Gary Girolimon, P.E.

Ted D. Miller Associates, Inc.

Open channels are those natural and man-made structures
through which water flows with a free surface.
Pressurized flow, flow in pressurized piping systems, does not have a
water surface exposed to atmospheric pressure.
Examples of open channel flow include streams, rivers, irrigation
ditches, canals, **partially full pipes**,
and water conveyance flumes.

Some of the more common open channel flow measurement methods are:

· Timed Gravimetric (Volumetric)

· Tracer-Dilution

· Manning’s Equation

· Hydraulic Structures

· Area-Velocity

**Timed gravimetric**
is the collection of an entire flow stream for a fixed amount of time. Technically, the contents of the container
are weighed to determine the volume of water.
This method of flow measurement is not typically practical for large
flow streams or for monitoring flow on a continuous basis. An exception exists when using a sewage lift
station monitor with software that can calculate the flow based on the fill and
discharge cycle times. These devices can
have an accuracy of +/- 2% over a longer time frame such as a month and are
considered accurate to +/- 8% at any moment.

**Tracer-dilution**
entails adding a known amount of tracer to the flow stream and measuring the diluted
concentration at some downstream point.
It is important that the tracer is added at a known and constant
discharge rate and is uniformly mixed prior to the measurement point. As with the time gravimetric method this method
is not practical for continuous flow measurement. An advantage of the tracer-dilution method is
that no flow channel geometry measurements are required. Making this method suitable for natural
stream channels.

**Manning’s equation**
is an empirically derived formula for estimating the average velocity of a liquid
flowing in an open channel. The formula
utilizes the cross-sectional average velocity, hydraulic radius, roughness
coefficient, and the slope of the channel.
Manning’s equation can be used for flow measurement through the use of
flow meters or can be used for spot flow measurements. However, the Manning equation is not expected
to provide results better than +/- 25-30% under field conditions.

**Hydraulic structures**
are fixed geometry devices that are placed into flow streams so that all of the
flow is directed through or over the device. The device produces a characterized
relationship between the liquid level and the flow rate at a single, defined
location (Ha) under free-flow conditions.
When the liquid level generated by the hydraulic structure is measured
by an additional device, that device is termed the secondary device. A flow meter is a secondary device that
converts the level measurement to a flow value and calculates the total volume
of flow over time.

Hydraulic structures are generally divided into two
categories: **flumes and weirs**.
Flumes are more adaptable in their sizing, configurations, and installation,
while weirs, on channels capable of developing a proper weir pools, are
typically less expensive. Of the two,
weirs show greater laboratory accuracy (+/-2-5%) than flumes (+/-2-6%). In practice under field conditions, the total
system accuracies for both tend to be similar at +/-10%.

**Area-velocity**
flow measurement does not require a primary device. The flow rate is calculated using the
continuity equation, Q = V x A. That is,
the wetted area is multiplied by the mean water velocity. The USGS and Colorado Division of Water
Resources are two entities that uses area-velocity flow measurement techniques
to measure flow in natural streams and manmade channels. Area-velocity flow meters usually measure the
flow velocity by Doppler or electromagnetic field and calculates the
cross-sectional area using the depth that is measured by a pressure transducer
or an ultrasonic sensor. With proper application
and installation, accuracies of better than +/- 10% are expected.

I would like to follow up with additional, hopefully shorter, articles on specific items of flow measurement. Future articles could be on flume types and selection, pressurized flow measurement, flow meter selection, or other topics you may recommend. Please feel free to contact me at gary@tdma-inc.com with any recommendation and comments.