Basic Hydrology & Hydraulics: DES 601

Module 16Open Channel Flow - II

Steady Uniform Flow• Steady flow means that the discharge at a

point does not change with time.

• Uniform flow means that there no change in

the magnitude or direction of velocity with distance, that the depth of flow does not change with distance along a channel.

• This uniform flow definition implies constant channel geometry – more importantly, geometry and flow are related.

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Steady Uniform Flow• Steady uniform flow is an idealized concept of open

channel flow that seldom occurs in natural channels and is difficult to obtain even in model channels.

• However, in many practical highway applications, the flow is assumed to be reasonably steady, and changes in width, depth, or direction (resulting in non-uniform flow) are assumed to be sufficiently small so that flow can be considered uniform.• Examples: Short sections of drainage

infrastructure, bridge deck drainage, etc.

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Steady Non-Uniform Flow• Steady non-uniform flow is flow that is steady (no

change in Q with time), but the flow geometry can (and does) change in space.

• Two kinds of non-uniform, steady flow are:• Rapidly varied flow:

• the changes take place abruptly over short distances. (Typically as flow changes between super- and sub-critical)

• Gradually varied flow:• the changes take place over long distances,

and occurs within one flow regime (sub- or super-critical)

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Gradually Varied Flow• Gradually varied flow (GVF) is important in drainage

engineering to account for:• Backwater effects (flow draining into a “pool”

situation) • Frontwater effects (flow accelerating over or

under a structure).• GVF conditions are characterized by relationships of

normal and critical depths, slope designations, and water surface profile “shapes”

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Slope Designation RelationsSlope Designation Critical to Normal Relationship RemarksSteep – S

Critical – C

Mild – M

Horizontal – H

Adverse – A

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Profile-Type RelationshipsProfile Type Logic

Type – 1 .AND.

Type – 2 .OR.

Type – 3 .AND.

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Slope/Profile Sketches• The GVF slope and profile designations convey

information on control (of flow) and are useful for:

• Selecting control sections for measurements

• Selecting geometries to produce desired flow depths near infrastructure

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M1 water surface profile

• Indicative of downstream control• Flow into a “pool” or forebay, flow approaching a

weir.

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M2 water surface profile

• Indicative of downstream control• Flow accelerating over a weir, waterfall, or

contraction but otherwise sub-critical

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M3 water surface profile

• Indicative of upstream control • Flow under a sluice gate, a jet from a culvert

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S1 water surface profile

• Indicative of downstream control

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S2 water surface profile

• Indicative of upstream control• Acceleration of flow just past a submerged weir on a

steep slope

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S3 water surface profile

• Indicative of upstream control• Flow under a sluice gate on an OGEE spillway

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Froude Number• Recall the specific energy diagram, the energy

minimum for a given discharge occurs when the dimensionless Froude number (Fr) is unity

• The Froude number is the ratio of inertial to gravitational forces in flow. In a wide channel or rectangular channel the number is well approximated by

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Froude Number• The Froude number also classifies the flow.

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Energy and Momentum• The short segment of open channel between two

sections is called a reach.

• The momentum change in a reach is related to the frictional forces of the channel on the water in the reach, the gravitational force on the water in the reach, and the difference in pressure forces at the upstream and downstream sections.

• Momentum change is important in computing forces of water on structures as well as determining the location of abrubt changes in flow regime.

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• Momentum equation for steady open channel flow is (after considerable algebraic simplification)

Energy and Momentum

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Example – Hydraulic jump• A hydraulic jump occurs as an abrupt transition from

supercritical to subcritical flow. There are significant changes in depth and velocity in the jump and energy is dissipated.• Specific energy changes across a jump.• Momentum however is nearly conserved, hence

computations would use the momentum equation

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Example – Hydraulic jump• The potential for a hydraulic jump to occur should

be considered in all cases where the Froude number is close to one (1.0) and/or where the slope of the channel bottom changes abruptly from steep to mild.

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