any compression or traction

b

b/3 central core of inertia

middle third

33

Equilibrium of moments

S h3+ SP

b3!P b

3= 0

16!wh

3 !16!db

2h+ 16µ!whb

2

b2 (µ!w !!d )h = !!wh3

b = h !w!d !µ!w

Pre-dimensioning for gravity dams with vertical upstream face (h

34

By comparison with other possible profiles you realize that the triangle minimizes the volume.

35

Sediment load The gradual accumulation of significant deposits of fine

sediment, notably silts, against the face of the dam generates a resultant

horizontal force, Ps. The magnitude of Ps, which is additional to water load

Pwh, is a function of the sediment depth, h3, the submerged unit weight !s

and the active lateral pressure coefficient, Ka,

PS = Ka12! sh3

2

It is active at h/3

Ka =1! sin!s1+ sin!s

where "s is the angle of shearing resistance of the sediment

Values of "s=18–20 kNm-3 and "s=30° are representative.

Accumulated depth h3 is a complex time-dependent function of suspended

sediment concentration, reservoir characteristics, river hydrograph and other

factors .

36

Hydrodynamic wave load The transient hydrodynamic thrust generated by

wave action against the face of the dam, Pwave, is considered only in

exceptional cases. It is of relatively small magnitude and, by its nature,

random and local in its influence. An empirical allowance for wave load may

be made by adjusting the static reservoir level used in determining Pwh.

Where a specific value for Pwave is necessary a conservative estimate of

additional hydrostatic load at the reservoir surface is provided by

Pwave = 2!wHs2

Hs is the significant wave height, i.e. the mean height of the highest third

of waves in a sample, and is reflected at double amplitude on striking a

vertical face

37

As a basis for a wave height computation, H (m) (crest to trough) can be

estimated from: H = 0.34F1 2 + 0, 76! 0.26F1 4

F (km) is the fetch (the maximum free distance which wind can travel

over the reservoir). For large values of fetch (F > 20 km) the last two

terms may be neglected.

Using the concept of significant wave height, Hs (the mean height of the

highest third of the waves in a train with about 14% of waves higher than Hs)

the use for the design wave height, Hd, is recommended.

Hd is a multiple of Hs ranging from 0.75Hs for concrete dams to 1.3Hs for

earth dams with a grassed crest and downstream slope and 1.67Hs for dams

with no water carryover permitted. Hs (m) can be determined quickly from

figure as a function of the wind velocity (m/s) and fetch (m) based on the

simplified Donelan/JONSWAP equation:

H =UF1 2

1760

38

39

Il Regolamento Italiano Dighe (R.I.D.) [D.P.R. n. 1363 del 1/11/59

(parte I) e D.M.LL.PP del 24/3/82] stabilisce precise norme per la

progettazione e la verifica statica di ogni tipologia di dighe.

In particolare, per le dighe a gravità ordinaria essa prescrive due

verifiche di sicurezza:

a)  Verifica a scorrimento

b)  Verifica di resistenza

Da effettuare sia in condizioni di lago vuoto, che di lago pieno alla

quota di massimo invaso, tenendo conto di tutte le forze che

agiscono sulla struttura.

40

La verifica di resistenza deve essere effettuata per le seguenti

condizioni di carico:

1) A serbatoio vuoto, considerando le azioni del peso proprio ed

eventualmente le azioni sismiche;

2) A serbatoio pieno, considerando le azioni del peso proprio, la spinta

idrostatica, spinta del ghiaccio ed eventualmente le azioni sismiche.

La verifica allo scorrimento deve essere effettuata per la condizione di lago pieno.

41

Weirs Barrages

fixed Earth

Brickwork Concrete

mobile

Plain Radial Drum Roller Flap

Dams

embankment

Earthfills rockfills

Concrete

gravity Gravity butress

Arch

Arch Arch-gravuty

Cupola

42

Weirs and barrages are relatively low-level dams constructed across

a river to raise the river level sufficiently or to divert the flow in full, or

in part, into a supply canal or conduit for the purposes of irrigation,

power generation, navigation, flood control, domestic and industrial

uses, etc. These diversion structures usually provide a small storage

capacity. In general, weirs (with or without gates) are bulkier than

barrages, whereas barrages are always gate controlled. Barrages

generally include canal regulators, low-level sluices to maintain a

proper approach flow to the regulators, silt excluder tunnels to control

silt entry into the canal and fish ladders for migratory fish movements.

