What is Stormwater Drainage?

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  • Written By: N. Madison
  • Edited By: C. Wilborn
  • Last Modified Date: 02 November 2019
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Stormwater drainage is the process of draining excess water from streets, sidewalks, roofs, buildings, and other areas. The systems used to drain stormwater are often referred to as storm drains, but they are also called storm sewers and drainage wells. Sometimes people confuse stormwater drainage systems with sanitation sewers, but storm drains often function separately from sewer systems created for sanitation purposes.

Stormwater collects because of precipitation, such as rain, snow, and sleet. Some of this water soaks into the ground, but without proper drainage, excess water may collect and present dangers to both people and physical property. For example, excess water can lead to flooding, making unsafe conditions for humans and animals and damaging cars and buildings. Also, bacteria may collect and grow in water that is allowed to sit for a long period of time, presenting a health hazard.

Many cities and towns have carefully planned stormwater drainage systems that consist of inlets, outlets, and pipes. The inlets of storm drains are often covered by protective grates that help to ensure that large items don't fall in while water can enter freely. Since it's important for large amounts of water to flow into these drains, the bars of the grates must be spaced some distance from each other. This concession means that some smaller objects do fall into the drain.


Once the water enters the storm drain, it usually flows to a catch basin, which catches small objects before the water continues its journey into the sewer. Next, the piping comes into play. Storm drainage systems can have different types of pipes. Some may be rectangular in shape while others are circular or oval. The materials may differ as well, with some systems using concrete or metal while others use plastic. Additionally, some drainage pipes have mechanisms in place for catching debris, such as pollutant traps.

The stormwater drainage systems maintained by cities and towns usually drain to a single point, and the outlet tends to be rather large and covered by another grate. Often, these systems drain into lakes, rivers, or reservoirs. In some cases, they may drain into a canal or ocean instead.

Though necessary, stormwater drainage can have a significant impact on the environment. Unfortunately, toxic substances, such as lawn fertilizers, cigarette butts, motor oil, pesticides, and other chemicals are often washed into stormwater drainage systems. These chemicals lead to polluted water that ends up in lakes, streams, rivers, oceans, and other bodies of water, where it proves unhealthy for fish, plants, and other water life, even killing them. In turn, humans and animals may be sickened by eating the contaminated fish.


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