GNITC
GURU NANAK INSTITUTIONS TECHNICAL CAMPUS
DEPARTMENT OF CIVIL ENGINEERING
WATER RESOURCES ENGINEERING-I
CLASS ROOM LECTURES
By
Dr. M. K. Mohan B.E,M.E,Ph.D,MISTE,MISCA,
Professor of Civil Engineering &
Member of Illuminati of IBC, Cambridge, England
WRE I – Unit V - Canals
October, 2015
UNIT V
Classification of canals, design of Irrigation canals by Kennedy’s and Lacey’s
theories, balancing depth of cutting, IS standards for a canal design canal
lining.
Design Discharge over a catchment, Computation of design discharge-rational
formula, SCS curve number method, flood frequency analysis introductory
part only – Stream Guaging – measurement and estimation of stream flow.
Irrigation Canals
A canal is an artificial channel, generally trapezoidal in shape constructed
on the ground to carry water to the fields either from the river or from a
reservoir.
Canals are classified into different types based on
1) nature of source of supply
2) financial output
3) function of a canal
Classification of canals
Classification of canals:
a) Based on nature of source of supply
i. Permanent canal
ii. Inundation canal
b) Based on financial output
i. Productive canal
ii. Protective canal
c) Based on function of a canal
i. Irrigation canal
ii. Carrier canal
iii. Feeder canal
iv. Navigation canal
v. Power canal
d) Based on discharge
i. Main canal
ii. Branch canal
iii. Major distributory
iv. Minor distributory
v. Water course
e) Based on canal alignment
i. Ridge canal or watershed canal
ii. Contour canal
iii. Side slope canal
Classification of Canals
Classification based on nature of source
Classification based on financial output
Classification based on function
Classification based on discharge
Classification based on discharge
Classification based on Canal alignment –Ridge canal
Classification based on Canal alignment – Contour canal
Classification based on Canal alignment – Side slope canal
CANAL ALIGNMENT
A canal has to be aligned in such a way that it covers the entire area
proposed to be irrigated, with shortest possible length, and at the same
time, its cost including the cost of cross drainage works is minimum.
General
considerations
alignment:
for
The alignment of the canal should be such
as to ensure
i.
ii.
iii.
the most economical way of
distributing water to the land
As high a command area as possible
Minimum number of cross drainage
works
Design of Irrigation channels: Silt theories
1. Normally a canal can be designed using Q = A V equation.
2. The canal which takes off from river carries water from the river to the
field.
3. Along with water, the canal has to carry a fair quantity of silt flowing in
the river which is carried in suspension state or along the bed of the
channel.
4. This silt load carried by the canal imposes a difficult problem in a channel
design on alluvial soils.
5. The velocity to be allowed in a channel design should not be too high,
and it should not be too less.
6. If it is too less, the silt load may be dropped on the bed.
7. If it is too high, the bed and banks of the canal may bed eroded away.
8. Therefore, a velocity which will just keep the silt in suspension without
scouring the channel is known as non-silting and non-scouring velocity.
9. There are 2 famous theories for the design of non-silting and nonscouring velocity 1. Kennedy’s theory 2. Lacey’s theory
Design of Irrigation canals by Kennedy’s theory
1. Eddies are generated from the bottom, because of the friction that is
coming out to the flowing water from the bed of the canal.
2. These eddies keep the silt in suspension state.
3. Eddies coming from sides of canal cannot have any silt carrying capacity.
4. Therefore, the silt carrying capacity depends upon the depth of the
canal.
5. He defined the critical velocity(VO) as non-silting and non-scouring
velocity.
6. The mean velocity of flow is given by the equation
V = 0.55 m D0.64
where V = Mean velocity of flow
D = Depth of flow
m = critical velocity ratio = V/VO
7. The value of ‘m’ depends on type of silt
8. For finding mean velocity of flow, using kutter’s formula
Where V = mean velocity of flow, N = Manning’s roughness coefficient
S = Bed slope, R = Hydraulic radius or mean depth
= A/P = wetted area/wetted perimeter
Kennedy’s Design Procedure – Case 1
Kennedy’s Design Procedure – Case 2
Kennedy’s Design Example 1
Kennedy’s Design Example 1
Kennedy’s Design Example 2
Kennedy’s Design Example 2
Design of Irrigation canals by Lacey’s theory
Fundamentals of Lacey’s theory:
1. Lacey put forward the concept of regime channel.
2. Lacey defined the regime channel as a stable channel transporting a
regime silt charge.
