Headworks

Any hydraulic
A h d li structure
t t which
hi h supplies
li water
t tto th
the off-taking
ff t ki
canal

Diversion headwork
Headwork
Storage headwork

Diversion headwork: To divert the required supply to canal from the river.

Storage headwork: Comprises the construction of a dam on the river (It

stores water during
g the p
period of excess supplies
pp and releases it when
demand overtakes available supplies).
 Diversion headworks

•A structure constructed across a river for the purpose of

raising water level in the river so that it can be diverted into
the off-taking canals.
•These are generally constructed on the perennial rivers
which have adequate flow throughout the year and,
therefore, there is no necessity of creating a storage
reservoir.

Minipe Anicut
Functions of a Diversion Headworks:
- R i
Raises th
the water
t llevell on itits upstream
t side
id
- Regulates the supply of water into canals
- Controls the entry of silt into canals
- Creates a small pond (not reservoir) on its upstream
and provides
pro ides some pondage

Two types:
• Weirs
• Barrages
 Weir
•A weir is a raised concrete crest wall constructed across the river.
•It may be provided with small shutters (gates) on its top.
•In the case of weir, most of the raising of water level or ponding
is done by the solid weir wall and little with by the shutters.
 p p
•A weir maintains a constant pond level on its upstream side 
so that the water can flow into the canals with the full 
supply level.
•In the case of a weir, the crest shutters are dropped during 
floods so that the water can pass over the crest.
•During the dry period, these shutters are raised to store 
water up to the pond level.
 Weir – Advantages and Disadvantages

Advantages:
-The initial cost is usually low
- Simple
Si l structure
t t

Disadvantages:
‐ There is a large afflux during floods which causes large
submergence
- Because
B the
th crestt is
i att high
hi h level,
l l there
th is
i greatt silting
ilti
problem
- The raising and lowering of shutters on the crest is not
convenient
- It requires considerable time and labour
- The weir lacks an effective control on the river during floods
- A roadway can not be conveniently provided over the weir
 Types of Weirs

Vertical drop weirs

Rockfill weirs
Concrete glacis
or sloping weirs
 Barrage
• A barrage has a low crest wall with high gates
• As the height of the crest above the river bed is low most of
th ponding
the di is i d
done b
by gates.
t
• During the floods the gates are opened, so afflux is very small
• The control of pondage and flood discharge is achieved with
the help of gates which are mechanically operated.
 Barrage – Advantages and Disadvantages
Advantages:
- Barrage has a good control on the river during floods. Outflow
can be easily regulated by gates.
- Afflux during floods is small and, therefore, the submerged area
is less.
- Good control over silt entry into the canal
canal.
- Good control over flow conditions, shoal formations and cross-
currents on the upstream of the barrage.
- A roadway can be conveniently provided over the structure at a
little additional cost.

Disadvantages:
-The initial cost of Barrage is very high.
Barrage
Polgalla Barrage
Gated barrage ‐
g concrete
Length = 145 m;  Height = 14.6 m;  Storage = 4.1 MCM,    Gate height = 6.4 m, width = 12.2 
m
 Comparison between weirs and Barrages 

Weir Barrage
High set crest Low set crest
Ponding is done against raised crest or 
is done against raised crest or Ponding is done by means of gates.
is done by means of gates
partly against the crest and partly by 
means of shutters.
No control of river in low floods. Perfect control of river.
Shutters are relatively smaller height,  Gates are of greater height
(~2m)
Shutters are dropped to pass floods
Shutters are dropped to pass floods Gates are raised to pass floods
Gates are raised to pass
Lack of speed and involve labour and time  Gates convenient to operate
to operate the shutters
Excessive afflux in high floods
afflux in high floods High floods can be passed with minimum afflux
High floods can be passed with minimum afflux
Not possible to provide road bridge Roads can be constructed across the river as the 
barrage is at low crest
Relatively cheaper structure
Relatively cheaper structure Costly structure
Costly structure
Raised crest causes silting upstream Less siltation u/s due to low set crest. 
Silt removal is done through under sluices.
 Storage Headworks; Dams
Dam  A hydraulic structure constructed across a river to
store water on its upstream side to create a reservoir for
impounding water for various purposes
 It can be an impervious or fairly impervious barrier
Storage Dams: These dams are constructed to impound water to
its upstream during the periods of excess supply in
the river and is used in periods of deficient supply.
Multi-purpose projects: Irrigation, Hydro-power generation
Water supply
Flood control
Recreation
Fish and wild life

