1. SUBJECT: IRRIGATION AND
HYDRAULIC STRUCTURES
YEAR & SEM : 4-1
COURSE OBJECTIVES:
1.To impart the knowledge of various irrigation techniques ,
requirements of the crops
2.To learn about distribution systems for canal irrigation, design of
unlined and lined irrigation canals design sediment problems
associated with canals.

2. UNIT 1
• RESERVOIRS AND ITS TYPES
• CONTENTS:
• .RESERVOIRS AND ITS CLASSIFICATION
• .FUNCTIONS OF RESERVOIRS
• SITE INVESTIGATIONS
• SITE SELECTION
• ZONES OF STORAGE
• ESTIMATION OF RESERVOIR YIELD
• RESERVOIR SEDIMENTATION
• DAMS& ITS TYPES
• FACTORS AFFECTING DAMS

3. What is a Reservoir?
 A reservoir is most commonly an enlarged natural or artificial
lake, pond or impoundment created using a dam or lock to store
water.
Tarbela Dam JungHua Dam (Taiwan)

4. Storage reservoir serve the following purpose:
 Irrigation
 Water supply
 Hydroelectric power generation
 Flood control
 Navigation
 Recreation
 Development of fish & wild life
 Soil conservation

5. Classification of Reservoirs:
• 1.Impoundment reservoirs:
• 2.Off stream reservoirs:

6. 1.Impoundement Reservoirs:
i.Storage Reservoirs:Storage reservoirs are also called conservation
reservoirs because they are used to conserve water. Storage reservoirs are
constructed to store the water in the rainy season and to release it later
when the river flow is low.
ii.Flood Control Reservoirs: A flood control reservoir is constructed for the
purpose of flood control. It protects the areas lying on its downstream side
from the damages due to flood.

7. iii.Retarding Reservoirs: A retarding reservoir is provided with spillways
and sluiceways which are ungated. The retarding reservoir stores a
portion of the flood when the flood is rising and releases it later when
the flood is receding.
iv.Detention Reservoirs : A detention reservoir stores excess water during
floods and releases it after the flood. It is similar to a storage reservoir but
is provided with large gated spillways and sluiceways to permit flexibility
of operation.

8. v.Distribution Reservoirs: A distribution reservoir is a small storage reservoir
to tide over the peak demand of water for municipal water supply or
irrigation. The distribution reservoir is helpful in permitting the pumps to
work at a uniform rate. It stores water during the period of lean demand
and supplies the same during the period of high demand.
vi.Multipurpose Reservoirs: The are constructed for more than single
purpose.
vii.Balancing Reservoirs: A balancing reservoir is a small reservoir
constructed d/s of the main reservoir for holding water released from the
dam.

9. 2.Off stream reservoirs:
• These are reservoirs not on a river course, but constructed to store the water
which is extracted and carried out from the reservoir through pipes.
• These are formed by partially or completely enclosed waterproof banks.

10. Site Investigations for reservoir planning:
 Engineering surveys
 Geological investigations
 Hydrological investigations

11. Engineering surveys:
 Conducted for dam, reservoir and other associated work.
 Topographic survey of the area is carried out and the contour plan is
prepared
 The horizontal control is usually provided by triangulation survey, and
the vertical control by precise levelling.
 To prepare area elevation curves which helps us to calculate storage
volume of reservoir.

12. Geological investigations:
 Geological investigations of the dam and reservoir site are done for
the following purposes.
(i) Suitability of foundation for the dam.
(ii) Watertightness of the reservoir basin
(iii)Location of the quarry sites for the construction materials
required for the dam construction.
(iv)Groundwater conditions in the region.
(v)Location of permeable and soluble rocks, if any.

13. Hydrological investigations:
 The hydrological investigations are conducted for the
following purposes :
(i)To study the runoff pattern at the proposed dam site, to determine
the storage capacity to a given demand.
(ii)To determine the maximum discharge at the site.
(iii)Determination of the hydrograph of the worst flood, to
determine the spillway capacity and design.

