Reducing Urban Runoff (College Board AP® Environmental Science): Study Guide

Methods to increase water infiltration

What is urban runoff?

• Urban runoff occurs when rainwater flows over impervious surfaces, such as roads and buildings, instead of infiltrating into the soil

• Excess runoff carries pollutants, sediments, and chemicals into waterways, causing water pollution

• Excess runoff also increases flood risks

  • This is because water cannot soak into the ground, causing it to accumulate quickly on surfaces

  • This overwhelms storm drains, rivers, and low-lying areas, leading to flash floods and water damage

• Reducing urban runoff helps prevent flooding, improves water quality, and supports groundwater recharge

Methods to reduce urban runoff and increase water infiltration

Permeable pavement

• Replaces traditional pavement with materials that allow water to soak into the ground

• Reduces surface runoff and stormwater drainage issues

• Examples include porous asphalt, pervious concrete, and interlocking pavers

• Advantages:

  • Reduces flooding and the transport of pollutants in runoff

  • Recharges groundwater by allowing infiltration in built-up areas

  • Can be retrofitted onto existing roads, paths, and parking surfaces

• Disadvantages:

  • Pores can clog with sediment over time and need regular cleaning

  • Not suitable for very heavy-traffic surfaces such as highways

  • More expensive per m² than standard asphalt

Green roofs

• Layers of soil and vegetation installed on the roofs of buildings

• Plants absorb rainfall through their roots and release it slowly through evapotranspiration

• Advantages:

  • Reduce stormwater runoff at source by trapping rainfall on the roof

  • Insulate buildings, lowering heating and cooling costs

  • Reduce the urban heat island effect

  • Provide habitat for pollinators and birds

• Disadvantages:

  • Require structural reinforcement of the building to support the added weight

  • High upfront construction costs

  • Need ongoing maintenance such as irrigation and weeding

Rain gardens

• Shallow planted depressions that collect runoff from roofs, driveways, or roads

• Native plants and porous soil absorb the water and filter pollutants from it

• Advantages:

  • Cheap and easy to install at residential or community scale

  • Remove pollutants such as oil, heavy metals, and excess nutrients from runoff

  • Create habitat and improve neighbourhood biodiversity

• Disadvantages:

  • Limited capacity — easily overwhelmed in heavy storms

  • Need regular weeding and occasional replanting

  • Perform poorly on heavy clay soils with low natural infiltration

Retention ponds

• Engineered ponds that hold stormwater before it is gradually released into nearby waterways

• Allow suspended sediments and pollutants to settle out before water leaves the site

• Advantages:

  • Handle large volumes of stormwater from a wide catchment

  • Filter pollutants effectively as water sits and sediments settle

  • Can support wildlife and recreation when well designed

• Disadvantages:

  • Require significant land area, limiting use in dense urban centres

  • Can attract mosquitoes and become health hazards if poorly maintained

  • Expensive to construct and require ongoing maintenance

Planting trees and vegetation

• Trees and plants absorb rainwater and slow the flow of runoff

  • Foliage intercepts rainfall, slowing the rate at which rain accumulates on impervious surfaces below

• Tree roots improve soil structure, increasing its ability to retain water

• Green spaces, rain gardens, and urban forests help filter pollutants from runoff

Increased use of public transportation

• Fewer cars on the road lead to less oil and heavy metal pollution in runoff

• Encouraging walking, biking, and public transit helps limit urban sprawl and conversion of natural land to impervious surfaces

Building up, not out

• Developing taller buildings instead of sprawling developments reduces land consumption

  • Further minimizes expansion of impervious surfaces, preserving natural landscapes

  • Allows more space for green infrastructure, parks, and wetlands, which help absorb runoff