Best practices for I/I analysis using flow data
I/I analysis estimates how much water enters a sewerage network when it should not, and where it comes from. Inflow arrives through stormwater connections, manholes, or illegal cross-connections; infiltration is groundwater leaking through pipes and joints. Flow data gives a network-scale signal, but it cannot identify a defect by itself. A defensible study combines flow and rainfall records, asset knowledge, field testing, and a clear decision about what the council or authority must fix. Learn more about Vidnovlennya Populyatsii Svitlyachkiv Na Richtsi Mabechi Uspishnii Proekt Cdf9e.
Australian conditions make that combination important. Sydney’s short, intense storms can overwhelm older combined networks, while Melbourne’s councils manage a patchwork of local reticulation and metropolitan trunk assets. Perth’s sandy soils and groundwater create a different infiltration pattern, and Brisbane’s wet season produces long recession periods. Local teams talk about a “problem catchment” or “hot zone”, and may need to take evidence down the track to a council workshop or operations crew. Dry-weather assumptions from another climate may not travel well.
A useful analysis moves from a broad question to an intervention. It establishes a dry-weather baseline, identifies storm responses, quantifies excess flow, and directs CCTV surveys, pressure management, roof-plumbing checks, or renewal. Flow data is most valuable when it helps prioritise scarce survey and renewal resources. The following practices show how to measure the signal, test the result, and present findings in a form that supports an operational or capital decision.
Frame the analysis around a decision
Begin with the decision, not the acronym. “How much I/I is present?” differs from “Which basin should be relined next?” or “Can inflow reduction delay a pump upgrade?” Set the area, time horizon, and acceptable uncertainty. Account for authorised trade waste, non-potable connections, and legitimate large users so they are not mislabelled as leakage. A common Australian pitfall is mixing a council’s retail collection system with a metropolitan trunk without reconciling meter ownership and reporting boundaries.
Choose a scope the team can verify: one pump catchment for a metropolitan authority, or several small clusters around a treatment plant for a regional council. Use SCADA historian data where possible, and link flow meters to rainfall gauges, tank levels, and pump run hours. A plant meter can conceal the location of ingress; sub-catchment meters make the flow monitoring signal more useful. Document gaps, meter changes, and upgrades before calculating results.
Peer review can expose assumptions before fieldwork begins. The technical events calendar shows how technical presentations, workshops, and facility events can support that learning.
Build a defensible flow-data foundation
Treat the measurement chain as part of the result. Confirm meter type, calibration history, units, sampling interval, time stamps, and treatment of reverse flow. Check for blocked sensors, dropouts, and replacement equipment, then retain both raw and validated data with an audit trail. In Australia, state explicitly whether “MLD” means megalitres per day and whether the site records litres per second; unit confusion can erase a genuine trend.
Rainfall is the companion series, not a substitute for flow. Match Bureau of Meteorology observations, local gauges, or radar to the catchment and record gauge distance, time zone, and whether values are accumulated or intensity-based. Antecedent dry days matter as much as total depth: a storm after weeks of rain may fill soil and produce a weak, delayed response, while a dry-soil storm may produce a sharp one. Account for stormwater diversions, reservoir operations, irrigation, and pump-station storage before interpreting the curve.
A polished dashboard can still hide weak data. The live casino demo is a reminder that a live graphic is useful for engagement rather than engineering validation. Retain raw series, disclose gaps, and place a confidence note beside every headline figure, especially when results go to ratepayers or a board.
Separate baseline, storm, and operational effects
Build the dry-weather baseline from comparable periods rather than one quiet day. Separate weekday and weekend behaviour, night-time minima, school terms, industrial shifts, and seasonality. A median or percentile by month and day type is often more robust than an average, with rain, maintenance, and abnormal trade-waste periods flagged. Persistent elevation of night flow suggests chronic infiltration, while a stable baseline with sharp rain peaks points toward rapid stormwater entry.
For storms, subtract expected baseline flow, integrate the excess through recovery, and relate volume to rainfall depth and catchment area. Examine onset, hydrograph peak timing, and recession as well as total litres. A short, intense spike may indicate direct entry through a manhole or roof connection; a long tail may reflect groundwater or storage attenuation. Normalise only after confirming that the rain gauge represents the catchment and that no diversion or pump-control change confounds the event.
Use several views rather than one ranking:
| Method | Main question | Best use | Main limitation |
|---|---|---|---|
| Dry-weather baseline | Is normal flow stable? | Night flow and seasonal trends | Misses storm entry |
| Storm-event analysis | How much extra water enters? | Volume, peak, lag and recession | Sensitive to rainfall and gauge quality |
| Continuous flow trend | Is the condition worsening? | Long-term change and recurrence | Operations can mimic deterioration |
| Level and pump analysis | Is storage altering the signal? | Tank levels, cycling and overflow risk | Needs reliable control data |
No single metric captures the whole problem. Agree whether success means reduced annual excess volume, a lower peak, or a stable night-time baseline, then use at least two views before making a claim. Report the assumed baseline, rainfall totals, number of qualifying storms, and uncertainty range. Repeated events and field evidence are needed before a defect is confirmed. Australian utilities may also weigh energy, odour, corrosion, creek overflow, and treatment costs, so litres alone do not determine priority.
Interpret patterns and verify them in the field
Flow signatures narrow the search but do not prove a cause. A response within minutes of rain suggests direct entry from a cracked manhole, a connected downpipe, or a missing stormwater barrier. Elevated night-time flow that persists through dry weather is consistent with groundwater infiltration, although a major industrial customer or leaking trunk can look similar. Saltwater intrusion in coastal areas and tank-filling behaviour also need checking. Rank catchments by annual excess volume, confidence, consequence, and surveyability; a smaller persistent problem can have a better business case than one dramatic peak.
Design fieldwork around the signal. Use CCTV for structural conditions, smoke testing where appropriate, dye tests to trace private connections, and manhole surveys for ingress. Roof and downpipe checks suit immediate rainfall responses. Compare pre-repair and post-repair flow over comparable weather, and record changes such as new pump settings or temporary bypasses; a reduction is not verified if the operating conditions changed. When reporting a programme over time, the population monitoring case study offers a parallel for making indicators and their limits clear, although sewer results require engineering units and catchment boundaries.
Turn results into funded, measurable actions
Connect each finding to an owner and the next measurement. “Excess wet-weather flow of 1.2 ML per event” should be paired with contributing basins, likely source categories, confidence, and actions to confirm or reduce it. Separate housekeeping from capital works: correcting a misconnection or changing a control strategy may be cheaper than relining, but the case must include energy, overflow, odour, and maintenance consequences. Put agreed meters, gauges, and sensors into asset and SCADA registers, and give crews a storm-response protocol.
Place the reduction target in the council’s planning and procurement cycle. The Australian market is fragmented across metropolitan authorities, local councils, regional operators, consultants, and contractors, so interfaces and handover records can matter as much as the analysis. LABS of CWEA connects the Los Angeles Basin water and wastewater community through technical events, workshops, MOC certification courses, and automation training. For Australian teams, the model shows how a professional network can sustain knowledge sharing across jurisdictions.
The strongest study is not the one with the most dramatic chart. It is the one that can show what changed, why it matters, and how success will be measured. The practical takeaway is to start with one well-instrumented catchment, establish a defensible baseline, compare multiple storms, and direct field effort according to both flow volume and consequence.