3. Lacey’s regime theory:
“Dimensions, width, depth and slope of a regime channel to carry a
given discharge loaded with given silt charge are all fixed by nature”.
4. Regime conditions:
A channel will be in regime if it flows in incoherent unlimited
alluvium of the same character as that transported and the silt grade
and silt charge are all constant.
5. A channel is said to be in regime when the following conditions are
satisfied.
i. The channel is flowing in unlimited incoherent alluvium of the
same character as that transported
ii. Silt grade and silt charge is constant.
iii. Discharge is constant.
6. If these conditions are met with fully then the channel is in the true
regime.
7. Incoherent alluvium: It is a soil composed of loose granular graded
material which can be scoured with the same ease with which it is
deposited.
8. Regime silt charge: It is the minimum transported load consistent
with fully active bed.
9. Regime silt grade: This indicates the gradation between the small
and the big particles. (difference in size of particles but not to be
taken as mean dia of particles)
10. If the above conditions are met with fully, then the channel is said to be
in true regime.
11. However, it is seldom that the above conditions are realized in the field.
12.Hence Lacey gave an idea of initial and final regime for a channel.
13. Initial regime: One of the conditions of attaining the regime of a
channel is that there should be freedom for the channel to form its own
section.
Initial regime is the state of channel that has formed its section only and yet
not secured the longitudinal slope.
The shape of the regime channel is semi-elliptical for course silt (semi
circle for fine silt).
14. Final regime: When a channel is constructed (with defective slope) it
tries to throw off the incoherent silt on the bed to increase their slopes and
attains longitudinal slope.
The channel after attaining its section and longitudinal slope, will be said to
be in final regime.
15. Permanent
regime: When a channel is protected on the bed and side
with some kind of protecting material, the channel section can not be
scoured up and so there is no possibility of change of section or
longitudinal slope, the channel will then said to be in permanent regime.
Design of Irrigation Canals by Lacey’s theory
1. Lacey states that the silt is kept in suspension due to the force of vertical
eddies generated not only from the bed but also from the sides.
2. Hence vertical component of eddies generated from sides will also
support the silt.
3. Lacey assumed hydraulic mean depth R = A/P as variable rather than
depth ‘D’ .
4. Lacey recognized the importance of silt grade and introduced a
parameter known as silt factor ‘f’.
5. Lacey gave an equation for finding the silt factor
where mr = mean diameter of silt in mm
Lacey’s Regime flow equation
By plotting a large mass of data from a number of different
sources, Lacey obtained the relationship
Perimeter-Discharge (P-Q) Relation
V-Q-f Relation
Regime-slope equation
Regime-slope equation
Lacey’s Channel Design Procedure
Shock losses due to irregularities
1. Lacey states that the value of absolute rugosity coefficient Na is
independent of channel conditions.
2. But it was observed that the absolute rugosity coefficient is dependent
on shock losses due to irregularities in channels.
3. Therefore, to account for this variation Lacey gave a new idea and
introduced the idea of shock losses.
4. He gave a relationship between Velocity ‘V’, Absolute rugosity
coefficient ‘Na’ and shock losses ‘s’ as given below.
where S = Bed slope s= shock losses, R= Hydraulic Radius
V = Velocity, Na = Absolute rugosity coefficient
Lacey’s Design Example
Lacey’s Derived Formulae
Balancing depth of cutting
Balancing depth is that particular cutting depth for which area of
cutting will be equal to area of filling for a given canal cross section.
When a canal is constructed partly in cutting and partly in filling,
maximum economy can be achieved, if the earthwork in excavation equals
the earthwork in filling.
For a given cross section of a channel, there can be only one depth, for
which such a balance between cutting and filling will occur. This depth is
known as the Balancing depth.
