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!