Chapter 13 - Open Channel Flow Measurement

The majority of industrial liquid flows are carried in closed conduits that flow
completely full and under pressure. However, this is not always the case for high
volume flows of liquids in industrial waste systems, waterworks, sanitary and
storm sewers, and irrigation systems. The flows in these situations are commonly
carried in open channels, which are characterized by low system heads and high
volumetric flow rates. This chapter will discuss the techniques used for measuring
flow in open channels.

Open Channel Flow

There are two basic types of flow systems: flow in closed conduits and flow in
open channels. Closed conduit flow is flow in completely filled pipes. Pipes are
usually used for fresh water lines or for industrial process lines, and flow through
them is often measured by some type of device inserted into the line.

Open channel flow is flow in any channel in which the liquid flows with a free
surface. Examples of this are rivers, irrigation ditches, canals, flumes, and other
uncovered conduits. Certain closed channels, such as sewers and tunnels when
flowing partially full and not under pressure, are also classified as open channels.
Open channels are used in most storm and sanitary sewer systems, sewage treatment
plants, many industrial waste applications, and some water treatment plants.
Most irrigation water is also distributed in open channels.

Methods of Open Channel Flow Measurement

There are several methods of determining the rate of flow in open channels,
including timed gravimetric, dilution, hydraulic structures, slope-hydraulic radiusarea,
and area velocity.

In the timed gravimetric method, the entire contents of the flow stream are collected
in some type of container for a fixed length of time. The weight of the fluid
is then determined, and the flow rate calculated. Weighing the fluid is a primary
standard and, because the accuracy of weighing devices is typically considered to
be better than ±0.1%, the timed gravimetric method is frequently used to calibrate
other flowmeters. In addition, this method is often used for occasional flow rate
measurements at a particular place and time, and for developing a liquid level flow
rate curve for a particular location. Practical considerations limit the use of
this technique to fairly low flow rates and, because of the nature of the measurement,
it is not suited for continuous measurement.

The majority of industrial liquid flows are carried in closed conduits that flow
completely full and under pressure. However, this is not always the case for high
volume flows of liquids in industrial waste systems, waterworks, sanitary and
storm sewers, and irrigation systems. The flows in these situations are commonly
carried in open channels, which are characterized by low system heads and high
volumetric flow rates. This chapter will discuss the techniques used for measuring
flow in open channels.

Open Channel Flow

There are two basic types of flow systems: flow in closed conduits and flow in
open channels. Closed conduit flow is flow in completely filled pipes. Pipes are
usually used for fresh water lines or for industrial process lines, and flow through
them is often measured by some type of device inserted into the line.

Open channel flow is flow in any channel in which the liquid flows with a free
surface. Examples of this are rivers, irrigation ditches, canals, flumes, and other
uncovered conduits. Certain closed channels, such as sewers and tunnels when
flowing partially full and not under pressure, are also classified as open channels.
Open channels are used in most storm and sanitary sewer systems,...