What is the minimum distance of “straight channel” required upstream of a flume?
There is no hard and fast rule governing the amount of upstream straight run of channel. Any run of channel that produces smooth streaming flow with a uniform velocity distribution prior to reaching the flume is long enough. How long this channel will need to be depends to a large extent on how extreme the disruption in the channel is, for example water will return to normal flow conditions more quickly downstream of slight change in pipe diameter than after a 90 degree elbow. If your flow appears to be smooth and streaming, free of white water and visible surface boils at the point being considered for flume installation then the upstream channel is adequate. If you are designing a channel for a new installation a safe bet is to allow an upstream distance of straight channel equal to 25 times the maximum head anticipated at the flume. This much distance will allow the flow to overcome the effects of even extreme conditions. When that much distance is unavailable it may be necessary to
