How Climate Change Impacts Stormwater and Sanitary Sewer Flows
Climate change is altering the timing, intensity, and reliability of rainfall across Southern California. For water and wastewater agencies in the Los Angeles Basin, that shift affects how quickly runoff reaches streets, channels, pump stations, treatment plants, and coastal outfalls. A storm that once produced manageable peaks may now create sharper surges, while longer dry periods can change groundwater conditions and pollutant buildup.
Stormwater and sanitary sewer systems are connected through the built environment, even where they are designed as separate networks. Cracked pipes, deteriorated manholes, illegal connections, and rising groundwater can allow inflow and infiltration into sanitary sewers. At the same time, intense rainfall can overwhelm storm drains and expose treatment facilities to hydraulic conditions beyond their original design assumptions.
Understanding these interactions helps engineers, operators, consultants, and agency leaders plan capital improvements and make better real-time decisions. Climate-informed asset management is becoming essential for protecting receiving waters, maintaining treatment capacity, and meeting increasingly demanding regulatory requirements.
Rainfall is becoming less predictable
Southern California’s annual precipitation may remain highly variable, but the pattern of rainfall is changing. Longer dry intervals can be interrupted by short, intense events that produce substantial runoff before soil, vegetation, and drainage systems can absorb it. Atmospheric rivers may bring large volumes of water within a narrow window, creating high peak flows even when the broader season has been dry.
Wildfires can intensify this response. Burned landscapes often have less vegetation and more water-repellent soil, increasing erosion and accelerating runoff. Sediment, ash, nutrients, metals, and organic debris can enter storm drains in unusually high concentrations. The result is a combined hydraulic and water-quality challenge for municipalities responsible for flood control and pollutant reduction.
Urban development magnifies these effects. Pavement and rooftops prevent infiltration, while dense neighborhoods send water toward inlets and channels quickly. When rainfall exceeds local conveyance capacity, streets can become temporary flow paths. Low-lying pump stations, underpasses, and treatment plant access roads are especially vulnerable to brief but severe flooding.
Sanitary sewers feel the pressure
Sanitary sewer systems are affected by climate conditions through inflow and infiltration, commonly called I/I. Inflow enters directly through improperly connected drains, manhole covers, cleanouts, or cross-connections during rainfall. Infiltration moves through cracks, defective joints, porous structures, and compromised service laterals when groundwater rises.
A heavy storm can therefore produce a rapid increase in wastewater arriving at lift stations and treatment facilities. This water may be relatively clean, yet it consumes collection and treatment capacity. Excessive I/I can trigger sewer overflows, bypasses, basement backups, higher pumping costs, and avoidable wear on mechanical equipment.
Longer wet periods can keep groundwater elevated after a storm has passed, extending the duration of increased sanitary sewer flows. Conversely, drought can worsen pipe deterioration as soils shrink, settle, or move around buried infrastructure. A dry year does not eliminate climate-related sewer risk; it may simply shift the dominant failure mechanism from hydraulic overload to structural vulnerability.
Treatment plants and receiving waters face compound risks
Wastewater treatment plants must manage both the volume and characteristics of incoming flow. Storm-related I/I can dilute wastewater, alter biological treatment conditions, and reduce the effective time available for settling and disinfection. Sudden surges may also challenge headworks, screening systems, grit removal, equalization basins, and pump capacity.
Stormwater carries pollutants from roads, industrial areas, construction sites, and residential properties. First-flush runoff can contain bacteria, trash, hydrocarbons, pesticides, nutrients, and metals. When coastal or inland receiving waters receive these discharges during intense events, water quality impacts may be concentrated over a short period rather than distributed across a longer, moderate storm.
Sea-level rise adds another dimension for coastal facilities. Higher tailwater levels can reduce gravity drainage, limit outfall performance, and increase the risk of backflow during high tides or coastal storms. Facilities near the shoreline may need additional flood protection, corrosion-resistant equipment, standby power, and pumping capacity.
