A storm sewer is a network of underground pipes, ditches, and drains designed to carry rainwater and snowmelt away from roads, parking lots, and other paved surfaces to prevent flooding. Unlike the sewer system connected to your home’s toilets and sinks, a storm sewer typically empties directly into nearby streams, rivers, wetlands, or the ocean without any treatment. That distinction has major implications for water quality and the environment.
How a Storm Sewer Works
When rain falls on hard, impermeable surfaces like driveways, streets, parking lots, and rooftops, the water has nowhere to soak in. Storm sewers collect that runoff through metal grates and curb openings you see along roadways, then channel it through underground pipes or open ditches. The system relies on gravity: pipes are sloped so water flows downhill toward a discharge point called an outfall, which releases the water into a local body of water.
Along the way, manhole covers provide access points for maintenance crews. The pipes themselves come in a range of materials depending on the size of the system and local conditions. Smaller residential lines often use PVC or corrugated polyethylene, while larger trunk lines under major roads are typically reinforced concrete. Steel pipe, sometimes coated with asphalt or polymer linings, is another option, particularly under highways.
Storm Sewers vs. Sanitary Sewers
This is the most important distinction to understand. Your home has two completely separate drainage paths, even though both involve underground pipes. Water from your sinks, bathtubs, washing machines, and toilets flows into the sanitary sewer system, which routes everything to a wastewater treatment facility. There, the water is cleaned before being released back into the environment.
Storm sewers handle only outdoor water: rain, snowmelt, and whatever that water picks up as it flows across surfaces. This water is not treated or cleaned before it reaches local waterways. The two systems are intentionally kept separate so that heavy rainstorms don’t overwhelm treatment plants. Some older cities, particularly in the Northeast and Midwest, still have combined systems that merge both types into one set of pipes, but most modern infrastructure keeps them apart. The formal name for a separated system is a Municipal Separate Storm Sewer System, or MS4.
What Ends Up in Storm Sewers
Because storm sewer water bypasses treatment plants, anything it picks up along the way goes straight into natural waterways. The list of common pollutants is longer than most people expect. As runoff flows across yards, roads, and parking lots, it collects trash, pet waste, yard debris, fertilizers, pesticides, motor oil, detergents, and household solvents like paint thinner.
Research has identified a more complex picture at the chemical level. Urban stormwater routinely contains heavy metals like copper, zinc, lead, and cadmium, largely from construction materials and vehicle wear. A study published in Environmental Science & Technology found 69 frequently detected organic chemicals in stormwater samples, including 21 pesticides, 16 combustion byproducts (several classified as probable human carcinogens), 11 industrial chemicals, and even nonprescription pharmaceuticals. Methylmercury turned up in 90% of samples tested. Mixtures of metals like cadmium, chromium, lead, and aluminum can pose health risks even at individually low concentrations.
These pollutants can harm aquatic life, particularly bottom-dwelling organisms in rivers and reservoirs. They also raise concerns for people who swim in or harvest food from affected waterways.
How Storm Sewers Are Regulated
In the United States, the EPA requires cities and counties that operate MS4 systems to obtain permits and develop stormwater management programs. These programs must address several specific areas: detecting and eliminating illicit discharges (anything entering the storm sewer that isn’t stormwater), controlling runoff from construction sites disturbing one acre or more of land, reducing pollutants from new development projects, and maintaining clean municipal operations.
Municipalities are also required to run public education programs about stormwater pollution and set measurable goals to track whether their efforts are working. If a local waterway is already impaired, the city may need to meet additional pollution reduction targets. The underlying principle is straightforward: since storm sewer water isn’t treated, the focus shifts to preventing pollutants from entering the system in the first place.
Maintenance and Upkeep
Storm sewers can clog with leaves, sediment, trash, and debris, reducing their capacity and increasing flood risk. Municipalities use two main cleaning approaches. Dry cleaning comes first: crews remove material by hand or with vacuums and shovels. Wet cleaning follows when needed, using high-pressure water jets to flush out stubborn buildup. For larger systems, cities rely on vactor trucks, specialized vehicles that combine vacuum suction with water jetting to clear pipes efficiently.
Regular maintenance matters because a partially blocked storm sewer can cause street flooding during moderate rain events that the system was designed to handle. Homeowners can help by keeping leaves and debris away from nearby grates and curb inlets.
Green Infrastructure Alternatives
Traditional storm sewers move water off the surface as fast as possible, but cities increasingly supplement them with green infrastructure that slows runoff down and filters it naturally. These approaches reduce the volume of polluted water reaching waterways and take pressure off aging pipe systems.
- Bioswales: Shallow, vegetated channels typically running alongside roads or parking lots. Plants and mulch slow the water, filter out pollutants, and let some of it soak into the ground.
- Permeable pavement: Porous concrete, asphalt, or interlocking pavers that let rain soak through the surface into gravel layers below instead of running off. One project in Sultan, Washington, eliminated over $260,000 in construction costs by using pervious concrete in place of traditional drainage infrastructure.
- Constructed wetlands: Engineered shallow pools that use natural settling and plant uptake to remove pollutants. They maintain a standing water level and work well in areas with enough space.
- Green streets: Roadway designs that weave together permeable pavement, bioswales, planter boxes, and trees to manage stormwater within the streetscape itself.
These systems don’t replace storm sewers entirely. They work alongside them, handling smaller rain events naturally and reducing the load on pipes during larger storms. For cities facing both aging infrastructure and tighter water quality regulations, green infrastructure offers a way to address both problems at once.