Flow monitoring practices that help prevent sanitary sewer overflows
Sanitary sewer overflows (SSOs) are rarely caused by a single event. Blockages, inflow and infiltration, pump failures, undersized conveyance, power interruptions, and extreme rainfall can combine to push a collection system beyond its hydraulic capacity. Reliable flow monitoring helps agencies see these conditions developing before wastewater reaches streets, waterways, or private property.
A well-designed monitoring program connects field measurements with maintenance, capital planning, emergency response, and regulatory reporting. It gives operators a clearer view of how much wastewater moves through each basin, when peak flows occur, and where observed conditions differ from the design assumptions used years earlier.
For water and wastewater professionals in the Los Angeles Basin, local climate, aging infrastructure, dense development, and seasonal rainfall create a strong case for practical, well-maintained instrumentation. Flow data becomes most valuable when engineers, operators, consultants, and agency managers can interpret it together and act on it quickly.
Why flow monitoring matters
Continuous or periodic flow measurement establishes a baseline for a sanitary sewer system. That baseline may include average dry-weather flow, minimum nighttime flow, wet-weather peaks, daily patterns, and unusual changes linked to industrial discharges or commercial activity. Without a baseline, an agency may mistake a normal seasonal fluctuation for a developing problem or overlook a gradual increase in wastewater volume.
Monitoring also supports early warning. A sudden rise in upstream flow can indicate rainfall-derived inflow, a broken lateral, or an illicit connection. A drop in flow downstream may point to a blockage, equipment failure, or sensor problem. When data from multiple locations is reviewed together, operators can narrow the likely source and prioritize field verification.
The strongest programs link flow meters with rainfall gauges, pump station status, level sensors, work-order records, and overflow alarms. This combination helps distinguish hydraulic conditions from instrumentation errors. It also creates a defensible record for capacity assessments, infiltration and inflow studies, emergency documentation, and long-term asset management.
Build a reliable monitoring network
Monitoring locations should be selected to answer specific operational questions. Useful sites may include the outlet of a drainage basin, the upstream and downstream sides of a suspected bottleneck, major pump station influent channels, trunk sewers, industrial discharge points, and locations near recurring overflow history. A small number of strategically placed meters can produce more useful information than a large network with poor maintenance and unclear objectives.
Site selection requires attention to hydraulics. A meter needs sufficient straight-run conditions, stable flow behavior, safe access, and an appropriate range of depth and velocity. Turbulence, surcharging, debris, sediment, grease, backwater effects, and submerged outlets can distort readings. Before installation, staff should inspect the pipe, review as-built drawings, examine historical complaints, and confirm that the selected technology fits the site.
Common technologies include area-velocity meters, insertion velocity sensors, ultrasonic devices, bubbler systems, and flumes or weirs. No instrument performs perfectly in every sewer. Area-velocity equipment can work well in variable, partially full pipes, while a primary device such as a flume may provide dependable control where installation conditions allow. Selection should consider accuracy, fouling risk, battery life, telemetry, maintenance access, and total cost over the monitoring period.
Turn measurements into operational intelligence
Raw readings require validation before they can guide decisions. Staff should establish acceptable ranges for depth, velocity, and calculated flow, then investigate flatlined signals, impossible negative values, abrupt step changes, and repeated gaps. Data checks should account for sensor drift, clock errors, communication interruptions, and periods when a meter is inaccessible because of high flow or unsafe conditions.
Calibration and verification should be documented from installation through removal. Field crews can compare sensor readings with independent depth measurements, portable instruments, pump runtime, tank levels, or known discharge volumes. A clear chain of custody for data helps technical staff explain why a value was accepted, adjusted, or excluded.
| Monitoring element | What it reveals | Useful operational response |
|---|---|---|
| Dry-weather flow | Baseline sanitary demand and possible infiltration | Compare trends by season and investigate persistent increases |
| Nighttime minimum flow | Continuous groundwater or stormwater entry | Prioritize smoke testing, CCTV inspection, or source removal |
| Rain-event hydrograph | Inflow, infiltration, and basin response | Check vulnerable assets and activate wet-weather readiness |
| Depth and velocity | Hydraulic behavior and surcharge risk | Inspect restrictions, bottlenecks, and downstream controls |
| Pump station influent and runtime | Capacity stress and equipment performance | Adjust maintenance, standby procedures, or pumping strategy |
| Sudden flow loss | Possible blockage, sensor fault, or diversion | Verify the site promptly and issue an operational alert |
Dashboards should emphasize exceptions rather than bury operators in excessive detail. Useful alerts may include rapid depth increases, sustained high flow, unusual nighttime volume, loss of communications, or a mismatch between adjacent monitoring points. Alert thresholds must be tested against historical data so that staff receive meaningful warnings instead of frequent nuisance notifications.