If this balance between cutting and filling can occur, then the need for spoil
banks or borrow pits is entirely eliminated.
Example Problem on Balancing Depth
Cross-Section of an Irrigation Canal
1. The irrigation canals are trapezoidal in cross section.
2. This section must be partly in cutting and partly in filling
and should aim at balancing the quantity of earthwork in excavation
with that in filling.
3. Some times, when the natural surface level is above the top of the
bank, the entire canal section will have to be in cutting and it shall
be called “canal in cutting”.
4. Similarly, when the natural surface level is lower than the bed level
of the canal, the entire canal section will have to be built in filling,
and is called “canal in filling” or “canal in banking”.
Design Elements in Cross-Section
There are various design elements in the cross section of an irrigation
canal other than bed with and depth which are given below.
1.
2.
3.
4.
5.
6.
7.
8.
Side slopes
Berms
Free board
Banks
Back berms or counter berms
Service Roads
Spoil banks
Borrow pits
Side slopes
1. The side slopes should be stable.
2. They depend upon the type of soil.
3. Steeper slope can be provided in cutting rather in filling, as the soil
in former case shall be more stable
4. Side slopes of 1H:1V to 1.5 H : 1V are adopted in cutting and 1.5
H:1V to 2H:1V are adopted in filling.
Berms
1. Berm is the horizontal distance left at ground level between the toe of the
bank and the top edge of the cutting.
2. As fine and impervious silt gets deposited, they serve as a good lining for
reducing losses and leakage.
3. They provide wider water way.
4. They help channel to attain regime conditions.
5. They provide additional strength to banks
6. They protect the banks from erosion, wave action and breaches
7. They provide scope for future
widening of the canal.
8. They can be used as borrow
pits.
Free Board
1. The margin between FSL and bank level is known as free board.
2. The amount of free board depends upon the size of the channel.
3. The generally provided values of free board are given below.
Banks
1. The primary purpose of banks is to retain water.
2. They can be used as means of communication and as inspection paths.
3. They should be wide enough, so that a minimum cover of 0.5 m above
the saturation line.
Back berms or Counter berms
1. Even after providing sufficient section for the bank embankment, the
saturation gradient line may cut the down stream end of the bank.
2. In such cases, the saturation line can be kept covered by atleast 0.5
metres with the help of counter berms.
Service Roads
1. Service Roads are provided on canals for inspection purposes, and
may simultaneously serve as the means of communication in
remote areas.
2. They are provided 0.4 m to 1 m above FSL or 0.15 m above TBL,
depending on the size of the channel.
Dowla
1. As a measure of safety in driving, dowlas 0.3 m high and 0.3 m to 0.6
m wide at top with side slopes of 1.5:1 to 2:1 are provided along the
banks.
2. They help in preventing slope erosion due to rain etc.
Spoil banks
1. When the earthwork in excavation exceeds earthwork in filling, even
after providing maximum width of embankments, the extra earth has
to be disposed of economically.
2. The extra soil is deposited in the form of heaps on both banks or only
one bank.
3. These heaps of soil are discontinued at suitable intervals and
longitudinal drains running by their sides are excavated for the disposal
of rain water.
4. Cross drains through the spoil banks may also be excavated, if needed.
Borrow pits
1. When earth work in filling exceeds the earthwork in excavation, the
earth has to be brought from some where.
2. The pits, which are dug for bringing earth, are known as borrow pits.
3. If such pits are excavated outside the channel, they are known as
“external borrow pits”, and if they are excavated with in the
channel, they are known as “internal borrow pits”.
Topic outside syllabus
Cross-Section of an Irrigation Canal
Topic outside syllabus
Cross-Section of an Irrigation Canal
Topic outside syllabus
Cross-Section of an Irrigation Canal
Topic outside syllabus
Longitudinal section of an Irrigation Canal
Topic outside syllabus
Longitudinal section of an Irrigation Canal
Topic outside syllabus
Schedule of Area Statistics and Channel Dimensions
Garret’s Diagrams
Topic outside syllabus
Garret’s Diagrams
Topic outside syllabus
IS standards for canal design
Canal Lining
Lining of canal is necessary to minimize seepage losses, to increase the discharge
in canal section by increasing the velocity, to prevent erosion of bed and side due
to high velocities and to reduce maintenance of canal.
Advantages of Canal Lining:
1. Prevents seepage losses
2. Reduces the problem of water logging
3. Provide smooth surface and increase velocity of flow
4. Higher velocity minimize loss due to evaporation
5. Higher velocity helps to provide narrow cross section
6. Higher velocity helps to provide flatter hydraulic gradient and flatter bed
slope
7. Higher velocity prevents silting of channel
8. Makes the banks more stable, Prevents weed growth
9. Reduces maintenance costs, Reduces breaching, Provides stability
10. Assures economical water distribution
11. Prevents water to come in contact with harmful salts.
Disadvantages of Canal Lining:
1.
2.
3.
4.
Requires heavy initial investment
Difficult to shift outlets very often
Difficult to repair the damaged lining
Berms are not provided in lined sections therefore additional safety
for pedestrians and vehicles is absent.
Requirements of good Lining
Suitability of lining
1.
2.
3.
4.
5.
The lining material should provide complete water tightness.
The lining material should have a low coefficient of rugosity.
The lining material should be strong and durable.
The lining should not have a very high initial cost.
The lining material should be able to resist growth of weeds and
attack of burrowing animals
6. The lining material should withstand high velocity
7. The lining material should permit construction of required slope
easily.
8. The lining material should be unaffected by tramping of cattles.
Types of Lining
The following are the important types of concrete lining used in India.
a) Hard surface type lining
i. Cement concrete lining
ii. Shotcrete lining
iii. Precast concrete lining
iv. Cement mortar lining
v. Brick lining
vi. Stone blocks or undressed stone block lining
vii. Asphaltic lining
b) Earth type lining
i. Soil cement lining
ii. Clay puddle lining
iii. Sodium carbonate lining
c) Buried and protected membrane type lining
i. Prefabricated light membrane lining
ii. Bentonite soil and clay membrane lining
iii. Road oil lining
Design Discharge over a catchment
Design Discharge and Peak Flow:
1. Floods and droughts are extreme hydrological events referring to
unusually high and low water supplies.
2. A flood represents a high stage of river due to runoff from rainfall
and/or melting of snow.
3. During a flood, peak flow takes place in a river and for the design of
hydraulic structures, this peak flow has to be taken into consideration.
4. There are 3 types of peak flood determination and designation.
1. Maximum Probable Flood
2. Standard Project Flood
3. Design Flood
1. Maximum Probable Flood:
This is defined as extreme flood that is physically possible in a region
as a result of severe most combination including extreme rare
combination of meteorological and hydrological factors.
2. Standard Project Flood:
It is defined as a flood that is physically possible in a region as a result
of the most severe combination of meteorological and hydrological
factors that are reasonably applicable to the basin excluding the extreme
rare combination.
A standard project flood may be taken as 80% of Maximum Probable
Flood.
3. Design Flood:
A design flood is the flood discharge adopted for design of hydraulic
structures after careful considerations of hydrologic and economic
factors.
Design flood in most of the cases may be less than MPF.
Computation of design discharge
A design flood is the flood discharge adopted for design of hydraulic
structures after careful considerations of hydrological and economical
factors.
Design flood in most of the cases may be taken as less than MPF.
The peak flood discharge in a stream may be determined by following
methods.
1.
2.
3.
4.
5.
6.
7.
By physical indication of past floods
By empirical flood formulae
By rational formula
By unit hydrograph
By enveloping curves
By hydraulic structures
By flood frequency analysis
Computation of design discharge
1. By physical Indication of Past floods:
Ancient monuments situated on river banks bear past flood marks.
2. By empirical formulae:
1. Dicken’s Formula
2. Ryve’s Formula
3. NawabJang Bahadur’s Formula
4. Fanning’s formula
Empirical Formulae
Empirical Formulae
5. By Rational Formula:
i.
