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 
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