What are Some Tips for Measuring Flows Underground?

Flow channels are relatively easy to measure given the right equipment, assuming they’re on the surface. For underground flows, however, the process is a bit more tricky. Fortunately, there are solutions you can employ to get the proper measurements you need. Learn about the most important tips for measuring flows underground.


Many underwater flow channels were constructed with flow measurement in mind, considering how important it is to the overall process and performance. To facilitate the measurement, vaults were crafted along with the channel. Vaults are large open areas underground in which the flow is exposed. If the underground flow you’re dealing with has a vault built-in, you can simply treat it as you would a flow channel on the surface. 

For a vault to function properly, however, the entire flume has to be integrated into the open area in addition to its end connections. Several different flumes can be used for this purpose, but Palmer-Bowlus flumes are specifically designed to integrate into in-line sewer pipes. Keep in mind that these vaults only exist among generally small flows so the flume can fit inside. 

Metering Manholes

If the underground flow doesn’t offer a vault for access, it may be better to have a metering manhole installed. These systems work to allow measurements for larger flumes that aren’t restricted by vault dimensions. Essentially any kind of large flow or flow traveling through a large pipe will need a new structure like a metering manhole to take flow measurements.

To use a metering manhole properly, the flow entering the attached flume must be simultaneously subcritical and under open channel conditions. For the latter, that means not having a full pipe flow. Fortunately, most sewer conditions meet these requirements already, but it’s an important factor to remember if you’re dealing with a high flow rate.

Energy-Absorbing Manholes

When the flow is too much for a metering manhole, you’ll have to resort to an energy-absorbing manhole. These systems are designed to help regulate the flow rate to allow for proper measurement. They’re installed upstream of a metering manhole or vault to allow proper conditions for flow measurement.

Energy-absorbing manholes work by using a plate that’s set up perpendicular to the flow direction. When the flow hits this plate, the velocity profile is broken, slowing the stream to the point of subcriticality by the time it reaches the underground flume. Keep in mind that energy-absorbing manholes are used in conjunction with vaults or metering manholes as a way to mitigate fast flows rather than acting as measuring devices themselves.

Fiberglass Manholes From Tracom

With these tips for measuring flows underground in mind, you may be in the market for a manhole. For quality and customizability you can always count on, fiberglass manholes from Tracom are your best bet. With numerous characteristics and functions to choose from, you can find the right solution for your flow measurement needs. Contact us today to get started and find the manhole that best fits your underground flow channel.