Use monitoring to guide maintenance and planning
Flow monitoring becomes valuable when it changes the maintenance schedule. If a basin shows recurring high nighttime flow, the agency can combine meter data with CCTV inspection, smoke testing, dye testing, private lateral investigations, and rainfall analysis. If a trunk sewer repeatedly approaches surcharge during moderate storms, cleaning alone may not solve the risk; hydraulic modeling or capacity improvements may be needed.
Trend analysis can also support targeted cleaning. A sudden increase in upstream depth with reduced downstream flow may justify immediate inspection for a blockage. Repeated flow irregularities near a grease-producing commercial area can support focused outreach and pretreatment enforcement. Linking monitoring data with computerized maintenance management systems helps agencies track whether corrective work actually improves hydraulic performance.
Field knowledge remains essential. Operators often recognize sounds, odors, access conditions, and flow behavior that a dashboard cannot capture. Sharing those observations with engineering and management teams creates a more complete picture of collection system performance. The LABS of CWEA gallery reflects the value of professional connection and field-based learning across the water environment community.
Prepare for storms and abnormal conditions
Wet-weather monitoring should begin before the first major storm. Agencies can inspect critical meters, clean sensor faces, test telemetry, confirm battery capacity, verify generator readiness, and review access restrictions. Rain gauges should be checked for accurate timestamps and representative placement, since a distant gauge may fail to reflect conditions in a particular drainage basin.
Rainfall intensity and duration should be evaluated alongside flow response. A fast rise immediately after rainfall often suggests direct inflow through manholes, roof connections, or cross-connections. A slower, prolonged increase may indicate groundwater infiltration through defects in pipes, joints, or laterals. These patterns can help determine whether the appropriate response is immediate debris control, targeted inspection, or a long-term rehabilitation program.
Emergency procedures should define who receives alerts, who verifies conditions in the field, and who has authority to mobilize crews or contractors. Plans should address traffic control, confined-space safety, public notification, sampling, documentation, and coordination with neighboring agencies. Lessons from major facility incidents, including the Hyperion fire lessons, reinforce the importance of clear communication, resilient procedures, and disciplined incident review.
Strengthen the monitoring program
A sustainable program needs defined ownership. One team may manage instruments and telemetry, another may review trends, and operations staff may lead field response. Those responsibilities should be written into standard operating procedures, with escalation thresholds and contact information kept current.
Training should cover sensor fundamentals, safe access, data interpretation, quality assurance, and emergency response. MOC certification courses, automation workshops, technical presentations, and facility tours can help staff connect instrumentation concepts with real collection system conditions. Cross-training is especially important when only a few employees understand a particular meter platform or data system.
Agencies should review program performance at least annually. Measures may include meter uptime, percentage of valid data, response time to alerts, number of avoidable alarms, completed inspections, overflow frequency, and reductions in unexplained flow. Reviewing these indicators helps justify replacements, improve specifications, and demonstrate that monitoring supports measurable risk reduction.
Recommended practices for SSO prevention
- Define the operational question before selecting a monitoring location or instrument.
- Establish dry-weather and wet-weather baselines using validated data and rainfall records.
- Inspect, clean, calibrate, and function-test meters on a documented schedule.
- Integrate flow, depth, rainfall, pump status, alarms, and maintenance records in one review process.
- Set alert thresholds around actionable conditions and verify every significant alarm in the field.
Flow monitoring is most effective when it becomes part of daily decision-making rather than a standalone data project. Agencies that combine dependable instrumentation, trained personnel, preventive maintenance, and clear response protocols can identify hydraulic stress earlier and reduce the likelihood that a blockage, storm, or equipment failure becomes an SSO.
LABS of CWEA provides a practical setting for water environment professionals to exchange methods, examine lessons from real facilities, and build the skills needed for resilient collection systems. Explore upcoming programs, technical activities, and professional events to strengthen your agency’s approach to flow data and overflow prevention.