Unit 5 · Topic 5.10 Beta

Impacts of Urbanization

Urbanization replaces soil and plants with impervious surfaces.

Practice 7: Environmental Solutions

Part 1 · Hook

Why this matters

A field of grass and a parking lot of the same size get the same summer storm. On the field, most of the rain soaks in. On the parking lot, almost all of it rushes into a storm drain, picking up oil, litter and heat, and reaches the river within minutes. Now picture a whole city built that way, and the river below it after every storm.

Part 2 · Before you start

What this builds on

Part 3 · Prerequisite check

Quick check before you start

1. What is infiltration?

Water soaking into the ground
Water flowing over the land surface
Water evaporating from leaves
Water vapor turning into droplets

Show the answer

Infiltration is water moving into the soil; runoff flows over the surface.

Correct: Water soaking into the ground:
Water flowing over the land surface:
Water evaporating from leaves:
Water vapor turning into droplets:

2. A surface with a low albedo

absorbs most sunlight and warms up
reflects most sunlight and stays cool
lets sunlight pass straight through
gives off no heat at night

Show the answer

Albedo is the share of sunlight reflected; dark, low-albedo surfaces absorb more and warm.

Correct: absorbs most sunlight and warms up:
reflects most sunlight and stays cool:
lets sunlight pass straight through:
gives off no heat at night:

3. What is a watershed?

The land area that drains into one stream, river or lake
A layer of rock that holds groundwater
A dam that stores water for a city
The line where fresh and salt water meet

Show the answer

A watershed is the land that drains to one body of water; an aquifer is underground.

Correct: The land area that drains into one stream, river or lake:
A layer of rock that holds groundwater:
A dam that stores water for a city:
The line where fresh and salt water meet:

Part 4 · See it

See it first

Line graph, invented data for practice. Stream flow in cubic meters per second after the same storm. Before development (fields and forest) the flow rises slowly to a peak of 14 at 10 hours and stays high for many hours. After the watershed is paved and built up, the flow rises fast to a peak of 34 at 3 hours, then drops quickly. — The same storm, before and after the watershed was paved: the stream peaks higher and sooner. Invented data for practice. LevlPrep original diagram.

Part 5 · Step by step

How it works, step by step

Land is covered with roofs, roads and parking lotsrain cannot soak in (less infiltration)
More water runs off, and storm drains carry it straight to streamsstreams rise higher and faster, so floods are more frequent
Less water soaks ingroundwater is recharged less, and streams run lower between storms
Runoff flows over pavementit carries oil, metals, salt, litter and heat into streams

Part 6 · Key ideas

Key ideas

Urbanization covers land with impervious surfaces. Less rain soaks in and more runs off, so floods rise higher and faster and groundwater is recharged less.
A coastal city that pumps groundwater faster than it is recharged can pull seawater into its aquifer: saltwater intrusion.
Cities are warmer than the countryside, an urban heat island: dark surfaces with low albedo absorb sunlight, and fewer plants cool the air.
Urban sprawl spreads low-density development onto farms and natural land, which adds roads, driving and paved area per person.

Part 7 · Misconception

A common mistake

The wrong idea: Cities flood because it rains more over cities.

What actually happens: The rain is about the same; the land is different. Impervious surfaces stop rain from soaking in, and pipes deliver the runoff to streams all at once, so the same storm causes a higher, faster flood.

Part 8 · Check yourself

Check yourself

Exam-style questions. Anything you miss goes into your review queue.

1. Which of these is an impervious surface?

A parking lot
A lawn
A forest floor
A farm field

Show the answer

Pavement does not let water soak through. Lawns, forests and fields let water infiltrate.

Correct: A parking lot: Right: water cannot soak through pavement.
A lawn: Lawns let water soak in, though less than a forest.
A forest floor: Forest soil soaks up water well.
A farm field: Field soil lets water soak in.

2. Why does a stream in a city flood more often than a similar stream in a forest?

Rain on roofs and pavement runs off instead of soaking in
Cities get more rain than forests because tall buildings pull in clouds
City streams are wider, so they hold more of the rain that falls
Trees in the forest release extra water into the stream during storms

Show the answer

Impervious surfaces and storm drains send rain to the stream at once, so water levels rise fast and high.

Correct: Rain on roofs and pavement runs off instead of soaking in: Right: runoff instead of infiltration.
Cities get more rain than forests because tall buildings pull in clouds: Rainfall is about the same; the land surface is what differs.
City streams are wider, so they hold more of the rain that falls: Width is not the reason; city streams often are straightened and fill fast.
Trees in the forest release extra water into the stream during storms: Trees soak up and slow water; they do not add floodwater.

