Solving Diverters' Headaches To Provide Peace Of Mind And Help Stay Out Of Trouble
Category: Accurate, Lasting Devices
There are standard measurement devices on the lower end of cost: weirs, orifices, and flumes. They can be easily purchased pre-fabricated, saving you time and money. Accuracy and speed of measurement make these great for reporting and for defending your diversion against challenges. High tech options like acoustic Dopplers, magnetic meters, and propeller meters cost more but save time for larger diversions. All are good to +/- 5% or better when installed, easily meeting any standard for field measurements.
When talking about new weirs, orifices, flumes, mag-meters, and acoustic Doppler devices, plus or minus (+/-) 5% accuracy is expected of new, properly installed, regularly maintained, correctly operated devices. What does that mean? If your diversion rate is measured at 1.00 cubic feet per second (cfs), then you would expect the true value to be between 0.95 and 1.05 cfs. If your diversion rate is 5.00 cfs, then the true value would be between 4.75 and 5.25 cfs. The total accuracy is 10%, we just don’t know if measured values are really up to 5% less, or 5% more than calculated.
New devices might actually have better accuracy than +/- 5%. Engineers never count on that because a bunch of factors, known and unknown, can stealthily make the accuracy worse. Accuracy also depends on the measurer – some are better than others, some are better trained and experienced, and most take the job seriously but some do not.
Of course, accuracy gets worse as measurement devices age. Why does this happen? There are a number of reasons:
Settling, so the device is not level front to back, or side to side, or both
Cracking, so water leaks out, or the cracked wall is not straight (planar)
Wear, spalling, chipping, and other roughening in the device floor and walls
The ditch fills in downstream, causing submergence
Old boards that warp and leak
Installed staff gages wear, making them harder to read correctly
The USBR Water Measurement Manual has 14 chapters, and all of Chapter 3 discusses accuracy in great detail. That’s the “Bible” of water measurement so we would expect it to be, well, accurate in its discussion of accuracy.
It is not clear to me yet whether the Board’s accuracy numbers are +/- values, meaning the allowed accuracy is +/- 15% for diversions less than 100 acre-feet (AF) per year, and +/- 10% for diversions greater than 10 AF per year. If so, that seems reasonable because that allows for some aging of measurement devices. Otherwise, the Board would expect measurement devices to always be in new condition for diversions greater than 100 AF per year or storage greater than 200 AF per year. That would be pretty expensive!
That brings up the subject of money – accuracy requirements hit your pocketbook. First you have to either install or pay for a measurement device to be installed. Hopefully the device will last 20 to 30 years, but high flows, getting walked on by cattle, freezing and thawing, settling faster than expected, and other events can wear them out faster. The replacement cycle might be 10 years for some diversions, or even 5 if wear and tear is bad.
This post may be more than most people want to read on the subject of accuracy. Still, it’s a lot shorter than Chapter 3 of the Water Measurement Manual!
That’s all for now, have a great rest of the week.
There have been 25 posts so far, on the types of California surface water rights, flow measurement devices, and how to measure diverted flows. You’ll see new posts once or twice a week.Please send suggestions for post topics! We have discussed:
This is a corner of a diversion box built by my wife’s grandfather, Emil Wigno, in 1922. The fleur de lis he brought with him from France. 🙂 Emil planted hay, peaches, prunes, and finally walnuts.
Hi, I’m Shawn Pike. 🙂
Now 2″ x 6″ boards, cut 1/2″ to 1″ shorter than the width of the board slots, are stacked up in the diversion box.
The weir board is cut 3-1/2″ deep. There are 2 weirs, one 1.0′ wide, and the other 0.5′ wide. These are contracted weirs, since water on the edge has to turn to go through.
The small weir is blocked off, so the 1.0′ weir is left. This is a 1.0′ contracted weir, and the flow can be read right out of the correct table in the Water Measurement Manual.
