APES · Unit 5 · Topic 5.13
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Cities replace natural land with pavement, rooftops, and roads that block water from reaching the soil. This lesson explores why that's a problem — and four science-backed strategies to fix it.
Before we can understand solutions, we need to understand the problem. Let's trace the path water takes in a city versus in nature.
Rain falls on soil and plants. Most soaks in — infiltration. Water recharges aquifers, flows slowly to streams, and is taken up by plants. Very little runs off the surface.
Pavement, rooftops, and roads are impervious surfaces — water can't pass through. Rain flows across the surface into storm drains or directly into streams.
Left: Natural land — most rain infiltrates into soil. Right: Impervious urban surfaces — rain runs off into storm drains.
Large runoff volumes overwhelm drainage systems quickly, flooding streets and properties downstream.
Fast-moving runoff erodes stream banks and carries pollutants — oil, fertilizers, sediment — directly into waterways.
When rain can't infiltrate, aquifers don't recharge. Cities can draw down groundwater, sometimes causing land to sink.
Older cities share stormwater and sewage pipes. Heavy rain overwhelms these, discharging raw sewage into rivers — a direct public health problem in Buffalo.
These are the four strategies in STB-1.B.1. Click each to read the full explanation — all four can appear on the AP exam.
Traditional asphalt and concrete are impervious — water can't pass through them. Permeable pavement is engineered with pores or gaps that let rainwater drain through the surface into a gravel base, then into the soil below. Examples: porous asphalt, pervious concrete, interlocking pavers with gravel gaps.
Water passes through the surface instead of running across it — converting surface runoff into groundwater recharge. The direct opposite of what regular pavement does.
More expensive than regular asphalt. Pores can clog and need maintenance. Not ideal for areas with heavy truck traffic.
Trees reduce runoff through two distinct mechanisms. First, leaf canopy intercepts rainfall before it hits the ground. Second, tree roots penetrate and loosen soil, creating channels that dramatically increase infiltration capacity. Urban forests, street trees, green roofs, and bioswales all use this principle.
Canopy catches and evaporates some rain before it reaches any surface. Root channels allow the rain that does reach the ground to soak in faster and deeper.
Trees absorb CO₂, reduce urban heat island effect, and provide wildlife habitat. This is "green infrastructure" — natural systems providing engineered services.
Rather than fixing existing impervious surfaces, expanding public transit reduces the demand for roads and parking infrastructure as a city grows. Fewer cars = fewer parking lots built = less new impervious surface added to the landscape over time.
Reduces vehicle use → decreases demand for new roads and lots → slows or prevents conversion of natural permeable land to impervious surface as population grows.
This is preventive, not corrective. It won't fix a parking lot that floods today. Most powerful as long-term urban planning.
This directly addresses urban sprawl (Topic 5.10). Sprawl spreads development outward at low density, converting large areas of natural land to impervious surface. Building "up not out" concentrates development vertically — same population, far less natural land disturbed.
Vertical development keeps more land undisturbed and available to absorb rain. Less road and parking infrastructure is needed to serve a concentrated population.
This is the solution to Topic 5.10 (urban sprawl). Urban sprawl = the problem; smart growth/compact development = the AP solution.
| Method | Primary Mechanism | Type | Best Scale |
|---|---|---|---|
| 🧱 Permeable Pavement | Water passes through surface → soil | Corrective | Site-specific |
| 🌳 Planting Trees | Canopy interception + root channels | Corrective | Neighborhood |
| 🚌 Public Transit | Reduces demand for new roads/lots | Preventive | City-wide |
| 🏙️ Build Up Not Out | Less land converted to impervious | Preventive | Regional |
Answer both questions before moving on.
Select a card from Column A, then one from Column B. You need all 4 correct to continue.
These mirror real AP question style and difficulty.
Use complete sentences. Your responses auto-save as you type.
The city of Riverside has experienced increased flooding and stream erosion over the past decade due to rapid suburban development. A city council member proposes two methods to reduce urban runoff. Identify TWO specific methods to increase water infiltration, and for each method, explain specifically how it would reduce urban runoff in Riverside.