River Exe - W&C Flashcards
Where is the river exe
South-West England, it rises in the moorland of Exmoor and travels to reach the sea at Exmouth on the South coast of Devon flowing through the towns of Tiverton and Exeter on route.
Where is all the data being used collected
On the upper area of catchment upstream of an Environment Agency level monitoring station at Thorverton
How long do the river travel
82.7km
What are the physical characteristics of the catchment area
Area of upper catchment is 601km^2.
Maximum elevation at 514km at river source (Exmoor).
Land is flatter in south, by the time it reaches Thorverton it’s elevation is 26km.
What is the geology of the catchment area
84.4% of catchment is underlaid by impermeable rock, usually Devonian sandstone, which accounts for extensive drainage network
What is the land-use of the catchment area
Source - used for sheep farming and shooting game birds.
67% of land is agricultural.
15% is woodland.
High group or Exmoor - 3% of land is peat bog and moor.
What is the water balance for the exe catchment
Precipitation (1295mm) = evaporation +/- soil water storage (451mm) + run off (844mm)
Why is the hydrological element of rainfall high for the river
Bc it lies in the wet south-west
What is much rainfall absorbed by
Peaty moorland soil. However if it is saturated or there are drainage ditches dug, water call flow off the hills rapidly.
Why does alot of rainfall form runoff in the exe catchment
Becsuse of the impermeable underlying rocks
How much of the water balance does runoff account for
65%
Why does rainfall run off in the catchment
Bc of impermeable rocks reduces percolation and base flow.
Drainage ditches on Exmoor reduces amount of soil water storage.
What is the regime of river exe
Not as flashy as expected but there is a clear response between rainfall events as this occurs relatively slowly and discharges recedes slowly following an event
What is a reason for the subdued regime of river exe
Becsuse of the rural natural of the catchment - 82% is either woodland or grassland
What is the reservoir
Wimbleball Reservoir
Where is the Wimbleball reservoir
On the River Haddeo - an upland tributary
Where does Wimbleball supply water
To Exeter and elsewhere in East Devon
What does Wimbleball regulate
The flow of the river smoothing out the more flashy peaks and troughs which may otherwise have been expected
What are human hydrological factors afecting the regime or the river
Water is abstracted for agriculture.
In some places, effluent is also returned to the river.
The river exe has experienced significant flooding in the last, why is it projected to experience more in the future
Climate change makes extreme weather events more likely
When were the last major floods
1960
What percentage of the houses in the Exe catchment are at risk of a 1 in 100 year flood
10% (this is quite high)
Why is blocking drainage ditches on Exmoor used to reduce flooding
It attenuates more water in the upper catchment rather than allowing it to enter the river channel and become flood water
What is the current amount of properties at risk from a 1 in 100 year flood (taking existing flood defences into account) in Exeter
Around 3500
What is the predicted (2100) amount of properties at risk from a 1 in 100 year flood (taking existing flood defences into account) in Exeter
Around 4500
What is the very upper catchment of the River Exe
Moorland with large amounts of peat
Why should peat be a carbon sink
As dead organic matter, largely sphagnum moss, is compacted in saturated conditions and is therefore unable to decompose much due to the anaerobic conditions
Why have large areas of peat been drained
Since the Second World War, it was an attempt to make the UK more agriculturally productive
How have large areas of peat been drained
By digging drainage ditches
How did digging drainage ditches help
It dried the peat out and made it more sustainable for conversion to farmland or for grazing more sheep on
How was digging drainage ditches a bad thing
It meant that water entered the river channel more quickly, reducing lag time and increasing flood risk downstream.
Dried peat was also susceptible to erosion and the carbon contained within it was dissolved in water and transported downstream.
Why is dissolved carbon in water bad
Creates problems with water quality. Dissolved organic carbon (DOC) makes water brown (DOC is what you’re seeing in Dales river when they’re the colour black tea) and therefore water companies have to spend money removing the brown colour.
It means carbon is no longer being sequestered in peat lands but is free to exchange from water with the atmosphere and become atmospheric carbon.
Due to the water quality problems and the loss of carbon on Exmoor, what has there been a move towards
Blocking up these drainage ditches and restoring the pest to it original saturated state in which it is able to sequester carbon. However, landowners have to be convinced of the benefits in order to invest in blocking drainage ditches and potentially lose money from agriculture as their land becomes saturated and boggy once again
Who did research into the effects of blocking drainage ditches between 2000 and 2015
On Exmoor the Exmoor Mires Project
What are the key findings of the Exmoor Mires Project into the effects of blocking drainage ditches
Restoration of peat bogs on Exmoor has resulted in a third less water leaving the moorland during heavy rainfall compared with three years ago.
By blocking up drainage ditches, the moorland can now hold more water and release it more slowly, reducing potential flooding elsewhere.
Extrapolated across the whole 2000 hectares of restored moorland - which was the 2015 target for the Exmoor Mires Project - the results indicated that the amount of storm water running off the moorland has reduced by a third, the equivalent of more than 6,630 Olympic size swimming pools less water entering downstream rivers.
It also indicates increased water storage in the peat of 260,000 cubic metres. Put another way, that’s 104 Olympic-sized swimming pools of water removed from the river system flowing down to major population centres like Exeter.
Prof Brazier (the lead academic) said: “across the experimental site we are seeing a rise in water table levels of up to 2.65cm that can be attributed to the ditch blocking and moorland restoration. This enhanced water storage could, when replicated across the whole of Exmoor, provide a significant buffer against downstream flooding in rivers like the Exe.”
It is thought that blocking ditches does reduce carbon transfers however the ditches haven’t been blocked for long enough to show conclusive evidence of this.
