WCC: River Catchment Case Study - River Exe

Question 1

How does the relief of the River Exe drainage basin impact on its hydrology?

Answer 1

Steep slopes increase the speed water gets to the river

Question 2

How does the area of the River Exe drainage basin impact on its hydrology?

Answer 2

Small area means water has less distance to travel to get to the river so runoff is faster

Question 3

How does the geology of the River Exe drainage basin impact on its hydrology?

Answer 3

Impermeable rocks reduce infiltration, increasing runoff and therefore the speed the water reaches the river

Question 4

How does the land use of the River Exe drainage basin impact on its hydrology?

Answer 4

Rural nature means more infiltration - relatively slow reaction to rainfall events

Question 5

What are the benefits of using a proportional circle map to display data?

Answer 5

It’s easy to interpret

Gives you a scale and location

Question 6

What are the problems of using a proportional circle map to display data?

Answer 6

Large margin of error - numbers aren’t exact

Hard to tell the difference between the sizes of circles

Question 7

Why were drainage ditches originally dug in the UK?

Answer 7

In an attempt to make the UK more agriculturally productive after WW2

Question 8

What are the 3 problems with drainage ditches?

Answer 8

Increases flood risk downstream
Dried peat susceptible to erosion
Carbon from peat was dissolved in water

Question 9

What are the key advantages of blocking drainage ditches?

Answer 9

The moorland can hold more water and release it more slowly, reducing potential flooding elsewhere.
Restoring the peat to its original saturated state means it can sequester carbon.

Question 10

What other data would you want before recommending blocking drainage ditches on a wider scale?

Answer 10

How cost effective it is

What exactly the impact is on water quality, over a long period of time and in different catchments

Question 11

Where is the River Exe?

Answer 11

Devon, SW England

Question 12

Describe the route of the River Exe

Answer 12

It rises in the moorland of Exmoor, travels 82.7km and through two towns to reach the sea at Exmouth on the south coast of Devon

Question 13

Which 2 towns does the River Exe flow through?

Answer 13

Tiverton

Exeter

Question 14

How large is the River Exe catchment?

Answer 14

601km^2

Question 15

Where is the River Exe’s source?

Answer 15

On Exmoor

Question 16

Describe Exmoor.

Answer 16

It is an upland area of moorland and peat bogs

Question 17

What is Exmoor used for?

Answer 17

Sheep farming and shooting game birds

Question 18

Why does the River Exe catchment have a high drainage density?

Answer 18

It is largely underlain by impermeable rocks

Question 19

What is the majority of the land use in the River Exe catchment?

Answer 19

Rural (grassland)

Question 20

What percentage of the River Exe catchment is grassland?

Answer 20

67%

Question 21

What is the elevation of the River Exe when it reaches the end of the upper course at Thorverton?

Answer 21

26m above sea level

Question 22

How much rainfall does the River Exe catchment receive annually?

Answer 22

1295m

Question 23

Why is rainfall high in the River Exe catchment?

Answer 23

It lies in the wet SW of the UK

Question 24

What happens to much of the rainfall in the River Exe catchment?

Answer 24

It forms runoff

Question 25

What are the 3 main reasons for the limited infiltration in the River Exe catchment?

Answer 25

Impermeable underlying rocks
Saturation of upland peat
Drainage ditches

Question 26

Why have drainage ditches been dug in the River Exe catchment?

Answer 26

To drain the moorlands so they are more suitable for farming and shooting

Question 27

Describe the river regime of the River Exe.

Answer 27

There is a clear response to rainfall events however this occurs relatively slowly and discharge recedes relatively slowly following rainfall events.

Question 28

What percentage of the River Exe catchment is rural?

Answer 28

82%

Question 29

What 4 factors reduce the flashiness of the River Exe’s regime?

Answer 29

Rural nature of the catchment
Wimbleball Reservoir
Water abstraction
Return of effluent

Question 30

How does Wimbleball Reservoir affect the River Exe’s regime?

Answer 30

It regulates the flow of the river, smoothing out the more flashy peaks and troughs

Question 31

Where is Wimbleball Reservoir?

Answer 31

On a tributary of the River Exe - Rivver Haddeo

Question 32

What percentage of the River Exe’s drainage basin is underlain by impermeable rocks?

Answer 32

84%

Question 33

What is the main rock underlying the River Exe’s drainage basin?

Answer 33

Sandstone

Question 34

Describe the relief of the River Exe drainage basin

Answer 34

It ranges from 514m ASL in the north to 26m in the south.

It’s hilly in the north but much flatter in the south.

Question 35

Describe the upper course of the River Exe.

Answer 35

Highest and steepest from 444m-330m.

There is a V-shaped valley in the centre due to vertical erosion, surrounded by flat moorland and bogs.

Question 36

Describe the middle course of the River Exe.

Answer 36

Less steep, with a wider valley starting to develop into a floodplain.

Question 37

Describe the lower course of the River Exe.

Answer 37

The least steep from 80m-35m.

Floodplain surrounded by remains of the valley sides

Question 38

Describe the pattern of flood risk in the River Exe.

Answer 38

Risk is highest in the lower course of the river, with a cluster around the estuary area where the river meets the sea. The biggest towns are at the most risk of flooding.

Question 39

What are the 2 major reasons flood risk in towns will increase in the future?

Answer 39

Climate change making extreme weather events more likely

Towns are likely to be larger in the future

Question 40

What is the flood risk in the Exe catchment?

Answer 40

10% of houses are at risk of a 1 in 100 year flood

Quite high

Question 41

Why does the high flood risk in the Exe catchment justify the blocking of drainage ditches on Exmoor?

Answer 41

It will attenuate more water in the upper catchment rather than allowing it to enter the river channel and become flood water.

Question 42

How would drainage ditches increase agricultural production?

Answer 42

They dry out the peat which makes it more suitable for conversion to farmland or for grazing more sheep on

Question 43

What are the 2 major problems with carbon from peat becoming dissolved in water?

Answer 43

DOC makes water brown so water companies have to spend money removing it
Carbon is free to become atmospheric carbon by diffusion

Question 44

What was the project that researched the effects of blocking drainage ditches between 2000 and 2015?

Answer 44

Exmoor Mires Project

Question 45

How much moorland has been restored on Exmoor?

Answer 45

2000 hectares

Question 46

How have restoration of peat bogs and moorland on Exmoor affected the rates of runoff?

Answer 46

The amount of water leaving the moorland during heavy precipitation has reduced by 1/3

Question 47

How much has the water storage of peat increased by due to blocking drainage ditches?

Answer 47

260,000 m^3

Question 48

What 3 pieces of evidence show the success of blocking drainage ditches on Exmoor?

Answer 48

Reduced runoff
Increased water storage in peat
Rise in the water table

Question 49

How much have water table levels risen by due to blocking drainage ditches?

Answer 49

Up to 2.65cm