Level , Seal, and Replace for Accurate Flow Measurements

Build measurement devices level, leak-proof outside and at boards inside, and replace old boards for accurate flows and to avoid fines for noncompliance.

Some operational considerations for flow measurements are the same as for your house.  When it was built, the foundation was made level.  Sure there can be transitions – changes in level where steps will be built – but the whole foundation had to be perpendicular to gravity.  Otherwise, the whole house would be tilted, nothing would fitfile000972749236 together correctly, doors would not swing easily, windows might get stuck open, or worse.

It’s the same when measuring flow.  For any manually-measured device, like a weir, orifice, or flume, structures have to be installed level and replaced when they are no longer level.  More particularly for this discussion, weir and orifice boards must be level.  If the bottom of the weir box is not level, some adjustment can be made with the bottom board so that the top board is level.

A house is sealed against the weather, so air doesn’t blow through the cracks and water can’t leak inside the house.  If roof shingles come loose, the damage may cost hundreds of dollars by the time the ceiling shows a wet spot.

Weirs, orifices, flumes, or any other device cannot leak around the edges.  Otherwise some flow is not measured – so the reported diverted amount would be less than the amountSeal_Diversionactually going past.  Inside the device, the boards have to be sealed.  They might be sealed with plastic and gravel, as shown below, or just well-fitted.  If the soil is sandy, it is easy to dump shovelfuls of sand along the backs of the boards until the sand fills the cracks.  As long as the boards are not changed out, they will stay sealed for the season.

Boards wear out, too.  Fortunately, lumber is inexpensive for the few boards needed in a weir or orifice.  They should be replaced each year, or every 2 years at the most.  Everyone knows why – boards warp, shrink, twist, and otherwise change so they won’t fit well03_Weir_Board_Going_Intogether anymore.  Besides fit, an old board can’t be leveled; one end will be level but the other end will not because of the warp or twist.

All of these considerations might seem like common sense, but 2/3 of diversions out in the field fail in at least one of these 3 tests.  Maintenance and replacement of measurement devices is not fun, but it’s like a house.  If a home is not built level and leak-proof, and rotted wood replaced, then damage only gets worse faster so it’s not a nice place to live.  In the case of surface water diversions in California, not only do these factors affect flow measurement, but they can affect compliance with the law, make neighbors upset, and possibly incur fines for inaccurate devices or misreported flows.

Demo Measuring Weir and Orifice; & Who To Call At The Board?

On January 16, we set up boards, this time videos show measuring flows over a weir and through an orifice!

Simple to set up weirs and orifices

Measuring Weir On Youtube: https://www.youtube.com/watch?v=-wOJrWIpPaM

Measuring Orifice On Youtube:  https://www.youtube.com/watch?v=-wOJrWIpPaM

Last time, on January 16, we looked at how to set up weir and orifice boards in a dry diversion.  This is more exciting – now we’re actually measuring flow over a weir and through an orifice!

Shawn_gesturing_in_fron_of_weir

Shawn_Sticking_WeirStanding in front of the Wigno Weir, getting ready to “stick the weir” with an engineering ruler.  The ruler has inches on one side and tenths and hundredths of a foot on the other side – which is how engineers and surveyors measure the world in English (non-metric) units.
Shawn_pointing_with_rule

 

Shawn_in_front_of_orifice

Sticking the weir with the ruler face-on shows that the depth is 0.31′, the same as the depth in the upstream pool.  The weir is 3.30′ wide and is suppressed or flat-sided – the water does not have to turn the corner while going over the weir.

Shawn_with_rule_in_front_of_orifice

 

 

With these measurements in hand, it’s a quick calculation using the suppressed weir equation:Supp_Weir_Eq_2

to find 1.90 cfs.

Here is the same weir, before being set up with orifice boards.  Flow is measured through a hole instead of over the top of the boards

 

The same engineering ruler is used, but this time measuring from the center of the hole, up to the top of the upstream water surface.

 

Actually, it’s easier to measurefrom the bottom of the hole and subtract off half the height of the hole.  The hole is 1.00′ wide, 0.30′ high, and the water height is 0.25′.