Diversion Dams: Raises the water level slightly in the river and thus,
provides
id h headd ffor di
diverting
ti water
t iinto
t canals
l or other
th
conveyance systems to the place of use
(~ smaller heights/ no reservoir).

Detention Dams: Used to store water during floods and release

gradually
g y at a safe rate,, when the flood recedes.
- to retard flood runoff & minimize the effect of sudden
floods.
 Type of Dams

 Masonry
M D
Dams
 Concrete Dams
 Embankment Dams
 Timber Dams
 Steel Dams

Concrete Dams: Embankment Dams:

 Gravity dams  Earth
E th dams
d
 Buttress dams  Rockfill dams
 Arch dams
- Single arch
- Double arch
 GRAVITY DAMS
• A gravity dam resists the water pressure and other forces
due to its weight (or gravitational forces).
• Usuallyy made of concrete and straight
g in p
plan.

Sediment
load

Moussakele Dam
Concrete gravity dam
Length = 187.5 m;
Height = 41.2 m;
Storage = 115 MCM
GRAVITY DAMS
Advantages
• Strong, stable and durable.
• 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 are available.
• Are well adapted
p for use as an overflow spillway
p y section. Earth
dams cannot be used as an overflow section. Even in earth
dams,, the overflow section is usuallyy a gravity
g y dam.
• Are specially suited to areas with very heavy rainfall. The slopes of
the earth dams might be washed away in such an area.
Advantages (Cont….)

• Low maintenance cost.

• Does not fail suddenly.

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.
GRAVITY DAMS
Disadvantages
• 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
dams,
especially when mechanised plants for batching, mixing and
transporting concrete are not available.

• Require more skilled labour than that in earth dams.

• Subsequent
S b t raising
i i iis nott possible.
ibl
ARCH DAMS
• An arch dam is curved in plan, with its convexity towards the
upstream side.
• T
Transfer
f the
th water
t pressure and
d other
th forces
f mainly
i l to
t the
th
abutments by arch action.
• Are 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.
• May have a single curvature or double curvature in the vertical
plane.
• Are subjected to large stresses because of changes in temperature
shrinkage of concrete and yielding of abutments.
ARCH DAMS
Ad
Advantages
t
 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
i ht ddue tto uplift
lift d
does nott affect
ff t the
th stability.
t bilit
 An arch dam can be constructed on a relatively less strong
foundation because a small part of load is transferred to base
base,
whereas in a gravity dam full load is transferred to base.
ARCH DAMS
Di d
Disadvantages
t
 An arch dam requires good rock in the flanks (abutments) to resist
the thrust. If the abutments yyield,, extra stresses develop
p which may
y
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
lli off concrete
t occurs due
d tot alternate
lt t freezing
f i and d thawing.
th i
 The arch dams are more prone to sabotage.
 The speed of construction is relatively slow.
 BUTTRESS DAMS

 Buttresses are triangular concrete walls which transmit the water

pressure from the deck slab to the foundation.
Buttress dams are of three types:
(i) Deck
D k ttype
(ii) Multiple arch-type
(iii)Massive head type
(iii)Massive-head
Flat-slab buttress dams (Deck Type)

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

BUTTRESS DAMS- More…
 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
massive head type buttress dam
dam, there is no deck slab
slab.
 Instead of the deck, the upstream edges of the buttresses are
flared to form massive heads which span the distance between
the buttresses.
Multiple arch-type
Massive-head type
BUTTRESS DAMS
Advantages
g
 Buttress dams require less concrete than gravity dams
 Uplift 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
g overturning
g and sliding
g 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 slabs
BUTTRESS DAMS
Disadvantages
• 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.