Transition Submergence and Hysteresis Effects in Three-Foot Cutthroat Flumes

Transcription

1 Transition Submergence and Hysteresis Effects in Three-Foot Cutthroat Flumes

2 Why Measure Water for Irrigation? (You had to ask.) Improve: Accuracy Convenience Economics

3 Water Measurement Manual (Door Prize) Published by Reclamation in 1997

4 Small mistakes at the beginning can result in big errors at the end

5 Measuring Water Has Been Going on For a Long Time Older techniques may be simple, but effective for some purposes.

6 Goal: No Math in this Presentation! This is not possible.

7 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 ft 2 and water traveling at 2 feet per second

8 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

9 More better Find the average velocity in a stream 60% of the way from the water surface to the bottom

10 Best Check the velocity everywhere (or at more than 1 point)

11 Velocity Distributions Depth Relative roughness determines the distribution shape Velocity (avg)

12 Velocity With Stage

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14 So how do you measure velocity? Show Flash Show wmv

15 Bernouilli s Principle Bernouilli says there are three forms of energy in water: Elevation head Pressure head Velocity head

16 Think of a swimming pool

17 Everything you need to know is in this slide (but the font is too small) NEH Flow measurement is based on specific predetermined hydraulic concepts. Measurement accuracy is strongly influenced 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.

18 Reclamation Video 4 types of measuring devices Cipolletti Weir Yakima Box Submerged Orifice Ramp Flume

19 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.

20 Critical-Flow Measurement Devices Flumes, sharp-crested weirs, broadcrested weirs

21 Critical Depth And why it is important in measuring flow rates

22 Subcritical Critical Supercritical Hydraulic Jump Tim McCabe, IA NRCS Subcritical

23 Subcritical Critical Supercritical Hydraulic Jump Subcritical Lynn Betts, IA NRCS

24 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

25 Subcritical or Supercritical Direction of Flow Waves Travel Upstream Point of entry of the Stone First Wave Second Wave Third Wave Waves Travel Downstream (a) Sub-Critical Flow (b) Critical Flow (c) Super-Critical Flow

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27 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

28 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

29 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

30 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

31 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

32 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

33 Ramp Flumes Also Known As Replogle Flume Long Throated Flume Broad Crested Weir

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46 Transition Submergence and Hysteresis Effects in Three-Foot Cutthroat Flumes

47 WINFLUME

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49 Weirs The importance of an aerated nappe

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53 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

54 Drop a ball Velocity = 2gh

55 Submerged Orifice Q=AV But the area is not what you might expect

56 Orifice Meter

57 Did I mention there s a test?