What if you have a small diversion, but grass or debris would interfere with a standard weir? A weir has to have unobstructed, free-flowing water over
the crest so measured depths accurately relate to a calculated flow. A weir with debris problems has to be cleared whenever flow is measured, which increases the time requirement.
When weirs have low flows, they trap debris more frequently, and they are less accurate when the depth over the crest drops below 0.2 feet (2.4 inches). Then the only way to measure flow is with a narrow suppressed weir, or with a contracted weir, typically half or less the maximum width. A V-notch weir can be used for measurement of low flows.
Changing the weir boards for different flows requires someone with experience,
who will recognize when the depth over the weir is 0.2 feet or less and then use a contracted weir board. However, people are busy when irrigating, and even busier when flows drop. Weirs are often neglected during the time they need more frequent maintenance visits.
A good flume for passing debris and measuring low flows is the HS flume. These are accurate right down to zero flow. For the maximum flow, they require more
material than a rectangular Winflume, Montana, or Parshall flume. However, they are more accurate than other flumes at very low flows – testing by the University of Minnesota found an average accuracy to be +/- 3.2% for ideal approach conditions. They will pass debris down to zero flow – the flume shown here has an opening of 0.05 feet, or 5/8 inch at the flat bottom, and the opening increases with height.
Why aren’t HS flumes common in California? I suspect that the early adoption of Parshall flumes here established the standard. I have seen a few hundred flumes, but I had never seen an operating HL (wide, high flow), H, or HS flume, prior to my installations.
Why go to the trouble of using an HS flume, if Parshall flumes are readily
available? A Parshall flume may be +/- 10% accurate down to perhaps 5% of its maximum flow. Below that, the accuracy decreases. An HS flume is +/- 10% accurate down to 1% to 2% of its maximum flow. If the flow regime is predominantly low with occasional high flows, it is important to measure those low flows with the best possible accuracy. Some places where low flow measurement is critical include field runoff where pollution is proportional to flow, small water rights, and dam leakage.
HS flumes are easier to construct than a Parshall, too. The HS flume bottom is flat, and it has 3 vertical planes. The photos of the Parshall flume here show
that it has 3 horizontal planes, and 5 vertical planes. An HS flume takes less time to build, and can be put together fairly quickly in any farm or ranch shop. Parshalls are complex enough that they are purchased, including design and shipping costs.