Flumes – installing for decades of flow measurement, Part 1

What is a flume?  Most people think of long flumes that carry water across a canyon, or along the edge of a mountAlong_Aqueductain, to get water through steep country.  These flumes are expensive and time-consuming to build so they have to make economic sense.  In early
California, flumes were used to get water from a stream to gold-bearing gravels Pipe_Flume_From_Endwhere there wasn’t water.  Gold was certainly worth the expense!  It takes water to wash gravel over a washboard so gold can settle out in the ribs or slats.  Flumes were then used to transport cut logs from the mountains down to mills in the valleys.  Lumber also brought in enough revenue to make flumes worth it.


The kind of flume for measuring flows is a concrete, metal, fiberglass, or wood structure built to exact dimensions.  The newly-built flume shown below is formed concrete.  It took 4 days for a crew of 5 people to make this.  This flume is 3.0′ wide, and will be used toFlume_newly_installed_edited measure diversions of up to about 16 cfs.  This device could last for 40 years before it becomes too worn to be accurate, or develops cracks that let parts of it settle.

Parshall_Flume_DimensionsFlumes are much more expensive than a weir box with boards.  It costs 3 or 4 times as much to install.  On the plus side, there are no boards to change, it measures a wide range of flows with good accuracy (+/- 5% in the first 10-15 years of its life), and it will pass debris and gravel through without clogging.

The photo below is of a flume that has been installed for 30 years.  It shows what can become a common problem: the ditch below has not been kept as deep as it should be, so the flume is “flooded out”.  The flow computed by using the staff gage depth is about 40% more than actually goes through the flume…so the ranchers who use theOld_Parshall_2 water could be shorting themselves.

Rehabilitating a flume is not impossible, but it is not often done.  The whole floor could be raised by pouring a higher concrete floor, making sure it slopes exactly the way the old floor sloped.  Usually a new measurement device is installed nearby, and the old flume is not used anymore.

More on the details and how-to’s of flumes later.  For now, we sure appreciate the snow and rain!

Life Of Reilly: If You Can’t Measure It, You Can’t Manage It!

A friend of mine, Chris Reilly, summarizes everything you need to know about measuring flows into your surface water diversion:  “If you can’t measure it, you can’t manage it!”  Except for riparian rights and some very small water rights, diverted flows have to be measured.  Why?


Dry_DitchLegally, to ensure your neighbors, the Board, and/or a Superior Court Judge that you are diverting no more than your water right.  Practically, how do you know if you are getting as much water as you should?  As surface flows decrease through the summer, every bit less means some pasture, hay, orchards, row crops, or something else does not get irrigated.

If you have nWMM_Cover_smallever measured flow into a ditch before, well, here goes, I am going to leak the secrets right here, I’m going to violate the Unspeakable Code Of The Water Measuring Brotherhood, the ve
ry deepest, most powerful wisdom of how to measure your flow will appear on this very page.  After this, who knows if you will ever hear from me again, once this classified information is made public?  Well, not really, but few people have heard of the Bible Of Water Measurement, the
 USBR Water Measurement Manual (WMM)

Let’s look at 3 common measurement devices detailed in the manual:  weirs, orifices, and flumes.  Properly installed and maintained, these devices can measure flow within plus or minus 5% of the actual amount.  The photos below are from the WMM, which has lots of diagrams that make it easy to see the details of how each device works.  First, the weir:


You have seen these before, they’re just a level plate or board of a specific width, with a relatively still pool behind them.  That’s it!  By measuring the height of the pool above the edge of the plate or board, you can use tables or equations from the WMM to determine what the flow is.


Above is shown an orifice.  Not much to see, is there?  In this case, it is just a hole, lower than the upstream flow.  That is physically all an orifice is.  Knowing the size of the hole, and how high the water is over the center of the hole, and how high the water is down the ditch, a table or equation can be used to figure out the flow.  The gentleman above is using a square gate with a certain width.  The area changes with how high the bottom of the gate is, not hard to figure out.


The photo above shows a Parshall Flume.  These are great for measuring high flows without needing a lot of “head” or the drop in the water from upstream to downstream.  By knowing the depth at a certain point, a table or equation can give the flow amount.

We’ll go into how to use tables for specific measurement devices in later posts.  It’s enough for now to know that if you have a decent measurement device, then you CAN manage your flow, as well as proving that you are taking no more than your legal water right.

All Water Rights, California

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