Shawn_with_rule_in_front_of_orifice

This time, the flow is less, at 0.73 cfs, using the equation: Supp_Orifice_Eq  WHY?  I did not wait the 5 minutes it would take for the upstream head to stabilize.  It was cold and about to get dark and the videographer was patient but getting cold.  🙂

A question I hear all the time is, “Hey, I got this letter from the ‘State Water Resources Control Board‘.  What am I supposed to do about measuring my flow?  How do I keep from getting in trouble?”  The main number for the Water Board is (916) 341-5300 – and these folks have much more work to do than time to do it.  Several calls may be required to reach a knowledgeable person who isn’t already talking to two telephone calls, or making three investigations in the field.  Since the most calls I get are about enforcement letters, calls, or visits from the Board, it’s probably most useful to have the phone numbers and emails from Enforcement Program Staff.  Here they are, from:

http://www.waterboards.ca.gov/waterrights/water_issues/programs/enforcement/compliance/

Enforcement Program Staff

Katherine Mrowka, Manager
(916) 341-5363
Kathy.Mrowka@waterboards.ca.gov

Enforcement Unit 1 Enforcement Unit 2

Laura Lavallee, Supervisor
(916) 341-5422
Laura.Lavallee@waterboards.ca.gov

Ramon Ruiz
(916) 341-5411
Ramon.Ruiz@waterboards.ca.gov

Kyle Wooldridge
(916) 323-9405
Kyle.Wooldridge@waterboards.ca.gov

Janelle Heinzler
(916) 323-9406
Janelle.Heinzler@waterboards.ca.gov

Dave LaBrie
(916) 341-5343
Dave.Labrie@waterboards.ca.gov

Paul Wells
(916) 323-5195
Paul.Wells@waterboards.ca.gov

Brian Coats, Supervisor
(916) 341-5389
Brian.Coats@waterboards.ca.gov

Chuck Arnold
(916) 341-5634
Chuck.Arnold@waterboards.ca.gov

Matt Quint
(916) 341-5380
Matthew.Quint@waterboards.ca.gov

Samuel Cole
(916) 341-5345
Samuel.Cole@waterboards.ca.gov

Jeff Yeazell
(916) 341-5322
Jeff.Yeazell@waterboards.ca.gov

Enforcement Unit 3 Enforcement Unit 4

Victor Vasquez, Supervisor
(916) 323-9407
Victor.Vasquez@waterboards.ca.gov

Michael Contreras
(916) 341-5307
Michael.Contreras@waterboards.ca.gov

Kathy Bare
(916) 327-3113
Kathy.Bare@waterboards.ca.gov

Oxcar Macias
(916) 341-5637
Oxcar.Macias@waterboards.ca.gov

Natalie Stork
(916) 322-8425
Natalie.Stork@waterboards.ca.gov

Tomas Eggers
916-327-8039
Tomas.Eggers@waterboards.ca.gov

Taro Murano, Supervisor
(916) 341-5399
Taro.Murano@waterboards.ca.gov

Michael Vella
(916) 327-3114
Michael.Vella@waterboards.ca.gov

Skyler Anderson
(916) 341-5355
Skyler.Anderson@waterboards

Kevin Porzio
(916) 323-9391
Kevin.Porzio@waterboards.ca.gov

Bill Rigby
(916) 341-5376
Bill.Rigby@waterboards.ca.gov

Stephanie Ponce
(916) 319-8107
Stephanie.Ponce@waterboards.ca.gov

Update to “Weirs – Planning, Building, & Measuring Flows”

This is an update and correction to the December 24 post, “Weirs – Planning, Building, & Measuring Flows“.  In that post explaining the essentials of installing a weir box, I had said to excavate the pad 4” deep and fill with base rock.  It should have said, excavate 8″.  I’m sure you already figured out why:  the weir bottom is about 4″ thick, so the weir floor would have been above the bottom of the ditch.  By excavating 8″, and then filling with 4″ and compacting a level pad of 3/4″-minus road base, when the weir is placed the floor of the weir will be level with the bottom of the ditch.  That way, the weir is not too high, where flow will undercut
Weir_Set_No_Backfill_edit_smallthe base, and it is not too low, requiring extra boards to get a still pool upstream of the weir.  The weir box in this photo is set – all it needs is for the water to be shut off, sides backfilled, and boards put in for easy measurement.

The important factor in figuring out where the weir gets placed along the ditch, is that the ditch needs to be straight upstream of the weir box.  You can see in the photo above that the weir is located in a straight section of the ditch.  When the box is placed in alignment with the straight ditch, the approaching water does not have to make a turn.  Water going around a bend rolls toward the outside of the bend, and rolling or turbulent water might give a false reading of depth over the weir boards.

How long does the straight section of ditch have to be?  The wider the weir, the longer the length of the ditch has to be straight.  For a 1.0 foot-wide (1.0′) weir, which would pass a maximum of 1.0 cubic feet per second (cfs) if it worked as a suppressed weir, the minimum distance should be about 10′.  For a wide weir box of 6.0′, the upstream distance should be 70′ or 80′.

