Velocity Flowmeter Aids Sanitary Wastewater Sewer System Evaluation Survey

Dec. 28, 2000
Flow Measuring Devices
The end of the sewer line for unincorporated Sedgwick County is the Kansas Coliseum. Home to two professional sports teams, the 10,000-seat events arena hosts concerts, exhibitions and livestock shows. A 25,000-seat outdoor amphitheater expansion is now being considered. In addition, a proposal is in effect for a nearby residential development.

To provide sanitary sewer service, a lift station was proposed that would discharge flow via a force main to the existing 8-inch-diameter gravity sewer that directly serves the Coliseum. Concerned with the proposed line's capacity to adequately transport future flows, Sedgwick County requested an impact study. George Butler Associates, Inc. (GBA) was selected to provide professional engineering services for the job.

"The question was, 'If we add more flow to this line, do we get in trouble with the capacity the Coliseum needs?'" said Jim Graham, P.E., head of field services division for GBA Utilities Management Group.

Responding to the Challenges

Graham reported that the only flow they were monitoring was generated from the Coliseum itself. Flow was most often minimal, intermittent and unpredictable. During events, when 5­p;6,000 people attended for 4­p;6 hours, conditions changed dramatically.

To meet the challenge, GBA selected an American Sigma 950 AV area velocity flow meter, configured with a single probe with a bubbler depth sensor and integral Doppler ultrasonic velocity sensor to collect flow data.

"The meter had to respond quickly to changing conditions and it had to provide complete data capture," Graham said. "During these events we had to make sure we acquired good depth and good velocity information."

In the past, measurement choices would have been limited to manual measurements with profiling and instantaneous depth measurements or a level-only arrangement with a weir or insert flume.

"We would have had somebody out on site during these events, or used a primary device to just measure level and not have attempted a velocity measurement," he said. "It would have been more expensive and more time consuming, and we wouldn't necessarily have obtained the information we wanted."

Continuous monitoring of flows from the Kansas Coliseum was performed from June 27 through July 30, 1996. Flows were measured at 5-minute intervals with an 8-inch-diameter vitrified clay pipe.

Weekly site visits made during the monitoring period ascertained the meter's status and verified its accuracy. Manual depth and velocity measurements compared well with real-time meter average velocity measurements, and no level offset or post-processing adjustment was made.

Survey Results

Flows proved to be minimal and nearly zero from midnight until 6:00 a.m. Daily peak flows were usually observed between 9:00 a.m. and 5:00 p.m.

Of particular interest for the study was quantification of peak flow rates and total flow volumes produced from a July 2 James Taylor concert and a July 26 Styx concert. The results clearly show that major events of extended duration with attendance in the thousands will produce excessive flows that can overburden the system.

Conclusions and Considerations

The county learned that the capacity of the existing sanitary sewer line serving the Kansas Coliseum was sufficient only to transport peak flow from events with attendance representing about 55 percent of the Coliseum's seating capacity. Events routinely taking place at the arena already posed capacity problems, and the prospect of adding residential development and a 25,000-seat amphitheater would require additional planning.

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