## Submerged Orifices From a Turned-Over Weir Board

A flow measurement weir has to have a nappe that springs free, leaving an air gap (photo on left).  What if your weir becomes submerged for some reason (photo below), and boards cannot be adjusted to make it work properly as a weir?

One thing you can do is turn the weir into a submerged orifice.  The weir board can be turned over, maybe

moved down a board position, so the same board serves as a rectangular submerged orifice.

To do this, all the flow needs to pass through a hole cut into the boards.  The depth of the hole doesn’t matter* as long as all of the hole is underwater enough to make it fully contracted.  The area of the hole is very important – it could be in square inches, but square-foot areas are much more convenient to read flows directly from the USBR Water Measurement Manual.  From Table A9-2, flows can be read directly if the area is a multiple of 1/4 square foot.  For example, a hole that is 0.5′ (6 inches) by 0.5′ results in a 0.25 square-foot hole, which is the first column of the cross-sectional area values.

*If the hole is on the bottom and full-width, use Table A9-3, since the bottom will be suppressed.

This is me with weir boards, starting to stack the boards for a weir.  I am using 2″ x 6″ lumber from Home Depot.  The lengths are cut about 2″ short of the slot width, so when they swell they won’t get stuck in the slots.  Once they’re all in, I end up with a 0.3′ high, 1.0’long, contracted weir.  The board took 5 minutes to measure and cut with a saw.

When we got flow in the ditch, there was too much flow to use the 1.0′ weir.  Instead, I used the full length of the board, 3.3′, to measure the flow.  Here I am sticking the weir to get an accurate depth.

I turned over the same weir board to make an orifice of 0.3′ x 1.0′, or 0.3 square feet.  That’s an area not in the tables of the Water
Measurement Manual, so I had to use the equation for a contracted, submerged, rectangular orifice.

This is a submerged orifice – you can’t see the 0.75 square-foot hole because it is underneath the water.  There are two staff gages, one on the upstream side, and one on the downstream side.  The difference between the depths shown on the staff gages gives the head.  In this photo the head differential is 0.10′, less than the acceptable 0.20′ or higher.  We’ll go ahead and read the flow from Table A9-2 above, 1.16 cfs.

A square or rectangular headgate makes a great rectangular submerged orifice, with bottom and side contractions suppressed (photo on left).  Table A9-3 is used to read the flow for this type of submerged orifice.  Weir boards can also be used this way, so the orifice is on the bottom instead of somewhere in the middle of the stack of boards.  I don’t use boards this way because it makes more work – all the boards have to be removed to reconfigure the orifice back to a weir instead of just one or two.

## 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 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!