How high do the boards have to be to provide an essentially still pool upstream of the weir?  Remember the rule that the static head going over a weir, or the height of water that climbs up a 1/2″ engineering ruler held face-on to the flow, should be a maximum of 0.45′.  A suppressed weir, with the flow width going from wall to wall as it goes over the weir, has to be 3 times that 0.45′, or 1.35′.  2  2″ x 8″ boards stacked up will get this height.  If the weir is contracted, or cut into the board, then the board height only has to be twice the static head, or 0.90′.  A 2″ x 12″ would take care of this.  However, to be sure, never use less than 2 2″ x 8″ boards.

One more thing – the weir has to keep from collecting dirt or sand behind the boards.  That means the boards may have to be lifted up every so often so the sediment can flush out.  Weeds have to be kept down all around the weir so they don’t affect the flow of water.  In the same way, sticks and grass have to be kept off the tops of the boards for the weir to work correctly.WMM_Cover_small

Where can you find all this information yourself?  As always, check the bible for measuring flows, the USBR Water Measurement Manual.

That’s enough for now, more to come soon!  Have a great week and I hope it rains today where you are.

Coming Soon, Free Ebooks on Measurement Device Installation and Flow Measurement

The All Water Rights Blog has most or all of the information needed for a farmer or rancher to install a simple measuring device that is compliant with the recent regulations of the State Water Resources Control Board.  I’ll have a couple of free ebooks by June, on how to Red_Books_Edited_3install pre-cast weirs and orifices, as well as how to use them correctly, and report flows from small diversions to the Water Board.  Actually, the ebooks will apply to larger diversions, too, except for the necessary pressure transducer needed to report hourly (and some day, 15-minute) flows to the Board.GE_PT878

There are certainly more complicated devices that require help, such as cast-concrete
Parshall and other flumes, mag-meters, acoustic Dopplers, or full-on gaging stations on rated sections of streams or ditches.

However, most diverters can (and many do) comply with the law with relatively simple devices.  That’s it for now, I just wanted to get the word out on this.

Oh, and there will be an ebook some time around August, which has a working title of “Practical And Applied Water Rights In California – The Non-Attorney Book For Diverting Your Water”.  That book will be more complete and will cost something, yet to be determined.  Let me know what YOU want to see included in a book like this.

Have a great day, pray for more rain and snow!

How Good Is Good Enough? Water Board Required Accuracy of Your Measurement Device

How accurate does your measurement device have to be?  The Water Board gives those numbers in the Fact Sheet at http://www.swrcb.ca.gov/press_room/press_releases/2016/pr12016_measurement.pdf; see the bottom of this post for the excerpt on accuracy.

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
  • Etc.

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.

http://www.usbr.gov/tsc/techreferences/mands/wmm/index.htm

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.

Board_FactSheet_MeasurementAccuracy

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.

Summary of Water Rights, Flow Measurement Posts So Far

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:

  1. All Water Rights, California
  2. Read Me My Rights (How do you know if you have a water right?)
  3. Reasonable And Beneficial Use Depends On Who You Are
  4. The Smartest Water Expert In California (Chuck Rich)
  5. Riparian Rules by Chuck Rich
  6. Water Rights – Why Do They Exist? Which Kinds Are There?
  7. Water Rights And Engineers
  8. California Water Right Holders Now Required To Have Measuring Device
  9. What Is Your Place Of Use?  (Where can you legally use your right?)
  10. Places Of Use – Adjudicated (Decreed) In The State Superior Court
  11. A Place For Permits And Licenses (Places of Use)
  12. Nothing Secret About It  (This is all public information.)
  13. Quick Change of Subjects: What’s a Water Right Permit Cost?
  14. Life Of Reilly: If You Can’t Measure It, You Can’t Manage It!
  15. How to Divide Up a Decreed Water Right – Part 1
  16. How to Divide Up a Decreed Water Right – Part 2
  17. Weirs – Planning, Building, Measuring Flows
  18. From weir to orifice in only an hour
  19. Chilean Water Rights at (darn near) the Driest Place on Earth
  20. Some Hope in Rain and Snow Totals
  21. Is John Stealing Water?? Orifices – Right Size and How to Measure
  22. Worried about SB 88? That’s what this blog is for! Get a device in, send a photo to the Board, record and report your diversions
  23. Flumes – installing for decades of flow measurement, Part 1
  24. Simple Weirs and Orifices, on video, and in photos!
  25. Diverters must report weekly, daily, or HOURLY starting 2017!

Diverters must report weekly, daily, or HOURLY starting 2017!

