Green infrastructure and the future of cleaner urban waterways
When rainfall moves across roofs, streets, parking lots, and other hard surfaces, it can overwhelm drainage systems in a matter of minutes. In communities with combined sewers, that surge may cause a combined sewer overflow (CSO), releasing a mixture of stormwater and untreated wastewater into nearby rivers, channels, or coastal waters. The result can include elevated bacteria, trash, nutrients, sediment, and other pollutants.
Green infrastructure offers a distributed way to manage that volume before it reaches underground pipes. Instead of treating every storm as a problem for larger tunnels and treatment plants, cities can use soil, vegetation, engineered media, and carefully designed landscapes to slow, store, filter, infiltrate, and reuse runoff.
For water and wastewater professionals in the Los Angeles Basin, the subject connects directly to stormwater quality, watershed management, asset planning, and climate resilience. The LABS of CWEA community provides a useful professional setting for exploring how these approaches affect operations, design standards, compliance, and public infrastructure investment.
How combined sewer overflows develop
A combined sewer carries sanitary sewage and stormwater through the same pipe. During dry weather, the system may have enough capacity to convey flows to a wastewater treatment facility. During an intense storm, however, runoff can enter faster than the collection network and plant can safely handle it.
When storage and conveyance capacity are exceeded, a relief structure may discharge excess flow. This protects the system from dangerous backups and structural stress, yet it transfers pollution to the receiving environment. CSO control therefore requires a balance between public health protection, treatment performance, flood prevention, and water quality goals.
The Los Angeles region includes many separate storm drain and sanitary sewer systems rather than one uniform combined sewer network. Even so, green infrastructure remains relevant because stormwater runoff can carry similar pollutants and can overload downstream channels, treatment processes, pumping stations, and receiving waters. The same practices used for CSO mitigation can support broader urban runoff and watershed objectives.
What green infrastructure changes
Green infrastructure manages rain where it falls or close to its source. Bioswales direct runoff through vegetated channels; rain gardens temporarily hold water in engineered soil; permeable pavement allows rainfall to pass through a surface; green roofs retain precipitation; and cisterns capture water for later use. Tree wells and urban tree canopies add interception, evapotranspiration, and shade.
These systems reduce the peak rate and total volume entering a sewer. That distinction matters. A short, intense flow peak can trigger an overflow even when the total rainfall volume is moderate. Delaying runoff by minutes or hours may keep pipes, storage tanks, and treatment facilities within their operating limits.
Performance depends on design and maintenance. Soil compaction, sediment accumulation, clogged inlets, dead vegetation, and insufficient drawdown time can reduce storage and infiltration. Green infrastructure should therefore be treated as a managed utility asset, with inspection schedules, access requirements, performance criteria, and responsible owners defined before construction.
Comparing distributed and conventional controls
No single intervention can address every overflow condition. Large-scale storage, sewer separation, treatment upgrades, and real-time controls may be necessary for high-volume events or densely developed areas. Green infrastructure is most effective when it is integrated with those investments and strategically placed to intercept runoff before it reaches critical bottlenecks.
| Approach | Primary function | Strengths | Planning considerations |
|---|---|---|---|
| Rain gardens and bioretention | Store, filter, and infiltrate runoff | Flexible at neighborhood scale; improves streetscapes | Requires suitable soil, overflow routing, and maintenance |
| Permeable pavement | Reduce surface runoff and peak flow | Uses existing paved areas; supports groundwater recharge where appropriate | Needs vacuuming, structural design, and protection from clogging |
| Green roofs | Retain and evapotranspirate rainfall | Useful where ground space is limited; reduces heat gain | Depends on roof load capacity, access, and plant survival |
| Cisterns and rain barrels | Capture water for later use | Provides water conservation benefits and predictable storage | Requires demand for reuse and routine inspection |
| Storage tanks and tunnels | Temporarily hold excess flow | Handles large volumes and can provide dependable capacity | Capital-intensive; requires major construction and operations planning |
| Sewer separation or treatment expansion | Increase conveyance or treatment capacity | Addresses systemic limitations directly | Disruptive, expensive, and dependent on available space |
A portfolio approach also improves resilience. Smaller installations distributed throughout a drainage area can provide value even when one component is offline. Conventional infrastructure supplies dependable high-capacity control, while natural and engineered landscape features reduce the frequency and intensity of the demand placed on it.