By simply flipping the weir board upside down, we have an orifice! If the downstream water is higher than the hole, then the orifice is “submerged”. If the flow out the orifice is free-flowing, then a different equation is used. Either way, calculating the flow is pretty easy because we know the orifice area, and depths of water upstream and downstream.
Here’s the pretty photographer and videographer, in the gold-mining town of Bodie. 🙂
What is a flume? Most people think of long flumes that carry water across a canyon, or along the edge of a mountain, 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 where 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 to 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.
Flumes 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 the 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!
Orifice devices are needed for flat ditches, where the fall may be as little as 0.20′ (2.4″) from upstream to downstream. An orifice is simply a hole through which water flows, so it can be accurately measured. The photo below shows a submerged weir, flowing from right to left. The water in the ditch downstream (left) is above the hole in the boards. You already noticed the amazing thing about this orifice, didn’t you? I could tell you are savvy that way. Yes, this is the same Briggs Manufacturing weir box as the ones in the previous post! It has the same 2″ lumber in the upstream board slot. Now the flow goes through a precisely cut hole in the boards, with a known area, instead of over the top of the boards.
Installation is just like with the weir boxes installed in the previous post, too. For convenience, staff gages may be attached to one side of the box so it is quick to read the water depths. So the precast concrete box is versatile, it can be used as both a weir and an orifice. Actually, some ditches need both a weir and an orifice. This is especially true in a ditch where a gate or boards may be put in the ditch below the weir box, to flood hay or pasture just below the measurement device. All it takes is a change of a couple of boards.
The big difference in measuring the flow is that, instead of “sticking” the weir boards, now the depth of the water must be measured upstream and downstream to use a weir equation or table. The “difference in head”, or water surface elevation, gives us a value needed to read the table or use an equation to figure out the flow. What tables or equations? These are out of the water measurement bible, the Water Measurement Manual. We will discuss these very soon in following posts.
This was a quick post to show how you can get 2 uses out of one device, to make your life simpler. That’s all for now, hope you had a Merry Christmas!
Weirs are the least expensive permanent measurement device you can install. Materials will cost the diverter in the range of $300 to $2,000; hiring the backhoe to set it in place probably costs more than the materials, unless the diverter already has a backhoe or crane.
The weir below was precast by Briggs Manufacturing in Willows. The weir is a cast concrete, 3-sided box with board slots for 2″ lumber. It’s pretty simple, and relatively easy to install. This particular weirneeded metal wing-walls to keep the dirt on the sides from washing out. Note that there are two board slots on each side, one for the boards to slide in, and the other to help make sure a nappe or air gap is created as water flows over the boards.
Step one is determining if there is enough fall in head from upstream to downstream. A weir needs 0.7 feet (0.7′), or 8.4 inches (8.4″) of fall to be sure it will work correctly. The 0.7′ figure is because the pool of water needs to be a maximum of 0.45′ above the top of the weir boards on the upstream side. Then, the water in the ditch downstream of the weir needs to be at least 0.25′ below the top of the boards so the water flows freely, separating from the boards and having an air gap on the downstream side. 0.45′ + 0.25′ = 0.70′.
The photo above shows a ruler in tenths of a foot, held vertically on top of the weir boards. This is called “sticking the weir”. When the ruler is turned face-on to the flow, the water will climb up to the same level as the flat pool upstream of the boards. It’s physics – standing water has an energy level equal to the height of the water surface. Moving water has both potential and kinetic energy, so the energy level or line is above the surface of the
water. Moving water stalls behind the face of the ruler, giving the height of the water if it were standing still. That is the water depth that has to be measured for weirs. The photo is showing a water level of 0.31′ – it wobbles up and down just a little – so we know this weir is flowing at about 0.6 cfs per foot of width.
If the ditch is very flat and shows no ripples when flowing, it’s probably too flat, and an orifice or a flume will be needed instead of a weir. Future posts will discuss those measurement devices, and others too.
