What is a storm sewer?

The storm drains system transports rainwater or stormwater and other runoff from roads, parking lots, and roof downpipes directly to nearby creeks, rivers, or the sea without any treatment.

A storm sewer, storm drain surface water drain/sewer, or stormwater drain is a structure that is designed to dispose-off excess rain and underground water from impermeable surfaces such as pavements, vehicular parking areas, footpaths, sidewalks, and rooftops. Storm drains change in their formation from small residential dry wells to vast municipal structures.

The storm drain is susceptible to getting water from roadside pipes, gutters, and other smaller drain branches in large quantities during monsoon season. Since some inundated drains cause subway and road flooding, the installation of gravity sewers that directly supply untreated stormwater into the main water bodies becomes indispensable. Detention tanks that temporarily hold water and combined sewers, which mix stormwater and sewage (in case of overflooding emergencies), are advisable.

While traveling to the exit of storm sewer drains, the water carries pollutants and other substances on the ground surfaces. So, it must be any kind of dumping and fertilizers since the water is directly drained untreated.

A storm drain discharging into the River Brent in the UK.
CC BY-SA 2.0 | Image Credits: https://en.wikipedia.org | J Taylor

Distinguish between a sanitary sewer and a storm sewer

The sanitary sewer is a system of underground pipes that carries sewage from bathrooms, sinks, kitchens, and other plumbing components to the Water Treatment Plant (WTP), where the withdrawal of most waste substances occurs before the treated stormwater is drained into the water sources. However, the WTP system cannot dispose off all the solvents, metals, or synthetic organic chemicals from wastewater. So, it is necessary to remove oil or greases and other hazardous materials through an authorized household hazardous waste disposal program.

The storm sewer is a system constructed to transfer rainfall-runoff at a controlled rate, but not any sewage. This runoff is transported to buried pipes, and discharges the untreated water into streams or other surface water bodies. The inlets to this system can be seen in curbs and low-lying outdoor places. Some old buildings have underground floor drains that connect to the storm sewer system.

Measures to prevent the contamination of storm sewers

  • Do not pour whatever comes in hand into the storm sewer drains
  • Save the floor drains that convey to storm sewers
  • Never pour paint down any of the open sinks or drains
  • Less use of pesticides and herbicides
  • Maintain drains and protect them from the entry of leaf and lawn litter
  • Gathering and recycling oil from vehicular maintenance
  • Clean up spills and never wash them into drains
  • Call the EHS (Environment, Health and Safety) to ask about sanitary or storm sewers

Structure and Components of a storm drain/storm sewer

  • Inlet

There are two main types of storm sewer drain inlets, one is the side inlet and the other one is the grated inlet. The side inlets are positioned right beside the curb and are dependent on the ability of the opening under the lintel to trap the flow. They are usually depressed on the invert of the channel to boost the capture capacity.

Most of the inlets have grates, grids, or narrow gaps, called the grated inlet, to stop people, vehicles, or large materials from falling into the storm drain. The bars are spaced at a considerable amount of distance for the water to enter, but they prevent any debris to pass through. The grate bars must not be too far-spaced that imposes a threat to the public, cycles, and so on. Also, storm drains in streets and parking areas must have enough strength to withstand the weight of vehicles and are often made of cast iron or reinforced cement concrete.

A catch basin serves the purpose of a trap for catching larger objects. Again, the ability of the catch basin for the removal of sediments and pollutants depends on its design and daily maintenance check. For mosquito breeding prevention, either insert an inverted cone filter or apply an oil layer on the surface for larvae control.

  • Piping

Pipes are available in various forms of cross-sectional shapes (rectangular, square, bread-loaf-shaped, oval, inverted pear-shaped, egg-shaped, and most commonly, circular). Storm drain systems may have pits for catching unwanted substances such as Gross Pollutant Traps (GPTs). Pipes are made of brick or concrete material and high-density polyethylene or galvanized steel. For drain pipes and fittings, fiber-reinforced plastic is used.

  • Outlet

Many of the drains have a single vast exit at their discharge point (usually covered by a grated inlet) to the main water outlet source. Other than catch basins, there are no treatment units in the piping system. Small storm drains may drain into the respective individual dry wells. Storm drains may be interconnected using pipe slots, to make a wider dry well system. Storm drains may drain into a man-made excavation called recharge basins or Retention Ponds. Retention Ponds capture the stormwater for a short period.

Combined sewers

The cities that constructed their sewage collection systems before 1930s generally used single piping systems to carry both runoff and sewage. Such a system is called a combined sewer system. The thought process of the people at that time was to build a single system that would be less expensive. However, during severe floods, the runoff water, mixed with sewer water, contaminates the main water outlet.

Storm drains are usually installed at shallower depths than the combined ones, since the combined sewers were made to obtain sewage flows from buildings with basements, along with receiving surface runoff from streets.

This overflow outlet is placed higher outside the detention tank that prevents the detention tank from filling.

Storm sewer detention

The stormwater must never be combined into the sewer system, as it can cause major overflows. Stormwater detention is when the rainwater flowing across ground surface areas such as driveways, paths, rocks or water, that is otherwise headed for the stormwater drain, is trapped or left to detain and stored before being discharged from the outlet.

A wet detention pond is a usual stormwater treatment. Wet detention ponds contain water every time and serve both purposes. This detention system treats stormwater runoff for the protection of the creeks and streams, and during severe storms, they help in slowing down the flooding by controlling detention volume.

Rainwater is carried away into stormwater drains that eventually lead the water into a nearby river. The purpose of a storm drain is to assemble stormwater runoff from the streets or roadways and convey it to the outfall.

Environmental impact factors include water quantity, water quality, landslides, floodplain caused by deposition as well as erosion, and mosquito breeding.

Context and Applications

The topic is taught in courses related to:

  • Bachelors of Technology (Civil Engineering)
  • Masters in Science (Environmental Studies)
  • Masters in Science (Hydraulic Engineering)
  • Masters of Technology (Environmental Engineering)

Practice Problems

Q1. What is the sewer that carries runoff water and sewage water known as?

  1. Storm sewer
  2. Sanitary sewer
  3. Combined sewer
  4. Stormwater retention

Correct Option: c

Explanation: The sewer that carries runoff water and sewage water is a combined sewer.

Q2. What are the components of a storm sewer?

  1. Inlet
  2. Piping
  3. Outlet
  4. All of these

Correct Option: d

Explanation: The components of a storm sewer are inlet, piping, and outlet.

Q3. What is the structure, where water is stored temporarily and then disposed off, known as?

  1. Retention tank
  2. Detention tank
  3. All of the above
  4. None of the above

Correct Option: b

Explanation: The structure where water is stored temporarily, and then disposed off is the detention tank.

Q4. What is the full form of GPTs?

  1. Gross Pollutant Traps
  2. Gross Prevention traps
  3. Gross Protection Traps
  4. Gross Pollutant Treatments

Correct Option: d

Explanation: The full-form of GPTs is Gross Pollutant Traps.

Q5. Which of the following statements is incorrect?

  1. Less use of pesticides and herbicides
  2. Call the EHS (Environment, Health and Safety) to ask about sanitary or storm sewers
  3. Save the floor drains that convey to storm sewers
  4. Leaves and lawn litter is permitted inside a storm sewer

Correct Option: d

Explanation: Leaves and lawn litter are not permitted inside a storm sewer.

  • Soil erosion
  • The detention system
  • Hydrology
  • The swale and riser
  • Infiltration

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