OPEN CHANNEL FLOW

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OPEN CHANNEL HYDRAULICS
OPEN CHANNEL FLOW

• Department of Water Resources and Irrigation
  Engineering
• Kombolcha Institiue of Technology

Ins.G.D
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CHAPTER ONEOPEN CHANNEL FLOW

• Question
• What is the most obvious difference between pipe
  flow and open channel flow?????????????
• (in terms of flow conditions and energy
  situation)

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Open Channel Flow

• Is a flow of liquid in a conduit with free space.
• (interface between water and air)
• is applied to understand the flow of a liquid in
  
• Artificial channel (flumes, spillways, canals,
  weirs, drainage ditch, culverts) and
• Natural channel (streams, rivers, flood plains).

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Open Channel Flow

• Open-channel flow must have a free surface,
  whereas pipe flow has none.

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Open Channel Flow

• A free surface is subject to atmospheric
  pressure.
• In Pipe flow there exists no direct atmospheric
  flow but hydraulic pressure only.
• it is much more difficult to solve problems of
  flow in open channels than in pipes.
• complication comes from the position of the free
  surface which will change with time and space.
• And also by the fact that depth of flow, the
  discharge, and the slopes of the channel bottom
  and of the free surface are all inter dependent.

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Summary between open channel flow and Pipe Flow
Table 1
Pipe flow Open channel flow
Flow driven by Pressure work Gravity(potential energy)
Flow cross section Fixed ,known Unknown in advance because flow depth vary
Characteristics of flow parameters Velocity deduced from continuity Flow depth deduced both from continuity and moment equation
Specific boundary Atmospheric pressure at free water surface
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Classification of open channel flow

• are made according to change in flow depth with
  respect to time and space.

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Classification of open channel flow

• Steady and Unsteady Time is the criterion.
• Flow is said to be steady if the depth of flow at
  a particular point does not change or can be
  considered constant for the time interval under
  consideration. i.e.
• The flow is unsteady if depth (flow x-ics)
  changes with time.

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Classification of open channel flow

• Uniform Flow and non Uniform Space as the
  criterion.
• Open Channel flow is said to be uniform if the
  depth and velocity of flow are the same at every
  section of the channel. i.e
• Hence it follows that uniform flow can only occur
  in prismatic channels.

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Classification of open channel flow

• Open Channel flow is said to be non uniform if
  the depth and velocity of flow varies at every
  section of the channel. i.e
• Non uniform flow classified as rapidly flow
  (R.V.F) and gradually varied flow (G.V.F)

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Classification of open channel flow

• For steady uniform flow, depth and velocity is
  constant with both time and distance.
• This constitutes the fundamental type of flow in
  an open channel.
• It occurs when gravity forces are in equilibrium
  with resistance forces.

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Classification of open channel flow

• Steady non-uniform flow
• Depth varies with distance but not with time.
• This type of flow may be either-
• (a) gradually varied -
• (requires the application of the energy and
  frictional resistance equations) or
• (b) rapidly varied-
• (requires the energy and momentum equations).

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Classification of open channel flow

• Unsteady non uniform flow-
• The depth varies with both time and space.
• This is the most common type of flow and requires
  the solution of the energy momentum and friction
  equations with time.
• In many practical cases the flow is sufficiently
  close to steady flow therefore it can be analysed
  as gradually varied steady flow.

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Classification of open channel flow summary
figure 2
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Types of Open channel

• Artificial channels
• are channels made by man.
• They include
• irrigation canals, navigation canals, spillways,
  sewers, culverts and drainage ditches.

CULVERT
Subsurface drain
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Types of Open channel

• In the field they are commonly constructed of
  concrete, steel or earth and have the surface
  rough nesses reasonably well defined (although
  this may change with age particularly grass
  lined channels.)
• Analysis of flow in such well defined channels
  will give reasonably accurate results.

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Types of Open channel

• Natural channels
• Streams, rivers and etc.
• are neither regular nor prismatic and their
  materials of construction can vary widely
  (although they are mainly of earth this can
  possess many different properties.)
• The surface roughness will often change with time
  distance and even elevation.
• It is more difficult to accurately analyze and
  obtain satisfactory results for natural channels
  than is does for manmade ones.
• The situation may be further complicated if the
  boundary is not fixed i.e. erosion and deposition
  of sediments.

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Velocity and
Pressure Distribution in open channel

• The measured velocity in an open channel will
  always vary (not uniformly distributed) across
  the channel section because of the presence of a
  free surface and frictional resistance along the
  boundary.
• Neither is this velocity distribution usually
  symmetric (as it is in pipe flow) due to the
  existence of the free surface.
• The velocity distribution at a solid boundary
  must be zero, and in open channel it generally
  increases with the distance from the boundaries.

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Velocity and
Pressure Distribution in open channel

• The maximum velocity doesnt occur at free
  surface instead it occurs at usually below the
  free surface water at a distance of 0.05 to 0.25
  of depth.
• The velocity distribution in a channel is
  measured either with the help of pitot tube or
  current meter.
• On account of non-uniform velocity distribution
  over channel section, correction factor will have
  to be applied while calculating Kinetic energy
  and momentum.

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Velocity and
Pressure Distribution in open channel

• The kinetic energy correction factor ?(alpha) is
  also known as coriolis coefficient in honors of
  G.coriolis who firs proposed it.
• The momentum correction factor ?(beta)Boussinesq
  coefficint after J.Boussinesq who first proposed
  it.
• Experimental data indicate that in fairly
  straight prismatic channel turbulent flow the
  alpha value ranges from 1.03 to 1.36 and that of
  beta ranges from about 1.01 to 12,however, for
  simplicity the value of both alpha and beta
  assumed to be unity.

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The figure below shows typical Velocity
Distribution across some channel cross section
.The number indicates percentage of maximum
velocity
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THE END!
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