Step 2 is figuring out how big a box is needed. Fortunately, there is an easy rule. 1.0′ feet of width is needed for every cubic foot per second (cfs) that will be diverted. For example, if the diversion will be a maximum of 3 cfs, then the diverter will need a 3′ wide weir. If in doubt, get the next larger size since the cost is not much more. The reason for this rule is that a weir can be accurate to plus or minus 5%, well within the accuracy needed for diversions in the field. If the pool upstream of the weir boards is more than 0.45′ over the top of the boards (or less than about 0.1′ over the top of the boards), the accuracy of the weir is worse than the standard.
Measurement devices need to be planned and operated correctly to assure the diverter (and ditch-tender, and neighbors, and the State Water Resources Control Board, andpossibly 10 other state and federal agencies, and possibly even the Superior Court in the very worst case) that the flow measurement is correct. It’s like a truck speedometer – they can get less accurate over time. It’s no problem if they read faster than the driver is actually driving, but if they read slower, the driver is in danger of unknowingly speeding and getting a ticket. Ouch.
The actual installation process is fairly simple to describe. Get 1 to 4 yards of 3/4″ minus road base rock delivered on site, trucked from the gravel plant. To save a lot of hassle, skip the forming up and pouring a concrete weir, and just callBriggs Manufacturing and order a weir to be delivered on site. Dig a shallow, level (flat), square hole in the bottom of the ditch, about 8″ deep, and 1′ longer and wider than the bottom of the weir. Shovel base rock into the hole about 2″ deep, and compact it. Rent a gas-powered thumper, or use the bucket of the backhoe. Pour another 2″ and compact it. Use a level and make sure the top of the base rock is level side to side, and along the ditch. Since it packed down during compacting, add the last 1″ and compact it, so the top of the road base is about 4″ below the bottom of the ditch upstream and downstream.
The installer needs to make sure to have a piece of 1″ steel bar that is about 1′ longer than the the width of the weir box. There is one hole through the top of each side of the weir – stick the rod through that and hook onto it with a chain to lift the weir. Set it in place, and make sure it is sitting level. The installer might have to gently press down on one side with the backhoe to get it completely level. Now the floor of the weir will be at the level of the bottom of the ditch. Remove the steel bar, and fill the weir box inside about 2′ deep with some dirt.
Next, install the wing-walls, if needed. These will keep the material on the outsides of the weir from washing out in a steeper ditch. Then backfill with the remaining road base on the sides, compacting it for each 6″ of depth. If thenative soil holds water well, it could be used instead of base rock to backfill, saving a little bit of money. Remember the dirt that was placed 2′ deep inside the weir? This will keep the weir weighted down so it does not move during backfilling. Also, it will keep the sides from being slightly bent in by the pressure of compacting the backfill. The reinforced concrete weir boxes are strong but the walls can be bent in with enough force.
That’s it! The weir box is installed and ready to go. New weir boards, usually 2″ x 6″ or 2″ x 8″, should be cut about 1″ shorter than the width inside the board slots. For example, a 3′-wide weir will have board slots about 2″ deep. The full width from inside of board slot, to inside of the opposite board slot, is 3′-4″. The boards should be cut about 3′-3″ long. That way, when they swell a little bit, they won’t get impossibly stuck.
Happy measuring! Good night to all, Merry Christmas, and blessings in the New Year.
Water laws are changing at lightning speed because California is in a historic drought. Groundwater law was passed requiring local agencies to be formed to manage groundwater. In 2012, I thought that would take 20 years to happen. The drought accelerated it to 2 years.
Surface water laws were passed in 2009, greatly increasing penalties for not reporting diversions, for misreporting, for overdiverting – in short, for evading, lying, and stealing. Suddenly tens of thousands of diverters who had been ignoring the State Water Resources Control Board started to worry. How do I report, am I in hot water if the Board sends me a letter, how do I figure out what my water right is?
