AP® Environmental Science review sheet from Aim for Five (aimforfive.com/enviro/units/5/5-13)
Unit 5 · Topic 5.13
5.13 Methods to Reduce Urban Runoff
Cities can reduce runoff by helping rain soak into the ground instead of rushing into storm drains. Permeable pavement, more trees and green space, building up instead of out, and better public transit all cut the amount of paved surface or let water through it.
Key terms
- urban runoff
- permeable pavement
- infiltration
- building up, not out
- public transportation
Why urban runoff is a problem
In cities, rain falls on roofs, roads and parking lots that it can't soak through. It runs off quickly, carrying oil, metals, fertilizer, bacteria, sediment and trash into storm drains, which usually empty straight into streams without treatment. Runoff causes flooding, erosion of stream banks, warmer water and pollution, and it reduces groundwater recharge.
Solutions work in one of three ways: they let water soak in (increase infiltration), they slow and store water so it doesn't all arrive at once, or they reduce how much land gets paved in the first place.
Letting water soak in
- Permeable pavement: special concrete, asphalt or pavers with gaps that let water drain through into a gravel layer and the soil below. It's often used for parking lots, sidewalks and driveways. It costs more than regular pavement and needs cleaning so the pores don't clog.
- Planting trees and green space: leaves catch rain and roots help water infiltrate. Trees also shade pavement and cool the city.
- Rain gardens and bioswales: shallow planted areas that collect runoff from roofs and streets and let it soak in, while plants and soil filter pollutants.
- Green roofs: roofs covered with plants that absorb rain and release it slowly.
- Retention ponds and wetlands: hold stormwater and release it slowly, letting sediment settle.
Reducing the paved footprint
Building up, not out, means constructing taller buildings closer together instead of spreading low buildings across the land. For the same number of people, vertical development covers less land with roofs, roads and parking, leaving more ground to absorb rain.
Public transportation reduces the need for roads, highways and huge parking lots. A bus or train line moves many people with far less pavement than the cars they'd otherwise drive. Transit also cuts air pollution and greenhouse gas emissions.
These approaches also protect farmland and habitat on the edges of cities by reducing urban sprawl.
Tradeoffs
Most runoff solutions cost more upfront than standard pavement and storm drains, and some need ongoing maintenance. Public transit requires large investments and enough riders to be practical. But they save money over time by reducing flood damage, water treatment costs and stream restoration, and they bring benefits like cooler cities and cleaner water.
Worked examples
Try each one yourself first, then open the solution.
- Example 1
Runoff from a parking lot
A storm drops 2 cm of rain on a 1-hectare (10,000 m²) parking lot. (a) How many liters of water fall on the lot? (b) If the lot were rebuilt with permeable pavement that lets 75% of the rain soak in, how many liters would still run off? (1 m³ = 1,000 L.)
Show the solutionHide the solution
- Step 1: Convert rainfall to meters: 2 cm = 0.02 m.
- Step 2: (a) Volume = area × depth = 10,000 m² × 0.02 m = 200 m³. In liters: 200 × 1,000 = 200,000 L.
- Step 3: (b) Runoff fraction = 100% − 75% = 25%.
- Step 4: Runoff = 0.25 × 200,000 L = 50,000 L.
Answer: (a) 200,000 L; (b) 50,000 L would still run off.
- Example 2
Choosing a strategy
A city council wants to reduce flooding downtown and also slow the loss of farmland at the city's edge. Recommend one strategy that addresses both, and explain.
Show the solutionHide the solution
- Step 1: Building up, not out, concentrates new housing and offices in taller buildings downtown.
- Step 2: That limits sprawl, so less farmland at the edge is paved.
- Step 3: For the same number of residents, less total land is covered with roofs, roads and parking, so less new runoff is created.
- Step 4: Pairing it with public transit means fewer parking lots are needed downtown, which could be replaced with green space or permeable surfaces.
Answer: Build up, not out (ideally with more public transit): it limits sprawl into farmland and reduces the paved area that creates runoff.
Common mistakes
- Saying storm drains solve runoff. They move water quickly to streams, which often makes downstream flooding and pollution worse.
- Forgetting to convert cm to m before multiplying by area in m².
- Listing a solution without explaining how it reduces runoff (more infiltration or less paved surface).
On the exam
- Questions often ask for a method to reduce urban runoff with an explanation. Name the method and say whether it increases infiltration or reduces impervious surface.
- Runoff volume calculations require consistent units. Write out the conversions.
Connected topics
Videos
Check yourself
5 questions on 5.13 Methods to Reduce Urban Runoff. Pick an answer to see if you got it, and why.
A city wants to reduce flooding from storm runoff. Which plan would best increase the amount of rain that soaks into the ground?
A city is deciding whether to house 10,000 new residents in tall apartment buildings near downtown or in single-family houses on the city's edge. Which statement best explains why the apartments would likely produce less storm runoff?
How can expanding public transportation help reduce urban runoff?
| Parking lot surface | Rain that fell (mm) | Runoff measured (mm) |
|---|---|---|
| Standard asphalt | 25 | 23 |
| Permeable pavers over a gravel bed | 25 | 6 |
Hypothetical data from one storm on two test lots of equal size and slope
By about what percentage did the permeable pavers reduce runoff compared with standard asphalt?
Why did the researchers use test lots of the same size and slope?
0 of 5 answered