Measuring drainage flow
By Timothy Allinson, P.E.,
Murray Co., Long Beach, Calif.
Many years ago I had a project where my client was required by the township to meter their sewage flow before connecting to the municipal sewer. At first this did not seem like a big deal, but the more I thought about it, the more I wondered, “How in the world are we going to meter the sewer?”
Given that this was in the early 90s, before the proliferation of the Internet, the resources for researching this were few and far between. After pulling out my old college text books, I stumbled across a device for measuring open channel flow in a civil engineering text called a Parshall flume.
In the early 20th century, an irrigation engineer with the USDA, named Ralph Parshall struggled with the measurement of irrigation stream flows. Over a period of years, he eventually developed the Parshall flume in 1922. This simple but effective device is ideal for measuring open channel and drainage flows. These flumes are available in a wide variety of sizes from 1 inch to 50 feet wide, produce little resistance, are not subject to blockages, and are the most widely used type of flume today for channel and drainage flow monitoring installations.
The flume consists of three sections: a converging section with a level floor; a throat section with a downward sloping floor; and a diverging section with an upward sloping floor. This results in flow at critical velocity through the throat, and a standing wave in the diverging section. Flow can then be calculated and monitored quite easily as a function of the throat width and upstream depth. The calculation for determining flow is:
Q = 4WHn where n = 1.52W0.026
Q is flow in CFS
W is throat width in feet
H is entry depth in feet
The point of measurement of the entry depth H is critical and is a function of the particular flume design.
Parshall flumes can be purchased with an ultrasonic transducer located at the critical point for measuring H. This information can then be fed into a recording device that will calculate, monitor, and cumulate flow data.
A subcategory of the Parshall flume is the Palmer-Bowlus flume, which is frequently used for temporary monitoring of sanitary flow applications. Its design is simpler and lends itself to manhole installation, but I would guess it is not as accurate as the Parshall flume, explaining its more limited use.
The geometry of these flumes is designed to force the occurrence of a critical flow through the throat. Downstream of that is a short section of super-critical flow followed by a hydraulic jump, much like that which occurs at the base of a drainage stack when it turns horizontal. If the ratio of the outlet depth to the inlet depth exceeds 70 percent, the flow has reached a submerged condition called transmission submergence. In such a state, the downstream flow will affect the upstream flow, making the flow measurement inaccurate. This flow condition should be avoided. If it cannot be avoided, submerged flow calibration curves can be used to correct the flow values.
The flume design does not allow for any dead-water regions where debris can accumulate, making it ideal for sanitary flows. However, the smaller sizes of 1" and 2" would be unwise to use as debris might then block the throat. A 3" Parshall flume can be used to measure flows from 15 to 830 gpm, while larger sizes go up from there.
As a sample calculation, consider a 4" flume with a 3" depth at inlet point H. The calculated flow rate would be:
n = 1.52 5 0.330.026 = 1.48
Q = 4 5 0.33 5 0.251.48 = 0.17 CFS
Q = 76 gpm
What’s remarkable about these flumes is that they have not changed in their design since 1922, and they remain the industry standard for monitoring sewer, storm and irrigation flows. Certainly the electronics for recording flow levels has improved greatly in that time, but the flume itself remains unchanged, which is not unlike many aspects of our plumbing industry.
Even though I have only had cause to use this device once in 25 years, with the advent of LEED I can see them being used more frequently. LEED has a point category regarding measurement and verification. It also has a point category for reducing sanitary flow rates below baseline values. I would imagine that if one wanted to couple these two point categories, a manhole flume would be required in order to perform the necessary measurements. However, that particular detail is yet to become a subject on any of my LEED projects.
Timothy Allinson is a senior professional engineer with Murray Co., Mechanical Contractors, in Long Beach, Calif. He is a professional engineer licensed in both mechanical and fire protection engineering in various states, and is a LEED accredited professional.