5.13 - Methods to Reduce Urban Runoff

Learning Objectives

I. Environmental Consequences of Urban Runoff

• Decreased groundwater infiltration: Impervious surfaces stop water from trickling down into the soil to recharge groundwater, which is an important source of drinking water.

• Pollutant collection: Runoff picks up pollutants, meaning harmful chemicals or substances, off the concrete surface.

• Contamination of waterways: That pollutant-loaded water enters a storm drain, which usually empties straight into a local body of water without any treatment.

Specific Pollutants and Their Effects

1. Salt: Used on roads in cold climates. It can kill local aquatic plants and insects that cannot tolerate high salt concentrations.

2. Sediments: Increase water turbidity, which decreases sunlight penetration, making it hard for plants to photosynthesize and for aquatic organisms to see.

3. Fertilizer: Washes off lawns into water sources and creates algae blooms that block sunlight. When the algae and plants die, bacteria break them down and cause hypoxia, a decrease in oxygen.

4. Pesticides: Can kill non-target species after washing in from lawns or industrial and agricultural fields.

5. Oil and gasoline: Drip from cars. They can suffocate fish by clogging their gills, decrease sunlight penetration, and are toxic to many aquatic species.

II. Solutions to Mitigate Urban Runoff

A. Permeable Pavement

• Description: Specially engineered pavement with large pores that let precipitation trickle through and reach the groundwater below.

• Benefits: Decreases runoff volume and the pollutants entering streams, recharges groundwater, and lowers the likelihood of flooding.

• Drawback: It costs more than standard concrete.

• How you'd test it (experiment design): Build two identical sloped plots, one with regular asphalt and one with permeable pavement, and measure the volume of runoff from each during the same storms. Pavement type is the independent variable, runoff volume is the dependent variable, and slope, plot size, and rainfall are kept the same (controls).

B. Rain Gardens and Trees (Green Infrastructure)

• Rain gardens: Gardens planted in urban areas, often near a storm drain, designed to soak up rainwater.
• They reduce the total volume of stormwater, and the soil and plant roots filter out pollutants.
• They also create habitat for pollinator species and foster a “sense of place.”

• Trees: Sequester carbon dioxide, which mitigates climate change. Trees also filter air pollutants like particulate matter and nitrogen dioxide through the stomata in their leaves, improving air quality.
• How trees reduce runoff: Tree canopies catch rain on their leaves so it drips down slowly or evaporates, roots loosen the soil and open channels so more water soaks in, and trees pull water back out of the soil through transpiration. All of that means less water rushes into storm drains.

C. Public Transportation

• Public transit reduces the number of cars on the road, which reduces the pollutants dripping onto road surfaces (motor oil, gasoline, antifreeze, tire bits).

• It is a far more efficient way to move people, since one bus carries as many people as many cars.

• It reduces $CO_2$ emissions and decreases traffic.

• Fewer cars means less need for impervious surface (fewer lanes, fewer parking lots), which decreases runoff. Those areas could be replaced with parks or rain gardens that allow groundwater recharge.

D. Building Up Versus Building Out

• Building up (vertically) decreases the total impervious surface needed compared to urban sprawl (building out).

• For example, in urban planner Alain Bertaud's well-known comparison (using 1990 data), sprawling Atlanta's built-up area covered about 4,280 square kilometers, while vertical Barcelona covered only about 162 square kilometers, even though the two metro areas had about the same population.

• Vertical cities are more efficient and cut transportation $CO_2$ emissions dramatically. Barcelona has barely 10% of Atlanta’s per-person transportation $CO_2$ because residents can walk or use effective transit.

• Vertical structures can also include green roofs or rooftop gardens to soak up rainwater and create habitat, combining efficient design with green infrastructure.