For small catchment areas, the rational formula is the most logical
and reasonable method for determination of flood discharge.
ii. It is considered that certain rainfall intensity which persists for a
time greater than or equal to period of time of concentration (tc)
generates extreme flood flow.
iii. When the entire area contributes the runoff at the basin outlet, the
extreme rate of runoff from watershed is observed.
iv. The runoff becomes constant at peak value, if the rainfall exceeds
time of concentration ‘tc’.
v. The rational formula is given below.
Limitation of Rational formula
1. The formula gives good results only for catchments up to 50 sq.km.
2. It is applicable only if the duration of the rainfall is equal to or more than
time of concentration tc.
3. Rainfall intensity (i) should be constant over the entire catchment.
4. It assumes a constant value of C for a given area for all storms.
5. If a plot is made between Qp and I, a straight line is obtained with zero
intercept but nature does not follow a linear relationship.
4. By Unit Hydrograph:
Using the stream flow data, a flood hydrograph can be constructed. A direct
runoff hydrograph can be derived from flood hydrograph. A unit hydrograph
can be derived from a direct runoff hydrograph. In the absence of stream flow
data, a synthetic unit hydrograph can be derived by knowing the basin
parameters. Peak flow can be determined using any of these hydrographs.
Enveloping curves
Flood frequency analysis
Some Basic Definitions
1. Recurrence Interval (T): It denotes the number of years in which
a flood can be expected once. It is denoted by ‘T’. In other words, it
can also be defined as the period of time between the equaling or
exceeding of a specific flood.
2. Return Period : It is the average of recurrence interval for a
certain event or flood.
3. Probability of occurrence (P): The probability of an event being
equaled or exceeded in any one year is the probability of its
occurrence.
It is given by the equation,
where T is the recurrence interval
Probability of non-occurrence ‘q” :
The probability that it will not occur in a year is known as probability of nonoccurrence.
It is given by the following formula,
Where q is probability of non-occurrence, p is probability of occurrence and T is
the recurrence interval.
Flood frequency (f):
It denotes the likelihood of flood being equaled or exceeded expressed as a
percentage.
The probability of occurrence of an event expressed as a percent is known as
frequency (f).
It is given by the following formula,
Where p is probability of occurrence.
Flood frequency studies
Flood frequency studies interpret a past record of events to predict
the future probabilities of occurrence.
It is based on the assumption that combination of the numerous
factors which produce floods are a matter of pure chance and
therefore subject to analysis according to mathematical theory of
probability.
There are two methods of compiling flood peak data
1. Annual duration series
2. Partial duration series
Annual duration series:
In the annual series, the largest flood observed in each water year is
taken.
It ignores the second and lower order events of each year which may
sometimes exceed many the annual maximum.
Partial duration series:
In the partial duration series, all flood events above a selected base value
are included.
The base is usually so chosen that more than 3 or 4 events are included
for each year.
For extreme flood, annual series is used
Ex: Spillway design
For small and frequent floods of return period 5 years, partial series is
used.
Ex: Design of coffer dams, urban drainage etc.
ANNUAL FLOOD SERIES
1. Annual flood series consist of the values of annual maximum flood from a
given catchment area, for large number of successive years.
2. The data of the series are arranged in the decreasing order of magnitude.
3. The probability ‘P’ of an event being equaled or exceeded is computed from
one of the following plotting position formulae.
SCS curve number method
1. The SCS-CN (soil conservation services-curve number)
method was developed in 1964 by U.S. Department of
Agriculture for calculating the depth of direct runoff from
the depth of rainfall.
2. The name of SCS-CN method was recently changed to
NRCS-CN method (Natural Resources Conservation –
Curve Number) method.
Runoff Curve Numbers
Fully developed urban areas (vegetation established)
Cover description
Open space (lawns,
parks, golf courses,
cemeteries, etc.)
