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.


FM technicians have years of experience observing flows within collection systems and they visit the collection system at multiple locations as part of their work. As a result, they often come across evidence or have strong suspicions regarding pipelines and basins contributing to I/I. If the consulting engineers and sanitary agencies agree to put a little extra into the budget up front, various I/I tracking techniques could be used to track down the source of the I/I right then and there. That way, small problems can be dealt with while they still are small, which saves the municipality a lot of time and effort in the long run.

Hydraulic modeling of wastewater collection systems occurs as part of the master planning process, and economic realities tend to dictate that FM is offered and regarded as a low-cost “commodity” service. Unfortunately, a waste-water collection system is neither uniform nor predictable, and a hydraulic model is only a theoretical construct. It can help predict how the system will behave under certain storm event conditions, and it can isolate certain basins and expose the system's limitations from a pipe-capacity point of view. But if what is needed is specific and actionable information, there's no substitute for “eyeball-verified” field data.

Public works departments should elevate FM from its customary status as a commodity service, and broaden its scope. This could be done in several ways.

Increase knowledge. Give FM technicians extra training and furnish them with information such as maps of the drainage basins, knowledge of problem areas, and prior sanitary sewer overflow locations. If team members are allowed to take action where warranted, they often can identify or eliminate potential problems. A little extra work can potentially accomplish what later would require many hours of effort to set up a separate study. The crews also could collect data for the district's permanent archive and confirm such seemingly obvious “details” as pipe diameters and locations, which often do not agree with existing documentation.

Include the engineer. In an ideal world, every FM crew might include an engineer, since the engineers—trained and interested in solving the municipality's problems from the master planning perspective—often can spot things that even a well-trained, experienced field technician might overlook. It's probably overoptimistic to suggest such a thing is possible, but the numbers do support the idea, since the hourly fee for an engineer is vanishingly small when measured against the value of the assets concerned.

Document the actual condition of the system. Being able to show that steps have been taken to identify and correct problems can keep public works or waste-water districts in good favor with the various regional, state, and federal oversight agencies.

Such an all-encompassing approach may not be possible in a medium or large city or district with its dense urban infrastructure. But even there it could be applied to one or two large basins that were already identified as problems in the previous master plan. And for small areas with limited resources, this approach can pinpoint problems far more quickly and cost-effectively than with other conventional methods.

— Krajewski is principal and manager of the sewer system evaluation services division of V&A Consulting Engineers, Oakland, Calif.

Taking control

How one city pinpointed the single biggest direct source of inflow and infiltration.

To northern California residents, Cotati, Calif.—located an hour north of San Francisco—is known for its annual jazz, blues, and arts festival in June and a two-day accordion festival in late August. To infrastructure professionals, the city also is known as one that runs its civil infrastructure with the discipline and proactive outlook of a private business enterprise.

Cotati's Water and Sewer Department retained the California firm Winzler & Kelly Consulting Engineers with V&A Consulting Engineers as a subconsultant to complete a master plan of its collection system, including an inflow and infiltration (I/I) study and reduction program that takes advantage of flow monitoring (FM).

The consulting firms partitioned the collection system into 10 basins and conducted dry- and wet-weather flow monitoring, for a total project cost of about $66,000. The data were analyzed to determine which basins contributed the most ground-water infiltration, rainfall-dependent infiltration, and stormwater inflow into the collection system.

When the analysis showed that Basin 9 had a significantly large source of stormwater inflow during the larger storm events, the basin was investigated using several techniques, including smoke testing, low-flow reconnaissance investigations, interviews, and closed-caption TV pipe inspections. Two months of investigation pinpointed the source of the offending inflow: a heavy-water-use business that had three outside floor drains situated adjacent to the property and directly connected to the sanitary sewer system.

At a low point within the watershed basin, the property flooded repeatedly during the larger storm events of the winter season, sometimes resulting in more than 24 inches of standing water. More than 500,000 gallons a day were flushed into the system via the floor drains. The connection of the floor drains to the sanitary sewer was later confirmed using a tracing dye test, and the characteristics of a flood event were consistent with the FM data.

By correcting this single defect, the city eliminated the biggest direct source of I/I in its wastewater system.