5.10 - Impacts of Urbanization

Learning Objectives

I. Urbanization Basics and Environmental Effects

A. Impact of Impervious Surfaces on the Water Cycle

• Urbanization replaces pervious land (soil and vegetation) with impervious surface.

• Impervious surfaces include the concrete and asphalt in sidewalks, roads, and buildings.

• These surfaces do not let water infiltrate into the ground, so the water is forced to leave as runoff.

• Water cycle comparison: In a natural landscape, up to 50% of rainwater infiltrates the ground through shallow or deep infiltration. In a city, that drops to around 15%.

• Consequences: This limits groundwater recharge and creates large amounts of urban runoff (also called stormwater runoff).

• Flooding: Because rain can't soak in, it rushes off roofs, roads, and parking lots into storm drains and streams all at once. Streams rise higher and faster after a storm, which causes flooding (including flash floods), and the runoff carries pollutants like oil, fertilizer, and road salt with it (5.13).

B. $CO_2$ Emissions and Climate Impact

• Producing the cement and asphalt needed to build the city.

• Machinery running on fossil fuels during construction.

• Deforestation to clear land, which means the loss of future carbon sequestration.

• Decomposition of the cut-down trees, which adds even more $CO_2$.

• Growing urban populations produce more waste, which requires landfills, and landfills release carbon dioxide and methane.

C. Resource Depletion

• Cities pack millions of people into a small area, so they pull huge amounts of water, energy, food, and building materials from the land around them, often faster than those resources can be replaced.

• Water: Many cities pump groundwater faster than rain can recharge it. Mexico City gets much of its water from the aquifer beneath it, and as the aquifer drains, the clay layers above it compact. Satellite measurements show parts of the city sinking more than 20 cm (about 8 inches) a year.

• Land: Cities often grow outward onto flat, fertile farmland and wetlands, so that land can no longer grow food or filter water.

• Saltwater intrusion: Pumping coastal aquifers too hard can also pull seawater into them (see Section II).

II. Saltwater Intrusion

• Mechanism 1, excessive groundwater use: As populations grow, they pull more water from groundwater resources, which lowers the water table. That decreases the downward pressure the freshwater exerts, so saltwater seeps into the fresh groundwater source and contaminates it.
• Real example: By 2011, Hallandale Beach, Florida, had stopped using six of its eight drinking-water wells because saltwater had moved into the aquifer beneath the city.

• Mechanism 2, sea level rise: Sea level rise causes the same problem.
• Warming oceans cause thermal expansion, meaning water molecules spread further apart.
• Melting polar ice caps increase ocean volume.
• The rising sea level overcomes the groundwater pressure, causing saltwater contamination.

• Consequence: The contaminated groundwater is no longer usable for drinking or for agricultural irrigation.

III. Urban Sprawl: Causes and Consequences

• Population shift: Since 1950 there has been a major shift from rural to urban and especially suburban communities. In a 2017 federal housing survey, 52% of US households described their neighborhood as suburban, compared with 27% urban and 21% rural.

• Urban sprawl: The movement of population away from dense urban areas out into the less dense suburbs.

Causes of Sprawl

1. Economic factors: Property values are cheaper in the suburbs, so residents can buy a bigger piece of land or a bigger home for the same price they would pay in a high-demand city.

2. Transportation: Affordable cars and gasoline make it easy to live in the suburbs while commuting into the city for work or entertainment.

3. Positive feedback loops (the domino effect):
• Residents leave, so the city’s tax base declines.
• The city cuts essential services like police patrols, bus routes, and park maintenance.
• More residents leave because those services are gone.
• Businesses follow residents to the suburbs because the consumer base shrank.
• The neighborhood experiences blight, meaning decline from abandoned homes and buildings.
• This is a vicious cycle where the original problem feeds itself and sprawl gets worse.

4. Highway expansion cycle: More driving raises gasoline tax revenue. That revenue funds more highways, which encourages people to drive even further, which continues the cycle.

Consequences of Sprawl

• Habitat loss and fragmentation: New subdivisions and roads spread into forests, wetlands, and farmland and break the remaining habitat into small pieces.

• More driving: Homes, jobs, and stores end up far apart, so people drive longer distances. That burns more gasoline and releases more $CO_2$ and air pollutants per person.

• More impervious surface per person: Spread-out roads, driveways, and parking lots cover more land, which means more runoff and flooding.

• Loss of farmland: Suburbs often spread onto the flat, fertile farmland at a city's edge.

• Higher infrastructure costs: Water pipes, sewers, and roads have to stretch farther to serve fewer people per mile, which takes more materials and energy.

IV. Solutions to Urban Sprawl

• Urban Growth Boundary (UGB): A zoning law that prevents further outward urbanization past a fixed point. It forces builders to go vertical or reuse existing abandoned buildings, which promotes efficient land use.

• Efficient public transportation: An effective, clean, on-time, affordable bus system can convince residents to stay by lowering their driving costs.

• Walkability: A city that is easy to walk around in encourages residents to stay.

• Mixed land use: Combining residential, business, and entertainment buildings in one area. This creates a “sense of place” and lets people live close to work and entertainment, which makes the community more attractive.