| Climate pressure | Likely flow response | Main operational concern | Useful planning response |
|---|---|---|---|
| Intense rainfall | Rapid stormwater peaks and runoff surges | Flooded streets, inlets, channels, and pump stations | Update intensity-duration-frequency assumptions and improve conveyance |
| Atmospheric rivers | Extended high inflow and elevated groundwater | Sanitary sewer I/I and treatment overload | Inspect vulnerable assets and use wet-weather flow monitoring |
| Longer droughts | Soil movement, pipe stress, pollutant buildup | Cracks, infiltration pathways, and concentrated first flush | Expand condition assessment and preventive maintenance |
| Wildfire and post-fire storms | Fast runoff with sediment and debris | Clogged infrastructure and impaired receiving waters | Coordinate watershed recovery, debris control, and sampling |
| Sea-level rise | Higher tailwater and reduced outfall capacity | Backflow, corrosion, and coastal flooding | Protect critical assets and evaluate tide-gate or pumping needs |
Data turns climate risk into operating decisions
Agencies can improve preparedness by combining rainfall records, radar data, stream gauges, groundwater observations, tide information, and flow monitoring. Comparing rainfall intensity with sanitary sewer response helps identify where I/I is entering the system and whether the source is rapid inflow or slower infiltration.
Remote monitoring can provide early warning at lift stations, diversion structures, detention facilities, and treatment plant headworks. Alarm thresholds should reflect local conditions and available response time. Operators may be able to adjust pumping sequences, inspect vulnerable locations, deploy portable equipment, or coordinate public messaging before an overflow occurs.
Asset management programs should also account for changing climate stressors. Historical performance remains valuable, but relying exclusively on past averages can understate future peak conditions. Scenario planning can test how facilities perform under combinations such as intense rainfall, elevated groundwater, high tide, and a power outage.
Planning must connect stormwater and wastewater programs
Stormwater management is increasingly shifting toward distributed solutions. Bioswales, infiltration galleries, permeable surfaces, rain gardens, cisterns, and detention areas can reduce runoff volume or delay its arrival at downstream infrastructure. Infiltration projects require careful groundwater, soil, contamination, and geotechnical analysis, especially in dense urban areas.
Sanitary sewer programs should prioritize areas with repeated wet-weather flow increases, known structural defects, and high consequences of failure. Smoke testing, closed-circuit television inspection, manhole surveys, flow isolation, and service-lateral programs can reveal sources that are invisible during dry-weather inspections.
Cross-department coordination is essential. Public works, wastewater, stormwater, transportation, emergency management, and environmental compliance teams often hold different pieces of the same risk picture. Joint planning can align green infrastructure, sewer rehabilitation, flood response, and treatment plant upgrades instead of addressing each problem in isolation.
Regulatory requirements are part of resilience
Climate-driven flow changes can complicate permit compliance. Higher wet-weather volumes may increase the likelihood of sanitary sewer overflows, while stormwater discharges can carry larger pollutant loads during intense events. Coastal agencies must also consider how changing receiving-water conditions affect monitoring, outfall performance, and discharge controls.
Regulatory planning should be integrated with engineering decisions from the beginning of a project. Professionals working with coastal facilities can review this NPDES permit guide when evaluating discharge requirements, documentation, and operational responsibilities. Clear records of monitoring, maintenance, inspections, and corrective actions support both compliance and long-term risk management.
Climate resilience does not mean designing every facility for the most extreme imaginable event. It means identifying credible scenarios, understanding consequences, and selecting measures that can be expanded or adapted. Flexible pump capacity, protected electrical systems, additional storage, modular treatment equipment, and improved telemetry can provide practical resilience without requiring every upgrade at once.
Practical priorities for water professionals
Agencies can begin with targeted actions that improve current performance while building a stronger foundation for future investment:
- Map stormwater and sanitary sewer assets exposed to flooding, sea-level rise, wildfire runoff, and elevated groundwater.
- Establish wet-weather flow monitoring at representative sewersheds, lift stations, and treatment plant influent points.
- Update hydraulic models with current land use, observed storm data, groundwater conditions, and realistic climate scenarios.
- Prioritize I/I reduction where capacity constraints, overflow history, or public health consequences are greatest.
- Train operations and maintenance teams on emergency pumping, telemetry response, debris control, and post-storm inspections.
Professional networks can help agencies compare methods and learn from projects in similar watersheds. Technical presentations, facility tours, workshops, and certification opportunities create a practical setting for exchanging operational experience. The LABS of CWEA events calendar connects water and wastewater professionals with programs that support this kind of ongoing development.
The strongest programs treat climate adaptation as a continuous management process. Flow data, inspection results, storm observations, and operator experience should feed back into capital planning and standard operating procedures. This approach allows agencies to improve reliability incrementally while responding to changing conditions.
Water professionals across the Los Angeles Basin can use these strategies to protect public health, reduce overflow risk, and preserve receiving-water quality. Participate in regional training and technical events, bring climate-related flow questions to colleagues, and turn local observations into coordinated action before the next extreme storm tests the system.