Introduction to Hydrology, Stream Gauging

HYDROLOGY & WATER
RESOURCE ENGINEERING
SPPU, Pune
T.E. Civil
UNIT-I
• Introduction to Hydrology
• Precipitation: Types & Forms
• Evaporation and Infiltration
• Stream Gauging
Prepared by : Prof. A. A. Inamdar
Associate Professor, KJCOEMR

INTRODUCTION TO HYDROLOGY
• Definition:- It is a science which deals with the
occurrence, circulation and distribution of water
over and beneath the earth surface.
2

SOME IMPORTANT DEFINITIONS
1. Precipitation:- It is the
fall of water in various
forms on the earth from
the clouds.
2. Evaporation:- It is the
process by which water
is changed to vapours
(gaseous state) at the
free surface below the
boiling point of water.
4

3. Transpiration :- It is the process by which water vapours
escape from the leaves of the plant & enter to the
atmosphere.
4. Evapo-transpiration :- It is the total water lost from the
forest land due to evaporation from the soil &
transpiration by the plants.
5. Infiltration :-It is the process by which water enters into
the soil from the ground surface.
6. Runoff :-The water that runs over the ground & enters into
streams and rivers.
5

Need for the Hydrologic Studies
Due to…
–Increased population,
–Increased standards of living of people,
–Unpredictable Rainfall.
Hydrological studies are very important
for planning and development of water resources
to meet these demands.
6

Importance of Hydrology in
Civil Engineering
• Design and Operation of water resources engineering
projects like
– Irrigation
– Flood control
– Water supply schemes
– Hydropower projects and
– Navigation
7

PRECIPITATION
• Types of Precipitation…
1) Convective Precipitation,
2) Orographic Precipitation,
3) Cyclonic Precipitation,
a) Frontal Precipitation,
b) Non-frontal Precipitation.
8

PRECIPITATION
• Forms of Precipitation
– Drizzle (dia. 0.1 to 0.5mm)
– Rain (dia. < 0.5mm)
– Glaze (Freezing Rain)
– Sleet (dia. 1 to 4mm snow pallets)
– Snow (Solid ice crystals)
– Hail (dia. <0.5mm lumps of ice)
– Dew (It forms due to condensation on ground)
9

MEASUREMENT OF PRECIPITATION
10

What if you need to know the
rainfall in a catchment?
- Type of rain gauges?
- Where to put gauges?
- How many gauges?
11

RAINGAUGES
• Following are the type of Rain Gauges…
- Non-recording type
a) Symon’s Rain gauge
b) IMD Standard Rain gauge
- Recording type
a) Tipping Bucket Rain gauge
b) Weighing Bucket Rain gauge
c) Float type Rain gauge
- Measurement by RADAR
12

Non-recording type
a) Symon’s Rain gauge b) Indian Meteorological Department
(I.M.D.) Standard Rain Gauge
13

Non-recording type…
• Working:-
 These type of rain gauges consists of a cylindrical vessel of dia.
12.7cm with funnel.
 The vessel is fixed in a masonry foundation block of size (60 x60 x
60)cm.
 The height of vessel is so fixed that the top rim is 30.5cm above the
ground.
 The glass bottle (receiver) is of 7.5 to 10cm in dia.
 The rain water collected in receiver is measured by measuring glass
jar in mm.
14

RECORDING TYPE
1. TIPPING BUCKET RAIN GAUGE
Working :-
1. It consist of two small buckets placed
below the funnel of 30 cm dia.
receiver.
2. The buckets are balanced in unstable
equilibrium.
3. So that, at one time only one bucket
remains below the funnel at
higher level.
4. When the higher bucket receives
0.25mm rainfall it becomes unstable
and tips to empty itself.
15

TIPPING BUCKET RAIN GAUGE…
Working…
5. At the same time other bucket comes below
the funnel for collection of rainfall.
6. The tipping of bucket actuate an electric
circuit which causes a pen to make a mark
on a drum chart.
7. Each mark indicates the rainfall of 0.25mm.
8. By counting the no. of marks & noting the
intensity & amount of rainfall can be
determined.
9. The amount of rainfall determined can be
verified by measuring the total water
collected in the measuring tube.
16

