In my time in the industry, I've encountered some "interesting" strategies for monitoring wastewater flows – from monitoring at the base of drop structures to dealing with pipes choked with silt. However, one of the most prevalent issues is the inclination to monitor large trunk sewers instead of collector lines, often driven by a desire to reduce upfront costs.
Typically, hydraulic modelers and consulting firms opt for the seemingly cost-effective route of installing flow monitors on large trunk sewers. This approach, while saving on equipment, relies on subtracting data from monitors upstream and downstream on the same line to estimate flows from contributing subdivisions. So what's the catch? Monitoring large trunks with a heavy reliance on subtracting flows can lead to errors that add up. It can even eventually result in what's sometimes referred to as a "flow imbalance."
I've encountered this question many times: "Why do the upstream flow monitors show more flow than the downstream one? That doesn't make sense." The answer lies in the errors that accumulate when subtracting flows between monitored sites, particularly when the net flow (from the collectors) is a small percentage of the total flow (of the trunk).
So, if your goal is RDII reduction, what's the solution? Monitor collector lines coming from subdivisions. Focus on smaller catchment areas, perhaps spanning 3 to 5 km of pipe. While this may involve higher upfront equipment costs due to an increased number of monitoring locations, the result will be significantly more accurate measurements (pending other hydraulic conditions). Better data paves the way for smarter decisions, ultimately saving money.
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General Superintendent TX/NM.
2moNice Work!