Green infrastructure vs. gray: how NJ can fix its CSO problem without just building bigger pipes

Published by NJ Clean Stream  |  Combined Sewer Overflows Series, Article 3 of 3

The traditional solution to combined sewer overflows has been a gray one: build larger pipes and deeper tunnels to contain the overflow. In some cities, this has worked — at enormous cost. But for most of New Jersey’s CSO-burdened communities, there is a more cost-effective path: managing stormwater at its source, before it ever reaches the drain.

What is green infrastructure and how does it reduce CSOs?

Green infrastructure, in the stormwater management context, refers to systems and practices that manage stormwater by mimicking natural hydrological processes — capturing, infiltrating, evapotranspiring, and slowly releasing precipitation where it falls, rather than routing it rapidly off impervious surfaces into sewer systems.

The core insight is this: modern urban development replaced the natural landscape’s capacity to absorb and slowly release precipitation with impervious surfaces that shed water immediately and rapidly. A forest receiving an inch of rain absorbs most of it through soil infiltration and plant uptake, releasing it slowly over hours and days. A city block receiving the same inch routes nearly all of it immediately to the nearest drain. Green infrastructure seeks to restore some of that natural absorptive capacity to the urban landscape — reducing the peak volume of stormwater entering the combined sewer during storm events, and thereby reducing the frequency and volume of overflows.

The green infrastructure toolkit

Rain gardens

Shallow, planted depressions that collect stormwater runoff from adjacent impervious surfaces and allow it to infiltrate into the soil. A well-designed rain garden can capture and infiltrate the stormwater from hundreds to thousands of square feet of adjacent impervious surface during a typical storm. They are typically planted with native perennial plants that provide habitat for pollinators and birds — combining stormwater management with aesthetic and ecological value.

Cost advantage: Rain gardens can be installed on existing landscaped areas without major excavation, making them among the most cost-effective green infrastructure tools for residential and commercial properties.

Bioswales

Vegetated channels designed to slow, filter, and infiltrate stormwater runoff. Where rain gardens are compact installations, bioswales are linear features — often installed in street medians, along parking lot edges, or in street rights-of-way — that slow and treat runoff as it flows through the vegetated channel. Particularly effective at managing stormwater from streets and parking lots, which are major contributors of both runoff volume and pollutant loads.

Green roofs

Roof surfaces covered with a growing medium and vegetation that capture and retain rainfall. A standard “extensive” green roof can retain 50 to 80 percent of annual rainfall, depending on climate and design. In dense urban environments where rooftop area is a substantial fraction of total impervious cover, widespread green roof installation can significantly reduce the stormwater contribution from buildings to the combined sewer system. Green roofs also provide energy efficiency benefits — reducing building heating and cooling costs.

Permeable pavement

Paving materials that allow water to infiltrate through the surface into a stone reservoir below, from which it infiltrates into the soil or is slowly released to a managed drainage system. Permeable pavement can replace conventional asphalt or concrete on parking lots, low-traffic streets, driveways, sidewalks, and pedestrian plazas. In areas where stormwater from parking and roadway surfaces is a major contributor to CSO events, widespread permeable pavement installation can substantially reduce runoff volumes.

Street trees and urban forestry

Often overlooked as stormwater management tools, mature urban trees can intercept hundreds of gallons of rainfall per year through canopy interception and evapotranspiration. The co-benefits — air quality improvement, urban heat island reduction, aesthetic and property value benefits — make urban forestry one of the highest-value investments a CSO-burdened city can make.

What Philadelphia’s program proved

Philadelphia’s Green City, Clean Waters program, launched in 2011, is the most extensively studied example of large-scale green infrastructure deployment for CSO management. The program committed to managing stormwater from 9,564 acres of impervious surface through green infrastructure over 25 years. The financial case was compelling: primarily green infrastructure would reduce CSO discharges by roughly the same amount as comparable gray infrastructure at approximately one-third the cost — $2.4 billion versus an estimated $6–8 billion for a conventional approach.

By the program’s ten-year mark, thousands of installations were in place across the city — rain gardens, tree trenches, green roofs, permeable pavement. Measured results showed reduced CSO volumes, reduced peak stormwater flows, and co-benefits including improved air quality, reduced urban heat, increased green space in underserved neighborhoods, and thousands of installation and maintenance jobs.

Washington, D.C.’s Clean Rivers Project has combined deep tunnel construction with significant green infrastructure deployment, demonstrating that green infrastructure can reduce the required size and cost of gray infrastructure by managing a portion of the stormwater that would otherwise require tunnel capacity. This complementary approach is increasingly the standard design for large CSO remediation programs.

What green infrastructure cannot do

NJ Clean Stream is a strong advocate for green infrastructure — but also clear-eyed about its limitations.

Green infrastructure cannot, on its own, eliminate CSOs from a large combined sewer system in a densely built-out urban environment. The stormwater volumes generated by major storm events are too great for even the most ambitious green infrastructure program to intercept without some gray infrastructure component.

Green infrastructure requires ongoing maintenance. Rain gardens must be inspected and replanted. Permeable pavement must be vacuumed periodically. Green roofs require maintenance. Cash-strapped municipal governments often struggle to sustain this work. Green infrastructure that is installed but not maintained degrades in performance and may fail entirely.

Green infrastructure cannot be installed everywhere. In the densest urban cores — where land is fully developed, basements are deep, and soils are often contaminated — finding suitable sites is genuinely difficult.

Green infrastructure still costs money that the most burdened communities in New Jersey cannot generate from their own resources. The same funding equity arguments that apply to gray infrastructure apply equally to green infrastructure programs.

What a comprehensive NJ CSO strategy would look like

  • A statewide green infrastructure mandate incorporated into all CSO Long-Term Control Plans, requiring green infrastructure to be evaluated as the primary stormwater management approach before gray alternatives are considered.
  • A Green Infrastructure for Clean Waters fund — a state revolving grant program capitalized by general obligation bonds and federal infrastructure funding — providing direct grants to combined sewer communities. Prioritized by CSO discharge volume, water quality impairment, and community financial capacity. Lower-income communities receive full grant funding.
  • Technical assistance for municipalities that lack the engineering staff to design, procure, and implement green infrastructure programs effectively.
  • Real-time CSO notification systems required for all CSO permittees, informing the public when a discharge has occurred and when the waterway is safe for use.
  • Firm, enforceable LTCP timelines with meaningful consequences for communities that fail to implement their plans — and the financial assistance necessary to make those timelines achievable.

What you can do

If you live in a municipality with a combined sewer system, attend public meetings on its Long-Term Control Plan and ask what green infrastructure commitments are included. Contact your state legislators and ask them to support the Green Infrastructure for Clean Waters fund. Ask your county health department what monitoring data exists for CSO discharge events near you.

If you live downstream from a combined sewer community — if you use the Passaic, the Raritan, the Delaware, or any waterway that receives CSO discharges — your voice in support of adequate funding for CSO remediation in those upstream communities matters, even if your own neighborhood has a separate sewer system.

And if you live in one of the communities that bears the heaviest CSO burden, NJ Clean Stream wants to hear from you. Your testimony at a public hearing or presence at a DEP comment meeting carries weight that a technical report cannot replicate.

This is Article 3 of 3 in NJ Clean Stream’s Combined Sewer Overflows Series. Article 1 explains what CSOs are and the scale of NJ’s problem. Article 2 examines the environmental justice dimensions of who bears the burden and why.