It depends!
When the flow is collected to a central point (like a field), the flow is usually directed into the a H Flume by means of flat, inlet wing walls. Here no appraoch section is present.
In a channel however, the approach section a H Flume should be at least 3-5 times the maximum anticipated head at the point of measurement of the flume. There are times, though, where there is insufficient space to allow for a standard length approach section.
For those times a variety of flow conditioners have been developed to install into the approach section of a H Flume. These include:
As a special case, where flow is rising up and then entering into the inlet of a H Flume, so long as the flow is quiescent, a shorter approach section may be used.
Note: The use of flow conditioners in the approach section is contingent upon the flow stream being free of debris that may accumulate or clog the flow conditioners.