14. Site selection for a reservoir:
 Large storage capacity
 River valley should be narrow, length of dam to constructed is less.
 Watertightness of reservoir.
 Good hydrological conditions
 Deep reservoir
 Small submerged area
 Low silt inflow
 No objectionable minerals
 Low cost of real estate
 Site easily accessible

15. Zones of storage

16.  Full reservoir level (FRL): The full reservoir level (FRL) is the highest
water level to which the water surface will rise during normal operating
conditions.
 Maximum water level (MWL): The maximum water level is the
maximum level to which the water surface will rise when the design flood
passes over the spillway.
 Minimum pool level: The minimum pool level is the lowest level up to
which the water is withdrawn from the reservoir under ordinary
conditions.
 Dead storage: The volume of water held below the minimum pool level
is called the dead storage. It is provided to cater for the sediment
deposition by the impounding sediment laid in water. Normally it is
equivalent to volume of sediment expected to be deposited in the
reservoir during the design life reservoir.

17.  Live/useful storage: The volume of water stored between the full reservoir
level (FRL) and the minimum pool level is called the useful storage. It
assures the supply of water for specific period to meet the demand.
 Bank storage: is developed in the voids of soil cover in the reservoir area
and becomes available as seepage of water when water levels drops down.
It increases the reservoir capacity over and above that given by elevation
storage curves.
 Valley storage: The volume of water held by the natural river channel in
its valley up to the top of its banks before the construction of a reservoir is
called the valley storage. The valley storage depends upon the cross
section of the river.
 Flood/Surcharge storage: is storage contained between maximum
reservoir level and full reservoir levels. It varies with spillway capacity of
dam for given design flood.

18. Dam illustration

19. Storage capacity and yield
 Specific Yield is the volume of water which can be withdrawn from a
reservoir in a specified period of time.
 Safe yield or firm yield is the maximum quantity of water
which can be supplied from a reservoir in a specified period of
time during a critical dry year.

20.  Secondary yield: is the quantity of water which is available during the
period of high flow in the rivers when the yield is more than the safe yield.
 Average yield:The average yield is the arithmetic average of the firm yield
and secondary yield over a long period of time.
 Design yield: The design yield is the yield adopted in the design of a
reservoir. The design yield is usually fixed after considering the urgency
of the water needs and the amount of risk involved.

21. Reservoir capacity
 depends upon the inflow available and demand
 inflow in the river is always greater than the demand, there is no
storage required
 if the inflow in the river is small but the demand is high, a large reservoir
capacity is required

22.  The required capacity for a reservoir can be determined by
the following methods:
1. Graphical method, using mass curves.
2.Analytical method

23. Graphical method
1. Prepare a mass inflow curve from the flow hydrograph of the site for a
number of consecutive years including the most critical years (or the
driest years) when the discharge is low.
2. Prepare the mass demand curve corresponding to the given rate of
demand. If the rate of demand is constant, the mass demand curve is a
straight line. The scale of the mass demand curve should be the same as
that of the mass inflow curve.

24. 3. Draw the lines AB, FG, etc. such that
(i)They are parallel to the mass demand curve, and
(ii)They are tangential to the crests A, F, etc. of the mass curve.
4. Determine the vertical intercepts CD. HJ, etc. between the tangential lines
and the mass inflow curve. These intercepts indicate the volumes by
which the inflow volumes fall short of demand.
Assuming that the reservoir is full at point A, the inflow volume during the
period AE is equal to ordinate DE and the demand is equal to ordinate CE.
Thus the storage required is equal to the volume indicated by the intercept
CD.
5. Determine the largest of the vertical intercepts found in Step (4). The
largest vertical intercept represents the storage capacity required.

