Measuring Pond or Lake Discharge with Weirs and Vertical Pipes

Sometimes measuring discharge from lakes and ponds can be challenging…particularly when there isn’t a convenient spot to put a discharge weir or flume.

One arrangement that has been used successfully is a vertical standpipe with a weir box mounted to the top. Now we use the term “weir box” but it isn’t the usual type that we would provide for an application like monitoring dam seepage. Instead the weir box is closed on three sides and has a weir plate mounted in the fourth.

Cipolletti Weir Structure Measuring Flow from a Pond

The idea is a to have a vertical standpipe located at a convenient point in the pond. A catchment structure is then attached to the top of the pipe into which a weir plate has been mounted.

The pond or lake itself acts as the weir pool – there is no need to artificially channel the flow before it reaches the weir plate.

The area forming the “upstream” portion of the channel is surrounded with an underflow baffle. The underflow baffle extends far enough from the weir plate to encompass the point of measurement – which extends 3-5 times the maximum anticipated head, Ha, back from weir plate. Additionally, an area of at least 2Hmax is enclosed around the point of measurement.

Underflow baffle surrounding Lake discharge Measurement Weir

The purpose of the underflow baffle is to calm any wind induced waves or surface turbulence that may form on the pond or lake. By forcing flow to go under the baffle, turbulence is eliminated, allowing accurate flow measurement to take place.

Now because there is no bank or sidewall from which to mount a flow meter sensor, usually an ultrasonic flow meter is rigidly suspended over the point of measurement. It is recommended that the ultrasonic sensor have a sunshade / wind guard as gusty conditions are frequent across open bodies of water.