Basic Civil Engineering first year Notes- Chapter 4 Building.pptx
Module 4
1. M O D U L E 4
OPEN CHANNEL FLOW
1
ATHIRA SURESH
I.C.E.T
2. INTRODUCTION
Flow of liquid through a free surface
Free surface is a surface having constant pressure such as
atmospheric pressure
Man made or natural
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6. TYPES OF FLOW IN OPEN CHANNELS
1. Steady flow and unsteady flow
2. Uniform flow and non uniform flow
3. Laminar flow and turbulent flow
4. Sub-critical, critical and super critical flow
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7. Steady and Unsteady Flow
In an open channel flow, if the flow parameters such as depth of flow, the
velocity of flow and the rate of flow at a particular point on the fluid do not
change with respect to time, then it is called as steady flow
If v is the velocity of the fluid, Q is the rate of flow and d is the depth of flow,
then for a steady flow:
dv/dt = 0; dQ/dt = 0; dy/dt = 0;
And is at any point on the open channel flow, the flow parameters like depth of
flow, the velocity of flow and rate of flow do change their value with respect to
time, then it is called as an unsteady flow. It is hence given by :
dv/dt ,dQ/dt and dy/dt not equal to Zero
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2. Uniform Flow and Non-Uniform Flow
The flow in the channel is said to be uniform, if, for a given
length of the channel, the velocity of flow, the depth of flow
remains constant. i.e.
dy/dS = 0 ; dv/dS=0;
In a Non-uniform flow, the flow parameters like velocity, depth
of flow, etc do not remain constant for a given length of the
channel.
dy/dS and dv/dS not equal to zero
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• The Non-uniform flow can be again defined as Rapidly varying flow
(R.V.F) and Gradually Varied Flow (G.V.F).
• In the case of R.V.F, the depth of flow rapidly changes over a smaller
length of the channel. It rises up suddenly for a short length and settles
back.
• While in a G.V.F, the depth of flow changes gradually over a longer
length of the channel.
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3. Laminar Flow and Turbulent Flow
Laminar and turbulent flow in open channel flow is defined based on the
Reynolds Number, Re. The Reynolds number is given by the relation:
If the Reynolds number Re is less than 500 or 600, then the flow is
called laminar flow. If the Reynolds number is more than 2000, then the flow
is said to be turbulent.
A flow that has Reynolds number between 500 and 2000 is said to be in the
transition state.
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4. Critical, Sub-Critical and Super - Critical Flow
The open channel flow is categorized as critical or sub-critical or super-critical
based on the Froude number Fe. Froude number is given by the relation:
Open channel flow is Sub-critical if the Froude number is less than 1. Sub-Critical
open channel flow is also defined as a tranquil or streaming flow.
An open channel flow with a Froude number equal to one is a critical flow.
super-critical flow in open channel has a Froude number greater than 1.
A supercritical flow is also termed as rapid flow or torrential flow or shooting
flow.
16. TYPES OF CHANNEL
Rigid boundary open channels
Loose boundary open channels
Prismatic open channels
Rigid boundary open channels can be said to be as the open channels
with the non-changeable boundaries.
While on the other hand if open channel has the boundaries which changes
due to scouring action or deposition of sediments, such channels are said
to be as loose boundary open channels.
The open channels in which shape, size of cross section and slope of the
bed remain constant are said to be as the prismatic channels. Opposite o
these channels are non-prismatic channels. Natural channels are the
example of non-prismatic channels while man made open channels are the
example of prismatic channels.
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17. Velocity distribution in open channels
Velocity is always vary
across channel(or not
uniformly distributed)
because of free surface &
friction along the boundary
It increases from zeroat the
invert of the channel to a
maximum value close to the
water surface.
the maximum velocity
usually occurs 0.05-0.25
from the free surface
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18. Velocity distribution in open channels
To measure velocity of open channel at required depth, Pitot tube
or current meter are used.
In general, to find average velocity of a particular open channel,
velocity at a depth of 0.6 m from free water surface is measured.
In the other case, velocity at 0.2 m depth, 0.8 m depth from free
water surface is taken and average velocity of these two values is
considered as channel average velocity.
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Mean velocity = (0.2y+0.8y)/2
y= depth of flow
19. Velocity distribution in open channels
Mainly depends upon the following factors.
Shape of the channel section
Roughness of channel
Alignment of channel
Slope of Channel bed
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20. 1. Shape of the Channel Section
Open channels may be naturally formed or
artificially developed.
Natural open channels do not have any particular
shape and they contain irregular sections while
artificial channels are provided with certain designed
shapes such as rectangular, circular, trapezoidal,
triangular etc.
Contour lines of equal velocities in different shapes
of channel sections are shown in below figure
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22. 2. Roughness of Channel
Roughness of channel is the measure of amount frictional resistance offered by
channel bed material against flow of water.
In natural channels, the flow velocity is affected by the presence of large
angular boulders as bed material, vegetation, obstructions etc.
If the channel is made of smooth clay or silt, its roughness is very low and
water flows faster.
In case of artificial channels, smooth finishing is required to maintain required
flow velocity.
The average velocity in open channels can be calculated using manning’s
formula mentioned below
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V = Average velocity of channel
R = hydraulic radius of channel =
Area/Perimeter
S = Slope of channel
n = Manning’s roughness coefficient
23. 3. Alignment of Channel
The velocity of flow in channel
also depends up on the alignment
of channel.
If the channel is straight there will
be no change is velocity with
respect to alignment.
In straight channels, maximum
velocity is generally occurs at 0.05
to 0.15 m depth from free water
surface.
If it is sinuous or meandering, the
velocity will vary at bends.
At bend, due to centrifugal action
of flow the velocity becomes more
at convex side compared to
concave side.
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24. 4. Slope of Channel Bed
Slope of channel bed or gradient of channel will also
effects the velocity of flow in open channels.
At steeper gradients, velocity increases while at
normal gradients velocity decreases.
Channel Slope
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25. GEOMETRIC ELEMNTS OF CHANNEL
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Flow depth, y -Vertical distance from the channel bottom to the free
surface.
Top width, T Width of the channel section at free surface.
Wetted perimeter, P Length of the interface between the water
and the channel boundary.
Flow area, A Cross-sectional area of the flow.
Hydraulic depth, D Flow area divided by top width, D = A/T.
Hydraulic radius, R Flow area divided by wetted perimeter, R = A/P.
Bottom slope, S0- Longitudinal slope of the channel bottom,
