River Engineering and Cross Drainage works

15CE5164
Advanced Water Resources Engineering
© 2016 KL University – The contents of this presentation are an intellectual and copyrighted property of KL University. ALL RIGHTS RESERVED 1

Stream Gauging:
• It is a technique used to measure the discharge, or the
volume of water moving through a channel per unit
time, of a stream.
• The height of water in the stream channel, known as a
stage or gage height, can be used to determine the
dischage in a stream.

• Streamflow representing the runoff phase of the
hydrological cycle is the most important basic data for
hydrologic studies.
• Streamflow is the only part of hydrological cycle that
can be measured accurately.
• Runoff from a catchment can be determined by
measuring the discharge of the stream draining it.
Stream Gauging:

Necessity of Discharge measurement:
•By measuring the discharge in river the irrigation
water can be distributed uniformly.
•By measuring flow in rivers flood warning can be
issued.

• By measuring flow in canal quantity of loses can be
known.
• The quantity of water flowing in the river helps for
future research.
Necessity of Discharge measurement:

Site selection for stream gauging:
•The bed and banks of the stream should be firm
and stable so as to ensure consistency of area-
discharge relationship.
•The bed and banks should be free from vegetal
growth, boulders or other obstructions like bridge
piers, etc.

•There should be no larger overflow section at flood
stage.
•To ensure good consistency between stage and
discharge there should be a good control section far
downstream of the gauging site.

•The section should be straight and uniform for a
length of about 10 to 20 times the width of the
stream.
•The stream gauging station should be easily
accessible.

Velocity-Area Methods:
• Discharge is the product of cross-sectional area and
velocity of water;
Q = v * A
where Q = discharge [m3/s], v = velocity [m/s],
and A = cross-section of flow [m2].
• Velocity of flow at a cross-section varies, it is not
enough to measure the velocity at a single point.

•Based on accuracy required, width of the stream
is divided into a number of vertical portions.
•In each portions, velocity is measured at one or
more points along the depth to get a
representative velocity.

•The area of the individual portion can be easily
calculated if the bed profile and stage are
known.
•The velocity may be measured by a conventional
method or by an advanced procedure.

•If the physical and hydraulic conditions at
the site permit, a fixed, undeformable
structure may be constructed to measure
river flow.
• A number of hydraulic structures are used
to measure flows in field conditions
Discharge Measurement Using Artificial
Structures:

Weirs:
•They are used to control upstream water level or
for measuring discharge or for both.
•They produce a critical relationship between
stage and discharge by obstructing channel flow.
•Weirs have a defined cross-section and hence
the computation of discharge is simple

Weirs:
• The size and cost of the structures increase
as the size of the river increases.
•Site requirements consist of a reasonably
straight approach channel which should be
free of excessive sedimentation, weeds and
other aquatic growth.

Weirs:
•The structure should be rigid, water-tight,
normal to the flow direction, and should be
capable of passing high flows without any
damage to its body.
•The stage-discharge relation at the site
depends on the geometrical characteristics
of such a structure.

Weirs:

Flumes:
• A flume is a flow measuring structure formed by a
constriction in a channel.
• The constriction can be either a narrowing section of
the channel or a narrowing section in combination
with a hump in the invert.
• A unique stage-discharge relationship exists
independent of the downstream conditions.

Flumes:
For a rectangular flume, the discharge of an ideal
fluid is expressed as -
here, H represents the upstream energy and b is
the typical width dimension for the particular
cross-section shape of the flume.

Flumes:

Flumes:
By introducing suitable coefficients, this equation can
be generalized to the following form –
Where, Cv = coefficient to take in to account the
velocity head in the approach channels, Cs =
coefficient to take account of the cross-section shape
of the flume, Cd = coefficient for energy loss, and h =
depth of water, upstream of the flume, measured
relative to the invert level of the throat.