Model or diagram

Stream flow after a storm, before and after development

3. How did the peak stream flow change after development?

It rose from 14 to 34 m³ per second and came 7 hours sooner
It fell from 34 to 14 m³ per second and came 7 hours later
It rose from 14 to 34 m³ per second but came at the same time
It stayed at 14 m³ per second but lasted much longer than before

Show the answer

Before: peak 14 at 10 hours. After: peak 34 at 3 hours, 7 hours sooner.

Correct: It rose from 14 to 34 m³ per second and came 7 hours sooner: Right: higher and sooner.
It fell from 34 to 14 m³ per second and came 7 hours later: The change is the reverse: up, not down; sooner, not later.
It rose from 14 to 34 m³ per second but came at the same time: The peak time moved from 10 to 3 hours.
It stayed at 14 m³ per second but lasted much longer than before: The peak rose to 34.

4. Write the setup for the percent change in peak stream flow after development, then give the answer.

Type the setup as on the exam, with brackets and units, then the answer.

Show the answer

$\frac{34 - 14}{14} \times 100$ $\approx 142.9\%$ : the flood peak is more than twice as high.

Setup: $\frac{34 - 14}{14} \cdot 100$ ; answer 142.9 %

5. Why does the "after" curve drop so quickly after its peak?

Little water soaked into the ground, so little seeps into the stream later
The storm stopped sooner after the land was developed
The stream channel was dammed upstream after development
Evaporation from pavement removed the water from the stream

Show the answer

Before development, water that soaked in fed the stream slowly for hours. With pavement, almost everything arrives at once and nothing is stored.

Correct: Little water soaked into the ground, so little seeps into the stream later: Right: no stored groundwater to keep the flow up.
The storm stopped sooner after the land was developed: The graph shows the same storm.
The stream channel was dammed upstream after development: A dam is not shown and would lower the peak.
Evaporation from pavement removed the water from the stream: Evaporation from pavement does not empty a stream in hours.

6. A town downstream of this watershed is planning flood control. What does the graph suggest?

Floods will come faster and higher than old records predict, so warning time is shorter
Floods will come slower and lower than before, so the old flood walls can be removed
Floods will be the same size as before, but they will last for many more hours
Floods will happen mostly in summer, because pavement heats the water

Show the answer

The developed peak is 2.4 times higher and arrives 7 hours sooner, so plans based on the old curve underestimate floods.

Correct: Floods will come faster and higher than old records predict, so warning time is shorter: Right: higher, faster floods mean less warning.
Floods will come slower and lower than before, so the old flood walls can be removed: The after curve is faster and higher.
Floods will be the same size as before, but they will last for many more hours: The after curve is shorter, not longer.
Floods will happen mostly in summer, because pavement heats the water: The graph is about one storm, not seasons.

Data

Where rain goes as a watershed is paved

Average share of yearly rainfall that runs off, soaks in or evaporates, by percent of the land covered with impervious surfaces (invented data for practice, typical of published estimates)
Impervious cover	Runs off (%)	Soaks in (%)	Evaporates or transpires (%)
Natural land (0%)	10	50	40
10-20%	20	45	35
35-50%	30	35	35
75-100%	55	15	30

7. Which relationship does the table show?

As impervious cover rises, runoff rises and the share that soaks in falls
As impervious cover rises, runoff falls and the share that soaks in rises
As impervious cover rises, each share stays about the same
As impervious cover rises, evaporation rises faster than runoff

Show the answer

Runoff: 10, 20, 30, 55%. Soaks in: 50, 45, 35, 15%.

Correct: As impervious cover rises, runoff rises and the share that soaks in falls: Right: more pavement, more runoff, less soaking in.
As impervious cover rises, runoff falls and the share that soaks in rises: The directions are reversed.
As impervious cover rises, each share stays about the same: Runoff more than quintuples.
As impervious cover rises, evaporation rises faster than runoff: Evaporation falls slightly, from 40 to 30%.

Part 9 · Summary

Summary

Urbanization replaces soil and plants with impervious surfaces. Rain runs off instead of soaking in, so floods rise higher and faster, groundwater recharge falls, and runoff carries pollutants and heat to streams. Coastal pumping can cause saltwater intrusion, dark surfaces create urban heat islands, and urban sprawl spreads all of this outward.

Part 10 · Up next

What comes next

Part 11 · Connections

Connections

Question set for this topic