Transition Submergence and

Hysteresis Effects in Three-Foot Cutthroat

Flumes

Why Measure Water for Irrigation?

• (You had to ask.)• Improve:

– Accuracy– Convenience– Economics

Water Measurement Manual

• (Door Prize)• Published by Reclamation in 1997

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

Measuring Water Has Been Going on For a Long Time

• Older techniques may be simple, but effective for some purposes.

Goal: No Math in this Presentation!

• This is not possible.

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

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

More better

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

Best

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

Dep

th

Velocity (avg)

Relative roughness determines the distribution shape

Velocity Distributions

Velocity With Stage

So how do you measure velocity?

• Show Flash• Show wmv

Bernouilli’s Principle

• Bernouilli says there are three forms of energy in water:– Elevation head– Pressure head– Velocity head

Think of a swimming pool

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.

Reclamation Video

• 4 types of measuring devices– Cipolletti Weir– Yakima Box– Submerged Orifice– Ramp Flume

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.

Critical-Flow Measurement Devices

• Flumes, sharp-crested weirs, broad- crested weirs

Critical Depth

• And why it is important in measuring flow rates

Tim McCabe, IA NRCS

Subcritical

Hydraulic Jump

Supercritical

Subcritical

Critical

Subcritical

Hydraulic Jump

Critical

Subcritical

Supercritical

Lynn Betts , IA NRCS

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

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

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

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

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

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

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

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

Ramp Flumes

• Also Known As• Replogle Flume• Long Throated Flume• Broad Crested Weir

Transition Submergence and

Hysteresis Effects in Three-Foot Cutthroat

Flumes

WINFLUME

Weirs

The importance of an aerated nappe

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

Drop a ball

ghVelocity 2=

Submerged Orifice

• Q=AV• But the area is not what you might

expect

Orifice Meter

Did I mention there’s a test?