• Working:-
1. The rain is collected in a weighing
bucket which is connected to the
spring balance.
2. As the rainwater increases in
bucket, the weighing platform(Pan)
moves downwards.
3. The movement of that pan is transmitted to pen through a link &
lever system.
4. The pen makes the trace of the accumulated amount of rainfall on a
chart attached to a drum revolved by a clock driven mechanism.
5. The record is continuous & the slope of line gives intensity of
rainfall.
2. WEIGHING BUCKET RAIN GAUGE
17

3. FLOAT-TYPE RAIN GAUGE
• Working:-
1. The rainfall collected in a float
chamber filtered & passes through
the funnel.
2. The filter prevents the dust debris
etc. from entering the float
chamber.
3. The float chamber contains a float
with a vertical stem fixed over it.
4. As the rain is collected in the float
chamber, the float rises & the pen
which is attached on the top of the
vertical stem moves on the drum
chart.
18

FLOAT-TYPE RAIN GAUGE
• Working…..
5. From the chart the intensity & duration
of rainfall is determined.
6. When the float chamber is fully filled,
the pen reaches the top of the chart
& the syphoning action occurs
automatically.
7. At the time of syphoning action of
15 seconds, the pen draws a
vertical line on the drum chart which
resets the pen to zero.
19

Rainfall measurement by
radar
1. Electromagnetic waves are
produced by a transmitter & are
radiated by a narrow beam
antenna.
2. These waves are reflected by the
clouds and are intercepted by the
same antenna, which now acts as
a receiver.
3. The receiver detects these
reflected signals and amplifies
them.
4. The returned signals are
transformed into a visual display
on the radarscope.
20

Estimation of missing rainfall data
1. Comparison Method,
2. Isohytal map Method,
3. Normal Ratio Method
Case I: When the mean annual rainfall at each of index station A, B, & C is
within 10% of the mean annual rainfall of station X, a simple avg. of the
values of the index station is taken…
Px = 1/3 (PA + PB + PC)
Case II: When the mean annual rainfall at each of index stations differs from
the station X by more than 10% the Normal Ration Method is used…
Px = Nx/M (PA/NA + PB/NB +……….+ PM/ NM )
Where M = No. of index stations,
N = Mean annual Rainfall(Avg. Annual Precipitation)
P = Precipitation
21

Mass curve & hyetograph
Mass curve hyetograph
Definition - The mass curve is a plot
between the accumulated rainfall at a
station as ordinate (Y-axis) & time as
abscissa (X-axis).
Definition – The hyetograph is a bar
diagram plotted between the avg.
intensity of rainfall as ordinate & time
as abscissa.
22

MASS CURVE
1. The recording type rain gauges
gives directly the mass curve.
2. It is used for determination of
the intensity, amount &
duration of the rainfall.
3. The amount & duration of the
rainfall are obtained directly
from the curve.
4. The rainfall intensity is obtained
from tangent to the mass curve.
HYETOGRAPH
1. The hyetograph is obtained
from the mass curve.
2. It is used for determination of
Average Rainfall Intensity with
a constant time interval.
3. The smaller is the time interval
the greater is the accuracy of
avg. rainfall intensity.
4. The area under the hyetograph
gives the total rainfall occurred
in that period.
• Rainfall intensity is expressed in mm/hr or cm/hr.
23

problems
- Mass Rainfall curve & Hyetograph
24
Time
(O’clock)
8:30 9:00 9:30 10:00 10:30 11:00 11:30
Rainfall (mm) 0 2 8 15 25 30 32
?? Draw mass-curve & hyetograph from the following data.

