Chapter 10

Question 1

What is a drainage basin?

Answer 1

The area of land drained by a river system.

Question 2

How does a typical drainage basin vary from it’s highest point to it’s lowest point?

Answer 2

At the high point - many small channel, steep slope

At the low point - One large channel, shallow gradient.

Question 3

What factors, natural and human, affect how quickly water will or will not collect in a drainage basin?

Answer 3

Different types of permeability of materials will affect how quickly water will collect in a drainage basin. For example compare a heavily forested area vs a bedrock canyon; the canyon will quickly fill up and empty of water because the water had no place to infiltrate into the ground and slow down, whereas a heavily forested area would have infiltrated through the leaves and the dirt and would have slowly percolated through the ecosystem before entering the drainage channel. And by comparison, the same applies to farmland vs dense urban cities.

Question 4

How does a river erode, transport and deposit sediment?

Answer 4

1. Weathering creates sediment
2. Erosion in a river occurs because of the impact of moving water, impact of other moving sediment and hydraulic life.
3. Transport:
   -Solution
   -Suspension
   -Saltation
   -Traction
4. Deposition occurs when water velocity drops.
   Solution deposition occurs with precipitation.

Question 5

When does a river erode, transport and deposit most of its sediment load?

Answer 5

Most sediment movement takes place during rare, brief episodes of increased discharge - a flood.

Question 6

There are two common types of river: 1) Braided; and 2) meandering. What are some of the key differences between braided and meandering rivers?

Answer 6

Feature Braided Meandering
# of channel >1 1
Size of channels Smaller Larger
Channel shape High W/D ratio Lower W/D ratio
Sediment Load Coarse, Poorly Fine, well sorted
Sorted - moving moving as suspended
As bedload
Discharge Variable Uniform

Question 7

What is discharge?

Answer 7

The volume of water moving in a river in a certain amount of time.

Question 8

What is bankfull discharge?

Answer 8

The maximum amount of water in the channel (often the dominant discharge)

Question 9

What is dominant discharge?

Answer 9

The volume of water in the channel that does the most work. Work is measured by the amount of sediment that is moved. (often the bank full discharge)

Question 10

What is base level flow?

Answer 10

The minimum amount of moving water in a channel

Question 11

What is a flood?

Answer 11

An event when there is more discharge than the channel can accommodate. This extra water leaves the channel and goes on to the flood plane.

Question 12

What is a floodplain?

Answer 12

The area of land next to the river channel used to store excess water temporarily and sediment long term.

Question 13

What is the purpose of a floodplain? (there are two)

Answer 13

Temporary storage of excess water

Long Term storage of sediment

Question 14

How often do rivers flood?

Answer 14

Rarely

Question 15

Based on the results of question 4. (how does a river erode, transport and deposit) which river, braided or meandering is most likely to produce catastrophic flood and why?

Answer 15

Both rivers are capable of creating a catastrophic flooding event.
While a flood in a braided river may be more violent (faster water), it will be shorter in duration.
A flood in a meandering system may be less violent (slower water) but last longer. Also, a flood in a braided system will arrive sooner, whereas in a meandering system it may arrive later after the precipitation event.

Question 16

Compare and contrast the behavior of a braided river when it floods with that of a meandering river when it floods.

Answer 16

Flooding in a braided system (see photo)
The excess water of a flood will enter the channel very soon after the rainfall event.

Discharge will increase quickly and reach a maximum value quickly

Just as quickly, discharge will drop as this excess water passes through the river.

Potentially a lot of sediment will be moved.

Braided rivers usually lack a flood plain so water and sediment continue to move during a flood.

Flooding in a meandering system (see photo)

Excess water from precipitation will enter the system more slowly.

This means that discharge will increase starting later after precipitation, reach a lower maximum value over a longer period of time, and then decrease slow over time.

In a meandering river, less sediment may be removed from the system. More might be deposited or simply moved within the system.

Question 17

Explain the effects that frozen ground has on flooding

Answer 17

-

Question 18

Explain the effects that saturated ground had on flooding

Answer 18

Any extra water will remain on the surface. As a result it will travel quicker into the river contributing to a more rapid increase in discharge.

Question 19

What is a hydrograph?

Answer 19

A hydrograph is a way to compare changes in river discharge over time.

Question 20

How does the crest of a flood behave over time?

Answer 20

-

Question 21

Explain the difference between rural flood hydrograph and an urban flood hydrograph

Answer 21

Spike in immediate flood, followed - urban

rural slow gradual spike

Question 22

Explain the unique circumstances surrounding the behavior of a north flowing river in the northern hemisphere when it floods. Give an example.

Answer 22

In Canada, many rivers flow north.
As they flow north the landscape changes; permafrost is encountered. Permafrost makes the ground impermeable.
Precipitation is therefore quickly channeled into the river adding to a faster increase in Q.
Also, if there is flooding, this excess water will remain on the surface, making the flood worse.
During the winter, most rivers in Canada are frozen over. They behave like a pipe in that the water moving in them is confined to a fixed channel site.
In spring, North flowing rivers in Canada thaw first in the south. Here the extra water from snow melt eventually flows into a channel to the North that is still sealed with ice. Here water velocity increases.
At the same time, water pressure under the ice increases eventually causing the ice to break, sometimes suddenly and violently.
(EX: Mackenzie river delta)

Question 23

Describe in detail 3 flood control measures in common use. Be sure to explain what they are, what they are intended to do and the impact they often have on a river ecosystem.

Answer 23

1. Artificial Levees: Raised banks of the river channel intended to increase the size of the channel and keep more water in it.
2. Artificial Channels: Extra river channels made to accommodate excess water and keep it moving.
   3.Flood Plains: The large areas of land next to the active channel intended to store water temporarily.
   All three of these control measures may alter the river system by either changing the size or shape of the river channel, the number of channels, or the nature of the whole system; including the flood plain.

Question 24

What is a levee?

Answer 24

The raised portion of the flood plain next to the channel.

Question 25

What are the three ways that water can escape a river channel during a flood?

Answer 25

• Over to top of the bank
• Through the bank
• Underneath the bank as groundwater

Question 26

What is avulsion and what type of land form does it create?

Answer 26

A breach in a bank caused by flood water. The deposit that’s created by an avulsion is a crevasse-splay. This is a fan shaped deposit that is thicker and coarser near the breach changing to thinner and finer away from the channel.

Question 27

What is channelization?

Answer 27

The modification of a natural river channel so as to control the flow of water in it. This may include straightening the channel.
Channelization can also mean changing the banks and bottom of a channel.

Question 28

What affect does channelization have on stream flow?

Answer 28

Channelization creates a rigid, fixed channel separate from the surround ground Groundwater cannot enter the channel no can river water leave as groundwater.

Question 29

What affects do fire, logging and animal grazing have on stream behavior?

Answer 29

Fire, logging and agriculture alter vegetation:
Most often this means vegetation is removed. Once vegetation is gone, there is nothing to intercept precipitation and nothing to keep this water from rapidly collecting on and moving across the land.
More surface water means more sediment that eventually reaches the river channel. A larger sediment load in a river can alter the river system.

Question 30

What affect does streambed sand and gravel mining have on the behavior of the stream?

Answer 30

-sand and gravel -mining of a river bed changes the size and shape of the channel and the flow conditions in it

Question 31

What effect does stream flow have on a concrete pillar that has been placed in a river channel?

Answer 31

Water moves around the concrete pillar in the river, turbulence increases and the water velocity drops on the lee side or down current side. Water velocity increases as it accelerates around the post. The end result is scouring (erosion) of sediment away from the front of the position and deposition of a pile of sediment behind it.