Using a flume is one of the best ways to get accurate measurements of pipe flow. If you want to make sure those measurements are correct, the pipe slope must be properly aligned to avoid energetic flows. Learn why pipe slope is such a crucial factor, and discover how pipe slope affects flume operation.

## How Do Flumes Work?

To understand why pipe slope matters, you must first understand how flumes work. The most essential function of a flume is to accelerate a relatively slow flow of water to a critical state. During the acceleration, the water will be channeled through a single point known as the throat. Given that uniform acceleration happens a bit upstream of that, that’s where the measurement is taken.

Because the flume’s measuring capabilities require the flow to accelerate after flowing slowly, it won’t work if the flow already exceeds a certain velocity when it reaches the flume. Instead of it reaching a critical state, a speedy flow going through a flume could reach a supercritical state. Essentially, the flow just gets faster, rendering it unable to change from one state to another.

## The Froude Number

When you’re adjusting pipe slope, the most important number to remember is the Froude number. This number is similar to the Mach number in terms of air travel. If the Froude number is 1, the flow has reached critical velocity. Any number less than 1 would be subcritical, and any number greater than 1 would be supercritical.

For a flume to operate properly, the Froude number of the incoming flow should be less than 1. For precise measurements, however, it takes a bit more. Between 0.5 and 1, the flow can develop surface waves, which may throw off measurements. Generally speaking, it’s best for the incoming flow never to exceed 0.5.

## Pipe Slope

The pipe slope and the Froude number are inextricably connected. Because the pipe slope determines the velocity of the water, and the Froude number essentially measures velocity in relation to criticality, the greater the slope, the greater the Froude number will be. While there are several acceptable slope ranges for any pipe size, it’s typically best to match the slope to the 0.5 Froude number.

Keep in mind, however, that the size of the pipe matters when determining the proper slope in relation to flow rate. As pipe sizes get bigger, the allowable slope options decrease. Generally, however, any pipe entering a flume shouldn’t have a slope greater than 2.83%, regardless of size. A higher slope will only lead to mismeasurement and likely supercritical results that throw off the measurement.