26.  The following points should be noted.
(i)The capacity obtained in the net storage capacity which must be
available to meet the demand. The gross capacity of the reservoir will be
more than the net storage capacity. It is obtained by adding the
evaporation and seepage losses to the net storage capacity.
(ii)The tangential lines AB, FG; etc. when extended forward must
intersect the curve. This is necessary for the reservoir to become full
again, If these lines do not intersect the mass curve, the reservoir will
not be filled again. However, very large reservoirs sometimes do not get
refilled every year. In that case, they may become full after 2-3 years.
(iii).The vertical distance such as FL between the successive tangents
represents the volume of water spilled over the spillway of the dam.

27. Analytical method
• capacity of the reservoir is determined from the net inflow and
demand.
• storage is required when the demand exceeds the net inflow.
• the total storage required is equal to the sum of the storage required
during the various periods.

28. 1.Collect the stream flow data at the reservoir site during the critical dry
period. Generally, the monthly inflow rates are required. However, for
very large reservoirs, the annual inflow rates may be used.
2.Ascertain the discharge to be released downstream to satisfy water
rights or to honour the agreement between the states or the cities.
3.Determine the direct precipitation volume falling on the
reservoir during the month.
4.Estimate the evaporation losses which would occur from the reservoir
The pan evaporation data are normally used for the estimation of
evaporation losses during the month.

29. 5.Ascertain the demand during various months.
6.Determine the adjusted inflow during different months as follows:
Adjusted inflow = Stream inflow + Precipitation - Evaporation –
Downstream Discharge
7.Compute the storage capacity for each months.
Storage required = Adjusted inflow – Demand
8.Determine the total storage capacity of the reservoir by adding the
storages required found in Step 7.

30. Determination of Yield of a Reservoir
 The yield from a reservoir of a given capacity can be determined by the
use of the mass inflow curve.
1.Prepare the mass inflow curve from the flow hydrograph of the river.
2.Draw tangents AB, FG, etc. at the crests A, F, etc. of the mass inflow
curve in such a way that the maximum departure (intercept) of these
tangents from the mass inflow curve is equal to the given reservoir
capacity.

31. 3.Measure the slopes of all the tangents drawn in Step 2.
4.Determine the slope of the flattest tangent.
5.Draw the mass demand curve from the slope of the flattest tangent (see
insect). The yield is equal to the slope of this line

33. Reservoir Sedimentation
• All the rivers carry certain amount of silt eroded from the catchment area during heavy rains.
The extent of erosion, and hence the silt load in the stream depends upon the following factors
i.e.,
• Causes of sedimentation
• Nature of the catchment area
• Topography of the catchment area
• Vegetation cover
• Intensity of rainfall
• Cultivation in catchment area

34. • 1.Channel erosion 2.sheet erosion

35. Sediment transport in rivers is classified into two heads
1.bed load: material that moves along the bottom of the channel (by
saltation and rolling) as a result of shear stress created by vertical velocity
gradients in the stream flow.
2.Suspended bed material load: The suspended load is kept in
suspension because of the vertical component of the eddies formed due to
friction of flowing water against the bed.
Generally bed load is much smaller i.e , 10 to 15% of the suspended load.
when the stream approaches the reservoir, the velocity is very much
reduced. Due to this coarser particles settle and finer particles are kept in
suspension.

36. Sedimentation process

37. Density currents:
• Density current may be defined as a gravity flow of fluid under another
fluid of approximately equal density . In case of reservoirs the water stored
is usually clear and the inflow during floods is generally muddy. The two
fluids have different densities and the heavy turbid water flows along the
channel bottom towards the dam under the influence of gravity. This is
known as density current.
• Measurement of sediment load :the amount of silt or sediment load carried
by a stream is determined by taking the samples of water carrying silt, at
various depths. The samples are then filtered and the sediment is removed
and dried (in terms of units ppm).There are no accurate devices to measure
bed load,which is estimated to about 15% of the suspended load.

