This document discusses open channel flow. It defines open channel flow and describes the different types of channels and flows that can occur, including steady/unsteady, uniform/non-uniform, laminar/turbulent, sub-critical/super-critical flows. It also discusses point velocity, how velocity varies across a channel, and how the average velocity is calculated.

1. Prepared By:
Parth Desani
Civil – 3rd Year

2. Contents
 Open Channel Flow
 Types of Channels
 Types of Flow in open channel
 Point Velocity
 Velocity Distribution in Open Channel

3. Open Channel Flow
 Open Channel Flow means the flow through channel
that is open to atmosphere and have a free surface.
The pressure on the free surface is atmospheric.
 The flow in a close conduit may also be categorized
as open channel flow, if the fluid level falls below the
crown of the pipe.
 Open channel flow is needed to study for the
following purposes :
 Estimate of discharge in a river or canal
 Development of relationship between depth of
flow and the discharge in a channel
 Design of canal
 Estimating the area of submerged due to
construction of dam on a river

4. Types of flow channels
 The different different flow channels are categorized
into two categories based on shape :
Non – Prismatic ChannelPrismatic Channel

5. Types of flow in open channel
 The different flow types in open channels are as follows :
 Stead flow :
 A flow is said to be steady if the flow characteristics i.e.
depth, discharge or velocity at a section do not change with
respect to time.
 The flow in a prismatic channel at constant discharge is an
example of the steady flow.
 Unsteady flow :
 If the flow characteristics i.e. depth, discharge and velocity
change with respect to time, then it may termed as
unsteady flow.
 The flow of river during flood is an example of unsteady
flow.

6.  Uniform flow :
 If the flow characteristics, i.e. depth, discharge or velocity
remain constant along the length of channel, the flow is
termed as uniform flow.
 Non – uniform flow :
 The depth of flow varies along the length of channel at
various sections, the flow is termed as non-uniform flow.
 The non-uniform or varied also subdivided into two types :
 Gradually varied flow
 Rapidly varied flow

7.  Laminar flow & Turbulent flow :
 Laminar flow refers to flow in the form of thin sheet
(Lamina). This is only possible when velocity of flow is
very small of the order of the few centimeters per
seconds.
 In the most of the practical cases, the velocity of flow
is large therefore paths of each particles are crossing
each other. This type of flow is defined as turbulent
flow.
 Flow through open channel for all practical cases
would be turbulent flow.

8.  In the case of flow through a circular pipe, the Reynold’s
number defines the criterion for the state of the state of
flow i.e. laminar or turbulent.
 If the value of Reynold’s number is less than 2000, then
is known as laminar flow or else it may said turbulent
flow.

9.  Sub-critical flow and Super-critical flow :
 Gravity is the force responsible for flows through open
channels. Hence, ratio of inertial to gravitational force
will play a major role in the analysis of open channel
flow. This ratio is defined as Froud number, which is
given as
 = 1  Critical velocity and Critical flow
 < 1  Sub-critical, streaming or tranquil flow
 > 1  Super-critical, rapid or shooting flow

10. Point Velocity
 A flowing fluid contains any velocity at every different
points, that velocity at a particular point during
flowing and at particular cross-section, which velocity
is obtained, that velocity can be termed as point
velocity.
 Velocity at any point can be carried out by,
 This velocity is determined for carrying out,
 the discharge
 pressure difference, etc.

11. Velocity Distribution in open
channel
 Looking to the flow in a channel at
first instant, one can say that flow
is occurring in longitudinal direction
or there is a velocity component in
longitudinal direction only.
 But due to presence of corners and
boundaries in an open channel, the
velocity vectors are having three
components viz; Vx, Vy, Vz
(corresponding to longitudinal,
lateral and normal or perpendicular
to flow).

12.  Mostly Vx is only considered and
small vectors Vy and Vz ignored.
 Velocity distribution is dependent on
the geometry of channel. Here
Velocity is 0 at bottom and gradually
increases with distance.
 Here is shown a typical profile in a
plane normal to direction of flow. It is
found that max velocity occurs at
0.4y from bed or 0.6y from top
where y is depth.
y
0.2y
0.8y
V0.8
V0.4
V0.2
Channel Bed

13.  Average velocity can be found by,
Vav = (V0.2 + V0.8) / 2
 Where V0.2 is velocity is at 0.2 y, V0.8 is velocity at
0.8y.
 This property of the velocity distribution commonly
used in the calculation of discharge of rivers and
canals using area-velocity method.