43

44

45

The fixed barrages can be constructed in masonry, concrete, clay and

many other materials (stone, wood) and they have a suitable profile to be

overflowed by excess capacities and are often equipped with gates to

evacuate gravel

The movable barrages consist normally of a work fixed in masonry or

concrete (threshold and driving piles) and real moving parts (gates) that

can be of various types.

46

Plain gates

Le paratoie piane sono costituite da pareti piane in legno o in acc ia io, scorrevol i in gu ide (gargami) con esse complanari; quelle in legno sono adatte per luci con larghezza massima di 3 metri e altezza di ritenuta di 2 o 3 metri, q u e l l e i n a c c i a i o p o s s o n o raggiungere valori più elevati (luce di 20 metri e ritenuta di 10 metri) perché vengono rinforzate con travi in profilati di acciaio che sopportano la spinta idrostatica.

Le tenute sul fondo e sui lati vengono realizzate per mezzo di travi con guarnizioni di gomma. Il problema delle paratoie piane, che ne limita le dimensioni, è quello del sollevamento a causa dei notevoli attriti in gioco.

47

Sector gates

Le paratoie a settore hanno la forma di un settore cilindrico girevole attorno ad un perno coincidente con l’asse del cilindro; esse sono costituite da una robusta ossatura a traliccio rivestita da una lamiera metallica; la tenuta sulla soglia e sulle pareti laterali è realizzata con strisce di gomma.

The advantages of radial over vertical lift gates are smaller hoist, higher stiffness, lower (but longer) piers, absence of gate slots, easier automation and better winter performance.

48

Roller gates

Le paratoie cilindriche sono indicate per luci fino a 40 metri ma con piccoli battenti. Sono costituite da un cilindro in lamiera di acciaio opportunamente irrigidito da appositi profilati. Il cilindro è disposto orizzontalmente ed il moto di sollevamento avviene per rotazione su una apposita cremagliera. Per aumentare l’altezza di ritenuta, che normalmente è pari al diametro del cilindro, si può dotare la paratoia di un becco inferiore e di uno scudo. La tenuta è realizzata con una trave di legno sulla generatrice di appoggio inferiore e con lamierino o gomma sui fianchi.

49

Flap gates

Le paratoie a ventola sono costituite da una

struttura piana in ferro, ricoperta di lamiera,

girevole intorno ad un asse orizzontale

coincidente con il bordo inferiore; esse si

prestano bene per luci fino a 15 metri e

altezza di ritenuta non superiore a 5 metri.

Le paratoie a ventola e a settore si prestano

assai bene al comando automatico.

Flap gates provide fine level regulation, easy flushing of debris and ice, and

are cost effective and often environmentally more acceptable than other

types of gates; they require protection against freezing and are particularly

sensitive to aeration demand and vibration

50

Reservoir storage zone and uses of reservoir

51

Minimum Drawdown Level (MDDL): It is the level below which the reservoir will not be drawn down so as to maintain a minimum head required in power projects. Dead Storage Level (DSL): Below the level, there are no outlets to drain the water in the reservoir by gravity. Maximum Water Level (MWL): This is the water level that is ever likely to be attained during the passage of the design flood. It depends upon the specified initial reservoir level and the spillway gate operation rule. This level is also called sometimes as the Highest Reservoir Level or the Highest Flood Level.

Full Reservoir Level (FRL): It is the level corresponding to the storage which includes both inactive and active storages and also the flood storage, if provided for. In fact, this is the highest reservoir level that can be maintained without spillway discharge or without passing water downstream through sluice ways.

52

Dead storage: It is the total storage below the invert level of the lowest discharge outlet from the reservoir. It may be available to contain sedimentation, provided the sediment does not adversely affect the lowest discharge. Outlet Surcharge or Flood storage: This is required as a reserve between Full Reservoir Level and the Maximum Water level to contain the peaks of floods that might occur when there is insufficient storage capacity for them below Full Reservoir Level.

Live storage: This is the storage available for the intended purpose between Full Supply Level and the Invert Level of the lowest discharge outlet. The Full Supply Level is normally that level above which over spill to waste would take place. The minimum operating level must be suf f i c ient ly above the lowest discharge outlet to avoid vortex formation and air entrainment. This may also be termed as the volume of water actually available at any time between the Dead Storage Level and the lower of the actual water level and Full Reservoir Level.