Impervious areas
Poor condition (grass cover <50%)
Fair condition (grass cover 50 to 75%)
Good condition (grass cover >75%)
Paved parking lots, roofs, driveways, etc. (excluding
right of way)
Paved; curbs and storm sewers (excluding right-ofway)
Paved; open ditches (including right-of-way)
Gravel (including right of way)
Dirt (including right-of-way)
Natural desert landscaping (pervious area only)
Curve numbers for hydrologic soil group
A
B
C
D
68
79
86
89
49
69
79
84
39
61
74
80
98
98
98
98
98
98
98
98
83
76
72
63
89
85
82
77
92
89
87
85
93
91
89
88
Western desert urban Artificial desert landscaping (impervious weed barrier,
areas
desert shrub with 1- to 2-inch sand or gravel mulch
and basin borders)
96
96
96
96
Commercial and business (85% imp.)
Industrial (72% imp.)
1⁄ acre or less (town houses) (65% imp.)
8
1⁄ acre (38% imp.)
4
1⁄ acre (30% imp.)
3
1⁄ acre (25% imp.)
2
1 acre (20% imp.)
2 acres (12% imp.)
89
81
77
61
57
54
51
46
92
88
85
75
72
70
68
65
94
91
90
83
81
80
79
77
95
93
92
87
86
85
84
8
Streets and roads
Urban districts
Residential districts
by average lot size
Cultivated agricultural lands
Cover description
Treatment[A]
Cover type
Bare soil
Fallow
Crop residue cover (CR)
Straight row (SR)
SR + CR
Contoured (C)
Row crops
C + CR
Contoured & terraced (C&T)
C&T + R
SR
SR + CR
C
Small grain
C + CR
C&T
C&T + R
SR
Close-seeded or
broadcast legumes or
rotation meadow
C
C&T
Curve numbers for hydrologic soil group
Hydrologic
condition
—
Poor
Good
Poor
Good
Poor
Good
Poor
Good
Poor
Good
Poor
Good
Poor
Good
Poor
Good
Poor
Good
Poor
Good
Poor
Good
Poor
Good
Poor
Good
Poor
Good
Poor
Good
Poor
Good
A
B
C
D
77
76
74
72
67
71
64
70
65
69
64
66
62
65
61
65
63
64
60
63
61
62
60
61
59
60
58
66
58
64
55
63
51
86
85
83
81
78
80
75
79
75
78
74
74
71
73
70
76
75
75
72
74
73
73
72
72
70
71
69
77
72
75
69
73
67
91
90
88
88
85
87
82
84
82
83
81
80
78
79
77
84
83
83
80
82
81
81
80
79
78
78
77
85
81
83
78
80
76
94
93
90
91
89
90
85
88
86
87
85
82
81
81
80
88
87
86
84
85
84
84
83
82
81
81
80
89
85
85
83
83
80
Stream Gauging
Stream:
A stream is that water body through which water flows naturally.
Stream Gauging:
The measurement of stream flow is called as stream guaging.
Objectives of Stream Gauging:
1.
2.
3.
4.
To assess available quantity of surface water accurately.
To study the relation between precipitation and runoff.
To study the variation in runoff.
To evaluate the maximum water level that may be reached in
case of bridge or culvert by extrapolation from stage-discharge
records.
Stage
Stage:
The depth of water flowing in a river is known as the stage of the river.
Measurement of stage:
There are several methods used in the measurement of stage.
1. Actual wading through the stream with staff guage.
2. Fixing the gauges along the cross section
1. vertical guage 2. inclined gauge
3. Suspension weight method
4. Water-stage recorder (stilling well)
5. Automatic water level recorder
1. Float method
2. Electrical resistance method
Actual wading through the stream
1. A person with a gauge in hand may move across the stream and record
the depth of flow at a location by the gauge in his hand.
2. This method is crude and may be adopted if the stream is small.
3. The person standing in the stream may cause an obstruction to the
flow and may affect the gauge reading.
4. The stage thus recorded will be at a specific location and at a specific
time.
Fixing the Gauges along the Cross Section
The stage of a stream can be measured by installing gauges in the stream.
There are two types of gauges.
1. Vertical gauges: Vertical gauges may be fixed along the cross section of the
river on one bank and may be correlated to each other with a minimum
overlap of 0.5 m between two successive staves.
However, these gauges form an obstruction to the flow. Alternatively, the
gauges may be fixed or painted on a bridge pier, a culvert or any other
hydraulic structure.
