How can large diversions be measured? Long-throated flumes are a good option, especially if the ditch has low banks, or a lot of sediment or debris could clog a weir or orifice. Premanufactured Parshall or Replogle flumes go up to around 20 cubic feet per second (9,000 gallons per minute). If they are made for larger flows than that, they are prohibitively expensive to ship or manufacture.
Recently, though, Watchman long-throated flumes have become available. They are made in Northern California, so shipping costs are lower. They typically go up to 20 cubic feet per second in size, but I have installed a 30-cfs Watchman flume. The manufacturer can easily make larger-capacity flumes, too – standard plans go up to 60 cfs, and they can be shipped in ready-to-assemble sections for up to 200 cfs.
Watchman flumes are made of 10-gauge steel, a little thicker than 1/8 inch. The premanufactured flumes I have seen ship from outside the state are made of 16-gauge steel, which is about 1/16″ thick. These can work well if care is taken during installation, but the Watchman’s heavier gauge steel can withstand more backfill and rougher treatment. They’ll last longer, too.
What about cost? It turns out that Watchman flumes are about the same cost per cubic foot per second, as flumes made from lighter-gauge steel. Some farmers and ranchers like concrete better than steel. Watchman flumes can be built inside Briggs pre-cast concrete rice boxes and weir boxes, if you need an installation to last for 30 years or more.
Where can you buy these? The manufacturer does not advertise – let me know and I can put you in touch with them.