Briggs Manufacturing weirs are a known quantity and work well. As we have covered before, weirs are supposed to flow at or less than 1 cubic foot per second (cfs) for every 1 foot of width for the highest accuracy. With adequate soil for backfill and some 6″ and larger rock for reinforcement, they might not require anything other than a backhoe, the box, and an expert installer to get a highly accurate weir box in place and working. Weirs are nice, too, because they don’t need a stilling well – a data logger in the upstream pool will measure the “still” pool depth nicely, and if placed at the weir entrance or just upstream, will not measurably affect accuracy.
A direct-reading staff gage could be installed on a weir, reading directly in cfs, but the boards would always have to be exactly the same for it to work right. In reality, weir boards are changed out for different flows, or a smaller weir is cut into a board (or sheet metal screwed to a board) to measure low flows. Boards configured as an orifice are usually submerged, so depths are measured upstream and downstream of the boards anyway. The point is that a calculation has to be done, or a table checked to get the flow for each measurement.
Also, weirs can be run at higher flows than 1 cfs / 1 ft. but the measured flow is less accurate. The stability of the box and backfill becomes more of an issue, too, as it does with a steeper ditch and inadequate backfill soils. The usual ways of increasing stability are pretty standard:
- Compacted 3/4″ minus road base may be needed for a solid base and flow-proof backfill.
- Wingwalls of sheet metal (or even plate steel) may be required upstream, and maybe also downstream. See the weir above – even though in a flat ditch, it still needs upstream sheet metal wingwalls.
- Rock reinforcement on the outside faces of the wingwalls is always a good idea.
I just made a proposal for a rancher, with options for EZ-Ramp (Nuway) Flumes as well as 4′ Briggs Twin-Track Weirs. The truth is, though, while I have installed other pre-fab metal flumes, I have not used the EZ-Ramp flumes yet. So, just how easy is EZ when it comes to actually installing and using these?
A couple of guys who have each installed more than a hundred EZ-Ramp flumes talked with me on the phone about their experience over the years. One encased them in concrete, the other put them on concrete slabs and poured concrete wingwalls on the upstream side. Both said their flumes have been used for years with few problems, and they are still being installed.
But, what about this photo from Intermountain Environmental showing only soil backfill? This is probably a temporary installation – for 1 or 2 years. To be permanent, it will need to be placed on and anchored to a concrete pad or railroad ties, as well as having extended sheet metal wingwalls, or bagged postcrete wingwalls. Flumes could have poured concrete walls but when expense is a consideration, bagged concrete often does the job.
The big advantage is, if there are ditch locations that are fairly narrow, and don’t have large rocks, EZ-Ramp flumes can sometimes be installed by hand. Especially the 3.5- and 7-cfs sized flumes come disassembled, are fairly lightweight, are 4′ long, and are a little less than 15″ (1.25′) high. Widths are just over 1 and 2 feet, respectively.
Reading flows is easy – the staff gages are in cfs already, so they are direct reading. Since the flow relationship changes a little at higher flows, Intermountain makes the staff gages non-linear so they show the correct flows up to the nominal flow. For flumes up to 10 cfs, they are accurate down to 0.1 cfs; to be really accurate at the bottom would take a little contracted weir or a Cipoletti weir.
Which measurement device to use depends on what equipment is available, ditch conditions, soil types, and who will be operating the device. Weirs and orifices are not hard to operate, once someone has a day or two of training, gives good attention to detail, and reviews the training periodically. The EZ-Ramp flumes take NO training, since they have direct-reading staff gages. However, metal flumes are not as tough as weir boxes, so installation takes more care, and exclusion fencing may be needed if cattle get in the ditch.
Next time, we’ll go into some detail on data loggers, since many diverters will need them starting in 2017. For now, have a great weekend!