The Basics of Runoff

March 1, 2016

The Basics of Runoff

Last summer on the blog, AMT’s Greg Fox told you about his stormwater management presentation to the Virginia Municipal Stormwater Association. It gave a great outline as to our skills in helping businesses and communities meet their stormwater management requirements, but why do these requirements exist in the first place?

Urban areas have been grappling with issues ranging from flooding to contamination of our rivers and waterways. The EPA, in 1972, passes the Clean Water Act, which sets forth aggressive pollution reduction standards using TMDLs, which stands for Total Maximum Daily Loads. These standards set quantifiable limits on the amount of sediment and pollutants, such as nitrogen that can be present in the water. To help meet those load reductions, communities were required to obtain National Pollution Discharge Elimination Permits. These NPDES permits control the amount of pollution from point sources into waters of the United States. They ensure that communities and entities are using the greenest methodologies practicable to minimize pollution in the waterway.

But let’s take a step back from here. What does all of this have to do with storms and rain? Simply put, when rain falls in open spaces such as forests or meadows, most of the water is absorbed into the ground pretty much where it falls. The earth acts as a giant sponge. The plants absorb and filter the water and the earth itself filters pollutants out of the water as it replenishes the water table.

In urban areas, things are different. Hard concrete, asphalt, and roof surfaces do not absorb rainfall–instead the water rolls over these surfaces and collects whatever dirt, oil and pollutants exist there. This is what people mean when they talk about runoff. To understand the kind of pollutants that runoff carries, think about a snow bank beside a roadway a week after a snowfall. What was once pristine white snow has become a blackened mess. That blackened mess when it melts, runs down into the storm drain system and is deposited directly into our waterways instead of back into the water table.

Cities everywhere are trying to mitigate this pollution using varying types of green infrastructure. Green roofs are gaining popularity as a way to reduce runoff, increase transpiration, and reduce the heat island effect which is caused by heat reflecting off of concrete, asphalt, and roof surfaces. In addition to green roofs, bioretention facilities and rain gardens are used frequently to collect and treat runoff from hard surfaces. These types of green infrastructure facilities rely on plants and soil media to filter and clean runoff.

AMT was fortunate enough to work with the District Department of Transportation and an excellent project team to create green infrastructure standards for the District of Columbia that outlined ways that green infrastructure methods could be used in the public right-of-way in the city.

These methods add bits of green space that can capture and filter runoff. They will also help the city to comply with its NPDES Permit. Other cities have developed their own standards and even more are likely to incorporate green infrastructure into their future plans to control runoff.


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