What is a Storm Drain? How Does it Work?

A storm drain, also known as storm sewer is a drainage system designed to drain excess rain or surface water from impervious surfaces like roofs, sidewalks, parking lots, car parks and streets through undergrounds pipes into rivers or streams. Unlike in sewer lines, storms drains drain the water without it being treated in a sewage treatment plant.


Storm Drain System: Key Points to Know

  • Purpose: Storm drains, also known as stormwater drains or storm sewer systems, are designed to manage and control excess rainwater, preventing flooding and directing runoff away from urban areas.
  • Components: A storm drain system typically consists of underground pipes, catch basins (grates), manholes, and outfall structures that discharge water into natural water bodies, such as rivers, lakes, or the ocean.
  • Collection: Storm drains collect rainwater from streets, parking lots, rooftops, and other impervious surfaces. This water can contain pollutants like oil, debris, and chemicals.
  • Gravity Flow: Stormwater flows into the catch basins and is carried away by gravity through a network of pipes, ultimately reaching the nearest natural water body or designated discharge point.
  • Separation from Sewage: Storm drains are separate from sanitary sewer systems (wastewater systems) to prevent sewage contamination. Stormwater is not treated before discharge.
  • Flood Control: Storm drains help prevent flooding by swiftly removing rainwater from urban areas, reducing the risk of property damage and road closures during heavy rainfall.
  • Pollution Control: Stormwater may pick up pollutants from surfaces it flows over. Some storm drain systems incorporate treatment measures, such as oil and sediment traps, to mitigate pollution.
  • Grates and Inlets: Catch basins, equipped with grates or inlets, capture debris and prevent larger objects from entering the storm drain system, reducing clogs.
  • Erosion Control: Storm drains are essential for preventing soil erosion caused by runoff, which can lead to sedimentation in water bodies and damage aquatic ecosystems.
  • Regulatory Compliance: Municipalities often have regulations in place to manage stormwater runoff and maintain water quality standards.
  • Design Variations: Storm drain systems can vary in design, including open channels, closed pipes, or a combination of both, depending on local needs and geography.
  • Maintenance: Routine maintenance is crucial to keep storm drains free from debris and functioning efficiently. This includes cleaning catch basins and inspecting pipes for damage.
  • Environmental Impact: Improperly managed stormwater can harm aquatic ecosystems by introducing pollutants and altering natural flow patterns.

In Private Homes


In most houses, homeowners use gutters to collect rain water from their house but since most of them are not keen on collecting the rain water (or are not allowed to do so by the state), they use gutter downspouts to drain out the water.

It is usually important to make sure that the gutter downspouts are installed in a sloping area where the water can drain out via gravity and not flow back and damage the foundation. The water then flows to the storm drain inlet near the street where it mixes with the runoff from the neighborhood and is drained out in the nearest water body.

Some people connect a hose/pipe at the bottom of the gutter downspout and direct water out into the driveway, yard or street. Others even install a French drain and connect it to the storm drain inlet at the street.

The important thing is to get water away from your house (to prevent damage to the foundation/basement flooding) and just to keep your property tidy.

Storm drains also ensures that when snow starts to melt, the water will have somewhere to drain out to. Pooling water is not only untidy but it invites a lot of flies/insects into your property.

Public Storm Drains


Storm drains in public spaces ensures that you are able to drive your car along the streets or freeway, you can park comfortably without splashing water to nearby cars or people in a parking lot and importantly there is no toxic wastewater accumulating around people’s properties.

Most people appreciate these drains when they are blocked and streets and roads are flooded with water especially after a heavy downpour.

This is why it is important to maintain storm drains. Don’t dump soil/debris inside storm drains as that will accumulate over a period of time resulting in clogs and wastewater backups.

Despite their usefulness, storm drains are the largest contributors to water bodies pollution in the United States. People still dump toxic waste like paints, acids and other industrial waste which end up polluting the water bodies where these drains drain to.

What is the Difference between a Storm Drain and Sewer Line?

Most people confuse between sewer lines and storm drains. As I had already mentioned, storms drains are also known as storm sewers so they wonder which type of sewer carries runoff and which carries domestic waste.

Sewer lines (also known as sanitary sewer systems) carry domestic waste away from houses (toilets, sinks, showers, tubs, washing machines drains etc.) to a sewer treatment facility.

Each house/property has it separate sewer line which runs underground through the yard and is connected to the city’s sewer lines near the street. Sewer lines from the house are installed sloping so that the waste can flow out via gravity.

Private sewer laterals (as the private sewer lines are called) are usually the responsibility of the homeowner. The city is responsible for the main sewer lines along the street.

Once the domestic waste exits the private sewer laterals, it joins the city sewer lines where it goes through a series of pumping stations until it reaches the sewage treatment plant.

You can therefore say that the difference between sewer lines and storms drains is that the wastewater in sewer lines goes through a treatment facility before being released while runoff in storm drains is drained directly into water bodies.

Difference between Storm Drains and Combined Sewer systems

As you already know now, sewer lines carry domestic waste from houses while storm drains carry rain and surface water from impervious surfaces.  There is a third sewer system which combines the 2 of them.

I guess this is where the confusion for most people starts. But how does a combined sewer system work?

A combined sewer system is designed to carry both domestic waste and storm runoff or melting snow water in one pipe to a sewage treatment plant. Sound nice; doesn’t it?

During the dry weather, this system works flawlessly. There is no wastewater being dumped in water bodies without being treated.

The problem starts during wet weather. As you can imagine, the volume of domestic waste and surface runoff is quite high and will overwhelm the system.

These systems are therefore designed to overflow into the nearest water body. The combined sewer overflows (CSOs) as they are called allows not only surface run off to overflow but also untreated raw sewage and sometimes even toxic industrial waste.

Increased water volume will also overwhelm the sewage treatment plant and thus it will be unable to treat the wastewater as well as it should.

It is estimated that more than 700 cities in the Unites States have combined sewer systems which is the highest source of water bodies pollution.

How they Work

I will now explain to you briefly how a storm drains work. As I mentioned, these drains are used to remove excess water from homes and business to avoiding flooding and house damage. Here I will just focus on residential areas storm drains.

Here is how water travels from the time it starts to rain until it ends up in the river or stream.

1. Roof and Gutters

The roof of your house is installed in a certain slope. That helps water to flow down via gravity and drain into the gutters.

The gutters prevent the water from dropping down from all over your house but instead flow from the gutter spouts. The gutters are also sloped towards the gutter downspouts.

2. Gutter Downspouts

A downspout, also known as downpipe or drain spout is a vertical pipe connected from the gutters towards the ground to remove rainwater from the roof. Close to the ground, the downspout will have an elbow or a 70 degrees angle and a pipe will be connected to it to divert water away from the house.

Water from the house will flow down through the yard where it will drain out via the storm drain/catch basin.

3. Catch Basin

A catch basin is a grated inlet also known as storm drain installed near the curb where runoff from the property drains into. The catch basin is connected to a series of underground pipes which drains out the runoff to the nearest water body.

Catch basins are usually recessed to ensure that water drains in with relative easy. They are also grated to ensure that people, pets, debris and basically any type of trash do not get into the drain and clog it (or cause injury to the people and animals).

4. Outlet/Discharge

Storm drains outlets comprises of a large pipe with a grated opening where they discharge the runoff into the river or stream. It is important to mention that small storm drains can discharge the water into a dry well but big storm drains will need an outlet into a water body.

And basically that is it in as far as storm drains are concerned. I hope this guide was helpful.

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