Why would a diverter even think about installing a weir, when cost-effective flumes are available? It sure would be nice to have just ONE flow measurement device to consider!
After all, a weir should ideally convey only 1 cubic foot per second (cfs) for every foot of interior width, per the USBR Water Measurement Manual. For example, the 3′ weir on the left should only convey 3 cfs.
A flume like the Nuway Ramp Flume to the left can convey up to 8 cfs per foot of width. A weir has to have boards correctly installed, and a flume has no boards.
Flumes can have some drawbacks, though, which is why in ditches with adequate slope, the California Department of Water Resources Watermasters have preferred weirs for 15 years. A formed-up flume that is poured in place is expensive compared to a prefabricated weir. The new, prefabricated, galvanized metal flumes will reduce the cost compared to concrete, so cost is not as much an issue today. Even so, these flumes probably won’t last 20 to 40 years like the older, well-built concrete flumes do.
Flumes have a history of the ditch downstream filling in. This isn’t the flume’s fault; it was cheaper for a landowner not to do ditch maintenance and over years of neglect, the bottom of the ditch gradually got higher, flooding out the flume a greatly reducing its capacity and accuracy. Weirs can handle the lack of maintenance better – boards 
can be stacked up a little higher and still have accurate measurements.
When a flume settles in the ground, or if a galvanized metal flume tips sideways or up- or downstream, its accuracy is seriously affected. It will only work right if a new calibration curve is developed by measuring flows in the ditch at different elevations within the flume.
However, when a weir settles or tips, boards can be cut so that the top board is still level. In fact, some settled or worn-out flumes have had weir racks and boards installed inside, to regain accurate flow measurement. It is not 
an ideal fix, but it is better than a +/- 30%-accurate flume.
If the ditch is significantly wider than a flume, then a couple of things have to be done to make it work. First, it has to have wide wingwalls, maybe much wider than the flume itself. These wingwalls have to be added upstream and downstream. Then, much more fill has to be used so water ca
nnot overtop or wear through the fill. If fill has to be trucked in, that means extra cost and work to install it.
A weir is already wider than a flume, so shorter wingwalls can be used, if they are necessary. Less fill is needed. Installation can be substantially less hassle, and less expensive, depending on the ditch and proximity of the property to main roads.
However, in some wide ditches, 2 weirs have been installed side-by-side to measure a larger flow than just one could handle. And, as Watermasters understood orifices better and worked with them more, they discovered that sometimes, more flow can be conveyed through an orifice than over a weir and still have accurate flow measurement, +/- 5%.
Finally, weir boards can be cut for a wide range of flows and elevations. A weir can become an orifice, which may be more appropriate for low flows or variable submerged conditions downstream.
We need both flumes and weirs! Which one is better, more durable, or more cost-effective, depends on the total diverted flow; width, depth, and slope of the ditch; proximity of the property to road access; downstream
flooding (backing up water) in the ditch, and other factors. That is what I do for a living: help you get rid of headaches, upset, and trouble, to comply with State law, make sure you are getting your water right, and cut off complaints from other diverters. The phone call is free to discuss your diversion, (530) 526-0134, and email is also free at RightsToWaterEng1@gmail.com.