John M.H. Barton, Joseph Kamalesh and Roger Jacobsen


This paper demonstrates a new tool capable of accurately measuring low flows in sewer pipes, the micromonitor. It solves three problems that have limited flow monitoring to mainly large scale basins, cost, low-flow accuracy, and debris. With the micromonitor, an I/I investigation program can be retooled to find I/I at a much lower cost. Instead of one regional flow monitor followed by comprehensive CCTV and SSES investigations, the micromonitor provides cost-effective reduction in basin size, down to a single street or segment. In the past year four separate I/I projects applied this approach, Seattle, WA; Cold Spring, KY; Florence, KY; and Clayton County, GA. Each of these projects was able to effectively isolate the I/I sources.

The micromonitor is a smooth weir with zero-slope leading edge. It uses a rating curve at very low flows, below 20 gpm, but transitions to the continuity equation at higher flows. This differs from other primary devices used in sanitary sewers such as a Thelmar weir or a Palmer-Bowlus flume, which maximize the range over which the rating curve applies, and thereby produce debris and ragging. The accuracy of the micromonitor will be demonstrated through the results of independent testing and micrometers placed in adjacent manholes. The lower cost will be shown empirically from the projects and explained by the reduced requirements on the data. Finally, actual CCTV results taken during wet-weather show how I/I sources can be found where micromonitoring indicates.

Micromonitoring offers the opportunity to refine hydraulic models upstream of regional flow monitors. Average loadings across a basin can be replaced with measurements. In cases where trunk sewer flow differencing is questionable, basin parameters would be measured not estimated from adjacent or similar basins. Though micromonitoring has not yet been used to refine hydraulic models it is on the horizon.

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