What is urban runoff?

Urban runoff is defined as the surface runoff created by rain in urban areas and is one of the main sources of water contamination in these communities, as well as the main cause of flooding.

main effects

In urban and industrial areas, natural soil is replaced by highly impermeable surfaces built with materials such as asphalt or cement. These surfaces, instead of allowing water to seep into the soil, cause most of the water to remain on the surface and carry urban soil waste to the watercourses where it is discharged (receiving waters).

Less water infiltration into the ground has two other direct effects: lowering of the water table and surface flooding as more water remains on the surface and underground aquifers are not recharged.

urban flood

Urban runoff is the main cause of flooding in urbanized areas. It occurs mainly when the volume of rainfall exceeds the capacity of drainage systems, but also when these systems are clogged for various reasons, mainly due to the accumulation of waste and lack of maintenance.

This type of flooding tends to repeatedly affect buildings and residences with levels underground or close to bodies of water, water entering drainage systems when the level of the channels into which it is discharged exceeds the level of the building. Urban runoff is the main cause of basement flooding.

The greater amount of water on the surface also causes faster rises in the level of nearby rivers, as well as a higher flow velocity, which further favors urban flooding in populations located further down the channels.

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Urban runoff carries all types of waste, such as plastics, garbage and polluting substances. Roads and parking lots are a major source of Polycyclic Aromatic Hydrocarbons, which are formed as a by-product of various fuels used by automobiles, along with heavy metals such as nickel, copper, zinc, cadmium, and lead.

Runoff from roofs and other construction can carry large amounts of synthetic organic compounds and heavy metals like zinc, used, for example, in galvanized structures. Fertilizers used at home, in public parks and other urban green areas are an important source of nitrates and phosphates.

Another very common pollutant in urban runoff is the salts used on roads and streets to melt snow during winters.

Urban runoff also generates thermal pollution, mainly thermal shock with a sudden increase in the temperature of water courses after the rains, which can considerably harm aquatic life.

Prevention and reduction of effects

Urban runoff must be treated as a waste that contains pollutants and negatively affects the quality of receiving waters. The effective control of urban runoff must contemplate both the reduction of runoff velocity and the reduction of the pollutants it carries.

There are several techniques that generally include retention systems and seepage ponds that retain runoff is a slower discharge and where it can be treated to remove contaminants. Another method is the use of permeable pavements and bioretention systems, known in some regions as rain gardens (rain garden), which can be installed in rain collectors before discharge, but also in streets, parking lots, highways and other paved areas.

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Rain gardens in a parking lot Components of a bioretention area Permeable Pavement Demonstration

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