Depth-area-duration curve
(Dad curve)
Depth-Area Relationship:-
- A storm over a particular
catchment never produce
uniform rainfall depth.
- Each storm has its centre called
as EYE where the precipitation is
maximum.
- As the distance of a point from
the eye increases, the
precipitation decreases thus the
depth of precipitation also
decreases.
- The depth-Area relation for
different storm durations are
different. (DAD curve-see fig.)
The Depth-Area-Duration Curve
25

Problem
DAD Curve
?? The computation of an Isohyet map of 1250 km2 basin, a
6-hr storm gave following data. Determine the avg.
precipitation for the basin and compute & draw depth-area
curve.
26
Isohyet (mm) 200 175 150 125 100 75
Area (km2) 100 275 400 520 600 650
650
km2
100
km2

Evaporation
- Definition: It is the process by which the precipitation that
falls on earth surface is returned to the atmosphere as
vapour.
- Factors affecting evaporation…
1. Temperature
2. Wind
3. Atmospheric Pressure
4. Impurities in water
5. Shape & size of Water surface
27

Evaporation
- Methods of Measurement of evaporation…
a) Evaporation pan (Evaporimeter) Method,
b) Empirical Method,
c) Analytical Method.
28
Evaporation pan
(Evaporimeter)

infiltration
• Factors affecting infiltration Rate,
1. Soil Moisture
2. Type of Soil Medium
3. Permeability
4. Vegetal Cover
5. Compaction of Soil
6. Temperature of water
29

infiltration
• Measurement of Infiltration capacity,
- Horton’s Method
- Field Methods…
a) By Flooding type Infiltrometer,
b) Rainfall Simulator type Infiltrometer.
30

1. Horton’s Method
• It is an empirical formula that says
- Infiltration starts at a constant rate f0 and is decreasing
exponentially with time, t. After some time when the soil saturation
level reaches a certain value, the rate of infiltration (ft) will level off to
the rate, fc.
ft = fc + ( fo - fc) e-kt
Where
31
ft = Infiltration rate at time t;
fo = Initial infiltration rate or maximum
infiltration rate;
fc = Constant or equilibrium infiltration rate
after the soil has been saturated or
minimum infiltration rate;
k = The decay constant specific to the soil.

2.a. Flooding type infiltrometer
1. It consists of a metal cylinder open
at both ends.
2. It has 22.5 cm diameter & 60 cm height.
3. The cylinder is driven into the ground
by hammer such that 10cm length
remains above the ground.
4. Water is filled in the cylinder.
5. A pointer is set to mark water level.
6. As the infiltration takes place, the
water level goes down.
7. Water level is maintained constant through the burette.
8. Reading on the burette are taken at a regular interval to determine
the rate & amount of infiltration.
9. The experiment is generally continued till a constant infiltration rate
is obtained. 32

2 b. Double-ring infiltrometer
1. It consists of two cylinder called rings.
2. The inner ring is of 22.5cm in dia. & outer
ring is of 35cm dia.
3. The inner ring is used to determine the
infiltration rate.
4. The water level in between the two rings
is maintained at same level to reduce the
other effects.
5. The outer ring provides a sort of water
jacket to the inner ring to infiltrate the
water vertically downward without
spreading.
6. It is more accurate than simple flooding
type infiltrometer.
33

Infiltration indices
- The infiltration capacity of the soil decreases with an increase of
duration of rainfall.
- For computing the runoff & flood discharge, the use of infiltration
capacity curve is not convenient.
- It is more convenient to use an average constant value of infiltration
rate called Infiltration index.
- There are two types of indices, ф-index & W- Index.
Infiltration Capacity Curve Ф- Line
34

Infiltration indices
ф-index
• It is the average rate of rainfall
such that the volume of rainfall
in excess of that rate is equal to
the volume of surface runoff.
• For determination of ф-index, a
horizontal line is drawn on a
hyetograph such that the shaded
area above that line is equal to
the volume of surface runoff.
• The unshaded area below the
line represents all losses
including interception,
depression storage & infiltration.
W- Index
35

PROBLEMS
Surface Runoff
ф-index & W-index
36
?? A 6-hours storm produced rainfall intensities of 5, 15, 25, 13, 9 & 2
mm/hr in successive 1 hr intervals over a catchment of 900 km. sq.
The resulting runoff is observed to be 2750 ha-m. Determine the ɸ-
index.

