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 ft 2 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)
Velocity Distributions Depth Relative roughness determines the distribution shape Velocity (avg)
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 9-206 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.
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, broadcrested weirs
Critical Depth And why it is important in measuring flow rates
Subcritical Critical Supercritical Hydraulic Jump Tim McCabe, IA NRCS Subcritical
Subcritical Critical Supercritical Hydraulic Jump Subcritical 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
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
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 Velocity = 2gh
Submerged Orifice Q=AV But the area is not what you might expect
Orifice Meter
Did I mention there s a test?