39. Effects of reservoir sedimentation
• Loss of reservoir storage
• Reduction in yield of reservoir and its reliability
• Loss of navigable depths on upstream of reservoir
• Degradation of channel downstream of reservoir due to erosion
loss of fertile lands
• Hydropower : abrasion of turbines
• Coastal areas: loss of coastal lines when silt supply is interrupted

40. Elements of sediment management
1. Proper selection of reservoir site:
• A stream collecting water from soft soil with steep slope may carry
more sediment load. Hence the site should be such that it excludes
runoff from easily erodable catchment area. If certain tributary is found
, then the dam should be constructed to the upstream of that tributary.
• 2. Proper designing and reservoir planning:
• The sediment trapped in the reservoir also depends upon the reservoir
planning. A small reservoir on a large river has lesser trap efficiency i.e
{f(capacity/inflow)}.hence if the dam is constructed lower in the first
instance, and is being raised in stages, the life of reservoir will be very
much increased. During the floods, the stream carries maximum
sediment so sufficient outlets should be provided at various elevations,

41. • 3.Control of sediment deposit:
• The sediment deposit in the reservoir can be controlled by proper operation of
the gates of scouring sluices and the head regulators of the canals. These should
be so designed and operated that selective removal of silt is affected. During the
floods, when sediment inflow is higher, the scouring sluices must be opened to
discharge the silt downstream.
• 4.Control of sediment inflow:
• Small check dams may be constructed across those tributaries which carry
more silt. By adopting cultivation also reduces the soil erosion and hence
sediment inflow is reduced.

42. • 5.Removal of sediment deposit:
• The undersluices may be used to scour the deposited silt, but not completely.
The silt deposit can also be removed by excavating and dredging process but
cost effective. Therefore by disturbing the deposited silt by mechanical means
so that silt is loosened and pushed towards the sluices, then opening the sluices
so that most of the silt is discharged to the downstream.
• 6.Erosion control in the catchment area:
• By adopting various methods of soil conservation, and provision of control
bunds , check dams , embankments , afforestation , regrassing and control of
grazing etc. this helps in reduction of silt deposition.

43. Catchment vegetation

44. Wooden barriers

45. Stepped watershed for sediment
control

46. Mechanical desilting from reservoir

47. Sediment sluicing

48. • DAMS
Types of dams
Selection of dam sites
Geological characters for investigation
Selection of the dam type

49. DAMS
• A dam may be defined as a solid barrier
constructed at suitable location across
river valley with a view of impounding
water flowing through that river.
• Dams are created for the following
objectives:
Generation of hydropower energy
Providing water for irrigation facilities
Fish farming
Fighting droughts
Controlling of floods
Water supply for domestic consumption
Providing navigational facilities

50. Types of dams
• Gravity dams
A gravity is a solid
concrete structure,
generally having a
triangular profile,
which is so designed
that it can safely stand
against the precalculated
volume of water by virtue
of its weight.

52. Butress dam
• Butress dams are
derived from
gravity dams. This
type of dams are
supported uses
thin concrete slab
which is supported
from downstream
side by butresses.
• It uses multiple
reinforced
columns to support
the dams. Since it
has relatively thin
structure so there
is considerable
amount of saving
of concrete
material.

54. Arch dams
It is an arch – shaped solid structure made up of concrete which is
designed in such a way that a major part of the thrust forces acting
on the dam are transmitted to arch.
There are mainly two types of arch dams :
• Constant radius arch dams : radius of curvature throughout the
structure is constant and upstream face is vertical.
• Variable radius dams : curvatures are different on upstream and
downstream sides.
•These types of dames are best suited for narrow valleys.
•An arch dam having a curvature both in horizontal and vertical
alignment is often called a CUPOLA dam.

56. Embankment dams
• These are non rigid
structures which are build
over wide valleys with
varying foundations. These
are trapezoidal in shape and
are build of single type of
material(such as earth fill or
rock fill) or combination of
more than one material.
• The main advantage of these
dams is that it can be
constructed on weak
foundation.
• Impervious core is placed in
the middle of the
embankment body.

