The accuracy of quantitative flow input is crucial for use of the hydraulic model to determine how much extra water is in various line segments due to infiltration and inflow (I & I).
As head of the I & I Investigative Unit of the Wheaton, Ill. Sewer Department, Mike Jankovic states, "Flow monitoring is critical to the accuracy of our hydraulic model. And the hydraulic model allows us to develop and prioritize cost-effective repairs for our overall sewer system." He goes on to say: "If there is a 30 to 50 percent inaccuracy up front, by the time all the mountains of mathematical calculations are made to develop the model, our cost effectiveness analysis is way off."
All Flow Meters Are Not Alike Given the serious nature of the I & I problem, a try-before-you-buy policy certainly seems wise. For example, in one "micro-management" instance in Wheaton, using the American Sigma 950 Area Velocity flow meter, Jankovic's Investigative Unit discovered a ten-fold increase in flow through 1,200 ft. of 8 in. sewer pipe, with only two-tenths of an inch of recorded rainfall over a 20 minute period.
Slow response affects accuracy in quickly changing flow conditions - the same type of conditions observed in Wheaton by Jankovic and his crew. Pipes that go from a dry condition to heavy flow during a rainfall event could easily have changes in level and velocity at a rate faster than some meters can respond (the time it takes to record a change in velocity). State-of-the-art signal response time is five seconds, and some meters can take two minutes or more to record a change in velocity. The Sigma 950 AV employs a patent-pending velocity compensation feature that corrects for the effects of increased velocity on level measurement. In the case of Wheaton, the entire rainfall event, producing a ten-fold increase in flow, lasted only 20 minutes. A response time in seconds versus minutes made a significant difference in accurately recording the flow for this event.
Accurately Measuring Base Flow Low flow and low velocities are common in many municipalities. For example, Jankovic states: "We use flow meters in the winter to monitor wet ground conditions and check the sewers for groundwater infiltration. When it's dry in the summer, we're getting what we consider to be our 'base' flow. Then, of course, we have our rainfall events. The difference between the base and peak flows gives us the quantity of water in our system that really shouldn't be there. We then investigate to find the sources of the extra water."
But if your base flow is 1.5 in. and your flow meter uses a default number below 3 in. or your flow meter requires profiling, how will you get accurate numbers in smaller pipes (6 in. to 18 in.)? Jankovic says he couldn't until he used Sigma's 950AV. "I've gotten good velocity data at six-tenths of an inch of water depth and as low as one foot per second of velocity. And that's phenomenal. What really gets me excited is that the performance of this flow meter gives us the ability to go look at bypasses, diversions and relief lines that don't flow all the time. This gives us the ability to micro-monitor in smaller and smaller basins."
The Problems with Weirs and Flumes With so many accuracy-affecting items to consider, one might have the tendency to fall back on a tried-and-true primary device. According to Jankovic, however, "If you're using a primary device, like a weir or flume, as long as you're measuring base flow during dry weather, the reading will be fine. But if you have a flow that increases ten-fold, as we've seen it do many times, the primary device can't measure because its capacity is vastly exceeded."
The Bottom Line: It Pays to Look Closely At Flow Meters The bottom line is that when it comes to I & I problems that can cost a municipality thousands (if not millions) of dollars to correct, it pays to look closely at the flow meters being used.
For the most accurate readings, consider flow meters possessing the following attributes.
- Ready-to-use: Today's technology allows quick installation without time-consuming profiling in any pipe size - wet or dry.
- Fast Response Time: To get readings from the first surge of a rainfall event, and to keep up with rapidly changing level and velocity, the meter's signal response time should be in the range of five seconds and compensate for velocity induced level measurement errors.
- Capable of Reading in Clean Water: Nighttime sanitary sewer flows and CSOs put meters to the clean water test. Doppler offers the best field accuracy and the more advanced higher frequency (1 MHz) Doppler maintains accuracy in clean water.
- Employs a Low Profile, Amplified Probe: The probe should be low enough and maintain signal strength as the area of water reduces to read base flows in dry conditions. The low profile will also avoid solids collection, thereby reducing probe maintenance.
Sampling and Monitoring Equipment