Open Channel Flow

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.