Dilution technique:
• Two dilution techniques are –
(1)the steady feed method and
(2)the instantaneous, point - source time indigenous
method.
For steady feed method, a solution of tracer material with
concentration C1 is injected at the constant injection rate
QT;

Dilution technique:

• The tracer disperses laterally into the flow and tracer concentration
distribution is similar to as shown in figure.
• At some point X2 downstream, where the tracer material is
approximately uniformly mixed, the flow is sampled continuously.
Dilution technique:

•If the tracer mixer has properties similar to
the water, so that there are no density
gradients, vertical mixing is very rapid due
to turbulence of the flow.
• Theoretically, complete lateral mixing
occurs at X but practically it occurs between
20 to 100 times the channel widths.
Dilution technique:

•By instantaneous injection method, a
quantity of tracer w, is injected,
instantaneously at section X and time t0.
•The cloud of tracer disperses laterally
and longitudinally as it moves
downstream.
Dilution technique:

Dilution technique:

• At the section X2, where the tracer is completely
mixed literally, the flow is sampled continuously.
• From the conservation of mass
• in which Q is nearly constant through sampling period
Dilution technique:

The common tracers used are –
(a) Salt solutions.
(b) Radio active tracer are detected by its scintillation
detectors.
(c) Fluorescent dyes with flourometers.
Dilution technique:

Advantage of dilution method-
• They condensed in closed conduits, such as
penstocks,
•Sewers pipe lines, where current-meter
measurements are difficult, and they are fast
and accurate.
Dilution technique:

Disadvantages-
•Expensive for measuring large stream.
•Special equipments required for the
measurements of concentration.
Dilution technique:

Velocity Measurement by Floats:
•A float is a distinguishable article that floats on
the water surface, such as a wooden log, a
plastic bottle partly filled with water, or branch
of a tree.
•Surface or near-surface floats are used for
streamflow measurement which are wooden
cylindrical rods of nearly 0.5 m length.

•They are shaped such that they float nearly
vertically with one third of the length
protruding above the water surface.
•The floats are painted in bright colours for
easy identification in muddy or turbulent
water.

• For a float measurement, two cross-sections
sufficiently far apart on a straight reach of river are
selected.
• The upstream and downstream cross sections should
be sufficiently far apart for accurate assessment of
float travel time (3 to 5 times the width of the river or
a minimum of 20 secs travel time).

• Both the cross sections should be clearly marked by
placing markers so that the exact time when the float
crosses the cross-section can be identified.
• The upstream channel cross section should be divided
into a number (preferably an odd number) of equal
segments as practically feasible in which the floats will
be placed.

• An observer each is positioned at upstream and
downstream ends of the reach such that they are visible
to each other.
• The downstream observer acts as timekeeper and carries
a stop watch.
• Floats may be tossed from a bridge or cableway; if there
is no such facility than they can be thrown in the water
from the river bank.

• When the float crosses the upstream cross-section,
the upstream observer gives a signal to the
downstream observer who notes the time taken by
the float to cover the distance.
• The velocity of the float is equal to the distance
between the two cross-sections divided by the time
taken by the float to cover this distance.

•The mean velocity in the vertical is determined
as the float velocity multiplied by a coefficient
which varies between 0.80 and 0.85.
•This coefficient depends on the shape of the
velocity profile of the river and the depth of
immersion of the float.

Pitot tube:
• It is used for velocity measurements in order to calculate
discharge in laboratory flumes or vey small streams.
• It is not recommended for rivers because it is very
difficult to support it when channel is very wide and
deep.
• The head generated by Pitot tube in open channel is
generally very small due to very low velocities thus,
discharge cannot be measured accurately.

•Formula for velocity
computation is -
V=(2gh)1/2
• where, h = Water
height in tube above
surface of water
Pitot tube:

Current Meters:
•It is used to measure velocity at a point in
the flow cross–section.
•Accurate measurements of the velocity
profile of the stream cross section are made
by current meters.

Working of Current Meters:
•It consists essentially of a rotating element
which rotates due to the reaction of the
stream current with an angular velocity
proportional to the stream velocity.
•The angular velocity acquired by the rotor is
proportional to the velocity of water.