2. Inclined gauges: Gauges along the
slope of the cross section of the stream
may be fixed .
While marking depth of flow on the
gauge, the side slope of the stream has
to be taken into consideration.
Such gauges will not cause any
obstruction to flow of water.
Suspension weight method
1. In this method a weight attached to a rope may be lowered from a
bridge, a culvert or any hydraulic structure, until it touches the
water surface.
2. Knowing the vertical length of rope up to the bed of the stream,
the stage (or depth) of river can be found out.
Water stage recorder
1. By constructing a stilling well on one of the banks of the stream, in
which a staff gauge is mounted.
2. Recorder with pulley arrangement is also provided.
3. Water is let in to the well with help of an intake pipe.
4. Staff gauge reading is noted.
Automatic water level recorder
The water level in a stilling well is automatically recorded by the following
methods.
1. Float method
2. Electrical resistance method
1. Float method:
In this method, a float is used to identify the water level in the well. The
level indicated by the float and the time is transferred by a suitable
mechanism by a pointer on a graph paper wound on a drum. The drum is
kept rotating uniformly by a clock work so that it makes one rotation a day
or a week. This type of mechanism is similar to the one used in automatic
rain gauge. The maximum and minimum water levels reached and the time
when reached are automatically recorded.
2. Electrical resistance method
1. In this method, two electrodes are dipped vertically in the observation
well keeping the distance between these two electrodes constant.
2. An electric current flows between these two electrodes.
3. The resistance between these two electrodes varies due to the change
in water level.
4. The change in resistance will be a measure of the water level and is
transferred by a suitable mechanism to a graph paper mounted on a
drum by a pen pointer.
5. The drum is kept rotating uniformly by a clock work and it makes a
rotation a day or a week.
6. The maximum and minimum water levels reached and the time when
reached are automatically recorded.
Measurement of Discharge
The different methods followed in the measurement of discharge in a
stream are
1.
2.
3.
4.
5.
6.
Area-slope method
Area-velocity method
Salt-titration method
Measurement by hydraulic structures
Hydraulic model method
Modern methods
a. ultrasonic method
b. Electro magnetic induction method
Moving Boat technique
Selection of site
Selection of site
1. The reach should be straight one
2. No other stream should join the stream in this
reach.
3. Cross sectional area of stream should be uniform
4. The reach should be accessible in all seasons
5. The site should be free from disturbance
Cross sectional area of a stream
It can be found out from two methods
1. Accurate survey
2. Echo sounder
Accurate survey
Hydrographic survey is carried out at 2 or 3 locations and using the data
obtained, following graphs are drawn.
Echo sounder
i.
When the stream is large, and depth is substantial, hydrographic
survey cannot be carried out.
ii. Echo sounder is used to find depth of flow and then cross
section.
iii. Echo sounder is an equipment is normally fitted to a boat.
iv. Sonic waves are transmitted from the water surface by a source
and they travel to the bed and reflect back and received by
receiver.
v. Knowing the time taken, and velocity of sound waves we can
find the distance.
Area-slope method
1. In this method, area of flow is measured and surface slope is measured
by conducting hydrographic surveys and echo sounder techniques.
2. Velocity is calculated using Manning’s formula, then discharge is
calculated using continuity equation
3. Select 2 cross sections of the stream at a distance of about 150 m.
4. Observe the depths of flow at these two cross sections as well as the
water surface level accurately between two sections.
5. Find the area of flow and also wetted perimeter at these two crosssections from the predetermined stage vs area and stage vs wetted
perimeter curves.
6. Calculate Hydraulic mean radius
7. Velocity can be calculated using manning’s formula
8. Discharge is calculated using continuity equation
Area-velocity method
In this method, the values of area ‘A’ and velocity ‘V’ are assessed
separately and then the discharge is calculated using continuity
equation Q= AV.
Area of flow:
Select two cross sections of the stream at a distance of about 150m. Observe the
depths of flow at these two cross sections as well as the water surface level
accurately between these two cross sections. Find the area of flow.
