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Channel Transitions

Channel Transitions. 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.

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Channel Transitions

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  1. Channel Transitions

  2. 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.

  3. 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.

  4. y y=E y1 y2 Dz yc E2 E1 E Specific Energy Representation of a Transition

  5. 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.

  6. 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%

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

  8. In analyzing hydraulic jumps we assume there is conservation of momentum, i.e. : We can algebraically manipulate this to find:

  9. y1 is known as the initial depth and is < yc (supercritical flow). • y2 is known as the sequent depth and is > yc (subcritical flow). • The energy loss in a hydraulic jump can be found by:

  10. 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?

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