SWRCB_Header

So on Tuesday, January 20, “The State Water Resources Control Board (State Water Board) adopted regulations Tuesday evening requiring all surface water right holders and claimants to report their diversions. Those who divert more than 10 acre-feet of water per year must also measure their diversions.”  Click the logo above to see the 2-page document on the Board’s website.

Well, how bad can it be?  Before January 20, most diverters had to report monthly diversions, so 12 volumes per year, plus the annual total.  That’s 13 numbers.  The required frequency a year from now will be increased quite a bit, to weekly, or daily, or hourly:

For instance, large diverters with a claimed right to take 1,000 acre feet of water or more per year are required to have a measuring device or measuring method capable of recording at least hourly in place by Jan. 1, 2017; those with claimed rights to divert 100 acre feet or more must comply by July 1, 2017 and record at least daily; and those with claimed rights to divert more than 10 acre feet must comply by Jan. 1, 2018 and record at least weekly.

How can flows even be reported hourly?  See the end of this post.  What if someone decides to skip reporting, and let the Board catch up with them later?  The FINES can be large enough to hurt – we’ll discuss this in a later post.

At the minimum reporting requirement of weekly, the volume is 10 acre feet (AF) to 100 AF.  What is 10 AF in terms of a seasonal agricultural diversion?  All the flows shown below are year-round; if the diversion only runs seasonally, the actual water right might be 2 to 10 times the calculated amount, depending on how long the season is and when the stream dries up.

10 AF  =  0.014 cubic feet per second (cfs) year-round, or 6.2 gallons per minute (gpm).  That’s a domestic right, enough for a family house, garden, and perhaps 15 trees or a yard.

100 AF = 0.140 cfs, or 62 gpm year-round.  Depending on soil, this is enough for 3 to 15 acres of pasture or hay, maybe 15 cows or steers, or maybe 30 acres of a mature walnut orchard with micro-sprinklers.  This is enough for a little extra money, still not enough to support a family.About_1.4_cfs_over_weir_edited_2_small

1,000 AF = 1.40 cfs or 620 gpm year-round.  This is enough for 30 to 150 acres of pasture or hay, or maybe 300 acres of orchard.  Water in this quantity could support a family and would be considered a ranch or farm.  The 4′ weir above has about 1.4 cfs going over it.  As mentioned above, if this diversion only runs 6 months of the year, and really only gets the full flow for 3 months, then the actual continuous water right might be 5 cfs.  It might be easier to reverse the thinking: a 5 cfs right might run at 5 cfs for 3 months,  3 cfs for a month, 2 cfs for 2 months, and be off the rest of the year.  That’s closer to a 2 cfs right year-round, or about 1,400 AF per year.

How is flow measured HOURLY?  The only practical ways to do this used to be an old mechanical recorder, like a Stevens F Recorder (pen on paper on rotating drum) you can still see on some creeks.

More likely today, it will require a battery-powered pressure transducer set inside a 2″ pipe bolted on the side of the weir, or headwall, or other permanent structure.  These cost from $400 to $1,200 or more, depending on the brand and more importantly, quality.  The higher the quality, the less they have to be checked, and have dirt removed from the bottom sensor.  The maintenance can be significant – in warm water with algae, the sensor might have to be cleaned once a week.  If it’s not maintained…well, then at some point it stops recording that data that the Board requires.

WaterLevelLogger_wl300_1

Here’s one that would do the job, from http://www.globalw.com/ products/levelsensor.html.  It sits there and records water levels night and day, for months at a time before it has to be downloaded to a computer.  The data file that is downloaded is what is actually sent to the Board – a spreadsheet of flows for 6 months would be half an inch think and unusable!

That’s enough for now, a good night to you all.

Simple Weirs and Orifices, on video, and in photos!

Simple to set up weirs and orifices!

On YouTube:  https://youtu.be/H2tOEV-zitk

01a_EW_1922_01This 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.

 

01_At_Diversion

 

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.

02_One_Board_In

 

 

 

 

03_Weir_Board_Going_In

 

 

 

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.04_1_Ft_Weir

 

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.

06_1_Ft_Orifice

07_0.5_Ft_Orifice

 

 

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.  🙂Wiggy_At_Bodie

From weir to orifice in only an hour

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.Orifice_Side_Top_2 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.

Staff_GageInstallation 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.

WMM_Cover_small

 

 

 

 

 

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 – Planning, Building, And Measuring Flows

Tomorrow is Christmas 2015!  Merry Christmas all.

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 weirWeir_Showing_Board_Slotsneeded 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′.Sticking_Weir_sharpened

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

Sticking_Weir_zoom_sharpenedwater.  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.contracted_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, andsuppressed_weirpossibly 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 call Briggs 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 tNew_Weirhe native 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.