Flooding

Question 1

Rain that falls is collected in what?

Answer 1

Drainage basins.

Question 2

The point where rainwater flows in opposite directions is the _____.

Answer 2

Drainage divide.

Question 3

Stream gradient refers to what? How is grade measured?

Answer 3

Steepness.

Measured by the ratio of drop in elevation of a stream per unit of horizontal distance.

Question 4

Changes in gradient result in what?

Answer 4

Deposition of heavier materials.

Question 5

Streams all eventually reach a lake or ocean, the base level. What is a base level?

Answer 5

Level below which stream cannot erode.

Question 6

What is the difference between graded and ungraded streams?

Answer 6

Ungraded: down cutting, smoothing irregular gradient. Rapids, waterfalls, flat segments.

Graded: static conditions, no erosion or deposition, close to base level.

Question 7

What is the calculation for stream discharge (Q)? What does ‘Q’ mean?

Answer 7

Q = Velocity x area

Q = flow rate at any point in stream.

Question 8

Q (discharge) varies with what four things?

Answer 8

Amount of rain/snow melt.

Size of drainage basin.

Input from groundwater.

Loss by evaporation (& irrigation).

Question 9

What is stream load?

Answer 9

Total mass of material moved by a stream at one time.

Question 10

What are the three types of stream load?

Answer 10

Bedload: materials that move along the stream bed (pebbles).

Suspended load: smaller particles suspended in the water (clay).

Dissolved load: ions in the water.

Question 11

What is stream capacity? What is it greater than, and propotional to?

Answer 11

Total amount of sediment a stream can transport.

Capacity is greater than the load.

Capacity is proportional to discharge.

Question 12

What is stream competency? What is it proportional to?

Answer 12

Maximum size of particles a stream can transport at one time.

Proportional to velocity and gradient.

Question 13

List three features of a meandering stream.

Answer 13

Relatively low gradient.

Sweep from side to side.

Diverted by irregularities.

Question 14

List four features of bedrock streams.

Answer 14

Fast moving.

High energy.

Steep mountainous areas.

Stream bed is rock.

Question 15

Where do braided streams occur? Describe their discharge and banks.

Answer 15

Dry areas with episodic precipitation.

Highly variable discharge and easily eroded banks.

Question 16

When the amount of water entering a stream exceeds the capacity of the channel, what occurs?

Answer 16

Overflow of banks, inundation of floodplain.

Question 17

During flooding, what increases?

Answer 17

Velocity, discharge, competency, capacity.

Question 18

When the level of the stream is the height of the river bank, this is known as _____.

Answer 18

Bankfull.

Question 19

Grain size a stream can carry is proportional to _____.

Answer 19

Velocity.

Question 20

As a stream spills over its floodplain it undergoes what channel transition?

Answer 20

Deep, fast to shallow, slow.

Question 21

What material drops out once the stream over-banks? What does this build?

Answer 21

Heavier material.

Natural levees.

Question 22

For the following recurrence intervals (in years), list the probabilities:

• 2
• 5
• 10
• 25
• 50
• 100
• 200
• 500

Answer 22

2: 50%
5: 20%
10: 10%
25: 4%
50: 2%
100: 1%
200: 0.5%
500: 0.2%

Question 23

Urbanization results in what two things?

Answer 23

Decreases amount of permeable surface.

Water forced to run off rapidly, without time to soak in.

Question 24

Why is it dangerous to drive a car in a flood?

Answer 24

Most cars weigh less than volume of water required to fill them.

Question 25

Regarding bridges, adding water can lead to mass wasting. What is a term for this?

Answer 25

Bridge scour.

Question 26

Deforestation, overgrazing, and fires can affect flooding in what way?

Answer 26

Excessive erosion, increasing stream load.

Question 27

Are dams long-term solutions? What are two results?

Answer 27

Short-term.

Streams react to restore equilibrium by deposition in reservoirs.

May make peak flood worse.

Question 28

Describe artificial levees and artificial waterways.

Answer 28

Levees: temporary, must be raised as a stream bed rises via deposition.

Waterways: concrete rivers, fill up with debris that must be cleared.

Question 29

How does dredging work? What is a consequence?

Answer 29

Silt is removed, increasing river capacity.

After extreme flood, river accumulates more silt as flow slows down.

Question 30

What do artificial cut-offs do?

Answer 30

Shortens the stream path and increases velocity.

Question 31

This flood control measure is temporary and involves a seal and a foundation.

Answer 31

Temporary flood barriers.

Question 32

What are floodways? What is an example of one? What is a consequence?

Answer 32

A channel for an overflow of water caused by flooding, gate opens to divert water into floodway.

E.g., Red River, Winnipeg, Manitoba.

Doesn’t change flooding downstream.

Question 33

What is a flood control measure related to city development?

Answer 33

Land use zoning.

Question 34

Which of these are temporary anthropogenic flood measures?

• Bridges
• Dams
• Levees
• Floodways

Answer 34

Everything but bridges.