Ch. 10. - Rivers and Flooding

Question 1

What is a drainage basin?

Answer 1

Areas of land that drain all streams/rainfall to a common outlet (reservoir, point in stream, bay) divided from neighbouring basins by ridges and highlands, major drainage basins consist of many small drainage basins

Question 2

How does a typical drainage basin vary from its highest point to its lowest point?

Answer 2

Many small channels and steep slope at the highest point and one large channel, shallow gradient at its lowest point.

Question 3

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

Answer 3

1. Permeability of ground - forested vs concrete - infiltration
2. artificial/natural drainage
3. rock type
4. slope steepness
5. level of urbanization
6. presence/absence of vegetation

Question 4

How does a river erode, transport and deposit sediment?

Answer 4

1. Weathering creates sediments
2. Erosion in a river occurs because of the impact of other moving sediment and hydraulic lift.
3. Transport a) solution b) suspension c)Saltation (bounces) d) 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

During the rare occasions of high discharge erosion and transport will occur and as water velocity drops deposition occurs

Question 6

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

Answer 6

1. Number of channels - Braided has more than 1, Meandering has 1
2. Size of channels - Braided are smaller, Meandering are larger
3. Channel shape - Braided higher width/depth ratio, Meandering lower width/depth ratio.
4. Sediment load - Braided coarse, poorly sorted, moving as bed load, Meandering fine, well sorted, moving in suspension
5. Discharge - Braided variable, Meandering uniform
6. Gradient - Braided steep, Meandering shallow

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

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. Dominant discharge is often the bankfull 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

A flood is an event when there is more discharge than the channel can accommodate. The extra water leaves the channel and goes on to the flood plain.

Question 12

What is a floodplain?

Answer 12

A floodplain is the area of land next to the river channel

Question 13

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

Answer 13

The purpose of a floodplain is to store excess water temporarily and sediment longer term

Question 14

How often do rivers flood?

Answer 14

River rarely flood. They only flood during/after a large precipitation event

Question 15

Based on the results of question 4, which river, braided or meandering is most likely to produce a catastrophic flood and why?

Answer 15

Both rivers are capable of creating a catastrophic flooding event. A flood in a braided river may be more violent (faster water) in 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 where as in a meandering system it may arriver later after the precipitation event.

Question 16

Compare and contrast the behaviour of a braided river when it floods with that of a meandering river when it floods?

Answer 16

Flooding in a braided system: the excess water of a flood will enter the channel very soon after the rain fall 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 river: 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 slowing 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

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 size.

Question 18

Explain the effects that saturated ground has on flooding?

Answer 18

When the ground is saturated with water, 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

The crest of a flood is the highest expected point that water will go. A flood will crest - reach its highest point soon after a precipitation event then decrease back to normal levels in the following days.

Question 21

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

Answer 21

An urban setting is less permeable. In an urban setting there are many land surfaces that prevent water from infiltrating into the ground. In addition there is artificial drainage (sewers) that will channel water quickly into a river. These both contribute to sudden increases and decreases in discharge.

In a rural setting there are a lot of permeable surfaces (fields, forests) that allow water to infiltrate into the ground. There is also little or no artificial drainage that will move water quickly. There is also lots of vegetation which can slower water down.

Question 22

Explain the unique circumstances surrounding the behaviour 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 discharge. 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 size. 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.

Question 23

Describe in detail three 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 system.

Answer 23

1. Artificial Levees - Raised banks of the river channel intended to increase the size of the channel and keep what 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 measure may alter the river system by either changing the size or shape of the channel, the number of channels, or the nature of the whole system including the flood plain.

Question 24

What is a levee?

Answer 24

A bank or a levee is a 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

1. Over the top the bank (levee). Once the bank full discharge is reached and exceeded water spills out of the channel over top of the bank. 2. Through the bank. Water exits the channel through a breech in the bank. 3. Underneath the bank as groundwater. Water exits the channel as ground water because of the weight of the higher water in the channel pushing outward

Question 26

What is avulsion and what type of landform does it create?

Answer 26

When flood water leaves the channel through a break in the bank it is called an avulsion. The deposit that's created by an avulsion is a crevasse-splay. This is a fan shaped deposit that is thicker and courser 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

Straightening a channel will make it shorter, straighter, steeper and the flow of water will be faster. Adding a concrete liner will create a rigid, fixed channel separator from the surrounding ground. Ground water cannot enter the channel nor can river water leave as groundwater.

Question 29

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

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 surface. 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 affects does streamed sand and gravel mining have on the behaviour of the stream?

Answer 30

Sediment sand and gravel mining of a river bed changes the size and shape of the channel and the flow condition in it.

Question 31

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

Answer 31

At (A) as water moves around the concrete pillar in the river, turbulence increases and water velocity drops on the levee side or down current side. At (B) 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.