transition submergence and hysteresis effects in three-foot … · 2013-12-28 · • all such...
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Transition Submergence and
Hysteresis Effects in Three-Foot Cutthroat
Flumes
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Why Measure Water for Irrigation?
• (You had to ask.)• Improve:
– Accuracy– Convenience– Economics
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Water Measurement Manual
• (Door Prize)• Published by Reclamation in 1997
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• Small mistakes at the beginning can result in big errors at the end
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Measuring Water Has Been Going on For a Long Time
• Older techniques may be simple, but effective for some purposes.
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Goal: No Math in this Presentation!
• This is not possible.
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Basic Open Channel Flow Measurement
• Q=AV• Flow Rate = Area of flow multiplied by
the (average) velocity of the flow.• e.g. Channel with a cross section of 10
ft2 and water traveling at 2 feet per second
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At its simplest
• Use a buoyant object– An orange
• Multiply velocity of float by a coefficient– e.g Depth of water is 3 feet: 0.70– Depth of water is 12 feet: 0.78– More values are available
• Multiple floats
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More better
• Find the average velocity in a stream 60% of the way from the water surface to the bottom
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Best
• Check the velocity everywhere (or at more than 1 point)
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Dep
th
Velocity (avg)
Relative roughness determines the distribution shape
Velocity Distributions
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Velocity With Stage
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So how do you measure velocity?
• Show Flash• Show wmv
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Bernouilli’s Principle
• Bernouilli says there are three forms of energy in water:– Elevation head– Pressure head– Velocity head
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Think of a swimming pool
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Everything you need to know is in this slide (but the font is too small)
NEH 652 9-206Flow measurement is based on specific predeterminedhydraulic concepts. Measurement accuracy is stronglyinfluenced by adherence to these concepts. • For open channel weirs and flumes, water must pass
through critical depth or two flow depths must be measured.
• With closed conduits the pipeline must be flowing full at the measuring device. This can be accomplished by dropping the pipeline below the hydraulic grade line.
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Reclamation Video
• 4 types of measuring devices– Cipolletti Weir– Yakima Box– Submerged Orifice– Ramp Flume
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Weirs and Flumes
• Weirs and flumes work on the principle that the flow over the weir or flume must go through the critical depth. It is the height of a weir or flume that determines whether or not the flow goes critical.
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Critical-Flow Measurement Devices
• Flumes, sharp-crested weirs, broad- crested weirs
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Critical Depth
• And why it is important in measuring flow rates
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Tim McCabe, IA NRCS
Subcritical
Hydraulic Jump
Supercritical
Subcritical
Critical
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Subcritical
Hydraulic Jump
Critical
Subcritical
Supercritical
Lynn Betts , IA NRCS
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Hydraulic Jump
• Change from supercritical to subcritical• Typical below dams or obstructions • Very high-energy loss/dissipation• Difficult to predict location• Water surface “jumps” up
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Point of entryof the Stone First Wave
Second WaveThird Wave
(a) Sub-Critical Flow (b) Critical Flow (c) Super-Critical Flow
WavesTravelUpstream
WavesTravelDownstream
Direction of Flow
Subcritical or Supercritical
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Flumes and Weirs
• This difference in elevation of the flow upstream from the structure with and without the flume or weir in place is the headloss caused by the device.
• Flumes tend to have less headloss than weirs
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Critical-Flow Measurement Devices
• Produce critical-depth flow in a control section– Critical depth occurs at locations where the
downstream depth does not “hold the flow back”• Minimum specific energy for a given flow• Shallow-water waves cannot travel upstream• Tailwater does not affect headwater elevation
– Flow rate through the critical section is a function of the upstream head, acceleration of gravity, and the control section size
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Long-Throated Flumes and Broad-Crested Weirs
• Long-throated flumes with a streamlined converging transition have one-dimensional flow in the control section -- Long-throated means the throat is long enough to eliminate lateral and vertical contraction of the flow at the control section, so streamlines are essentially parallel to one another
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Long-Throated Flumes and Broad-Crested Weirs
– Can be calibrated using well-established hydraulic theory
• No laboratory testing needed– Calculations are iterative, but computer
models that do the calculations have made long-throated flumes reasonable to implement in recent years
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Traditional Critical-Flow Devices• Most critical-flow devices have curvilinear,
three-dimensional flow fields in the control section
• All such devices require laboratory calibration• Flumes
– Parshall flumes, cutthroat flumes, H-flumes, etc.• Sharp-Crested Weirs
– V-notch weirs, Cipoletti weirs, contracted and suppressed rectangular weirs, etc.
• Broad-Crested Weirs– If they do not have a streamlined approach
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Flumes and Weirs
• Permanent or portable installation• Can be very accurate• They are obstructions that produce
backwater that extends upstream and raises the water surface in the approach channel
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Ramp Flumes
• Also Known As• Replogle Flume• Long Throated Flume• Broad Crested Weir
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Transition Submergence and
Hysteresis Effects in Three-Foot Cutthroat
Flumes
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WINFLUME
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Weirs
The importance of an aerated nappe
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WEIR CONCERNS
• Debris on crest—intuitive and obvious• Approach conditions and sediment
buildup• Head measurement location—avoid
measuring in the drawdown zone• Submergence on the downstream side
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Drop a ball
ghVelocity 2=
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Submerged Orifice
• Q=AV• But the area is not what you might
expect
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Orifice Meter
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Did I mention there’s a test?