On free-spilling discharges from pipe where more accuracy (+/- 3-5%) is needed that the California Pipe Method can supply (+/-10%), it is common to see H flumes used.
Unlike the California Pipe Method which uses the pipe itself as the flow measurement structure, the H flume is attached to the end of the pipe and the water is metered in it before it spills off the end. The drawback to this approach is that space is required after the end of the pipe to allow the flume to be installed. In space constrained applications, other flow measurement approaches must be used.
The typical arrangement is the pipe (straight and level for at least 5 pipe diameters), the flume’s inlet bulkhead with pipe stub or flange, the approach section (usually 3-5 times the depth of the flume), and then the flume itself.
When mounting the flume, it is no sufficient to secure to the flume to the end of the pipe alone, other support must be installed under the flume.
One advantage of using H flume to measure end of pipe flows is that the size of the flume can (generally) be sized for the flow rates (not for the pipe size). So long as the width of the approach section is at least as wide as the pipe’s diameter, any size HS / H flume can be used to measure a pipe’s flow. Simply choose the flume who’s flow range best fits the anticipated discharge from the flume.