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 squ
are-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 the
y’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 i
s 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.
That’s all for now, and may all your flow measurements be accurate! Please leave a comment below, on flow measurement devices or anything else: