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.

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.

Riparian Rules by Chuck Rich

Riparian Rules by Chuck Rich, State Water Resources Control Board, 2007


  1. A riparian right exists by reason of ownership of land abutting upon a stream or body of water and affords no basis of right to use water upon nonriparian land.
  2. A parcel of land generally loses its riparian right when severed from the stream channel via a parcel split (i.e., “physical severance”) unless the right is specifically reserved for the severed parcel in the deed of transfer or other conveyance document. However, the California Supreme Court has held that where a physical severance has previously taken place, if the severed tract was receiving water from the creek at the same time the conveyance created the severance, that fact can be used in court to argue that the grantor and grantee did not intend any severance of riparian rights notwithstanding the physical severance, and the riparian right might be preserved as a result – if the court so decides. The riparian right also may be lost when transferred apart from the land by grant, contract, or condemnation. Once lost or severed, the riparian right can NEVER be restored.
  3. Riparian water right holders may only divert a share of the “natural streamflow” of water in the stream. “Natural streamflow” is the flow that occurs in a watercourse due to accretions from rainfall, snowmelt, springs and rising groundwater. To the extent that flow in its natural state reaches or flows through their property, riparian right holders have a proportional right, based on need, to the use of the natural flow.
  4. A riparian right does not allow diversion of water that is foreign to the stream source. Water that is: a) imported from another watershed; b) stored and subsequently released later in time into the stream system from upstream dams; or c) irrigation runoff generated from the application of percolating groundwater applied to upstream lands; is not available for diversion under a riparian claim of right.
  5. Water diverted under claim of riparian right may only be used on the parcel of land that abuts the stream – – unless the severed parcel’s riparian status has been somehow retained (see #2 above), and then only on that portion of the parcel that drains back into that portion of the stream from which the water was originally diverted.
  6. In order to divert water under claim of riparian right, the diverter must use the water on riparian land but need not own the land at the point of diversion. That is, the diversion may be made at a point upstream (or downstream) from the land being served so long as permission is granted to use that point of diversion and intervening land owners between the point of diversion and place of use are not adversely affected by such practice. However, water cannot be diverted upstream or downstream under a riparian claim of right if this water would not have reached the diverter’s land in the “natural” state of affairs. (In other words, the land is only riparian to the stream when the stream, in the natural state, would actually reach or touch the parcel in question.)
  7. Riparian rights are not lost by nonuse of the water.
  8. “Seasonal storage” of water cannot be accomplished under a riparian claim of right. “Seasonal storage” is generally defined as the collection of water during a period of excess flow for use during a period of deficient flow. However, water may be retained for strictly “regulatory” purposes. “Regulatory storage” of water means the direct diversion of water to a tank or reservoir in order that the water may be put to use shortly thereafter at a rate larger than the rate at which it could have been diverted continuously from its source. Regulatory ponds should generally be drained at the end of the season of use (e.g., irrigation season).
  9. If there is insufficient water for the reasonable, beneficial use requirements of all riparian owners, they must share the available supply. Apportionment is governed by various factors, including each owner’s reasonable requirements and uses. In the absence of mutual agreement, recourse to a determination in the Superior Court may be necessary.
  10. The riparian diverter is subject to the doctrine of reasonable use, which limits the use of water to that quantity reasonably required for beneficial purposes. The method of diversion and conveyance must also be reasonable and non-wasteful.
  11. A diverter who possesses a valid riparian claim of right does NOT need to obtain a permit from the State Water Resources Control Board for the act of diverting water. However, any alteration made to a natural channel in order to divert the water will probably require acquisition of a “streambed alteration agreement” from the Department of Fish and Game and may require a Section 404 Permit from the Army Corps of Engineers or a waste discharge requirement from the appropriate Regional Water Quality Control Board. Compliance is also required with any other local, state, or federal requirements regarding construction and operation of the diversion facilities.
  12. Water Code section 5100, et seq. requires that a “Statement of Water Diversion and Use” be filed with the Division for any diversion under riparian right if no other entity reports this use. As of 2007, there is no charge to file this document and forms are available upon request from the Division of Water Rights.

All Water Rights, California

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