river exe Flashcards
where is the river exe?
flows for 82.7km from its source in the hills of Exmoor through Tiverton and Exeter to the sea at Exmouth on the south coast of Devon.
characteristics of the exe upper catchment
physical - the area of the catchment is 601km2 - its maximum elevation of 514m is in the north. the land is much flatter in the south - the lowest elevation is 26m.
geology - 84.4% - underlain by impermeable rock, predominantly Devonian sandstones, which accounts for the extensive drainage network
land use - most of the land is agricultural grassland (67%) with some woodland (15%) and arable farmland. on the high ground of Exmoor, there are moors and peat bogs (3%)
annual rainfall - 90inches in upland areas - 32 inches coast
the rivers are confined with steep, narrow valleys that respond rapidly to rainfall
- floodplain widens to provide floodwater storage reducing peak flows
what is the water balance of the river exe?
- rainfall is high, especially over Exmoor
- much is absorbed by peaty moorland soils
- however, if saturated or where drainage ditches have been dug, water can flow off the hills rapidly
- runoff = 65% of the water balance - higher than average UK rivers
reasons: - impermeable nature of the most of the bedrock reduces percolation and baseflow
- drainage ditches on Exmoor reduce the amount of soil water storage
How does the river Exe respond to rainfall?
- relatively slowly - after flooding, discharge recedes slowly reflecting the rural nature of the lower catchment - 82% of the land use is either woodland or grass
what are the two recent developments that have affected the water cycle
- Wimbleball Reservoir
- Restoration of peatland on Exmoor
What is wimbleball reservoir
- In 1979 the River Haddeo, an upland tributary of the River Exe, was dammed to create the Wimbleball Reservoir
- It covers 150 hectares
- It provides water for Exeter and parts of East Devon, while ensuring a steady flow regime and regulating the peaks and flow of discharge that causes flooding
- This slows down discharge and reduces flooding downstream
However, the build up of water in the reservoir and the channel upstream could cause upstream flooding
What is the restoration of peatland on Exmoor
- For decades drainage ditches have been dug in the peat bogs of exmoor to make it sustainable for farming
- Increased speed of water flow to the Exe therefore reduces water quality because more silt is carried downstream
- Peat has also been dug for fuel, leaving ugly scars on the landscape
- When peat surfaces dry out, decomposition occurs and releases carbon dioxide and methane
- Peat also has a high carbon content (40-60%)
- The Exmoor Mires project works to restore peat bogs (mires) by blocking drainage ditches with peat blocks. This increases water content and returns the ground to the saturated, boggy conditions that would naturally occur
What is the Exmoor Mires Project?
This aims to restore 2000ha of Exmoor to boggy conditions that would naturally be present by blocking drainage ditches with peat blocks and moorland bales.
IMPACT:
Water table is going to rise as by blocking drainage ditches with peat blocks and moorland bales, percolation will occur, increasing amount of water in there
BENEFITS:
· Water storage in the upper catchment
Increasing storage capacity and reducing any flows of water away which benefits the peat ecosystem
· Water quality
Improves water quality
Where the through flow is quicker, it will tend to take with it bits of silt and that will affect the water quality
Slower through flow - when wetter- means it will take less sediment with it into the rivers so the water is generally cleaner when it ends up in the rivers meaning it is better for wild life and easier to treat.
· Carbon storage
Improves as peat is essentially carbon and water but if peat dries out through oxidation carbon is released, but if we can keep peat wet then increase of carbon storage and reduced amount released into atmosphere.
· Habitats and animals
Improved water supply for animals and plant growth
Improved grazing particularly during summer months where vegetation may have dried out
How does the landscape affect the water cycle
- 82% of the land is woodland or grass:
○ After flooding discharge recedes slowly
○ Exe catchment responds slowly due to high levels of interception from trees in woodland area
○ Grassland also leads to infiltration, some prevented by the impermeable rock = surface runoff Most of catchment is underlain by impermeable rock
○ Grass intercepts relatively small amount of interception so runoff into streams and rivers is greater
○ Despite woodland and little urbanisation (6%), bedrock is still very impermeable
○ Runoff is prominent leading to fast flow into main channel - Only 0.6% of the catchment is urbanised:
○ Less surface runoff, as the surface soil is permeable, and not covered by impermeable built environment.
○ Greater proportion of precipitation
○ Higher proportion of any rainfall will infiltrate and percolate but percolation may be reduced due to impermeable bedrock - Most of the catchment is underlain by permeable rock:
○ Reduces amount of percolation
○ Soil will reach field capacity, soil moisture capacity - soil quickly becomes saturated when rain does fall therefore higher levels of surface runoff when soil is saturated
How do the projects affect the water cycle
- An extensive network of ditches drains Exmoor’s peatlands
○ Carries water away quickly
○ Increases volume in main rivers and river flow because of drainage ditches working effectively - Wimbleball reservoir has been built on an upland tributary of the River Exe:
○ Increased store of water in catchment area
○ Discharge will decrease because water flow is being regulated therefore flow below dam is likely to be reduced
○ Flashy river may turn less flashy as water flow is controlled
Unlikely - large amounts of water may be released quickly which could cause flooding