57. COMPOSITE DAMS
• Composite dams are
combination of one or more dam
types
• most often a large selection of a
dam will be either an embankment
or gravity dam, with the selection
responsible for power generation
being a buttress or arch dam.

59. SELECTION OF DAM SITES
Selection of sites is based on following basis:
Topographically
: most suitable place must be chosen for construction.
Ideally it must be a narrow or a small valley with enough
catchment area available behind so that calculated
amount of water can be easily stored in the reservoir
created upstream.

60. Location of spillway : All dams should have an adequate
spillway for passing
flood flows. If a river gorge is narrow,
then there may not be sufficient spillway
width available and a suitable location on
the periphery of the reservoir has to be
found to locate a spillway.
Construction: The way, river can be
diverted at a particular site for making
way for construction of the dam may
affect the design of the dam and also the construction
schedule.
Sedimentation possibilities :The average
quantity of sediment carried by the river
has to be known, as precisely as
possible, which would give an idea of the
rate at which a proposed reservoir way
get filled up.

61. Technically: the site must be strong, impermeable
and stable. Strong rocks make the job of designer
easy.Impermeable sites ensure better storage
inventories. Site must be stable with respect to
seismic shocks slope failures around dam.
Human welfare : site selection should be done in
such a way that it must cause minimum damage to
public in the of destruction or failure.
Economically : the creation of the dam must not
create ecological disorder. Fish culture is the first
sector that suffers the major shock due to dam
construction. It indirectly affects the other population.

62. Geological characters for investigation:
• Geology of the site
a. Litho logy : surface and subsurface studies must be carried out.
These studies reveal the type, the composition and texture of
the rocks along the valley floor.
b. Structures:
1.Dip and strike: the resultant force due to weight of the dam
and the up thrust of the impounded water is always inclined the
downstream as shown in the figure.
•So, gently upstream dipping layers
offer best resistance to the resultant force of the dam.

63. •So most unfavorable strike direction
is the one in which the bed strike is
parallel to the axis of the dam and the
slip is downward.

64. 2. Faults: Dams founded on the fault
zones are most liable to the shocks
during an earthquake. Generally the
small scale fault zones can be
treated effectively by grouting.

65. 3. Folds: the effects on of fold on rock
are shattering and jointing along the
axial planes and stressing of limbs.
In the synclinal region dams placed
on the upstream limbs have the risk
of leakage from beneath the dam.

66. Selection of the dam type:
Material availability:
If the cost of transportation of construction material is excessively
high, then an alternate design with locally available materials, have
to be considered.
Seismicity :
It is very important to analyse the behaviour of the dam under earth
quake vibrations thereby making it possible for the designer to
check if a particular section of the dam is suitable or not.

67. • Geology and foundation strength- The existence
of joint patterns in the abutments (their
orientation, inclination and infilling) may indicate
the possibility of instability under loading from an
arch dam and reservoir water.
• Where the possibility exists of differential
deformation of the foundation along the axis of a
dam, a gravity or arch dam would not be a suitable
choice because of their inherent rigidity due to
their construction in concrete. Instead, an
embankment dam may be proposed, which is
more flexible.
• Further, it may be noted that the stresses
expected at the base of a dam may have to be
checked with the bearing capacity of the
foundation material.

68. • Hydrology - If, during the construction
season, there are possibilities of the
partially constructed dam being overtopped
by the floods of the river water, then a
concrete dam section would be preferred
then an embankment dam section.
• If an embankment dam section is still
proposed to be built, then adequate
diversion works have to be provided for
diverting the river flood water.
• Unavailability of skilled workers-In case of
sophisticated dam section, skilled workers
are an absolute necessity. Unavailability of
such workers at proposed dam construction
site may have to force the designer to adopt
a more easy to construct a type of dam.

69. Queries …..???
•THANK YOU