• By placing a current meter at a
point in a stream and counting the
number of revolutions of the rotor
during a time interval, the velocity
of water at that point is
determined.
Working of Current Meters:

There are two main types of current meters –
(a) Vertical-axis meters.
Current Meters:

•These instruments consists of a series of
conical cups mounted around a vertical axis.
•The cups rotate in a horizontal plane and a
cam attached to the vertical axis spindle
records generated signals proportional to
the revolutions of the cup assembly.
Vertical - Axis Meters:

• The normal range of velocities is from 0.15 to 4.0
m/sec.
• The accuracy of these instruments is about 1.5% at
the threshold value and improves to about 0.3% at
speeds in excess of 1.0 m/sec.
• These have the disadvantage that they cannot be used
in situations where there are appreciable vertical
components of velocities.
Vertical - Axis Meters:

(b) Horizontal-axis meters.
Current Meters:

•It consists of a propeller mounted at the end
of horizontal shaft.
•These come in a wide variety of size with
propeller diameters in the range 6 to 12 cm,
and can register velocities in the range of
0.15 to 4.0 m/sec.
Horizontal - Axis Meters:

•These meters are fairly rugged and are not
affected by oblique flows of as much as 15°.
•The accuracy of the instrument is about 1%
at the threshold value and is about 0.25% at
a velocity of 0.3 m/sec and above.
Horizontal - Axis Meters:

The number of rotations are measured and
correlated to velocity using the formula-
•v = a + bN where N is the rotation of the
propeller (revolutions per sec {rps}).
•a and b are coefficients determined by
calibration in an experimental flume.
Rating of the Current Meter:

•These constants differ from one current
meter to the other as a result of
manufacturing variations as well as change
with time due to wear and tear.
•Therefore, each current meter should be
recalibrated periodically.

• Considering the velocity profile with depth, average
value of velocity can be obtained at 0.6 of the depth.
i.e. V = average velocity is at about 0.6 D.
• An alternative of using the 0.6 D velocity is to take 0.2
and 0.8 velocities and obtain the averages.
• The latter method is more accurate but in a shallow
cross-section, the velocity at 0.2 D may be difficult to
measure so use a single value at 0.6 D.

1
b
Propeller Rotation, N
Velocity
a
Surface Velocity
Average Vel
0.6 D
D

Measurement of Area of Flow:
1. Measurement of Width-
(Pivot point method)
•This method is based on the
principle of similar triangles.
•X and X are two points fixed
on the cross-section line on
the bank of the channel.

•From point X a pivot line is erected at right
angles to cross-section XX.
•The length of the pivot line should be about half
the width of the channel or 300 m whichever is
more.
•Other extremity of the pivot line is called a pivot
point, marked P in Fig. 15.10.

•At 1/5 length of the pivot line from the pivot point a
direction line DD1 is drawn at right angles to pivot
line.
•The direction line DD1 is divided in suitable parts of
equal length by points d1, d2, d3 etc. Let the length of
each part be 3 m.
•Then from similar triangles Pd1d2 and Ps1 s2 the
length of s1 s2 is 5 times the length of d1, d2 (since PX
is 5 times PD).

• The length of each strip s1s2, s2s3 etc., is 15 m.
• The points etc., can be located on the cross-section line
very accurately with the help of a theodolite placed at P.
• When the gauge site is to be made permanent the points
D, d1, d2 etc., on the direction line, points and on the
cross-section line and the pivot point should be
constructed with masonry in the form of blocks with a
hole in the centre to fix a flag exactly on the point

2.Measurement of depth-
(a) Sounding rod
• It is a wooden rod 5 to 8 cm in diameter with markings on it.
A bamboo pole may also be used as a sounding rod.
• Many times flat iron of 5 cm x 0.6 cm size may also be used
for the purpose.

• The graduations are generally in tenth of a metre.
•To prevent sinking of a rod and to achieve accuracy in
measurement the rod is provided with a flat base
plate.
•The base plate is in the form of an iron disc 10 to 15
cm in diameter attached to the lower end of the rod.

(b) Eco sounder
• It is based on an electrical principle.
• It consists in transmitting a sound impulse (by a
transmitter) from the surface of water level to the bed of
the river.
• When the sound waves reflect back in the form of an echo
they are arrested by a receiver.

•There is an automatic arrangement for plotting
the time of transmission and the time of
reception.
•The velocity of sound in water is known (1470
m/sec).
• From this fact the depth of water is
automatically computed.

•This method is of
very common use on
ships for observing
depths.
• It can also be used
successfully on rivers.

River Engineering and Cross Drainage works

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