UPDATE #2: Thank you to Mr. Jon Wachter with OpenChannelFlow, for correcting my mistake; I could not get your comment to show up so here is your comment verbatim (my emphasis):
Name: Jon Wachter
Comment: Nice pics. One thing to point out though: What you have in the first two pics is a Montana flume and not a Montana Cutthroat flume.
The Montana flume is a modification of the standard Parshall flume with the throat and discharge sections removed. The advantage – other than cost and ease of installation over a flume length flume – is that you use the same flow equations as the Parshall flume.
The Cutthroat flume – correctly identified in the last two pictures – is a different beast entirely. Keep in mind that the Cutthroat flume can be quite sensitive to upstream conditions – which has somewhat limited its use in recent years.
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More good news for diverters who will need to install measurement devices…as promised in a previous post, the friend who has started making cutthroat flumes is now making Montana Cutthroat flumes.
Here are a couple of photos of a 3-inch flume he just completed. That sounds tiny, but it is nearly 1.0 feet tall, and it can measure up to about 0.85 cfs. This flume is small enough for one person to install by hand, with some expertise. Installation goes relatively fast.
What is the advantage of small flumes? In a ditch with some slope, where there are ripples in the flowing water, a weir can work well. Weirs usually need 1′ to 1.5′ of stacked boards, with 0.45′ for water above the crest of the weir boards. The minimum head (upstream water depth) required is about 1.5′ to 2′. If a ditch is flatter, where the flowing water surface has few ripples, even more head will be required. These 2 flumes need half or less the head of a weir.
Montana Cutthroat flumes require the minimum of material, that can still be highly accurate with care. They cut out the expansion section of the cutthroat and Parshall flumes, so preparation of the ditch at the exit requires more care. That is usually accomplished with some larger rock placed for several feet downstream of the end of the flume. If the ditch bottom is already gravel or rock, then little extra work is required.
For a short-term trial installation, the flume might be installed with just dirt back-fill, and for permanent installations, it will require some added sheet-metal flanges on the upstream size. Bagged pre-mixed concrete may be needed to back the upstream flanges.
All manufacturers I know, including my good friend who makes these, aim for better than +/- 5%, more like +/- 2 – 3%. +/- 5% is the Water Board‘s requirement for devices certified by the manufacturer, and devices certified in the field must have +/- 10% accuracy.
The prices on these are competitive, especially because shipping will cost less. These flumes give you, the diverter, more options for your particular budget and ditch conditions.
Flumes are just one option – maybe you have one that works well or you favor something else. Please let us know in the poll below:
Original Post Excerpt:
A good friend of mine, also a water measurement expert, does professional work with sheet metal, and he has come up with accurate, slightly lower cost cutthroat flumes for lower flows! Posts in this blog have already covered
EZ-Ramp flumes (3.5, 7,0, 10, and 20 cfs) several times, so what is different about the cutthroats? The name comes not from cutthroat trout, which I loved to catch in Wyoming when I was a kid. Instead, it is because the throat section is cut out of what would otherwise be a Parshall flume, while still having high accuracy. These particular cutthroats are for LOW flows, say 0.05 to 0.68 cfs with high accuracy AND still reading flow directly in cfs. The manufacturer is working to include higher flow ranges (up to 3 cfs, and more) with very stiff but still relatively low-weight construction.
The neat thing is, the costs are a little less than the EZ-Ramp flumes, comparing the same sizes. They are shipped fully built, but the manufacturer’s location is much closer than the bigger companies in Idaho and Utah, so shipping is less, too. Who doesn’t want to save some money?
My friend is working on a couple of other types of flumes, too, including a Montana Cutthroat. Each flume has advantages and disadvantages, depending on the flow range, site, soils, geology, sediment transport, and application. What are the advantages of each type of measurement device? We have discussed weirs, flumes, and orifices in posts here, and later we’ll discuss differences in flumes.
Please leave a comment!