Open Channel Flow measurement


Thread Starter


Need to calculate flow in a long open channel.
One Venturi is installed at the entrance and three Dall Orifices are installed at the exit.
Variations in readings are large.

A 22 km channel, 20 m wide, 4.5 m deep, with a trapezoidal profile is used to convey water from a dam to a large water purification plant.
Operating flow range: 2000 to 3000 Ml/day

Need to re calculate the Venturi and the Dall Orifices. Current reading ariations amongst these equipment is +/- 7%.
A) Does anybody has data on Dall Orifice calculations, installation? and accuracies?
b) Any idea how to measure the flow in the canal
by other improved means?


Al Pawlowski

You did not say whether the variation you mentioned was % of reading or FS. Keep in mind that +/- 7% of Full Scale (FS) in a non-laboratory
open-channel meter is (in my experience) about average. With enough resources, of course, it can be bettter. How much, and how costly, will depend on too many factors of your exact situation to really get into here.

In such a large channel, however, the general approach is to install velocity and level (head or stage) sensors, determine the channel
dimensions in the section being measured and use a computer to calculate the flow as average velocity times cross sectional area. Often, the channel dimensions can be determined with good accuracy so the trick is to get a good velocity at a price you can live with.

Commonly used velocity sensors are submerged ultrasonic transducers, "stream" type propeller meters, insertion magnetic sensors, pitot tube
wands, and timed tracers such as dye, radioactive material or even sticks. Commonly used head sensors are submersible pressure transmitters,
ultrasonic, floats, capacitance and staff gauges. IME, overall accuracy is usually limited by the average velocity measurement so many point readings have to be made, or multiple sonic paths implemented, over the cross section. Tracer methods can use concentration to replace the cross section measurement. Ultrasonic sensors are usually used for permanent installations with the other sensors used for surveys.

I have not checked recently, but Ocean Research Equipment in Falmouth MA used to manufacture large open-channel ultrasonic meters. A good reference on open-channel measurements is the U. S. Department of the Interior's "Water Measurement Manual".