Channel Transitions

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Channel Transitions
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• Channel transitions occur where there is a change
  in width, shape, slope, roughness, bottom
  elevation of the channel.
• For changes in slope and roughness we can use
  backwater curves to evaluate the effects of the
  changes.
• For other types of smooth transitions
  (transitions were energy loss is minimal), we can
  use energy relationships to evaluate the impact
  of the transition.

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EGL
v2/2g
y
yc
Dz
datum
1
2

• If we assume no energy loss through the
  transition we can say

which says for a constant total energy E, there
is a specific energy loss between 1-2 of Dz.
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y
yE
y1
y2
Dz
yc
E
E1
E2
Specific Energy Representation of a Transition
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• If the flow must pass through critical depth, the
  assumption of no energy loss may not be valid.
• This is especially true when going from
  supercritical to subcritical flow.
• A hydraulic jump accompanied by considerable
  energy loss occurs.

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Example
Problem 4.27 A concrete rectangular channel goes
through a smooth transition. The flow depth is
originally normal depth. The flow rate is 50
cfs. If there is no energy loss what will be the
depth of flow immediately after the transition?
The channel properties are Upstream Downstream
b 10 ft b 8 ft s 0.05 s 0.05
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Hydraulic Jumps

• Occurs when there is a sudden transition from
  supercritical (y lt yc) to subcritical (y gt yc)
  flow.
• Examples of where this may occur are
• At the foot of a spillway
• Where a channel slope suddenly turns flat.

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• In analyzing hydraulic jumps we assume there is
  conservation of momentum, i.e.

We can algebraically manipulate this to find
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• y1 is known as the initial depth and is lt yc
  (supercritical flow).
• y2 is known as the sequent depth and is gt yc
  (subcritical flow).
• The energy loss in a hydraulic jump can be found
  by

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Example
Problem 4.31 A hydraulic jump occurs in a wide
rectangular channel where the flow is initially
at a depth of 1 ft and a flow velocity of 14 fps.
What is the depth after the jump? What is the
energy loss within the jump?