This document discusses irrigation and various flow measurement methods in open channels, focusing on techniques such as the velocity area method, direct discharge methods (including weirs, orifices, and flumes), and specific types of weirs like rectangular, trapezoidal, and V-notch weirs. It emphasizes the principles of conservation of mass and energy that underlie flow measurement, along with formulas for calculating discharge through these structures. Additionally, the document covers tools like current meters and Parshall flumes and provides guidelines for their effective installation and use.

1. Lecture - 12
Irrigation - measurement of flow in open channels -
velocity area method - Rectangular weir -
Cippoletti weir - V notch - Orifices - Parshall flume

2. IRRIGATION
• It is the process of artificial application of water to plants at regular interval and it is
help to grow agricultural crops.
Measurement of flow in open channel:
a) Velocity area method
• Float method
• Current meter
• Tracer
b) Direct discharge method
• Weir – Rectangular and Cippoletti weir
• V-notch
• Orifices
• Parshall flume
Measurement of flowing water is usually expressed as litre/second or cubic
metre per second

3. Fundamental Equations
• Flow measurement are based on the principle of conservation of mass
and energy
1. Considering water is an incompressible liquid, the principle of
conservation of mass leads to continuity equation
Q = A x V
2. The principle of conservation of energy is stated by the Bernouill’s
equation
P = pressure
V = velocity
Z = Elevation about datum head
ϒ = specific weight of liquid
g = acceleration due to gravity

4. Velocity area method
Float method:
• Cheap and simple and gives only approximate measure of the
rate of flow
• Velocity of flow is made by determining the velocity of an
object floating
• A straight uniform section of the channel about 20 to 25 m long
is selected and marked on the banks
• The time required to traverse the distance by the floating object
is measured and the velocity is calculated
• Mean velocity in the whole of the cross section is obtained by
applying coefficient 0.80 to surface velocity
• Rate of flow = area of cross section x average velocity of flow

6. Current meter
• Current meter consists of a
wheel having several cups or
wheels attached to a streamlined
weight and assembly suspended
by a cables or mounted on
straight rods
• The wheel in the current meter is
rotated by the action of flowing
water
• The rate at which wheel revolves
varies with the velocity of water

8. Direct Discharge methods
• The rate of flow are measured directly by using certain devices
they are weirs, orifices and flumes
Weir:
A weir is a notch of regular form through which water flows.

9. Construction and Placement
of weirs
• The weir should be installed in a
straight line at right angles to the
direction of flow
• The crest and side of the edge should be
sharp edged
• The velocity of approach of water to the
weir should be negligible
• The head over the weir should be
measured on the upstream side at the
distance of at least 4 times the head of
flow from the weir
• The weir crest should be placed no
closer than 2H from the bottom of the
channel
• The distance between end of notch and
sides of the channel should not be less
than 2 times depth of flow of water over
the weir
• The falling water surface from the weir
should have a free flow

11. • The basic formula for calculating discharge through a weir is
Q = C x L x Hm
Q – discharge
C - coefficient dependent on the nature of crest and approach
conditions
L – length of crest
H – head of water over the crest
m – an exponent, dependent on the weir opening

12. Different types of Weir
Rectangular weir –
• Level crest and vertical sides
• Measures for large discharge
• Rectangular weir, the discharge is
given by the formula
Q = 0.0184 L H3/2
Q = discharge in m3
/sec
L = length of weir crest in cm
H = height of head over crest in cm

14. Trapezoidal weir or Cipolleti weir
• Level crest and sides of the notch
sloping outward from the vertical at one
horizontal to four vertical
• The trapezoidal weir with 1:4 side slope
is known as cipolleti weir
• The discharge through Cipoletti weirs
may be computed by the following
formula:
Q = 0.0186 L H3/2

16. Triangular or V- notch
• The triangular or V- notch weir
is made with different angles
like 450
, 900
and 1200
• The 900
V – Notch is most
commonly used
• More accurate measurements
for lower discharge
• The discharge through 90° V
notch weirs may be computed
by the following formula:
Q = 0.0138 H5/2

18. Orifices
• An orifice plate can be used to measure water in
comparatively small areas.
• Orifice is an opening with closed perimeter and of a regular
shape through which water flows
• The stream of water coming out of the orifice is known as Jet
• If this jet discharges into the air, the orifice is said to have free
discharges and if the discharge is under water is called a
submerged orifice.
• The depth of water producing the discharge is called head
• Orifice may be circular, square, rectangular or any other
regular shape. Circular and rectangular orifice are most
common for the measurement of water on the farm

19. • The equation of discharge through a sharp edge orifice is
given by
Q = C.a (2gH)0.5
Q = Discharge in litres per second
a = Area of the orifice opening in sq.cm
G = acceleration due to gravity
H = Head of water causing the flow in cm
C = coefficient (taken as 0.62)

20. Parshall Flume
• Parshall flume consists of three parts:
• Converging section on upstream end, Constricted section
known as throat and diverging section on the downstream side
• The floor of converging section is level, the floor of throat
inclines downward and the floor of diverging section slopes
upwards
• The size of the parshall flume is specified by its throat width
• To measure the small discharge from a very small fraction of
cu.m per sec. to more than 100cu.m per sec.

23. THANK YOU