” (i) On and after July 1, 2016, the measurement of a diversion of 10 acre-feet or more per year shall comply with regulations adopted by the board pursuant to Article 3 (commencing with Section 1840) of Chapter 12 of Part 2. “
That doesn’t sound too bad. But what does CWC 1840 say?
” 1840 (a) (1) Except as provided in subdivision (b), a person who, on or after January 1, 2016, diverts 10 acre-feet of water per year or more under a permit or license shall install and maintain a device or employ a method capable of measuring the rate of direct diversion, rate of collection to storage, and rate of withdrawal or release from storage. The measurements shall be made using the best available technologies and best professional practices, as defined in Section 5100, using a device or methods satisfactory to the board, as follows:
(A) A device shall be capable of continuous monitoring of the rate and quantity of water diverted and shall be properly maintained. The permittee or licensee shall provide the board with evidence that the device has been installed with the first report submitted after installation of the device. The permittee or licensee shall provide the board with evidence demonstrating that the device is functioning properly as part of the reports submitted at five-year intervals after the report documenting installation of the device, or upon request of the board.
(B) In developing regulations pursuant to Section 1841, the board shall consider devices and methods that provide accurate measurement of the total amount diverted and the rate of diversion. The board shall consider devices and methods that provide accurate measurements within an acceptable range of error, including the following:
(i) Electricity records dedicated to a pump and recent pump test.
(ii) Staff gage calibrated with an acceptable streamflow rating curve.
(iii) Staff gage calibrated for a flume or weir.
(iv) Staff gage calibrated with an acceptable storage capacity curve.
(v) Pressure transducer and acceptable storage capacity curve.
(2) The permittee or licensee shall maintain a record of all diversion monitoring that includes the date, time, and diversion rate at time intervals of one hour or less, and the total amount of water diverted. These records shall be included with reports submitted under the permit or license, as required under subdivision (c), or upon request of the board.
(b) (1) The board may modify the requirements of subdivision (a) upon finding either of the following:
(A) That strict compliance is infeasible, is unreasonably expensive, would unreasonably affect public trust uses, or would result in the waste or unreasonable use of water.
(B) That the need for monitoring and reporting is adequately addressed by other conditions of the permit or license.
(2) The board may increase the 10-acre-foot reporting threshold of subdivision (a) in a watershed or subwatershed, after considering the diversion reporting threshold in relation to quantity of water within the watershed or subwatershed. The board may increase the 10-acre-foot reporting threshold to 25 acre-feet or above if it finds that the benefits of the additional information within the watershed or subwatershed are substantially outweighed by the cost of installing measuring devices or employing methods for measurement for diversions at the 10-acre-foot threshold.
(c) At least annually, a person who diverts water under a registration, permit, or license shall report to the board the following information:
(1) The quantity of water diverted by month.
(2) The maximum rate of diversion by months in the preceding calendar year.
(3) The information required by subdivision (a), if applicable.
(4) The amount of water used, if any, for cannabis cultivation.
(d) Compliance with the applicable requirements of this section is a condition of every registration, permit, or license.
(Amended by Stats. 2016, Ch. 32, Sec. 98. Effective June 27, 2016.) “
Now THAT has a punch. There are exceptions in following paragraphs, but the Board now wants “continuous monitoring”, meaning one of the older, mechanical Stevens Recorders and the like, or newer, electronic pressure transducers. Now we are talking $500 and up just for recording data, in addition to a measurement weir, flume, or orifice.
And the diverter has to provide “evidence”. How is that done? Is a photo good enough? A video? A drawing? A statement by the local ditch tender, the Resource Conservation District, a technician, or an engineer?
Of course, the Board has higher priorities with larger diversions, and streams with anadromous (chinook and steelhead) fisheries. Still, it is an open question about when the Board will get to your or my diversion.
Complaints from neighbors with a grudge tend to elevate problems that the Board considers. But, water is nothing to argue over, is it? Or have grudges?