Hydrology Notes - G1 WJEC

Notes on the hydrology section of the G1 section on the WJEC board. 

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  • Created on: 01-05-13 18:18
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Physical Characteristics of the Drainage Basin that either Increase/Decrease the risk of Flooding.
1. Permeable and Impermeable rocks.
Impermeable rocks like slate and marble do not allow water to enter and so the precipitation stays
on the ground and is more prone to surface run off. This is a much faster route to the river and so
there is an increased chance of flooding. This is as there is no sort of percolation or interception and
so the water is not being stored in the ground or intercepted by the trees and so there is an
increased chance of flood risk.
Permeable Rocks on the other hand do allow water to go through and allow absorption of water for
example limestone, so the risk of flooding would decrease as percolation and interception occur and
the roots of trees intercept the water and the time in which the water reaches the river decreases.
All in all the risk of a flood decreases as less water reaches the river. As a result a decrease in
percolation water logging of soil can lead to an increased overland flow.
2. Character of Precipitation.
This is where the length and amount of rainfall happens and over what area receives this level of
precipitation. Heavy short rainfall can lead to an increase in the risk of flooding as the amount of
water entering into the river is at a quick rate and can cause the banks of the river to burst. On the
other prolonged precipitation levels reduce the risk of flooding as the river levels rise at a steady
rate and not in quick flashes.
3. Vegetation Available at time of Rainfall.
Vegetation largely affects the risk of flooding as the amount of interception depends on the amount
of vegetation in that area. Areas with less vegetation will have an increased risk of flood as majority
of the water from the rainfall will reach the river, however if there is increased vegetation there is
less chance of there being aq flood as the trees and plant life are intercepting the water and so less
water is reaching the river reducing the risk of the flood.
However this may not be the case all year as vegetation may depend on the biome indigenous to
that area for example the temperate deciduous forest is a prime example as during winter times
there is an increase in the risk of flooding as the flora of the deciduous forest shed their leaves
annually as temperatures decrease this decreases the interception rates and so more of the water
from the rainfall reach the river. On the other hand in summer the risk of flood is decreased in the
temperate deciduous forest as the forest grows again with the leaves being broad and so
interception rates increase as the deciduous forest is intercepting more of the water from the rainfall
and so less reaches the river and so decreasing the risk of flooding.
4. The Relief of the Drainage Basin
The nature of slopes in the drainage basin can also be a reason for the decrease or increase in the
flood risk. If the relief of the land was steep then there would be a quick surface run off and so the
water reaches the river much quicker than if the relief of the land was shallower and so in case would
mean that there would be decreased chance of flooding as the surface run off would be at a much
slower rate and so not all of the water reaches the river in one go.

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Indirect Human Factors Affecting the Risk of Floods:
1. Urbanisation
Urbanisation is a human factor which influences the flood risk in that. Urbanisation is the conversion of
the land from rural land in to housing, offices, factories and roads.…read more

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Case Study: Boscastle, North Cornwall.
When: On 16th of August estimated 2 million tonnes worth of precipitation fell over a small river
catchment of 20km squared.
Causes of the flood:
Characteristic of the Precipitation accounted for the flooding of the River Valency where 70mm
of rain fall which is more than a month's average rainfall fell in 2 hours meaning that there was a
vast increase in the amount of water than the Valency could accommodate.…read more

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Management: Flood Defence Scheme for the Valancy:
The diagram
below shows the
many various
implemented to
decrease the risk
of flooding in
Flood Defence Schemes:
Removing trees so they don't act as debris.
Dredging of the Valancy downstream in order to allow the Valancy to contain more water.
Deflection of the water through the River Jordan relieving the amount of water that is in the River
Valancy.…read more

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flood event is the same and so the trees may not be an issue again and they may not be debris the
next time a flood happens.
Flood Management for such an extreme flood event cannot be done without spoiling the character
of the village.…read more

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Channel Dredging would not be effective in Shrewsbury as the channel would eventually silt up
again and it would be environmentally damaging. Also it would cost 45 million pounds, and so this
method was not used.
Case Study: Brisbane River, Queensland Floods. Australia.
The Brisbane River banks broke on the 11th of January 2011 and river levels peaked on the 13th at 4.46
At 4am the river had fallen to 2.75 metres, from its peak of 4.46 metres yesterday.…read more

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Land was not usable as a large percentage of land was still covered in water in the aftermath of the
flooding.…read more


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