Stream gauging
• Definition:- The process of measuring discharge of a
stream is called Stream Gauging.
• Stream gauging is done for determination of Runoff
from catchment by measuring discharge of river at
outlet of catchment.
CURRENT METER
PROPELLERE TYPE
CURRENT METER
37

Selection of stream gauging site
The following factors are considered while
selecting a stream gauging…
1. The site should be easily accessible.
2. The river reach at the site should be straight & uniform.
3. The c/s of the river at the site should be well-defined.
4. The banks & bed should be firm & stable.
5. There should be a good permanent control section located at the
downstream of the site.
6. The best site is where the river c/s is regular & V-shaped with
sufficient depth for immersing the current meter.
38

Measurement of discharge
of stream
Methods used for measurement of discharge
of river…
A) Direct Measurement Methods:
1. Area-Velocity Method,
2. Moving boat Method,
3. Salt concentration/Tracer method,
B) Indirect Measurement Methods:
1. Slope-Area Method,
2. By using Notches, Weirs, Venturi-flumes & Spillways.
39

1. AREA-VELOCITY METHOD
1. In this method, the discharge is
determined from the area of c/s &
the mean velocity.
2. The area of c/s of the river is determined
from the profile of the river bed
obtained by sounding.
3. The river c/s is divided into a suitable number of vertical
segments (generally 15-25 segments).
4. The discharge in each segment is equal to the area of the
segment multiplied by mean velocity of flow.
5. The total discharge in the river is computed as the sum
of the discharges in various segments.
A. DIRECT MEASUREMENT METHOD…
40

a) Mid-Section Method b) Mean-Section Method
There are two methods…
41

• Mid-Section Method :-
1. In this method, the area of c/s is
divided into several segments & the
mean velocity is measured on a
vertical line in the middle of the
segment.
2. The mean depth of the segment is
taken at the middle of the segment.
3. Now by multiplying mean depth to
area of segment determine the
discharge in that segment.
4. The total discharge is the sum of all
segment discharge.
• The discharge in the end tow
triangular strips is usually neglected.
42

• Mean-Section Method :-
1. In this method, the segment is taken
between two vertical lines on which
the velocity & depth are measured.
2. The velocity in the segment is taken as
the average of the mean velocities of
the two adjacent vertical lines.
3. Similarly, depth is also taken as the
average of two depths of vertical lines.
4. The discharge of segment is
determined by multiplying avg.
velocity to avg. depth of segment.
5. The total discharge is calculated by
sum of all discharges of segments.
43

2. Moving boat METHOD
Fig.a
Fig.b
1. Discharge measurement of large rivers by
velocity-area method is quite time
consuming & tedious.
2. This method is very useful when river is
in spate (full & flowing fast).
3. In this method, the boat is towed rapidly
to cross the river normal to the direction
of flow.
4. A current meter is fixed to the boat by
vertical axis at a constant depth of 0.5m
from the surface.
5. The velocity recorded by the current
meter is the resultant velocity of the boat
& the stream. (as shown in fig.a)
6. An angle indicator is attached to measure
the angle.
7. An echo sounder is used to record the
depth of flow at different sampling
points.
44

3. Salt concentration or
tracer METHOD
1. This method is used for small & turbulent
streams in mountainous regions.
2. The main advantage of this method is
that it gives discharge directly.
3. In this method, a reach is selected & a
solution of a common salt or sodium
dichromate is injected of a known
quantity at a constant rate at the
injection point of reach.
4. The solution get mixed & diluted with
river water.
5. The concentration of salt at the measurement point of reach is determined after taking a
sample of water.
6. The total discharge is determined from the continuity equation assuming that the total
amount of salt remains constant.
45

