Case Study - River Exe

Location and how long?

South West of England - Devon, 82.7km long, flows through Tiverton and Exeter

Where is the source?

Exmoor National Park in the North of the catchment. This is the highest part of the catchment. 

Where is the mouth?

near the town of Exmouth in the South of the catchment. This is the lowest part of the catchment

Where receives the highest amount of rainfall?

Simonsbath near Exmoor (eg 2166mm) this area is in the north of the catchment where it’s mountainous.

Where receives the lowest amount of rainfall?

Exmouth (770mm)

What is the drainage density like?

High drainage density due to extensive network of tributaries.

Where are the main urban areas?

Exeter, Exmouth, Tiverton

Types of land cover?

67% agricultural grassland, 15% woodland, some arable farmland, high grounds of Exmoor =  moors and peat bogs 3% 

Rock types?

 84.44% is underlain by impermeable rocks, mostly Devonian Sandstone

Area with the highest population density?

Exeter followed by Exmouth

Areas at greatest risk of flooding?

In the south of the Exe catchment. Based on the population map this area also has the highest population density in the catchment. We can see on the terrain map that this area is very flat and low-lying. Expansive areas like this which are prone to flooding are called floodplains and are typically found in the lower course of the river.

Flood likelihood

typical range = 0.1m-2.2m has been at these levels 90% of time, highest ever recorded = 3.38m on 23 Dec 2012 2:30am, flood warning level = 3.4m. River Exe is relatively slow at responding to rainfall events. After flooding, discharge recedes slowly reflecting rural nature of lower catchment where 82% of landuse is woodland or grass

Water balance

• Rainfall is high and much is absorbed by the peaty moorland soils

• Rainfall = 65% of water balance - high compared to many other rivers as:

  • Impermeable bedrock reduces percolation and baseflow

  • Drainage ditches on Exmoor reduce amount of soil water storage 

Wimblewall Reservoir (surface area? supplies water to?)

• 1979 River Haddeo, upland tributary of River Exe was dammed creating the Wimblewall Reservoir

• Reservoir surface area = 150ha

• Supplies water to Exeter and parts of East Devon

• Regulates water flow throughout year preventing peaks and lows of river discharge that can make flooding or droughts more likely = negative feedback loop

Exmoor Peatland Restoration

• Drainage ditches have been dug in the peat bogs to make it suitable for farming increasing the flow of water into the River Exe and reducing water quality as more silt is carried downstream = positive feedback loop 

• Peat surface has dried out, decomposition has occurred releasing carbon + methane

Exmoor Mires Project

• Works to restore peat bogs by blocking the drainage ditches, increasing water content and returning the ground to saturated boggy conditions and retain carbon = negative feedback loop 

• Aim to restore 2000ha

Benefits of EMP

• More water storage and water transfer is slowed

• Improved water quality as less sediment is carried into the river - it is less expensive to treat and better for wildlife

• More carbon storage - rewetting peat means CO2 is naturally absorbed

• Improved opportunities for education, leisure and recreation

• Improved grazing and water supplies for animals 

Results of EMP

• By 2015 over 1000ha had been restored raising the water table by 2.65cm = more moisture is retained within the soil

• Storm flows and flood peaks have been reduced and baseflow has increased