Designing for Los Angeles conditions
Southern California presents specific constraints. Rainfall is seasonal, impervious surfaces are extensive, and many storms arrive in bursts that produce rapid runoff. Long dry periods can leave accumulated pollutants on streets and roofs, creating a concentrated first flush when rain begins. Designs must also account for water conservation, wildfire-related sediment, drought-tolerant plant selection, and limited groundwater infiltration in locations with contamination concerns.
Site assessment should precede facility selection. Engineers and agencies need information about drainage areas, soil permeability, groundwater depth, utility conflicts, slopes, pollutant sources, and expected maintenance access. Infiltration is not automatically appropriate everywhere; underdrains, liners, or controlled discharge may be safer options where groundwater protection or geotechnical conditions limit direct recharge.
Equity and public use belong in the design process as well. A curb extension with trees and bioretention can improve pedestrian comfort, reduce localized flooding, and create a more attractive public realm. A facility that blocks sidewalks, collects litter, or lacks an accountable maintenance program can produce the opposite result. Community participation helps identify practical concerns that hydraulic models may miss.
Measuring performance and accountability
Effective programs establish measurable outcomes before installation. Useful indicators include runoff volume retained, peak flow reduction, drawdown time, pollutant load removed, overflow frequency, maintenance completion, vegetation survival, and lifecycle cost. Monitoring can combine flow meters, water-level sensors, rainfall data, inspections, and periodic water-quality sampling.
Modeling tools help compare scenarios across a watershed. A city can test how many bioretention cells, permeable surfaces, or storage facilities are needed to meet a target for a particular design storm. Models should be calibrated with local observations where possible and updated as land use, rainfall patterns, and system conditions change.
Operations teams should receive clear documentation, training, and access to replacement materials. Asset registries can record facility locations, dimensions, drainage connections, inspection dates, defects, and responsible departments. Professional development resources and CWEA newsletters can help practitioners track technical discussions, local programs, and emerging approaches in water environment management.
Making green infrastructure part of capital planning
Green infrastructure has the greatest impact when it is included early in transportation, housing, parks, streetscape, and redevelopment projects. Retrofitting an already finished site is often more expensive and less effective than reserving space for stormwater management during initial planning. Public works departments can use standard details, approved plant palettes, and maintenance agreements to make implementation more consistent.
Funding strategies may combine stormwater utility revenues, transportation budgets, water quality grants, climate resilience funds, development requirements, and regional partnerships. Lifecycle analysis should account for installation, inspections, vegetation care, sediment removal, rehabilitation, and eventual replacement. A low construction price does not guarantee value if the facility fails after several neglected maintenance cycles.
Recommended steps for agencies and project teams include:
- Map runoff sources, sewer bottlenecks, receiving waters, and vulnerable communities before selecting facilities.
- Match each green infrastructure type to local soils, slopes, groundwater conditions, utilities, and maintenance capacity.
- Pair distributed controls with storage, treatment, or conveyance upgrades where storm volumes exceed landscape-based capacity.
- Create inspection, reporting, and funding responsibilities that remain clear after construction is complete.
- Use pilot projects and measured results to refine design standards for larger watershed programs.
Turning watershed goals into action
Reducing combined sewer overflows is a long-term infrastructure objective, but each street retrofit, roof installation, rain garden, and storage connection can contribute to measurable progress. Green infrastructure works best when hydraulic performance, ecological function, public access, and reliable maintenance are planned together.
Water and wastewater professionals can move these efforts forward through technical exchange, facility tours, workshops, certification courses, and collaboration across agencies and disciplines. Engage with LABS of CWEA to connect local experience with practical strategies for managing runoff, protecting receiving waters, and building a more resilient water environment.