Launch Slideshow

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Ahead of the flow

Ahead of the flow

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    Smoke signalsA typical smoke test encompasses a small portion of a sewer line. Areas showing inflow and infiltration can pop up anywhere along the system. Source: V&A Consulting Engineers

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    Smoke pours from a residential area drain illegally connected to a sanitary sewer system. When the yard becomes flooded, the stormwater is drained directly into the sanitary sewer. This is a direct inflow location. Photo: Oro Loma Sanitary District

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    Smoke from a below-grade sanitary sewer manhole structure showed pipe defects near a stormwater drainage channel. Further inspection revealed cracks and defects in structure. Photo: V&A Consulting Engineers

The old adage “an ounce of prevention is worth a pound of cure” should be updated to “a million gallons of waste-water saved through flow monitoring is worth millions of dollars, fewer headaches, and happy constituents.”

The new phrase doesn't really roll off the tongue, but the idea makes sense.

Strictly speaking, hydraulic modeling involves using a computer program that will accurately predict the flow characteristics of wastewater through the pipelines of a sanitary sewer system. Weaknesses within the system are located by running simulations of design storm events (typically 5-or 10-year events) through the model to estimate the reaction to increased flows due to inflow and infiltration (I/I). To calibrate the model, engineers use flow data collected from various strategic locations.

The result is a report that can be used to assure the municipality or sewage district that the system meets current and future needs, or warns if parts of the system are under-capacity or require rehabilitation and/or replacement.

Looked at more broadly, however, “hydraulic modeling” also includes other supporting functions.

In hydraulic modeling, a key support function is flow monitoring (FM) done by public works personnel or—more commonly—a FM service retained by a consulting engineering firm under contract to the district. FM equipment is placed in selected manholes to measure and record the depth and velocity of the wastewater moving past that point to determine the flow rate. These measurements, combined with collected rainfall data, can be used to determine the change in flow rates and the intensity of I/I during rainfall events.

The responsibilities of the FM service typically end at providing the engineers with the level and velocity data. But for the consultants and their clients, stopping at FM alone often results in their missing out on a valuable opportunity, because in many cases a little extra investigative work could provide huge dividends. With a more comprehensive approach—bundling FM services with other inspection and facility assessment capabilities—it's often possible to pinpoint specific I/I sources during the initial phase of the project rather than years later, as often happens, when repairs will be far more difficult and costly.