5.11 Urban Runoff

What is urban runoff?

Urban runoff happens when rain falls on impervious surfaces like roads, parking lots, and rooftops instead of soaking into soil. Because these surfaces don't allow infiltration, water flows quickly across land, picking up pollutants (oil, trash, fertilizer, pesticides, sediment) and carrying them into storm drains, streams, and eventually larger water bodies.

Impacts on ecosystems

Urban runoff increases flooding risk because less water infiltrates and more flows rapidly downstream. It also causes nonpoint source pollution, degrading water quality with heavy metals, oil, and nutrients that can trigger eutrophication. Increased flow volume and speed erode stream banks and disrupt aquatic habitats, reducing biodiversity.

Reducing runoff impacts

Solutions increase infiltration and slow water flow. Permeable pavement, green roofs, rain gardens, and retention/detention ponds capture and filter stormwater. Riparian buffer zones with vegetation trap sediment and pollutants before they reach waterways. City planning that limits impervious surface coverage also reduces runoff volume and pollution.

Key terms

Urban runoff

Rainwater that flows over impervious urban surfaces instead of infiltrating, carrying pollutants into waterways.

Impervious surface

A surface like pavement or rooftops that water cannot penetrate, increasing runoff volume.

Nonpoint source pollution

Pollution from diffuse sources, like runoff collecting contaminants across a wide area rather than one pipe.

Rain garden

A vegetated, shallow depression designed to capture and filter stormwater, increasing infiltration.

Permeable pavement

Pavement material that allows water to pass through it into the ground, reducing surface runoff.

Try an AP-style question

Why does urban runoff often carry more pollution than rural runoff?

1. It is filtered well by the city's soil
2. It washes oil, metals, and trash off pavement
3. It flows through more forests on its way
4. It carries much less sediment overall