Velocity of stream flow
Velocity of flow:
The velocity of flow is not uniform over the depth. It is very low at the
bottom. It increases with the depth. However, it slightly reduces at the
surface. The normal velocity profile is given below. The average
velocity is calculated by using the formula given below.
Velocity of stream flow
Velocity of stream flow can be found out by any one of the following
methods.
1. Float method
i. Surface floats
ii. Canister floats
iii. Rod float
2. Current meter method
i. Cup type current meter
ii. Propeller type current meter
3. Pitot tube method
Float method
1.
2.
3.
4.
Topic outside syllabus
Float is released in to the stream.
Distance travelled and time taken is noted.
Velocity can be calculated as distance travelled per unit time.
Some of the floats are shown below.
Current meter
Topic outside syllabus
A current meter is an instrument used to measure velocity of flow.
Current meters are of two types.
1. Cup type current meter
2. Propeller type current meter
Cup type current meter Topic outside syllabus
Cup type current meter Topic outside syllabus
Topic outside syllabus
Propeller type current meter
Topic outside syllabus
Propeller type current meter
Topic outside syllabus
Current meter lowered from a trolley
Pitot tube
Topic outside syllabus
Pitot tube is an instrument used to measure velocity of flow
in a stream
Salt-titration method
In this method, a concentrated solution of a salt or a chemical in water is
added to a stream at a constant rate ‘q’ and is allowed to mix with the flow
of the stream.
Samples of water before and after mixing salt are collected and tested in
the laboratory and the concentration is determined accurately.
Topic outside syllabus
Measurement by hydraulic structures
The discharge can be calculated by hydraulic structures like notches and
weirs.
Hydraulic Model Method
1. A three dimensional composite model of the stream to a suitable scale may be
constructed in the laboratory.
2. By running the model, a stage-discharge curve may be obtained for the crosssection where the discharge in the prototype is required.
3. This stage discharge curve may be used for the prototype by using the
discharge scale.
4. This curve may be extrapolated if required
5. Before using the stage-discharge curve observed on the model, it should be
seen that the model is proved to prototype conditions.
Ultrasonic Method
1. In this method, a reach of the stream is reduced to a rectangular
section.
2. This length of reach ‘L’ should be approximately equal to the bed
width of rectangular channel.
3. Two transducers are located on each bank at a depth ‘d’.
4. The line joining the transducers should make an angle of 45O with the
axis of the stream.
5. These two transducers emit and receive ultrasonic signals.
6. Ultrasonic signal is emitted by source and returned by receiver.
7. The time taken for each signal is calculated
8. Knowing the velocity of ultrasonic sound, we can calculate the
velocity of water using the following equation.
Topic outside syllabus
Moving Boat Technique
1. When a river is in spate, it is very difficult to take discharge
observations particularly because it is very difficult to keep the boat
steady at a location, to lower the current meter and take the velocity
observations at the same time.
2. A technique called “Moving boat technique” for discharge
measurement was developed by USGS.
3. In this method, the boat is kept moving and velocity observations are
continuously taken with current meter.
Stage Discharge Curves
ASSIGNMENT QUESTIONS
ASSIGNMENT QUESTIONS
ASSIGNMENT QUESTIONS
TEXT BOOKS
1. Engineering Hydrology by Jayaram Reddy, Laxmi publications pvt. Ltd., New
Delhi
2. Irrigation and water power engineering by Punmia & Lal,
Laxmi publications pvt. Ltd., New Delhi
REFERENCE BOOKS
1. Elementary hydrology by V.P.Singh, PHI publications.
2. Irrigation and Water Resources & Water Power by P.N.Modi, Standard Book House.
3. Water resources engineering –I by Dr. G.Venkata Ramana, academic publishing
company.
4. Irrigation Water Management by D.K. Majundar, Printice Hall of India.
5. Irrigation
and hydraulic structures by S.K.Garg
6.Applied hydrology by Ven Te Chow, David R.Maidment larry W. Mays Tata Mc. Graw
Hill
7. Introduction to hydrology by Warren Viessvann, Jr, Garyl. Lewis, PHI.
Have a Good Luck.
THANK YOU.