Case Study: River Exe

Question 1

Which part of the River Exe forms the case study area?

Answer 1

• Upper catchment
• Lies above the gauging station at Thoverton

Question 2

What is the total annual precipitation input?

Answer 2

• 1295mm
• Relatively high, particularly over Exmoor.

Question 3

What proportion of the precipitation input leaves the catchment area as discharge?

Answer 3

• Approximately 2/3s or 65%.
• This is above average for a river

Question 4

What is the water balance for the River Exe? Give figures.

Answer 4

• Precipitation = evaporation +/- storage + runoff
• 1295 mm = 451 mm + 844mm

Question 5

Spearman rank correlation coefficient can be used to test the relationship between monthly precipitation and discharge. Outline the findings.

Answer 5

• Rs value was 0.36, suggesting a weak positive correlation.
• At 10 degrees of freedom, the confidence level was well below 5%.
• The result was not statistically significant and could have occurred by chance.

Question 6

Identify and explain a physical factor that accounts for the relatively high runoff.

Answer 6

• Geology
• 85% of the upper catchment is underlain by impermeable bedrock (Devonian sandstones).
• Water cannot percolate, resulting in very limited groundwater storage.
• Peaty moorland soils quickly become saturated.
• Rainfall rapidly becomes surface runoff.

Question 7

Describe the main land use in the upper catchment (include percentages).

Answer 7

• 67% agricultural grazing - grassland
• 15% woodland
• 3% moors and peat bog

Question 8

Explain how the land use affects the water balance.

Answer 8

• The low % of woodland means less interception and less evapotranspiration so more runoff.
• Grassland transpires and intercepts less than woodland and is kept short by grazing sheep and cattle who also trample soil, so a high % of rainfall runsoff.

Question 9

How many houses in the Exe catchment are at risk of a 1 in 100-year flood? Where are they located?

Answer 9

• 11,000, or approximately 10%
• Mostly in towns in the lower catchment e.g., Exeter and Exmouth.

Question 10

What is meant by the regime of a river?
Describe the regime of the River Exe.

Answer 10

• Regime: The pattern of a river’s discharge over the course of a year.
• Data from the gauging station at Thoverton shows:
• The River Exe is flashy but less flashy than might be expected.
• There is a clear response to rainfall events (indicated by a steeper rising limb).
• The response is not sudden; discharge recedes relatively slowly (gently sloping falling limb).

Question 11

Outline the key physical features of the drainage basin that contribute to the relative flashiness of the regime (shown by a relatively short lag time), which increases flood risk.

Answer 11

Geology:
- Upper catchment underlain by impermeable rock (85% Devonian sandstone).
- Precipitation runs off the surface into rivers rather than percolating to become groundwater.
- This shortens lag time.

High drainage density:
- Extensive network of tributaries.
- Transfers rainfall quickly downstream.
- Also shortens lag time.

Question 12

Identify the key physical feature of the drainage basin that helps explain why the regime is less flashy than expected.

Answer 12

Rural nature of the upper catchment:
- 82% of the area is grassland or woodland.
- Vegetation intercepts and slows the transfer of water to rivers.
- Vegetation encourages infiltration and soil moisture storage.
- This lengthens lag time.

Question 13

Identify the key human activities in the past that have contributed to the relative flashiness of the regime, which increases floor risk.

Answer 13

• Drainage ditches dug in the peat on Exmoor to make the land easier to use for grazing.
• Replacement of woodland with grazing land for livestock.

Question 14

Explain the impact of drainage ditches on Exmoor on the regime of the river.

Answer 14

• Channels precipitation more quickly off the moorland and into rivers.
• Reduces potential infiltration
• So less water is stored on the moorland, and more is transferred downstream via channel flow
• This increases the flood risk for properties in the lower lying lower catchment.

Question 15

Identify a human feature on the Exe and explain how it makes the river is less flashy than expected.

Answer 15

Wimbleball Reservoir:
- Built in 1979 on the River Haddeo, an upland tributary of the Exe.
- Regulates water flow, ensuring a steady regime throughout the year
- Prevents peaks and troughs that might cause flooding or drought.
- Acts as a significant water store.

Question 16

Outline what is being done to reverse damage done to the River Exe catchment in the past and to further reduce flood risk.

Answer 16

• Areas of peat moorland are being restored by the Exmoor Mires Partnership.
• Over the past two decades hundreds of kilometres of old ditches have been blocked to increase water storage on the moorlands and create wetter, healthier peatland.

Question 17

Evaluate the impact of the peatland restoration project on reducing flood risk downstream. Positives

Answer 17

• Water storage on the moorland:
• Water table has risen by 2.65 cm in some areas.
• 33% reduction in storm flow leaving restored sites.
• Hydrograph shows a clear increase in base flow.

Question 18

Outline the role played by the peatland in the upper catchment of the Exe in the local carbon cycle.

Answer 18

Moorland on Exmoor:
- Contains large amounts of peat, a major natural carbon sink.
- Peat is made up of dead organic matter, including sphagnum moss.
- Saturated conditions compact the organic matter, reducing decomposition.

Question 19

Outline the changes in the carbon cycle caused by the digging of drainage ditches in the past.

Answer 19

Impact of ditches:
- Ditches caused the peat to dry out, making it more susceptible to erosion.
- Carbon in the peat dissolved in water and was transported downstream, affecting water quality (making the water brown).
- The loss of carbon has the potential to turn the moorland peat from a carbon sink to a carbon source.

Question 20

Evaluate the impact of the peatland restoration project on the carbon cycle and water quality

Answer 20

Blocking ditches:
- Results in wetter, healthier peatland.
- Increases carbon storage, as less carbon is dissolved in water running off into ditches.
- Improves water quality.

Early results:
- Show an overall reduction of carbon leaving restored sites by up to 50%.
- Longer-term data is needed to get a fuller picture.

Question 21

Evaluate the impact of the peatland restoration project on reducing flood risk downstream. Limitations

Answer 21

• Relatively small scale of the project limits its overall impact.
• Project could be extended in the future if landowners agree.
• More intense storms, linked to climate change, may counteract recent gains.