B) Indirect Measurement Method
• It is commonly used to determine the maximum discharge in the
river during flood.
• In this method, discharge is computed using Manning’s formula
(or Chezy’s Formula) from the area of c/s & the slope of water
surface.
• Manning’s Formula, V = 1/n R2/3 S1/2
• Discharge is Q= A x V = A/n R2/3 S1/2
Where, A = c/s Area determined upto High Flood Level(HFL),
n = Manning’s Roughness coefficient,
R = Hydraulic Radius = A/P, P= Perimeter of River Bed,
S = Water surface slope= h/L, h= difference in
water levels in the Length L of the reach
V = Velocity of Water
1. Slope-Area Method
46

• The c/s area (A) is obtained by taking soundings at intervals
of 3 to 6m. The c/s is area plotted & determined upto the
HFL.
• The water surface slope (S) is determined by noting water
levels at upstream 1 km & also downstream 1km of site
( h) with measured length (L) of that reach (e.g. 2km).
• The Manning’s Coefficient (n) depends on the nature of
the bank, bed surface & condition of river.
• Sometimes, Chezy’s Formula is used to determine
Discharge of river…
Q = A C R1/2 S1/2
1. Slope-Area Method
47

MODERN STREAM GAUGING METHODS
1. Conventional measurements of river flows are costly,
time-consuming, and frequently dangerous.
2. The purpose of the modern methods are to directly
measure the parameters necessary to compute flow,
surface velocity (converted to mean velocity) and cross-
sectional area,
3. The aim of these methods is to avoid the uncertainty,
complexity, and cost of maintaining rating curves.
4. They are also called as Non-contact methods.
5. The results are accurate & methods are safe.
48

Advance techniques/ equipment used in
gauge discharge measurement
1. Radar System,
2. ADCP (Acoustic Doppler Current Profiler),
49

1. Types of Radar systems used
1. Continuous Wave Radar,
2. UHF Doppler Radar,
3. Pulsed Doppler Radar,
4. Ground Penetrating Radar
50

• River surface water velocity is obtained by
- Pulse Doppler Radar,
- Continuous Microwave Radar,
- UHF Doppler radars
• River channel cross sections is measured by
- Ground Penetrating Radar (GPR)
Microwave Radar GPR Reflections Display
51

Current meters
• It is generally used to measure velocity of water in the river.
• ADCP is the hydro acoustic current meter used for measure
velocity profile of water in river.
• BoogieDopp (BD) is a fairly new acoustic instrument is
designed for small and shallow rivers.
PROPELLERE TYPE
CURRENT METER
CUP TYPE
CURRENT METER
ADCP
52

2. Acoustic Doppler current Profiler
(ADCP or ADP)
• It is a Type of current meter, attempting to
measure water current velocities over a depth
range using the Doppler effect of sound
waves scattered back from particles within
the water column.
• It is used to determines water velocity
profiles by transmitting sound pulses at a
fixed frequency and measuring the frequency
(or phase) shift in acoustic echoes reflected
back from scatterers in the water.
Head of an ADCP with
the four transducers53

Velocity Contours at c/s of river by using
ADCP & BD
Complete velocity distribution in a cross-section of San Joaquin River at Vernalis, California.54

System used for river Staging
1. Shaft Encoders,
2. Bubblers System.
55

Bubbler system
used in river staging
1. In a bubbler system, an orifice is
attached securely below the water
surface and connected to the
instrument by a length of tubing.
2. Pressurized gas (usually nitrogen or
air) is forced through the tubing and out
the orifice.
3. Because the pressure in the tubing is
depends on the depth of water over the
orifice, a change in the stage of the river
produces a corresponding change in
pressure in the tubing.
4. Changes in the pressure in the tubing are recorded and are converted to a record
of the river stage.
56

Shaft encoder
used for river staging
• Shaft encoders convert shaft rotation
to pulses for accurate and precise
measurements of river stage.
• Mechanical float and pulley assembly
rotates with fluctuating water level.
• As a float rises or falls, a pulley rotates
the shaft of the encoder, generating
signals for both rotation direction and
amount.
• Encoders read the signals and send a
calculated water level to the datalogger.
57

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