Build measurement devices level, leak-proof outside and at boards inside, and replace old boards for accurate flows and to avoid fines for noncompliance.
Some operational considerations for flow measurements are the same as for your house. When it was built, the foundation was made level. Sure there can be transitions – changes in level where steps will be built – but the whole foundation had to be perpendicular to gravity. Otherwise, the whole house would be tilted, nothing would fit
together correctly, doors would not swing easily, windows might get stuck open, or worse.
It’s the same when measuring flow. For any manually-measured device, like a weir, orifice, or flume, structures have to be installed level and replaced when they are no longer level. More particularly for this discussion, weir and orifice boards must be level. If the bottom of the weir box is not level, some adjustment can be made with the bottom board so that the top board is level.
A house is sealed against the weather, so air doesn’t blow through the cracks and water can’t leak inside the house. If roof shingles come loose, the damage may cost hundreds of dollars by the time the ceiling shows a wet spot.
Weirs, orifices, flumes, or any other device cannot leak around the edges. Otherwise some flow is not measured – so the reported diverted amount would be less than the amount
actually going past. Inside the device, the boards have to be sealed. They might be sealed with plastic and gravel, as shown below, or just well-fitted. If the soil is sandy, it is easy to dump shovelfuls of sand along the backs of the boards until the sand fills the cracks. As long as the boards are not changed out, they will stay sealed for the season.
Boards wear out, too. Fortunately, lumber is inexpensive for the few boards needed in a weir or orifice. They should be replaced each year, or every 2 years at the most. Everyone knows why – boards warp, shrink, twist, and otherwise change so they won’t fit well
together anymore. Besides fit, an old board can’t be leveled; one end will be level but the other end will not because of the warp or twist.
All of these considerations might seem like common sense, but 2/3 of diversions out in the field fail in at least one of these 3 tests. Maintenance and replacement of measurement devices is not fun, but it’s like a house. If a home is not built level and leak-proof, and rotted wood replaced, then damage only gets worse faster so it’s not a nice place to live. In the case of surface water diversions in California, not only do these factors affect flow measurement, but they can affect compliance with the law, make neighbors upset, and possibly incur fines for inaccurate devices or misreported flows.
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